Adaptive Modulation Technique for cooperative diversity in Wireless Fading Channels

We consider the power and rate allocation for a transmission in a wireless network where the transmitter knows the channel state of the desired receiver but only knows the statistics of the interferes. While this scenario is often encountered in practice, and extremely important in dense networks, its optimal solution was not yet known. The analysis considers nodes that are distributed according to a homogeneous Poisson point process, and hence applies both to cellular and to ad‐hoc networks. We derive (for the first time) the optimal power allocation at a node, based only on the channel gain to its desired receiver. We also derive the optimal rate allocation in networks that are characterized by the outage rate model. Considering sub‐optimal allocations, we advocate the use of the simple threshold scheduling scheme for power allocation and linear rate adaptation for rate allocation. We prove that these suboptimal schemes are close to optimal, for any channel fading. We also present simulation results over Rayleigh fading channels that show a maximum loss of 1.5% between the advocated schemes and the optimal schemes.

We consider a wireless relay network with multiple antennas terminals over Rayleigh fading channels, and apply distributed space-time coding (DSTC) in amplify-and-forward (A&F) mode. It turns out that, combined with power allocation in the relays, A&F DSTC results in an opportunistic relaying scheme, in which the best relay is selected to retransmit the source's space-time coded signal. Next, assuming M-PSK or M-QAM modulations, we analyze the performance of the cooperative diversity wireless networks using A&F opportunistic relaying with the multiple-antennas source and destination. We first derive the probability density function (PDF) of the received SNR at the destination. Then, the PDF is used to determine the symbol error rate in Rayleigh fading channels. Then, we derived closed-form approximations for SER in high SNR scenario, from which we find the diversity order of system Rmin{Ns, Nd}, where R, Ns, and Nd are the number of the relays, the source antennas, and the destination antennas, respectively. Simulation results show that the proposed system obtain 2 dB gain in SNR over DSTC for BER 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , when R = 2, N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> = 2, N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> = 2.

We study power allocation for the decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly distributed node locations, we aim to find the power allocation that minimizes the outage probability under a short-term power constraint, wherein the total power for all nodes is less than a prescribed value during each two-stage transmission. Due to the computational and implementation complexity of the optimal solution, we derived a simple near-optimal solution. In this near-optimal scheme, a fixed fraction of the total power is allocated to the source node in stage I. In stage II, the remaining power is split equally among a set of selected nodes if the selected set is not empty, and otherwise is allocated to the source node. A node is selected if it can decode the message from the source and its mean channel gain to the destination is above a threshold. In this scheme, each node only needs to know its own mean channel gain to the destination and the number of selected nodes. Simulation results show that the proposed scheme achieves an outage probability close to that for the optimal scheme obtained by numerical search, and achieves significant performance gain over other schemes in the literature

Cooperative Diversity transmission, a newly upcoming field in the area of wireless communication, has been receiving great attention. This diversity transmission exploits the spatial diversity created by antenna sharing to improve the performance of a wireless network. The working of a cooperative diversity system is such that a source node communicates with a destination node with the help of another partner node. One important question usually raised regarding the operation of this system is that of the amount of power allocation among partnering nodes. Initial stages of research in this area had assumed equal distribution of power resources between the nodes. This approach has been proven as clearly suboptimal. Several works to allocate power optimally are coming up. In this project, optimal power allocation is used as a key approach to analyze the rate performance of the system. This is done with the help of a parameter called cooperative ratio which is the ratio of the power used for cooperative transmission to total power. Simulation results to support the analysis have also been provided.

In wireless fading channels, the network outage probability is defined as a probability that there is no power allocation such that each user can achieve its SIR target. In this paper several lower and upper bounds on the network outage probability under the assumption of Rayleigh fading channels are presented. Moreover, we analyze the asymptotic behavior of the bounds as the number of users tends to infinity linearly and with the square root of the bandwidth.

In this paper, the combination of orthogonal frequency-division multiplexed (OFDM), multi-input multi-output (MIMO) and adaptive modulation can effectively eliminate or resist various fading in wireless channels, thus it can improve the capacity of system and transmission quality. Depending on the optimal bit and power allocation strategy based on greedy algorithm, in this paper, the performance of adaptive bit and power allocation MIMO-OFDM system based on greedy algorithm is completely studied. And the performance comparison among greedy algorithm, Chow algorithm and average algorithm is represented. We also compare the performance of MIMO-OFDM system under different antennas numbers. It is shown that the bit error rate (BER) of MIMO-OFDM system based on greedy algorithm is better than that based on Chow algorithm and average algorithm. And the spectral efficiency of greedy algorithm is superior to that of Chow and average algorithm. Ultimately, we show that the performance of MIMO-OFDM system is also superior to that of OFDM system with single transmit antenna and receive antenna which is under the same scheme as the former.

We study the Shannon capacity of adaptive transmission techniques in conjunction with diversity-combining. This capacity provides an upper bound on spectral efficiency using these techniques. We obtain closed-form solutions for the Rayleigh fading channel capacity under three adaptive policies: optimal power and rate adaptation, constant power with optimal rate adaptation, and channel inversion with fixed rate. Optimal power and rate adaptation yields a small increase in capacity over just rate adaptation, and this increase diminishes as the average received carrier-to-noise ratio (CNR) or the number of diversity branches increases. Channel inversion suffers the largest capacity penalty relative to the optimal technique, however, the penalty diminishes with increased diversity. Although diversity yields large capacity gains for all the techniques, the gain is most pronounced with channel inversion. For example, the capacity using channel inversion with two-branch diversity exceeds that of a single-branch system using optimal rate and power adaptation. Since channel inversion is the least complex scheme to implement, there is a tradeoff between complexity and capacity for the various adaptation methods and diversity-combining techniques.

Multipath fading is one of the primary factors for degrading the performance in a wireless network. Information theoretic and past research suggest the use various diversity techniques to combat fading in wireless networks. Antenna diversity, a form of diversity technique, when incorporated in a wireless transceiver increases the system capacity and is one of the effective methods to combat fading in wireless systems. Also, recent research by Laneman et.al., Sendonaris et.al. suggests that cooperation among users in a wireless networks is an effective approach for a better signal reception in multipath fading environments. The diversity gains obtained by cooperation among the users of a wireless network is termed as cooperative diversity . Although, prior research in cooperative diversity considers users equipped with single antenna, in practical scenarios users may be able to accommodate multiple antennas due to the recent advanced research in semiconductor industry. Hence, the primary purpose of this thesis is to design, simulate and analyze an end-end performance of multi-antenna wireless systems employing cooperative multi antenna relay nodes so as to exploit the cooperative diversity and antenna diversity simultaneously in a wireless networks. Three main contributions to the area of cooperative multiple-input multiple-output (MIMO) wireless systems is presented in this thesis. First, we perform information theoretic analysis to study the impact of antenna arrays on cooperative wireless networks and propose the best possible distribution of antenna arrays among the three terminals of a simple three terminal cooperative relay network. Second, we design, simulate, and analyze a cooperative multiple-input multiple-output (MIMO) wireless systems employing orthogonal space-time block codes as proposed by Alamouti in 1998 with a decode-and-forward (DF) relay terminal. We implement a maximal ratio combining receiver that provides almost twice the diversity gain with respect to point-point multiple input multiple output link. Finally, we implement a practical receiver for cooperative reception using multiple antennas at all nodes based on Bell-Labs Layered Space Time architecture (BLAST). We incorporate a practical adaptive decode-and-forward (DF) relaying technique for reliable signal retransmission for both Alamouti space-time coding and the BLAST schemes. Results presented in terms of bit error rates and throughput show that remarkable performance gains are achievable by combining the concepts drawn from space-time coding, cooperative relaying and array processing.

In multi-access wireless communication systems, power control and adaptive modulation are two important means to increase spectral efficiency, combat the time varying fading environment, and reduce cochannel interference. The overall uplink transmitting power is optimized while the overall network throughput is fixed. Each link can select a range of targeted signal-to-interference-plus-noise ratio (SINR) according to its current channel condition and each link's time average SINR is maintained as a constant to ensure fairness. Adaptive M-QAM or M-PSK modulation with antenna diversity is applied to increase spectral efficiency. The scheme can be interpreted as "water filling" each link's SINR in the time domain and allocating network throughput to different links at each time. From simulation results, our scheme reduces about 40% of overall transmitting power and increases average spectral efficiency by about 0.9 bit/s/Hz.

Optimisation of average spectral efficiency for adaptive multilevel quadrature amplitude modulation (MQAM) with service outage and instantaneous bit error rate constraints is studied over fading channels with variations in two time-scales. The authors obtain the corresponding optimal schemes for power and rate adaptation based on signal-to-noise ratio for continuous and discrete rate conditions. For continuous rate MQAM they show that the optimal power allocation scheme is a combination of generalised water-filling and channel inversion. For discrete rate MQAM the optimal power will be piecewise channel inversion. The authors also show that using discrete-rate adaptation with five different rates around 1 dB power is lost compared with continuous rate adaptation scenario. In the optimisation problems, they consider block flat fading channels with Nakagami-distributed variations within each block.

Performance of 5G NR-polar QAM-OFDM in nonlinear distortion plus Non-Gaussian noise over Rayleigh fading channel

Wireless physical layer security has attracted great attention in recent years. Although mobility is an intrinsic property of wireless networks, most of the existing works only consider static scenarios and the impact of mobility on wireless physical layer security is not well understood. To fill this gap, in this paper, we investigate physical layer security in the scenario with a random mobile receiver under Rayleigh fading channel. We consider a common scenario where a base station or access point communicates to a random mobile receiver with a passive eavesdropper in a circular region. The secrecy performances under three typical random mobility models are studied: random waypoint model (RWP); random direction model (RD); and border move model. We investigate the secrecy characteristics of the mobile user under steady-state running and provide a general analytical framework for computing the ergodic secrecy capacity. We derive tractable closed-form expressions of positive secrecy capacity probability and secrecy outage probability for RWP and RD mobility users, respectively. By comparing the secrecy performance of mobility with static case, we find that the RWP mobile user can achieve much better secrecy performance than the others. We then investigate the secrecy performance of RWP mobile user with pause time. Furthermore, the two types of secrecy improvement strategies for random mobile users are proposed. In the first strategy, a transmit subcell for RWP users is bounded. We allow the transmitter to communicate with the mobile user only when he is moving within the subcell. In the other strategy, the transmitter is turned on only when the evaluated secrecy performance reaches to a required level. We strike a good trade-off between the secrecy improvements and transmit outage probability. Extensive simulation results validate our theoretical analysis. Finally, we extend our framework to other realistic scenarios, including nodes moving in other shapes of areas, and multiple non-cooperative and cooperative eavesdroppers.

Call admission control (CAC) is a significant component in wireless networks to guarantee quality-of-service (QoS) requirements and also to enhance the network resilience. In the literature, CAC is normally evaluated by considering the limited bandwidth in the radio interface while the physical fading wireless channel is not considered. This work proposes a cross-layer analytical technique to study a CAC mechanism in wireless networks over Rayleigh fading channel. The tele-traffic parameters are relaxed to follow general distributions in stead of exponential variables for the sake of general applicability in various scenarios. The explicit relationship is derived between the performance metrics and the Rayleigh fading channel characteristics. Numerical investigation demonstrates that the fading channel has a substantial impact on the network performance.

In the presence of channel fluctuation, rate adaptation is one way to maintain the quality of the link at a desired level. This is especially important in ad hoc wireless networks, where temporary channel fluctuations might create frequent needs for rerouting that would result in severe overhead and adversely affect the performance. We study an unusual method of passive rate adaptation in which some bits are dropped at the receiver end of a link. The symbol-error probability decreases as some bits are dropped. In terms of the distortion for a realtime analog signal, the tradeoff is between more reliable detection of fewer bits and less reliable detection of more bits. Our scheme achieves smaller distortion for a certain region of signal-to-noise ratio (SNR) values when compared with the original scheme without rate adaptation. Two examples, uniformly spaced, uncoded pulse amplitude modulation and quadrature amplitude modulation, are studied and compared for both a Gaussian channel and a Rayleigh fading channel. We conclude that our scheme is more suitable to use in a fading channel than in a Gaussian channel. We also verify that our scheme has a larger applicable region of SNR values when a nonuniform constellation is used, since the important bits are given additional protection.

Cooperative diversity is proposed to combat the detrimental effects of channel fading. In this paper, we investigate the effectiveness of cooperative diversity in interference limited ad hoc networks. The throughput performance of ad hoc networks that employ cooperative diversity techniques is examined. The negative effects of relay transmission blocking and extra time delay due to using the relay node, on the network throughput are investigated. We show that the relay blocking problem is mainly dependent on the relay selection criterion. Furthermore, we examine a two-channel approach to solve the delay problem and hence improve the overall network throughput.