Adaptive Equalization and Phase Tracking for Simultaneous Analog/Digital Data Transmission

Recently, considerable interest arose in finding ways to transmit and receive digital and analog data simultaneously over the 2-wire voiceband telephone channel. We investigate a system using quadrature amplitude modulation (QAM) where digital and analog data modulate the two quadrature channels. T h e same scheme was independently proposed in Ref. 1. The effects of various channel impairments, as well as an imbalance of signal powers between the digital and analog signals, on the error probability of the system has been studied in Ref. 2. In this paper, we analyze an adaptive equalizer for this type of hybrid modulation, using a cross-coupled transversal filter as described in detail by Falconer in Refs. 3 and 4. The difference here is that one of the two quadrature channels transmits analog, i.e. continuous amplitude, data. If the communication channel is time invariant, it is possible to train the equalizer with digital data on both quadrature 2039 channels and freeze the equalizer taps after convergence. Analog data can then be sent. However, possible changes in channel characteristics warrant the use of an adaptive equalizer to update the taps continually. Since references for the analog data are not available, especially when the receiver is in a decision-directed mode, the update algorithms reported in Refs. 3, 4 are not applicable in this case. We propose a modification which only minimizes the mean-square error (mse) in the digital data path. Thus, only the error signal from this path is required for adaption.