When a QAM (quadrature amplitude modulation) data signal is transmitted over a channel that includes PCM codecs, it has frequently been observed and shown that the resulting noise around the final received constellation points grows with the distance of the point from the origin. The authors extend the analysis of this effect, and further show that the noise distribution about each point is elliptical, with greater variance in the radial direction. Formulas giving the signal-dependent noise components as a function of the channel characteristics of the analog transmit and receive access legs are derived. Experimental results confirm the analysis.>

An analytical approach for investigating the effect of noise correlation on the performance of trellis-coded communication systems is described. Several trellis codes are investigated in the presence of colored noise, and computer simulation and real-time experimental results are given to confirm the analysis. In some practical cases, the actual coding gain could be as small as a fraction of 1 dB as opposed to the nominal 4-5 dB of coding gain that can be obtained on additive white Gaussian noise (AWGN) channels. A filter swapping technique which eliminates the performance degradation due to the effect of noise correlation is proposed. It is shown that if the equalizer is moved from the receiver to the transmitter and is scaled to satisfy an average power constraint, then the noise at the Viterbi decoder input is white and the signal-to-noise ratio remains the same. Doing this allows the nominal coding gain to be achieved.>

Adaptive LMS (least mean squares) filter are applied to cancel harmonic noise. Simulations were done using actual noise recorded at a distribution power substation. Two implementation methods are discussed. Using the characteristic properties of the noise, a constrained filter is proposed which greatly reduces computation without reducing its effectiveness. It is shown that both constrained and unconstrained adaptive noise cancellation filters can be used effectively. For either case, significant improvement in the error rate is obtained.>

Analytical and experimental results are presented for the performance of one echo canceller arrangement. It consists of a data-driven echo canceller having a so-called cross-coupled structure, which is followed by a rotator and a phase-locked loop (PLL). A cross-coupled echo canceller structure without a PLL is analyzed first. Expressions for speed of convergence and achievable echo-return-loss enhancement (ERLE) in the presence of frequency offset are derived. These results are compared in previously published results for a noncross-coupling echo canceller structure. Specifically, it is shown that the cross-coupled structure converges twice as fast as the noncross-coupled structure and provide an achievable ERLE that is about 6 dB better. The joint adaptation of the echo canceller and the PLL is then studied. It is shown that it is always possible to choose design parameters for the echo canceller which are consistent with adaptation requirements under double-talking conditions, provided that the PLL is properly engineered. The sensitivity of the performance of PLL to the power level of the far echo, as well as possible solutions to this problem, are discussed.>

The equivalence of three fast fixed order reeursive least squares (RLS] algorithms is shown. The FAEST and FTF algorithms are derived by eliminating redundancies in the fast Kalman algorithm. Exact equivalence is obtained by careful selection of the initial conditions. This equivalence suggests a new rescue variable which can perform no worse than previous ones and can test other symptoms of divergence as well.