1. Multiplexing & Multiple Access
 Multiplexing
 SDM
 FDM/FDMA
 TDM/TDMA
 CDM
 CDMA
1

2. MC Influence to the Layer Model
 service location
Application layer
 new applications, multimedia
 adaptive applications
Transport layer  congestion and flow control
 quality of service
 addressing, routing,
Network layer device location
 hand-over
 authentication
Data link layer  media access
 multiplexing
 media access control
 encryption
Physical layer  modulation
 interference
 attenuation
 frequency
2

3. Multiple transmitters sending signals
at the same time through the shared medium “air”
How to share the medium (common channel) with
other transmitters?
Multiplexing
Goal: Minimize the degree of interferences and
maximize the bandwidth for data transmissions
3

4. Multiplexing
•Capacity of transmission medium usually exceeds
capacity required for transmission of a single signal
•Multiplexing - carrying multiple signals on a single
medium
•More efficient use of transmission medium
4

5. Reasons for Widespread Use of Multiplexing
 Cost per kbps of transmission facility declines with an
increase in the data rate
 Cost of transmission and receiving equipment declines
with increased data rate
 Most individual data communicating devices require
relatively modest data rate support
5

6. Multiplexing Techniques
 Frequency-division multiplexing (FDM)
 Takesadvantage of the fact that the useful bandwidth of the
medium exceeds the required bandwidth of a given signal
 Time-division multiplexing (TDM)
 Takesadvantage of the fact that the achievable bit rate of the
medium exceeds the required data rate of a digital signal
6

7. Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
7

8. Frequency-division Multiplexing
8

9. Time-division Multiplexing
9

10. Multiplexing channels ki
Multiplexing: Multiple transmitters send
signals at the same time
k1 k2 k3 k4 k5 k6
Multiplexing in 4 dimensions
 space (si)
c
 time (t)
 frequency (f) t c
 code (c)
t
Goal: supporting multiple users on a
s1
shared medium (more channels) f
 Maximize channel utilization s2
f
(higher total bandwidth)
c
Important: guard spaces needed t
What will be the problem if the separation is
s3
small and large? Small, the receiver cannot distinguish signals/noises. Large, a waste
of bandwidth f
Fr. Schiller
10

11. Space Division Multiple Access
 Use space division multiplexing
 Frequency reuses to increase the total system
bandwidth
 Segment space into sectors
 Use directed antennas or limited communication range
signals from base stations
 Mobile stations may receive signals from base stations with
different quality (select the best one => it is the closet one)
 May combine with other schemes, i.e., FDM
11

12. Frequency Multiplexing
Separation of the whole spectrum into smaller frequency bands (consider the whole
spectrum as the multiple lanes of a road)
The same station uses different frequencies for sending signals for different users
A channel gets a certain band of the spectrum for the whole time
Advantages:
 Simple
 No dynamic coordination
necessary k1 k2 k3 k4 k5 k6
Disadvantages: c
 Waste of bandwidth f
if the traffic is
distributed unevenly
 Inflexible
 Guard spaces
(adjacent channel interference)
t
12

13. Frequency Division Multiple Access
 Assign a certain frequency to a transmission channel between a
sender and a receiver (use frequency division multiplexing)
 Channels can be assigned to the same frequency at all times
(permanent), i.e., in radio broadcast
 Channel frequency may change (hopping) according to certain pattern
 Slow hopping (e.g., GSM) and fast hopping (FHSS, Frequency
Hopping Spread Spectrum)
 Frequency division duplex (FDD): simultaneous access to medium by
base station and mobile station using different frequencies
 Uplink: from a mobile station to a base station
 Downlink: from a base station to a mobile station
13

14. Time Multiplexing
A channel gets the whole spectrum for a certain amount of time
Advantages:
 Only one carrier in the
medium at any time (constant time period)
 Throughput high even k1 k2 k3 k4 k5 k6
for many users (RR)
c
Disadvantages:
 Time quantum normally very small f
 Precise synchronization
necessary (timing)
t
14

15. Time and Frequency Multiplexing
Combination of both methods (time & frequency)
A channel gets a certain frequency band for a certain amount of time
Example: GSM (a 2G cellular network)
Advantages:
 Better protection against
tapping (more complicated)
 Protection against frequency k1 k2 k3 k4 k5 k6
selective interference
c
But: precise coordination
required f
t
15

16. Code Multiplexing
Each channel has a unique code (encoding and decoding) => d1 -
> (encoding function f(d1,key)) -> p1
k1 k2 k3 k4 k5 k6
After encoding, noises can be identified as noises
All channels use the same spectrum
at the same time
c
Advantages:
 Bandwidth efficient
 No coordination and synchronization necessary
 Good protection against interference and tapping (different
coding schemes)
Disadvantages: f
 Lower user data rates
 More complex signal regeneration
What is the guard space? Keys for coding
t
16

17. FDD/FDMA - General Scheme
Example GSM
f
960 MHz 124
935.2 MHz 1 200 kHz
20 MHz
915 MHz 124
1
890.2 MHz Fr. Schiller
t
GSM: 900MHz
Uplink: 890.2MHz to 915MHz
Downlink: 935.2MHz to 960MHz
Each channel 0.2MHz separated. Totally 124 channels for
each direction 17

18. Time Division Multiple Access
 Assign a fixed sending frequency to a transmission channel between a sender
and a receiver for a certain amount of time
 The receiver and transmitter use the same frequency all the times (simplified the
design of receivers)
 How to do the time synchronization is the problem? Fixed time slot or assigned
dynamically
 Fixed TDM:
 Allocating time slots for channels in a fixed pattern (fixed bandwidth for each
channel)
 Fixed time to send and get data from a channel
 Fixed bandwidth is good for constant data traffic but not for bursty traffic
 TDD (time division duplex): assign different slots for uplink and downlink using
the same frequency
 Dynamic TDM requires coordination but is more flexible in bandwidth allocation
18

19. TDD/TDMA - General Scheme
Example DECT
10-6
417 µs
1 2 3 11 12 1 2 3 11 12
t
downlink uplink
417 x 12 = 5004
Fr. Schiller Fixed period of 5ms
19

20. Polling Mechanisms
 If one terminal can be heard by all others, this “central” terminal can
poll all other terminals according to a certain scheme, i.e. round-robin
or random
 Now all schemes known from fixed networks can be used (typical
mainframe - terminal scenario)
 Example: Randomly Addressed Polling
 Base station signals readiness to all mobile terminals
 Terminals ready to send can now transmit a random number
without collision with the help of CDMA or FDMA (the random
number can be seen as dynamic address)
 The base station now chooses one address for polling from the list
of all random numbers (collision if two terminals choose the same
address)
 The base station acknowledges correct packets and continues
polling the next terminal
 This cycle starts again after polling all terminals of the list
20

21. ISMA (Inhibit Sense Multiple Access)
 Current state of the medium is signaled via a “busy tone”
 The base station signals on the downlink (base station to terminals)
if the medium is free or not
 Terminals must not send if the medium is busy
 Terminals can access the medium as soon as the busy tone stops
 The base station signals collisions and successful transmissions
via the busy tone and acknowledgements, respectively (media
access is not coordinated within this approach)
 Mechanism used, e.g., for CDPD (USA, integrated into AMPS)
21

22. Code Division Multiple Access
 All terminals send on the same frequency probably at the same time
and can use the whole bandwidth of the transmission channel
 So, how the receivers identify the data/signals for them?
 Each sender has a unique random number (code), the sender XORs
the signal with this random number
 Different senders use different codes
 The codes separate the signals from different senders
 The encoded signals are concatenated together for sending, i.e., as a
signal stream of signals
 The receiver “tunes” into this signal stream if it knows the pseudo
random number. Tuning is done via a correlation function
 The received decodes the signal stream using the known code to
identify the data for it
 Different receivers received different data as they use different codes
22

23. Code Division Multiple Access
 Disadvantages:
 Higher complexity of a receiver (receiver cannot just listen into the
medium and start receiving if there is a signal)
 All signals should have the same strength at a receiver
 Advantages:
 All terminals can use the same frequency, no planning needed
 Huge code space (e.g. 232) compared to frequency space
 Interferences is not coded
 Forward error correction and encryption can be easily integrated
23

24. CDMA Encoding
Fr. Schiller
 Each user is assigned a unique
tb
signature sequence (or code),
denoted by (c1,c2,…,cM). Its user data
component is called a chip 0 1 XOR
tc
 Each bit, di, is encoded by multiplying chipping
sequence
the bit by the signature sequence: 01101010110101 =
Zi,m = di cm resulting
signal
01101011001010
 XOR of the signal with pseudo-
random number (chipping sequence) tb: bit period tc: chip period
tc = 1/m x tb
0 : +1 1: -1
 One bit is now sent as multiple bit =>
0 (1) X 0 (1) = 1; 0 (1) X 1 (-1) =
higher bandwidth is required -1
1 (-1) X 0 (1) = -1; 1 (-1) X 1 (-1)
=1 24

25. Encoding Example
 Data bit
d1 = –1 (0: +1; 1 = -1)
 Signature sequence
(c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1)
 Zi,m = di cm = (-1) x (+1), (-1) x (+1), …, (-1) x (-1)
 Encoder Output
(Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1)
25

26. CDMA Decoding
 Without interfering users, the receiver would receive the encoded bits,
Zi,m, and recover the original data bit, di, by computing:
M
1
di =
M
∑Z
m =1
c
i ,m m
26

27. CDMA Decoding Example
M
1
(c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1)
di =
M
∑Z
m =1
c
i ,m m
(Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1)
i = 1, m = 1 i = 1, m = 8
multiply
(+1)x(-1) (-1)x(+1)
(–1,–1,–1,–1,–1,–1,–1,–1)
add and
-8/ m, m = 8 divide by M
di = –1 27

28. 28

29. Multi-User Scenario
 If there are N users, the signal at the receiver becomes:
N
Z *
i ,m = ∑Z n
i ,m
n =1
 How can a CDMA receiver recover a user’s original data bit?
29

30. 2-Senders
example
Multiplied by the signature
sequence of user 1
N
Z i*,m = ∑ Z inm
,
n =1
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31. Signature Sequences
 In order for the receiver to be able to extract out a particular sender’s
signal, the CDMA codes must be of low correlation
 Correlation of two codes, (cj,1,…, cj,M) and (ck,1,…, ck,M) , are defined by
inner product:
M
1
M
∑c
m =1
c
j ,m k ,m
 Code 1: 1, 1, 1, -1, 1, -1, -1, -1
 Code 2: 1, -1, 1, 1, 1, -1, 1, 1
 Inner product: 1 + (-1) + 1 + (-1) + 1 + 1 + (-1) + (-1) /8 = 0
31

32. Meaning of Correlation
What is correlation?
 It determines how much similarity one sequence has with another
 It is defined with a range between –1 and 1
Correlation Value Interpretation
1 The two sequences match each other exactly.
0 No relation between the two sequences
–1 The two sequences are mirror images of each other.
Other values indicate a partial degree of correlation.
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33. Orthogonal Codes
Orthogonal codes
 All pair wise cross correlations are zero
 Fixed- and variable-length codes used in CDMA
systems
 For CDMA application, each mobile user uses one
sequence in the set as a spreading code
 Provides zero cross correlation among all users
Types
 Welsh codes
 Variable-Length Orthogonal codes
33

34. Walsh Codes
Set of Walsh codes of length n consists of
the n rows of an n x n Walsh matrix:
 Wn Wn 
 W1 = (0) W2 n =
W 
 n Wn 

n = dimension of the matrix
 Every row is orthogonal to every other row
 Requires tight synchronization
 Crosscorrelation between different shifts of
Walsh sequences is not zero
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35. Example
35

36. Typical Multiple Spreading Approach
 Spreaddata rate by an orthogonal code
(channelization code)
 Provides mutual orthogonality among all users in the
same cell
 Further
spread result by a Pseudo-Noise (PN)
sequence (scrambling code)
 Providesmutual randomness (low cross correlation)
between users in different cells
36

37. Comparison
SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA
Idea segment space into segment sending segment the spread the spectrum
cells/sectors time into disjoint frequency band into using orthogonal codes
time-slots, demand disjoint sub-bands
driven or fixed
patterns
Terminals only one terminal can all terminals are every terminal has its all terminals can be active
be active in one active for short own frequency, at the same place at the
cell/one sector periods of time on uninterrupted same moment,
the same frequency uninterrupted
Signal cell structure, directed synchronization in filtering in the code plus special
separation antennas the time domain frequency domain receivers
Advantages very simple, increases established, fully simple, established, flexible, less frequency
capacity per km² digital, flexible robust planning needed, soft
handover
Dis- inflexible, antennas guard space inflexible, complex receivers, needs
advantages typically fixed needed (multipath frequencies are a more complicated power
propagation), scarce resource control for senders
synchronization
difficult
Comment only in combination standard in fixed typically combined still faces some problems,
with TDMA, FDMA or networks, together with TDMA higher complexity,
CDMA useful with FDMA/SDMA (frequency hopping lowered expectations; will
used in many patterns) and SDMA be integrated with
mobile networks (frequency reuse) TDMA/FDMA
37

38. References
 Schiller: Ch. 2.1, 2.2, 2.4, 2.5, 2.6.1-2.6.4
 Schiller: Ch 3.1, 3.2, 3.3, 3.4.1, 3.4.8 ,3.4.9, 3.4.10, 3.5, 3.6
 Schiller, Mobile Communications, sections 4.1 (except 4.1.7) and
4.4.2, 4.4.4 – 4.4.6 (except the protocol stack)
 Wireless Communications & Networks, 2Edition, Pearson, William
Stallings (Ch 7)
38