2. BACKGROUND
Multiple Access techniques are used to allow many mobile
users to share simultaneously a finite amount of radio
spectrum.
The sharing of spectrum is required to achieve high
capacity by simultaneously allocating the available
bandwidth (or the available amount of channels) to multiple
users.
There are five basic access or multiple access methods:
frequency division multiple access (FDMA), time division
multiple access (TDMA), code division multiple access
(CDMA), orthogonal frequency division multiple access
(OFDMA), and spatial division multiple access (SDMA).
3. DIFFERENCE BETWEEN MULTIPLE ACCESS
AND MULTIPLEXING
MULTIPLE ACCESS MULTIPLEXING
It is a technique that permit
many users to simultaneously
access a given frequency
allocation.
It is process to combine
multiple signals for transmit it
over a single channel or media.
5. INTRODUCTION
FDMA is the division of the frequency bands allocated for
wireless cellular telephone communication into channels.
Each channel can be assigned to only one user at a time
All users transmit and receive at different frequencies
because every user receives an individual frequency slot.
6. HOW IT WORKS?
FDMA allows multiple users to send data through a
single communication channel, such as a coaxial
cable or microwave beam, by dividing the bandwidth of the
channel into separate non overlapping frequency sub
channels.
Then allocating each sub channel to a separate user.
Users can send data through a sub channel by modulating
it on a carrier wave at the sub channel's frequency.
7. HOW IT WORKS
FDMA is implemented at the media access control (MAC)
layer of the data-link layer in the Open Systems
Interconnection (OSI) reference model for networking
protocol stacks.
In FDMA, the user is assigned a specific frequency band in
the electromagnetic spectrum, and during a call that user is
the only one who has the right to access the specific band.
In the AMPS cellular phone system, these frequency bands
are allocated from the electromagnetic spectrum as follows:
8. In FDMA method, guard bands are used between the adjacent
frequency slots to minimize crosstalk between the channels. A
specific frequency band is given to one person, and it will
received by identifying each of the frequency on the receiving
end. It is often used in the first generation of analog mobile
phone.
CONT..
9. DIFFERENCE BETWEEN FDMA AND FDM
FDMA is also different from Frequency-division multiplexing
(FDM).
FDM refers to a physical layer method that blends and
transmits low-bandwidth channels via a high-bandwidth
channel.
FDMA, in contrast, is a channel access technique in the
data link layer.
10. USES OF FDMA:
FDMA is used in the Total Access Communication System
(TACS).
It is used in satellite communication systems and
telephone lines.
The Digital-Advanced Mobile Phone Service (D-AMPS)
also uses FDMA but adds time division multiple
access(TDMA)
11. ADVANTAGES OF FDMA
Due to the frequency filtering, FDMA is not sensitive to
near- far problem.
Many of the problems experienced in TDMA due to different
propagation delays are eliminated in FDMA as there is no
need for network timing
Fairly efficient when the number of stations is small and the
traffic is uniformly constant
No restriction regarding the type of baseband or type of
modulation
12. DISADVANTAGES OF FDMA
One disadvantage of FDMA is crosstalk, which can cause
interference between frequencies and interrupt the
transmission.
Between the different used frequency channels is a small
amount of bandwidth not used.
The maximum flow rate per channel is fixed and small.
Relatively expensive and complicated band pass filters
required
13. EXAMPLES
CABLE TELEVISION SYSTEM
The best example of this is the cable television system. The
medium is a single coax cable that is used to broadcast
hundreds of channels of video/audio programming to
homes.
The coax cable has a useful bandwidth from about 4 MHz
to 1 GHz. This bandwidth is divided up into 6-MHz wide
channels.
14. EXAMPLES
ANALOG TELEPHONE SYSTEM
One of the older FDMA systems is the original analog
telephone system, which used a hierarchy of frequency
multiplex techniques to put multiple telephone calls on
single line.
17. WHAT IS TDMA TECHNOLOGY?
Time Division Multiple Access(TDMA) is a digital cellular
telephone communication technology.
It facilitates many users to share the same frequency without
interference.
It’s technology divides a signal into different time slots.
Hence increase the data carrying capacity.
18. HOW IT WORKS?
TDMA takes a cellular communication channel(frequency
band) and slices it in to a series of time segments.
Each cellular user is assigned the time slices with a given
number.
Each user transmit information only for the duration of their
time segments using TDMA scheme.
By Small time segments and High slicing frequency user
Perceives a continuous Communication channel.
19. FEATURES OF FDMA
Shares a single channel by several users
High synchronization overhead is required
Data transmission is not continuous
Possible to allocate different number of slots to a user
20. FDMA VS TDMA
FDMA TDMA
Entire band of frequencies is
divided into multiple RF
channels/carriers.
Each carrier allocated to
different users.
It’s entire bandwidth is shared
among different subscribers at
fixed predetermined or
dynamically assigned time
intervals.
21. CDMA VS TDMA
CDMA TDMA
Entire bandwidth is being used
by users all the time.
Each have their unique code
to recover data.
It’s entire bandwidth is shared
among different subscribers at
fixed predetermined or
dynamically assigned time
intervals.
22. ADVANTAGES
Carry data rates from 64 kbps to 120 Mbps
More number of users can use same spectrum.
Operational costs are lower than traditional FDMA.
Different types of traffics easily achieved by allocation of
different time slots.
Resources utilized effectively as per need basis and battery
life enhanced.
23. DISADVANTAGES
Network and spectrum planning require more efforts.
Multipath interference affects call quality.
Switching from one base station cell to the other cell results
into dropped calls.
25. ROAD MAP
What is CDMA technology?
Advantages and Disadvantages
Working of CDMA
Steps in CDMA modulation
Code-division multiplexing (Asynchronous CDMA)
Code-division multiplexing (synchronous CDMA)
Applications of CDMA technology
26. WHAT IS CDMA TECHNOLOGY?
Code-division multiple access (CDMA) is a channel access
method used by various radio communication technologies.
CDMA is an example of multiple access, where several
transmitters can send information simultaneously over a
single communication channel.
This allows several users to share a band of frequencies.
28. ADVANTAGES
Improvement in capacity and security
Improvement in hand over/ hand off
Use of Wide Bandwidth
It has more number of users can share the same bandwidth.
It is well-matched with other cellular technologies.
29. DISADVANTAGES
The system is more complicated.
Guard band and guard time both are required to be provided.
As the number of users increases, the overall quality of
services decrease
30. WORKING OF CDMA
Code Division Multiple Access is entirely a different approach
from the Time Division Multiple Access. CDMA, after digitizing
the data, spreads out the data over the entire available
bandwidth. Multiple calls are overlapped to each other on a
channel which is assigned with a unique sequence code.
CDMA is a form of Spread-Spectrum technique, which
means data can be sent in small pieces over a number of
frequencies available to use at any time in the specified
range.
31. CONT…
Below diagram show that there is only one channel which is accessing by four
stations (i.e. mobile) at the same time.
Each channel has it’s own Code (i.e C1) and each channel wants to transmit data
(i.e. D1=0=-1).
Code will be assigned if it satisfies following to properties.
Property 1: Pick any two stations’ code up and multiply them you will get a
resultant and sum up the resultant if the it becomes to zero ‘0’, it will satisfy first
property.
i.e. C1 x C2 = 0, (1111)(1-111-1) = (1-11-1)=0.
Property 2: Pick any station’s code up and multiply it with ownself if you get the
answer as equal to total number of stations, it will satisfy property two as well.
i.e. C1 x C1 = total No. stations. C1xC1 = 1111x1111 = 1111 = 4
32. CONT…
The next step is to multiply each Station’s code with it’s own data (i.e C1xD1) you
will get a resultant and sum up the resultants of all stations you will get a Channel
code (i.e. 0 -4 0 0)
33. CONT…
Once you have done with above steps at the Transmitter side the next step is to
decode the data at receiver side.
At the reciver side you will have to multiply the channel code with Spreading code
you will get a resultant
The next step is to sum up the resultant and divide the answer with total no. of
stations you will get the actual data as shown in figure.
34. SPREAD SPECTRUM TECHNIQUES
Types of Spread Spectrum Communications: There are two
types of spread spectrum communications:
i. Frequency Hopping
ii. Direct Sequence
35. FREQUENCY HOPPING
Frequency hopping is the easiest of all the spread spectrum
modulation technique to use.
The idea behind frequency hopping is to transmit data across a
broad spectrum; the frequency can be rapidly switched from one to
another.
The transmitter and receiver are synchronized every time, and an
accurate clocking system, and pseudo generating system make this
frequency hopping very simple.
36. Cont…
Following diagram shows that frequency is changing after each interval of time
So that, It would be difficult for any third party to know about which frequency is
being used
37. CONT..
FHSS can be done by using two techniques
Slow-FHSS: Here one frequency hop transmits several number of Symbols (data).
Fast-FHSS: Here one frequency hop will transmit only one symbol.
38. DIRECT SEQUENCE
Direct sequence is the most famous spread spectrum
technique in which the data signal is multiplied by a Pseudo-
random noise code.
A PN code is a sequence of chips which is given values as -1
and 1 where -1 indicates 0.
The data encodes at the transmitter side and decodes at
receiver side by using the same Spreading code.
39. HOW DSSS WORKS?
In the following diagram we have data (1 -1 1) and Spreading code (100001) and
we also know that 0 = -1 so that spreading code becomes (1-1-1-1-11).
The next step is to encode the data by multiplying each bit of the data with entire
spreading code
40. CONT…
At the receiver side to decode the data you will just have to multiply the encoded
data (as shown in diagram) with spreading code.
After multiplying you will get actual data.
41. APPLICATIONS OF CDMA TECHNOLOGY
It is used in military and some commercial application.
It is used in mobile communication.
It is used in Radar and navigation systems.
CDMA is considered as the highest mode of wireless
communications and is responsible for fast and safe mode of
data exchange such as 3G.
