Influence of coding and hardware imperfections on the bit error structure of multilevel QAM systems

When putting together 140-Mb/s 16-ary quadrature amplitude modulation (16-QAM) modems, radio-relay sets, and the appertaining total multiplex/demultiplex equipment, different and even higher bit error rates (BERs) have been measured at the primary low-level 64-kb/s outputs as compared to that at 140 Mb/s. The authors show that this error multiplex effect is mainly caused by the coding of signal points and by hardware imperfections such as carrier and clock phase jitter, nonideal regenerator threshold voltages, etc. They derive the mathematical tools necessary for calculation of the bit error structure at the modem output under optimum and nonoptimum receiver conditions. It is shown how the errors are distributed among the demultiplexer channels. QAM space diagrams of distorted signals are used for calculation of the BERs in the subsystems. Some experimental results emphasize the fact that the multiplex equipment must not be excluded from design considerations for QAM modems. The 16- and 64-QAM systems are compared with respect to their susceptibility to the error multiplex effect, and a scrambling technique is proposed as a possible means to reduce the BER differences