Efficient Maximum-Likelihood Based Clock and Phase Estimators for OQPSK Signals

In this paper we propose an algorithm for joint carrier phase and timing estimation with OQPSK modulations. The derivation is based on the maximum-likelihood criterion, and exploits a very efficient algorithm for the detection of differentially encoded $M$-PSK symbols already described in literature. Though we are mainly interested in measuring the phase and clock parameters, estimates of the transmitted symbols are also obtained as by-products. The resulting scheme has a feedforward structure and provides phase and timing information in a fixed time, differently from closed-loop architectures. It can be implemented in digital form and is particularly suitable for burst mode transmissions. Its performance is investigated by simulation and the results are compared with Cramér-Rao bounds. It turns out that the estimation accuracy is very close to the theoretical limits, even with short observation intervals and small values of the excess bandwidth. In such conditions, the proposed estimators largely outperform other schemes already known in literature. Their superiority becomes less significant as the signal bandwidth increases

Synchronization Techniques for Burst-Mode Continuous Phase Modulation

Synchronization is a critical operation in digital communication systems, which establishes and maintains an operational link between transmitter and the receiver. As the advancement of digital modulation and coding schemes continues, the synchronization task becomes more and more challenging since the new standards require high-throughput functionality at low signal-to-noise ratios (SNRs). In this work, we address feedforward synchronization of continuous phase modulations (CPMs) using data-aided (DA) methods, which are best suited for burst-mode communications. In our transmission model, a known training sequence is appended to the beginning of each burst, which is then affected by additive white Gaussian noise (AWGN), and unknown frequency, phase, and timing offsets. Based on our transmission model, we derive the Cramer-Rao bound (CRB) for DA joint estimation of synchronization parameters. Using the CRB expressions, the optimum training sequence for CPM signals is proposed. It is shown that the proposed sequence minimizes the CRB for all three synchronization parameters asymptotically, and can be applied to the entire CPM family. We take advantage of the simple structure of the optimized training sequence in order to design a practical synchronization algorithm based on the maximum likelihood (ML) principles. The proposed DA algorithm jointly estimates frequency offset, carrier phase and symbol timing in a feedforward manner. The frequency offset estimate is first found by means of maximizing a one dimensional function. It is then followed by symbol timing and carrier phase estimation, which are carried out using simple closed-form expressions. We show that the proposed algorithm attains the theoretical CRBs for all synchronization parameters for moderate training sequence lengths and all SNR regions. Moreover, a frame synchronization algorithm is developed, which detects the training sequence boundaries in burst-mode CPM signals. The proposed training sequence and synchronization algorithm are extended to shaped-offset quadrature phase-shift keying (SOQPSK) modulation, which is considered for next generation aeronautical telemetry systems. Here, it is shown that the optimized training sequence outperforms the one that is defined in the draft telemetry standard as long as estimation error variances are considered. The overall bit error rate (BER) plots suggest that the optimized preamble with a shorter length can be utilized such that the performance loss is less than 0.5 dB of an ideal synchronization scenario

Timing synchronization for cooperative wireless communications

In this work the effect of perfect and imperfect synchronization on the performance of single-link and cooperative communication is investigated. A feedforward non- data-aided near maximum likelihood (NDA-NML) timing estimator which is effective for an additive white Gaussian noise (AWGN) channel and also for a flat-fading channel, is developed. The Cramer Rao bound (CRB) and modified Cramer Rao bound (MCRB) for the estimator for a single-link transmission over an AWGN channel is derived. A closed form expression for the probability distribution of the timing estimator is also derived. The bit-error-rate (BER) degradation of the NDA-NML timing estimator with raised cosine pulse shaping for static timing errors over an AWGN channel is characterized. A closed form expression is derived for the conditional bit error probability (BEP) with static timing errors of binary phase shift keying modulation over a Rayleigh fading channel using rectangular pulse shaping. The NDA-NML timing estimator is applied to a cooperative communication system with a source, a relay and a destination. A CRB for the estimator for asymptotically low signal-to-noise-ratio case is derived. The timing complexity of the NDA-NML estimator is derived and compared with a feedforward correlation based data-aided maximum likelihood (DA-ML) estimator. The BER performance of this system operating with a detect-and-forward relaying is studied, where the symbol timings are estimated independently for each channel. A feedforward data and channel aided maximum likelihood (DCA-ML) symbol timing estimator for cooperative communication operating over flat fading channels is then developed. For more severe fading the DCA-ML estimator performs better than the NDA- NML estimator and the DA-ML estimator. The performance gains of the DCA-ML estimator over that of the DA-ML estimator become more significant in cooperative transmission than in single-link node-to-node transmission. The NDA-NML symbol timing estimator is applied to three-node cooperative communication in fast flat-fading conditions with various signal constellations. It is found that timing errors have significant effect on performance in fast flat-fading channels. The lower complexity NDA-NML estimator performs well for larger signal constellations in fast fading, when compared to DA-ML estimator. The application of cooperative techniques for saving transmit power is discussed along with the related performance analysis with timing synchronization errors. It is found that power allocations at the source and relay nodes for transmissions, and the related timing errors at the relay and the destination nodes, have considerable effect on the BER performance for power constrained cooperative communication. The performance of multi-node multi-relay decode-and-forward cooperative com- munication system, of various architectures, operating under different fading con- ditions, with timing synchronization and various combining methods, is presented. Switch-and-stay combining and switch-and-examine combining are proposed for multi-node cooperative communication. Apart from the proposed two combining methods equal gain combining, maximal ratio combining and selection combining are also used. It is demonstrated that synchronization error has significant effect on performance in cooperative communication with a range of system architectures, and it is also demonstrated that performance degradation due to synchronization error increases with increasing diversity. It is demonstrated that decode-and- forward relaying strategy with timing synchronization, using a very simple coding scheme, performs better than detect-and-forward relaying with timing synchronization. Analytical expressions are derived for BEP with static and dynamic timing synchronization errors over Rayleigh fading channels using rectangular pulse shaping for amplify-and-forward and detect-and-forward cooperative communications. Moment generating function (MGF) based approach is utilized to find the analytical expressions. It is found that timing synchronization errors have an antagonistic effect on the BEP performance of cooperative communication. With the relay intelligence of knowing whether symbols are detected correctly or not, detect- and-forward cooperative communication performs better than the low complexity amplify-and-forward cooperative communication

Carrier Recovery in burst-mode 16-QAM

Wireless communication systems such as multipoint communication systems (MCS) are becoming attractive as cost-effective means for providing network access in sparsely populated, rugged, or developing areas of the world. Since the radio spectrum is limited, it is desirable to use spectrally efficient modulation methods such as quadrature amplitude modulation (QAM) for high data rate channels. Many MCS employ time division multiple access (TDMA) and/or time division duplexing (TDD) techniques, in which transmissions operate in bursts. In many cases, a preamble of known symbols is appended to the beginning of each burst for carrier and symbol timing recovery (symbol timing is assumed known in this thesis). Preamble symbols consume bandwidth and power and are not used to convey information. In order for burst-mode communications to provide efficient data throughput, the synchronization time must be short compared to the user data portion of the burst. Traditional methods of communication system synchronization such as phase-locked loops (PLLs) have demonstrated reduced performance when operated in burst-mode systems. In this thesis, a feedforward (FF) digital carrier recovery technique to achieve rapid carrier synchronization is proposed. The estimation algorithms for determining carrier offsets in carrier acquisition and tracking in a linear channel environment corrupted by additive white Gaussian noise (AWGN) are described. The estimation algorithms are derived based on the theory of maximum likelihood (ML) parameter estimation. The estimations include data-aided (DA) carrier frequency and phase estimations in acquisition and non-data-aided (NDA) carrier phase estimation in tracking. The DA carrier frequency and phase estimation algorithms are based on oversampling of a known preamble. The NDA carrier phase estimation makes use of symbol timing knowledge and estimates are extracted from the random data portion of the burst. The algorithms have been simulated and tested using Matlab® to verify their functionalities. The performance of these estimators is also evaluated in the burst-mode operations for 16-QAM and compared in the presence of non-ideal conditions (frequency offset, phase offset, and AWGN). The simulation results show that the carrier recovery techniques presented in this thesis proved to be applicable to the modulation schemes of 16-QAM. The simulations demonstrate that the techniques provide a fast carrier acquisition using a short preamble (about 111 symbols) and are suitable for burst-mode communication systems

New advances in synchronization of digital communication receivers

Synchronization is a challenging but very important task in communications. In digital communication systems, a hierarchy of synchronization problems has to be considered: carrier synchronization, symbol timing synchronization and frame synchronization. For bandwidth efficiency and burst transmission reasons, the former two synchronization steps tend to favor non-data aided (NDA or blind) techniques, while in general, the last one is usually solved by inserting repetitively known bits or words into the data sequence, and is referred to as a data-aided (DA) approach. Over the last two decades, extensive research work has been carried out to design nondata-aided timing recovery and carrier synchronization algorithms. Despite their importance and spread use, most of the existing blind synchronization algorithms are derived in an ad-hoc manner without exploiting optimally the entire available statistical information. In most cases their performance is evaluated by computer simulations, rigorous and complete performance analysis has not been performed yet. It turns out that a theoretical oriented approach is indispensable for studying the limit or bound of algorithms and comparing different methods. The main goal of this dissertation is to develop several novel signal processing frameworks that enable to analyze and improve the performance of the existing timing recovery and carrier synchronization algorithms. As byproducts of this analysis, unified methods for designing new computationally and statistically efficient (i.e., minimum variance estimators) blind feedforward synchronizers are developed. Our work consists of three tightly coupled research directions. First, a general and unified framework is proposed to develop optimal nonlinear least-squares (NLS) carrier recovery scheme for burst transmissions. A family of blind constellation-dependent optimal "matched" NLS carrier estimators is proposed for synchronization of burst transmissions fully modulated by PSK and QAM-constellations in additive white Gaussian noise channels. Second, a cyclostationary statistics based framework is proposed for designing computationally and statistically efficient robust blind symbol timing recovery for time-selective flat-fading channels. Lastly, dealing with the problem of frame synchronization, a simple and efficient data-aided approach is proposed for jointly estimating the frame boundary, the frequency-selective channel and the carrier frequency offset

Implementation of Four Real-Time Software Defined Receivers and a Space-Time Decoder using Xilinx Virtex 2 Pro Field Programmable Gate Array

This paper describes the concept, architecture, development and demonstration of a real time, high performance, software defined 4-receiver system and a space time decoder to be implemented on a Xilinx Virtex 2 Pro Field Programmable Gate Array. It is designed and developed for research into receiver diversity and multiple input and multiple output (MIMO)wireless systems. Each receiver has a Freescale DSP56321 digital signal processor (DSP) to run synchronization, channel state estimation and equalization algorithms. The system is software defined to allow for flexibility in the choice of receiver demodulation formats, output data rates and space-time decoding schemes. Hardware, firmware and software aspects of the receiver and space time decoder system to meet design requirements are discussed

Optimum symbol-by-symbol detection of uncoded digital data over the Gaussian channel with unknown carrier phase

10.1109/26.310614IEEE Transactions on Communications4282543-2552IECM