I've been studying the basics of digital communications using the free book at desktopsdr.com.
One of the examples is a 4-QAM demodulator which where symbol timing recovery is done with some Early-Late Gates after match-filtering with RCC.
I thought that Early-Late Gates were only useful when all pulses had triangular shape. However, binary data filtered with Raised Cosine does not:
For non-triangular signals, the Early-late algorithm seems to output an error even when the sampling point is exactly right:
- When there is a single symbol, I understand that the error L - E == 0
- If there is a transition to a sequence of repeated symbols, however, I see that the estimated error is non-zero, which is wrong.
What am I missing?