Multiple access in wireless domains

Wireless networks are multiuser systems in whichreverse direction. The point-to-multipoint
information is conveyed by means of radiocharacteristic of the downlink facilitates the
waves. In a multiuser environment, accesssynchronous approach, because one reference
coordination can be accomplished via severalchannel, broadcast by the base station, can be
mechanisms: by insulating the various signalsused by all mobile stations within its service area
sharing the same access medium, by allowing thefor synchronization purposes. On the other hand,
signals to contend for the access, or by combiningthe implementation of a similar feature on the
these two approaches. The choice for thereverse link is not as simple because of its
appropriate scheme must take into account amultipoint-to-point transmission characteristic. In
number of factors, such as type of traffic undertheory, the use of orthogonal codes eliminates
consideration, available technology, cost,the multiple-access interference. Therefore, in an
complexity. Signal insulation is easily attainable byideal situation, the forward link would not present
means of a scheduling procedure in which signalsmultiple-access interference. The reverse link, in
are allowed to access the medium according to aturn, is characterized by multiple-access
predefined plan. Signal contention occurs exactlyinterference. In practice, however, interference still
because no signal insulation mechanism is used.occurs in synchronous systems, because of the
Access coordination may be carried out inmultipath propagation and because of the
different domains: the frequency domain, timeother-cell signals. The
domain, code domain, and space domain. Signalmultipathphenomenonproduces delayed and
insulation in each domain is attained by splitting theattenuated replicas of the signals, with these
resource available into nonoverlapping slotssignals then losing the synchronism and, therefore,
(frequency slot, time slot, code slot, and spacethe orthogonality. The other-cell signals, in turn,
slot) and assigning each signal a slot. Four mainare not time-aligned with the desired signal.
multiple access technologies are used by theTherefore, they are not orthogonal with the
wireless networks: frequency division multipledesired signal and may cause interference.
access (FDMA), time division multiple accessChannels in the forward link are identified by
(TDMA), code division multiple access (CDMA), andorthogonal sequences, i.e., channelization in the
space division multiple access (SDMA).forward link is achieved by the use of orthogonal
Frequency Division Multiple Accesscodes. Base stations are identified by pseudonoise
FDMA is certainly the most conventional method(PN) sequences. Therefore, in the forward link,
of multiple access and was the first technique toeach channel uses a specific orthogonal code and
be employed in modern wireless applications. Inemploys a PN sequence modulation, with a PN
FDMA, the available bandwidth is split into acode sequence specific to each base station.
number of equal subbands, each of whichHence, multiple access in the forward link is
constitutes a physical channel. The channelaccomplished by the use of spreading orthogonal
bandwidth is a function of the services to besequences. The purpose of the PN sequence in
provided and of the available technology and isthe forward link is to identify the base station and
identified by its center frequency, known as ato reduce the interference. In general, the use of
carrier. In single channel per carrierFDMAorthogonal codes in the reverse link finds no direct
technology, the channels, once assigned, are usedapplication, because the reverse link is intrinsically
on a non-time-sharing basis. Thus, a channelasynchronous. Channelization in the reverse link is
allocated to a given user remains allocated untilachieved with the use of long PN sequences
the end of the task for which that specificcombined with some private identification, such as
assignment was made.the electronic serial number of the mobile station.
Time Division Multiple AccessSome systems, on the other hand, implement
TDMA is another widely known multiple-accesssome sort of synchronous transmission on the
technique and succeeded FDMA in modernreverse link. In such a case, orthogonal codes
wireless applications. In TDMA, the entiremay also be used with channelization purposes in
bandwidth is made available to all signals but on athe reverse link.
time-sharing basis. In such a case, theSeveral PN sequences are used in the various
communication is carried out on asystems, and they will be detailed for the several
buffer-and-burst scheme so that the sourcetechnologies. Two main orthogonal sequences are
information is first stored and then transmitted.used in all CDMA systems:Walsh codes and
Prior to transmission, the information remainsorthogonal variable spreading functions (OVSF)
stored during a period of time referred to as a(see Appendix C).
frame. Transmission then occurs within a timeSpace Division Multiple Access
interval known as a (time) slot. The time slotSDMA is a nonconventional multiple-access
constitutes the physical channel.technique that finds application in modern wireless
Code Division Multiple Accesssystems mainly in combination with other
CDMA is a nonconventional multiple-accessmultiple-access techniques. The spatial dimension
technique that immediately found wide applicationhas been extensively explored by wireless
in modern wireless systems. In CDMA, the entirecommunications systems in the form of
bandwidth is made available simultaneously to allfrequency reuse. The deployment of advanced
signals. In theory, very little dynamic coordinationtechniques to take further advantage of the
is required, as opposed to FDMA and TDMA inspatial dimension is embedded in the SDMA
which frequency and time management have aphilosophy. In SDMA, the entire bandwidth is made
direct impact on performance. To accomplishavailable simultaneously to all signals. Signals are
CDMA systems, spread-spectrum techniques arediscriminated spatially, and the communication
used. (Appendix C introduces the concept oftrajectory constitutes the physical channels. The
spread spectrum.)implementation of an SDMA architecture is based
In CDMA, signals are discriminated by means ofstrongly on antennas technology coupled with
code sequences or signature sequences, whichadvanced digital signal processing. As opposed to
correspond to the physical channels. Each pair ofthe conventional applications in which the locations
transmitter–receivers is allotted oneare constantly illuminated by rigid-beam antennas,
code sequence with which a communication isin SDMA the antennas should provide for the
established. At the reception side, detection isability to illuminate the locations in a dynamic
carried out by means of a correlation operation.fashion. The antenna beams must be electronically
Ideally, the best performance is attained with zeroand adaptively directed to the user so that, in an
crosscorrelation codes, i.e., with orthogonal codes.idealized situation, the location alone is enough to
In theory, for a synchronous system and fordiscriminate the user.
equal rate users, the number of users within aFDMA and TDMA systems are usually considered
given bandwidth is dictated by the number ofto be narrowband, whereas CDMA systems are
possible orthogonal code sequences. In general,usually designed to be wideband. SDMA systems
CDMA systems operate synchronously in theare deployed together with the other
forward direction and asynchronously in themultiple-access technologies.