Frequency-selective Slow Fading Channels Research Articles

Energy and delay constrained scheduling of real time traffic over OFDM communication system with non-causal channel information

A point to point wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM) is considered, wherein R bits of information have to be scheduled within a hard delay deadline of T time slots over a frequency selective slow fading channel. The scheduling objective is to minimize the energy expenditure with non-causal knowledge of channel state over a limited time horizon, number of bits to be served and delay constraints. We assume a convex monomial energy-bit cost function and propose novel efficient two stage heuristic algorithm that allocate the R bits on the time slots, followed by greedy optimal bit loading over the OFDM subcarriers. An analytical model is developed and numerical simulations are performed to validate the proposed approach as well as the results show that our algorithms outperform existing approaches in published literature by up to 10 dB in power margin on an average in high load conditions while meeting the deadline of T timeslots.

Performance of multi-relay coded cooperative diversity in asynchronous code-division multiple-access over fading channels

In this study, multi-relay decode-and forward (DAF) cooperative networks employing convolutional coding are studied for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems over frequency-selective slow fading channels. The authors show that the full benefits of coded cooperative diversity cannot be achieved if no multi-user interference suppression is employed at the cooperative end. The authors consider two scenarios; perfect and imperfect inter-user channels. In that, the bit-error-rate performance of the cooperative system is investigated for an uplink transmission where a decorrelator detector is used at both the relay and base station receivers. Both simulation and analytical results are presented to demonstrate the diversity gains of the convolutionally coded cooperative network.

A Channel-Estimation and Data-Detection Scheme for Multiuser MIMO-CDMA Systems in Fading Channels

In this paper, we examine the effect of channel estimation errors on the performance of multiple-input-multiple-output (MIMO) systems that employ code-division multiple-access (CDMA) transmission. Channel estimation based on training techniques has widely been considered throughout the literature. However, employing these training techniques in MIMO-CDMA systems degrades system performance due to multiuser interference. This degradation is clear as the diversity advantage of the MIMO system diminishes with the increased level of interference. As a remedy to this problem, we propose a channel-estimation and data-detection scheme based on the superimposed training technique for space-time spreading systems. The proposed scheme enhances the performance of the space-time system by eliminating the interference effect from both the channel and data estimates using two decorrelators: channel and data decorrelators. We investigate the performance of the proposed estimation technique considering an asynchronous CDMA uplink transmission over frequency-selective slow-fading channels. In particular, we analyze the bit-error-rate (BER) performance of the multiuser system with two-transmit-antenna and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> -receive-antenna configuration over Rayleigh fading channels. Compared with other conventional estimation techniques, our results show that the proposed estimation technique is more robust to channel-estimation errors. Furthermore, both simulations and analytical results indicate that full system diversity is achieved.

Adaptive interleaver based on rate-compatible punctured convolutional codes

This letter focuses on the design of an adaptive bit-interleaved coded modulation (BICM) scheme for frequency selective slow fading channels where the transmitter has certain knowledge of the channel response. In particular, we consider the design of the interleaver stage for a specific convolutional code operating with OFDM modulation. The adaptive interleaver uses the puncturing tables of the rate-compatible punctured convolutional codes (RCPC codes) to rearrange the bits as a function of the fading values and the specific constellation. The performance of different interleavers are compared, revealing that the adaptive RCPC-based interleaver produces larger Euclidean distances between the received codewords and reduces the packet error rate (PER), specially when the number of deep-faded subcarriers increases. Numerical results also evidence the importance of the interleaver choice when comparing the performance of different power allocation strategies.

Analysis of Electric Field and Potential Distributions along Surface of Silicone Rubber Insulators under Various Contamination Conditions Using Finite Element Method

Abstract —A fast adaptive Tomlinson Harashima (T-H) precoderstructure is presented for indoor wireless communications, wherethe channel may vary due to rotation and small movement of themobile terminal. A frequency-selective slow fading channel whichis time-invariant over a frame is assumed. In this adaptive T-Hprecoder, feedback coefficients are updated at the end of everyuplink frame by using system identification technique for channelestimation in contrary with the conventional T-H precoding conceptwhere the channel is estimated during the starting of the uplink framevia Wiener solution. In conventional T-H precoder it is assumedthe channel is time-invariant in both uplink and downlink frames.However assuming the channel is time-invariant over only one frameinstead of two, the proposed adaptive T-H precoder yields betterperformance than conventional T-H precoder if the channel is variedin uplink after receiving the training sequence. Keywords —Tomlinson-Harashima precoder, Adaptive channel es-timation, Indoor wireless communication, Bit error rate.

On Slow-Fading MIMO Systems With Nonseparable Correlation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In a frequency-selective slow-fading channel in a multiple-input multiple-output (MIMO) system, the channel matrix is of the form of a block matrix. A method is proposed to calculate the limit of the eigenvalue distribution of block matrices if the size of the blocks tends to infinity. Asymptotic eigenvalue distribution of <emphasis><formula formulatype="inline"><tex>$HH^*$</tex> </formula></emphasis> is also calculated, where the entries of <emphasis><formula formulatype="inline"><tex>$H$</tex></formula></emphasis> are jointly Gaussian, with a correlation of the form <emphasis><formula formulatype="inline"><tex>$E[h_{pj}\bar h_{qk}]= \sum_{s=1}^t \Psi^{(s)}_{jk}\mathhat\Psi^{(s)}_{pq}$</tex></formula></emphasis> (where <emphasis><formula formulatype="inline"><tex>$t$</tex></formula></emphasis> is fixed and does not increase with the size of the matrix). An operator-valued free probability approach is used to achieve this goal. Using this method, a system of equations is derived, which can be solved numerically to compute the desired eigenvalue distribution. </para>

Performance analysis of cellular CDMA high‐speed data services

AbstractThis paper investigates the forward‐link peak and average data rates, throughput, and coverage of a cellular CDMA system for delivering high‐speed wireless data services. The analysis takes into account major aspects commonly found in the forward data channel and applies the generalized Shannon capacity formula for multi‐element antenna (MEA) systems. The study focuses on the physical layer and is flexible for various propagation environments, antenna configurations, multicode allocations, user distributions, and cell site configurations. Numerical results for various multicode allocations are presented for a system model with two‐tier interfering cells operating under a frequency selective slow fading channel with propagation environments specified in the Recommendation ITU‐R M.1225. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Efficient image transmission scheme in FMT system

An efficient image transmission scheme is proposed based on byte partition and adaptive sub-channel distribution technique in Filtered MultiTone (FMT) system over frequency selective slow fading channel. According to the simulation results and analysis of a typical image with matlab, improvement in Peak Signal to Noise Ratio (PSNR) of the received image and low complexity for equalization is demonstrated remarkably. Comparing with no adaptive and no actual channel equalization scheme, the proposed scheme saves over 6 dB when PSNR=40 dB.

Channel precoding with small envelope variations for π=4-QPSK and MSK over frequency-selective slow fading channels

This paper presents channel precoding schemes to combat intersymbol interference (ISI) over a frequency-selective slow fading channel in wireless communication systems using /spl pi//4 shifted quadrature phase-shift keying or minimum shift keying. Based on the dimension partitioning technique, the precoders predistort the phase of the transmitted symbol in the forward link to combat ISI, keeping the transmitted symbol amplitude constant. The proposed schemes can ensure the stability of the precoder even in equalizing a nonminimum-phase channel, combat ISI without increasing the complexity of portable unit receivers, and reduce the envelope variations of transmitted signals such that power-efficient nonlinear amplifiers can be used for the precoded signals without causing a significant undue distortion. Theoretical analysis and simulation results are presented to demonstrate the effectiveness of the proposed channel precoding schemes.

Soft-detection phase precoding with MPSK-TCM for ISI channel

Trellis-coded modulation (TCM) concatenated with phase precoding is proposed to alleviate the effect of intersymbol interference (ISI) in frequency selective slow fading channels in wireless communications. Simulation results show that gains of 6.5 and 2.5 dB at a bit error rate (BER) of 10/sup -6/ are achieved by the hard detection method for 8PSK-TCM with four and eight state codes, respectively. The comparison is made with respect to a system employing post-equalization with a sub-optimum soft output equalizer and the same TCM schemes. Moreover, soft detection of the precoded signal is also carried out, which results in an additional gain of 4 dB. Finally, the simulation performance is compared with the analytical bound and they match quite well.

On the diversity, bandwidth, and performance of digital transmission over frequency-selective slow fading channels

The issues of diversity reception, bandwidth utilization, and performance evaluation of digital transmission over slowly time-varying frequency-selective fading channels are investigated, with a transmit pulse design that is more general than previously considered. In particular, the tradeoffs among performance, bandwidth, and complexity are addressed. Three types of transmit pulses are considered and compared, including the raised cosine (RC), square-root RC (SRRC), and Gaussian pulses. Union bounds on the symbol and burst error rate are derived for maximum likelihood sequence estimation (MLSE) based on exact pairwise error probabilities for burst-mode transmission. Explicit and implicit diversity are employed to improve the system performance. The results show that the implicit diversity is flexible and easy to exploit and is a good complement to the space or other explicit diversity to further improve the system performance.

Adaptive bit loading scheme using COFDM at low elevation over LEO satellite communication channel

Much interest has been shown in employing LEO satellite communication systems. A novel algorithm is proposed for adaptive bit loading (multilevel modulation assignment) between COFDM subcarriers in a frequency selective fading channel. Two-dimensional alignment has been carried out. The system performance is greatly improved at low elevation angles compared to that using fixed bit loading. Furthermore, the comparison shows that this adaptive bit loading COFDM system is suitable for frequency selective slow fading channels, which have time-varying deep nulls in the frequency response.

A new lattice polynomial perceptron and its applications to frequency-selective fading channel equalization and ACI suppression

It is expected that the available spectrum for the proposed personal communication networks will soon be at a premium as the user population increases, and using multilevel quadrature amplitude modulation (MQAM), an bandwidth efficient transmission method for digital signals, may significantly ease the problem. A new lattice polynomial perceptron (LPP), with faster and less input signal-dependent convergence behavior, is presented and applied to frequency-selective slow fading channel equalization and adjacent-channel interference (ACI) suppression in a 16-QAM system. Computer simulation results are given, which shows that, in a 16-QAM system, the performance of LPP is clearly superior to that of the other structures.