Loop control architectures for symbol timing synchronization in sampled data receivers

Symbol timing synchronization is an important component in a receiver designed to recover data from a digitally modulated waveform. Current trends favor sampled data architectures to perform the synchronization, matched filtering, and detection required to recover the data. Symbol timing synchronizers in sampled data receivers differ from their continuous-time counterparts: the functionality in a sampled data receiver is as an adaptive interpolator as opposed to a sample and hold. This paper explores architectures that can be used for loop control (i.e. adjustment of the fractional interpolation interval) by contrasting and comparing three different methods using numerically controlled oscillators. Assuming the Nyquist sampling rate is N samples/symbol, the first method operates at MN samples/symbol where M is an upsample factor. The second method operates at N samples/symbol. It is shown that these two methods automatically compensate for misadjustments between the sample clock and symbol clock frequencies. The third method operates at 1 sample/symbol. In this case, we show that auxiliary control must also be included to adjust the clocking of data into the filter bank to account for small differences in the sample clock and the data clock.