Downlink User Capacity of Cellular Systems: TDMA vs Dirty Paper Coding

Dirty paper coding has been shown to achieve the capacity region of a Gaussian broadcast channel. However, it is a complicated scheme and assumes knowledge of the interfering signals at the transmitter. In this paper, we compare the downlink user capacity of dirty paper coding and time-division multiple access (TDMA) in a single cell of a cellular network with single antennas at the transmitter and each of the receivers. Here we define user capacity as the expected value of the number of users/receivers to which the transmitter can simultaneously transmit at an equal rate. The system that we consider selects the users to which it transmits from a population of mobile users. We derive closed-form expressions for the downlink user capacity using dirty paper coding and TDMA under a total transmit power constraint and a common target information rate at all the mobile users, as the user population tends to infinity. The results indicate that DPC provides a gain of at most 12% in user capacity over TDMA for the scenarios that we consider. Thus TDMA may be an attractive alternative to DPC because of the complexity of the latter