We propose a robust and low complexity scheme to estimate and track carrier frequency from signals traveling under low signal-to-noise ratio (SNR) conditions in highly nonstationary channels. These scenarios arise in planetary exploration missions subject to high dynamics, such as the Mars exploration rover missions. The method comprises a bank of adaptive linear predictors (ALP) supervised by a convex combiner that dynamically aggregates the individual predictors. The adaptive combination is able to outperform the best individual estimator in the set, which leads to a universal scheme for frequency estimation and tracking. A simple technique for bias compensation considerably improves the ALP performance. It is also shown that retrieval of frequency content by a fast Fourier transform (FFT)-search method, instead of only inspecting the angle of a particular root of the error predictor filter, enhances performance, particularly at very low SNR levels. Simple techniques that enforce frequency continuity improve further the overall performance. In summary we illustrate by extensive simulations that adaptive linear prediction methods render a robust and competitive frequency tracking technique.

Performance of the mobile user objective system is dominated by the channel conditions characterized by ionospheric scintillation and multipath fading. Detail analytical modeling for flat channel is provided in part I of this paper, while the emphasis for part II of this paper is on wideband waveforms. Efficient computer simulation techniques for the selective scintillation and terrestrial multipath fading channel impairment have been presented.

A new lock detection scheme for NASA's Mars Reconnaissance Orbiter (MRO) Project Electra payload has been proposed. Compared to the conventional lock detector of phase locked loops (PLLs) and Costas loops, this proposed scheme has better performance. It can be used to indicate lock status during acquisition as well as cycle slip during tracking mode.

This paper studies feasibility of using one-way and two-way Doppler to accomplish rendezvous for Mars Sample Return project. Acquisition and tracking requirements of the second and third order phase-locked loop (PLL) under flight and rotational dynamics conditions is presented.

In this paper, adaptive modulation and coding structures are presented that are appropriate for the MUOS environment. The proposed approach adaptively controls the modulation format, FEC coding rate, transmission power and even possibly bandwidth spreading gain in response to changing channel conditions in an attempt to optimize throughput while maintaining acceptable error rate performance. Being able to estimate and then predict the channel is vital to the system performance. Channel prediction algorithms; have been developed and the advantages of the adaptive modulation/coding techniques are presented together with implementation issues.

For planetary lander missions, the most challenging phase of the spacecraft to ground communications is during the entry, descent, and landing (EDL). As each 2003 Mars Exploration Rover (MER) enters the Martian atmosphere, it slows dramatically. The extreme acceleration and jerk cause extreme Doppler dynamics on the X-band signal received on Earth. When the vehicle slows sufficiently, the parachute is deployed, causing almost a step in deceleration. After parachute deployment, the lander is lowered beneath the parachute on a bridle. The swinging motion of the lander imparts high Doppler dynamics on the signal and causes the received signal strength to vary widely, due to changing antenna pointing angles. All this time, the vehicle transmits important health and status information that is especially critical if the landing is not successful. Even using the largest Deep Space Network antennas, the weak signal and high dynamics render it impossible to conduct reliable phase coherent communications. Therefore, a specialized form of frequency-shift-keying must be used. This paper describes the EDL scenario, the signal conditions, the methods used to detect and frequency-track the carrier and to detect the data modulation, and the resulting performance estimates.

The Joint Services Advanced Range Telemetry (ARTM) Program at Edwards Air Force Base has been evaluating FQPSK-B for possible upgrades to the existing telemetry equipment. It has been found in the wideband channel sounding experiments sponsored by ARTM that the in-flight fading channel can be modeled as a 3-ray multipath channel. Delay spread for a typical in-flight channel is in the order of 300 nanoseconds. Furthermore, the pre-flight channel is characterized by much more severe multipath, in which the delay spread is in the order of microseconds covering one or more symbols when the FQPSK-B transceiver operates at a rate of millions of symbols per second. This adverse channel condition inevitably causes tremendous distortion in the received signals due to severe inter-symbol interference (ISI) from the multipath. This paper provides an assessment of the potential ability of blind equalization to reduce the FQPSK-B system susceptibility to degradation caused by dynamic frequency selective fading in the aeronautical telemetry environment. In particular, a blind equalizer applique that can be inserted prior to the demodulator without knowledge of the received signal such as carrier frequency, symbol timing and sequence, etc, is proposed. Since it is desired that the equalizer applique operate independently of the carrier frequency and given that the modulation of interest is constant envelope (PCM-FM or FQPSK-B), we have selected the constant modulus algorithm (CMA) cost function for implementation. Extensive tests on both simulated and recorded FQPSK-B data transmitted over different ARTM channels have been conducted and the blind equalizer structure has shown substantial improvements, even on the difficult ARTM pre-flight channels. The CMA adapts the equalizer coefficients to minimize the deviation of the output envelope from an arbitrary constant level. This paper depicts the pre-flight and in-flight channel conditions using time and spectral domain measurement. It quantifies the benefit of the blind CMA tapped delay line equalizer. Due to the extensive signal processing requirements associated with the very high sampling rate (100 MHz) of the FQPSK-B system, hardware implementation complexity is very high. Complexity reduction issues regarding the implementation of the CMA using field programmable gate array (FPGA) are also presented.

For NASA's Mars sample return (MSR) program, there is a strong need to enhance the received signal strength for the radio direction finding (RDF) application. We consider the possibility of linearly combining the outputs from two orthogonal, polarized antennas to prevent polarization loss. The problem is complicated due to the dynamics of the beacon transmitter. In particular, it is assumed that the transmitter is rotating and that it radiates a linearly polarized wave. Initial results are based on the assumption of perpendicular incidence. It is also extended for arbitrary incidence, in which case there are gain and phase differences between the orthogonally polarized antenna pair outputs. An innovative algorithm for polarization combining is proposed and performance data in a multipath environment are presented with simulation results. The SNR improvement using this algorithm translates into a more favorable channel condition for the overall RDF system design.

Various arraying techniques are studied focusing on the very low received signal SNR channel conditions commonly found in deep space communications applications. These include correlation-based blind approaches as well as a sub-space based superresolution approach. In addition to weak received signals, atmospheric turbulence and spatially correlated interference from nearby planets (and possibly quasars) creates additional channel impairment. It is demonstrated that the sub-space based MUSIC algorithm is a strong candidate for this application that can provide great angle separation accuracy and interference suppression capability. Adaptive beamforming techniques in combination with the MUSIC algorithm provide a flexible platform to combat channel impairment.

This paper describes a programmable DSP-based testbed that is employed in the development and evaluation of blind demodulation algorithms to be used in wireless satellite or terrestrial communications systems. The testbed employs a graphical user interface (GUI) to provide independent, real-time control of modulator, channel and demodulator parameters and also affords realtime observation of various diagnostic signals such as carrier, timing recovery and decoder metrics. This interactive flexibility enables an operator to tailor the testbed parameters and environment to investigate the performance of any arbitrary communications system and channel model. Furthermore, a variety of digital and analog interfaces allow the testbed to be used either as a stand-alone digital modulator or receiver, thereby extending its experimental utility from the laboratory to the field.

We describe a testbed that is being developed for the evaluation of demodulation algorithms in communication systems. This testbed is intended to provide a research tool in the area of signal demodulation and is built around easily programmable, general purpose and digital signal processors. A variety of analog and digital modulations can be evaluated in real and non-real-time scenarios and under different channel conditions ranging from intersymbol interference to modulated or unmodulated co-channel interference. Testbed architectural and design details are presented along with preliminary results of our algorithm development and testing program.

This paper presents a new open-loop technique for estimating and correcting Doppler frequency shift in K/Ka-band communication systems with special reference to the advanced communications technology satellite (ACTS) mobile terminal (AMT) modem, which utilizes square-wave pulse-shaped, binary differential phase shift-keyed (DPSK) modulation. The novelty of this estimation scheme is that it exploits the Doppler-induced phase shift over a fraction of a symbol interval to provide an estimate of the Doppler offset, without requiring symbol synchronization. Furthermore, by utilizing time-differential detection (delay-and-multiply), the proposed technique can tolerate much larger frequency offsets than existing open- or closed-loop techniques. Analytical results are provided for the variance of the above estimator and the error probability performance of the AMT is evaluated in the presence of the Doppler correction. Practical design considerations are also discussed, including a method for modifying the front end, digital bandlimiting filter in such a way that Doppler bias effects in the new estimator are eliminated. Simulation results reveal that, in general, performance improves with increasing data rates, i.e., the new frequency offset estimation/compensation algorithm induces a degradation from ideal of approximately 1 dB at a 6 dB energy per data symbol (bit) and a 2.4 kbps data rate. However, there is no appreciable degradation when the data rate is increased to 9.6 or 19.2 kbps.

A rain compensation algorithm (RCA) has been developed for use in the advanced communications technology satellite (ACTS) mobile terminal (AMT) system. The basic goal of the RCA is to control the transmitted data rates (9·6, 4·8 or 2·4 kbps) in the forward and return links so that a 3 dB link margin is maintained at the highest possible transmitted data rate. In this paper, analyses of both theoretical and practical issues relating to the RCA are presented. In addition, sample simulations of a one-dimensional version of the RCA at the MT are presented which illustrate typical RCA performance using both simulated and recorded pilot fade field data. It is found that with suitable post-processing, the RCA can provide reasonable (conservative) data rate estimates without making excessive data rate changes, i.e., data rate change fluctuations. It is anticipated that the results presented here will not only be useful for the eventual operation of the RCA, but more generally will be useful in the design and operation of other rain compensation techniques for K/Ka-band communication systems.

We present the application of wavelet-based de-noising techniques to the demodulation of digital communication signals. These techniques are related to those originally devised by Donoho and Johnstone (see Biometrika, v.81, p.455, 1994, and IEEE Trans. Info. Theory, vol.41, p.613, 1995) for minimum L/sub 2/-risk signal reconstruction. The important minimax property of wavelet de-noising filters yields significant noise reduction while retaining the essential signal features. However, the two problems are quite different, i.e., the objective is not signal reconstruction but rather minimizing bit error rate. This generally results in different design rules for optimizing performance. We show that wavelet de-noising can be effectively used for data demodulation and we propose simple design rules for its implementation.

A simple new estimator is proposed for direction finding applications which extends conventional phase-only interferometry to incorporate both calibrated phase and amplitude response data from antenna arrays. This is done by appropriately weighting the square of the baseline phase differences with the antenna gains. The incorporation of amplitude data generally provides significant performance improvement over phase-only interferometry with only a modest increase in computational complexity. Furthermore, this performance improvement increases with increasing additive noise and with increasing deviation of the antenna array response from an ideal geometric array response. As such, the new estimator strikes a nice compromise between phase-only interferometry and maximum likelihood estimation (MLE), this latter yielding nearly-optimal performance but at significant computational expense. Performance results are derived analytically for sufficiently high signal-to-noise ratio (SNR) or sample count, and are also demonstrated using antenna array responses simulated with the numerical electromagnetic code (NEC). The simulation results corroborate the analysis and clearly demonstrate that significant variance reduction in direction of arrival (DOA) estimation error can be achieved with the new estimator.

JPL has developed a K-/Ka-band mobile terminal and is currently conducting a series of mobile experiments to explore the potential of K-/Ka-band to meet the needs of future mobile satellite services. In support of the AMT field experiments, data analysis software algorithms have been developed for processing the various data received at the fixed and mobile terminals. This processing consists of both quick-look analysis of events recorded at a single terminal as well as in-depth analysis of data events collected from both terminals. In this paper, we provided an overview of these data analysis techniques and provide some examples.

In this paper, we provide an asymptotic precision analysis of blind adaptive filter coefficients derived from a wide class of scale-invariant cost functions implicitly implemented in a batch processing mode. This analysis is based on a first-order Taylor expansion of the cost functions in the vicinity of their maxima and represents an extension of Donoho's (1981) classic asymptotic precision analysis to the complex case. Through this analysis we have a means of discriminating among different nonlinear cost functions in the sense of yielding more precise estimates with N finite samples. We also find that cost functions based on very large order statistics tend to have highly desirable convergence properties over a wide range of constellations

An alternative basis for blind equalization is presented which utilizes uniformly most powerful statistical tests as the cornerstone for generating candidate equalizer cost functions. In addition, general criteria for selecting admissible equalizer cost functions are provided, as well as a simple asymptotic figure-of-merit for comparing the performance that can be achieved with different equalizer cost functions. As an example of our methodology, we present specific cost functions for both PAM and QAM modulations which exhibit extremely rapid convergence properties.