Robust control of decision directed loops

A series of algorithms that significantly extend the range of control for decision-directed structures are presented. Experimental results show that increased control ranges of approximately 20 dB can be obtained for 64-QAM systems. The algorithms can eliminate digital radio outage due to hysteresis during dispersive multipath fading and allow for enhanced control of cross-pol interference cancelers in dual-polarized digital radio systems. The performance of 64-QAM digital radios utilizing these techniques verifies that the substantial improvements predicted by theory are realizable. Examples of performance improvements with thermal noise, FM interference, and multipath fading are given to demonstrate the power of the algorithms. Examples taken from standard production AT&T DR6-30-135 digital radio show that timing and carrier synchronization are easily maintained for 64 QAM systems at signal-to-noise ratios approaching 0 dB. The results can be applied to other communications systems and may eliminate data training sequences in many applications such as voice band modems.<>