1

Multiplexing & Multiple Access

Multiplexing SDM FDM/FDMA TDM/TDMA CDM CDMA

2

MC Influence to the Layer Model

service location new applications, multimedia adaptive applications congestion and flow control quality of service addressing, routing,

device location hand-over authentication media access multiplexing media access control encryption modulation interference attenuation frequency

Application layer

Transport layer

Network layer

Data link layer

Physical layer

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

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

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

6

Multiplexing Techniques

Frequency-division multiplexing (FDM) Takes advantage of the fact that the useful bandwidth of the

medium exceeds the required bandwidth of a given signal

Time-division multiplexing (TDM) Takes advantage of the fact that the achievable bit rate of the

medium exceeds the required data rate of a digital signal

7

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

8

Frequency-division Multiplexing

9

Time-division Multiplexing

10

Multiplexing: Multiple transmitters send signals at the same time

Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c)

Goal: supporting multiple users on a shared medium (more channels)

Maximize channel utilization(higher total bandwidth)

Important: guard spaces needed

What will be the problem if the separation is small and large? Small, the receiver cannot distinguish signals/noises. Large, a waste of bandwidth

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

Fr. Schiller

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

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 usersA channel gets a certain band of the spectrum for the whole timeAdvantages:

Simple No dynamic coordination

necessaryDisadvantages:

Waste of bandwidth if the traffic is distributed unevenly

Inflexible Guard spaces (adjacent channel interference)

k2 k3 k4 k5 k6k1

f

t

c

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

14

f

t

c

k2 k3 k4 k5 k6k1

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

for many users (RR)

Disadvantages: Time quantum normally very small Precise synchronization

necessary (timing)

15

f

Time and Frequency Multiplexing

Combination of both methods (time & frequency)

A channel gets a certain frequency band for a certain amount of timeExample: GSM (a 2G cellular network)

Advantages: Better protection against

tapping (more complicated) Protection against frequency

selective interference

But: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

16

Code Multiplexing

Each channel has a unique code (encoding and decoding) => d1 -> (encoding function f(d1,key)) -> p1

After encoding, noises can be identified as noisesAll channels use the same spectrum

at the same time

Advantages: Bandwidth efficient No coordination and synchronization necessary Good protection against interference and tapping (different

coding schemes)Disadvantages:

Lower user data rates More complex signal regeneration

What is the guard space? Keys for coding

k2 k3 k4 k5 k6k1

f

t

c

17

FDD/FDMA - General SchemeExample GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

Fr. Schiller

GSM: 900MHz

Uplink: 890.2MHz to 915MHz

Downlink: 935.2MHz to 960MHz

Each channel 0.2MHz separated. Totally 124 channels for each direction

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

19

TDD/TDMA - General SchemeExample DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

Fr. Schiller

417 x 12 = 5004

Fixed period of 5ms

10-6

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

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)

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

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

24

CDMA Encoding Each user is assigned a unique

signature sequence (or code), denoted by (c1,c2,…,cM). Its component is called a chip

Each bit, di, is encoded by multiplying the bit by the signature sequence:

Zi,m = di cm

XOR of the signal with pseudo-random number (chipping sequence)

One bit is now sent as multiple bit => higher bandwidth is required

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit period tc: chip periodtc = 1/m x tb

0 : +1 1: -1

0 (1) X 0 (1) = 1; 0 (1) X 1 (-1) = -1

1 (-1) X 0 (1) = -1; 1 (-1) X 1 (-1) = 1

Fr. Schiller

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)

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

mmmii cZ

Md

1,

1

27

CDMA Decoding Example

(c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1)

(Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1)

(+1)x(-1) (-1)x(+1)

(–1,–1,–1,–1,–1,–1,–1,–1)

di = –1

multiply

add and divide by M

∑=

=M

mmmii cZ

Md

1,

1

i = 1, m = 1 i = 1, m = 8

-8/ m, m = 8

28

29

Multi-User Scenario

If there are N users, the signal at the receiver becomes:

How can a CDMA receiver recover a user’s original data bit?

∑=

=N

n

nmimi ZZ

1,

*,

30

Multiplied by the signature sequence of user 1

2-Senders example

∑=

=N

n

nmimi ZZ

1,

*,

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:

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

∑=

M

mmkmj cc

M 1,,

1

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

The two sequences are mirror images of each other.–1

No relation between the two sequences0

The two sequences match each other exactly.1

InterpretationCorrelation Value

Other values indicate a partial degree of correlation.

33

Orthogonal Codes

Orthogonal codes All pair wise cross correlations are zeroFixed- and variable-length codes used in CDMA

systemsFor CDMA application, each mobile user uses one

sequence in the set as a spreading codeProvides zero cross correlation among all users

TypesWelsh codesVariable-Length Orthogonal codes

34

Walsh Codes

Set of Walsh codes of length n consists of the n rows of an n x n Walsh matrix:

W1 = (0)

n = dimension of the matrix

Every row is orthogonal to every other rowRequires tight synchronization

Cross correlation between different shifts of Walsh sequences is not zero

=

nn

nnn WW

WWW2

35

Example

36

Typical Multiple Spreading Approach

Spread data 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)Provides mutual randomness (low cross correlation)

between users in different cells

37

Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMAIdea segment space into

cells/sectorssegment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km²

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

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)