Notice that the power of the channel modeling error is more constant. Future work will quantify the suboptimality of this method as compared to true joint maximum likelihood sequence estimation. The figure characterizes the SER for cochannel QPSK signals assuming the worst case carrier and symbol phase offsets as described above. This curve shows that at high SNR, an SIR greater than 6.4 dB is required to achieve an SER of better than . Similarly, to achieve this SER at high SIR, the SNR must be greater than 8.2 dB Figure 6.
Performance contours for tentative primary and secondary symbol streams before error correction.
The performance contours for a given SER in the secondary symbol stream enclose a small subset of the corresponding region for the primary symbol stream. This is true for two reasons. This causes the secondary SER contours to approach a diagonal asymptote at high SIR instead of a vertical one as in the case of the primary. This diagonal asymptote corresponds to the conditions where perfect cancellation of the primary signal has occurred and the remaining secondary signal has an SNR which yields the SER of the contour. Here secondary symbol errors are mainly due to imperfect cancellation of the primary modulation resulting from primary symbol errors.