Multiple Acess Techniques for Wireless Communications

8/13/2019 Multiple Acess Techniques for Wireless Communications

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MULTIPLE ACESS TECHNIQUESFOR WIRELESS

COMMUNICATIONS


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Multiple access schemes are used to allow

many mobile users to share simultaneously afinite amount of radio spectrum .

Sharing of spectrum is required to achievehigh capacity by simultaneously allocating the

available bandwidth to multiple users.


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INTRODUCTION:

In wireless communications systems, it is oftendesirable to allow the subscriber to sendsimultaneously information to the base stationwhile receiving information from base station.

This effect is called DUPLEXING. Duplexing may be done using frequency or time

division techniques.

Frequency division multiplexing provides two

distinct bands of frequencies.Forward band and reverse band

Forward band: provides traffic from BSMS

Reverse band: provides traffic from MSBS


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In FDD,duplex channel consists of two simplex channel(a forward and

reverse) and a device called duplexer is used inside each subscriber unit

and base station to allow simultaneous bi directional radio transmission

and reception.

Frequency separation between forward and reverse channel is constant.

TDD uses time instead of frequency to provide both forward andreverse link.

In TDD, multiple users share a single radio channel by taking turns in the

time domain.

Individual users are allowed to access the channel in assigned time slots

and each duplex channel has both a forward time slot and reverse time

slot to facilitate bi-directional communications.


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Illustration of FDD and TDD techniques:


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There are several trade off between FDD and TDD approach.

FDD is geared towards radio communications that allocateindividual radio frequencies for each user.

TDD enables each transceiver to operate either as a

transmitter or a receiver on same frequency and eliminatesthe need for separate forward and reverse frequency bands.

Because of the rigid timing required for time slotting, TDD is

generally limited to cordless phone or short range portableaccess.


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The three major access techniques:

Frequency division multiplexing access(FDMA),

Time division multiplexing access (TDMA) Code division multiple access(CDMA)

These techniques can be grouped as narrowband

wideband systems

depending on available bandwidth.


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Narrowband TDMA allows users to share the same radio

channel but allocates a unique time slot to each user in a

cyclical fashion on the channel.

For narrowband TDMA systems, there generally are large

number of radio channels allocated using either FDD/TDD.

Such systems are called TDMA/FDD or TDMA/TDD accesssystems.


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Wideband systems: In wideband systems, the

transmission bandwidth of a single channel is

much larger than the coherence bandwidth ofthe channel.(B >> Bc)

Here a large number of transmitters are

allowed to transmit on the same channel.


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Frequency division multiple access:

FDMA assigns individual channels to individual

users.


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These channels are assigned on demand to users

who request service.

During the period of the call, no other user can share

the same channel

In FDD systems, the users are assigned a channel as

a pair of frequencies, one for forward channel and

other for reverse channel.


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FEATURES OF FDMA ARE AS FOLLOWS:

If an FDMA channel is not in use, then it sits

idle and cannot be used by other users toincrease or share capacity.

After the assignments of voice channel, thebase station and mobile station transmit

simultaneously and continuously.

FDMA is a continuous transmission scheme.


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The bandwidthsof FDMA channels are relatively narrow i.e.

FDMA is implemented in narrowband systems.

FDMA systems have higher cell site system costs as compared

to TDMA systems.

FDMA mobile unit uses duplexers. This result in an increasein the cost of FDMA subscribers units and base stations.

FDMA requires tight RF filtering to minimize adjacent channel

interference. Ts>>tau, thus ISI is less and little or no equalization is

required

Complexity is less


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Nonlinear effects in FDMA:

In a FDMA,many channels share the same antenna at the base station.

The power amplifiers or the power combiners when operated at or near

saturation for maximum power efficiency are non linear.

The non linearity's cause signal spreading in frequency domain and

generate intermodulations(IM) frequencies.IM is undesired RF radiation

which can interfere with other channels in FDMA systems.

Spreading of the spectrum results in adjacentchannel interference.

Inter modulation is the generation of undesirable harmonics.


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The number of channels that can be

simultaneously supported in FDMA systems is

given by

Bt = total spectrum allocation

Bguard = guard band

Bc = channel bandwidth


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Time division multiple access:

TDMA systems divide the radio spectrum into time

slots, and each slot only one user is allowed to either

transmit or receive.


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Here each user occupies a cyclically repeating

time slot, so a channel may be thought of as a

particular time slot that recovers every framewhere N time slots compromise a frame

TDMA systems transmit data in a buffer andburst method hence transmission is

noncontious

Hence digital data and digital modulation

must be used with TDMA


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TDMA frame structure:


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A frame consists of a number of slots.

Each frame is made up of preamble, information and tail bits.

In TDMA/TDD ,half of the time slots in the frame informationmessage would be used for the forward link channels and rest halffor reverse link channel.

In TDMA/FDD an identical/similar frame structure would be usedsolely for either forward or reverse link (fc is different for F link andR Link).

In TDMA frame, preamble contains the address and synchronizingbits/information.

Guard times are utilized to allow synchronization of the receiversbetween different slots and frames.


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FEATURES OF TDMA ARE AS FOLLOWS: TDMA shares a single carrier frequency with several users, where each

user makes use of non over lapping time standards.

Data transmission for users of TDMA system is not continuous but occursin bursts.

Handoff process is much simpler in TDMA(because of discontinousTransmissions).

TDMA uses different time slots for transmission and reception, thusduplexers are not required.

Adaptive equalization is necessary in TDMA systems.

In TDMA, guard size must be minimized.

High synchronization is required because of burst transmision.

It is not possible to allocate different number of time slot per frame todifferent users.


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Efficiency of TDMA:

Efficiency of TDMA is the measure of the percentage of transmitteddata that contains information as opposed to providing overheadfor the access scheme.

The frame efficiency f, is the percentage of bits per frame which

contain transmitted data(source and channel coding bits)


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The frame efficiency can be found as follows.

No. of overhead bits per frame is

Where

Nr=no. of reference bits

Nt=no.of traffic bursts

br=overhead bits per reference burst

Bp=over head bits per preamble

Bg=no. of equivalent bits in each guard interval.


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Total bits per frame bTis

where

Tf=frame duration

R=channel bit rate

The frame efficiency is thus


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No. of channels in TDMA system: Multiplying

the number of TDMA slots per channel by thenumber of channels available

where

m = maximum no of TDMA users on each radio channel.


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Spread spectrum multiple Access:

Spread spectrum multiple access(SSMA) uses signals whichhave transmission bandwidth that is several orders of

magnitude greater than the minimum required RF bandwidth.

A pseudonoise(PN) Sequence converts a narrow bandwidthsignal to a wideband noise like signal before transmission.

Provides immunity to multipath interference and robust

multiple access capability.

Not very bandwidth efficient when used by a single user but

efficient in a multiuser environment


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There are two main types of spread spectrum

multiple access: Frequency hopped multiple access(FHMA)

Code division multiple access(CDMA)

FREQUENCY HOPPED MULTIPLE ACCESS:

Frequency hopped multiple access is a digitalmultiple access system in which the carrierfrequencies of the individual users are variedin a pseudorandom fashion within a widebandchannel.


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FHMA allows multiple users to simultaneously occupy the

same spectrum at the same time, where each users dwell at a

specific narrowband channel at a particular instance of time,

based on particular PN code of the user.

The digital data of each user is broken into uniform sized

bursts which are transmitted on different channels within the

allocated spectrum band.

The instantaneous bandwidth of any one transmission burst is

much smaller than the total spread bandwidth.

The difference between FHMA and FDMA is that the

frequency hopped signal changes channel at rapid intervals.


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Code division multiple access:

In code division multiple access systems, thenarrowband message signal is multiplied by a very

large bandwidth signal called the spreading signal.

The spreading signal is a pseudo noise code sequence

that has a chip rate which is orders of magnitudes

greater than the data rate of the message.

All users in a CDMA system use the same carrier

frequency and may transmit simultaneously.


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Multipath fading may be substantially reduced becausethe signal is spread over a large spectrum

Channel data rates are very high in CDMA systems.

Since CDMA uses co-channel cells, it can use

macroscopic spatial diversity to provide soft handoff. Selfjamming is a problem in CDMA systems.

THE FEATURES OF CDMA ARE AS FOLLOWS:

Many users of CDMA system share the same frequency.

Either TDD/FDD may be used.

Unlike TDMA or FDMA,CDMA has a soft capacity limit.


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Hybrid spread spectrum technologies:

Hybrid FDMA/CDMA(FCDMA):Thistechniquecan be used as an alternative to the DS-CDMA

techniques .


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The available wideband spectrum is divided into a

number of subspectras with smaller bandwidths. Each of these smaller sub channels becomes a

narrowband CDMA system having processing gainlower than the original CDMA system.

This hybrid system has an advantage in that therequired BW need not be contiguous anddifferent users can be allotted different subspectrum bandwidths depending on their

requirements. The capacity of this CDMA /FDMA technique is

calculated as the sum of capacities of a systemoperating in the sub spectra.


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HYBRID DIRECT/FREQUENCY HOPPED

MULTIPLE ACCESS(DS/FHMA):

This technique consists of a direct sequencemodulated signal whose center frequency is

made to hop periodically in a pseudo random

fashion


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Direct sequence, frequency hopped systems have

an advantage in that they avoid the near-fareffect.

TIME DIVISION CDMA(TCDMA):

Different spreading codes are assigned todifferent cells. Within each cell, only one CDMA

user is transmitting in each cell.

When a handoff takes place, the spreading code

of the user is changed to that of the new cell.

Using TCDMA has an advantage in that it avoids

the near-far effect since only one user transmits

at a time within a cell.


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TIME DIVISION FREQUENCY HOPPING(TDFH):

This technique has an advantage in severe multipathor when severe co-channel interference occurs.

It has been adopted for the GSM standard.

Avoids co-channel interference. The use of TDFH can increase the capacity of GSM

by several fold.


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Space division multiple access(SDMA):

SDMA controls the radiated energy for eachuser in space.


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SDMA serves different users by using spot beam antennas.

These different areas covered by the antenna beam may be served by the

same frequency or different frequencies.

Sectorized antennasare primitive applications of SDMA. Later adaptive

antennasare likely to be used.

The base stations has complete control over the power of all the transmittedsignals on forward link.

The reverse link presents the most difficulty in cellular systems for several

reasons.

Because of different radio propagation paths between each user and base station,

transmitted power must be dynamically controlled to any single user from driving up

interference level for all other users.

Tx pwr is limited by battery consumption at the SS unit which limits the power control

on reverse link

Capacity of cellular systems:


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Capacity of cellular systems:

Channel capacity for a radio system can be defined as the maximum

number of channels or users that can be provided in a fixed frequencyband.

Radio capacity is a parameter which measures spectrum efficiency of

wireless systems.

This parameter can be determined by C/I and Bc (Channel bandwidth).

In a cellular systems, the interference is of 2 types

Reverse channel interference(Interference due to subscriber units in

the surrounding cells at BS Rx)

Forward channel interference (Interference due to surrounding co

channel base stations at subscriber unit)


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Consider Forward channel interference problem:

The minimum ratio of D/R that is required to providea tolerable level of co-channel interference is the co-

channel reuse ratio

The co-channel reuse ratio is given by

Ddistance between the 2 co-channel cells

Rcell radius


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The carrier to interference ratio is given by

Where Mclosest co channel cells

n0path loss exponent in the desired cell

D0 distance from the desired base station to the

mobile

Dk distance of kth cell from the mobile

nkpath loss exponent to the kth interfering BS


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Illustration of forward channel interference for

a cluster size N =4


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C/I i l i b


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C/I is also given by

Now if we assume D0=R, then

Therefore

The radio capacity of a cellular system is definedas

N is related to Q = 3N

From the above equations ,we can conclude that

F th if Bt d k t t t it b


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Further if Bt and m are kept constant ,it can bewritten as

Further

(C/I)= EbRb/I = EcRc/I

From the above equations, it can be written as


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Capacity of digital cellular TDMA:

TDMA systems improve capacity by a factor of 3 to 6

times as compared to analog cellular radio systems.

Powerful error control and speech coding enable betterlink performance in high interference environment.

By exploiting speech activity, some TDMA systems areable to utilize each radio channel.

Mobile assisted handoff(MAHO)allows subscribers tomonitor the neighboring base stations.

TDMA also makes it possible to introduce adaptivechannel allocation.

GSM,PDC have adopted digital CDMA for high capacity.


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C it f ll l CDMA


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Capacity of cellular CDMA:

Capacity of cellular CDMA is interference limited, while it is BWlimited in FDMA and TDMA.

Therefore any reduction in the interference will cause a linearincrease in the capacity of CDMA.

Interference can be reduced by the use of multi sectorised

antennas which results in spatial isolation of users.

The directional antennas receive signals from only a fraction ofcurrent users, thus leading to the reduction of interference.

CDMA capacity can be increased by operating in discontinuoustransmission mode(DTX).

In DTX mode , the Tx is turned off during the periods of silence inspeech


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For evaluating the capacity of a CDMA system ,first consider asingle cell system.

Let the no. of users be N,

The SNR at the base station receiver can berepresented in terms of Eb/N0


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Taking noise into considerations,

The no of users that can access the systems is

thus given as

further


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The number of users that can access thesystem is given as

Interference can be reduced by using antenna

sectorization.

Eb/No within a sector is given by


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When the number of users is large and the system is

interference limited rather than noise limited, thenumber of users is


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Capacity of CDMA with multiple cells:

In CDMA cellular systems , neighboring cells share

the same frequency and each base station controls

the transmit power of each of its own in-cell users.

A particular base station is unable to control the

power of users in neighboring cells and these users

add to noise floor and decrease capacity on the

reverse link of the particular cell .

Ill i f i hi CDMA ll l


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Illustration of users within a CDMA cellular

radio systems:


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Fig shows how the users in adjacent cells may bedistributed over the coverage area

Transmit powers of each out-of-cell user will add to thein-cell interference (where users are under powercontrol) at the base station receiver

The amount of out-of-cell interference determines thefrequency reuse factor, f, of a CDMA cellular system

Each cell shares same frequency and maximum possible

Value of f = 1 is achieved

The frequency reuse factor of a CDMA cellular


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The frequency reuse factor of a CDMA cellular

system on reverse link is.

No is the total interference power receivedfrom N-1 in-cell users

Naiis the average interference power for a user

located in the ith adjacent cellUi is the number of users in the ith adjacent

cell


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and frequency reuse efficiency F

the average interference power for a user located in

the ith adjacent cell

Nijpower received at the BS from the jth userin the ith cell

A geometric technique called concentric circle cellular


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A geometric technique called concentric circle cellular

geometric is used to determine how the propagation path loss

impacts the frequency reuse of a CDMA system considering

the interference from both in cell and out of cell users

Thi t h i ifi ll f i t t t b i l ll l t d t th


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This technique specifies cell of interest to be circular cell located at the

centre and the interfering cells are wedge shaped and arranged in layers

Let centre cell have Radius = R

d0be the close in distance such that all users in the cell are >= d0

All the users in the cell are located at distance d from BS such that d0


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The concentric circle cellular geometry was

proposed by Rapport.

The area of the centre cell,(if d0


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For each first layer to possess U =KA users apply weight factors such

that


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that

K

user density

W1inweighting factors for the user density within the

inner

W1out

weighting factors for the user density within the

outer

If W1in=1 & W1out =1 then 3/8 of users are in inner sector and

5/8 users in outer sector

To find the capacity of multicell CDMA systems, the concentric


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p y y ,

circle geometry can be used in conjunction with a propagation

path loss model to determine the interference from adjacent

cell users.

The in-cell interference power N0 is

Where P0power received from any one of the U users inthe centre cell

Assume all adjacent cells contain U users and receiving P0

power from each user of its cell and id at a distance dfrom BS

In the forbidden zone it is assumed not to contain users


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Approximation for d is made while computing the power of

the adjacent cell user to its BS

By considering the figure, using the law of cosines,

The interference power at the centre of the cell from jth user in

the ith interfering cell is

where P0=interference power received.

npath loss exponent ,d is a function of theta


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f can range from 0.316 to 0.707 depending on

the path loss exponent and distribution of

users.

Single Cell CDMA system offers ideal

frequency reuse of f=1

Capacity of space division multiple access:


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Capacity of space division multiple access:

For interference limited CDMA operating in an AWGN

channel, with perfect power control with no interference from

adjacent cells and with omnidirectional antennas, the

average bit error rate, Pb, for a user can be found from the

Gaussian approximation.

K-number of users in the cellN - spreading factor

Q(x)Standard Q-function

Illustration showing different antenna Patterns:


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Illustration showing different antenna Patterns:


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Directive pattern can be formed at the base station using N

element adaptive array antenna

Assuming that a beam pattern, G() is formed such that max.

gain is in the direction of the desired user

Beam pattern is as shown in the figure without variation inthe elevation plane


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An idealized flat topped pattern with 60 deg beam

width and -6db side lobe level:

As the antenna beam pattern is made more narrower D and I


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As the antenna beam pattern is made more narrower D and Ireduces proportionally

The average bit error rate for user 0 is given by

Thus Pb is dependent on beam pattern of a receiver

Further using the fact that additional interfernce from adjacent cellssimply adds to the interference level in a multi cell environment

Where f=frequency re-use factor

A fi ill t ti th b bilit f f diff t


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A figure illustrating the average probability of error for different

propagation path loss exponents:

Multiple Acess Techniques for Wireless Communications

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