Techniques for improving power and bandwidth efficiency of UHF MILSATCOM waveforms

Multi-h continuous phase modulation (CPM) employing a constant envelope modulation technique is being used over UHF military satellite communication (MILSATCOM) channels. It was implemented as a result of the Advanced Digital Waveform (ADW) program. The goal of this program was to increase throughput on UHF SATCOM channels. The resulting waveform, described in MIL-STD-188-181B, is a 4-ary, 2-h, linear shaped, full response CPM. This paper describes techniques for further improving the power and bandwidth performance of multi-h CPM. Improving power efficiency will benefit disadvantaged platforms by allowing link closure at lower C/N₀ levels. Improving bandwidth efficiency will allow higher data rates to be used. Three methods for accomplishing these goals are explored. The first is to increase the symbol alphabet size from 4 to 8 symbols by encoding 3 bits per symbol instead of 2, thus increasing bandwidth efficiency by 50 percent. The second area of interest is to implement a partial response waveform, meaning that each symbol is shaped over multiple symbol periods. This second method will add more memory to the trellis structure and should also cause the spectrum to become more compact. The third method investigated is to increase the number of modulation indices. The idea is that using three or more indices will spread the trellis more, increasing the minimum Euclidean distance. The first two areas of research, 8-ary CPM and partial response CPM, show promising results, in some cases improving both power and bandwidth efficiency. Of these two, 8-ary CPM seems the more practical path for immediate further development. Partial response is actually more promising in theory, but is much more complex and requires further research. Early results of three-h CPM show that adding another h-value does not improve performance and does not appear to be a promising avenue for further research.