Seminar Report on SDMA

7/23/2019 Seminar Report on SDMA

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Chapter 1

SDMA(SPACE DIVISION MULTIPLE

ACCESS)

1.1 Introduction

Space division multiple access (SDMA) controls the radiated energy for

each user in space. It can be seen from Figure that SDMA serves different

users by using spot beam antennas. These different areas covered by the

antenna beam may be served by the same freuency (in a TDMA or !DMA

systern) or different freuencies (in an FDMA system). Sectori"ed antennas may be thought of as a primitive application of SDMA. In the future# adaptive

antennas $ill li%ely be used to simultaneously steer energy in the direction of

many users at once and appear to be best suited for TDMA and !DMA base

station architectures

A patia!!" #i!tered $ae Station Antenna er%in& %ariou MS uin& pot

'ea

The reverse lin% presents the most difficulty in cellular systems for several

reasons . First# the base station has complete control over the po$er of

all the transmitted signals on the for$ard lin%. &o$ever# because of

radio propagation paths bet$een each user and the base station# the transmitted

po$er from each subscriber unit must be dynamically controlled to prevent any

single user from driving up the interference level for all other users. Second#

transmit po$er is limited by battery consumption at the subscriber unit#therefore

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there are limits on the degree to $hich po$er may be controlled on the

reverse lin%. If the base station antenna is made to spatially filter each desired

user so that more energy is detected from each subscriber# then the reverse lin%

for each user is improved and less po$er is reuired.

Adaptive antennas used at the base station (and eventually at the subscriber units) promise to mitigate some of the problems on the reverse lin%. In

the limiting case of infinitesimal beam$idth and infinitely fast trac%ing ability#

adaptive antennas implement optimal SDMA# thereby providing a uniue

channel that is free from the interference of all other users in the cell. 'ith

SDMA# all users $ithin the system $ould be able to communicate at the same

time using the same channel. In addition# a perfect adaptive antenna system

$ould be able to trac% individual multipath components for each user and

combine them in an optimal manner to collect all of the available signal energy

from each user.The perfect adaptive antenna system is not feasible since it reuires infinitely

large antennas.

1. ADVANTA*ES O+ SDMA

DMA increases the capacity of the system and transmission uality by focusing

the signal into narro$ transmission beams. Through the use of smart antennas

$ith beams pointed at the direction of the mobile station# SDMA servesdifferent users $ithin the same region.

Mobile stations operating outside the bounds of these directed beams eperience

a near "ero interference from other mobile stations operating under the same

base station $ith the same radio freuency.

Since the beams are focused# the radio energy freuency can have increased

base station range. This attribute of SDMA allo$s base stations to have larger

radio coverage $ith less radiated energy.

nder traditional mobile phone net$or% systems# the base station radiates radio

signals in all directions $ithin the cell $ithout %no$ledge of the location of the

mobile station. SDMA technology channels radio signals based on the location

of the mobile station.

The main advantage of SDMA is freuency reuse. *rovided the reuse distance is

preserved in the net$or% architecture# interference can be near "ero# even if

mobile stations use the same allocated freuencies.

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C,APTE-

PACET -ADIO

.1 INT-ODUCTION

In pac%et radio (*+) access techniues# many subscribers attempt to access

a single channel in an uncoordinated (or minimally coordinated) manner.

Transmission is done by using bursts of data. !ollisions from the simultaneous

transmissionsof multiple transmitters are detected at the base station receiver# in

$hich case an A!, or -A!, signal is broadcast by the base station to alert the

desired user (and all other users) of received transmission. The A!, signal

indicates an ac%no$ledgment of a received burst from a particular user by the

base station# and a -A!, (negative ac%no$ledgment) indicates that the

previous burst $as not received correctly by the base station. y using A!, and

-A!, signals# a *+ system employs perfect feedbac%# even though traffic

delay due to collisions may be high.

*ac%et radio multiple access is very easy to implement but has lo$ spectraldensity and may induce delays. The subscribers use a contention techniue to

transmit on a common channel. A/0&A protocols# developed fo1 early satellite

systems# are the best eamples of contention techniues. A/0&A allo$s each

subscriber to transmit $henever they have data to send# The transmitting

subscribers listen to the ac%no$ledgment feedbac% to determine if transmission

has been successful or not. If a collision occurs# the subscriber $aits a random

amount of time# and then retransmits the pac%et. The advantage of pac%et

contention techniues is the ability to serve a large number of subscribers $ith

virtually no overhead. The performance of contention techniues can beevaluated by the throughput (T)# $hich is defined as the average number of

3



messages successfully transmitted per unit time# and the average de%y (D)

eperienced by a typical message burst.

. P-OTOCOLS

.1.1 PU-E ALO,A

The pure A/0&A protocol is a random access protocol used for data transfer.

A user accesses a channel as soon as a message is ready to be transmitted.

After a transmission# the user $aits for an ac%no$ledgment on either the same

channel or a separate feedbac% channel. In case of collisions# (i.e.# $hen a

-A!,

is received)# the terminal $aits for a random period of time and retransmits the

message. As the number of users increase# a greater delay occurs because the

probability of collision increases.

For the A/0&A protocol# the vulnerable period is double the pac%et duration at

n 2 3

0ne may evaluate the mean of euation (4.53) to determine the averagenumber of pac%ets sent during 6t (This is useful in determining the average

offered traffic). The probability of no collision is *r (3) 2 The throughput

of the A/0&A protocol is found by using 7uation as

T 2 +e8r

.1. SLOTTED ALO,A

In slotted A/0&A# time is divided into eual time slots of length greater

than the pac%et duration t. The subscribers each have synchroni"ed cloc%s and

transmit a message only at the beginning of a ne$ time slot# thus resulting in a

discrete distribution of pac%ets. This prevents partial collisions# $here one

pac%et collides $ith a portion of another As the number of users increase# a

greater delay $ill occur due to complete collisions and the resulting repeated

transmissions of those pac%ets originally lost. The number of slots $hich a

transmitter

$aits prior to retransmitting also determines the delay characteristics of

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the traffic. The vulnerable period for slotted A/0&A is only one pac%et

duration#

since partial collisions are prevented through synchroni"ation. The probability

that no other pac%ets $ill be generated during the vulnerable period is The

throughput for the case of slotted A/0&A is thus given by T 2 +e8r

throughput for delays.

.1./ CA--IE- SENSE MULTIPLE

ACCESS(CSMA) P-OTOCOLS

A/0&A protocols do not listen to the channel before transmission# and

therefore do not eploit information about the other users. y listening to the

channel before engaging in transmission# greater efficiencies may be achieved.!SMA protocols are based on the fact that each terminal on the net$or% is able

to monitor the status of the channel before transmitting information. If the chan

nel is idle (i.e.# no carrier is detected)# then the user is allo$ed to transmit a

pac%et based on a particular algorithm $hich is common to all transmitters on

the net$or%.

In !SMA protocols# detection delay and propagation delay are t$o important

parameters. Detection delay is a function of the receiver hard$are and is

the time reuired for a terminal to sense $hether or not the channel is idle.

*ropagation delay is a relative measure of ho$ fast it ta%es for a pac%et to travelfrom a base station to a mobile terminal. 'ith a small detection time# a terminal

detects a free channel uite rapidly# and small propagation delay means that a

pac%et is transmitted through the channel in a small interval of time relative to

the pac%et duration.

*ropagation delay is important# since 9ust after a user begins sending a

pac%et# another user may be ready to send and may be sensing the channel at

the same time. If the transmitting pac%et has not reached the user $ho is poised

to send# the latter user $ill sense an idle channel and $ill also send its pac%et#

resulting in a collision bet$een the t$o pac%ets. *ropagation delay impacts the performance of !SMA protocols. If is the propagation time in secnds# +b is

the channel bit rate# and m is the epected number of bits in a data pac%et then

the propagation delay td (in pac%et transmission units)

can be epressed as

td 2tp+:m

There eist several variations of the !SMA strategy;

5<persistent !SMA = The terminal listens to the channel and $aits for

transmission until it finds the channel idle. As soon as the channel is idle#

the terminal transmits its message $ith probability one.

5



> non<persistent .!SMA = In this type of !SMA stratea after receiving a

negative ac%no$ledgment the terminal $aits a random time before

retransmission

of the pac%et. This is popular for $ireless /A- applications# $here

the pac%et transmission interval is much greater than the propagation delayto the farthermost user.

> p<persistent !SMA = p<persistent !SMA is applied to slotted channels.

'hen a channel is found to be idle# the pac%et is transmitted in the first

available slot $ith probabilityp or in the net slot $ith probability i<p.

> !S.MA?!D = In !SMA $ith collision detection (!D)# a user monitors its

transmission for collisions. If t$o or more terminals start a transmission at

the same time# collision is detected# and the transmission is immediatelyaborted in midstream. This is handled by a user having both a transmitter

and receiver $hich is able to support listen<$hile<tal% operation. For a single

radio channel# this is done by interrupting the transmission in order to sense

the channel. For duple systems# a full duple transceiver is used.

Data sense multiple access (DSMA) = DSMA is a special type of !SMA

that relies on successfully demodulating a for$ard control channel before

broadcasting data bac% on a reverse channel. 7ach user attempts to detect a

busy<idle message $hich is interspersed on the for$ard control channel.

'hen the busy<idle message indicates that no users are transmitting on the

reverse channel# a user is free to send a pac%et.

.1.0 -ESE-VATION P-OTOCOLS

-eer%ation ALO,A is a pac%et access scheme based on time division

multipleing.In this protocol# certain pac%et slots are assigned $ith priority# and it

is possible for users to reserve slots for the transmission of pac%ets. Slots can be

permanently reserved or can be reserved on reuest. For high traffic conditions#

reservations on reuest offers better throughput. In one type of reservation

A/0&A# the terminal ma%ing a successful transmission reserves a slot

permanently until its transmission is complete# although very large duration

transmissions may be interrupted. Another scheme allo$s a user to transmit a

reuest on a subslot $hich is reserved in each frame. If the transmission is

successful (i.e# no collisions are detected)# the terminal is allocated the netregular slot in the frame for data transmission.

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Pacet -eer%ation Mu!tip!e Acce (P-MA)

*+MA uses a discrete pac%et time techniue similar to reservation A/0&A

and combines the cyclical frame structure of TDMA in a manner that allo$s

eachTDMA time slot to carry either voice or data# $here voice is given priority.

*+MA $as proposed in as a means of integrating bursty data and human

speech. *+MA defines a frame structure# much li%e is used in TDMA systems.

'ithin each frame# there are a fied number of time slots $hich may be

designated

as either @itserved@ or @available@# depending on the traffic as determined

by the controlling base station.

./ CAPTU-E E++ECT

*ac%et radio multiple access techniues are based on contention $ithin a

channel. 'hen used $ith FM or spread spectrum modulation# it is possible for

the strongest user to successfully capture the intended receiver# even $hen many

other users are also transmitting. 0ften# the closest transmitter is able to capture

a receiver because of the small propagation path loss. This is called the near<tbr

effect. The capture effect offers both advantages and disadvantages in

practical systems. ecause a particular transmitter may capture an intended

receiver# many pac%ets may survive despite collision on the channel. &o$ever#

a

strong transmitter may ma%e it impossible for the receiver to detect a much

$ea%er transmitter $hich is attempting to communicate to the same receiver.

This problem is %no$n as the hidden transmitter problem.

A useful parameter in analy"ing the capture effects in pac%et radio protocols

is the minimum po$er ratio of an arriving pac%et# relative to the other colliding

pac%ets# such that it is received. This ratio is called the capture ratio# and

is dependent upon the receiver and the modulation used.In summary# pac%et radio techniues support mobile transmitters sending

bursty traffic in the form of data pac%ets using random access. Ideal channel

throughput can be increased if terminals synchroni"e their pac%et transmissions

into common time slots# such that the ris% of partial pac%et overlap is avoided.

'ith high traffic loads# both unslotted and slotted A/0&A protocols become

inefficient#

since the contention bet$een all transmitted pac%ets eposes most of the

offered traffic to collisions# and thus results ih multiple retransmissions and

increased delays. To reduce this situation !SMA can be used $here thetransmitter

7



first listens either to the common radio channel or to a separate dedicated

ac%no$ledgment control channel from the base station. In a real $orld mobile

system# the !SMA protocols may fail to detect ongoing radio transmissions of

pac%ets sub9ect to deep fading on the reverse channel path. tili"ation of an

A/0&A channel can be improved by deliberately introducing differences bet$een the transmit po$ers of multiple users competing for the base station.

C,APTE- /

CAPACIT2 O+ CELLULA- S2STEMS

/.1 INT-ODUCTION

!hannel capacity for a radio system can be defined as the maimum number

of channels or users that can be provided in a fied freuency band. +adio

capacity is a parameter $hich measures spectrum efficiency of a $ireless

system.

This parameter is determined by the reuired carrier<to<interference ratio

(!:I) and the channel band$idth In a cellular system the interference at a basestation receiver $ill come from the subscriber units in the surrounding cells.

This is called reverse channel

interference. For a particular subscriber unit# the desired base station $ill

provide

the desired for$ard channel $hile the surrounding co<channel base stations

$ill provide the for$ard channel interference. !onsidering the for$ard channel

interference problem# let D be the distance bet$een t$o co<channel cells and +

be the cell radius. Then the minimum ratio of D:+ that is reuired to provide a

tolerable level of co<channel interference is called the co<channel reuse ratio

and

is given by

2D:+

The radio propagation characteristics determine the carrier<to<interference

ratio (! :5) at a given location.

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/. CAPACIT2 O+ DI*ITAL CELLULA-

TDMA

In practice# TDMA systems improve capacity by a factor of B to C times as

compared to analog cellular radio systems. *o$erful error control and speech

coding enable better lin% performance in high interference environments. y

eploiting speech activity# some TDMA systems are able to better utili"e each

radio channel. Mobile assisted handoff (MA&0) allo$s subscribers to monitor the neighboring base stations# and the best base station choice may be made by

each subscriber. MA&0 allo$s the deployment of densely pac%ed microcells#

thus giving substantial capacity gains in a system. TDMA also ma%es it possible

to introduce adaptive channel allocation (A!A). A!A eliminates system

planning since it is not reuired to plan freuencies for cells. arious proposed

standards such as the ESM# .S digital cellular (SD!)# and *acific Digital

!ellular (*D!) have adopted digital TDMA for high capacity.

/./ CAPACIT2 O+ CELLULA- CDMA

The capacity of !DMA systems is interference limited# $hile it is band$idth

limited in FDMA and TDMA. Therefore# any reduction in the interference

$ill cause a linear increase in the capacity of !DMA. *ut another $ay# in a

!DMA system# the lin% performance for each user increases as the number of

users decreases. A straightfor$ard $ay to reduce interference is to use

multisectori"edantennas# $hich results in spatial isolation of users. The directional

antennas receive signals from only a fraction of the current users# thus leading

to the reduction of interference. Another $ay of increasing !DMA capacity is to

operate in a discontinuous transmission mode (DT)# $here advantage is ta%en

of the intermittent nature of speech. In DT# the transmitter is turned off during

the periods of silence in speech. It has been observed that voice signals have a

duty factor of about B:4 in landline net$or%s GraC4H# and 5:6 for mobile

systems#

$here bac%ground noise and vibration can trigger voice activity detectors. Thus#the average capacity of a !DMA system can be increased by a factor inversely

9



proportional to the duty factor. 'hile TDMA and FDMA reuse freuencies

depending on the isolation bet$een cells provided by the path loss in terrestrial

radio propagation# !DMA can reuse the entire spectrum for all cells# and this

results in an increase of capacity by a large percentage over the normal

freuencyreuse factor.

For evaluating the capacity of !DMA system# first consider a single cell system

. The cellular net$or% consists of a large number of mobile users

communicating

$ith a base station (In a multiple cell system# all the base stations

are interconnected by the mobile s$itching center). The cell<site transmitter

consists

of a linear combiner $hich adds the spread signals of the inthvidual users

and also uses a $eighting factor for each signal for for$ard lin% po$er control purposes. For a single cell system under consideration# these $eighting factors

can be assumed to be eual. A pilot signal is also included in the cell<site

transmitter

and is used by each mobile to set its o$n po$er control for the reverse

lin%. For a single<cell system $ith po$er control# all the signals on the reverse

channel are received at the same po$er level at the base station.

/et the number of users be -. Then# each demodulator at the cell site

receives a composite $aveform containing the desired signal of po$er S and

(- = I) interfering users# each of $hich has po$er# S. Thus# the signal<to<noise

ratio is

S-+

2 (-<I)S 2 (-< I)

In addition to S-+# bit energy<to<noise ratio is an important parameter in

communication systems. It is obtained by dividing the signal po$er by the

baseband

information bit rate# +# and the interference po$er by the total +F band$idth#

'. The S-+ at the base station receiver can be represented in terms of

7:-3 given by7## < S:+ < ':+ -3 < (-<I)(S:') < -<5

7uation does not ta%e into account the bac%ground thermal noise#

in the spread band$idth.

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/.0 CAPACIT2 O+ CDMA 3IT, MULTIPLE

CELLS

In actual !0MA cellular systems that employ separate for$ard and reverse

lin%s# neighboring cells share the same freuency# and each base station controls

the transmit po$er of each of its o$n in<cell users. &o$ever# a particular base

station is unable to control the po$er of users in neighboring cells# and these

users add to the noise floor and decrease capacity on the reverse lin% of the particular cell of interest. The transmit po$ers of each out<of<cell user $ill add

to the in<cell interference ($here users are under

po$er control) at the base station receiver. The amount of out<of<cell

interferencedetermines the freuency reuse factor# f# of a !DMA cellular

system. Ideally#

each cell shares the same freuency and the maimum possible value of f

!f 2 5) is achieved. In practice# ho$ever# the out<of<cell interference reduces f

significantly.

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-E+E-ENCES

3ire!e Counication4 Princip!e

and Practice '" Theodore S. -appaport

Ce!!u!ar Mo'i!e Counication $"

*ottapu Sai'huhana -ao

555.techopedia.co

http466555.decode"te.co6t678#e

'6

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Seminar Report on SDMA