The effects of capacity limitations on frequency diversity gain in FH/TDMA systems

Wireless local loop (WLL) and personal communications systems (PCS) provide services over a radio channel. The quality of the channel is dependent on the propagation environment which varies over both time and frequency. One of the ways to improve the quality of service (QoS) (i.e. link quality) of the channel is through the transmission of the information using diversity. Frequency diversity mechanisms are a popular method for increasing the system gain in wireless systems. We discuss the effects that capacity limitations have on this gain for a number of different algorithms which allocate frequency diversity channels to users below a given threshold. We discuss complete partitioning (CP), complete sharing (CS), and reservation algorithms. We calculate the gains due to diversity and the limitations due to the allocation algorithms analytically and plot the results. The results are compared to the upper bound where there is no limit on the number of channels that may have frequency diversity. We show that the gain due to frequency diversity is about the same as the margin for all cases. While the CS policy most closely follows the system gain upper bound for all margin values, the CS GoS deteriorates as the offered load goes up. As a result, we suggest that at low levels of offered load, the CS algorithm be used and that as the load increases, the reservation policy be used. We additionally discuss the relationship between the capacity and margin for the CS policy and the relative behavior of the different allocation algorithms for different margins.