Close Packing PCSFSK Signals - Model and Simulation Results

In many signal processing satellite applications, it is necessary to pack as many bit-time unsynchronized, power imbalanced digital FDM users as possible into the available bandwidth. In certain important cases the on-board demodulator (which simultaneously detects each user bit-stream individually) is not able to maintain phase reference. Phase comparison sinusoidal frequency shift keying (PCSFSK), a phase incoherent demodulation method with low adjacent-channel crosstalk properties, has been proposed as one way of meeting the requirements mentioned above. This paper describes a mathematical model and computer simulation which have been used to analyze the effects of adjacent-channel crosstalk on bit-error rates for a PCSFSK demodulator. Both the model and the simulation are flexible enough to incorporate many users of various strengths and center frequency separations and may be modified to yield results for any phase comparison MSK-type modulation. The results of this work show very close agreement between the model's prediction and the simulation. User scenarios of interest involve from two to eight users both with and without additive white Gaussian channel noise. Of particular interest is the performance of one weak user among many stronger users. A typical example involves eight users, seven of which are 20-dB stronger in power than the user whose probability of error was monitored. In this case, a bit-error rate of 10
^-3
requires that adjacent users be separated in frequency by two times the data rate. If, however, all users are of equal strength, the minimum frequency separation need only be equal to the data rate. The above results indicate that close packing of signals is possible when incoherent detection (PCSFSK) is used, and that optimum user spacings can be determined if the relative power ratio between users is known.