Conference Paper

Common detectors for shaped offset QPSK (SOQPSK) and Feher-patented QPSK (FQPSK)

Dept. of Electr. & Comput. Eng., Brigham Young Univ., Provo, UT
DOI: 10.1109/GLOCOM.2005.1578470 Conference: Global Telecommunications Conference, 2005. GLOBECOM '05. IEEE, Volume: 6
Source: IEEE Xplore

ABSTRACT Symbol-by-symbol detection of SOQPSK and FQPSK using detectors designed for offset QPSK represents a simple common detector architecture for these two interoperable waveforms. Unfortunately, this detection method results in a 2 dB loss in bit error rate performance. This paper describes detection methods for recovering this loss without the need for knowing which modulation is used by the transmitter. An equivalent cross-correlated trellis-coded quadrature modulation (XTCQM) representation for SOQPSK is developed which forms the basis of a common TCM detector. An equivalent CPM representation for FQPSK is developed which forms the basis for a common CPM detector. The common XTCQM detector performs slightly better than the common CPM detector, but achieves this gain at the expense of higher complexity

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    • "This is not the case for FQPSK, which cannot be exactly represented with a CPM model. However, a very close CPM approximation can be obtained with x JR (t; a) ≈ s JR (t; a), where s JR (t; a) has the frequency pulse [27] "
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    ABSTRACT: We investigate the performance of Feher-patented quadrature phase-shift keying (FQPSK) and shaped-offset QPSK (SOQPSK) when serially concatenated with an outer code. We show that the receiver complexity for FQPSK and SOQPSK can he greatly reduced by viewing them as continuous phase modulation (CPM) waveforms. We use the pulse amplitude modulation (PAM) representation of CPM, which allows near-optimum detection of both modulations using a simple 4-state trellis. We compare the performance of the PAM-based approximation with another common approximation known as frequency/phase pulse truncation (PT). We use both of these reduced-complexity designs in serially concatenated coding schemes with iterative detection. In the end, we show that the PAM approximation has a slight performance advantage over PT, but both approximations achieve large coding gains in the proposed serially concatenated systems
    Military Communications Conference, 2005. MILCOM 2005. IEEE; 11/2005
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    • "This is not the case for FQPSK, which cannot be exactly represented with a CPM model. However, a very close CPM approximation can be obtained with x JR (t; a) ≈ s JR (t; a), where s JR (t; a) has the frequency pulse [12] "
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    ABSTRACT: We investigate the performance of Feher-patented quadrature phase-shift keying (FQPSK) and shaped-offset QPSK (SOQPSK) when serially concatenated with an outer code. We show that the receiver com-plexity for FQPSK and SOQPSK can be greatly reduced by viewing them as continuous phase modulation (CPM) waveforms. We use the pulse amplitude modulation (PAM) representation of CPM, which allows near-optimum detection of both modulations using a simple 4-state trellis. We compare the performance of the PAM-based approximation with another common approximation known as frequency/phase pulse truncation (PT). We use both of these reduced-complexity designs in serially concatenated coding schemes with iterative detection. In the end, we show that the PAM approximation has a slight performance advan-tage over PT, but both approximations achieve large coding gains in the proposed serially concatenated systems.
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    ABSTRACT: Shaped offset quadrature phase shift keying (SOQPSK) is a highly bandwidth efficient modulation technique used widely in military and aeronautical telemetry standards. This work focuses on symbol timing recovery for SOQPSK. Continuous phase modulation (CPM) based detector models for SOQPSK have been developed only recently. The proposed timing recovery schemes make use of this recent CPM interpretation of SOQPSK, where SOQPSK is viewed as a CPM with a constrained (correlated) ternary data alphabet. One roadblock standing in the way of these detectors being adopted is that existing symbol timing recovery techniques for CPM are not always applicable since the data symbols are correlated. Here, we derive timing error detectors (TED) that are extended versions of existing non-data-aided (blind) and data-aided TED's for CPM, where the proposed extensions take the data correlation of SOQPSK explicitly into account. Further, for the nod-data-aided case, the merits of the modified TED are demonstrated by comparing its performance with and {\em without} taking the data correlation into account. A simple quantization scheme has also been discussed and implemented for the blind TED to yield an extremely low-complexity version of the system with only negligible performance losses. The S-curves of the proposed TED's are given, which rule out the existence of false lock points. Numerical performance results are given for the two versions of SOQPSK: MIL-STD SOQPSK and SOQPSK-TG. These results show that the proposed schemes have great promise in a wide range of applications due to their low complexity, strong performance and lack of false lock points; such applications include timing recovery in noncoherent detection schemes and false lock detectors.
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