Combined Frame Alignment and Timing Recovery in OFDM Communications Systems

摘要

Timing recovery circuitry for a digital subscriber line (DSL) modem, including a combined frame and timing function for adjusting frame alignment, and for adjusting sample frequency for frequency offset and for phase offset, is disclosed. Frame alignment is adjusted by the frame and timing function by averaging estimates of the phase offset over a plurality of tone pairs within a frame, and then averaging that average estimated phase offset over a plurality of frames to produce a frame offset measurement. Frequency offset is derived by analyzing the extent to which phase error in the received signal varies at a constant rate over time, specifically over a sequence of frames; the sample frequency of the modem is adjusted in response to the detected frequency offset. Phase offset is determined by averaging the phase offset over a plurality of tones within a frame, and integrating differences in this phase offset from frame to frame. The combined frame alignment and sample frequency adjustment may be performed during initialization, or during DSL communications.