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.