Data throughput on MILSATCOM channels using military standard 188-184 and 188-181B

The physical layer continuous phase modulated (CPM) waveforms that are defined in Mil-Std-188-181B can now be used with network layer protocols such as those defined in Mil-Std-188-184 and the UHF SATCOM high performance waveform (HPW). These CPM waveforms provide higher throughput than the standard, ubiquitous 16 kbps FSK waveform and this paper documents the performance of the combination of the two military standards as used in a tactical SATCOM radio (AN/PRC-117F). In addition, the uncoded CPM waveforms Mil-Std-188-181B also provide optional interleaving and Reed-Solomon outer codes which can be added to considerably enhance BER performance. This paper also documents the bit error rate performance of all of the Reed-Solomon coded waveforms in the tactical SATCOM radio. Unfortunately the improvement in bit error rate does not correspond to an equivalent improvement in data throughput. For example, since the Mil-Std-188-184 network layer provides an external "keyline" to a radio, the radio is required to wait until an interleaver is completely filled before transmitting an encoded set of data. This holdoff time also results in an equivalent wait time at the receiver while the entire codeword and interleaver block is demodulated and decoded at the receiver. The 184-capable receiver must wait for this end-to-end hold off time before it can respond with an ACK/NACK to the transmitter. This round-trip holdoff time consequently reduces the ultimate throughput of the message. This paper provides an analysis of this weakness in the strict layering (network/physical) approach to data throughput and provides recommendations for modifications to the Mil-Std-188-184 and HPW waveforms.