**3.1.1.2 Symbol timing synchronizing algorithm**

Symbol timing synchronizer is a critical module of the digital receiver design of the channel sounder based on sliding correlation channel measurement. Gardner's symbol timing recovery method is used in this system [18][19]. The structure of the symbol timing synchronizer is shown in Fig. 4. All the processing of this synchronizer is done in digital domain. No interaction between analog and digital part of the system is needed. This synchronizer is capable of compensating sampling phase and frequency offset and is independent of carrier phase [20].

Fig. 4. Symbol timing synchronizer.

The sampled data *ym* , which is filtered by matching filter, is then feed into the interpolator to compute the desired sampled strobe *<sup>k</sup> y* . This is done by digital interpolation, controlled by NCO (Numerical Controlled Oscillator) and fraction interval *<sup>k</sup>* . Ideally, the period of the NCO is / *TT K i symb* , where *K* is an integer. The loop consisting of timing error detector, loop filter and NCO function just like a DPLL, where *un* , *wk* and*<sup>k</sup>* represents timing error signal, NCO control word and NCO register content respectively.

In this design, the DTTL algorithm [20] is used to compute the timing error signal. This choice specifies /2 *T T i symb* . In order to avoid up-sampling in the interpolator, *Ts* should be smaller than*Ti* . Thus, the sampling frequency s*f* should be larger than two times the symbol rate *R*symb . The interpolator performs linear interpolation, which is easy to implement. The loop filter is a proportional-plus-integral structure.
