Extending the capacity of multiple access channels

Multiple access techniques which allow a communication medium to be shared between different users represent one of the most challenging topics in dig ital communications. In terms of the number of users that can be accommodat ed on a given channel, there are two distinct classes of multiple access te chniques. The first class includes the well-known FDMA, TDMA, and OCDMA. On a channel whose bandwidth is N limes the bandwidth of the individual user signals, these techniques can accommodate N users without any mutual interf erence, but not a single additional user can be supported beyond this limit ing number. The second class includes CDMA with pseudo-noise spreading sequ ences (which we refer to as PN-CDMA) and some other related schemes. PN-CDM A does not have a hard limit on the number of users that can be accommodate d, but is subject to multi-user interference which grows linearly with the number of users. In this article, after reviewing the capacity limits of ex isting multiple access techniques, we describe some newly introduced concep ts which allow us to accommodate N users without any interference while als o accommodating additional users at the expense of some SNR penalty.