Chapter 1 SDMA(SPACE DIVISION MULTIPLE ACCESS) 1.1 Introduction Space division multiple access (SDMA) controls the radiated energy foreach 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 ofmany users at once and appear to be best suited for TDMA and !DMA base station architectures A patia!!" #i!tered $ae Station Antenna er%in& %ariouMS uin& pot 'eaThe reverse lin% presents the most difficulty in cellular systems for several reasons . First# the base station has complete control over the po$er ofall the transmitted signals on the f or$ard lin%. &o$ever# because of radio propagation paths bet$een each user and the base station# the transmitted po$er from each subscriber unit must be dynamicall y controlled to prevent any single user from driving up the interference level for all other users. Second# tr ansmit po$e r is li mi ted by batt ery consumpt ion at the subsc ri ber unit # therefore 1
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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
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 $ith probability one.
first listens either to the common radio channel or to a separate dedicated
ac%no$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$o co<channel cells and +
be the cell radius. Then the minimum ratio of D:+ that is reuired to provide a
tolerable level of co<channel interference is called the co<channel reuse ratio
and
is given by
2D:+
The radio propagation characteristics determine the carrier<to<interference
In practice# TDMA systems improve capacity by a factor of B to C times as
compared to analog cellular radio systems. *o$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#
$here bac%ground noise and vibration can trigger voice activity detectors. Thus#the average capacity of a !DMA system can be increased by a factor inversely
proportional to the duty factor. 'hile TDMA and FDMA reuse freuencies
depending on the isolation bet$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$idth#
'. The S-+ at the base station receiver can be represented in terms of
In actual !0MA cellular systems that employ separate for$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