Tom Nelson

Near Optimal Common Detection Techniques for Shaped Offset QPSK and Feher's QPSK

A detector architecture capable of detecting both shaped offset quadrature phase shift keying (SOQPSK-TG) and Fehers quadrature phase shift keying (FQPSK-JR) is developed and analyzed. Both modulations are embodied as fully interoperable modulations in the Interrange Instrumentation Group (IRIG) standard IRIG-106. It is shown that the common detector achieves near optimal bit error rate performance without knowledge of which modulation is used by the transmitter. The detection techniques are based on a common trellis-coded modulation representation and a common continuous phase modulation (CPM) representation for these two modulations. In addition the common pulse amplitude modulation (PAM) decomposition of the common CPM representation is developed. The common PAM-based detector offers the best performance- complexity trade-off among the detectors considered.

A unified perspective on ARTM tier I waveforms - part I: common representations

The constraints placed on aeronautical telemetry channels require the use of bandwidth efficient, constant envelope modulations. Two such modulations have been adopted into the standard for telemetry channels: Feher-patented QPSK (FQPSK) and shaped offset QPSK (SOQPSK). These modulations are known as ARTM tier I modulations. In this paper we explore the similarities and differences between these two modulations. We show that both modulations can be represented as a cross correlated trellis coded quadrature modulation (XTCQM) as well as a continuous phase modulation (CPM). The XTCQM representation is exact for both FQPSK and SOQPSK and the CPM representation is exact for SOQPSK and a good approximation for FQPSK. The observation that these modulations are very similar suggests the possibility of a common detector which can detect either modulation with near optimal bit error rate performance. The bit error performance and complexity of three candidate common detector architectures are explored in the part II companion paper

Coded FQPSK and SOQPSK with iterative detection

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.

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

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

MIMO communications using offset modulations

This paper explores the use of offset QPSK in a space-time coded multi-input multi-output channel by demonstrating its use with the 2 × 1 Alamouti space-time code. It is shown that the desirable orthogonalization produced by the Alamouti space-time code when used with non-offset QPSK does not carrier over to the case of offset QPSK. A least-squares detector is formulated whose performance is shown to be very close to optimal for slowly varying channel gains in an AWGN environment. The basic trellis structure of the detector is presented and applied to unshaped offset QPSK and shaped offset QPSK.

A unified perspective on ARTM tier I waveforms - part II: common detectors

In the part I companion paper we showed that the tier 1 waveforms FQPSK-JR and SOQPSK-TG - can both be viewed as either an XTCQM or a CPM (although the CPM representation of FQPSK-JR is only an approximation). In this paper we examine the performance of three architectures for a common detector: a symbol-by-symbol detector, an XTCQM detector, and a CPM detector. The symbol-by-symbol detector suffers a 2.0 dB loss relative to the optimum detector for SOQPSK-TG and a 2.2 dB loss for FQPSK-JR when an integrate and dump filter is used and a 1.5 dB loss for SOQPSK-TG and a 1.6 dB loss for FQPSK-JR when a detection filter matched to the average waveform is used. The common XTCQM detector gives a bit error rate performance that is 0.1 dB worse than optimum for SOQPSK-TG and approximately equal to optimum for FQPSK-JR. The common CPM detector gives a bit error rate performance that is 0.25 dB worse than optimum for SOQPSK-TG and about 0.05 dB worse than optimum for FQPSK-JR. The common XTCQM detector provides the best overall performance among the candidate common detectors, but this detector has the highest complexity of the three

Detection of Alamouti Encoded Shaped Offset QPSK

Unitary space-time block codes (STBCs) are attractive because they provide transmit diversity while allowing low complexity detection. Most work with STBCs has been done with non-offset modulations such as QPSK and quadrature amplitude modulation (QAM), but offset modulations such as shaped offset QPSK (SOQPSK) can be attractive in systems employing non-linear power amplifiers. In this paper we explore the application of STBCs to SOQPSK. We show that the offset in SOQPSK complicates the detection of STBC-SOQPSK. Using the Almouti STBC as an example, we present two architectures for detecting unitary STBCs that provide a trade off between decoder complexity and bit error rate performance.

Detection of Alamouti Space-Time Encoded Offset QPSK

The Alamouti space-time code (STC) is attractive because of the low complexity of its associated detection algorithm. However, when this code is applied to offset modulations such as offset QPSK (OQPSK), the offset complicates the detection process. The Maximum Likelihood (ML) detector for Alamouti encoded OQPSK is much more complex than the ML detector for QPSK. We present two alternate detection algorithms: one which duplicates the simplicity of the non- offset QPSK Alamouti detector with a cost in bit error rate performance and one which matches the performance of the ML detector with less complexity than the ML detector.

Space-Time Coding for Aeronautical Telemetry: Part II—Decoder and System Performance

This paper describes the use of Alamouti-encoded-shaped offset QPSK version TG (SOQPSK-TG) to solve the two-antenna problem in aeronautical telemetry. The Alamouti space-time block code is used to encode the phase states in the complex exponential representation of SOQPSK-TG. Because SOQPSK-TG possesses memory, the Alamouti decoder is a sequence estimator. Maximum likelihood and least squares sequence decoders are derived. To reduce the number of states, the eight-waveform cross-correlated trellis-coded quadrature modulation (XTCQM) approximate representation of SOQPSK-TG is used. A prototype decoder based on the least squares decoder and the estimators described in Part I and operating at a data rate of 10 Mb/s was tested in the laboratory in test flights at the Air Force Test Center, Edwards AFB. The test flights demonstrate that Alamouti-encoded SOQPSK-TG, as described in this paper, using the least squares decoder based on the estimators described in Part I solves the two antenna problem in aeronautical telemetry.

Detection of SOQPSK in a space-time coded system with arrival time differences

This paper presents an algorithm suitable for space-time codes (STC) in power-limited, high rate data links subject to shadowing due to antenna placement (such as aeronautical telemetry channels). Two difficulties arise when using STC on such channels-they employ offset modulations (such as shaped offset QPSK or SOQPSK) due to the need for a constant envelope waveform and the spacing between multiple transmit antennas can lead to arrival time differences among the received signals. The algorithm presented in this paper deals with these two difficulties and provides a solution to this situation. The performance of the algorithm is simulated and presented, and it is shown that the use of STC along with the algorithm presented here eliminates the occurence of data dropouts which result when multiple antennas are used at the transmitter (without STC).