The effects of multipath and fading on the performance of direct-sequence CDMA systems

We show the following results concerning the maximum number N max of simultaneous users supportable by an asynchronous direct-sequence code-division multiple-access (CDMA) system using DPSK. (a) For a network of single-path nonfading links in which all users' signals arrive at any receiver with equal energies, N max is 10-20 percent of the number of chips M in the system's codes at bit error rates of 10-3to 10-5. (b) If typical urban/suburban multipath and fading phenomena occur and no power control ameliorates them, N max falls to 1-5 percent of M even under the best of circumstances (availability of multipath diversity and ideal multipath combining receivers). (c) If the links consist mostly of only single fading paths and no power control is available, direct-sequence CDMA becomes unusable. Power-control policies can be devised to overcome shadowing (slow fading) or near/far problems, thus partially restoring the N max of (a). It is unlikely that power control will effectively combat fast fading, especially in single-path situations.