Introduction

The radio channel is fundamentally a broadcast communication medium. Therefore, signals transmitted by one user can potentially be received by all other users within range of the transmitter. While this high connectivity is very useful in some applications, like broadcast radio or television, it requires stringent access control in wireless communication systems to avoid, or at least limit, interference between transmissions. Throughout, the term wireless communication systems is taken to mean communication systems which facilitate two-way communication between a portable radio communication terminal and the fixed network infrastructure. Such systems range from mobile cellular systems over personal communication systems (PCS) to cordless telephones.

The objective of wireless communication systems is to provide communication channels on demand between a portable radio station and a radio port or base station which connects the user to the fixed network infrastructure. Design criteria for such systems include capacity, cost of implementation, and quality of service. All of these measures are influenced by the method used for providing multiple-access capabilities. However, the opposite is also true: the access method should be chosen carefully in light of the relative importance of design criteria as well as the system characteristics.

Multiple access in wireless radio systems is based on insulating signals used in different connections from each other. The support of ``parallel'' transmissions on the up-link and down-link, respectively, is called multiple access, while the exchange of information in both directions of a connections is referred to as duplexing. Hence, multiple-access and duplexing are methods which facilitate the sharing of the broadcast communication medium. The necessary insulation is achieved by assigning to each transmission different components of the domains that contain the signals. The signal domains commonly used to provide multiple access capabilities include:

Spatial Domain

All wireless communication systems exploit the fact that radio signals experience rapid attenuation during propagation. The propagation exponent on typical radio channels lies between and with a typical value. As signal strength decays inversely proportional to the -th power of the distance far-away transmitters introduce interference which is negligible compared to the strength of the desired signal. The cellular design principle is based on the ability to re-use signals safely if a minimum re-use distance is maintained. Directional antennas can be used to enhance the insulation between signals. We will not focus further on the spatial domain in this treatment of access methods.

Frequency Domain

Signals which occupy non-overlapping frequency bands can be easily separated using appropriate bandpass filters. Hence, signals can be transmitted simultaneously without interfering with each other. This method of providing multiple-access capabilities is called frequency-division multiple-access (FDMA).

Time Domain

Signals can be transmitted in non-overlapping time slots in a round-robin fashion. Thus, signals occupy the same frequency band but are easily separated based on their time of arrival. This multiple access method is called time-division multiple-access (TDMA).

Code Domain

In code-division multiple-access different users employ signals which have very small cross-correlation. Thus, correlators can be used to extract individual signals from a of signals even though they are transmitted simultaneously and in the same frequency band. The term code-division multiple-access (CDMA) is used to denote this form of channel sharing. Two forms of CDMA are most widely employed and will be described in detail below: frequency-hopping (FH) and direct-sequence (DS).

System designers have to decide in favor of one, or a combination, of the latter three domains to facilitate multiple-access. The three access methods are illustrated in Figure 1. The principal idea in all three of these access methods is to employ signals which are orthogonal or nearly orthogonal. Then, correlators which project the received signal into the subspace of the desired signal can be employed to extract a signal without interference from other transmissions.

     
Figure 1: Multiple-access methods for wireless communication systems.

Preference for one access method over another depends largely on overall system characteristics as we will see in the sequel. No single access method is universally preferable and system considerations should be carefully weighed before the design decision is made. Before going into the detailed description of the different access methods, we will discuss briefly the salient features of some wireless communication systems. This will allow us later to assess the relative merits of the access methods in different scenarios.