A new channel coding algorithm for in-band spectral suppression in PAM and QAM systems

An improved low-frequency suppression (LOFS) code is described which increases by 10 dB or more the critical spectral suppression near DC achieved by line-balanced or feedback-balanced type codes currently used in digital microwave systems. The LOFS code can also achieve equivalent suppression in baseband multilevel PAM (pulse amplitude modulated) and modulated QAM (quadrature amplitude modulated) signals with up to four times less overhead, allowing the insertion of an IF carrier pilot or analog service channel. Pilot-insertion techniques provide a recovered carrier that can be more robust than conventional synchronization techniques in a faded environment and can better track system-induced phase noise. Hardware results from a four-level PAM system and simulations show the increased effectiveness and robust performance of the LOFS code.<>