Pre-equalization for MISO DS-UWB systems with pre-Rake combining

In this paper, we propose two novel pre-equalization schemes for multiple-input single-output (MISO) direct-sequence ultra-wideband (DS-UWB) systems with pre-Rake combining and symbol-by-symbol detection. The first scheme employs one pre-equalization filter (PEF) per transmit antenna, whereas in the second scheme, the simplified PEF (S-PEF) scheme, all transmit antennas share the same PEF. For both schemes the optimum finite impulse response (FIR) and infinite impulse response (IIR) PEFs are calculated based on the minimum mean squared error (MMSE) criterion. Our approach is sufficiently general to include also reduced-complexity versions of pre-Rake combining that employ a limited number of Rake fingers. We show that under certain conditions the S-PEF scheme achieves the same performance as the more complex PEF scheme. We also demonstrate that a single-input multiple-output (SIMO) DS-UWB system with post-Rake combining and MMSE post-equalization is the dual system to the considered MISO DS-UWB system with pre-Rake combining and MMSE pre-equalization. This uplink-downlink duality can be exploited for efficient calculation of the PEFs and for complexity reduction. Our simulation results show that the proposed PEF schemes achieve significant performance gains over pre-Rake combining without equalization, even if only short PEFs are employed.