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