Wireless Fading Channels Research Articles

Human Body Shadowing in Cellular Device-to-Device Communications: Channel Modeling Using the Shadowed $\kappa-\mu$ Fading Model

Using device-to-device communications as an under- lay for cellular communications will provide an exciting opportu- nity to increase network capacity as well as improving spectral efficiency. The unique geometry of device-to-device links, where user equipment is often held or carried at low elevation and in close proximity to the human body, will mean that they are particularly susceptible to shadowing events caused not only by the local environment but also by the user's body. In this paper, the shadowed κ − μ fading model is proposed, which is capable of characterizing shadowed fading in wireless communication channels. In this model, the statistics of the received signal are manifested by the clustering of multipath components. Within each of these clusters, a dominant signal component with arbitrary power may exist. The resultant dominant signal component, which is formed by the phasor addition of these leading contributions, is assumed to follow a Nakagami-m distribution. The probability density function, moments, and the moment-generating function are also derived. The new model is then applied to device-to-device links operating at 868 MHz in an outdoor urban environment. It was found that shadowing of the resultant dominant component can vary significantly depending upon the position of the user equipment relative to the body and the link geometry. Overall, the shadowed κ − μ fading model is shown to provide a good fit to the field data as well as providing a useful insight into the characteristics of the received signal.

Distortion Minimization in Multi-Sensor Estimation With Energy Harvesting

This paper presents a design methodology for optimal energy allocation to estimate a random source using multiple wireless sensors equipped with energy harvesting technology. In this framework, multiple sensors observe a random process and then transmit an amplified uncoded analog version of the observed signal through Markovian fading wireless channels to a remote station. The sensors have access to an energy harvesting source, which is an everlasting but unreliable random energy source compared to conventional batteries with fixed energy storage. The remote station or so-called fusion centre estimates the realization of the random process by using a best linear unbiased estimator. The objective is to design optimal energy allocation policies at the sensor transmitters for minimizing total distortion over a finite-time horizon or a long term average distortion over an infinite-time horizon subject to energy harvesting constraints. This problem is formulated as a Markov decision process (MDP) based stochastic control problem and the optimal energy allocation policies are obtained by the use of dynamic programming techniques. Using the concept of submodularity, the structure of the optimal energy allocation policies is studied, which leads to an optimal threshold policy for binary energy allocation levels. Motivated by the excessive communication burden for the optimal control solutions where each sensor needs to know the channel gains and harvested energies of all other sensors, suboptimal decentralized strategies are developed where only statistical information about all other sensors' channel gains and harvested energies is required. Numerical simulation results are presented illustrating the performance of the optimal and suboptimal algorithms.

Distortion Minimization in Multi-Sensor Estimation Using Energy Harvesting and Energy Sharing

This paper investigates an optimal energy allocation problem for multisensor estimation of a random source where sensors communicate their measurements to a remote fusion center (FC) over orthogonal fading wireless channels using uncoded analog transmissions. The FC reconstructs the source using the best linear unbiased estimator (BLUE). The sensors have limited batteries but can harvest energy and also transfer energy to other sensors in the network. A distortion minimization problem over a finite-time horizon with causal and noncausal centralized information is studied and the optimal energy allocation policy for transmission and sharing is derived. Several structural necessary conditions for optimality are presented for the two sensor problem with noncausal information and a horizon of two time steps. A decentralized energy allocation algorithm is also presented where each sensor has causal information of its own channel gain and harvested energy levels and has statistical information about the channel gains and harvested energies of the remaining sensors. Various other suboptimal energy allocation policies are also proposed for reducing the computational complexity of dynamic programming based solutions to the energy allocation problems with causal information patterns. Numerical simulations are included to illustrate the theoretical results. These illustrate that energy sharing can reduce the distortion at the FC when sensors have asymmetric fading channels and asymmetric energy harvesting processes.

A Simulation Model for Nakagmi-M Fading Channel with M>1

In this paper, we propose a model to simulate a wireless fading channel based on Nakagami-m distribution with m>1. The Nakagami-m fading channel is the most generalized distribution as it can generate one-sided Gaussian distribution, Rayleigh distribution and Rician distribution for m equals to 0.5, 1 and >1 respectively. In this work we have proposed a method to generate a wireless fading channel based on Nakagami-m distribution as this distribution fits to a wide class of fading channel conditions. Simulation results were obtained using Matlab R2013a and compared with the analytical results.

Performance Evaluation of a New Cooperative MAC Protocol with a Helper Node Selection Scheme in Ad Hoc Networks

A new cooperative MAC protocol called the busy tone cooperative medium access control (BT-COMAC) protocol is proposed to overcome the drawbacks and maximize the advantages of existing schemes. This scheme uses a new metric called decibel power to decide an appropriate helper node. Using received power strength is more efficient in selecting an appropriate helper node, especially in a densely populated network, than the effective transmission rates used in conventional schemes. All communication nodes in a communication service area are assumed to move independently. Two performance metrics are used: System throughput and channel access delay. A performance evaluation of the BT-COMAC protocol is conducted using a computer simulation over a slow fading wireless channel, and its performance results are compared with those of four existing schemes. The numerical results show that the BT-COMAC protocol improves the system throughput by approximately 15% as compared to the best existing scheme.

A Hardware Based Ricean Fading Radio Channel Simulator

The rise of the need of fading radio channel simulators is due to time and cost of field tests of wireless communications systems. There exist two approaches to design a hardware based fading simulators, namely the filter based and sum of sinusoids (SOS) methods. This paper introduces a hardware based Ricean fading simulator that is based on SOS and utilizes empirical formulas that were derived based on extensive measurements to estimate the Ricean distribution K factor. The simulator presented here can simulate the Ricean fading radio channel under different parameters including antenna height, antenna beam width, season and distance between the transmitter and the receiver.

Energy Efficiency of Space and Polarization MIMO Communications With Packet Erasures Over Wireless Fading Channels

The transmit energy efficiency (EE) of space- polarization multiple-input multiple-output (SpPol-MIMO) architectures, dual-polarized MIMO architectures (DP-MIMO), and conventional co-polarized MIMO (CP-MIMO) schemes are compared for packet-based communication systems using full multiplexing without channel state information at the transmitter. We develop a closed-form approximation for assessing the EE of single-input single-output (SISO) and MIMO systems and show tradeoffs that exist between EE, spectral efficiency, modulation schemes, and packet designs. Using a hardware/software testbed, we also compare the EE among SISO, 2 × 2 CP-MIMO, 2 × 2 DP-MIMO, and 4 × 4 SpPol-MIMO architectures in frequency-selective MIMO channels. Emulation results from the testbed indicate that in the high SNR-regime, SpPol-MIMO provides the best EE performance in spatially uncorrelated channels, with EEs approximately four and two times better than SISO and 2 × 2 MIMO architectures, respectively. In Rician channels exhibiting either partial or high sAbstract-The transmit energy efficiency (EE) of space- polarization multiple-input multiple-output (SpPol-MIMO) architectures, dual-polarized MIMO architectures (DP-MIMO), and conventional co-polarized MIMO (CP-MIMO) schemes are compared for packet-based communication systems using full multiplexing without channel state information at the transmitter. We develop a closed-form approximation for assessing the EE of single-input single-output (SISO) and MIMO systems and show tradeoffs that exist between EE, spectral efficiency, modulation schemes, and packet designs. Using a hardware/software testbed, we also compare the EE among SISO, 2 × 2 CP-MIMO, 2 × 2 DP-MIMO, and 4 × 4 SpPol-MIMO architectures in frequency-selective MIMO channels. Emulation results from the testbed indicate that in the high SNR-regime, SpPol-MIMO provides the best EE performance in spatially uncorrelated channels, with EEs approximately four and two times better than SISO and 2 × 2 MIMO architectures, respectively. In Rician channels exhibiting either partial or high spatial correlation, DP-MIMO offers more than a 2× performance improvement relative to the other architectures at high SNR. At low SNR, SISO systems offer the best energy efficiency but also exhibit the lowest data rates. For all of the architectures, operating points are identified that provide near achievable data rates while operating at a local minimum in energy consumption.patial correlation, DP-MIMO offers more than a 2× performance improvement relative to the other architectures at high SNR. At low SNR, SISO systems offer the best energy efficiency but also exhibit the lowest data rates. For all of the architectures, operating points are identified that provide near achievable data rates while operating at a local minimum in energy consumption.

Research on Interference Model of Cooperative Cognitive Radio Networks over Rayleigh Fading Channel

Cooperative spectrum sensing technology has been identified as an effective approach to mitigate the interference generated by cognitive radio users on primary networks. In this paper, based on the rigorous mathematical derivation, we proposed an interference model of cooperative cognitive radio networks on the licensed user over Rayleigh fading wireless channel. In the case of the cognitive radio users independently and cooperatively sensing the existence of the licensed user, the proposed model analyzes the effect of some key parameters on the interference such as the missing detection probability of spectrum sensing scheme, spatial density of cognitive radio users etc. This model not only is a theoretical expression but also can be put into applications. The numerical results are given to verify the proposed model.

IJARCCE - Computer and Communication Engineering

This paper presents the spectrum sharing network consisting of a pair of primary users (PUs) and a pair of cognitive users or secondary user (SUs) in fading wireless channels. Cognitive radio networks are the spectrum utilizing networks. When both the links (primary user link and secondary user link) utilized the spectrum simultaneously networks are called the spectrum sharing networks. During the spectrum sharing, the interference will occur at the primary user link and secondary user degrades the QoS of the primary user link due to same frequency band utilization from the PUs and SUs. The priority of the primary user's spectrum utilization will be higher than secondary user's spectrum utilization. This study discussed about the QoS assurance of primary users, improved the performance of the secondary users and minimization of bit rate in the spectrum sharing cognitive radio network.

Optimized OFDM Transmission of Encrypted Image Over Fading Channel

This paper compares the quality of diffusion-based and permutation-based encrypted image transmission using orthogonal frequency division multiplexing (OFDM) over wireless fading channel. Sensitivity to carrier frequency offsets (CFOs) is one of the limitations in OFDM transmission that was compensated here. Different OFDM diffusions are investigated to study encrypted image transmission optimization. Peak signal-to-noise ratio between the original image and the decrypted image is used to evaluate the received image quality. Chaotic encrypted image modulated with CFOs compensated FFT-OFDM was found to give outstanding performance against other encryption and modulation techniques.

Scheduling Heterogeneous Real-Time Traffic Over Fading Wireless Channels

We develop a general approach for designing scheduling policies for real-time traffic over wireless channels. We extend prior work, which characterizes a real-time flow by its traffic pattern, delay bound, timely throughput requirement, and channel reliability, to allow clients to have different deadlines and allow a variety of channel models. In particular, our extended model consider scenarios where channel qualities are time-varying, the access point may not have explicit information on channel qualities, and the access point may or may not employ rate adaptation. Thus, our model allows the treatment of more realistic fading channels as well as scenarios with mobile nodes and the usage of more general transmission strategies. We derive a sufficient condition for a scheduling policy to be feasibility optimal, and thereby establish a class of feasibility optimal policies. We demonstrate the utility of the identified class by deriving a feasibility optimal policy for the scenario with rate adaptation, time-varying channels, and heterogeneous delay bounds. When rate adaptation is not available, we also derive feasibility optimal policies for both scenarios where the access point may or may not have explicit knowledge on channel qualities. For the scenario where rate adaptation is not available but clients have different delay bounds, we describe a heuristic. Simulation results are also presented, which indicate the usefulness of the scheduling policies for more realistic and complex scenarios.

Review on STBC coded MISO and MIMO in Frequency Selective Wireless Fading Channel

Diversity methods offer the receiver with separately faded copies of the transmitted signal with the expectation that at least one of these replicas will be received properly. There are ranges of techniques available during which these faded copies can be recovered and used as part of preparations for better reception. Transmit diversity uses multiple transmit antennas to offer the receiver with multiple uncorrelated replicas of the transmitted signal. The advantage is that the difficulty of having multiple antennas is located on the transmitter, which can be common among many receivers for the downlink of a wireless system, even as still providing diversity benefits. This technology is called as MIMO system. Hence the multipath fading channels are measured as an advantage scenario in MIMO. This is the most preferred technique in fourth generation cellular communication, as higher data rates can be achieved. Our objective is to review concatenating various error control codes in the MIMO setting using BER and EDR as performance indicators.

APP-MAC-PHY Cross-Layer Video Streaming Technique over Wireless Channels

In this letter, we propose a cross-layer technique jointly considering modulation coding schemes (MCSs) of medium access control (MAC) layer, source significance information (SSI) and error concealment unit of application (APP) layer, and channel quality information (CQI) of physical (PHY) layer. We demonstrate the improved video quality by the proposed technique when H.264 videos are streamed over Rayleigh fading wireless channels.

Spectrum-Map-Empowered Opportunistic Routing for Cognitive Radio Ad Hoc Networks

Cognitive radio (CR) has emerged as a key technology for enhancing spectrum efficiency by creating opportunistic transmission links. Supporting the routing function on top of opportunistic links is a must for transporting packets in a CR ad hoc network (CRAHN) consisting of cooperative relay multiradio systems. However, there lacks a thorough understanding of these highly dynamic opportunistic links and a reliable end-to-end transportation mechanism over the network. Aspiring to meet this need, with innovative establishment of the spectrum map from local sensing information, we first provide a mathematical analysis to deal with transmission delay over such opportunistic links. Benefitting from the theoretical derivations, we then propose spectrum-map-empowered opportunistic routing protocols for regular and large-scale CRAHNs with wireless fading channels, employing a cooperative networking scheme to enable multipath transmissions. Simulations confirm that our solutions enjoy significant reduction of end-to-end delay and achieve dependable communications for CRAHNs, without commonly needed feedback information from nodes in a CRAHN to significantly save the communication overhead at the same time.

Polarization Division Multiple Access on NLoS Wide-Band Wireless Fading Channels

A cellular down-link polarization division multiple access (PDMA) scheme with collaborative transmitter-receiver-polarization adjustment and polarization filtering detection is proposed for polarized wide-band non-line-of-sight (NLoS) wireless fading channels. The fundamental novelty of the proposed scheme is that it can adjust transmit polarization to yield more desirable per-subcarrier complex cross-polarization discrimination (XPD) observed at the receivers. This is a significant difference between the proposed scheme and conventional multiple-input multiple-output (MIMO) systems. The capacity analysis of the proposed PDMA scheme is provided, and the feasibility and satisfactory performance of the scheme are demonstrated even when serious time-variant channel depolarization and polarization mismatch between receivers occurs. The proposed cellular down-link PDMA scheme uses synchronous orthogonal frequency division multiple access (OFDMA) with an equally spaced pilot symbol allocation and a channel estimator. Excellent symbol error rate (SER)/effective signal-to-interference plus noise ratio (SINR) performance is shown despite the presence of substantial channel depolarization. The scheme can be used to extend the capacity of any cellular system that uses uni-polarized antennas and can be extended and applied to MIMO arrays as well.

On the Throughput Capacity of Cooperative Multi-hop Wireless Ad hoc Networks with Multi-flow

Cooperative relaying techniques are known to provide spatial diversity for wireless fading channels. In contrast to non-cooperative transmission (direct transmission), they increase link reliability, provide higher capacity, reduce transmit power, and extend transmission range. Mostly the gains of cooperative relaying are shown for single flow scenarios in the absence of inter-flow interference. In this paper we study the effect of inter-flow interference on the capacity of cooperative networks in multi-hop multi-flow settings. We used the conflict graph to model the interference and find the additional constraints introduced due to cooperative transmission by using the concept of cliques on the conflict graph, which can be used to capture the interference relation among links. We formulated the multi-commodity flow problem for network capacity using linear programming, and employed a clique based analysis of the conflict graph to compute interference constraints. It is observed that the throughput drops significantly when cooperative transmissions are used in the network. We also found that the hop counts increase when cooperative links are used that is due to avoiding interfering links, which results in losing the benefits of shortest-path routing.

A low computational complexity V-BLAST/STBC detection mechanism in MIMO system

Abstract The idea of multiple antenna arrays has evolved into multiple-input multiple-output (MIMO) system, which provides transmit and receive diversities. It increases robustness of the effect of multipath fading in wireless channels, besides yielding higher capacity, spectral efficiency and better bit error rate (BER) performance. The spatial diversity gain is obtained by transmitting or receiving multiple copies of a signal through different antennas to combat fading and improves the system BER performance. However, the computational complexity of MIMO system is inevitably increased. Space-time coding (STC) technique such as Alamouti’s space-time block code (STBC) that combines coding, modulation and signal processing has been used to achieve spatial diversity. Vertical Bell Laboratories Layered Space-Time (V-BLAST) uses antenna arrays at both the transmitter and receiver to achieve spatial multiplexing gain. Independent data streams that share both frequency bands and time slots are transmitted from multiple antennas and jointly detected at the receiver. The theoretical capacity of V-BLAST increases linearly with the number of antennas in rich scattering environments. It's well-known that maximization of spatial diversity gain leads to degradation of spatial multiplexing gain or vice versa. In order to achieve spatial multiplexing and diversity gains simultaneously, the V-BLAST/STBC scheme has been introduced. This hybrid scheme increases MIMO system capacity and maintains reliable BER performance at the same time. However, both V-BLAST and STBC layers, in this hybid scheme, assume each other as an interferer. Thus, the symbols must be decoded with a suitable detection mechanism. In this paper, a new low complexity detection mechanism for V-BLAST/STBC scheme based on QR decomposition, denoted as low complexity QR (LC-QR) decomposition, is presented. The performance of the proposed LC-QR decomposition detection mechanism in V-BLAST/STBC transceiver scheme is compared with other detection mechanisms such as ZF, MMSE and QR decomposition. It is shown that the BER performance in V-BLAST/STBC scheme is better than V-BLAST scheme while its system capacity is higher than orthogonal STBC scheme when the LC-QR decomposition detection mechanism is exploited. Moreover, the computational complexity of proposed LC-QR decomposition mechanism is significantly lower than other abovementioned detection mechanisms.

Optimal Power Management in Wireless Control Systems

This paper considers the control of a linear plant when plant state information is being transmitted from a sensor to the controller over a wireless fading channel. The power allocated to these transmissions determines the probability of successful packet reception and is allowed to adapt online to both channel conditions and plant state. The goal is to design plant input and transmit power policies that minimize an infinite horizon cost combining power expenses and the conventional linear quadratic regulator control cost. Since plant inputs and transmit powers are in general coupled, a restricted information structure is imposed allowing them to be designed separately. Under this information structure the standard LQR controller becomes the optimal plant input policy, while the optimal communication policy follows a Markov decision process minimizing transmit power at the sensor and state estimation error at the controller. The optimal power adaptation to channel and plant states is examined qualitatively for general forward error correcting codes. In the particular case of capacity achieving codes event-triggered policies are recovered, where the sensor decides whether to transmit or not based on plant and channel conditions. Approximate dynamic programming is employed to derive a family of tractable suboptimal communication policies exhibiting the same qualitative features as the optimal one. The performance of our suboptimal policies is shown in simulations and is contrasted to other simple transmission policies.

Goodput improvement for multipath transport control protocol in cooperative relay‐based wireless networks

Multipath transport control protocol (MPTCP) standardised by Internet Engineering Task Force offers a promising solution to support simultaneous delivery of transport control protocol packets over multiple interfaces of multi-radio mobile devices. Even when there is no multiple access coverage, it is still possible to enable multipath transmission by involving mobile devices in vicinity as cooperative relays. However, the available bandwidth provided by relays can be highly varying because of wireless channel fading or dynamic local traffic load. Although MPTCP can pool the available bandwidths of multiple paths, it is challenging to address the dynamics of the cooperative wireless network and ensure a stable performance. In this study, the authors extend MPTCP for the long-term evolution network with cooperative relays by an integrated solution in three aspects, namely, subset ‘sum-based relay selection’, ‘adaptive congestion control’ and ‘differentiated packet forwarding’. Extensive simulations in NS-3 show that the proposed modules can achieve a stable aggregate throughput by engaging a small number of relays and significantly improve the goodput in various scenarios.

Cooperative OFDM for semi distributed detection in wireless sensor networks

In this paper, parallel distributed detection in wireless sensor network (WSN) is investigated. Sensors are assumed to be transmitting their local decisions to a fusion center through a wireless fading channel using cooperative transmission. To enable cooperative transmission, sensors are divided into groups where sensors in each group help each other in transmitting their decisions in a way that the fusion center receives each local decision as an orthogonal frequency division multiplexed (OFDM) block. The fusion center detects all OFDM blocks sent by all groups to process them in order to obtain a final (global) decision. Using this cooperative transmission scheme enables the fusion center to apply diversity combining methods in order to reduce the fading effects of the channel. Optimal and sub-optimal fusion rules are derived for such system. Simulation results are provided to show the performance improvement that can be obtained compared to the conventional system where each local decision is transmitted to the fusion center individually and no diversity technique is applied at the fusion center.