Symbol Synchronization

As we have seen in other chapters, the operation and performance of various receiver functions can be quite sensitive to knowledge of the timing (data transition epochs) of the received data symbols. Thus, the ability to accurately estimate this parameter and continuously update the estimate, i.e., perform symbol synchronization (sync), with little knowledge of other parameters is critical to successful operation of an autonomous receiver. Traditionally, symbol synchronization techniques have been developed assuming that the data symbols are binary, the modulation format, e.g., non-return to zero (NRZ) or Manchester data, is known a priori, and carrier synchronization is perfect. Thus, the symbol synchronization problem has been solved entirely at baseband, assuming perfect knowledge of the carrier phase and frequency. Among the various symbol sync schemes that have been proposed in the literature, by far the most popular in terms of its application in binary communication systems is the data-transition tracking loop (DTTL) [1,2]. The scheme as originally proposed in the late 1960s is an in-phase–quadrature (I-Q) structure where the I arm produces a signal representing the polarity of a data transition (i.e., a comparison of hard (±1) decisions on two successive symbols) and the Q arm output is a signal whose absolute value is proportional to the timing error between the received signal epoch and the receiver’s estimate of it. The result of the product of the I and Q signals is an error signal that is proportional to this timing error, independent of the direction of the transition. Although originally introduced as an efficient symbol synchronization means for tracking an NRZ data signal received in additive white Gaussian noise (AWGN), it was later demonstrated (although not formally published) that the closed-loop DTTL structure can be obtained from a suitable interpretation of the maximum