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The locking mechanism for transparent tone-in-band (TTIB) was analyzed. The major issue addressed was the phase error in recombining the subbands, especially when phase-lock is applied to data. The effect on BER (bit error rate) performance was evaluated, and it was demonstrated that the phase error phi should be held to 0.1 radians or less if degradation is not to exceed 1 db. The most significant finding was the high level of self-noise in the PLTTIB (phase-locked TTIB) phase detector output. Its variance is only halved when the SNR (signal/noise ratio) increases from 7 dB to infinity. The self-noise forces a very small loop bandwidth if phi is to be small enough not to degrade BER performance. The effect on system design is a slow pull-in time, up to 1500 symbols, even with infinite SNR. It is concluded that PLTTIB is unsuitable for burst transmission of data. On the other hand, for continuous transmission or for very long blocks, PLTTIB has a significant advantage: phase locking can be achieved for any signal format, as long as there is sufficient power near 0 Hz.>
J.K. Cavers (Mon,) studied this question.