16-ary amplitude phase shift keying (16APSK) is a candidate modulation for aeronautical mobile telemetry because it has better spectral efficiency and lower sensitivity to adjacent channel interference compared SOQPSK-TG. This paper compares the post-equalizer bit error rate performance of 16APSK and SOQPSK-TG in the aeronautical mobile telemetry multipath channel. The linear polarizations produced by the orthogonal dipoles in the receive antenna feed are combined in two different ways: using a 90◦ hybrid coupler to produce RHCP or LHCP and maximum likelihood combining. The first combining method is the common practice at test ranges and the second combining method is new. The results show the following. First, 16APSK with MMSE equalization outperforms SOQPSK-TG with CMA equalization for the RHCP and LHCP channels. Second, 16APSK with MMSE equalization outperforms SOQPSK-TG with CMA equalization for the maximum likelihood channels at moderate to high signal-to-noise ratios. For lower signal-to-noise ratios, SOQPSK-TG shows the best performance. Third, 16APSK with MMSE equalization operating the maximum likelihood combined channel outperforms SOQPSK-TG with CMA equalization operating on the RHCP or LHCP channels.

The outputs of a cross-polarized antenna can produce a pair of different parallel frequency-selective channels. The optimum combining strategy is derived from maximum likelihood principles and used to define an equivalent discrete-time model. The simulated post-equalizer BER results show that optimum combining produces the best results, selection diversity can provide reasonably good results, and that both optimum combining and selection diversity can be superior to linear equalizer operating on the channel obtained by combining the antenna outputs before applying a channel matched filter.