Iterative detection of turbo-coded offset QPSK in the presence of frequency and clock offsets and AWGN

The key contribution of this paper is to develop transmitter and receiver algorithms in discrete-time for turbo-coded offset QPSK signals. The procedure for simulating a clock offset between the transmitter and receiver is described. Due to the use of up-sampling, matched filtering and a differential correlation approach at the receiver, the time required for detecting the start of frame (SoF) is just around 500 symbols, which is also the length of the preamble. The initial estimate of the SoF and the frequency offset, obtained using the differential correlation approach, is improved using an iterative process. A novel two-step maximum likelihood (ML) frequency offset estimation is proposed, which significantly reduces the complexity over the conventional ML estimation. The decision-directed carrier and timing recovery algorithms use simple first-order IIR filters to track the carrier phase and clock slip. The proposed synchronization and detection techniques perform effectively at an SNR per bit close to 1.5 dB, in the presence of a frequency offset as large as 30% of the symbol-rate and a clock offset of 25 ppm (parts per million). It is shown via simulations that the performance loss with respect to the bare turbo code is only about 0.5 dB, for a preamble length of 500 and a BER of 10−7. The proposed techniques are well suited for software implementation.