Everything about wireless networks


 

Multiple access in wireless domains

Wireless networks are multiuser systems inasynchronously in the reverse direction. The
which information is conveyed by means ofpoint-to-multipoint characteristic of the
radio waves. In a multiuser environment,downlink facilitates the synchronous
access coordination can be accomplished viaapproach, because one reference channel,
several mechanisms: by insulating the variousbroadcast by the base station, can be used by
signals sharing the same access medium, byall mobile stations within its service area
allowing the signals to contend for thefor synchronization purposes. On the other
access, or by combining these two approaches.hand, the implementation of a similar feature
The choice for the appropriate scheme muston the reverse link is not as simple because
take into account a number of factors, suchof its multipoint-to-point transmission
as type of traffic under consideration,characteristic. In theory, the use of
available technology, cost, complexity.orthogonal codes eliminates the
Signal insulation is easily attainable bymultiple-access interference. Therefore, in
means of a scheduling procedure in whichan ideal situation, the forward link would
signals are allowed to access the mediumnot present multiple-access interference. The
according to a predefined plan. Signalreverse link, in turn, is characterized by
contention occurs exactly because no signalmultiple-access interference. In practice,
insulation mechanism is used. Accesshowever, interference still occurs in
coordination may be carried out in differentsynchronous systems, because of the multipath
domains: the frequency domain, time domain,propagation and because of the other-cell
code domain, and space domain. Signalsignals. The multipathphenomenonproduces
insulation in each domain is attained bydelayed and attenuated replicas of the
splitting the resource available intosignals, with these signals then losing the
nonoverlapping slots (frequency slot, timesynchronism and, therefore, the
slot, code slot, and space slot) andorthogonality. The other-cell signals, in
assigning each signal a slot. Four mainturn, are not time-aligned with the desired
multiple access technologies are used by thesignal. Therefore, they are not orthogonal
wireless networks: frequency divisionwith the desired signal and may cause
multiple access (FDMA), time divisioninterference.
multiple access (TDMA), code division
multiple access (CDMA), and space divisionChannels in the forward link are identified
multiple  access  (SDMA).by orthogonal sequences, i.e., channelization
in the forward link is achieved by the use of
Frequency  Division  Multiple  Accessorthogonal codes. Base stations are
identified by pseudonoise (PN) sequences.
FDMA is certainly the most conventionalTherefore, in the forward link, each channel
method of multiple access and was the firstuses a specific orthogonal code and employs a
technique to be employed in modern wirelessPN sequence modulation, with a PN code
applications. In FDMA, the availablesequence specific to each base station.
bandwidth is split into a number of equalHence, multiple access in the forward link is
subbands, each of which constitutes aaccomplished by the use of spreading
physical channel. The channel bandwidth is aorthogonal sequences. The purpose of the PN
function of the services to be provided andsequence in the forward link is to identify
of the available technology and is identifiedthe base station and to reduce the
by its center frequency, known as a carrier.interference. In general, the use of
In single channel per carrierFDMA technology,orthogonal codes in the reverse link finds no
the channels, once assigned, are used on adirect application, because the reverse link
non-time-sharing basis. Thus, a channelis intrinsically asynchronous. Channelization
allocated to a given user remains allocatedin the reverse link is achieved with the use
until the end of the task for which thatof long PN sequences combined with some
specific  assignment  was  made.private identification, such as the
electronic serial number of the mobile
Time  Division  Multiple  Accessstation. Some systems, on the other hand,
implement some sort of synchronous
TDMA is another widely known multiple-accesstransmission on the reverse link. In such a
technique and succeeded FDMA in moderncase, orthogonal codes may also be used with
wireless applications. In TDMA, the entirechannelization  purposes in the reverse link.
bandwidth is made available to all signals
but on a time-sharing basis. In such a case,Several PN sequences are used in the various
the communication is carried out on asystems, and they will be detailed for the
buffer-and-burst scheme so that the sourceseveral technologies. Two main orthogonal
information is first stored and thensequences are used in all CDMA systems:Walsh
transmitted. Prior to transmission, thecodes and orthogonal variable spreading
information remains stored during a period offunctions  (OVSF)  (see  Appendix  C).
time referred to as a frame. Transmission
then occurs within a time interval known as aSpace  Division  Multiple  Access
(time) slot. The time slot constitutes the
physical  channel.SDMA is a nonconventional multiple-access
technique that finds application in modern
Code  Division  Multiple  Accesswireless systems mainly in combination with
other multiple-access techniques. The spatial
CDMA is a nonconventional multiple-accessdimension has been extensively explored by
technique that immediately found widewireless communications systems in the form
application in modern wireless systems. Inof frequency reuse. The deployment of
CDMA, the entire bandwidth is made availableadvanced techniques to take further advantage
simultaneously to all signals. In theory,of the spatial dimension is embedded in the
very little dynamic coordination is required,SDMA philosophy. In SDMA, the entire
as opposed to FDMA and TDMA in whichbandwidth is made available simultaneously to
frequency and time management have a directall signals. Signals are discriminated
impact on performance. To accomplish CDMAspatially, and the communication trajectory
systems, spread-spectrum techniques are used.constitutes the physical channels. The
(Appendix C introduces the concept of spreadimplementation of an SDMA architecture is
spectrum.)based strongly on antennas technology coupled
with advanced digital signal processing. As
In CDMA, signals are discriminated by meansopposed to the conventional applications in
of code sequences or signature sequences,which the locations are constantly
which correspond to the physical channels.illuminated by rigid-beam antennas, in SDMA
Each pair of transmitter–receivers isthe antennas should provide for the ability
allotted one code sequence with which ato illuminate the locations in a dynamic
communication is established. At thefashion. The antenna beams must be
reception side, detection is carried out byelectronically and adaptively directed to the
means of a correlation operation. Ideally,user so that, in an idealized situation, the
the best performance is attained with zerolocation alone is enough to discriminate the
crosscorrelation codes, i.e., with orthogonaluser.
codes. In theory, for a synchronous system
and for equal rate users, the number of usersFDMA and TDMA systems are usually considered
within a given bandwidth is dictated by theto be narrowband, whereas CDMA systems are
number of possible orthogonal code sequences.usually designed to be wideband. SDMA systems
In general, CDMA systems operateare deployed together with the other
synchronously in the forward direction andmultiple-access technologies.



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