MULTIPLE-ACCESS TECHNIQUES

Cellular systems divide a geographic region into cells where a mobile unit in each cell communicates with a base station. The goal in the design of cellular systems is to be able to handle as many calls as possible (this is called capacity in cellular terminology) in a given bandwidth with some reliability. There are several different ways to allow access to the channel. These include the following.

As mentioned earlier, FDMA was the initial multiple-access technique for cellular systems. In this technique a user is assigned a pair of frequencies when placing or receiving a call. One frequency is used for downlink (base station to mobile) and one pair for uplink (mobile to base). This is called frequency division duplexing. That frequency pair is not used in the same cell or adjacent cells during the call. Even though the user may not be talking, the spectrum cannot be reassigned as long as a call is in place. Two second generation cellular systems (IS-54, GSM) use time/frequency multiple-access whereby the available spectrum is divided into frequency slots (e.g., 30 kHz bands) but then each frequency slot is divided into time slots. Each user is then given a pair of frequencies (uplink and downlink) and a time slot during a frame. Different users can use the same frequency in the same cell except that they must transmit at different times. This technique is also being used in third generation wireless systems (e.g., EDGE).

Code division multiple-access techniques allow many users to simultaneously access a given frequency allocation. User separation at the receiver is possible because each user spreads the modulated waveform over a wide bandwidth using unique spreading codes. There are two basic types of CDMA. Direct-sequence CDMA (DS-CDMA) spreads the signal directly by multiplying the data waveform with a user-unique high bandwidth pseudo-noise binary sequence. The resulting signal is then mixed up to a carrier frequency and transmitted. The receiver mixes down to baseband and then re-multiplies with the binary { 1} pseudo-noise sequence. This effectively (assuming perfect synchronization) removes the pseudo-noise signal and what remains (of the desired signal) is just the transmitted data waveform. After removing the pseudo-noise signal, a filter with bandwidth proportional to the data rate is applied to the signal. Because other users do not use completely orthogonal spreading codes, there is residual multiple-access interference present at the filter output.

This multiple-access interference can present a significant problem if the power level of the desired signal is significantly lower (due to distance) than the power level of the interfering user. This is called the near-far problem. Over the last 15 years there has been considerable theoretical research on solutions to the near-far problem beginning with the derivation of the optimal multiuser receiver and now with many companies (e.g., Fujitsu, NTT DoCoMo, NEC) building suboptimal reduced complexity multiuser receivers. The approach being considered by companies is either successive interference cancellation or parallel interference cancellation. One advantage of these techniques is that they generally do not require spreading codes with period equal to the bit duration. Another advantage is that they do not require significant complexity (compared to a minimum mean square error-MMSE-detector or a decorrelating detector). These interference cancellation detectors can also easily be improved by cascading several stages together.

As a typical example, Fujitsu has a multistage parallel interference canceler with full parallel structure that allows for short processing delay. Accurate channel estimation is possible using pilot and data symbols. Soft decision information is passed between stages, which improves the performance. Fujitsu's system uses 1-2 stages giving fairly low complexity. Fujitsu claims that the number of users per cell increases by about a factor of 2 (100%) compared to conventional receivers and 1.3 times if intercell interference is considered.


Published: July 2000; WTEC Hyper-Librarian