Aeronautical Telemetry Research Articles

A Soft-Output STBC Decoder for Aeronautical Telemetry

Alamouti encoding is a well-known space time block encoding technique used to improve the received signal quality in Rayleigh fading channels. In aeronautical telemetry, this encoding technique is applied to shaped offset quadrature phase shift keying tier generation (SOQPSK-TG) modulation in order to handle the two-antenna issue. It is provided for in telemetry-related IRIG standards. In this paper, we propose a unique decoding architecture for Alamouti-encoded SOQPSK-TG signals, taking advantage of pulse amplitude modulation decomposition with soft and hard outputs. We exploit this result to obtain a Viterbi algorithm (VA) for hard decoding and a soft output Viterbi algorithm (SOVA) for soft and hard decoding, with a twofold benefit: operation using one trellis structure, unlike decoders that are based on the 8-waveforms cross-correlated trellis-coded quadrature modulation (XTCQM) approximation, and very attractive bit error rate performance, as well as a complexity trade-off

Coding for Error Correction

In aeronautical telemetry, if it looks too good to be true, it probably is - unless it's LDPC

On the Effects of Channel Sparsity on Joint Estimators in Aeronautical Telemetry

Minimum mean-squared error (MMSE) equalizers are a viable solution to mitigate the frequency selectivity of the aeronautical telemetry channel. Because the MMSE equalizer filter coefficients are a function of the carrier frequency offset (CFO), the equivalent discrete-time channel, and the noise variance, reliable estimates of those parameters are required. The CFO is due primarily to the high velocity of the airborne transmitter. Because the equivalent discrete-time channel in aeronautical telemetry is sparse, the performance of the joint estimator based on a sparse channel estimator is superior to the performance of a traditional nonsparse maximum-likelihood-inspired joint estimator. The improvement is seen in lower estimator error variances for the parameters, particularly for the CFO and channel estimate, and in the postequalizer bit-error rate. Simulation results demonstrate that the postequalizer bit-error rate using the sparse estimator is almost as good as that using ideal estimators.

On Sparse Channel Estimation in Aeronautical Telemetry

This paper examines the application of sparse estimation techniques for the estimation of a discrete-time equivalent multipath channel in the aeronautical telemetry context. The sensing matrix comprises samples of shaped offset QPSK-TG (a continuous phase modulation) based on the pilot bit sequence currently defined in the aeronautical telemetry standard. Representative algorithms from the three broad classes of sparse estimators were examined side by side using computer simulations to estimate the postequalizer bit error rate (BER). Ideal and nonideal frequency offset synchronization were assumed in the simulations. The results show that the performance of the matching pursuit (MP) algorithms seemed to be better suited to this application in the sense that no additional steps were required and the postequalizer BER of the best MP algorithm was slightly better than that of the other sparse estimation techniques. In the case of both ideal and nonideal frequency offset synchronization, the postequalizer BER achieved by the generalized orthogonal MP algorithm was approximately 1.5 dB better than that obtained using the nonsparse-constrained maximum likelihood channel estimate.

Sparse Equalization in Aeronautical Telemetry Using Two Transmit Antennas

This correspondence explores the application of sparse equalizers in generalized time-reversed space-time block codes (GTR-STBCs) for a two-transmit/one-receive antenna aeronautical telemetry link. It is shown that the jointly optimum design of the sparse equalizer coefficients and the power sharing parameter $\rho$ in GTR-STBCs is computationally challenging because the active tap ratio of the sparse equalizers is a non-convex function of $\rho$ . A search algorithm based on a grid search followed by an interpolation is shown to yield impressive results. Our numerical results demonstrate that depending on signal-to-noise ratio and channel conditions, the number of non-zero taps of sparse equalizers for GTR-STBCs can be reduced by 53% to $86\%$ at the cost of 0.25 dB relaxation in the mean-squared error.

Approximate MLSE Equalization of SOQPSK-TG in Aeronautical Telemetry

An approximate maximum likelihood sequence estimator (MLSE) equalizer for shaped offset QPSK, version TG (SOQPSK-TG) is derived based on the pulse amplitude modulation (PAM) approximation of SOQPSK-TG using the two principal PAM pulses. The resulting equalizer is a generalization of the equalizer based on the Ungerboeck observation model, where the generalization is to two parallel, cross-correlated symbol streams over parallel intersymbol interference channels. The bit error rate (BER) performance and computational complexity of the approximate MLSE equalizer operating over three example channels, motivated by multipath propagation in aeronautical telemetry and with increasing selectivity, are presented. The BER performance was evaluated using computer simulations and shown to be approximately 5–8 dB better than the best frequency-domain minimum mean-squared error (MMSE) equalizers published in the open literature. The computational complexity, measured by the number of real-valued multiplications, is comparable to that of the frequency-domain MMSE equalizers.

Spectrum allocations: The aeronautical telemetry story in the USA

Aeronautical telemetry encompasses all the technologies necessary to successfully monitor airborne test articles, such as fixed and rotary wing aircraft, missiles, and drones. Aeronautical telemetry is primarily focused the final stages of development when the test article performs the maneuvers required to operate its deployed environment. Necessarily, the test article is in the air while the flight test engineers who monitor its performance are on the ground. The key enabling resource for aeronautical telemetry is spectrum: bandwidth is needed to transmit the measurements from the test article to the ground so flight test engineers can monitor the test, the safety of which is paramount.

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.

On frame synchronization in aeronautical telemetry

Frame synchronizers, suitable for use with continuous phase modulation (CPM) in the presence of phase and frequency offsets, are derived for periodically inserted preamble bits in continuous-mode transmission. The resulting frame synchronizers involve single or double correlations of the received samples with a local copy of the CPM samples corresponding to the preamble bits. Of particular interest in aeronautical telemetry is shaped offset QPSK, telemetry group version (SOQPSK-TG), with the preamble defined in the integrated network enhanced telemetry (iNET) standard. Two low-complexity versions that leverage the special properties of the iNET preamble and SOQPSK-TG operating at 2 samples/bit are developed. Simulation results show that most frame synchronizers are capable of achieving acceptable performance for Eb/N0 ≥ 0 dB and the maximum frequency offset that can be tolerated increases with Eb/N0 to about 3%of the bit rate.

Frequency-domain Equalization Techniques for Multi-h Continuous Phase Modulation

Two receiver architectures are presented for equalization of multi-h continuous phase modulation (CPM). In the first approach, the receiver front end is a matched-filter bank, which is derived from Laurent decomposition of the signal. A circulant-matrix model derived for the samples of the matched filter is used to perform the equalization in frequency domain. The second technique employs a receiver with a simple front end consisting of a low-pass filter and a fractional sampler. We also develop a circulant matrix model for the latter approach, which relates the Laurent pseudo-symbols to each polyphase component at the output of the sampler. A linear frequency-domain equalizer (FDE) is also proposed using this model. In addition to providing the details of these approaches, the paper proposes several simplifications applicable to each of the two cases, and provides a detailed discussion on the performance/complexity trade-off. Bit-error rate (BER) results are presented for the IRIG-106 aeronautical telemetry dual-h CPM waveform.

FEC Systems for Aeronautical Telemetry

We consider the design of capacity-approaching forward error correction (FEC) systems for use in the aeronautical telemetry environment. The modulation format is the bandwidth-efficient telemetry-group version of shaped-offset quadrature phase-shift keying (SOQPSK-TG). The FEC codes are a low-density parity-check (LDPC) code and a serially concatenated convolutional code (SCCC). The block structure of the two types of FEC is designed such that the two code words are formatted as similarly as possible. This unified format allows the flexibility of the FEC-encoded signal to be received by legacy test-range assets that are not equipped with FEC?although no coding gain is realized in such a case. We also show how a wide range of near-optimal SOQPSK demodulators can be paired with the FEC decoders; this includes the most widely deployed SOQPSK demodulator (the so-called symbol-by-symbol (SxS) demodulator), which so far has not been considered for use in FEC applications. In all of our demodulator/decoder designs, we apply techniques that are simple and robust and that fully decouple the demodulator's tasks (e.g., synchronization, filtering) from the decoder's tasks (e.g., path metrics, soft outputs). For both LDPC and SCCC, we show that very attractive system designs can perform within 0.5 dB of optimum, while requiring only a fraction of the complexity of the optimum system.

고속 자료전송을 위한 비행체용 원격측정링크 개발

본 논문은 초고속 비행체에 탑재하여 대용량 자료를 송수신하는 원격측정링크 개발에 대하여 기술하였다. 고속 이동체 환경에서 고속 자료전송을 위해 기존에 주로 사용되던 PCM/FM 변조방식에 비해 대역폭 효율성, 주파수 선택적 페이딩, 도플러주파수 옵셋 등을 고려하여 DQPSK 변조방식을 사용하였다. 또한 통신채널 장애의 주요 원인이 되는 멀티패스 간섭을 최소화하기 위해 다수의 수신안테나를 사용하여 공간다이버시티를 구현하였으며 추가적으로 터보코드를 오류정정부호로 사용하여 양질의 데이터를 수신할 수 있도록 개발하였으며 비행시험을 통하여 성능을 확인하였다.

On the Reversibility of Randomizers and Derandomizers in Aeronautical Telemetry

Data may be recovered when randomization and derandomization are applied in the reverse order as long as the initial register conditions for both the randomizer and derandomizer are identical. In the (very realistic) case where the initial register contents for the derandomizer are not known, simple procedures for identifying the correct initial conditions are described. The procedures leverage the presence of frame synchronization words in the telemetry data.

Linear Frequency Domain Equalization of SOQPSK-TG for Wideband Aeronautical Telemetry Channels

This work explores multipath interference mitigation for a single transmit antenna, single receive antenna system with SOQPSK-TG using a linear single-carrier frequency domain equalization (SCFDE) technique. Linear SCFDE with maximum likelihood (ML) channel estimation is shown to greatly improve system Bit Error Rates (BER). Simulation results also demonstrate that three samples per bit at the receiver are adequate to achieve the best BER performance.

Fade Mitigation for Air-to-Ground Communication in Aeronautical Telemetry

Telemetry links were comprised of a transmit antenna mounted on the underside of fuselage and a fixed ground station equipped with a tracking antenna. Air-to- ground channel for this telemetry link can be estimated as line of sight propagation. But there is link loss observed at ground control station for certain instant of time. Possible causes of link loss in this case are due to aircraft maneuvering and fading. So analysis of various fade mitigation techniques as well as bit error rate (BER) and signal to noise (SNR) ratio performance comparison between selection diversity, equal gain combining and maximal ratio combining scheme is done. Finally bit error rate performance comparison curve between maximal ratio scheme (MRC) and Alamouti dual antenna transmission scheme is plotted, in order to find out the best scheme for fade mitigation, among all the above mentioned schemes.

Highly-Dynamic Cross-Layered Aeronautical Network Architecture

Highly-dynamic wireless environments present unique challenges to end-to-end communication networks, caused by the time-varying connectivity of high-velocity nodes combined with the unreliability of the wireless communication channel. Such conditions are found in a variety of networks, including those used for tactical communications and aeronautical telemetry. Addressing these challenges requires the design of new protocols and mechanisms specific to this environment. We present a new domain-specific architecture and protocol suite, including cross-layer optimizations between the physical, MAC, network, and transport layers. This provides selectable reliability for multiple applications within highly mobile tactical airborne networks. Our contributions for this environment include the transmission control protocol (TCP)-friendly transport protocol, AeroTP; the IP-compatible network layer, AeroNP; and the geolocation aware routing protocol AeroRP. Through simulations we show significant performance improvement over the traditional TCP/IP/MANET protocol stack.

Turbo-Coded APSK for aeronautical telemetry

The performance of turbo-coded Amplitude-Phase Shift Keying (APSK) in an aeronautical telemetry system with a non-linear power amplifier is investigated. The AM/AM curves for four L-band power amplifiers are modeled and used to simulate the performance the turbo-coded APSK system over a non-linear channel, Spectral efficiency and bit error rate performance as a function of back-off are quantified and compared with a system based on the existing aeronautical telemetry standard using uncoded SOQPSK. The results show that the -50 dBc bandwidth can be matched with 2.5 to 4.1 dB of back-off, the -60dBc bandwidth can be matched with 4.3 to 6.6 dB of back-off, and the 99%. bandwidth can be matched with 0.8 to 1.6 dB of back-off. The bit error rate results show that each power amplifier has an optimal back-off and that this back-off is 3 dB for the four amplifiers used in this study. When put together, this system is capable of outperforming an uncoded system based on SOQPSK by matching (or exceeding) the detection efficiency while simultaneously achieving a spectral efficiency in excess of three times that of SOQPSK.

Calculation of the communication angles between an air vehicle and receiver antenna in aeronautical telemetry

To determine the immediate effect of an air vehicle's manoeuvre on the radio link of aeronautical telemetry, it is needed to calculate instant angles between such an air vehicle and a receiver antenna. Having determined the communication angles of the air vehicle, the instantaneous gains from transmitter antennas can be calculated, thus evaluating the radio link status in terms of bit error rate (BER). Through calculating its communication angles, a receiver that is equipped with a system of auto tracking the air vehicles can be assisted in better tracking. For non-tracking systems, having determined the communication angles of the receiver antenna, the instantaneous gain from such a receiver antenna can be attained. However, availability of the communication angles is absolutely vital for evaluation of the real-time radio link in aeronautical telemetry. This paper proposes an algorithm and its relevant equations to calculate these angles. By a proper methodology, the proposed algorithm is then validated and the results are depicted using a hypothetical missile and an assumed airplane flight starting from earth's geographical coordinates (where longitude and latitude are set to zero) and extending towards the North Pole.

Decision-Directed Symbol Timing Recovery for SOQPSK

Shaped-offset quadrature phase shift keying (SOQPSK) is a highly bandwidth efficient modulation technique used widely in military and aeronautical telemetry standards. It can be classified as a form of continuous phase modulation (CPM), but its major distinction from other CPMs is that it has a constrained (correlated) ternary data alphabet. CPM-based detection models for SOQPSK have been developed only recently. While these detectors offer an appreciable performance gain over current detection schemes, one roadblock standing in the way of their being adopted is the lack of a CPM-based symbol timing recovery scheme that will work with SOQPSK. We show how an existing CPM-based timing error detector (TED) can be adapted to the unique characteristics of SOQPSK. The TED is formulated for a coherent detector but can be extended to the noncoherent case. We apply this timing recovery method to the versions of SOQPSK used in military (MIL-STD SOQPSK) and telemetry group (SOQPSK-TG) standards. We derive the theoretical performance limits on the accuracy of timing recovery for SOQPSK, as given by the modified Cramer-Rao bound (MCRB), and show that the proposed decision-directed TED (DD-TED) performs close to these bounds in computer simulations and is free of false-lock points. We also show that the proposed scheme outperforms a non-data-aided TED (NDA-TED) that was recently developed for SOQPSK. These results show that the proposed scheme has great promise in a wide range of applications due to its low complexity, strong performance, and lack of false-lock points.

Multipath Channel Model for Over-Water Aeronautical Telemetry

A multipath propagation model for aeronautical telemetry over water at 8.0 GHz is presented. Data recorded during channel sounding experiments, conducted over the Pacific Ocean near Point Mugu, CA was used to model the multipath propagation. The modeling results show that a three-ray model consisting of line-of-sight propagation and two reflected propagation paths is a good fit for the measured channel frequency responses. One of the reflected propagation paths is a strong specular reflection determined by the geometry defined by the airborne transmitter, the ground-based receiver, and the sea surface. For the flight profiles used, the delay of this path is between 10 and 50 ns. The other propagation path is a more diffuse, random reflection whose amplitude is significantly smaller than the amplitude of the specular reflection and whose mean excess delay is 57 ns with an rms delay spread of 25 ns.