Phase reference & synchronization
The phase reference symbol (PRS) is the second OFDM symbol in the synchronization channel and follows the NULL symbol. It constitutes the reference for the differential modulation for the next OFDM symbol and is the same for all DAB frames.
The receiver can also employ the PRS for more precise DAB frame synchronization and frequency offset correction (integer part).
Better DAB frame synchronization is achieved by timing offset. It is estimated by cross-correlation in time between the received PRS and the theoretical PRS.
Frequency offset normalized to the subcarrier spacing can be divided into an integer part and fractional part. The fractional part of the frequency offset is estimated by using special algorithm not related to PRS. Integer frequency offset causes a cyclic shift of the FFT output. When the frequency offset lies between 0.5 and 1.5, the FFT output is shifted by 1 in either the counterclockwise or clockwise direction.
Jaehee Cho, Namshin Cho, Keukjoon Bang, Myunghee Park, Heeyoung Jun, Hyuncheol Park and Daesik Hong,
PC-Based Receiver for Eureka-147 Digital Audio Broadcasting,
IEEE TRANSACTIONS ON BROADCASTING, VOL. 47, NO. 2, JUNE 2001 95
K. J. Bang, N. S. Cho, J. H. Cho, K. C. Kim, H. C. Park, and D. S. Hong,
A coarse frequency offset estimation in an OFDM system using the concept of the coherence phase bandwidth,
in ICC2000, New Orleans, June 2000, pp. 1135–1139.
K. Taura et al., A digital audio broadcasting (DAB) receiver,
IEEE Trans. Consumer Electronics, vol. 42, no. 3, pp. 322–327, Aug. 1996.
ETSI EN 300 401