PPM performance using a concatenated code over an optical-rf relay link

Abstract

Use of an optical-rf relay deep space communication link is being considered1 that transmits optical MPPM data from a deep space vehicle to a satellite, then retransmits the data via microwave to ground. High-order (255,127) Reed-Solomon (RS) codes2 over the MPPM optical link for improved error correction have been considered. In this work we show that either timing errors or optical power constraint may cause severe degradation in system performance using high-order RS codes. Using an (8,4) extended Bose-:Chaudhuri-Hocquenhem (BCH) code as an inner code in a concatenated code in which the outer code is a low-order (15,7) RS code and then transmitting the data using optical BPPM may get rid of the previous two limitations. Furthermore, we show that this concatenated code is superior to different orders of RS codes till (127,63) RS codes and is an attractive competitor to the (255,127) RS code from the point of view of performance. It also requires a lower bandwidth expansion factor and simpler system implementation. The price paid is the need for more energy which may be supplied by solar cells. It is also shown that using an interleaver after the inner encoder and then transmitting the data as M-ary PPM optical pulses with low values of M(M= 16) gives performance which competes with both (255,127) RS codes and needs lower bandwidth and low energy values but at the expense of complex system implementation.