On RAKE receivers for ultra-wideband binary block-coded PPM in dense multipath channels

Ultra-wideband communications with a balanced orthogonal binary block-coded pulse-position-modulation scheme are considered over dense multipath channels. The structure and the performance of all-RAKE (A-RAKE) and selective-RAKE (S-RAKE) receivers are studied under the minimum bit-error rate (MBER), the maximum signal-to-noise ratio (MSNR), and the maximum-energy-capture (MEC) criteria. For the A-RAKE reception, the amount of the pulse-position shift is identified as the key design parameter that significantly affects the BER performance of the MBER and, equivalently, the MSNR receivers. It is also shown that the amount of energy capture does not well characterize the BER performance of the RAKE receiver in a very dense multipath channel. For the S-RAKE reception, although suboptimal, the RAKE receivers that allow template-matched filtering are mainly studied. The condition for the equivalence of the MSNR S-RAKE receiver to the MEC S-RAKE receiver is derived. It is shown that the amount of the pulse-position shift is again the key design parameter that determines whether the MSNR S-RAKE receiver is equivalent to the MBER S-RAKE receiver. Various schemes for finger-location selection are also studied, and a heuristic selection scheme is proposed for digital implementation of the RAKE receivers.