THE CAPACITY FOR A DISCRETE-STATE CODE-DIVISION MULTIPLE-ACCESS CHANNEL

One of the proposed techniques for the third generation of mobile communications is code division multiple access (CDMA). Some attributes of CDMA which are important to indoor systems include desirable anti-multipath properties, the lack of a need for frequency management or assignment, coexistence with other systems and suitability for micro-cell and in-building systems. The anti-multipath feature turns out to be a key issue for indoor systems and derives from the property that a signal with path delay greater than one chip interval is treated much like an interfering user. The number of users that share the same spectrum, and still maintain an acceptable performance, is determined by the interference generated by the set of remaining users and leads to soft degradation as the number of users increases. This work describes and analyzes a model for the discrete-input continuous-output Gaussian multiple-access channel, that uses spread spectrum techniques. The notion of sum capacity is used to characterize the system performance. Bounds on the sum capacity of this essentially binary input, continuous output channel, where the variance of the output noise is dependent on the number of users present, are obtained. The users are noncooperative in that no common codebook or synchronization is assumed. A novel feature of the work is the use of the modeling of the activity of the user community as a birth-death process. An information theoretic approach is used and the sum capacity is then considered in light of the various regimes to determine the effect of this modeling.