The performance of coherent BPSK in the presence of oscillator phase noise for ka-band pico-terminal inbound links

Abstract

In microwave systems, the phase noise is well known to be a significant source of degradation especially for low data rate transmission, giving rise to a significant irreducible error rate. In an earlier study [1,2] based on the European Space Agency's Olympus Satellite 30/20 GHz payload parameters, it was found that several thousands of simultaneous transmissions of 50 to 200 bits/sec each could be supported from hand-held pico-terminal earth stations. It is important to use the most appropriate modulation scheme in the presence of oscillator phase noise for such low data rate systems. It is generally assumed that noncoherent or differentially coherent modulation schemes will perform best in applications where the coherence time of the oscillators is short compared to the bit period. However, our analysis of these schemes has shown that they are subject to irreducible error rates and minimum useable data rates. This has led us to reconsider coherent BPSK (CPSK). It is shown that the choice of the optimum noise bandwidth of the phase locked loop (PLL) in the carrier recovery circuit is very important when considering the coherent schemes. The PLL bandwidth must be wide enough to track the oscillator phase noise at its input, but, on the other hand, it must be as narrow as possible to exclude the thermal noise. Results show that coherent BPSK in the phase noise contaminated channel is also subject to an irreducible error rate, which increases as the noise bandwidth is reduced. For the optimum loop bandwidth, however, this appears to be lower than the corresponding figure for DPSK [3]. There is a minimum useable data rate for any given bit error rate (BER) requirement. Results of a complete analysis are presented in the form of BER versusE _b/N _o plots.