Multiple Access Techniques forWireless Communications

By- Gaurav Khanna13/PIT/082

M.Tech ( WCN)

Table of Contents:

• Introduction• FDMA• TDMA• CDMA• SS (Spread Spectrum)

• FHSS• DSSS• Hybrid

• Packet Radio• Pure ALOHA• Slotted ALOHA• CSMA• Reservation Protocol

• Reservation-ALOHA• PRMA• NC-PRMA

Introduction:

• Multiple Access:• Enable many mobile users to share simultaneously

radio spectrum.

• Provide for the sharing of channel capacitybetween a number of transmitters at differentlocations.

• Aim to share a channel between two or moresignals in such way that each signal can bereceived without interference from another.

• In conventional telephone systems, it is possible totalk and listen simultaneously, called duplexing.

• Duplexing• Allow the possibility of talking and listening

simultaneously.• Frequency Division Duplex (FDD)

• Provides two distinct bands of frequenciesfor every user

• Time Division Duplex (TDD)• Multiple users share a signal channel by

taking turns in time domain• Each duplexing channel has both a forward

time slot and a reverse time slot tofacilitate bidirectional communication.

Multiple Access Techniques in use

Cellular System Multiple Access Technique

Advanced Mobile Phone System (AMPS) FDMA/FDD

Global System for Mobile (GSM) TDMA/FDD

US Digital Cellular (USDC) TDMA/FDD

Digital European Cordless Telephone (DECT) FDMA/TDD

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

• Each transmitter is allocated a channel with a particularbandwidth.

• All transmitters are able to transmit simultaneously.• Allocation of separate channels to FDMA signals

Frequency Division Multiple Access (FDMA)

Time-frequency characteristic of FDMA

Features of FDMA

• If an FDMA channel is not in use, then it sits idle and can’t be used by otherusers.

• Transmit simultaneously and continuously.

• FDMA is usually implemented in narrowband systems.• Its symbol time is large as compared to the average delay spread.

• For continuous transmission, fewer bits are needed for overhead purposes(such as synchronization and framing bits) as compared to TDMA.

• FDMA uses duplexers since both TX and RX operate at the same time.

Time Division Multiple Access (TDMA)

• Transmitter share a common channel.

• Only one transmitter is allowed to transmit at a time.

Synchronous TDMA: access to the channel is restricted to regular.

Asynchronous TDMA: a station may transmit at any time that thechannel is free.

Allocation of time slot in TDMA

Time-frequency characteristic of synchronousTDMA

Features of TDMA

TDMA systems divide the radio spectrum into time slots. Each user occupies a cyclically repeating time slot. Transmit data in a buffer-and-burst method, thus the transmission for any

user is not continuous. TDMA has TDD and FDD modes.

TDMA Frame Structure

A single carrier frequency for several users. Low battery consumption. Handoff process much simpler. FDD : switch instead of duplexer. Very high transmission rate. High synchronization overhead. Guard slots necessary.

TDMA Frame Structure

In TDMA, the preamble contains the address and synchronizationinformation that both the base station and the mobiles use toidentify each other.

Different TDMA standards have different TDMA frame structures.

Share a single carrier frequency with several users.

Data transmission is not continuous, but occurs in bursts.

No duplexers is required since users employ different time slots fortransmission and reception.

TDMA can allocate different numbers of time slots per frame todifferent users, allowing bandwidth be supplied on demand todifferent users.

Combined use of synchronous TDMA and FDMA

Asynchronous TDMA: Carrier-Sense Multiple Access(CSMA)

Allows a transmitter to access the channel at any time that is not being usedby another transmitter.

FDMA compared to TDMA

• Fewer bits for synchronization

• Fewer bits for framing

• Higher cell site system costs

• Higher costs for duplexer used in base stationand subscriber units

• FDMA requires RF filtering to minimize adjacentchannel interference

Space Division Multiple Access

• Controls radiated energy for each user in space usingspot beam antennas.

• Base station tracks user when moving.• Cover areas with same frequency: TDMA or CDMA

systems.• Cover areas with same frequency: FDMA systems.

Space Division Multiple Access

• Primitive applications are“Sectorized antennas”

• In future adaptive antennassimultaneously steer energy inthe direction of many users atonce

Reverse link problems

• Different propagation path from user to base.• Dynamic control of transmitting power from each user to the base

station required.• Limits by battery consumption of subscriber units.• Possible solution is a filter for each user.

Solution by SDMA systems

• Adaptive antennas promise to mitigate reverse link problems.• Limiting case of infinitesimal beam width.• Limiting case of infinitely fast track ability.• Thereby unique channel that is free from interference.• All user communicate at same time using the same channel.

Disadvantage of SDMA

• Perfect adaptive antenna system:infinitely large antenna needed.

• Compromise needed.

SDMA and PDMA in satellites

• INTELSAT IVA• SDMA dual-beam receive

antenna.• simultaneously access from

two different regions of theearth.

SDMA and PDMA in satellites

• COMSTAR 1• PDMA• separate antennas.• simultaneously access

from same region.

SDMA and PDMA in satellites

• INTELSAT V• PDMA and SDMA• Two hemispheric coverage

by SDMA.• Two smaller beam zones by

PDMA.• orthogonal polarization.

Code Division Multiple Access (CDMA)

Transmitter may transmit at the same time, in the same channel.

Each signal is modified by spreading it over a large bandwidth.o This spreading occurs by combining the transmitter signal with a

spreading sequence.

Near-far Problem

Users may be receivedwith very different powers:

Users near the base station arereceived with high power. Users far from the base stationare received with low power. Nearby users will completelyswamp far away users.

Advantages of CDMA

• Capacity is CDMA's biggest asset. It can accommodate moreusers per MHz of bandwidth than any other technology.3 to 5 times more than GSM.

• CDMA has no built-in limit to the number of concurrent users.• CDMA uses precise clocks that do not limit the distance a

tower can cover.• CDMA consumes less power and covers large areas so cell size

in CDMA is larger.• CDMA is able to produce a reasonable call with lower signal

(cell phone reception) levels.• CDMA uses Soft Handoff, reducing the likelihood of dropped

calls.• CDMA's variable rate voice coders reduce the rate being

transmitted when speaker is not talking, which allows thechannel to be packed more efficiently.

• Has a well-defined path to higher data rates.

Disadvantages of CDMA

• Most technologies are patented andmust be licensed from Qualcomm.

• Breathing of base stations, wherecoverage area shrinks under load. As thenumber of subscribers using a particularsite goes up, the range of that site goesdown.

• Currently CDMA covers a smaller portionof the world as compared to GSM whichhas more subscribers and is in morecountries overall worldwide.

CDMA example

Low-Bandwidth Signal:

High-Bandwidth Spreading Code:

...repeated...

CDMA example

Low-Bandwidth Signal:

High-Bandwidth Spreading Code:

Mix is a simple multiply

… and transmit.

CDMA example

To Decode / Receive, take the signal:

CDMA example

To Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ...

… which we should recognize as...

CDMA example

To Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ...

What if we use the wrong code?

Take the same signal:

Multiply by the wrong Spreading Code:

… for example, let's just shift the same code left a bit:

Take the same signal:

Multiply by the wrong Spreading Code:

… We get ...

… which clearly hasn't recovered the original signal.Using wrong code is like being off-frequency.

What if we use the wrong code?

Features of CDMA

Many users of a CDMA system share the samefrequency.

The symbol (chip) duration is very short andusually much less than the channel delayspread.

The near-far problem occurs at a CDMA RX ifan undesired user has a high detected poweras compared to the desired user.

Frequency Hopping

Frequency hopping is a form of FDMA.

Each transmitter is allocated a group of channels, known as hop set .

The transmitter transmits data in short bursts, choosing one of thesechannels on which to transmit each burst.

Time-frequency characteristic of a singletransmitter.

Signal received form a pair of frequency-hoppingtransmitters.

Time Hopping

Each bit is transmitted as a single pulse, with the value of j-th bitdetermined by whether it arrives before or after the reference time tj.

TH-PPM

i

Ns

jicjiNfstrtr dTcjTiTtwtS

s

1

0

Spread Spectrum Multiple Access

A transmission technique in which a PN code,independent of information data, is employed asa modulation waveform to “spread” the signalenergy over a bandwidth much greater than thesignal information bandwidth.

At the receiver the signal is “de-spread” using asynchronized replica of the PN code.

Direct Sequence Spread Spectrum (DSSS)

Frequency Hopping Spread Spectrum (FHSS)

General Model of Spread Spectrum System

Pseudo-Noise Sequence/Maximum LengthSequence/Spreading Sequence

Noise-like wideband spread-spectrum signals are generatedusing PN sequence.

In DS/SS(direct-sequence spread-spectrum) , a PN spreadingwaveform is a time function of a PN sequence.

In FH/SS(frequency-hopping spread-spectrum), frequencyhopping patterns can be generated from a PN code.

PN sequences are deterministically generated, however theyalmost like random sequences to an observer.

The time waveform generated from the PN sequences alsoseem like random noise.

M-sequence (binary maximal length shift-register sequence)- Generated usinglinear feedback shift-register and exclusive OR-gate circuits.

Linear generator polynomial g(x) of degree m>0

Recurrence Equation ( gm = g0 =1)

If gi =1, the corresponding circuit switch is closed, otherwise gi 1, it is open.

Output of the shift-register circuit is transformed to 1 if it is 0, and –1 if it is 1.

Important PN Properties

• RandomnessUniform distributionBalance propertyRun propertyIndependenceCorrelation property

• Unpredictability

Direct Sequence Spread Spectrum (DSSS)

A carrier is modulated by a digital code in which the code bit rateis much larger than the information signal bit rate. These systemsare also called pseudo-noise systems.

Also called code division multiple access (CDMA)

A short code system uses a PN code length equal to a data symbol.

Along system uses a PN code length that is much longer than adata symbol.

Basic principle of DSSS

For BPSK modulation

Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping Example:

Frequency Hopping Spread Spectrum System(Transmitter):

Frequency Hopping Spread Spectrum System(Receiver):

Frequency Hopping Spread Spectrum (FHSS)

It divides available bandwidth into N channels and hops betweenthese channels according to the PN sequence.

1. Fast hopping2. Slow hopping

Slow and Fast FHSS:

Slow Frequency Hopping Spread Spectrumusing MFSK (M=4, K=2):

Fast Frequency Hopping Spread Spectrumusing MFSK (M=4, K=2):

Modulation

Performance in the presence of interference

Narrowband interference Wideband interference Gaussian noise

Narrowband interference

Wideband interference

Gaussian noise

Hybrid FDMA/CDMA (FCDMA):

The available wideband spectrum is divided into a number of sub-spectras with smaller bandwidths.

Each of these smaller sub channels becomes a narrowband CDMAsystem having processing gain lower than the original CDMA system.

Hybrid Direct Sequence/Frequency HoppedMultiple Access (DS/FHMA)

This technique consists of a direct sequence modulated signalwhose center frequency is made to hop periodically in apseudorandom fashion.

Having an advantage in that they avoid the near-far effect.

Time Division Frequency Hopping (TDFH)

The subscriber can hop to a new frequency at the start of a newTDMA frame.

Has been adopted in GSM.

Packet Radio

In packet radio (PR) access techniques, manysubscribers attempt to access a single channel inan uncoordinated (or minimally coordinatedmanner.

Collision from the simultaneous transmissions ofmultiple transmitters are detected at the BS, inwhich case an ACK or NACK signal is broadcast bythe BS to alert the desired user of receivedtransmission.

PR multiple access is very easy to implement buthas low spectral efficiency and may includedelays.

The subscribers use a contention technique totransmit on a common channel.

ALOHA protocols, developed for early satellite systems, alloweach subscriber to transmit whenever they have data to sent.

The transmitting subscribers listen to the acknowledgementfeedback to determine if transmission has been successful ornot.

If a collision occurs, the subscriber waits a random amount oftime, and then transmits the packet.

The performance of contention techniques can be evaluatedby throughput (T), which is defined as the average number ofmessage successfully transmitted per unit time, and theaverage delay (D) experienced by a typical message burst

Packet Radio Protocols

Vp , vulnerable period isdefined as the time intervalduring which the packets aresusceptible to collisions withtransmission form other user.

Packet A suffer a collision ifother terminals transmitpackets during the period t1to t1+ 2τ

Assume that packet transmissions occur with Poissondistribution having mean arrival rate of λ packets per secondand τ is the packet duration in seconds. The traffic occupancyor throughput R is given by R= λ τ .

R is the normalized channel traffic (measured in Erlangs) and ifR > 1, then the packets generated by the users exceed themaximum transmission rate of the channel. For reasonablethroughput, 0 < R < 1.

Under normal loading, the throughput T is the same as thetotal offered load L.

The load L is the sum of the newly generated packets and theretransmitted packets that suffered collisions.

The normal throughput is given as the total offered loadtimes the probability of successful transmission, i.e.

Pr[ ] Pr[ ]T R nocollision nocollision

The probability that n packets are generated by the userpopulation during a given packet duration interval isassumed to Poisson distributed and is given as:

Pr( )!

Ren

n

The probability that zero packets are generated (i.e., no collision)during this interval is given by :

Pr(0) Re

Type of Access

Contention protocols are categorized as:

Random Access: there is no coordination among that users andthe messages are transmitted from the users as they arrive atthe transmitter.

Scheduled Access: based on a coordinated access of users onthe channel and the users transmit messages within allottedslots or time intervals.

Hybrid Access: a combination of random access and scheduled.

Pure ALOHA

The pure ALOHA protocol is random access protocol used fordata transfer and a user accesses a channel as soon as amessage is ready to be transmitted.

After a transmission, the user waits for an acknowledgment oneither the same channel or a separate feedback channel.

In case of collisions, the terminal waits for a random period oftime and retransmits the message.

For pure ALOHA, the vulnerable period is double the packetduration:

Slotted ALOHA

In slotted ALOHA, time is divided into equal time slotsof length greater than the packet duration τ .

The subscribers each have synchronized clocks andtransmit a message only at the beginning of a new timeslot.

The vulnerable period of slotted ALOHA is only onepacket duration, since partial collisions are preventedthrough synchronization.

The probability that no other packets will be generatedduring the vulnerable period is е-R.

The throughput for the case of slotted ALOHA is thusgiven by T=R.е-R .

Carrier Sense Multiple Access (CSMA)

CSMA protocols are based on the fact that eachterminal on the network is able to monitor thestatus of the channel before transmittinginformation.

In CSMA, detection delay and propagation delayare two important parameters.

o Detection delay is a function of he receiverhardware and is the time required for aterminal to sense whether or not the channelis idle.

o Propagation delay is a relative measure ofhow fast it takes for a packet to travel froma BS to a MS.

Several variations of the CSMA strategy

persistent CSMA

Non-persistent CSMA

p-persistent CSMA

CSMA/CD

Data sense multiple access(DSMA)

Reservation Protocols

Reservation ALOHA (R-ALOHA): R-ALOHA is a packet scheme based on

time division multiplexing. Two phase: contention phase and

transmission phase Mobiles contend the channel in

reservation phase (slotted-ALOHA) Mobiles that succeed in making

reservation can transmit withoutinterference

ACK

Frame N

. . .B-M B-M

M-B M-B

ACK

. . .

Reservationphase

Transmissionphase

PRMA (Packet Reservation Multiple Access)

A combination of TDMA and reservationALOHA

Ask channel resource in the packet spurt. Release channel resource in the silent gap Permission probability Effect of voice activity detector

Frame N

B->M ACK B->M B->MACK ACK...........

M->B M->B ... M->B

NC-PRMA (Non-Collision Packet ReservationMultiple Access)

The existing users inform the BSabout their demands in a non-collision manner (time-frequencysignaling scheme)

I i : Information slotCM i : Control minislotC : Control slotUID : Uplink user identifierDID : Downlink user identifier