This document provides an overview of CDMA (Code Division Multiple Access) technology. It is divided into 5 sections that cover: 1) how CDMA works using spreading codes, 2) how CDMA systems function on the forward and reverse links, 3) call processing procedures in CDMA networks, 4) data transmission over CDMA, and 5) an introduction to the Lucent BSS CDMA infrastructure components. The document uses diagrams and explanations to illustrate key CDMA concepts such as spreading codes, processing gain, orthogonal sequences, and network architecture.
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Cdma basics
1. WELCOME TO CDMA OVERVEIW
INTRODUCTION TO CDMA RADIO INTERFACE
CDMA
2. SECTION 1
CODE DIVISION MULTIPLE ACCESS
Section Introduction
•The CDMA frequency band
•Frequency Allocation in CDMA
•Understanding the DSSS
•Codes and their functions in CDMA
•Generation of Codes
•Spreading And Despreading with Codes
3. SECTION 2
HOW CDMA WORKS
LET US PUT EVERYTHING TOGETHER
Section Introduction
•Forward link Architecture
•Reverse Link Architecture
•Logical Channels on Forward Link
•Logical Channels on Reverse Link
4. SECTION 3
CALL PROCESSING IN CDMA
Section Introduction
•Mobile Initialization
•Mobile Registration
•Handoff Types
•Rake Receiver
•Power Control
•Vocoding
5. SECTION 4
INTRODUCTION TO DATA IN CDMA
Section Introduction
•DATA Layers
•Data and Quality
•FCH and SCH
•Dormant Mode
•MAC and RLP
•SARA
9. “Shalom” “Guten Tag”
“Hello”
Û
“Time division”
“Frequency division!”
“CHAOS”
“Buenos Dias”
“Bonjour”
The SYMPHONY!
10. GSM Vs CDMA
FREQUENCY REUSE IN CDMA & TDMA
F 5
F 6 F 4
F 7 F 3
TYPICAL TDMA SYSTEM
EACH CELL USES DIFFERENT
FREQUENCY
THE PATTERN IS REPEATED
FOR THE NEXT SET OF CELL
SITES
F 1
F 1 F 1
F 1
F 1
F 1
F 1
TYPICAL CDMA SYSTEM
EACH CELL USES SAME
FREQUENCY
F 1
F 2
11. Frequency Reuse
7 cell re-use pattern
f7
f7
f2
f2
f6
f6
f1
f5 f3
f4
f1
f5 f3
f4
12. f1
f1
FREQUENCY REUSE IN in CDMA
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1 f1
13. Spread Spectrum Concept
In GSM small time slots of the spectrum (200 kHz) are used by different users as channels.
User 1
User 2
User 3
User 4
User n
Code 1
Code 2
Code 3
Code 4
Code n
1800 MHz 1850 MHz 1910 MHz 1930 MHz 1990 MHz 2000 MHz
Mobile Tx Cell Tx
Spread spectrum uses much larger slice (1.25 MHz) of the available bandwidth.
Same slice is used for all user with no time multiplexing but each user is
assigns with a different code to uniquely identify them.
14. CDMA Cellular Spectrum
846.5
MHz
825
MHz
824
MHz
835
MHz
845
MHz
849
MHz
A’’ A B A’ B’ Reverse link
891.5
MHz
870
MHz
869
MHz
880
MHz
890
MHz
894
MHz
A’’ A B A’ B’ Forward link
2 - 7
16. DIRECT SEQUENCE SPREAD SPECTRUM
A System is said to be using DSSS if it follows the two basic rules mentioned
•The Bandwidth of the Carrier frequency must be much larger than the
Bandwidth of the baseband Signals to be transmitted.
•The same codes that are used for coding the signal must also be used for
decoding the signals.
19. The Processing Gain and Capacity Relation
# USERS PROCESING GAIN (dB)
1 21 dB
2 18 dB
3 15 dB
8 12 dB
16 9 dB
32 6 dB
CDMA Spreading Gain
Consider a user with a 9600
bps Vocoder talking on a
CDMA signal 1,228,800 Hz wide.
The processing gain is
1228800/9600=128,
which is 21 db.
What happens if additional
users are added?
S/N = G/N
2 Users S/N = ___1___ = 128
1/128
3 Users S/N = ___1___ = 64
2/128
5 Users S/N = ____1___ = 32
4/128
9 Users S/N = ___1___ = 16
8/128
17 Users S/N = ____1____ = 8
16/128
Capacity Quality Related
24. Orthogonal Sequences
• Definition:
Orthogonal functions have zero correlation. Two binary sequences
are orthogonal if the process of “XORing” them results in an equal
number of 1’s and 0’s. EExxaammppllee::
00000000
((XOR) 00110011
------------
00110011
• GGeenneerraattiioonn SSeeqquueennccee::
- Seed
0 0
- Repeat: right & below 0 1
- Invert: diagonally
0 0
0 1
0 0
0 1
0 0
0 1
1 1
1 0
33. Pseudorandom Noise (PN) Codes
Two Short Codes (215 = 32,768)
Termed “I” and “Q” codes (different taps )
Used for Quadrature Spreading
Unique offsets serve as identifiers for a Cell or a Sector
Repeat every 26.67 msec (at a clock rate of 1.2288Mcps)
One Long Code (242= 4400 Billion)
Used for spreading and scrambling
Repeats every 41 days (at a clock rraattee ooff 11.22228888MMccppss))
34. PN Code Generation
1 1 0
Out
• Seed Register with 001
• Output will be a 7-digit sequence that repeats
continually : 1001011
45. SECTION 2
HOW CDMA WORKS
LET US PUT EVERYTHING TOGETHER
Section Introduction
•Forward link Architecture
•Reverse Link Architecture
•Logical Channels on Forward Link
•Logical Channels on Reverse Link
54. Pilot Channel
I Pilot PN sequence
1.2288 Mcps
BB
BB
All 0’s
Walsh W0
1.2288 Mcps
To QPSK
Modulator
Q Pilot PN sequence
1.2288 Mcps
26.67 ms frame period, repeated 75 times a second.
Pilot channels are kept at 4-6 dB higher then rest of the channels
55. Sync Channel - W
64
32
Needed to achieve code synchronization and
timing information.
Sync Channel Message includes :
• System Identification
• Network Identification
• Supported protocol revision levels
• Pilot PN sequence offset index
• Long code state
• System time
• Leap seconds
• Offset local time
• Daylight savings time indicator
• Page channel data rate
56. S
O
M
Sync Channel Frames
31 Information Bits
32 bits / 26.67 ms
I Pilot PN sequence
Convolutional encoder not zeroed out after each frame
No CRC bits at frame level, SOM (Start Of Message)
BB
BB
Sync Channel
Message Walsh W32
1.2288 Mcps
1.2288
Mcps
Q Pilot PN sequence
1.2288 Mcps
To QPSK
Modulator
Convolutional
Encoder
Rate=1/2, K=9
Symbol
1.2 2.4
Repetition
Kbps
Ksps
Block
Inter-19.2
leaver
Ksps
19.2
Ksps
57. 64
1-7
Page Channel - W
Base station communicates with the mobiles during
Idle Mode.
Page channel message includes :
• System and access parameters
• Neighbor list
• Channel list
• Page
• Order
• Channel Assignment
• Data Burst
• Authentication challenge
• SSD (Shared Secret Data) update
• Feature notification
• Null
58. I Pilot PN
1.2288 Mcps
BB
To QPSK
Modulator
BB
Paging Channel
Message
Walsh W1-7
1.2288
Mcps
Q Pilot PN
1.2288 Mcps
Convolutional
Encoder
Rate=1/2, K=9
Symbol
4.8/ 9.6/
Repetition
9.6
19.2
Kbps
Ksps
Block
Inter-19.2
leaver
Ksps
19.2
Ksps
Long Code
Decimator
Long Code
Generator
1.2288
Mcps
64:1
Long-code Mask
for
Paging Channel
Paging Channel
59. I Pilot PN
1.2288 Mcps
BB
To QPSK
Modulator
Power Control
Bits (800bps)
Walsh Wn
1.2288
Mcps
Q Pilot PN
1.2288 Mcps
Convolutional
Encoder
Rate=1/2, K=9
Symbol
Repetition
Block
Inter-leaver
19.2
Ksps
Long-code 64:1
Mask
Decimator
Long Code
Generator
1.2288
Mcps
Mux
24:1
Decimator
BB
Forward Traffic Channel
64. Walsh Code Administration
Walsh Codes have to be Orthogonal
• Walsh codes on the same “row” are Non-
Orthogonal
Reserved Walsh Codes
• F-PICH W64
0
• F-SYNC W32
64
• F-PCH W1
64
• F-TDPICH W16
128
• F-QPCH W30
128
- 2nd and 3rd F-QPCH W48
128 and W112
128
- Non-provisioned F-QPCH Walsh codes is
available for traffic
Walsh Functions Lengths
65. Access Channel
Mobile communications with base station during Idle
Mode
Access channel message includes :
• Registration
• Order
• Data Burst
• Origination
• Page response
• Authentication challenge response
66. Access Channel Frames
Long-code 1.2288 Mcps
Mask
88 Information Bits
8 Tail
Bits
20 ms
To QPSK
Modulator
• Tail Bits Zero Convolutional Encoder, No CRC Bits At Frame Level
• Preamble Comprised of Zero Filled Frames
BB
BB
Access Channel
Message
1.2288
Mcps
I Pilot PN
1.2288 Mcps
Q Pilot PN
1.2288 Mcps
Convolutional
Encoder
Rate=1/3, K=9
Symbol
4.8/ 14.4
Repetition
9.6
Ksps
Kbps
Block
Inter-28.8
leaver
Ksps
64-ary
Orthogonal
Modulator
Long Code
Generator
67. BB
To QPSK
Modulator
BB
1.2288
Mcps
I Pilot PN
1.2288 Mcps
Q Pilot PN
1.2288 Mcps
Convolutional
Encoder
Rate=1/3, K=9
Reverse Traffic Channel
Symbol
RS1/ Repetition
RS2
Block
Inter-28.8
leaver
Ksps
64-ary
Orthogonal
Modulator
Data
Burst
Randomizer
Long Code
Generator
28.8
Ksps
1.2288
4.8
Ksps
Long-code Mcps
Mask
68.
69. Reverse Channel
All MS transmit on same frequency but with different PN
codes to create different logical channels. Some channels
marked for Access are used for signaling and control.
Access Channel 1
PNA A ccess Channel 2
PNB Access Channel n
PNX Traffic Channel 1
PNH Traffic Channel 2
PNI Traffic Channel 3
PNJ Traffic Channel m-1
PNY Traffic Channel m
PNZ
70. SECTION 3
CALL PROCESSING IN CDMA
Section Introduction
•Mobile Initialization
•Mobile Registration
•Handoff Types
•Rake Receiver
•Power Control
•Vocoding
72. MOBILE TRANSITION INTO DIFFERENT STATES
Power - up
Mobile
Initialization
State
Mobile
Idle
State
Mobile
System
Access
State
Mobile
Traffic
Channel
State
Mobile Idle
Handoff or enable
to receive Paging
Channel.
Receives acknowledgement to an
Access Channel transmission other
than Origination or Page response I.e.
registration acknowledgement.
Mobile has fully
acquired system
timing.
Receives Paging
Channel Message :
Originates a cell Registration
Directed to a
Traffic
Channel
Ends use of
Traffic Channel
74. MOBILE TRANSITION INTO THE IDLE
STATE
MOBILE SWITCHED
ON
SCAN PN OFFSET 1- 512
MEASURE PILOT SIGNAL
STRENGTH
SELECT BEST SIGNAL PN
OFF SET PILOT
READ SYNCH
CHANNEL
NO
SID/NID
O.K
YES
READ PAGIING CHANNEL MOBILE IS NOW IN IDLE MODE
79. Mobile Idle State
Mobile
Initialization State
Page
Channel Monitor
Pilot search
Sub state
Mobile
System Access
State
Idle Handoff complete
Idle Handoff
Sub state
Mobile unable to
receive Paging
Channel
Mobile receives
Acknowledgement to
an Access transmission
other than Origination
or Page Response.
• Call Termination
• Call Origination
• Registration
Another pilot stronger
than current pilot
Mobile has fully
acquired system
timing
80. Idle State
System Parameters Message
Neighbor List
Access Parameters Message
CDMA Channel List Message
Global service Redirection Message
81. Slotted Mode Quick Paging Channel
Objective
• To extend the battery life of a mobile in slotted mode by
reducing the time the mobile spends monitoring paging channels.
Slotted Mode
• Paging channel divided into slots and slot cycles.Mobile
monitors specific slot in a slot cycle.
F-QPCH Functionality
• Paging or configuration change indicators is sent out on the F-QPCH
100ms prior to the message on the F-PCH.
• If the mobile cannot detect an indicator to be “OFF”, the
mobile will read the F-PCH slot immediately following the F-QPCH
slot.
83. Access Channel Protocol
Used when mobile contact base station
• Quickly
•Avoid Interference
Two protocols
• Message or order response access
• Request access
Trial – and - error
85. Access Handoff Features
Mobile (MS) searching for pilots Active-,
Neighbors-,Remaining State
Origination
No
MS perform access attempt.Up to
6 strongest pilots ind.
Active pilot in msg capsule
Access
Handoff
Mobile Traffic
Channel State
BS sends general
Page to MS
Access Entry
Handoff
Channel Assignment into
Soft Handoff (CAMSHO)
Yes
86. The Traffic Channel State
From Mobile System
Access State
Traffic
Channel
Initialization
Sub state
MS call termination MS
receives an Acknowledge
Order on forward traffic
Waiting for
Order
Sub state
Conversation
Sub state
Waiting for
Mobile
Answer
Sub state
Release
Sub state
To Mobile
Initialization State
MS cell
origination MS
receives an
Acknowledge
Order on
forward Traffic
Channel
MS user
disconnects or
MS receives
Release Order
MS receives an
Alert Order
MS receives
Release Order
MS user
Answers
call
channel
MS receives
Release Order
87. Traffic Channel Assignment Algorithm
Access seizure received on carrier
frequency Fk
Calculate downlink loading for F1,
F2,…Fk….,Fn normalized
on max_power
Subtract tca_weight from downlink
Loading for Fk
Select RF carrier frequency with
Available traffic channel elements,
available packet pipe capacity, and
minimum downlink loading.
Example:
tca _weight = 20
F1 : 30%
F2 : 45% (originating carrier)
F2 vs. F1 : (45-20=25) vs. 30
Traffic Channel assigned to F2
Called Cross Carrier Frequency
Traffic Channel Assignment when
selected RF carrier frequency = Fk
88. Traffic Channel Associated Signaling
When on a traffic channel, associated signaling is used for
communication of messages between mobile and base
station.
In addition to certain messages transmitted in Idle Mode,
other messages are send on the traffic channel :
• Handoff direction
• In-traffic system parameters
• Neighbor list update
• Mobile station registered
• Pilot strength measurement
• Power measurement report
• Handoff completion
89. Conversation Substate
Messages being sent on the traffic channel
Continuous confirmation of the traffic channel
Locating handoff candidates
Performing handoffs
Power control
Overload control
Other call activities
91. Overview of Registration
•NAME
•MIN
•ESN
•Location
•Desired Slot Cycle
•Station Class Mark
•Billing Information
92. Types of Registration
Power Up
Power Down
Timer Based
Distance Based
Zone Based
Parameter Change
Ordered
Implicit
Traffic Channel
93. CDMA2000 Handoff Related to Call Processing
States
Idle Handoff
Access Entry Handoff **
Dormant Handoff
Dormant Handoff
Access Probe Handoff
Access Handoff
Idle State
Request or
Response to send
on Access
Channel
Update Overhead
Information
System Substate
Access
States Allowed
Handoff
Types
Mobile
States
Page Response or
Origination or
Order/Message
Response Substate
Channel Assignment
Message or Extended
Channel Assignment
Message Received
Mobile Station
Control on the
Traffic Channel
State
Dormant Handoff
Soft Handoff
Softer Handoff
Soft Handoff
Softer Handoff
Hard Handoff
Dormant
Handoff
**Access Entry Handoff
allowed after receiving a
message requiring a
response or
acknowledgment
95. Access Handoffs
TIA/EIA-95
Improvements
Perform Idle Handoff
here if required
Tx strength of several
neighbors
Perform Idle Handoff between
probes if necessary
Perform Idle Handoff between
probes if necessary
Access Handoff
Channel Assignment into Soft
Handoff
Receipt of Page or
Subscriber dials #
Update Overhead Information
Begin Access Attempt
(TX of 1st Probe)
Continuous Access
Attempt(TX of 2nd Probe)
Continuous Access Attempt
(TX of 3rd Probe)
Probe is Acknowledged
Channel Assignment Message
IDLE
STATE
ACCESS
STATE
110. Why Power Control ?
Objectives
• Maintain QOS
• Maximize capacity
• Minimize interference
Power Control Algorithms
• Forward Link Power Control (FLPC,FPC)
- a.k.a Downlink Power Control
• Reverse Link Power Control (RPLC,RPC)
- a.k.a.Uplink Power Control
111. Power Control Is Required ?
Near-far Problem
Path Loss
Fading
Performance Objectives
112. Power Control
Mobile Tx Power (dBm) =
OPEN LOOP
k-Mobile Receive Power (dBm)
+ Parameters
+ Access robe Corrections (dB)
+ CLOSED LOOP Corrections (dB)
113. Reverse Link Power Control
Two separate algorithms
Reverse Open Loop (Autonomous Control)
• Performed in the mobile
• Adjust for pathloss
- Output power based on received signal strength
- Tx power = - [Mean Received Signal Strength] + correction_factors
Reverse Closed Loop (Base Station Directed Control)
•Base station directs mobile to adjust power
•Controls frame error rate of signal received at the serving base station
•Consists of an inner loop and an outer loop
114. Reverse Link Closed Loop Power Control
Base station sends power control bits
• 800 controls per second (800 Hz)
Closed Outer Loop (at base station)
• Calculates Ec/Io set point (for R-PICH)
- Based on R-FCH frame error rate
Closed Inner Loop (at base station)
•Compare Ec/Io set point with measured Ec/Io
- Send power control bits (up/down) to mobile
Mobile Adjust its power
•Based on power control bits from base station
•Step size of is adjustable
115. Forward Link Power Control
Reverse Link Closed Lop process is adopted
Mobile sends power control bits on R-PICH
• 800 control per second (800 Hz)
• Based on Eb/Nt and FER objectives
Base station received power control bits
• Variable power step size controlled by the base station
132. Packet Data Traffic
Packet data is bursts of data followed by periods of
inactivity
Resources from instantaneously inactive users are
reassigned
Inherently a best – effort system
• The system makes the best effort to provide an adequate service to multiple users
consistent with scheduling policies and user priorities.
135. 3G-1X Airlink Overview
Fundamental Channel - 9.6 Kbps
Supplemental Channel - 19.2 - 153.6 Kbps
. . .
Supplemental channels
Inactivity
Timer
Inactivity
Timer Session End
PPP Disconnect
Data Call
Reconnections
time
Fundamental
Channel
Burst Burst Dormant Burst Data Call
Origination
Data Call Data Call Data Call
Data Session
137. Supplemental Air Recourse Allocation
• SARA determines data rate and burst duration of SCH
- Goal is to maximize throughput based on QoS objectives
• First maximum data rate is determined
- Channel elements, Walsh codes, packet pipes etc.
• Then data rate and burst duration
- Maximum data rate and RF conditions.
138. Supplemental Channels -
Configuration
• No SCH
- Voice and low speed data
• Dedicated SCH
- High speed packet or circuit data for one user per SCH at a
time
• Shared SCH
- High speed packet data for multiple users on one or more
SCHs
145. Coverage
(Eb/Io) = (Eb/Io) + Margin
received required
Strong coding enables
operation at lower Eb/Io.
Soft Handoff enables
CDMA to provide
acceptable level of service
with smaller margin.
146. Radio Frequency Impairments
Total impairment = Thermal noise NO
+ Co – channel interference from
mobiles served by the same
physical antenna face
+ Co – channel interference from
mobiles served by nearby
physical antenna faces
NT
Thermal noise = NO
Total impairment = NT
149. Link Budget Impact of 3G-1X Data
• Traffic channel activity of SCH is assumed to be 1.0
• Data device is expected to be away from the user’s
head – body loss is 0dB
• Different data rates and corresponding Eb/Nt
Data rate FER Eb/Nt
19.2kbps 2% 3.4dB
38.4kbps 3% 2.6dB
76.8kbps 5% 1.8dB
153.6kbps 10% 1.0dB
• Turbo Codes
The major difference between wire line and wireless networks is the choice of access techniques.While in Landline based Telecommunication systems the users can be very conveniently divided by independent wire lines,in cellular communications since the medium of communication is air interface the subscribers have to be separated with respect to the radio frequency that they are using for communication with the base station.Bus as we know the frequency spectrum is a very limited resource.So if we divided the subscribers based on frequencies then the no of subscribers will be very limited and it will become extremely difficult to increase the capacity of the network.To overcome this problem the pundits of Cellular communications came up with various access techniques to separate the subscriber communication over the air interface.As we will see in the following section the type of access technique used plays a major role in determining the Radio capacity of the network or speaking in plain terms it decides the number of users that can simultaneously make calls in a cell.The most popular Multiple access techniques are
Frequency Division Multiple access(FDMA)
Time Division Multiple Access(TDMA)
Code Division Multiple Access(CDMA)
FDMA
FDMA is the oldest and most familiar method of radio communication
• used since 1890 in broadcasting, two-way radio, and cellular systems
Each user has a private frequency for the duration of their call
Distant users are far enough that they cause no interference
When the call is finished, the channel is released and available for a new call FDMA is the method used in the original cellular systems
• “AMPS” Advanced Mobile Phone System.
TDMA
Each user has a specific frequency but only during an assigned time slot. The frequency is used by other users during other time slots.
INTERNATIONAL VERSION
GSM: Groupe Special Mobile
Developed in Europe, used in roughly 50% of all wireless systems
worldwide.
8 timeslots, 7 or 8 users occupy in rotation
Have you got into a group, say at a street corner, hotly discussing something. Well most of the time you can’t understand a word of what is being discussed. Ask someone who have joined the group before you did, and normally “I don’t know” would be the reply.
Contrary to this imagine your staff meeting, people coming up with ideas, speaking one at a time. Everyone is heard and understood – that time division multiplexing. Or have you seen judges in a gymnastics competition – all showing their score at the same time but on different sheets, like frequency division.
And what happens when a cosmopolitan crowd gets together. Everyone speaks, everyone listens – how. Because everyone speaking uses a different language, which her/ his listener understands. We are not multiplexing time or frequency, but codes – language in this case.
Take a symphony, all the instruments are played but they remain distinct in the symphony. Each instrument have a special tone, pitch, timbre, etc. – simple each have a code. This is Code division multiplexing.
Frequencies have to be reused systematically as otherwise it will lead to co channel and adjacent channel interference which will degrade the quality of the network.
SO for this purpose standardized Frequency Reuse patterns are implemented in any cellular network though slight modifications will be there.The most common reuse patterns are
3/9 Reuse Pattern
4/12 Reuse pattern or
7 cell reuse pattern
The reuse layout pattern basically decides the cluster size.Cluster is a group of cells where all the cells are having different frequencies that means to say that there is no frequency reuse in the cluster.These clusters are then rotated and repeated through out the network.The amount of rotation and the frequency reuse distance is decided by mathematical models.
It is quite obvious that the smaller the cluster size more will be the no of repetitions of the radio frequencies and hence greater will be the capacity achieved .But frequency reuse give rise to a very imminent radio evil called interference.Smaller cluster sizes and greater repetitions will lead to lower frequency reuse distances and hence higher levels of interference.It is very important to optimize between capacity/reuse and interference.
Universal Frequency Reuse
CDMA Frequency Reuse
The principal attribute of a CDMA System is that all subscribers can use the same frequency. This underlies all other attributes. With spread spectrum, universal frequency reuse applies not only to users in the same cell, but also to those in all other cells. The advantage here is that complicated reuse patterns are not necessary.
CDMA IS A SCHEME IN WHICH MULTIPLE USERS ARE ASSIGNED RADIO RESOURCES USING THE DIRECT SEQUENCE – SPREAD SPECTRUM (DSSS) TECHNIQUES.
ALTHOUGH ALL USERS ARE TRANSMITTING IN THE SAME RF BAND, ALL USERS ARE SEPARATED FROM EACH OTHER VIA THE USE OF ORTHOGONAL CODES (WALSH CODE).
EACH USER HAS FULL TIME USE OF THE ENTIRE SPECTRAL ALLOCATIONS.
EACH USER’S SIGNAL ENERGY IS SPREAD OVER THE ENTIRE BANDWIDTH AND CODED SO AS TO APPEAR LIKE BROADBAND NOISE TO EVERY OTHER USER.
The 850MHz CDMA band is most popularly used all over the world.This band as mentioned in the previous slide works between
824-849MHz Used for the Reverse link communication
869-894MHz Used for the Forward link communication
The CDMA band is divided into sub bands as shown above.The Total Band of 25MHz is divided into small channels of 30KHz each.An actual CDMA carrier will be using a multiple of the 30KHz channels.
That means for an actually utilized bandwidth of 1.23MHz we will need 41X30KHz channels.
The Following equation gives the relationship between the channel numbers and the actual frequency.
Reverse Link Frequency = (825 + N0.03)MHz
Forward Link Frequency = (870 + N0.03)MHz
Where N = CDMA channel number
CDMA uses a modulation technique called “spread spectrum” to transport a narrowband voice
signal over a wide bandwidth channel. The wide bandwidth for IS-2000 is 1.23 MHz.
The CDMA modulation technique uses three methods for spectrum spreading:
•FH (frequency hopping)
•TH (time hopping)
•DS (direct sequence)
Because Lucent systems operate only with DS spreading, it is the only spreading technique
discussed throughout the remainder of this course, so whenever CDMA is mentioned DS
CDMA is implied.
While FH is more popular in CDMA systems used for military purposes in commercial CDMA system DSSS is popularly used.
Spreading
In a spread spectrum system the data information signal, b(t), is multiplied by a wideband
signal, c(t), which is the output signal of the Direct Sequence (DS) generator – a
pseudorandom noise (PN) output signal. The signal which will eventually be transmitted,
y(t)=b(t)c(t), will occupy bandwidth far in excess of the minimum m bandwidth to transmit the
data information.
Tb is the bit interval of the information stream and Tc is the bit interval of the DS
stream. Tc is also called a chip time. It should also be noted that the ratio of Tb to Tc is referred
to as the processing gain.
Spreading gain, or processing gain, is achieved when noise components, or noise-like
components, remain spread when the original signal (user 1 in the figure) is despread.
The original signal appears to have gained energy relative the noise. It can also be seen as if the noise has been suppressed.
By filtering out most of the wideband noise energy the original signal can be extracted,provided sufficient bit energy over noise ratio, Eb/NT. It can be seen that the “signal to noise” ratio after despreading will favor user 1 by a factor of
G = BW/bw (or Fc/Fb or Tb/Tc).
G is then called spreading gain or processing gain.
Shannon's work suggests that a certain bit rate of information deserves a
certain bandwidth If one CDMA user is carried alone by a CDMA signal, the
processing gain is large - roughly 21 db for an 8k vocoder.
• Each doubling of the number of users consumes 3 db of the
processing gain
• Somewhere above 32 users, the signal-to-noise ratio becomes
undesirable and the ultimate capacity of the sector is reached.
Practical CDMA systems restrict the number of users per sector to ensure
processing gain remains at usable levels.
PN Long Code
The long code gets its name from the fact that it takes about 41.4 days for the code to repeat.
Information about the long code is broadcast to the mobile station by the Sync Channel to help the mobile lock onto the base station and helps provide separation from other base stations. The long code is used to scramble the interleaved signal to provide additional security against interception and interference. An additional advantage of the long code is that it allows the transmitter to use less power, maintaining control over the ambient RF environment and increasing the overall capacity of the cell.
PN Short Code
One of the codes used in conjunction with the Walsh Code is the PN (pseudorandom noise)
code. The pseudorandom noise code, more commonly referred to as the PN code, is used to
provide the base station with a unique identification that the mobile station uses to identify the serving base station. The PN code can be further modified with a time offset which allows additional PN codes to be generated. The time offsets used for the PN code is based on orthogonal coding in which the spread signal is split and sent to a quadrature spreader whose output is offset by ninety degrees. The resulting offset outputs are combined and transmitted to the mobile.
Walsh Function
The user signal (or control channel) is multiplied by the Walsh code. The Walsh code provides each user or channel with a unique identifier and, in DS spreading, spreads the frame across the entire 1.23 MHz bandwidth.
The above table of 64 Walsh codes is generated by repeating the Orthogonal sequence method over and over again. These 64 codes are used to identify 64 forward channels (BTS – MS communication channels).
Walsh functions allow multiple access interference among users in the same cell to be
eliminated on the forward link using the transmitter/receiver combination shown. On the
forward link, all Walsh functions in the same cell are synchronized and have zero correlation between each other. The functions are orthogonal (correlation between two different function equal 0) and orthonormal (correlation between the same function equal 1).
The same walsh codes are reused in all cells.
•The input user data (e.g. speech) is multiplied by an orthogonal Walsh function.
•IS-2000 can use up to 128 (256 for 3G-3X) orthogonal Walsh functions.
•The user data is then spread by the base station PN code and transmitted on the carrier.
These Walsh functions are not used for user identification on the reverse link since each
mobile in the same cell experiences a different delay and the Walsh functions that are received would not be synchronized. Walsh functions that are shifted in time with respect to one another do not always produce zero correlation.
Orthogonal spreading is achieved by taking one signal bit & n-chip Walsh Code (called symbol). The signal bit is expanded to n-bits (this is simple – just repeat the signal bit, 1 or 0, n times). Then this n-bit signal are XORed with the n-chip Walsh Code and we get a n-bit Symbol.
Lets take a simple case, using a 5-bit signal string (10011) and 4-chip Walsh Code (say 0110). The first operation is of expanding the signal and XORing with Walsh Codes. That gives us:1001 0110 0110 1001 1001. This we transmit
At the receiver, we again XOR with the same code (0110) and we get 1111 0000 0000 1111 1111, that after compression (remember 4 chips in the above string represents one signal bit) gives 10011 (Wow! Just what we transmitted).
And what happens at the other receivers who also receives this string but XOR with it’s own code (say 1100)? We get 1100 0011 0011 1100 1100, well taking average over 4 chips we arrive at 00000 – that’s no signal.
PN SHORT CODE ON FORWARD LINK
On the Forward Link the PN Short code is used for quadrature spreading a.The same PN Short Code is used to generate are used as I and Q components .The PN Short Code is also used to identify the sectors by the mobile.This is achieved as mentioned below.
Each Base Station transmits a pilot signal used for acquisition, system synchronization, cell selection and coherent demodulation of the traffic channels. All base stations transmit a unique pilot signal using the same PN spreading code (which is also known as short code) but with different time offsets. So, basically, with the help of the PN short codes, the MS can identify the BTS from which the pilot is coming.
A 15 bit PN code would generate a sequence of 32,768 chips. Each PN offset has a time delay of 64 chips. The offset for a given pilot PN sequence from the zero shift pilot PN sequence equals the index value multiplied by 64. e.g. If the pilot PN sequence offset index is 11, the pilot PN sequence offset will be 11 X 64 = 704 PN chips.
Each offset being 64 chip wide, there are a total of 512 phase offsets possible. That means 512 BTS’s can be uniquely identified.PN offsets can be reused if there is sufficient separation between cells using the same offset.
PN SHORT CODE ON REVERSE LINK
On the Reverse link the PN Short Code is used only for Quadrature spreading without any offset as on the reverse link the the PN Short Code is used only for tracking the mobile,that means to calculate the round trip delay or path delay for the mobile.
PN Code Generation
PN Code Generation
PN codes are generated from prime polynomials using modulo 2 arithmetic. The state machines generating these codes are very simple and consist of shift registers and XOR gates.
Masking
PN Offset (Masking)
Masking provides the shift in time for PN codes. Different masks correspond to different time shifts. In cdmaOne systems, Electronic Serial Numbers (ESN) are used as masks for users on the traffic code channels.
As you can see by using a 3-bit mask of different values we can generate 7 (23-1) offsets.
The reason for the quadrature spreading is really to make sure that the mutual interference between users and between stations is uniformly distributed in phase. It otherwise contributes nothing to performance.
For each user (up to M users) the digital information signal is processed and then scrambled
using the PN long code.
Each channel is assigned a unique Walsh function.
All the bit streams from the active channels are then summed together.
On the resulting bit stream the PN short code is used to perform quadrature spreading
Quadrature Spreading
Quadrature spreading* is employed in the IS-2000 standard. The quadrature spreading ensures
that other-user interference appears as though it has both random phase and amplitude
information (i.e., looks like band limited Gaussian noise).** In the transmitter, user data, is
multiplied by an in-phase (PN-I(t)) and a quadrature (PN-Q(t)) spreading waveform. The
spreading waveforms are chip synchronous, but otherwise totally independent of one another.
They are modulated by a quadrature carrier, summed and transmitted.
At the receiver, quadrature carriers demodulate the transmitted carrier and the quadrature
spreading is removed. The despread signals are summed and the user data is detected. The
detected signal-to-noise ratio is greater than if quadrature spreading is not used.
The logic of 64 chip offset is simple. While the signal is spread by a 64 chip Walsh Code, it is decoded by a 64 chip code. The decoding sample is of 64 chips.
By modulating with a PN sequence which has a minimum offset of 64 chips with any other PN sequence, the sample would have near zero correlation with signals from other BTS’s.
Signals transmitted from a single antenna in a particular CDMA radio channel share a
common PN code phase. Including the zero offset sequence, PN-I-0(t) and PN-Q-0(t) there are 512 possible time offset indices to identify cells. Each time offset is 64 chips and the PN-I and PN-Q are identified by an offset index, 0 through 511, from the zero offset PN sequence, PNI-0, PN-Q-0. This can be expressed as:
PN-I-i-(t) = PN-I-0 (t -i x 64Tc )
PN-Q-i(t) = PN-Q-0 (t-i x 64Tc )
where i = 0, 1, 2,...511, and Tc is the chip duration.
The forward link of the IS-2000 system provides for orthogonal user channels by using Walsh functions. Within a sector the Walsh functions are unique but they are reused from sector to sector.
Generated in a 42-bit register, the PN Long code is more than 40 days
long (~4x1013 chips) -- too big to store in ROM in a handset, so it’s
generated chip-by-chip using the scheme shown above
PN LONG CODE ON THE FORWARD LINK
The PN Long Code is used by the cell to scramble the users signal on the forward link for enhanced security.Scrambling does not increase the signal rate it only rearranges the user data in a random sequence.This is similar to encrypting.
PN LONG CODE ON THE REVERSE LINK
On the Reverse Link each handset codes its signal with the PN Long Code, but at a unique
offset computed using its ESN (32 bits) and 10 bits set by the system
• this is called the “Public Long Code Mask”; produces unique shift
• private long code masks are available for enhanced privacy
Integrated over a period even as short as 64 chips, phones with different
PN long code offsets will appear practically orthogonal.This public long code mask is used to identify the users on the Reverse link just like the walsh codes are used to identify the users on the Forward Link.We can conclude that all Public Long Code Masks are Reverse Link traffic channels.
Coherent / Non-coherent Detection
Coherent Detection of the Forward Link
Coherent detection requires that the carrier signal used in the transmitter and the receiver are perfectly matched in both frequency and phase. In cdmaOne systems, the forward link detection in the mobile is coherent. (Forward Link is BTS>>>mobile)
Non-coherent Detection of the Reverse Link
Non-coherent detection refers to the case where the phase information is not available to the receiver. In cdmaOne systems, the reverse link is a non-coherent link since the phase information is not available. As a result, the detection process at the base station becomes non-coherent. (Reverse link is mobile>>>BTS)
In digital comm. system CRC Check bits are added to the sampled digital data. The redundant bits increase the noise resistance in the channel. In CDMA convolution coding and bit interleaving is resorted to protect the data against random errors. The resultant group of bits are called as symbols. Each symbol bit is then modulo 2 added with the chips of the spreading code in the spreader. The spreaded data acts as the modulating signal and is used for PSK modulation of the carrier
Forward Link Channels
•Pilot Channel (F-PICH) - Similar to the downlink pilot channel in IS-95, it is used at the
mobile to provide continuous time and phase reference. Each base station transmits the short
PN code using Walsh code W0 over the pilot channel with a unique base station timing offset.
•Sync Channel (F-SYNC) - In addition to providing system timing and network identification,
the sync channel identifies the state of the long PN code so that the generation of the long PN code in the mobile is synchronized with the generation of the long PN code at the base station.
•Paging Channel (F-PCH) - Provides notification of incoming calls to idle mobiles. In addition, the paging channel may be used to broadcast messages.
•Fundamental Channel (F-FCH) - For low data rates and voice calls that operate in the same
way as 2G traffic channels for backwards capability. Channel carries message and control data.
Some of the additional channels that are introduced in IS-2000 are:
•Supplemental Channel (F-SCH)* - For high data rate transmission, such as multimedia.
Channel carries user data only and must be transmitted with either the fundamental channel
and/or dedicated control channel.
•Dedicated Control Channel (F-DCCH) - For transmitting control data to individual users on
the SCH. The control data includes signaling for soft handoff, power control, and MAC protocol.
•Common Control Channel (F-CCCH) - For broadcasting control data and messages to all
mobiles within the service area
•Quick Paging Channel (F-QPCH)* - For extending the battery life of the mobile.
Forward Traffic Code Channel
Forward Traffic Code Channels
The forward traffic code channels are used to transmit user data and signaling information. The Forward Traffic Code Channels are separated by their unique Walsh code assignments. Once the mobile is assigned a Walsh code in an omni cell (or sector), the code cannot be assigned to any other mobile in that omni cell (or sector) for the entire duration of the call. A Forward Traffic Channel can be comprised of a Fundamental Code Channel and Supplemental Code Channels.
The Fundamental Code Channel
The Fundamental Forward Code Channel is used to transmit user data, signaling, and the power control sub-channel.
The Supplemental Code Channel
Supplemental Code Channels may be used to provide the subscriber with a high speed data capability. The bit rate of a single Fundamental Code Channel is limited by the Rate Set frame formats. A Forward Traffic Channel may include several Supplemental Channels to provide the required bit rate. Each Supplemental Code Channel requires an additional unique Walsh Code assignment. The Supplemental Code Channels always transmit at the maximum rate for the rate set in use and do not carry any signaling or power control sub-channel information. Supplemental Code Channels are a TIA/EIA-95 capability and are not defined in IS-95A.
The Pilot channel produces a string of 0’s, which get spread by Walsh Code W0. The resultant string at 1.288 Mcps is modulated by two PN codes in I & Q channels. The resultant is than Sin & Cos modulated and transmitted.
As one short PN code sequence takes 26.67 ms to repeat, the Pilot Frame is also taken as the same. That means the Pilot frame repeats 75 times a second.
The Sync channel always uses Walsh code 32 of length 64. After the mobile has locked onto a
sector or Base Station, it tunes to the Sync channel for timing synchronization and other
essential information, such as the Long Code State.
Sync Channel Message
The Sync Channel Message contains the parameters shown.
•System identification is the identifier number for this cellular system.
•Network identification serves as a sub-identifier for this cellular system.
•The pilot PN sequence offset index, PILOT_PN is the offset index, in units of 64 chips, for
this base station.
•The long code state is the state of the long code at the time given in the system time
parameter.
•The leap seconds is the number of leap seconds that have occurred since the start of system
time (given in the system time parameter).
•The offset of local time is the offset from the system time.
•The page channel data rate is either 4.8 kbps or 9.6 kbps.
Up to seven paging channel can be used depending on the capacity need of the paging
channel. The page channel is used to notify a specific mobile of an incoming call or message.
Page Channel Messages
Typical message on the page channel are listed.
•The system parameters message provides overhead information such as, pilot PN sequence
offset index, base station identifier, and number of paging channels..
•The access parameters message defines the parameters needed by a mobile to transmit on an
access channel.
•The neighbor list message provides information about neighbor base station parameters (e.g.,
the neighbor pilot PN sequence offset index, i.)
•The CDMA channel list message gives the designations of the supported CDMA channels.
•Typical order messages are: reorder, intercept, base station acknowledgment, lock until
power-cycled, release, registration accepted, registration request.
•The channel assignment message informs the mobile of the new CDMA frequency to use
•The data burst message is a data message sent by the base station to the mobile.
•The authentication challenge message allows the base station to validate the mobile’s
identity. The unique mobile authentication keys and/or shared secret data (SSD) for each mobile registered in the system will be used to perform the authentication calculation.
•The SSD update message is request by the BS for the mobile to update the shared secret data.
•The feature notification message allows the network to supply display information to be
displayed by the mobile such as: to identify the calling party’s number and to indicate the
number of messages waiting.
EXCEPT W1,W0,W32 ALL OTHER CHANNELS ARE USED FOR TRAFFIC CHANNEL.
Reverse cdmaOne Link
The Reverse cdmaOne Link
The reverse cdmaOne link is substantially different than the forward link. The difference is primarily due to the power control requirements and to the non-coherent nature of the reverse link. Reverse link code channels are identified by unique time shifts of the long PN code. Recall that time shifted versions of a PN code have very little correlation with each other. Hence, the subscribers in the reverse direction are channelized using a unique shift of the long PN code. There are two types of code channels in the reverse link:
Traffic
Access
IS-2000 Reverse Link Channels
To reach the higher data rates and allow more flexibility on the services required for 3G, in IS-
2000, more channels are used in the reverse link. The channels are:
•Fundamental Channel (R-FCH)* - For low data rates and voice calls that operate in the same
way as 2G traffic channels . Channel carries both message and control data.
•Supplemental Channel (R-SCH)* - For high data rate transmission, such as multimedia. The
channel carries data only and messages must be transmitted with either the fundamental
channel and/or dedicated control channel.
•Dedicated Control Channel (R-DCCH) - For signaling and short messages for the
supplemental channel
•Pilot Channel (R-PICH)* - Similar to the downlink pilot channel, it is used at the base station
to provide phase reference for the received uplink signal. The pilot channel allows the mobile
to transmit at a lower power level to reduce the overall interference level.
•Supplemental Code Channel (R-SCCH) - For high data rate transmission, such as multimedia,
and is used in a similar manner as the supplemental channel.
•Access Channel (R-ACH)* - Used when the mobile must access the system to initiate
communication or respond to a direct message sent from the base station.
•Enhanced Access Channel (R-EACH) - For better access, using 5-ms mini-frames.
•Common Control Channel (R-CCCH) - For sending data or messages without setting up the dedicated traffic channel
The reverse link physical channels are distinguished by a separate Walsh code within the
mobile. The Walsh codes are used to insure orthogonality. Spectrum spreading is done with
the long code, which is used to distinguish between mobiles as done in 2G. Because only four
channels are being separated, Walsh codes of shorter chip lengths can be used. The Walsh
codes for the reverse channels are given in the table.
Walsh Code Administration :
Because 3G-1X can co-exist with 2G on the same carrier and Walsh codes of different length can be used.Walsh code administration is an important function in the base station.The original 2G Walsh functions are all 64-bit Walsh functions WkN,
- where the superscript is the number of bits N (otherwise known as the length of the Walsh function) in the Walsh function and the superscript is the Walsh function k of Walsh length N.
Walsh function are used by the cell in the forward direction to organize traffic into different channels that can be isolated and decoded by the target mobiles. Orthogonally is the property that enables the independent channels to be isolated.All Walsh functions in use,must be orthogonal with each other,including those of a different length. The table shows how Walsh functions of different length are related.Only Walsh functions that do not share one or more rows with another Walsh function are orthogonal, e.g. W32128 is orthogonal to W064 but not to W032.The entries with dark gray shading indicate those Walsh functions in use or reserved and entries with light grey shading show which related non-orthogonal Walsh functions that cannot be used as a result.
The access channel has no specific Walsh code assignment, but uses the paging channel Walsh
codes (or number) as part of its long code mask. The access channel is used by the mobile
station for registration and call origination.
Access Channel Message
Typical message on the access channel are listed:
•A registration message informs the base station about the mobile’s location status,
identification, and other parameters. This is necessary so that the base station can efficiently
page the mobile when establishing a call to the mobile.
•Typical order message on the access channel are: base station challenge, SSD update
confirmation, SSD update registration, mobile station acknowledgment, local control response,
mobile station reject (with and without a reason).
•The data burst message is a user generated data message sent by the mobile to the base
station.
•An origination message allows the mobile to place a call - sending dialed digits.
•The page response message is the mobile response to a page or slotted-page in continuing the
process for receiving a call.
•The authentication challenge response message contains the information necessary to
validate the mobile’s identity.
Walsh Code Administration :
Because 3G-1X can co-exist with 2G on the same carrier and Walsh codes of different length can be used.Walsh code administration is an important function in the base station.The original 2G Walsh functions are all 64-bit Walsh functions WkN,
- where the superscript is the number of bits N (otherwise known as the length of the Walsh function) in the Walsh function and the superscript is the Walsh function k of Walsh length N.
Walsh function are used by the cell in the forward direction to organize traffic into different channels that can be isolated and decoded by the target mobiles. Orthogonally is the property that enables the independent channels to be isolated.All Walsh functions in use,must be orthogonal with each other,including those of a different length. The table shows how Walsh functions of different length are related.Only Walsh functions that do not share one or more rows with another Walsh function are orthogonal, e.g. W32128 is orthogonal to W064 but not to W032.The entries with dark gray shading indicate those Walsh functions in use or reserved and entries with light grey shading show which related non-orthogonal Walsh functions that cannot be used as a result.
While Walsh Codes identifies different channels, PN codes are used to identify Cells (by 15 bit code) and MS’s (by 42 bit codes).
As each MS receives a bundle of signals from a BTS, which is meant for all the channels (maximum 64) for that BTS, it identifies which one to accept depending on the Walsh Code.
The MS could also be receiving similar bundles from more than one BTS’s. Which BTS should it talk to? That is identified by a PN Code (called Short code), an unique sequence of codes allotted to each BTS.
Similarly a BTS may be receiving two or more MS’s transmitting with same Walsh Code (because a BTS can hear a MS of a neighboring cell talking to the neighboring BTS using the same Walsh code it has allotted to a MS within it’s cell). The BTS identifies which MS’s are talking to it by another set of PN code (called Long code). Long PN codes are another unique sequence of codes identifying the MS’s.
Overview of Call Processing
Initialization
The subscriber unit performs initialization when the phone is turned on and at other times during normal operation. During initialization, the subscriber unit searches for a usable pilot signal then reads the Sync Channel that is broadcast from that site.
Idle
In the Idle State the mobile is mainly listening to the Paging Channel for incoming messages. The Sync Channel is not monitored in this state.
Access
When the mobile must signal the Base Station, a transmission on the Access Channel is required. The mobile transitions to the Access State, but continues to monitor the Paging Channel.
Traffic
Once assigned to a Traffic Channel, the mobile is no longer monitoring the Paging Channel.
A mobile will in one of four states after it is powered on.
Initialization State
In the Initialization State, the mobile tunes to the first preferred RF frequency channel and
attempts to decode the Pilot Channel. If a pilot is not found, the mobile tries to decode a pilot
on another RF channel or frequency band.
Idle State
In the Idle State, the mobile decodes the Page Channel, receives additional information about
the base station, and then may stay in this state and monitor the Channel for pages or other
Messages or the mobile may change the Page Channel to monitor. This is called an idle handoff. The mobile enters the Access State if the mobile needs to communicate with the base station.
System Access State
The mobile will enter the Mobile System Access State if it receives a Page Message,
originates a call, registers, or needs to send some other message to the base station. Upon
successfully accessing the base station for either a Page Response or Origination, the mobile
will be directed to enter the Traffic Channel State. Receiving an acknowledgment to any other
order or message while in the System Access State will cause the mobile to re -enter the Idle State.
Traffic Channel State
In the Traffic Channel State, the mobile communicates with the base station using the Forward
and Reverse Traffic Channels.
Overview of Protocol Layers (continued)
Functions of the cdma2000 Signaling Services Layer
The Signaling Services Layer processes all messages exchanged between the mobile and the base station. These messages control such things as call setup and teardown, handoffs, feature activation, system configuration, registration and authentication.
In the mobile, the Signaling Services Layer is also responsible for maintaining the mobile’s call processing states:
Mobile Station Initialization State
Mobile Station Idle State
System Access State
Mobile Station Control on the Traffic Channel State
System Determination Substate
In the System Determination Substate, the mobile selects the system to use. The mobile
usually has user selectable preferences that prioritize the mobile initialization sequence. These
functions are manufacturer dependent but may allow user preferences, such as going to an
analog band if CDMA is not available.
Pilot Channel Acquisition Substate
The Pilot Channel is assigned Walsh code zero, which is all zeros, so the Pilot Channel is
simply the short PN codes. If the mobile fails to find the PN codes within 15 seconds, it will
return to the System Determination Substate.
Sync Channel Acquisition Substate
Having obtained a Pilot Channel (bit synchronization), the mobile tries to decode the Sync
Channel, Walsh code 32, to obtain basic timing and long code information from the base
station (code synchronization).
Mobile Timing Change Substate
In the Mobile Timing Change Substate, the mobile uses the Sync Channel information to set
its internal timing to the system by synchronizing its long code generator to the system, and
offsetting its System Time with respect to the short code PN sequence.
Introduction
In the Initialization State, the mobile selects a serving system, mode of operation, and
synchronizes to the selected system. The Initialization State has four sub-states:
Introduction
In the Initialization State, the mobile selects a serving system, mode of operation, and
synchronizes to the selected system. The Initialization State has four sub-states:
•System Determination Substate
•Pilot Channel Acquisition Substate
•Sync Channel Acquisition Substate
•Mobile Timing Change Substate
The state diagram shows the states and the events that cause the mobile to change to a new
state.
Sync Channel Acquisition Substate
Having obtained a Pilot Channel (bit synchronization), the mobile tries to decode the Sync
Channel, Walsh code 32, to obtain basic timing and long code information from the base
station (code synchronization).
Mobile Timing Change Substate
In the Mobile Timing Change Substate, the mobile uses the Sync Channel information to set
its internal timing to the system by synchronizing its long code generator to the system, and
offsetting its System Time with respect to the short code PN sequence.
In the Idle State, the mobile reads the paging channel information and measure the Pilot
Channel on the serving base station, as well as other Pilot Channels used in the system.
Page Channel Monitoring
The mobile can monitor all the paging information, or just selected frames, to conserve battery
life in a handheld unit. This is known as the slotted mode. If the mobile is using the slotted
mode, the system can only page a mobile when the mobile is active. The mobile and base
station have to negotiate the frames to be used before the mobile can enter the slotted mode.
The negotiations are conducted between the base station and the mobile on the Page Channel
and Access Channel.
While in the idle state, the mobile can originate a call, respond to a page, or synchronize to a
stronger base station. The mobile will re -enter the initialization state if it loses the signal from
the active base station, and none of the other pilot channels are adequate.
Introduction
In the Idle State,the mobile reads the paging channel information and measure the Pilot Channel on the serving base station,as well as other Pilot Channels used in the system.
Page Channel Monitoring
The mobile can monitor all the paging information or just selected or just selected frames to conserve battery life in a handheld unit. This is known as the slotted mode.If the mobile is using the slotted mode,the system can only page a mobile when the mobile is a active.The mobile and base station have to negotiate the frames to be used before the mobile can enter the slotted mode.The negotiations are conducted between the base station and the mobile on the Page Channel and Access Channel.
While in the idle state,the mobile can originate a call,respond to a page,or synchronize to a stronger base station.The mobile will re-enter the initialization state if it loses the signal from the active base station and none of the other pilot channels are adequate.
CDMA Channel List Message/Extended CDMA Channel List Message
The CDMA Channel List Message (CLM) or Extended CLM (ECLM) include the frequency channel assignments for all carriers with supported Paging Channels,the ECLM applies to 3G mobiles.Once the CLM/ECLM shows more than one carrier the mobiles will use the their IMSI to”hash” to an available carrier,spreading the mobiles over the various Paging channels,and their associated Access Channels.
Idle Handoff
When the mobile synchronizes to a stronger base station it is called idle handoff.the mobile monitors the strength of the active pilot,as well as the strength from a number of neighbor pilots,and any other pilot channels.If another pilot channel exceeds the active pilot channel by 3dB for more than one second the mobile will re-enter the initialization state and attempt to acquire the stronger base station a handoff while in the idle state
Idle State
Idle State
The mobile’s first action in the idle state is to read the configuration messages on the paging code channel. Paging code channel 1 is the default paging channel. When a mobile enters the idle state, it listens to paging channel 1 and reads the configuration messages. The paging channel messages include: configuration messages, page messages, mobile station directed orders, data burst messages, and acknowledgments for access channel messages. In the idle state, the mobile may listen to the Paging Channel continuously or may use a discontinuous reception technique called “Slotted Paging”.
Paging Channel Overhead Information
Overhead messages are transmitted on the paging channel. The mobile station uses the information in these messages to configure itself for proper operation in the serving system.
Mobile Station Directed Messages
In addition to the overhead messages, the base station also sends messages on the paging channel directed to a particular subscriber unit. The figure lists some of the messages that are sent on the paging channel in a PCS system.
Slotted Mode
The Paging Channel (F-PCH) is divide into 80-ms slots called Paging Channel Slots. A mobile is able to operate in two modes when reading the paging channel.In the first mode the mobile reads every paging message,responds to messages addressed to the mobile, and ignores all the rest. In the second mode, called Slotted Mode,mobile will only read the paging channels messages in selected paging channel slots and ignores all messages in the remaining slots.This allows a mobile unit to power down until a selected slot is being transmitted extending battery life in handheld units.
Quick Paging Channel
The quick Paging Channel (F-QPCH) is a feature that reduces the amount of time a mobile requires to monitor page channels,that result in an extended battery life for the mobile.F-QPCH contains a 2-bit message that directs slotted mode.3G-capable mobile stations to monitor their assigned slot on the page channel.When the F-QPCH is activated in a cell,every carrier with 3G hardware capability and operating page channel receives one F-QPCH.
F-PCH
The Paging Channel protocol provides for scheduling the transmission of messages for an individual mobile in certain assigned paging slots.The mobile may not operate in the slotted mode in any state except the mobile idle state.The mobile calculates the page channel slot by using the hashing algorithm specified in the IS-95A standard.The base station calculates the same page channel slot from the International Mobile Station Identification (IMSI) as specified by IS-95/IS-2000 and only transmits messages in that page slot.
The slot cycle is a multiple of 1.28 seconds and is specified by the slot cycle index.The length of the slot cycle.T in units of 1.28 seconds is given by:
T= 2 i
Where I is the slot cycle index.There are 16 T slot cycle.
F-QPCH
A mobile monitoring the F-QPCH will be notified about a page or updated overhead messages on the regular page channel (F-PCH) using a 2-bit indicator 100 ms prior to the assigned slot on the F-PCH.
The slot cycle structure of the F-QPCH is the same as F-PCH but shifted 100 ms. See the figure.
The F-QPCH slot is divided into four parts. A paging indicator is transmitted two times in part 1 & 3 or part 2 & 4. What parts to transmit the indicator in is determined by the hashing algorithm defined in IS-2000.5 (Section 2,6,7,1).The purpose of the hashing algorithm is to spread out the mobile over the F-QPCH slot so that not every mobile is monitoring the same indicators.
The hashing algorithms determines the paging indicator bit position relative to the start of the F-QPCH slot.
If the mobile cannot detect a paging indicator to be “OFF”, the mobile will read the F-PCH slot immediately following the F-QPCH slot.The mobile needs only to detect one indicator to be “OFF”, I.e. if the mobile detect the first indicator to be “OFF” the mobile does not have to check another.
In the System Access State the mobile attempts to access the base station for a number of reasons :
-The user originates a call
-The mobile has received a page
-The mobile needs to communicate with the base station
The mobile first reads the Page Channel overhead message stream until it has received a current set configuration messages,and then transmits a message to the base station on the access channel using the Access Channel Protocol.
There are 2 Access Channel Protocols,both having slightly different configuration parameters:
-Mobile is responding to a message or order (e.g. page message).
-Mobile is requesting access (e.g., origination)
Introduction
The mobile accesses the base station on the Access Channel using a random procedure to minimize the chance of interference to other users.The access protocol is controlled by base station translation parameters that are transmitted to the mobile in the Access Parameters Message on the Page Channel.The figure shows a simplified Access Channel Protocol.
Access Attempt
The entire process of accessing a base station is called an access attempt. Each transmission in the access attempt is called an access probe. The mobile transmits the same data in each access probe in the access attempt.
Within an access attempt, access probes are grouped into access probe sequences. Each sequence consists of a number of access probes, all transmitted on the same access channel.The first access probe of each sequence is transmitted at a specified power level relative to the nominal open loop power level, IP.Each subsequent access probe is transmitted at a higher power level until the base station acknowledges the transmission.
Access Probe Structure
The actual access probe transmission consists of a preamble.Walsh modulation symbol ), for a number of frame ,followed by the actual message consisting of a number of frames.The preamble is used to synchronize the base station to the mobile.The preamble plus the message capsule is called one access slot.
Persistence Delay
The persistence delay depends on,among other parameters, the access overload class assigned to a subscriber (as programmed into the mobile ) and its persistence value. The purpose of the persistence test is to increase the probability a high priority subscriber will gain access over a lower priority subscriber during emergency or overload conditions.
Introduction
IS-95B introduced a set of features aimed at lowering the origination and termination failures, by making sure the mobile is always communicating with the best server or servers. In the Lucent architecture, three IS-95B features called the Access Handoff features are implemented. Th 3 features are :
Access Entry Handoff
Access Handoff
Channel Assignment Into Soft Handoff
The Access Handoff features operate from the time the base station sends a page message to the mobile, until a Channel Assignment Message is sent to the mobile.
Access Entry Handoff
The Access Entry Handoff (AEHO) optional feature allows IS-95B mobiles to improve termination performance by transferring reception of the paging channel from one base station to another before the mobile enters the system access state during a termination.
Access Handoff
The Access Handoff optional feature allows IS-95B mobiles to transfers reception of the paging channel from one base station to another after a successful access attempt. When a mobile sends a message to a cell and receives an acknowledgement from the cell, a successful access attempt has been made.
Channel Assignment Into Soft Handoff
Another of the IS-95B Access Handoff features is the Channel Assignment Into Soft Handoff (CAMSHO). This features gives the capability for an IS-95B complaint mobile station (MS) to set up multiple traffic channels with multiple IS-95B complaint cells when the traffic channel assignment to as many as six traffic channels during the call setup phase to reduce the chance of failed mobile call attempts.
Introduction
When the mobile station has accessed the system,a number of substates are visited to complete the call
The substates are :
Traffic Channel Initialization Substate
Waiting for Order Substate
Waiting for Mobile Answer Substate
Conversation Substate
DetailsAfter the mobile successfully accesses the base station and receives a traffic channel assignment the mobile verifies that the Forward Traffic Channel can be decoded and begins transmitting on the Reverse Traffic Channel. If the call is mobile originated the mobile enters the conversation Substate until the call is ended by either party. For mobile-terminated calls,the mobile enters the Waiting for order Substate,and waits for an order usually the alert.he mobile now enters the Waiting for Mobile Answer Substate until the user answers the call.The mobile enters the conversation state when the user answers the alert. Finally, the mobile resets back to the initial System Determination Substate when either party “hangs up.”If the party calling the mobile terminates the call before the mobile party answers, the mobile will enter the release Substate.
Traffic Channel Initialization Substate
Once the mobile has successfully accessed the base station, the system takes over the process of getting the mobile assigned to a traffic channel.
When a carrier frequency is selected, the first step is selecting a channel element(CE) controller to load balance traffic. An idle CE is selected and assigned a Walsh code and sector. The base station will activate the CE and set the initial CE gain. Additional negotiations of the details of the service request may occur on the traffic channel. The base station notifies the mobile about the traffic channel using the Channel Assignment Message on the page channel.
Traffic channel Assignment Algorithm
When operating with multiple RF carrier frequencies the base station must also perform traffic carrier frequency selection using the Traffic Channel Assignment(TCA) algorithm to balance the load of traffic between carriers. If the carriers are equally loaded, then their coverage is similar, which is important for multi-carrier call processing, especially call setup and handoff.
How does it work ?
The figure shows the flowchart for the TCA algorithm. While operating with multiple RF carrier frequencies, the base station may receive a seizure from a mobile on the access channel associated with a particular RF carrier frequency,FK, but the mobile may be served by a traffic channel associated with a different RF carrier frequency. This is called “ cross carrier assignment ”. The decision as to selection of the actual traffic carrier frequency is affected by the comparative downlink loading across RF carrier frequencies. This comparison can be controlled by weighting with the RF Loading Weight translation parameter, tea_weight.
The downlink loading is calculated by estimating the total power transmitted on the downlink. Ptotal , on a per carrier basis , and then normalizing by dividing by max_power translation parameter. Finally, the value of the tea_weight translation parameter is subtracted from the normalized value calculated for the carrier frequency that the access seizure was received, FK .
The least loaded, after the weight factor has been subtracted, and unblocked carrier will be selected for the traffic channel.
Traffic channels may use up the rest of the available Walsh codes (not used by overhead
channels), although the system typically requires far fewer.
Traffic Channel Messages
Some messages on the forward traffic channel are listed.
•The handoff direction message provides the mobile with information to begin the handoff
process. The analog handoff direction message tells the mobile to switch to the analog mode
and begin the handoff process.
•The in-traffic system parameters message updates some of the parameters set by the system
parameters message in the page channel.
•The neighbor list update message updates the neighbor base station parameters set by the
neighbor list message on the page channel.
•The pilot strength measurement message send information about the strength of other pilot
signals that are not associated with the serving base station.
•The power measurement report message sends frame error rate statistics to the base station.
The report is made at specified intervals or when a threshold is reached.
•The handoff completion message is the mobile response to a handoff direction message.
A mobile station (MS) on a traffic channel is said to be in the Conversation Substate. When the MS is in the conversation Substate, many activities,besides traffic information between the MS and the base station (BS), takes place.
The MS no longer monitors the page channel.Also, the access channel is no longer used to send information to the base station (BS).Messages between the BS and MS are instead sent on the traffic channel. This is called Associated Signaling.
The traffic channel is continuously being confirmed so that resources are not unnecessarily tied up to a traffic channel that is not functioning. Other call processing activities that take lace are location of handoff candidates as well as performing handoffs. To maximize capacity and performance, power control is done. To protect resources and performance, overload control also takes place.
The Traffic Channel State
Frame Formats
The traffic channel carries user voice, user data, and call control signaling. This is accomplished using a multiplex option. Signaling on the traffic channel is important for the Handoff and power control processes.
Overview of Registration
Definition
Registration refers to the process by which mobile stations make their whereabouts known to the cellular system. Cellular systems use registration as a means to balance the load between the Access Channel and the Paging Channel.
Types of Registration
The CDMA specifications offer multiple ways of initiating registration. The different registration procedures can be enabled or disabled independently allowing the cellular carriers to tailor any subset of registration methods to optimize the use of their systems.
Registration is the process by which the mobile station notifies the base station of its location, status,identification, slot cycle and other characteristics. This is needed so that the base station knows the capabilities of the mobile and can efficiently page the mobile.The CDMA system supports nine different forms of registration :
Power-up registration : When the mobile powers on or switches from using an alternative serving system, the mobile station registers.
Power-down : The mobile registers when it power off.
Timer-based Registration : It causes the mobile to register at regular intervals.The system can also automatically deregister mobile that did not perform a successful power-down.
Distance-based Registration : It causes the mobile to register when the distance between the current BS and the BS in which the mobile is last register exceeds a threshold. The mobile can calculated the distance based on the base stations latitude and longitude.
Zone-based Registration : It is done when a mobile enters a new zone. Zones are groups of base stations within a given system and network. A base station’s zone assignment is identified by the REG_ZONE field of the System Parameters Message.
Parameter-change Registration : It is performed when a mobile station modifies any of the following stored parameters :
- preferred slot cycle index- station class mark- call termination enabled indicator
Types of CDMA Handoffs (continued)
Dormant handoff pertains to packet data calls, and will be discussed in Section 10, Wireless Data. Soft and softer handoffs are allowed in the System Access State when using the Reservation Access Mode of the Enhanced Access Channel.
Ordered Registration : The BS can request the mobile to register Implicit Registration : When a mobile successfully sends sends an origination or Page Response Message, the base station can infer the mobile station’s location.
Traffic Channel Registration : Whenever the BS has registration information for a mobile assigned to a traffic channel, the BS can notify the mobile that it is registered.
Non-Autonomous Registration Parameter Change Registration: Certain parameters in the mobile station directly affect the process of delivering calls to the mobile station and therefore, should be updated in the system whenever a change in them occurs. These parameters are the mobile station’s SCM (Station Class Mark), the mobile station’s preferred slot cycle, and the mobile terminated call indicator.
Ordered Registration: When the cellular system orders the mobile station to register, using the registration order message, the mobile station responds with a registration message on the access channel.
Implicit Registration: Implicit registration occurs when the mobile station and base station exchange messages that are not directly related to registration but convey sufficient information to identify the mobile station and its location (to within a base station coverage area) to the cellular system.
Traffic Channel Registration: Traffic channel registration relates to a method in which the mobile station receives registration related information while on the traffic channel. IS-95A provides for transmission of registration information on the traffic channel, preventing, in many cases, an automatic registration following a call.
Idle Handoffs
Idle Handoff
While in the Idle state, the mobile may move from one cell to another. Idle handoff arises from the transition between any two cells. Idle handoff is initiated by the mobile when it measures a pilot signal significantly stronger than the current serving pilot (3 dB stronger).
Access Handoffs
Handoff During Access
Handoff in the access state is specifically prohibited in IS-95A. This prohibition made access processes easier to implement during the initial development of the early CDMA systems. Performance was sacrificed for simplicity. Access failures in the handoff region were a significant performance deficiency, however, and TIA/EIA-95 includes the following handoff techniques to improve performance:
Access Entry Handoff
Access Probe handoff
Access handoff
Channel Assignment into soft handoff
Soft Handoff
Soft Handoff
Soft handoff refers to the state where the mobile is in communication with multiple base stations at the same time. Soft handoff is a make-before-break type of handoff whereby a mobile acquires a target code channel before breaking an existing one. Soft handoff is a special attribute of CDMA and is enabled by universal frequency reuse. The advantages of soft handoff are several:
Fewer dropped calls.
Soft handoffs in general require less mobile transmit power.
Increases capacity.
Improved Clarity.
Inter-frequency handoff is when the mobile has to retune to a different CDMA RF carrier frequency. When a mobile is on a traffic channel, there are two reasons why the mobile would perform an inter-frequency handoff.
Carrier is discontinuing (border carrier ).If the mobile is on a carrier that is discontiuing and moving out from that carrier’s coverage area, an inter-frequency handoff must be performed to avoid losing the call.
Current carrier may be blocked(handoff escalation ). A mobile may not be able to perform a soft/softer handoff because there are no resources available at the target sector.
To avoid dragging the call too far and risking a dropped call, the mobile may perform an inter-frequency handoff.
Traffic Channel Handoffs
Soft Handoff
Soft Handoff is Mobile Assisted
Soft Handoff is the process of establishing a link with a target cell before breaking the link with a serving cell. Mobile stations continuously search for Pilot code channels on the current frequency in order to detect potential candidates for handoff.
Requires Both Cells to Be on the Same Frequency
The mobile typically contains only one RF receiver. Therefore soft handoff requires that both the serving and the target cells be transmitting on the same CDMA frequency.
All Cells Deliver Vocoded Frames to the BSC
All BTSs participating in a soft handoff transmit identical frames. The mobile station combines these frames and then forwards a single frame to the vocoder. On the reverse link, the BTSs decode and then deliver vocoded frames to the BSC independently.
Soft-Softer Handoff
Multiple cells and sectors may be involved in a handoff in a variety of ways. The Figure depicts a scenario where a portable unit is in softer handoff with two sectors of one cell and is also in soft handoff with another cell. The BSC will receive a vocoded frame from each cell and choose the error free one.
Traffic Channel Handoffs (continued)
“Softer” Handoff
Softer handoff is a soft handoff between two sectors of the same cell. Signals received by different sectors can be combined by the rake receiver in the BTS. It should be noted, however, that only one voice frame is eventually forwarded to the BSC. Softer handoff enables greater efficiency in the use of hardware since only one channel element is used to support such a handoff.
Softer handoff occurs between two sectors of the same cell and using the principles of CE( channel element) pooling allows one CE to control the call on two different sectors. Softer handoff is a special case of soft handoff mode, since it is communicating on two CDMA traffic channels in the same 1.23Mhz band The BS uses the CE’s rake receiver to combine the traffic frames from both sector into a single traffic fame that is send to the MSC. The MSC treats this traffic frame as if it were the only frame received by the base station CE. The BS CE also generates two forward traffic channel frames, each with a different Walsh code.
Traffic Channel Handoffs (continued)
Hard Handoffs
A Hard handoff entails a brief disconnection from a current serving cell prior to establishing a connection with a target cell. Hard handoffs can occur for several reasons. The figure illustrates a hard handoff from a CDMA system to an analog system. Hard handoffs, however, may also occur between CDMA cells. CDMA-to-CDMA hard handoffs are due to frequency mismatches, frame offset misalignment, or disjoint cells (cells served by different BSCs).
Semi-Soft
A semi-soft handoff occurs when the mobile is commanded to retune to a different CDMA RF channel or to change the frame offset but the frame selector and MSC Vocoder (speech handler) remains assigned to the call. Semi-soft handoffs require a connection from the frame selector to both base stations. Either both base stations are connected to the same Digital Cellular Switch (DCS) or there exists a packet connection between DCSs when the base stations are connected to different DCSs.
Hard Handoff
A CDMA-to-CDMA hard handoff occurs when a new frame selector and Vocoder are assigned to the call by the MSC. A change of CDMA RF channels may also occur, which is referred to as an inter-frequency handoff. This type of handoff is used when the call is transferred to a base station connected to a different DCS that does not have a ATM or packet bus connection to the old DCS ( for e.g. : Non-networked adjacent systems.)
CDMA soft handoff is driven by the handset
• Handset continuously checks available pilots
• Handset tells system pilots it currently sees
• System assigns sectors (up to 6 max.), tells handset
• Handset assigns its fingers accordingly
• All messages sent by dim-and-burst, no muting!
Each end of the link chooses what works best, on a frame-by-frame basis!
• Users are totally unaware of handoff
Each BTS sector has unique PN offset & pilot
Handset will ask for whatever pilots it wants
If multiple sectors of one BTS simultaneously serve a handset, this is
called Softer Handoff
Handset can’t tell the difference, but softer handoff occurs in BTS in
a single channel element
Handset can even use combination soft-softer handoff on multiple
BTS & sectors
Introduction
The IS-95B soft handoff algorithm enables the network to reduce the average no. of legs per call, improves forward link capacity and saves the backhaul (packet pipes) capacity without impacting the RF performance. The algorithm allows the mobile to use the dynamic add/drop threshold defined by standard.
Background
The field data shows that under some conditions there may be more soft handoffs occurring than are necessary when using the IS-95A hand-off algorithm. Such handoff overheads may also overuse system recourses, thereby degrading total system capacity. The new soft handoff algorithm is intended to improve these situations by introducing the dynamic handoff threshold determined by combining the pilot strengths from all pilots in the active set. Under this algorithm, the mobile will send out a PSMM Message to request the BS to add a pilot into the active set only, when the pilot is worthy of being added. The mobile will request the base station to drop a pilot from the active set if the pilot contributes little.
Benefits
The IS-95B soft handoff algorithm introduces improvements that will reduce the time a call is in soft handoff and also filter out unnecessary handoffs from each call, therefore the average no. of legs for each call is reduced and the forward link capacity is increased
Dynamic Thresholds
Three additional parameters are defined for IS-95B: namely. ADD_INTERCEPT. DROP_INERCEPT, and SOFT_SLOPE. The parameters contribute to the calculation of the Dynamic Add and Drop thresholds (DAT and DDT. Respectively)
The Pilot Searching Process
The Mobile Searches for Strong Pilots
The searching process is continuous and is conducted not only to find handoff candidates, but also to identify usable multipath arrivals from the serving cell.
The Mobile Reports
The handoff process is “mobile assisted”: When the mobile detects a pilot of sufficient strength, it reports the event to the base station.
The Base Station Directs
When the base station receives a report from the mobile, a handoff decision is made and directions are sent to the mobile to perform the handoff.
The Pilot Searching Process (continued)
Search Windows
The system operator determines the size of the search windows used by the mobile. Searching over a window of chips accommodates unpredictable changes in propagation delay due to varying multipath conditions and propagation delay differences between the serving cells and other cells that may be useful in the future.
The Pilot Searching Process (continued)
Multipath Arrivals
The figure depicts the signals arriving from 3 different cells. The horizontal axis is time, in PN chips. The vertical axis is the pilot signal to noise ratio, Ec/I0,in dB. This is a typical display found on QUALCOMM’s Mobile Diagnostic Monitor (MDM).
The Pilot Searching Process (continued)
Pilots are Grouped Into Sets
Pilots are grouped into four sets. These sets are used to prioritize the pilots and increase the efficiency of searching. The searching process is not standardized but pilots are generally searched in the following order:
Active Set
Pilot Channels associated with forward Traffic Channels currently assigned to the subscriber station. This is a search for additional multipaths of the same Pilot Channels.
Candidate Set
Pilot Channels whose strength, as measured by the subscriber station, exceeds an over the air given threshold.
Neighbor Set
Pilot Channels transmitted by cells in the vicinity of the cells currently transmitting to the subscriber station. The contents of the Neighbor Set is normally configured by the system operator using the Neighbor List Message.
Remaining Set
All other Pilot Channels that are possible within the current system. This search is conducted to allow the system to configure itself as well as to account for special coverage spots within the cell.
Handoff Signaling
Handoff Signaling
The Pilot Strength Measurement Message (PSMM)
The Handoff Direction Message (HDM)
The Handoff Completion Message (HCM)
Handoff-Signaling Parameters
T_ADD: The threshold for signaling a PSMM to the Base Station. This threshold may be fixed for all mobiles in a sector or may be dynamically calculated by each mobile individually depending on the protocol revision supported in the mobile.
T_DROP: The threshold for signaling a PSMM to the Base Station. This threshold may be fixed for all mobiles in a sector or may be dynamically calculated by each mobile individually depending on the protocol revision supported in the mobile.
T_TDROP
Handoff Signaling
The Mobile adjusts the priority of pilots as necessary
When the strength of a pilot rises above T_ADD, the mobile will autonomously add that pilot to its Candidate Set and signal the system by sending a PSMM. If the system directs the mobile to handoff, the new pilot will be added to the mobile’s Active Set. When the strength of the pilot falls below T_DROP for a sufficient period of time, T_TDROP, the mobile signals the base station with a PSMM.
Power control is a continuous process in CDMA. The entire foundation of CDMA is rested on accurate power control. Remember, “everyone receives everything” – every MS receives transmission related to itself plus those related to other MS in the vicinity. The success of recovering it’s own signal is based on the fact that other signals (which is noise to this MS) are just as strong but carry different codes. That is why when the composite signal is operated by it’s relevant code only the intended signal remains, rest gets cancelled out.
But if any rogue MS transmit with such high power than what other receive may just get flooded with the rogue’s noise (like a violin getting violent in a symphony – the symphony is lost) then all efforts of recovering respective signals may become futile. Thus power recovery is a must in CDMA.
Even when every unit is behaving perfectly certain power control is required for various reasons. Power update is done in the MS’s based on specific instructions from the BSC every 1.25 ms.
Power Control (continued)
Fast and Accurate Power Control
Control of the mobile’s transmission power is more critical than control of the base station transmit power. The mobile uses the equation shown to calculate its transmission power. Both the open loop and the closed loop power control can impact the level of transmit power on a continual basis as they work together to adjust power.
Introduction
CDMA systems use two forms of reverse link power control,open and closed.Open power control is used by the mobile to evaluate the strength of s base station’s received signal during the access cycle.The mobile uses these measurements to determine its initial power level for call origination.
By using power control, the base station is able to control interference.
Reverse Link Open Loop Power Control
The mobile measures the total signal power received from all base station at the mobile’s location.The combined signals are used by the mobile to adjust its transmitter power, the stronger the combined received signals, the lower the transmitter power.Reception of a strong signal from the base station indicates that the mobile is either close to the base station or has a good path to the base station. This means the mobile can use less transmitter power and still produce a good signal at the base station reducing interference to other stations.
Reverse Link Closed Loop Power Control
The IS-200 reverse power control scheme is a generalization of the IS-95 version.The system supports the same open and closed loops, but it integrates the functionality of power control of the fundamental and supplemental channels in a simple scheme, which is easy to expand.
The key factor in the simplification is the introduction of the reverse pilot R-PICH, which is used as a reference for measurement and scaling in the open and inner loops. The system computes the R-PICH power control corrections needed by the open and inner loops, and then translates them to corrections that apply to the R-FCH. The scaling is done pr channel and per data rate, so that rate equalization can be performed easily.
Introduction
Power Control, specifically Forward Link Power Control (FPC), is one of the capacity limiting factors in 2G.Therefore,the FPC algorithm has been redesigned for 3G and is fundamentally different from its IS-95 predecessor. Its main objective is to increase the voice call capacity in the forward link by a series of new enhancements :
High speed forward power control
Closed loop with fast time response
Variable power step size controlled by the base station
3G Forward Link Power Control
3G-1X FPC for voice is similar to the IS-95 Reverse Link Closed Loop Power Control, with an inner loop and an outer loop. The 3G FPC consists of a closed loop that operates at a rate of 800 Hz. The closed loop is used to compensate for power fluctuations due to fast Rayleigh fading in the forward link. The closed loop involves both the base station and the mobile.
3G FPC Mobile Operation
Similarly to the IS-95 Reverse Link Closed Loop power Control, the 3G FPC operates with an inner loop and an outer loop. The parameters of the outer loop are configurable by the base station; however, the operation of the outer loop itself is not specified in IS-2000.
3G FPC Base Station Operation
The base station operation of the 3G FPC is a simple algorithm that increases the traffic channel output power when a power up control command is received on the R-PICH. When a power down command is received, the traffic channel gain is reduced. The traffic channel gain will vary between a maximum and minimum value.
Multipath
Better Use of Multipath
One of the main advantages of CDMA systems is the capability of using signals that arrive in the receivers with different time delays. This phenomenon is called multipath. FDMA and TDMA, which are narrow band systems, can not discriminate between the multipath arrivals, and resort to equalization to mitigate the negative effects of multipath. Due to its wide bandwidth and Rake Receivers, CDMA uses the Multipath signals and combines them to make an even stronger signal at the receivers.
Multipath
Rake Receiver
CDMA subscriber units use rake receivers. The Rake receiver is essentially a set of four or more receivers. One of the receivers (fingers) constantly searches for different multipaths and helps to direct the other three fingers to lock onto strong multipath signals. Each finger then demodulates the signal corresponding to a strong multipath. The results are then combined together to make the signal stronger.
This is what the MS’s do to your speech. They take the parts where you are actually speaking (creating non-zero signals), sample, digitize and transmit them are they are. They codify the periods of silence into a few bytes which basically indicates the period of silence. At the receiving end, the MS regenerates the speech by converting the digitized samples punctuated by periods of silence, creating from the codes, at right places.
This technique, aptly called Vocoding, is optimized for human speech. It may not be suitable for other forms of signal – say music.
Several techniques have evolved for vocoding – Residual Excited Linear Prediction (RELP) & Code Excited Linear Prediction (CELP) are two, which reproduces voice almost like PSTN lines.
The main purpose of variable-bit-rate operation is to reduce interference caused by each transmitter when there is little or no speech activity. It allows the transmit power to be decreased while maintaining a constant Eb/Io. A reduction in transmit power decreases the level of interference imposed on other users of the system.
Eb= Energy per bit, Io= Interference Power spectral density.
Variable Rate Vocoder
Variable Rate Vocoder
The vocoder compresses the output of the codec to a lower bit rate to reduce bandwidth. The variable rate vocoder takes advantage of low speech activity and transmits at lower rates, thus reducing the average transmission to about 4 kbps. The second diagram illustrates how the vocoder outputs frames at full, half, quarter, and one-eighth rate.
Rate Sets
Primary Traffic Frame Sizes - Rate Sets 1 and 2
In cdmaOne, voice data is formatted into 20 msec frames. The frames are appended with cyclic redundancy check (CRC) bits for error detection purposes and with extra eight “0” bits to zero out the convolutional encoder that is used for error correction purposes. As the data rate decreases, the number of CRC bits also reduces. Notice that there are no CRC bits for 1/4 or 1/8 rates in Rate Set 1. The information content of these frames is very minimal.
Multiplexing
Multiplex Option 1
CDMA One systems multiplex signaling and data with the voice signal. Multiplexing may also be used to transmit a secondary user data signal over the same traffic channel. Note that for Rate Set 1, multiplexing is done at Full Rate only. The transmission of data over CDMA channels is standardized in IS-707.
Overview of Protocol Layers
ISO/OSI Seven Layer Network Reference Model
In 1978, the International Standards Organization (ISO) introduced the ISO model for Open Systems Interconnection (OSI). This model serves as a international standard for designing protocols for network communications. It defines a layered architecture consisting of seven layers, and specifies the functions of each layer.
Conceptually, each node of a network contains a protocol “stack” consisting of the seven layers. Each layer of a given node communicates with its “peer” layer at another node of the network. In reality, all communication flows down the stack within a node, until it reaches the physical layer. At the physical layer, the data flows across a physical medium to another node, where it then flows up the stack.
It is important to remember that this is a reference model. Not all layers are present in every network. For example, cdma2000 networks specify Layers 1 and 2, and an upper layer of signaling that is loosely referred to as Layer 3.
MAC on Dedicated Channels (continued)
Quality of Service (QoS)
The Quality of Service (QoS) function of the MAC sublayer is responsible for meditating conflicting requests from competing services and prioritizing those requests.
For example, suppose that a voice service and a data service application were running simultaneously. It is the responsibility of the MAC sublayer to ensure that the voice service receives real-time throughput, since voice packets cannot be retransmitted. However, it also attempts to ensure that the data service gets some share of the available bandwidth.
QoS is an important function of the MAC layer, but it is not specified in the cdma2000 standard. This is an unspecified area that gives equipment manufacturers an opportunity to distinguish themselves in the marketplace.
MAC on Dedicated Channels (continued)
Radio Link Protocol
The Radio Link Protocol (RLP) is a protocol layer used in cdma2000 for data service applications. It provides segmentation and reassembly of the transmitted data stream, delivering the octets in the correct order. It substantially reduces the error rate exhibited by CDMA channels.
RLP may operate in transparent or non-transparent mode. Transparent mode does not provide any retransmission of erased frames. Non-transparent mode is a NAK-based protocol. Using sequence numbers, the receiving side detects missing frames and sends a negative acknowledgment (NAK). The transmitting side retransmits those frames that were lost, and the receiving side must re-assemble the data stream in the correct order.
A new type of RLP, called RLP Type 3, has been specified to be used with cdma2000 traffic channels. RLP Type 1 supports TIA/EIA-95A traffic channels, while RLP Type 2 supports Medium Data Rate in TIA/EIA-95B.
Architecturally, the RLP function is part of the MAC sublayer. However, because it is an integral part of Data Services, it is specified in the IS-707 Data Services specifications.
CDMA Capacity
CDMA Reverse Link Capacity Estimate:
The parameters in the CDMA capacity equation are defined as follows:
N = Number of users
W = Bandwidth of the CDMA System
R = Rate of the vocoder
GS = Sectorization Gain
GV = Voice activity gain
Eb = Energy per bit (Joules)
I0 = Interference Power Spectral Density
f = Percentage of interference from other cells
Coverage
Greater Coverage
Due to high processing gain (strong coding and interleaving techniques), CDMA cells can cover a larger area for the same amount of available power. This, along with the capacity advantages, makes CDMA the most economical system for cellular and PCS.