A pragmatic Rake receiver for direct sequence spread spectrum coded troposcatter transmissions

For tactical troposcatter transmissions, explicit diversity techniques (such as spatial, angular or frequency diversity) are not usable due to severe constraints on equipment size and cost. Multipath fading can be efficiently overcome by using implicit diversity techniques such as spread spectrum where the use of a signal bandwidth much wider than the channel coherence bandwidth permits the resolution of independently faded components at the receiver. This paper presents a novel scheme using on the one hand, Hadamard sequences and direct sequence spread spectrum associated with a Rake receiver and on the other hand multilevel codes associated with multi-stage decoding for tactical data rates (64 to 512 kbit/s). First, computer simulations show that the method introduced in this paper leads to great improvement of performance compared to conventional techniques. Bit error rate (BER) performance is presented for a frequency selective slowly fading Rayleigh channel for several values of coherence bandwidth. Then, a hardware architecture is proposed built on COTS RISC processors.