EEE 309 Communication TheorySemester: January 2017Semester: January 2017

Dr. Md. Farhad HossainAssociate Professor

Department of EEE, BUETDepartment of EEE, BUET

Email: mfarhadhossain@eee.buet.ac.bdOffice: ECE 331, ECE Building

Part 08

M lti l i dMultiplexing and Multiple AccessMultiple Access

TechniquesTechniques

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Multiplexing and Multiple Access

Multiple cars are sharing the same road Requires proper traffic management policies

Improper traffic management: 

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accidents, traffic jam, delay, inefficient utilization of resources

Multiplexing and Multiple Access (MA)

Multiple users shares the same channel Multiple users under a single base station 

Multiplexing: Multiplexing technique combine signals from several sources Thus allows one channel to be used by multiple sources to send multiple messages without Thus allows one channel to be used by multiple sources to send multiple messages without interfering each other It works on the physical layer (L1) of OSI model

M lti l A (MA)Multiple Access (MA):Decides on - Who will transmit? Whom to transmit? When to transmit? How to transmit? MA techniques are channel access methods based on some principles including multiplexing All t h l t diff t d l h dl th it ti h th

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Allocates channels to different users and also handles the situation when there are more message sources than available channels It works on the data link layer (L2) of OSI model

Multiplexing Techniques Multiplexing techniques allow sharing a channel by keeping the transmitted signals from various sources separate so that they do not interfere with one another

This separation is accomplished by making the signals orthogonal to one another This separation is accomplished by making the signals orthogonal to one another in the dimensions of frequency, time, code, space, etc.

Various types: Time division multiplexing (TDM) Frequency division multiplexing (FDM) Frequency division multiplexing (FDM) Wavelength division multiplexing (WDM) Code division multiplexing (CDM) Space division multiplexing (SDM) Orthogonal frequency division multiplexing (OFDM): a variety of FDM Polarization division multiplexing (PDM)

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Polarization division multiplexing (PDM)

FDM

P

t

3D view

fSub-channel NSub-channel 2Sub-channel 1

Channel

User 1 User 2 User N

2D view

Available bandwidth of the common channel is divided into bands Signals are orthogonal (separated) in frequency domain

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Requires guard bands to avoid adjacent-channel interference Requires filtering to minimize adjacent channel interference: costly

FDM (2)

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Block Diagram of an FDM System

TDM (1)

A digital transmission technology

Transmission time is divided into time-slots and unique time slot(s) are allocated to q ( )each user

Different users can transmit or receive messages, one after the next in the same bandwidth but in different time slots: Orthogonal in time-domain

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g

TDM (2)Block Diagram of a TDM System

Increases the transmission efficiency (i.e., better resource utilization) P it th tili ti f ll th d t f di it l t h i di it l h Permits the utilization of all the advantages of digital techniques: digital speech interpolation, source coding, channel coding, error correction, bit interleaving, etc. Suitable for asymmetric (i.e., unequal uplink and downlink data rate) data rate E i t i b i i i l h Equipment is becoming increasingly cheaper

Requires a significant amount of signal processing for synchronization as the t i i f ll t b tl h i d

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transmission of all users must be exactly synchronized Requires guard times between time slots to compensate clock instabilities and transmission time delay

TDM Frame: Four Signals

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1 1 1

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1 12 2 2

3 3 3TimeTime

TDM Frame TDM Frame TDM Frame

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TDM Frame

One TDMA Frame

Preamble Information Message Trail Bits

Slot 1 Slot 2 Slot 3 Slot NSlot 1 Slot 2 Slot 3 … Slot N

Trail Bits Sync. Bits Information Data Guard Bits

11Typical TDMA frame formation

Example 1Following figure shows synchronous TDM with a data stream for each inputand one data stream for the output. The unit of data is 1 bit. Find (a) the inputbit duration, (b) the output bit duration, (c) the output bit rate, and (d) theoutput frame rateoutput frame rate.

a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μsp p μb. The output bit duration is one-fourth of the input bit duration, or 1/4 μsc. The output bit rate is the inverse of the output bit duration, i.e., 4 Mbpsd The frame rate is always the same as any input rate So the frame rate is 1 000 000

12School of Electrical and Information Engineering

d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000 frames per second

Example 2We have four sources, each creating 250 8-bit characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame findsynchronizing bit is added to each frame, find –(a) the data rate of each source(b) the duration of each character in each source(b) the duration of each character in each source(c) the frame rate(d) the duration of each frame(e) the number of bits in each frame(f) the data rate of the link

SolutionTh d t t f h i 250 8 2000 b 2 kb

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a. The data rate of each source is 250 × 8 = 2000 bps = 2 kbps

Example 2b. Each source sends 250 characters per second. Therefore,

the duration of a character is 1/250 s, or 4 ms.

c. Each frame has one character from each source, which means the link needs to send 250 frames per second to k th t i i t f hkeep the transmission rate of each source.

d. The duration of each frame is 1/250 s, or 4 ms. Note that the duration of each frame is the same as the duration of eachduration of each frame is the same as the duration of each character coming from each source.

e Each frame carries 4 characters and 1 extra synchronizinge. Each frame carries 4 characters and 1 extra synchronizing bit. This means that each frame is 4 × 8 + 1 = 33 bits

f 33 bits are transmitted in 4 ms Hence the data rate = 33 x

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f. 33 bits are transmitted in 4 ms. Hence the data rate = 33 x 1000 /4 = 8250 bps

Digital Carrier Systems using TDM Two main systems for voice communications:

1. T-carrier D l B ll L b USA Developer: Bell Labs, USA Used in North America, Japan and South Korea US system based on DS-1 signaling format US system based on DS-1 signaling format ITU-T use a similar (but different) system Formats: T-1, T-2, T-3, T-4

2. E-Carrier Developer: European Conference of Postal and

Telecommunications Administrations (CEPT)Telecommunications Administrations (CEPT) With some revisions, ITU-T has accepted it Used throughout Europe and most of the rest of the world

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Used throughout Europe and most of the rest of the world

* DS = Digital Signal,  ** ITU‐T = ITU Telecommunication Standardization Sector

T-Carrier (1) 24 channels per frame 1 bit per frame (The first bit of a frame) is framing bit used for

synchronizationsynchronization 8 kHz sampling rate and 8 bits/sample = 64 kbps per channel Uses μ-law with μ = 255

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T-1 Lines for Multiplexing Telephone Lines

T-Carrier (2)T-1 Frame Structure

(Frame duration: 125 µs)(Bit duration: 0.6477 µs)

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T-Carrier (3) Can also interleave DS-1 channels:

For example, DS-2 is four DS-1 giving 6.312 Mbps

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E-Carrier (1) E-Carrier system multiplexes 32 DS-0 channels (time slots

each carrying 8 bits) together to form an E-1 circuit Ti l t 0 i d t d t t i i t d ti Time slot 0 is devoted to transmission management and time

slot 16 for signaling The rest slots are assigned for voice/data transport The rest slots are assigned for voice/data transport Data rate: 32*8*8 kbps = 2.048 Mbps Uses A-law

19** DS = Digital Signal

E-Carrier (2)

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E-Carrier (3)

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Joint TDM and FDM For certain applications, such as synchronous optical network (SONET) or synchronous digital hierarchy (SDH), both TDM and FDM can be employed simultaneously

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WDM Block Diagram of an

WDM System

Conceptually same as FDM, except that multiplexing and demultiplexing involves light signals transmitted through fibre-optic channels

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Combines different frequency signals (same as FDM). However, the frequencies are very high.

WDM is designed to utilize the high data rate capability of fibre-optic cable

Multiple Access (MA) Techniques Decides on - who will transmit? whom to transmit? when to transmit? How to transmit?

Random access (contention methods): No station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send.

Controlled access: The stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations.

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Channelization techniques: The available bandwidth of a link is shared in time, frequency, or through code, between different stations. Usually, it is controlled by a system administrator.

Multiple Access (MA) Techniques Various forms of channelization techniques:

Frequency division multiple access (FDMA): e g 1G cellular system Frequency division multiple access (FDMA): e.g., 1G cellular system

Time division multiple access (TDMA) : e.g., 2G GSM system

Wavelength division multiple access (WDMA)

Code division multiple access (CDMA): e.g., 2G CDMA, 3G UMTS system

Orthogonal frequency division multiple access (OFDMA): e.g., LTE, WiMAX

Space division multiple access (SDMA)

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These techniques can be used in combination

Case Study: GSM TDMA Frames

Hyperframe

Superframe

Frame

Multiframe

Frame

Bursts

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CDMA A spread spectrum (SS) multiple access technique, which allows multiple signals occupying the same bandwidth to be transmitted simultaneously without interfering with one another

In a CDMA system, each user is assigned a particular code, named as pseudo-noise (PN) code which are ideally supposed to be unique for each usercode, which are ideally supposed to be unique for each user

This unique code enables the desired message to be extracted at the receiver

The transmissions from other users looks like interference

What is a spread spectrum (SS) system?What is a spread spectrum (SS) system?

Spreads a narrowband communication signal over a wide range of frequencies

Si l di i d b f t i i b i

PowerNarrowband

(High Peak Power)

Signal spreading is done before transmission by using a spreading sequence

De-spreads it into the original data bandwidth at the receiver

Spread Spectrum(Low Peak Power)

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receiver

Same sequence is used at the receiver to retrieve the signal

Frequency

FDMA, TDMA and CDMA

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CDMA: Principle (1)Two types: Direct sequence CDMA (DS-CDMA) Frequency hoping CDMA (FH-CDMA)

DS-CDMA System:1 0 1

Datab(t)

Symbol Duration TSBit duration Tb

Datad(t) 0b(t)

PNSequence

a(t)

d(t)

PN sequence

c(t)

PowerNarrowband

b(t)

a(t)

b(t)a(t)

c(t)

d(t)c(t)

b(t)a(t)

( )

Spread SpectrumTime

Chip Duration T

( ) ( )

29Processing gain, G = No. of chips per bit = Tb/TcFrequency

Chip Duration TC

CDMA System

Modulator

Spreaded signal foruser 1, bS1

Data of Modulator

PN code ofUser 1

user 1, b1PN 1

Transmitted signal of user 1, TX1

Input Signal ofReceiver 1 beforeDespreading, bS1'

Modulator

PN 2

Transmitted signal of user 2, TX2

Data ofuser 2, b2

Demodulator

PN d f

Output ofReceiver 1, b1'

bS2 Despreading

p g, S1

PN 1

PN code ofUser 2

PN code ofUser 1

bSK Receiver

Modulator

PN code ofUser K

PN NTransmitted signal

of user K, TXK

Data ofuser 2, bK

30Transmitter

CDMA: Principle (2)

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CDMA: Principle (3)

PN code

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CDMA: Principle (4)

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CDMA: Principle (5)Detection by receiver (station) 2:

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FH-CDMA (1)

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FH-CDMA (2)

Transmitter

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FH Spread Spectrum: InventionInvention (1941): For controlling radio-controlled torpedoesUS patent: “Secret Communication System”, August 1942 First implementation (modified form): For the sake of national defense, government did not allow publication of its details First implemented by US Defense during ‘Cuban Missile Crisis’ in 1962.publication of its details. First implemented by US Defense during Cuban Missile Crisis in 1962.Award: Pioneer Awards, Electronic Frontier Foundation, 1997

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George Antheil (1900‐1959) Composer, pianist, author, and 

inventorActress and inventor

(1914‐2000)

CDMA: AdvantagesSome of the advantages:

Hard to intercept: secure communications

Difficult to jam

Improved interference rejection and suppression

No guard-band like FDMA or guard-time like TDMA

Easy addition of more users

Can accommodate more users than TDMA and FDMA Can accommodate more users than TDMA and FDMA

Improved multi-path effect mitigation

Graceful degradation of performance as the number of simultaneous Graceful degradation of performance as the number of simultaneous users increases

Less susceptible to effects induced from a changing environment

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CDMA: Drawbacks Requires high bandwidth

Self-jamming problem due to spreading sequences not being exactly orthogonalorthogonal

Power control necessary for mitigating near-far problem

Inappropriate for ultra high rate wireless access because pp p g

Tremendous width of BW necessary

Hardware complexity

Synchronization problem

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DuplexingDuplexing refers to the technique of separating the transmitting and receiving channels

Communication Systems: Simplex, Half‐duplex, Full‐duplex

FDD TDD

Frequency‐division duplexing (FDD):  Time‐division duplexing (FDD): 

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Transmitter and receiver operate at different carrier frequencies

p g ( )Transmitter and receiver operate at 

same carrier frequencies, but through different time‐slots

MA and Duplexing Schemes in Use

System Multiple Access

(Don’t need to memorize the followings for exam) 

Advanced Mobile Phone System (AMPS) FDMA/FDD

2G Global System for Mobile (GSM) TDMA/FDDy ( )

US Digital Cellular (USDC) TDMA/FDD

Digital European Cordless Telephone (DECT) FDMA/TDD

US Narrowband Spread Spectrum (IS-95) CDMA/FDD

Satellite Communication TDMA, FDMA, CDMA

3G WCDMA/FDD3G WCDMA/FDD

LTE OFDMA/FDD or TDD

WiMAX OFDMA/FDD or TDD

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OFDM OFDM dates back to the 1960’s, whereas its implementation in commercial silicon onlybecame possible in the late 1990’s A variant of FDM: Sub‐carriers are orthogonal to each other requiring no guard‐band

l fApplications of OFDM/OFDMA:

LTEWiMAXDSLADSLPLC

Di it l TV

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Digital TVWLAN

OFDM System

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