Multiple Access Techniques

Unit III - Multiple Access Techniques for Wireless Communications

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What is Duplexing?**When a subscriber can send as well as receive the information simultaneously from the base station.

Narrowband SystemsUsed to relate the BW of single channel to the expected coherence BW of the channelAvailable radio spectrum is divided into large number of narrowband channels.Maximum frequency separation in the frequency spectrum to minimize interference between forward and reverse channels.

Narrowband TDMA

Each user is assigned a unique time slot in cyclical fashionThe time slots are allotted on the same radio channel.Large no. of radio channels allocated using either FDD or TDDNarrowband FDMA

Each user is assigned a particular channel which is not shared by other users in the vicinity .FDMA/FDD: The single channel is actually two simplex channels which are frequency duplexed with 45 MHz split**

Wideband SystemsTransmission bandwidth of single channel is much larger than the coherence bandwidth of the channel.Multipath fading doesnt greatly vary the received signal power.Large number of transmitters are allowed to transmit on one channel

Wideband CDMA

Allows all tranmitters to access the channel at the same timeWideband TDMA

Allots time slots to many transmitters on the same channel.Only one transmitter is allowed to access the channel at one instant of time.**

Multiple AccessTechniquesTo accommodate a number of users, many traffic channels need to be made available

In principle, there are three basic ways to have many channels within an allocated bandwidth

Frequency Division Multiple Access (FDMA)Time Division Multiple Access (TDMA)Code Division Multiple Access (CDMA)

Concepts and Models of FDMA, TDMA and CDMAIn one BS radio service range, many MSs are located.MS must distinguish which signal meant for itself among many signals being transmitted by other users.BS should be able to recognize the signal sent by a particular user.In cellular system, MS not only can distinguish a signal from a serving BS but also can discriminate the signal from adjacent BS

Concepts and Models of FDMA, TDMA and CDMA (cont.)A Radio Signal can be presented as a function of frequency, time and code.

s (f,t,c) = s(f,t)c(t)Where s(f,t) a function of frequency and time c(t) a function of codeWhen c(t) = 1 then

s (f,t,c) = s(f,t)

Concepts and Models of FDMA, TDMA and CDMA (cont.)System employs different carrier frequency FDMA system.System uses distinct time TDMA system.System uses different code CDMA system.In wireless communications, it is necessary to utilize limited frequency bands at the same time, allowing multiple users (MSs) to share radio channel simultaneously.To provide simultaneous two-way communication (duplex communication) :

Frequency division duplexing (FDD)Time Division Duplexing (TDD)FDMA uses FDD, TDMA & CDMA uses TDD or FDD

Preamble: contains address and sync information that BS and subscriber use to identify each other.Guard times: Utilized to allow synchronization of the receivers between different slots and frames.

**Time Division Multiple Access

Slot 1Slot 2Slot 3 Slot N Frame Structure in TDMAPreamble Information Message Trail BitsOne TDMA FrameTrail Bits Sync. Bits Information Data Guard BitsThe frame is cyclically repeated over time.

Features of TDMAa single carrier frequency for several userstransmission in burstslow battery consumptionhandoff process much simplerFDD : switch instead of duplexervery high transmission ratehigh synchronization overheadguard slots necessary

FDMA compared to TDMAfewer bits for synchronization fewer bits for framinghigher cell site system costshigher costs for duplexer used in base station and subscriber unitsFDMA requires RF filtering to minimize adjacent channel interference

Nonlinear Effects in FDMAmany channels - same antennafor maximum power efficiency operate near saturationnear saturation power amplifiers are nonlinearnonlinearities causes signal spreading intermodulation frequencies

Nonlinear Effects in FDMAIM are undesired harmonicsinterference with other channels in the FDMA systemdecreases user S/I - decreases performanceinterference outside the mobile radio band: adjacent-channel interferenceRF filters needed - higher costs

Number of channels in a FDMA systemN number of channels Bt total spectrum allocationBguard guard bandBc channel bandwidth

N=Bt -2BguardBc

Example: Advanced Mobile Phone SystemAMPSFDMA/FDDanalog cellular system12.5 MHz per simplex band - BtBguard = 10 kHz ; Bc = 30 kHz

N=12.5E6 - 2*(10E3)30E3= 416 channels

Number of channels in a TDMA systemN number of channelsm number of TDMA users per radio channelBtot total spectrum allocationBguard Guard BandBc channel bandwidth

N=m*(Btot - 2*Bguard)Bc

Example: Global System for Mobile (GSM)TDMA/FDDforward link at Btot = 25 MHz radio channels of Bc = 200 kHzif m = 8 speech channels supported, andif no guard band is assumed :

N=8*25E6200E3= 1000 simultaneous users

Efficiency of TDMApercentage of transmitted data that contain informationframe efficiency fusually end user efficiency < f ,because of source and channel coding

Efficiency of TDMAbOH number of overhead bitsNr number of reference bursts per framebr reference bits per reference burstNt number of traffic bursts per framebp overhead bits per preamble in each slotbg equivalent bits in each guard time interval

bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg

Efficiency of TDMAbT = Tf * RbT total number of bits per frameTf frame durationR channel bit rate

Efficiency of TDMA

f frame efficiencybOH number of overhead bits per framebT total number of bits per frame

f = (1-bOH/bT)*100%

Types Of SSMA**

SSMASpread-spectrum multiple access transmission technique wherein data occupy a larger bandwidth than necessary

There are two basic types of implementation methodologies:

Direct Sequence (DS)Frequency Hoping (FH)

CDMA is a system based on Direct Sequence spread-spectrum technology.

Frequency Hopped Multiple AccessAllows multiple users to simultaneously occupy same spectrum at the same time.

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It is a digital multiple access technique.Here each user is assigned a narrowband channel depending on its Pseudo-Noise code. The digital data from the user is broken into uniform sized bursts and transmitted on different channels within the allocated bandwidth. This allows to accomodate multiple user over wide range of frequency. A locally generated Pn code also synchronizes the frequency of the receiver to that of the transmitter.*

Frequency Hopping Spread Spectrum (FHSS)

A pseudorandom sequence is used to change the radio signal frequency across a broad frequency band in a random fashion

The modulation technique implies that the radio transmitter frequency hops from channel to channel

The RF signal is dehopped at the receiver end using a frequency synthesizer control by a pseudorandom sequence generator

Frequency Hopping Spread Spectrum (FHSS)

A frequency hopper may beFast hopping frequency spectrumSlow hopping frequency spectrum

Multiple simultaneous hops from several users is possible using FH as long as each uses different FH sequences and do not collide.

Frequency Hopped Multiple AccessDifference between FDMA and FHMA is that, in FHMA, the signal changes channels at rapid intervals

For Fast Frequency Hopping system

Rate of change of carrier frequency > Symbol rate

For Slow Frequency Hopping System

Rate of change of carrier frequency < Symbol rate**

Frequency Hopped Multiple AccessFEATURESIt employs energy efficient constant envelope modulationIt provides high level of securityIt is immune to fadingUsed for military operations, walkie talkies, bluetooth, etc.

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CDMA: Spread SpectrumSpread spectrum a transmission technique wherein data occupy a larger bandwidth than necessary

Bandwidth spreading is accomplished before transmission using a code that is independent of the transmitted data. The same code is used to demodulate the data at receiving end

Originally designed for military used to avoid jamming

Direct Sequence Spread Spectrum (DSSS)

CDMA: Spread Spectrum (cont.)Signal transmission consists of the following steps:

1. A pseudo-random code is generated, different for each channel and each successive connection.

2. The Information data modulates the pseudo-random code (the Information data is spread).

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and broadcast.

CDMA: Spread Spectrum (cont.)Signal reception consists of the following steps

1. The carrier is received and amplified. 2. The received signal is mixed with a local carrier to recover the spread digital signal3. A pseudo-random code is generated, matching the anticipated signal4. The receiver acquires the received code and phase locks its own code to it. 5. The received signal is correlated with the generated code, extracting the Information data.

CDMA: Spread Spectrum (cont.)Direct Sequence Spread Spectrum (DSSS)The radio signal is multiplied by a pseudorandom sequence whose bandwidth is much greater than the signal itself spreading its bandwidth.Pseudorandom sequence directly phase modulates a carrier increase the bandwidth of transmission and lowering the spectral power density.The resulting RF signal has a noise like spectrum. Noise to others but not to the intended receiverThe received signal is despread by correlating it with the local identical pseudorandom sequence to spread the carrier at the receiver.

CDMA: AttributesSoft capacity LimitNear Far EffectMultipath ResistanceSoft HandoffSelf JammingHigh Channel Data Rates

Soft capacity LimitIncreasing the number of users in CDMA system raises the noise floor in a linear mannerThere is no absolute limit on the number users in CDMASystem performance gradually degrades for all users as the number of users is increased and improves as the number of users is decreased

Resistance to Multipath FadingIt is reduced because the signal is spread over a large spectrumIf the spread spectrum bandwidth is greater than the coherence bandwidth of the channel, the inherent frequency diversity will mitigate the effects of small scale fading

Near Far EffectOccurs when many mobile users share the same channelStrongest received mobile signal will capture the demodulator at base stationStronger received signal levels raise the noise floor at the base station demodulators for the weaker signals, thereby

decreasing the probability that weaker signals will be receivedTo combat the near-far problem, power control is used in most CDMA implementations Power control is provided by each BS and assures that each mobile within the BS provides the same signal level to the BS receiver

Self JammingSelf jamming is a problem in CDMAIt arises from the fact that the spreading sequences of different users are not exactly orthogonalHence in the despreading of a particular PN code, non-zero contributions to the receiver for a desired user arise from the transmissions of other users in the system

Soft HandoffCDMA uses co-channel cells, it can use macroscopic spatial diversity to provide a soft handoff Soft handoff is performed by MSC, which can simultaneously monitor a particular user from two or more base stations.The MSC may choose the best version of the signal at any time without switching frequencies

Channel Data RatesChannel data rates are very high in CDMA systemsThe symbol (chip) duration is very short and usually much less than a channel delay spreadSince PN sequences have low autocorrelation, multipath which is delayed by more than a chip will appear as noiseA Rake receiver can be used to improve reception by collecting time delayed versions of the required signal

Space Division Multiple Access (SDMA)SDMA controls the radiated energy for each user and serves different users by using spot beam antennas may be served by same or different frequenciesReverse links present difficultyDifferent areas covered by antenna beam

Transmitted power from each subscriber must be controlled to prevent any single user from driving up the interference levelTransmit power is limited by battery consumption at the subscriber unit

SDMA(SPACE DIVISION MULTIPLE ACCESS)

Space Division Multiple AccessControls radiated energy for each user in spaceusing spot beam antennasbase station tracks user when movingcover areas with same frequency: TDMA or CDMA systemscover areas with same frequency:FDMA systems

Space Division Multiple Accessprimitive applications are Sectorized antennas

in future adaptive antennas simultaneously steer energy in the direction of many users at once

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It is a digital multiple access technique.Here each user is assigned a narrowband channel depending on its Pseudo-Noise code. The digital data from the user is broken into uniform sized bursts and transmitted on different channels within the allocated bandwidth. This allows to accomodate multiple user over wide range of frequency. A locally generated Pn code also synchronizes the frequency of the receiver to that of the transmitter.*

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Multiple Access Techniques