Improving Capacity in LMDS Networks Using Trellis-Coded Modulation

Abstract

Local multipoint distribution systems (LMDS), which are operating worldwide in the frequency range of 26 to 43 GHz, have large bandwidths of 0.1 up to 2 GHz but a limited area coverage of a few kilometers only. This is due to line-of-sight (LOS) constraints for reliable point-to-multipoint links as well as due to large propagation losses. The size of the macrocells illuminated by the base stations, where LOS exists, is approximately 1 to 5 km. As suggested in the CRABS report (1999), the maximal spectral efficiency can be obtained with a dual frequency and polarization reuse plan. This frequency and polarization reuse leads to interference. In this paper, we report a new technique that uses trellis-coded modulation (TCM) for increasing the capacity of LMDS networks. Analytical expressions have been derived for pairwise error probability for both high and low SNR scenarios. Numerical simulations have shown that using the proposed TCM schemes in cochannel cells have resulted in a large decrease in interference, thereby allowing us to reduce the frequency reuse distance. In this paper, we present the strategy using two TCM schemes that allow the frequency reuse factor of unity, without compromising on the QoS caused by increased interference. Design rules for constructing TCM schemes are also proposed in this paper.