Capacity of a cellular time division duplex CDMA system

A time division duplex (TDD) technique is considered for a CDMA cellular system. Its use enables all the multipath combining (using a RAKE type receiver) to be located in the base site rather than in the mobile unit. This is possible due to the reciprocal uplink and downlink channels which are created by time division, and employ the same carrier frequency. Simulations show that at low Doppler fading rates TDD is only marginally lower in cell capacity than conventional frequency division duplex (FDD) techniques. However, at higher Doppler fading rates (for high speed mobiles) the cell capacity degrades in a nonlinear manner and will breakdown suddenly, unlike that of a comparable FDD CDMA system. The limitation of the TDD technique results from the noncontinuous receiver training caused by the bursty nature of the communications. Losses in cell capacity are also due to the guard times inserted between the uplink and downlink transmissions