Multiple-Access Techniques
In Section 1.2.1 we discussed ways in which a symbol stream associated with a single user can be transmitted. Many wireless channels, particularly in emerging systems, operate as multiple-access systems, in which multiple users share the same radio resources.
There are several ways in which radio resources can be shared among multiple users. These can be viewed as ways of allocating regions in frequency, space, and time to different users, as shown in Fig. 1.1. For example, a classic multiple-access technique is frequency-division multiple access (FDMA), in which the frequency band available for a given service is divided into subbands that are allocated to individual users who wish to use the service. Users are given exclusive use of their subband during their communication session, but they are not allowed to transmit signals within other subbands. FDMA is the principal multiplexing method used in radio and television broadcast and in first-generation (analog voice) cellular telephony systems, such as the Advanced Mobile Phone System (AMPS) and Nordic Mobile Telephone (NMT), developed primarily in the 1970s and 1980s (cf. [458]). FDMA is also used in some form in all other current cellular systems, in tandem with other multiple-access techniques that are used to further allocate the subbands to multiple users.
Figure 1.1. Multiple-access schemes.
Similarly, users can share the channel on the basis of time-division multiple access (TDMA), in which time is divided into equal-length intervals, which are further divided into equal-length subintervals, or time slots. Each user is allowed to transmit throughout the entire allocated frequency band during a given slot in each interval but is not allowed to transmit during other time slots when other users are transmitting. So, whereas FDMA allows each user to use part of the spectrum all of the time, TDMA allows each user to use all of the spectrum part of the time. This method of channel sharing is widely used in wireless applications, notably in a number of second-generation cellular (i.e., digital voice) sytems, including the widely used Global System for Mobile (GSM) system [178, 407, 408] and in the IEEE 802.16 wireless MAN standards. A form of TDMA is also used in Bluetooth networks, in which one of the Bluetooth devices in the network acts as a network controller to poll the other devices in time sequence.
FDMA and TDMA systems are intended to assign orthogonal channels to all active users by giving each, for their exclusive use, a slice of the available frequency band or transmission time. These channels are said to be orthogonal because interference between users does not, in principle, arise in such assignments (although, in practice, there is often such interference, as discussed further below). Code-division multiple access (CDMA) assigns channels in a way that allows all users to use all of the available time and frequency resources simultaneously, through the assignment of a pattern or code to each user that specifies the way in which these resources will be used by that user. Typically, CDMA is implemented via spread-spectrum modulation, in which the pattern is the pseudorandom code that determines the spreading sequence in the case of direct sequence, or the hopping pattern in the case of frequency hopping. In such systems, a channel is defined by a particular pseudorandom code, so each user is assigned a channel by being assigned a pseudorandom code. CDMA is used, notably, in the second-generation cellular standard IS-95 (Interim Standard 95), which makes use of direct-sequence CDMA to allocate subchannels of larger-bandwidth (1.25 MHz) subchannels of the entire cellular band. It is also used, in the form of frequency hopping, in GSM to provide isolation among users in adjacent cells. The spectrum spreading used in wireless LAN systems is also a form of CDMA in that it allows a number of such systems to operate in the same lightly regulated part of the radio spectrum. CDMA is also the basis for the principal standards being developed and deployed for 3G cellular telephony (e.g., [130, 361, 362, 407]).
Any of the multiple-access techniques discussed here can be modeled analytically by considering multiple transmitted signals of the form (1.1). In particular, for a system of K users, we can write a transmitted signal for each user as
where xk(·), {bk[0], bk[1], ..., bk[M-1]}, and wi,k(·) represent the transmitted signal, symbol stream, and ith modulation waveform, respectively, of user k. That is, each user in a multiple-access system can be modeled in the same way as in a single-user system, but with (usually) differing modulation waveforms (and symbol streams, of course). If the waveforms {wi,k(·)} are of the form (1.2) but with different carrier frequencies {wk}, say, this is FDMA. If they are of the form (1.2) but with time-slotted amplitude pulses {pk(·)}, say, this is TDMA. Finally, if they are spread-spectrum signals of this form but with different pseudorandom spreading codes or hopping patterns, this is CDMA. Details of these multiple-access models will be discussed in the sequel as needed.