Simulation performance of optimal and suboptimal nonsynchronizedreceivers

Two optimal receivers, along with their approximations derived for the optical on-off keying channel, are briefly described. These receivers make symbol decisions in the absence of synchronization. The various approximations to the optimal rules are derived. Computer simulations are presented that compare the performance of suboptimal receivers to that of jointly optimal receivers, as well as to receivers utilizing a separate synchronizer. It is observed that one of the approximations, which is rather simple to implement, performs as well as the significantly more complicated receiver that utilizes a separate, optimal, synchronizer. Further computer simulations show that the suboptimal receivers are quite robust to nonperfect knowledge of the received signal power     

Performance based on selective multipath reception

As wireless data rate requirements increase, multipath delay spread becomes an increasingly significant limitation on the performance of wireless systems. Techniques such as RAKE reception combat time dispersion by combining multipath components. Alternative implementations of RAKE receivers isolate the strongest multipath components and then shift each component to a common timing reference. The optimal timing reference in frequency-selective fading channels remains an open problem. This paper examines the impact pulse shaping and multipath delay spread on both signal-to-noise ratio (SNR) and bit-error rate performance. The receiver being considered achieves symbol synchronization to the strongest multipath component. The performance when synchronization is achieved based on the first multipath component arrival is also found and used to illustrate performance differences. Multipath delay distributions used on the performance calculations are derived from indoor measurements. Pulse shapes considered in the analysis include root-raised cosine, raised cosine, and Gaussian filters. SNR losses are shown to range between 1-6 dB for bit rates of 10 Mb/s. Results show that synchronization of the receiver to the strongest multipath component gives a 1-3 dB advantage over synchronization to the first arriving multipath component.     

Maximum-likelihood synchronization, equalization, and sequenceestimation for unknown time-varying frequency-selective Rician channels

This paper develops a receiver structure to perform jointly maximum-likelihood (ML) synchronization, equalization, and detection of a linearly modulated signal transmitted over a time-varying frequency-selective Rician-faded channel, corrupted by additive white Gaussian noise (AWGN). The receiver is particularly suited to a fast-fading channel, where other receivers that rely on estimating the channel cannot track it quickly enough. The signal mean and autocovariance are needed, and a scheme is proposed for estimating these quantities adaptively. The receiver processes the specular and diffuse components (corresponding to the signal mean and autocovariance) separately. Processing the known specular component is the classical detection problem. The unknown diffuse component is processed by predictors. We show that the predictors can achieve synchronization in a novel manner, if synchronization is required. A union bound on the receiver's bit-error rate (BER) is derived, and it tightly bounds simulated BERs in fast-fading at high signal-to-noise ratios (SNRs)     

Rapid combined synchronization/demodulation structures for DS-CDMA systems - part II: finite data-record performance analysis

For pt.I see ibid., vol.51, p.983-94 (2003). We investigate the coarse synchronization performance of blind adaptive linear self-synchronized receivers for asynchronous direct-sequence code-division multiple-access communications under finite data record adaptation. Based on transformation noise modeling techniques, three alternative methods are developed, leading to analytical expressions that approximate the probability of coarse synchronization error of matched-filter-type and minimum-variance distortionless-response-type receivers. The expressions are explicit functions of the data record size and the filter order and reveal the effect of short data-record sample matrix-inversion implementations on the coarse synchronization performance. Besides their theoretical value, the derived expressions provide simple, highly-accurate alternatives to computationally demanding performance evaluation through simulations. The effect of the data record size on the probability of coarse synchronization error is further quantified through the use of a receiver synchronization resolution metric. Numerical and simulation studies examine the accuracy of the theoretical developments and show that the derived expressions approximate closely the actual coarse synchronization performance.     

Error Probablity for Optimal and Suboptimal Quadratic Receivers in Rapid Rayleigh Fading Channels

The exact performance of optimal and suboptimal quadratic receivers in a binary hypothesis test between jointly distributed zero mean complex Gaussian variates is derived. The probability of error is given as a function of the characteristic values of a generalized eigenvalue problem set up in terms of the covariance matrix of the received signal-plus-noise and in the matrix of the quadratic form of the receiver. The results include the exact performance of various types of suboptimal receivers including those previously derived for the envelope matched filter for MFSK and the noncoherent DBPSK receiver in rapid Rayleigh fading, nonfrequency-selective channels. Also, the performance of near-optimal stationary process-long observation time [SPLOT] receivers, "energy detectors," and other approximately optimal receivers may be calculated for noncoherent signaling in the same channel.     

Pilot-symbol-assisted coded transmission over the block-noncoherent AWGN channel

In this paper, pilot-symbol-assisted transmission in conjunction with high-performance coding over the block-independent noncoherent additive white Gaussian noise channel is investigated. Several approximate iterative receivers are proposed, which either perform carrier-phase estimation separately from detection, or joint carrier-phase estimation/decoding in an iterative fashion. The performance of the proposed receivers is analyzed using density evolution. The power allocation to the pilot symbol is quantified, and it is shown that an optimal allocation scheme exists that minimizes the overall information bit signal-to-noise ratio required for error-free communication. This optimal power allocation, which could be utilized in code design, is found to be sensitive to the channel coherence interval, as well as to the particular receiver used. In addition, a simple upper bound on the performance of any receiver that performs joint iterative carrier-phase estimation and detection, is derived. The obtained results are compared with the simulated performance of the proposed receivers.     

Diversity-assisted channel estimation and multiuser detection for downlink CDMA with long spreading codes

In downlink communication of a direct-sequence (DS) code-division multiple-access (CDMA) system, each user's short spreading codes are superimposed by base station's common long codes. This situation creates much difficulty in blind signal detection when multipath propagation occurs. However, when spatial/temporal diversity is available at the receiver, it is shown in this paper that subspace technique can be directly applied to estimate the common downlink multipath channel. Then, typical linear receivers, such as zero-forcing (ZF), minimum mean-square-error (MMSE) and RAKE receivers can be designed to detect the desired signal. Since the data covariance matrix is used but estimated from finite data samples, performance of both channel estimator and receivers gets perturbed. It is thus thoroughly and jointly analyzed by perturbation analysis. Justification of analysis and comparison of different receivers are also made through simulations.     

Combined synchronization and power control for differentially-encoded di-symbol time-division multiuser impulse radio

The differentially-encoded, di-symbol time-division multiuser impulse radio (d²TD-IR) with delay-sum autocorrelation receivers is a low complexity, high efficiency short range wireless communication technology for infrastructure networks. The d²TD-IR system is designed with the assumption that the users are perfectly synchronized. In this letter, we propose a recursive algorithm of combined synchronization and power control. Computer simulation results show that the proposed algorithm has significant performance improvement over the algorithm, in which synchronization and power control are performed separately.     

Some optimal and suboptimal receivers deriving information fromnonsynchronized Poisson data

The author derives receivers that make optimal sequence decisions from observations of modulated data in the absence of symbol synchronization for the direct-detection optical channel. He shows that, for soft-decision sequence estimation, the average number of likelihoods that must be evaluated need increase only linearly with the sequence length, with the average number of likelihood evaluations actually needed being smaller than the sequence length. Finally, the author presents computer simulations that compare the performance of two of the receivers to the performance of a receiver utilizing a separate synchronizer and to that of a receiver that is perfectly synchronized     

Compensation of IQ imbalance and phase noise in OFDM systems

Nowadays, a lot of effort is spent on developing inexpensive orthogonal frequency-division multiplexing (OFDM) receivers. Especially, zero intermediate frequency (zero-IF) receivers are very appealing, because they avoid costly IF filters. However, zero-IF front-ends also introduce significant additional front-end distortion, such as IQ imbalance. Moreover, zero-IF does not solve the phase noise problem. Unfortunately, OFDM is very sensitive to the receiver nonidealities IQ imbalance and phase noise. Therefore, we developed a new estimation/compensation scheme to jointly combat the IQ imbalance and phase noise at baseband. In this letter, we describe the algorithms and present the performance results. Our compensation scheme eliminates the IQ imbalance based on one OFDM symbol and performs well in the presence of phase noise. The compensation scheme has a fast convergence and a small residual degradation: even for large IQ imbalance, the overall system performance for an OFDM-wireless local area network (WLAN) case study is within 0.6 dB of the optimal case. As such, our approach greatly relaxes the mismatch specifications and thus enables low-cost zero-IF receivers.     

Slot Synchronization of a WDM Packet-Switched Slotted Ring

In this paper, we present two different strategies of slot synchronization in wavelength-division-multiplexing (WDM) packet-switched slotted-ring networks. Emphasis is given to the architecture behind the WDM optical network demonstrator over rings (WONDER) project, which is based on tunable transmitters and fixed receivers. The WONDER experimental prototype is currently being developed at the laboratories of Politecnico di Torino. In the former strategy, a slot-synchronization signal is transmitted by the master station on a dedicated control wavelength; in the latter, slave nodes achieve slot synchronization aligning on data packets that are received from the master. The performance of both synchronization strategies, particularly in terms of packet-collision probability, was evaluated by simulation. The technique based on transmitting a timing signal on a dedicated control wavelength achieves better performance, although it is more expensive due to the need for an additional wavelength. However, the technique based on aligning data packets that are received from the master, despite attaining lower timing stability, still deserves further study, particularly if limiting the number of wavelengths and receivers is a major requirement. Some experimental results, which were measured on the WONDER prototype, are also shown. Measurement results, together with theoretical findings, demonstrate the good synchronization performance of the prototype.     

Impact of synchronization on performance of enhancedarray-receivers in wideband CDMA networks

The synchronization performance of the receivers may severely impact the capacity or spectrum efficiency of wideband code division multiple access (CDMA) networks. Evaluations of the uplink capacity for improved spatio-temporal array receivers (STAR) and the two-dimensional RAKE (2-D-RAKE) with both perfect and active synchronization indicate that synchronization with the early-late gate component of a RAKE-type receiver may constitute a bottleneck to performance improvement. Enhancement of synchronization remains a key issue. Results also suggest that STAR offers a promising alternative to the 2-D-RAKE with early-late gate, offering an average increase in spectrum efficiency up to 100% in the presence of synchronization errors at both data rates of 9.6 and 128 kb/s. This gain further increases in high Doppler and fast multipath delay drifts. Data oversampling above the chip-rate favors STAR even more. Significant simplifications are introduced in the formulation of the STAR receiver which result in a complexity comparable to the 2-D-RAKE     

Quantized UWB Transmitted Reference Systems

Digital implementation of ultra-wideband receivers requires analog-to-digital conversion (ADC) at an extremely high speed, thereby limiting the available bit resolution. Herein, the effect of low bit resolution quantization on the performance of UWB transmitted reference receivers is investigated. It is verified that the gain of the automatic-gain-control (AGC) has a significant effect on the achievable performance. Because of the considerable performance loss of conventional transmitted reference receivers in the presence of a low resolution ADC a new family of receiver structures optimized and tailored to quantized observations is presented. In particular, the generalized- likelihood ratio test (GLRT) based on the quantized samples is derived and shown to provide modest performance gains relative to the infinite resolution GLRT rule employed on the quantized received signal suggesting that conventional receiver structures can also be employed in the presence of a low resolution ADC. Results reveal that four bits of resolution in combination with an optimal choice for the AGC gain are sufficient to closely approach the performance of an infinite resolution receiver.     

MMSE receivers for multirate DS-CDMA systems

Minimum-mean squared error (MMSE) receivers are designed and analyzed for multiple data rate direct-sequence code-division multiple-access (DS-CDMA) systems. The inherent cyclostationarity of the DS-CDMA signal is exploited to construct receivers for asynchronous multipath channels. Multiple- and single-bandwidth access are treated for both single and multicarrier scenarios. In general, the optimal receiver is periodically time-varying. When the period of the optimal receiver is large, suboptimal receivers are proposed to achieve a lower complexity implementation; the receivers are designed as a function of the cyclic statistics of the signals. In multiple chipping rate systems, the complexity of receivers for smaller bandwidth users can also be controlled by changing their front-end filter bandwidth. The effect of front-end filter bandwidth on receiver performance and system capacity is quantified for a variable chipping rate system. Analysis and simulation show that significant performance gains are realized by the periodically time-varying MMSE receivers over their time-invariant counterparts     

Novel diversity receivers in the presence of Gaussian channel estimation errors

Novel diversity receivers that operate in the presence of Gaussian channel estimation errors are proposed for L independent and identically distributed fading channels. Previous work concerned with channel estimation errors has mainly examined the performance of maximal ratio combining (MRC) with estimation errors. It is shown here that MRC is not optimal when estimation errors occur. Moreover, it is shown that better diversity receivers that operate in the presence of Gaussian channel estimation errors can be obtained by using knowledge of the channel estimate statistics. Numerical results show that the derived new diversity receivers can perform as much as 2.0 dB in signal-to-noise ratio better than the conventional MRC receiver in some cases.     

ML iterative soft-decision-directed (ML-ISDD): a carrier synchronization system for short packet turbo coded communication

Modern communication systems are required to provide services based on high data rates burst-mode packet-data transmission, capable of operating at very low SNR conditions. Turbo codes enable the operation at low SNR, close to the Shannon limit. However, carrier frequency and phase synchronization, needed for optimal coherent performance of the receiver, still remains a problem in low SNR and short bursts conditions. This paper proposes a new carrier synchronization method, the Maximum-Likelihood Iterative-Soft-Decision-Directed (ML-ISDD), which uses the turbo-decoder soft decisions to improve the carrier synchronization performance at low SNR values. The ML-ISDD method operates iteratively and jointly with the turbo decoder, enhancing both the turbo-decoder and the synchronization performance. The ML-ISDD method has been shown by simulation to significantly increase the allowed initial frequency and phase uncertainty region, thus allowing the use of very short training sequences for initial carrier synchronization.     

Terahertz Bistatic Synthetic Aperture Radar for 1-D Near-Field High-Resolution Imaging

Considering the difficult transceiver-isolation problem of the monostatic synthetic aperture radar (SAR) in the terahertz (THz) band, this paper proposes a compact THz bistatic SAR (BiSAR) geometry. The system allows the separately distributed transmitter and receivers. At the receiving end, there are a direct-wave receiver and an echo receiver, both operating at the heterodyne and in-phase mode. The echo receiver runs along a linear rail to fulfill the scene scanning, while the direct-wave one is fixed as a reference. Furthermore, assuming that the receivers are synchronized, both the problem of synchronization between the separated transmitter and receivers and the problem of timing at the signal acquisition would be solved by utilizing the high coherence between the echo and the direct wave. Based on such a system, the application of THz BiSAR for one-dimensional imaging is taken into consideration. Then, a high-resolution imaging algorithm is proposed benefitting from the total least squares estimating signal parameters via rotational invariance techniques (TLS-ESPRIT) and the spatial smoothing process (SSP). The imaging performance is then demonstrated by both simulations and the experiments in the 0.183 THz.     

Ultra-wideband transmitted reference systems

This paper derives optimal receiver structures for an ultra-wideband transmitted reference (UWB TR) system in multipath environments, based on the average likelihood ratio test (ALRT) with Rayleigh or lognormal path strength models. Several suboptimal receivers are obtained by either applying an approximation to the log-likelihood function without any specific channel statistical models or by approximating two ALRT optimal receiver structures. It is shown that the generalized likelihood ratio test optimal receiver is one of the suboptimal receiver structures in the ALRT sense. Average bit error probabilities of ALRT receivers are evaluated. Results show that ALRT optimal and suboptimal receivers derived from Rayleigh and lognormal models can perform equally well in each other's environments. This paper also investigates ad hoc cross-correlation receivers in detail, and discusses the equivalence between cross-correlation receivers and one theoretically derived ALRT suboptimal receiver. Results show that the noise /spl times/ noise term in a cross-correlation receiver can be modeled quite accurately by a Gaussian random variable when the noise time/spl times/bandwidth product is large, and cross-correlation receivers are suboptimal structures which have worse performance than ALRT receivers.     

Cross-Modulation Interference and Mitigation Technique for Ultrawideband PPM Signaling

The detection issues of ultrawideband (UWB) signals depend on the type of modulation scheme that is used during the transmission. Cross-modulation interference (CMI) is a problem that is specific to UWB pulse-position-modulation (PPM) signaling. In this paper, the effects of CMI on the performance of noncoherent UWB receivers are analyzed. The probabilities of error for transmitted-reference (TR) and energy detector (ED) receivers in the presence and absence of CMI are derived. Optimal and suboptimal CMI avoidance algorithms, which are based on novel acquisition techniques, are proposed for Rake receivers. The results show that the performance degradation in both receivers, which is due to the CMI effects, can be significant, depending on the modulation index. TR receivers still can be functional in the presence of CMI, and the target performance level determines the modulation index to be used. It is unlikely that effects of CMI on the performance of ED receivers in the presence of CMI are more severe relative to TR receivers, and the performance level is not acceptable. As a result, PPM signaling is not an appropriate modulation technique for ED receivers that are operating in the CMI region, unless CMI mitigation algorithms can be developed. Furthermore, the proposed optimal and suboptimal algorithms are two promising schemes for avoiding the CMI effects and, consequently, for improving the performance of Rake receivers operating in the CMI region.     

Performance analysis of asymptotically optimal noncoherentdetection of trellis-coded multi-amplitude/-phase modulation signals inGaussian noise and ISI channels

Performance upper bounds for noncoherent receivers employed in conjunction with single and multi-amplitude/-phase signals, transmitted over time dispersive and Gaussian noise channels are derived. Based upon a metric which has been previously derived by the authors, we present analytical expressions and computer generated results for the performance of asymptotically optimal noncoherent detection over such channels. As a typical application of the developed theoretical analysis, we consider wideband telecommunication systems. Where time dispersion resulting in intersymbol interference (ISI) is one of the significant sources of system performance degradation. Numerical evaluation of the optimal noncoherent decoding algorithms, shows the proposed bounds to be an effective and efficient means of evaluating the performance of the noncoherent receivers under investigation. Using the derived bounds, performance evaluation results for modulation schemes such as π/4-shift DQPSK (differential quadrature phase shift keying), 8- and 16-DQAM (differential quadrature amplitude modulation), at very low bit-error rates (BER), which would otherwise pose impractically high computational loads when using Monte-Carlo error counting techniques, are readily obtained. At BER>10^-4 evaluation results generated via computer simulation have verified the tightness of the bounds