Severe Fading Environments Research Articles

Reweighted Error Reducing Channel Estimator for QoS Enhancement in Wireless Nautical Radio Networks

Maritime explorations may suffer from unwanted situations such as delays, insecurity, congestions, and collisions, etc., which may arise from severe environmental conditions. Thus, there is a need to develop proper techniques that will improve the overall quality of service (QoS) of marine users. This work aims to address the limitations of wireless transmissions over maritime communication systems using channel estimation (CE) by designing and verifying the performances of two estimators named inter-symbol interference/average noise reduction (ISI/ANR) and reweighted error-reducing (RER) for aggrandizing the quality of nautical radio transmissions. To show that adopting accurate and stable CE methods can considerably increase the QoS requirements of marine networks, the performances of the proposed estimators are analysed in comparison to traditional methods under signal propagations assuming both line of sight (Rician) and Non-line of sight (Rayleigh) conditions. The adoption of a reweighting attractor in addition to the introduction of a variable leakage factor controlled log-sum penalty function to our proposed RER estimator provides additional stability for the estimation of oceanographic channels. Results obtained highlight that the proposed estimator demonstrates a performance gain of over 1 dB at a data rate of 100 bps under severe fading environments in comparison to the customary RLS technique. At an MSE of 10−2, the RER method under slow fading channels show a performance gain of about 1 dB when compared to the traditional RLS method while similarly showing superiority gain of about 0.6 dB over RLS method assuming fast fading Rayleigh channel conditions.

Observed time difference of arrival based position estimation for LTE systems: simulation framework and performance evaluation

Precise user equipment (UE) location is paramount for the reliable operation of location-based services provided by mobile network operators and other emerging applications. In this paper, the Long Term Evolution (LTE) network positioning performance based on mobile assist Observed Time Difference of Arrival (OTDoA) method is considered. The received signal time difference (RSTD) measurements are estimated by the UE using dedicated position reference signal (PRS) transmitted in the downlink frame where the reported time measurements are used by the network for location calculation. A simulation framework for the position estimation in LTE networks is presented where the LTE downlink communication link is implemented. The correlation-based method for the time of arrival measurement is used for the implementation of OTDoA. The simulation framework provides different configurations and adjustments for the system and network parameters for evaluating the performance of LTE positioning using OTDoA over multipath fading channels. Different simulation scenarios are conducted to identify the influence of various parameters of LTE system and positioning procedure setup on the positioning accuracy. Simulation results demonstrated that the positioning accuracy is highly affected by the channel fading condition where the accuracy of time of arrival measurements is deteriorated in severe fading environments; however, the positioning accuracy can be significantly improved by increasing the positioning sequences involved in the estimation process either in the frequency domain or in the time domain.

A real-time locating system for localization of high-speed mobile objects

This article addresses a novel real-time locating system for localization of high-speed mobile objects in fading environments. The proposed locating system exploits time difference of arrival measurements based on ultra-wideband signals. However, the ultra-wideband signals cause a frequency-selective fading due to their short time duration, which induces severe inter-symbol interference. Moreover, high-speed objects cause fast fading due to large Doppler spread. Therefore, the fading cases considerably reduce the localization performance. The presented locating system relies on a new localization approach in order to overcome the fading issues, which utilizes a modification of extended Kalman filtering. Especially, the suggested locating method works well even in the zero time difference of arrival case, which occurs due to a very deep fading. Experiment results verify that the proposed real-time locating system gives excellent localization performance in severe fading environments. The results also exhibit that the presented locating system is superior to the conventional locating systems in the localization of high-speed mobile objects under fading environments.

Feasibility Investigation on Cognitive Radio Network Systems: Hearsay Algorithm

In this paper Specifies, how effectively utilization of radio spectrum is important for accommodating the rapid growth of wireless communications. Cognitive Radio (CR) refers to a wireless architecture that enables dynamic spectrum access, where unlicensed devices are allowed to operate in temporally/spatially unused licensed channels. The main challenge for CR is a robust protection mechanism to guarantee an adequate Licensed Users (LU) system performance at all times. This requires a reliable spectrum sensing technique to accurately identify frequency opportunities; and an autonomous transmit power control algorithm that remains effective in severe fading environments. Cooperation among CR devices to assist the LU identification process is shown to be an imperative CR attribute for maximising the CR performance and the overall spectral utilisation The Hearsay Algorithm is a novel Transmit Power Control algorithm for CRs in a cooperative network. Each node seeking to transmit is informed of the status of the LU channel by a chain of other CR nodes, hence the term “Hearsay”.

An Improved Energy Detection Scheme based on Channel Estimation

Energy detection is most commonly employed technique for spectrum sensing in cognitive radio networks due to its low computational and implementation complexities. However, the performance of energy detector deteriorate considerably under severe fading environment especially in low SNR region. The performance metrics used for energy detector are probability of detection, probability of false alarm and number of samples requirements. The detection threshold plays a significant role in PU signal detection and thus must be chosen appropriately to meet tradeoff between probability of detection and probability of false alarm under severe fading. In this paper, we have proposed adaptive threshold detection scheme for severe fading environment to work under low SNR. The simulated results are presented to validate the proposed scheme and it has been shown that proposed scheme is more robust against noise uncertainty under severe fading environment and performs better than conventional energy detection scheme to yield same detection performance.

Analysis of Link Lifetime With Auto-Correlated Shadowing in High-Speed Railway Networks

In high-speed railway (HSR) networks, wireless link connection is fragile because of fast train speed and severe fading environment. Link lifetime is an important metric for many safety and non-safety applications that require a radio connection to be maintained for an entire communication session. An analytical modeling structure is proposed for estimating the probability density function (PDF) of link lifetime under auto-correlated shadow fading environment in HSR networks. A discrete-time Markov chain (DTMC) based model is proposed to model both pathloss and auto-correlated shadowing components into a transition process. Based on the proposed DTMC model, the PDF of the link lifetime between a communication pair can then be more accurately derived. Performance of the model is verified by a measurement campaign conducted along a part of the Zhengzhou-Xi'an HSR networks in China.

Performance of multilevel‐turbo codes with blind/non‐blind equalization over WSSUS multipath channels

AbstractIn this paper, in order to improve error performance, we introduce a new type of turbo codes, called ‘multilevel‐turbo codes (ML‐TC)’ and we evaluate their performance over wide‐sense stationary uncorrelated scattering (WSSUS) multipath channels. The basic idea of ML‐TC scheme is to partition a signal set into several levels and to encode each level separately by a proper component of the turbo encoder. In the considered structure, the parallel input data sequences are encoded by our multilevel scheme and mapped to any modulation type such as MPSK, MQAM, etc. Since WSSUS channels are very severe fading environments, it is needed to pass the received noisy signals through non‐blind or blind equalizers before turbo decoders. In ML‐TC schemes, noisy WSSUS corrupted signal sequence is first processed in equalizer block, then fed into the first level of turbo decoder and the first sequence is estimated from this first Turbo decoder. Subsequently, the other following input sequences of the frame are computed by using the estimated input bit streams of previous levels. Here, as a ML‐TC example, 4PSK 2 level‐turbo codes (2L‐TC) is chosen and its error performance is evaluated in WSSUS channel modelled by COST 207 (Cooperation in the field of Science & Technology, Project #207). It is shown that 2L‐TC signals with equalizer blocks exhibit considerable performance gains even at lower SNR values compared to 8PSK‐turbo trellis coded modulation (TTCM). The simulation results of the proposed scheme have up to 5.5 dB coding gain compared to 8PSK‐TTCM for all cases. It is interesting that after a constant SNR value, 2L‐TC with blind equalizer has better error performance than non‐blind filtered schemes. We conclude that our proposed scheme has promising results compared to classical schemes for all SNR values in WSSUS channels. Copyright © 2005 John Wiley & Sons, Ltd.

A simple joint estimation-detection technique for OFDM systems

In this work a simple approach for the joint estimation-detection in a frequency selective severe fading environment of OFDM signals adopting PSK constellations is presented. A linear predictor of suitable order is adopted for the channel estimation in the frequency domain. The predictor coefficients are estimated on the basis of a sample estimation of the autocorrelation of the channel frequency response, aided by a preliminary differential decoding, in a blockwise manner. The detection technique proposed here is based on a simple tree search where a small number of best survivor paths are maintained at each step. Despite the simplicity of above detection approach, the simulation results show its effectiveness when the number of carriers is sufficiently high. Appealing features of this method are that it does not rely on any detailed model of the fading channel and that the channel coefficients are allowed to change independently in correspondence of different blocks of OFDM symbols.

New concepts and technologies for achieving highly reliable and high-capacity multimedia wireless communications systems

The rapid growth of multimedia wireless communications services forces the development of advanced digital wireless systems with high reliability and high speed as well as flexibility for varying traffic conditions. To achieve such advanced wireless systems. New system design concepts different from the conventional ones aiming at increasing system capacity for voice transmission would be required. Since multimedia wireless communications require high quality, high speed, and high flexibility as well as temporary and spatial control of traffic under severe fading environments, the so-called conventional system design concept will be insufficient. New system design concepts and techniques for achieving highly reliable and high-capacity multimedia wireless communications are discussed using both time division multiple access (TDMA) and code division multiple access (CDMA) systems. The demands for high-speed and high-reliability mobile, personal, and multimedia wireless communications services strongly require matching with the trunk network constructed by a broadband optical fiber system or wired system.