Recently, zero-intermediate frequency (IF) zero-crossing demodulators (ZIFZCDs) have been developed for many applications to digital phase and/or frequency modulation, e.g., minimum shift keying (MSK), generalized MSK (GMSK), π/4-differentially encoded quadrature phase shift keying (π/4-DQPSK), and multi-level continuous phase frequency shift keying (CPFSK). The ZIFZCDs have exhibited better than and nearly the same performance as the existing demodulators for FM modulation index h ≥ 1.5 and 0.5 ≤ h < 1.5, respectively. The implementation complexity of the ZIFZCDs is much simpler than existing demodulators, e.g., limiter discriminator integrate & dump (LDI). Perfect symbol synchronization was assumed in the previous analyses of the ZIFZCDs. In this paper, a new efficient symbol synchronization scheme is proposed for the practical applications of the ZIFZCDs. The bit error rate performance of a four-level CPFSK with the ZIFZCDs using the proposed symbol synchronization shows 2 dB and 1 dB (average 1.5 dB) better than that of an existing scheme for the phase bit and magnitude bit, respectively. In addition, the ZIFZCDs with the proposed synchronization algorithm reduces the implementation complexity significantly compared to the existing synchronization schemes. Therefore, battery life can be increased and the size of a portable receiver (a pager or cellular phone) can be smaller.
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