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In this paper, we discuss an algorithm for mitigating the deleterious effects of nonlinear intersymbol interference (ISI) in nonlinear satellite channels. We call this algorithm the RAM-search/Viterbi detector (RS-VD) algorithm because it borrows the idea of employing a random-access memory (RAM) from RAM-DFE, it involves search, and it is incorporated into the Viterbi detector. The RS-VD algorithm has been devised to mitigate both precursor and postcursor (nonlinear) ISI. It also mitigates the error propagation problem that arises in making feedforward decisions (for precursor terms) in ISI cancellers used in combination with partial-response signaling. Simulation results indicate that the proposed algorithm achieves a significant performance improvement over the no-equalization case. Similar positive results are expected regardless of the nonlinearity involved as the RAM can model, via training, the nonlinearity of arbitrary nonlinear channels.

We consider the concatenation of a high rate punctured convolutional code and a precoded intersymbol interference (ISI) channel separated by an interleaver. The goal in this work is to develop design techniques for the interleaver and precoder for a given ISI channel where the design criterion is to obtain the lowest error rate floor. Our results show that an S-random interleaver is an appropriate choice for this system and that the optimal precoder may be selected through the use of what we call truncated conditional distance enumerators. Simulation results support the techniques developed.

A number of papers have recently been published on the concatenation of an outer code with a partial response (PR) channel, where the outer code is a turbo code, a convolutional code, or a low density parity check code. This paper deals with the second case assuming EPR4 and modified extended EPR4 (MEEPR4) partial response targets. The goal in this work is the joint optimization of interleaver and precoder for a fixed outer convolutional code and PR target. We introduce mathematical and algorithmic tools for accomplishing this goal and present simulation results which support our approach. Among the positive results is the ability to lower the well-known error rate floor of these concatenated schemes for arbitrary partial response channels.

We modify the iterative soft-output Viterbi algorithm (SOVA) for partial response (PR) channel and examine its performance for parallel and serial concatenated codes on a precoded Class IV partial response (PR4) channel. We present a SOVA that differs slightly from what appears in the literature and compare the performance of the two versions. Code rates of the form k/sub 0//k/sub 0/+1 (k/sub 0/=4, 8, and 64) are considered. Our simulations indicate that the loss at P/sub b/=10/sup -5/ suffered by the SOVA, relative to the APP algorithm, is at most 1.2 dB for parallel concatenations and at most 1.6 dB for serial concatenations.

Qatar is a leading natural gas producer and exporter in the world. Most of the natural gas (and oil) of Qatar is extracted from offshore wells, and then it is transferred to onshore for processing. In addition, Qatar is connected to UAE by one of the world's longest underwater pipelines (managed by Dolphin Energy), to transfer processed gas from the offshore north field to the UAE. Security of such critical offshore infrastucture against threats along with the environmental and preventive maintenance monitoring (e.g., pollution, leakage) are of utmost importance. A wireless underwater sensor network can be deployed for the security and safety of underwater pipelines. However, underwater acoustic communication brings its own challenges such as limited transmission range, low data rates and link unreliability. In this paper, we propose “cooperative communication” as an enabling technology to meet the challenging demands in underwater acoustic communication (UWAC). Specifically, we consider a multi-carrier and multi-relay UWAC system and investigate relay (partner) selection rules in a cooperation scenario. For relay selection, we consider different selection criteria, which rely either on the maximization of signal-to-noise ratio (SNR) or the minimization of probability of error (PoE). These are used in conjunction with so-called per-subcarrier, allsubcarriers, or subcarrier grouping approaches in which one or more relays are selected. In our simulation study, we choose an offshore area in the North Eastern side of Qatar (which coincides with the North Field) and conduct an extensive Monte Carlo simulation study for the chosen location to demonstrate the performance of the proposed UWAC system. Our channel model builds on an aggregation of both large-scale path loss and small-scale fading. For acoustic path loss modeling, we use the ray-tracing algorithm Bellhop software to precisely reflect the characteristics of the simulation location such as the sound speed profile, sound frequency, bathymetry, type of bottom sediments, depths of nodes, etc (See Fig.1 ). Our simulation results for the error rate performance have demonstrated significant performance improvements over direct transmission schemes and highlighted the enhanced link reliability made possible by cooperative communications (See Fig.2 ).

The performance of space-time trellis codes with antenna selection over fast fading channels is studied. A selection criterion is adopted that is based on selecting L out of the available M receive antennas that results in maximising the instantaneous signal-to-noise ratio (SNR) at the receiver, where L/spl les/M. It is shown that the resulting diversity order with antenna selection deteriorates and becomes a function of the number of selected antennas. This is unlike the case for slow fading in which the diversity order is always preserved with antenna selection when the underlying space-time code is full-rank.

In CDMA systems, assignment of the RAKE fingers to the correct multipath components is crucial for the receiver to combat fading and to take advantage of the multipath diversity. This is particularly important since the number of fingers available is normally limited in order to maintain low receiver complexity. In this paper, we introduce a new RAKE receiver finger assignment algorithm (FAA) based on estimates of the signal-to-interference ratio (SIR) per path, as opposed to signal strength in the conventional schemes. We also introduce a simple algorithm to produce these SIR estimates. A performance comparison between the proposed scheme and the already existing schemes is presented. We show that the proposed scheme provides a significant performance improvement relative to that of the conventional schemes. For instance, the proposed scheme provides gains of up to 3.0 dB at bit error rate 10/sup -4/, relative to the conventional scheme.

We investigate the impact of imperfect multipath detection on the performance of CDMA systems employing space-time spreading. Conventionally, multipath components are detected and assigned to the RAKE fingers based on their average energy content. It is shown that the errors produced by the conventional scheme in detecting the potential multipath components severely impact the performance of the receiver. To boost the performance, we introduce an improved multipath detection scheme based on estimating the interference power in the resolved paths. Our results show that the proposed scheme not only improves the bit error rate performance significantly, but also utilizes the pilot power much more efficiently.

Null-steering transmit beamformers aim to maximize the received signal power at the direction of the access point (AP) while inflicting minimal interfering effect on unintended receivers. However, existing null-steering beamformers may not be directly applied to wireless sensor networks (WSNs) as they require every node to be aware of the locations of other nodes in the network. In this paper, a novel null-steering beamformer is introduced that can be implemented in uniformly distributed WSNs in which each node is oblivious of other nodes' locations. The average beampattern expression of the proposed beamformer is derived and its properties are analytically studied. In particular, it is shown that the average beampattern of the proposed beamformer is similar to that of the conventional beamformer in directions with far angular distance from any unintended receiver. The analytical results are also adopted to design a null-steering beamformer with a single null and required average beampattern properties. The optimal null position is obtained which minimizes the maximum sidelobe of the average beampattern while having negligible deteriorating effect on the received power at the AP. Finally, computer simulations are used to validate the analytical results as well as to confirm the effectiveness of the design approach.

This paper addresses the problem of multipath detection in CDMA systems. In conventional CDMA receivers, the detection of multipath components and RAKE finger management is normally based on the received signal energy per path. These energy-based schemes essentially overlook the interference component contaminating the total received power. Consequently, they exhibit poor detection capability especially at low signal-to-interference-plus-noise ratio (SINR). In this paper, we present a new scheme for multipath detection and RAKE finger assignment that takes into consideration the interference level in each resolved path individually. The proposed scheme utilizes information provided by the pseudo random code acquisition circuit to estimate the interference power per path. To account for the hardware limitations of the receiver, a low complexity version of the proposed scheme is designed and incorporated into the receiver structure. Analytical and simulation results show that the proposed scheme provides significant improvements in the detection probability of multipath components over the energy-based schemes. For instance, our results show that the proposed scheme can achieve the same detection probability of all multipath components as that of the energy-based scheme with a saving of at least 2 dB in E/sub b//N/sub 0/. In some cases, it is shown that the improvement can be as high as 3 dB.

In this paper, we propose the concept and the means for practical testing of TCP congestion window. With this respect, we present the architecture of the hardware and expert-system-based distributed protocol analysis that we used for practical data acquisition and testing, which we conducted on a major network with live traffic (Electronic Financial Transactions data transfer), as well as the appropriate algorithm for estimating the actual congestion window size from the measured data - mainly decoding with precise time-stamps (100 ns resolution locally and 1 μs with GPS clock distribution) and expert-system-processed results of the accordingly filtered data from the special-hardware-based capture buffer. We used statistical analysis for evaluation whether the data belonged to the specific (in this case assumed normal) cumulative distribution function, or whether two data sets exhibited the same statistical distribution - the condition-sine-qua-non for a stable interval. Finally, as the measured data-based congestion window values exhibited fitting, with satisfactory statistical significance, to the truncated normal distribution, we applied the appropriate model for estimation of the relevant parameters of the congestion window distribution: its mean value and variance.