The signal may be described in several domains, and an appropriate manipulation of the signals within these domains may lead to effective use of the transmission medium. The signal domains most commonly used for access purposes are frequency, time, code, and space.

Frequency Domain

The radio spectrum is a precious resource, the quintessence of wireless communication systems. Because radio propagation does not recognize geopolitical boundaries and because political, economic, and social aspirations may vary from country to country, international cooperation leading to an intelligent and efficient use of the frequency spectrum is mandatory. Because of the explosion of the demand for and the diversification of wireless services, special attention have been given to spectrumallocation issues.Wireless communications systems have been driven to use high frequencies due to the congestion at the lower portion of the frequency spectrum, where the available bandwidth is insufficient to satisfy the great demand for mobile services.

However, dealing with high frequencies usually leads to intricate problems that are severely aggravated by the mobility of the users. Services are assigned fixed bandwidth, not necessarily in a contiguous fashion. For competition purposes, for a given service the frequency band is split into subbands, each of which is allotted to different service operators. Each one of these bands is then further split into two halves, one for the forward link and the other for the reverse link. Subsequent divisions are carried out to form the nonoverlapping frequency slots (channels). The channel bandwidth (channel spacing) is determined according to criteria such as the services to be provided and the available technology. Each channel—the physical channel—is identified by a carrier placed in the middle of the channel band.

Time Domain

Signal insulation in the time domain is accomplished by allowing the information to use the frequency band during a specific period of time (time slot)—the physical channel. Nonoverlapping time slots constitute the orthogonal channels. For any given piece of information, the aim is to transmit the information in as short a period of time as possible, so that more information can be conveyed in the same frequency band; this is achieved by including more time slots per carrier. As before, this certainly depends on the services to be provided and on the available technology. Access in the timedomaincharacterizes the transmission occurring in bursts because for the same source the information will occupy the carrier only in specific periods of time.

Code Domain

Signal insulation can also be accomplished by assigning each signal a different code (a key, a password)—the physical channel. A code is built as a sequence of symbols belonging to an alphabet. In an ideal situation, these codes must present zero cross-correlation so that they can be univocally discriminated (e.g., for different sources, different passwords are assigned). For a finite alphabet, the number of codes is obviously finite. Therefore, the larger the alphabet, the larger the number of orthogonal codes in the alphabet and the longer each code. For a given transmission rate, the longer the code, the longer the time to transmit the code and the longer the time to detect the code. Should it be transmitted, and detected, in a shorter period of time, the transmission rate must be increased as well as the required bandwidth. Therefore, for a limited bandwidth the number of orthogonal codes (code slots) is also limited.

Space Domain

Signal insulation in the space domain can be achieved in two possible dimensions: distance and angle—the physical channels. The distance dimension exploits the fact that the propagation loss increases with the increase of the distance between transmitter and receiver. Thus, signals using the same frequency but transmitted by sources sufficiently apart from each other may not strongly interfere with each other. In the same way, a given signal may reach the receiver through different paths (due to multiple reflections, for example). Each multipath signal suffers different attenuations and different delays, according to the length of the path traveled. Therefore, both attenuation and delay, jointly or independently, can be used to detect each multipath signal. The angle dimension exploits the fact that, by illuminating wedges of a circular area, signals simultaneously using the same frequency may be discriminated by these very wedges within which they are located. Smart antennas may be used to keep track of these signals.

Brief Remarks on Signal Domains

The most commonly used and most straightforward way of accomplishing radio signal insulation is by assigning different frequency carriers to different signals. This technique is widely employed by both analog and digital wireless systems. Insulation in the time domain has been boosted by the digital technology and is widely used in wireless communications. Insulation in the code domain is a well-known technique that has long been used for military as well as satellite communications applications. The move toward high-capacity wireless systems has found great support in this technique. Insulation in the space domain is widely used in wireless communications. More specifically, the cellular concept with its frequency reuse philosophy constitutes an example of such an application.