Study of alternative approaches to non-coherent detection of FQPSK and other quadrature modulated systems

For telemetry applications requiring high spectral efficiency, robust performance and fast synchronization in a frequency selective environment, various non-coherent demodulator architectures are proposed, presented, and compared. Previously studied methods including differential detection, limiter discriminator (LD) with maximum likelihood sequence detection (MLSD), and LD with multiple symbol correlation incorporating Viterbi algorithm (VA) based receivers are reviewed. These previously studied methods offer performance within approximately 3 dB of the coherent detection of US DoD-RCC-IRIG 106/00 and international CCSDS standardized Feher (see US Patents: 4,567,602; 4,644,565; 5,491,457; 5,784,402; 6,198,777, International Patent Cooperation Treaty (PCT) No: PCT US99/17995; International Patent No. WO 00/10272, February 2000) quadrature phase shift keying (FQPSK). virtual diversity, comparator-counter, and other architectures are introduced as other alternatives for robust operation and fast synchronization in a frequency selective environment. Virtual diversity employs several "free-running" and offset carriers placed in distinct receiver paths. Each signal is evaluated in its BER performance with selection of the path with the best performance. This architecture has the potential of better BER performance for high bit rate applications. The comparator-counter method utilizes a comparator to detect zero crossings and counters placed in quadrature paths to detect the number of zero crossings. The information obtained from these counters is then evaluated to determine the transmitted bit. This method potentially offers simplicity with a relative ease of implementation. Areas for comparison of all of the above mentioned methods include overall BER performance, simplicity with ease of implementation, and robust operation.