Extending the capacity of next generation wireless LANs using space division multiplexing combined with OFDM

With the increasing amount of bandwidth-hungry services like video on demand and video conferencing, the need for broadband communication systems is obvious. Furthermore, with the growing demand for flexible workplaces at the office (office hotelling) and university, wireless broadband communication systems are no luxury anymore. The main goals in developing new wireless communication systems are increasing the bit rate and increasing system capacities. Because the available frequency spectrum is limited, this means that future systems should be characterised by improved spectrum efficiency. Recent information theory research has revealed that the multipath wireles.s channel is capable of enormous capacities, provided that the multipath scatterIng is sufficiently rich and is properly exploited. A possible way to exploit the multipath scattering properly is Space Division Multiplexing (SDM) or Space Division Multiple Access (SDMA). Basically, these techniques transmit different signals simultaneously on different transmit antennas. The parallel streams of data are mixed-up in the air, but can be recovered at the receiver by using the SDM algorithms proposed in this report, like the Zero Forcing or Minimum Mean Square Error method, with or without Decision Feedback Decoding, or the Maximum Likelihood Decoding (MLD) technique. When the multipath is exploited properly, both the data rate and the Signal-to-Noise Ratio performance can be increased. For BLAST (Bell laboratories LAyered Space Time), a system based on Minimum Mean Square Error decoding with Decision Feedback, it has be shown that spectrum efficiencies of 20 40 Bit/slHz can be achieved in an indoor propagation environment at realistic Signal-to-Noise Ratios and error ratios. In this report it is shown that Maximum Likelihood Decoding (based on detection) achieves a diversity order equal to the number of receive antennas. This is (much) better than the diversity order of Zero Forcing for which it is shown in literature that the diversity order equals the number of receive antennas minus the number of transmit antennas plus one. The big disadvantage ofMLD is that its complexity grows exponentially with the number of transmit antennas. For a reasonable number of transmit antennas (~ 5), however, its complexity is shown to be comparable with other Space Division Multiplexing methods. Furthermore, in this thesis, single carrier Space Division Multiplexing based on MLD is combined with the multicarrier technique Orthogonal Frequency Division Multiplexing (OFDM) to make the system more robust against delay spread impairments. A basic introduction to OFDM is given and it is described how to combine SDM with OFDM. This system is simulated in C++ using the parameters of the new IEEE 802.11 OFDM standard for wireless LANs. The delay spread channel is modelled by exponentially-decayed Rayleigh fading. By performing Monte Carlo simulations, Bit Error Rate performances for different antenna configurations and different delay spreads are shown in this report. To achieve a better performance, forward error correction coding based on a convolutional code is implemented. However, if a soft-decision input Viterbi decoder, which should get soft-decision values from the receiver detection algorithm, is used to decode this convolutional code, an even better performance is obtained. Therefore, the MLD algorithm is adapted in order to produce softdecision outputs. A simple intuitive adaptation and a complex adaptation based on the loglikelihood ratio are proposed and their performance is compared. From all the results presented in this report, it can be concluded that Space Division Multiplexing in combination with OFDM is a promising solution for increasing the bit rate and system capacity.