Replica Of Signal Research Articles

The Extended Invariance Principle for Signal Parameter Estimation in an Unknown Spatial Field

This paper treats the problem of joint estimation of time-delay, Doppler frequency, and spatial (direction-of-arrival or DOA) parameters of several replicas of a known signal in an unknown spatially correlated noise field. Both spatially unstructured and structured data models have been proposed for this problem and corresponding maximum likelihood (ML) estimators have been derived. However, structured models require a high computational complexity and are sensitive to the antenna array response, while unstructured models are unable to achieve good performance in some scenarios. In this paper, it is shown how the extended invariance principle (EXIP) can be applied to obtain estimates with the quality of a spatially structured model, but with much lower complexity than directly utilizing a structured model and with greater robustness to errors in the model of the array response. EXIP improves the quality of the time-delay and Doppler frequency estimates obtained with a spatially unstructured model by introducing DOA estimates which are obtained in a second step through an innovative reparametrization. Simulation results for time-delay and Doppler frequency estimation for Global Positioning System (GPS) signals are presented and confirm that the proposed two-step approach attains the Cramer-Rao lower bound (CRLB) of the spatially structured model.

A waveguide invariant adaptive matched filter for active sonar target depth classification

This paper addresses depth discrimination of a water column target from bottom clutter discretes in wideband active sonar. To facilitate classification, the waveguide invariant property is used to derive multiple snapshots by uniformly sub-sampling the short-time Fourier transform (STFT) coefficients of a single ping of wideband active sonar data. The sub-sampled target snapshots are used to define a waveguide invariant spectral density matrix (WI-SDM), which allows the application of adaptive matched-filtering based approaches for target depth classification. Depth classification is achieved using a waveguide invariant minimum variance filter (WI-MVF) which matches the observed WI-SDM to depth-dependent signal replica vectors generated from a normal mode model. Robustness to environmental mismatch is achieved by adding environmental perturbation constraints (EPC) derived from signal covariance matrices averaged over the uncertain channel parameters. Simulation and real data results from the SCARAB98 and CLUTTER09 experiments in the Mediterranean Sea are presented to illustrate the approach. Receiver operating characteristics (ROC) for robust waveguide invariant depth classification approaches are presented which illustrate performance under uncertain environmental conditions.

SEP calculations for coherent M-ary FSK in different fading channels with MRC diversity

In this paper, the authors derive symbol error probability (SEP) expressions for coherent M-ary frequency shift keying (MFSK) modulation schemes in multipath fading channels. The multipath or small-scale fading process is assumed to be slow and frequency non-selective. In addition, the channel is also subjected to the usual degradation caused by the additive white Gaussian noise (AWGN). Different small-scale fading statistics such as Rayleigh, Rician (Nakagami-n), Hoyt (Nakagami-q), and Nakagami-m have been considered to portray diverse wireless environments. Further, to mitigate fading effects through space diversity, the receiver front-end is assumed to be equipped with multiple antennas. Independent and identically distributed (IID) as well as uncorrelated signal replicas received through all these antennas are combined with a linear combiner before successive demodulation. As the detection is coherent in nature and thus involves phase estimation, optimum phase-coherent combining algorithms, such as predetection maximal ratio combining (MRC), may be used without any added complexity to the receiver. In the current text, utilizing the alternate expressions for integer powers (1≤n≤4) of Gaussian Q function, SEP values of coherent MFSK are obtained through moment generating function (MGF) approach for all the fading models (with or without MRC diversity) described above. The derived end expressions are composed of finite range integrals, which can be numerically computed with ease, dispenses with the need of individual expressions for different M, and gives exact values up to M=5. When the constellation size becomes bigger (M≥6), the same SEP expressions provide a quite realistic approximation, much tighter than the bounds found in previous literatures. Error probabilities are graphically displayed for each fading model with different values of constellation size M, diversity order L, and for corresponding fading parameters (K, q, or m). To validate the proposed approximation method extensive Monte-Carlo simulations were also performed, which show a close match with the analytical results deduced in the paper. Both these theoretical and simulation results offer valuable insight to assess the efficacy of relatively less studied coherent MFSK in the context of the optimum modulation choice in wireless communication. Copyright © 2010 John Wiley & Sons, Ltd.

Environmentally tolerant waveguide‐invariant target depth classification for active sonar.

Shallow‐water environments produce active sonar returns with many target‐like returns from bottom clutter. Scatterer depth classification methods which can discriminate bottom clutter from water column targets are thus critical for controlling false alarms. In recent work, the waveguide invariant (WI) property of shallow‐water channels has been used to obtain multiple snapshots of frequency‐domain target return data from a single active sonar ping. These snapshots are, in turn, used to estimate a waveguide invariant spectral density matrix (WI‐SDM), which can serve as a basis for depth classification. One method employing the WI‐SDM performs minimum variance filtering (MVF) matched to depth‐dependent signal replicas derived from a normal mode model. While MVF is capable of depth classification when the environment is known, it is sensitive to mismatch when the channel parameters are uncertain. In this paper, robustness of the WI MVF to mismatch is achieved by using environmental perturbation constraints derived from a WI‐SDM for the signal averaged over the uncertain channel parameters. Simulated and real data results are presented from the CLUTTER‐09 experiment in the Mediterranean Sea. [Work sponsored by ONR.]

Interpretation of the compressed pulse output for broadband acoustic scattering from inhomogeneous weakly scattering objects.

Pulse compression signal processing techniques are commonly used to identify dominant scattering features for weakly scattering marine organisms. In cases where the organism can be accurately represented as homogeneous (e.g., eupausiids), the separation of two distinct peaks in the compressed pulse output (CPO) envelope produced by the front and back water‐tissue interfaces can be used to estimate the size of the organism at some angles of orientation. In order to investigate the effect of internal inhomogeneities on the scattering, a 3‐D distorted‐wave Born approximation‐based numerical model has been used to model the CPO due to scattering from weakly scattering inhomogeneous objects. It has been found that when the organism is highly inhomogeneous (e.g., sea‐water filled cavities in squid), the CPO envelope does not provide an accurate estimate of size. Characteristics of the replica signal can further complicate the issue by introducing spurious peaks with high side lobes or smearing out the CPO peaks from internal inhomogeneities when the main lobe is wide.

Cooperative Multi-relay Scheme for Secondary Spectrum Access

In this paper, we propose a cooperative multi-relay scheme for a secondary system to achieve spectrum access along with a primary system. In the primary network, a primary transmitter (PT) transmits the primary signal to a primary receiver (PR). In the secondary network, N secondary transmitter-receiver pairs (ST-SR) selected by a centralized control unit (CCU) are ready to assist the primary network. In particular, in the first time slot, PT broadcasts the primary signal to PR, which is also received by STs and SRs. At STs, the primary signal is regenerated and linearly combined with the secondary signal by assigning fractions of the available power to the primary and secondary signals respectively. The combined signal is then broadcasted by STs in a predetermined order. In order to achieve diversity gain, STs, SRs and PT will combine received replicas of the primary signal, using selection combining technique (SC). We derive the exact outage probability for the primary network as well as the secondary network. The simulation results are presented to verify the theoretical analyses.

Outage Probability of Selection Cooperation With MRC in Nakagami-$m$ Fading Channels

Selection cooperation has been shown to be an effective way to implement cooperative diversity in wireless networks. In this letter, we investigate the outage performance of decode-and-forward selection cooperation with optimal maximal ratio combining (MRC) receiver at the destination to combine the signal replicas available from the source and the selected relay in Nakagami-/mbim fading channels with arbitrary real-valued/mbim. In particular, an exact closed-form expression for the outage probability is derived for integer values of fading severity parameter/mbim and an infinite summation expression for the noninteger situation. Our analysis extends previous results pertaining to selection combining (SC) receiver at the destination. Simulation and numerical results are presented to show the validity of the analytical results, as well as the effect of the two hops unbalanced fading conditions on the performance gain of MRC with respect to SC.

A Contribution to Study the Effect of Multipath Coherent Crosstalk Due to Optical Network Nodes on DPSK Signals

Multipath coherent crosstalk, due to multiple replicas of the main signal in optical networks, is examined in terms of the potential impact on the performance of optical differential phase-shift keying signals. Analytical expressions are derived and presented for both the moment generating function of the decision variable and the average error probability. A detailed comparison with incoherent crosstalk is also performed. Using symbol-conditioned analysis for this crosstalk, it is shown that for low optical signal-to-noise ratio penalties, the coherent crosstalk is more damaging than the incoherent one.

MRC in the Presence of Asynchronous Cochannel Interference Over Frequency-Selective Rayleigh Fading Channels

This paper studies the performance of Rake receivers with maximal ratio combining (MRC) in the presence of cochannel interference over slowly varying frequency-selective fading channels with combined spatial receive antenna diversity and temporal multipath diversity. Two models for the instantaneous powers of the residual multipath components and the resolved cochannel interfering signals, which represent the overall interfering signals, are employed. The first model considers the phase asynchronism between the desired signal replicas and interfering signals, whereas the second model assumes that the instantaneous powers of interfering signals incoherently add up. The desired signal replicas and interfering signals are assumed to experience statistically independent Rayleigh fading processes with arbitrary statistics. For the system model previously described, new closed-form expressions for the probability density function (pdf) of the combined signal-to-interference-plus-noise ratio (SINR) are derived. These expressions are then used to obtain formulas for various performance measures. The case study under consideration and the corresponding derived expressions for the statistical measures generalize many previous results and can usefully be employed to investigate the impact of various system and channel parameters on the performance of Rake receivers with MRC in the presence of cochannel interference.

Performance of Cellular Mobile Systems Employing SNR-Based GSC in the Presence of Rayleigh and Nakagami-$q$ Cochannel Interferers

This paper studies the performance of cellular mobile systems employing a signal-to-noise ratio (SNR)-based generalized selection combining (GSC) algorithm in the presence of background white noise and multiple cochannel interfering signals. Closed-form expressions are derived for the received signal-to-interference-plus-noise ratio (SINR) distribution and outage probability, considering the case when the interfering signals experience mixed Rayleigh and Nakagami-q fading conditions, and the desired signal replicas are attenuated by flat Rayleigh fading. These expressions are obtained via the new representation of the total interference-to-noise ratio (INR) distribution for the mixed mode of fading experienced by different interferers. The derived expressions for the aforementioned statistical measures are valid for an arbitrary number of interferers and system diversity order and can be employed to investigate the impact of various system and fading parameters on the performance measures. Several case studies can be deduced from the results presented in this paper, including interference-limited systems, wherein the impact of white noise can be neglected.

Contrast agent response to chirp reversal: simulations, optical observations, and acoustical verification

Active response of a microbubble is characterized by its resonance behavior where the microbubble might oscillate after the excitation waveform has been turned off. We investigate in this paper an excitation approach based on this resonance phenomenon using chirps. The technique, called chirp reversal, consists in transmitting a first excitation signal, the up-sweep chirp (UPF) of increasing frequency with time, and a second excitation signal, the down-sweep (DNF) that is a replica of the first signal, but time reversed with a sweep of decreasing frequency with time. Simulations using a modified Rayleigh-Plesset equation were carried out to determine bubble response to chirp reversal. In addition, optical observations and acoustical measurements were carried out to corroborate the theoretical findings. Results of simulations show differences between bubbles' oscillations in response to up-sweep and down-sweep chirps mainly for transmitted center frequencies above the bubble's resonance frequency. Bubbles that are at resonance or far away from resonance engender identical responses. From the optical data, the larger bubbles showed different dynamics when up-sweep or down-sweep chirps were transmitted. Smaller bubbles (< 2 microm diameter) appear to be less sensitive to frequency sweep at 1.7 MHz center frequency. However, driven at a higher center frequency, smaller bubbles tend to be more sensitive. These results were confirmed through the acoustical measurements. We concluded that simulations and experimental data show that significant differences might be observed between bubbles' responses to UPF and DNF chirps. We demonstrate in this study that, for an optimal use of chirp reversal, the transmit frequency should be higher than the resonance frequency of the contrast microbubbles.

MLD INTERFERENCE CANCELER FOR M-ARRAY QAM SIGNALS APPLIED TO FWA SYSTEMS

We propose a maximum likelihood detection (MLD) interference canceler applied to fixed wireless access (FWA) systems. There are two major degradation factors associated with a conventional MLD canceler. One is the decision error caused by constellation ambiguity when distinguishing between desired and interference signal components for M-array QAM signals. The novelty of our interference canceler is its use of our antenna switching technique to reduce constellation ambiguity. The phase difference between the desired and interference signals received by the main and sub antennas depends on the path gap between desired and interference signal transmitters to the two receiving antennas. Cancelation performance is improved by using the phase difference that maximizes the separation of the received signal points. Two key points of the proposed technique are the path gap and the threshold for antenna switching. The second degradation factor is the degradation in interference signal replica accuracy caused by the phase rotation of the interference signals. An automatic phase control (APC) circuit estimates and compensates for this phase rotation, enabling the interference canceler with APC to achieve excellent bit error rate (BER) performance. Computer simulations show that our interference canceler improves the BER performance.

Complexity reduction of signal detection by exploiting correlation characteristics of spreading sequences

AbstractIn this paper, we propose an approach to reduce the multi‐user detection (MUD) complexity based on user grouping and signal replica classification by exploiting the correlation characteristics of spreading sequences in multipath fading channels. The spreading sequences are constructed from inter‐group complementary codes with a sparse and regular correlation matrix and inherit its attractive auto/cross‐correlation properties. Users are first partitioned into independent user groups according to whether or not there is interference among them, and then the replicas of user signals from the same user group are further classified into independent replica classes. The MUD is carried out within each low‐dimensional user group or replica class, respectively, reducing the MUD complexity substantially. This approach can be applied to most of the existing MUD algorithms for complexity reduction, and in this paper optimal MUD and multi‐stage MUD are exemplified. The analytical and simulated results demonstrate that this approach can reduce the MUD complexity significantly under any load conditions without performance loss. Copyright © 2009 John Wiley &amp; Sons, Ltd.

A spectrally efficient frequency diversity technique for single‐carrier modulations with frequency division multiplexing

AbstractWhen the number of relevant propagation paths between the transmitter and the receiver is small and/or when they are not separable (i.e. the difference between the corresponding delays is smaller than the symbol duration), we can have deep fading effects and poor performance, especially for slow‐varying channels. In this case, the use of diversity techniques is strongly recommendable.In this paper, we consider an single‐carrier‐based (SC) block transmission, employing frequency domain equalisation (FDE) receivers and we present a frequency diversity technique that allows high spectral efficiencies. As with conventional frequency diversity techniques, L replicas of a given signal are transmitted at L different frequencies. However, contrarily to conventional frequency diversity schemes, up to L users can share this set of L frequency bands. Since this leads to strong interference levels, we propose an iterative, frequency‐domain receiver where all the users sharing the L frequencies are jointly detected.Our performance results show that the proposed frequency diversity scheme allows high diversity gains, while keeping the spectral efficiency of conventional SC schemes. Moreover, we can cope with channels that can be severely time‐dispersive and/or can have deep fading effects. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Performance of Single and Multichannel Coherent Reception under Rician Fading

In the present work, simple closed-form series solutions for the average error rate of several coherent modulation schemes such as, binary phase shift keying (BPSK), binary frequency shift keying (BFSK), differential binary phase shift keying (DBPSK), quadrature phase shift keying (QPSK), offset-QPSK, minimum shift keying (MSK), and square M-ary quadrature amplitude modulation (M-QAM), operating over frequency non-selective slow Rician fading channel and corrupted by additive white Gaussian noise (AWGN) are derived. Further, to improve the link quality, receiver antenna space diversity is considered, where multiple independent and identically distributed (i.i.d.) as well as uncorrelated signal replicas are combined before successive demodulation. The proposed linear predetection combiner follows optimum maximal ratio combining (MRC) algorithm. Starting from a novel unified expression of conditional error probability the error rates are analysed using probability density function (pdf) based approach. The derived end expressions, consisting of rapidly converging infinite series summations of Gauss hypergeometric function, are accurate, free from any numerical integration and general enough, as it encompasses as special situations, some cases of non-diversity, non-fading AWGN and Rayleigh fading. Symbol or, bit error probabilities (SEP/BEP) are graphically displayed against signal to noise ratio (SNR) per bit per channel for all the digital modulation schemes stated above with different values of diversity order L and varying values of the channel specular-to-scatter ratio or, the Rician parameter K, as found from the measured statistics of mobile and indoor wireless channels. In addition, to examine the dependence of error rate performance of M-QAM on the constellation size M, numerical results are plotted for various values of M. Selected simulation results are also provided to verify the analytical deductions. The series solutions presented in current text realize a trade-off between precision and complexity and offers valuable insight into the performance evaluation over a fading channel in a unified manner.

A Novel SNR Estimation Technique Associated with Hybrid ARQ

By using multiple repeated signal replicas to formulate the accumulative observed noisy signal sequence (AONSS) or the differential observed noisy signal sequence (DONSS) in the hybrid ARQ system, a novel data-aided maximum likelihood (DA ML) SNR estimation and a blind ML SNR estimation technique are proposed for the AWGN channel. It is revealed that the conventional DA ML estimate is a special case of the novel DA ML estimate, and both the proposed DA ML and the proposed blind ML SNR estimation techniques can offer satisfactory SNR estimation without introducing significant additional complexity to the existing hybrid ARQ scheme. Based on the AONSS, both the generalized deterministic and the random Cramer-Rao lower bounds (GCRLBs), which include the traditional Cramer-Rao lower bounds (CRLBs) as special cases, are also derived. Finally, the applicability of the proposed SNR estimation techniques based on the AONSS and the DONSS are validated through numerical analysis and simulation results.

SNR and SINR-based selection combining algorithms in the presence of arbitrarily distributed co-channel interferers

The performance of selection combining (SC) diversity in the presence of additive white Gaussian noise as well as multiple co-channel interfering signals over fading channels is analysed. The analysis considers two SC algorithms, namely, the signal-to-noise ratio-(SNR) based SC algorithm and the signal-to-interference-plus-noise ratio (SINR)-based SC algorithm. Closed-form expressions for the distribution of the resulting SINR and the outage probability performance are derived for both SC algorithms. The derived expressions are valid for arbitrary number of interfering signals, and when the desired signal replicas experience independent Rayleigh fading with generalised statistics. In addition, they are applicable for the general case when the interfering signals are subject to arbitrary fading models, which include the cases of identical, non-identical, correlated and mixed-mode fading scenarios. Numerical results are presented to examine the impact of different fading conditions and system parameters on the outage performance for both SC algorithms under consideration.

Four-Wave Mixing of Pulsed Signal in Dispersion-Shifted Fiber With Pump Depletion

The output signal and idler pulses generated by dispersion-shifted-fiber-based four-wave mixing (FWM) with pulsed signal input are analyzed. A set of analytical expressions for the amplitude and phase of the output pulses is derived. Analytical analysis shows that the generated idler pulse is far away from the phase conjugation of the input signal, and that the output signal is not an amplified replica of the input signal. These phenomena are due to both the phase distortion caused by self- and cross-phase modulation and the temporal envelope broadening resulting from pump depletion. Besides, in the frequency domain, the spectral inversion between signal/idler will be destroyed. The implications of the phase and shape distortions in all-optical signal processing by FWM are discussed. The induced phase distortions of idler pulses in dual pumps case are found to be much smaller than those of the single-pump case.

Analysis of an Adaptive Wideband Duplexer With Double-Loop Cancellation

An active wideband duplexer reduces the need for multiple switched duplexing filters in multiband software radio implementations. The technique is based on a two-step isolation process: 1) a low isolation device and 2) an active double-loop canceling technique that leads to two nulls, one at the desired receive frequency and the other at the transmit frequency. Two cancellation paths generate replicas of the residual transmitter interference signal and noise signal for subtraction from the output of the low isolation device. The feasibility of wideband cancellation mainly depends on the duplexing offset and the operating range of delays in the main path and the cancellation paths. The adjustment range of the coefficients in the cancellation path puts further constraints on these delays. This paper presents a theoretical analysis of double-loop cancellation using geometrical representations. The delay boundaries are derived for a given adjustment range of the vector attenuators (coefficients) used in the cancellation paths. The relationship between cancellation bandwidth and residual cancellation level is shown to be linear (6 dB/octave) and related to the loop delays and the duplexing offset.

Performance Analysis of Decode-and-Forward Relaying with Selection Combining

Distributed spatial diversity systems utilize multiple communication nodes to create independent signal replicas that are combined at the destination. Practical diversity receivers often employ simple combining techniques especially if comparable performance to more complex optimum combining can be achieved. The performance of decode-and-forward relaying that uses selection combining is investigated. In particular, a closed- form expression for the outage probability of a system with an arbitrary number of relay nodes is derived. By comparing the performance of a system that uses selection combining to one that uses optimum maximal ratio combining, it is shown that the performance loss due to using a less complex combiner is not substantial.