One of the well-known advantages of wideband CDMA systems is their ability to exploit frequency-selective diversity by the resolution of multipaths. However, we showed that, somewhat surprisingly, the performance of CDMA will degrade instead when the spreading bandwidth is too wide. In particular, we proved that the information theoretic capacity of a very wideband CDMA system is inversely proportional to the number of resolvable multipaths, and in fact go to zero if the number of paths is too large! The limitation comes from the fact that when there are many multipaths, it is very difficult to track the channel. This result has ramifications to the design of ultra wideband communication systems.
E. Telatar and D. Tse, "Capacity and Mutual Information of Wideband Multipath Fading Channels", IEEE Transactions on Information Theory, vol 46(4), July 2000, pp. 1384-1400.
This work is supported by the National Science Foundation under grant #NCR-9734090.