Optimal Power Adaptation Research Articles

Distributed QoS Constraint based Resource Adaptation Strategy for Cognitive Radio Networks

This paper primarily investigates and addresses the optimal power adaptation strategy problem for multi-user cognitive radio network. The need to determine the optimal power transmission for the secondary user (SU) in a distributed network environment is challenging and important. This requires an efficient adaptive strategy with high convergence capability in order to meet up with the multi-users quality of service (QoS) requirements in a cognitive network and ultimately ensure more efficient resource utilization amongst users. In this paper, a distributed QoS contraint scheme which considered both the transmission power and outage probability has been proposed to maximize the network performance and to control the SUs transmission power. Firstly, the QoS constraint optimization problem for distributed secondary users is formulated and solved in order to adapt with the network dynamics of the cognitive networks. Subsequently, the solution has been used to dynamically adjust the radio power of the SUs to conform with the stringent network constraint and ensures coexistence amongst the users. More importantly, the simulation result shows that the scheme increases the overall network utility when compared to the scheme without adaptation.

A New Closed-Form Expressions of Channel Capacity with MRC, EGC and SC Over Lognormal Fading Channel

In this work, we derive the closed-form expressions of channel capacity with maximal ratio combining, equal gain combining and selection combining schemes under different transmission policies such as optimal power and rate adaptation, optimal rate adaptation, channel inversion with fixed rate (CIFR) and truncated CIFR. Various approximations to the intractable integrals have been proposed using methods such as Holtzman and Gauss–Hermite approximations and simpler expressions are suggested. Moreover, as an application, the channel capacity of lognormally distributed fading channel in the interference-limited environment is discussed. The obtained closed-form expressions have been validated with the exact numerical results.

Capacity analysis of α − η − μ channels over different adaptive transmission protocols

In this study, the authors consider the α − η − μ fading channels and derive a new form for the cumulative distribution function of the signal-to-noise ratio (SNR) in terms of the bivariate Fox H-function. They also derive closed-form expressions for the moments and the amount of fading of the SNR. The capacity of α − η − μ fading channels is analysed assuming different adaptive transmission protocols. To this end, they provide analytical closed-form expressions for the average channel capacity with optimal rate adaptation (C ORA), optimal power and rate adaptation (C OPRA), channel inversion with fixed rate (CIFR) (C CIFR), and truncated CIFR (C TIFR). The derived expressions are valid for arbitrary values of the α, η, and μ fading parameters. The derived expressions can also be reduced to study the capacity of other fading distributions such as η − μ, α − μ, Weibull, and Nakagami. Numerical results are provided and compared with simulations to validate of the derived expressions.

Adaptive Transmission in Spectrum Sharing Systems With Alamouti OSTBC Under Spatially Correlated Channels

This paper presents a performance analysis for adaptive transmission techniques in a spectrum sharing system in which a primary user (PU) and a secondary user (SU) employ an Alamouti orthogonal space-time block coding (OSTBC) system under correlated antennas over Rayleigh fading channels. The considered adaptive transmission techniques include practical rate-adaptive modulation techniques and classical adaptive transmission techniques, namely, optimal power and rate adaptation (opra), constant transmit power with optimal rate adaptation (ora), and channel inversion with fixed rate (cifr). Closed-form expressions are derived for the following performance metrics: the average spectral efficiency, the outage probability, the average channel capacity, and the average bit error rate (BER) of the system. Using numerical evaluation results, the impacts of antenna correlation on the performance of each adaptive technique in different cases and under various parameters are evaluated. These results provide comprehensive and useful insights about the performance of the considered system under various adaptive transmission techniques.

A tight approximate analytical framework for performance analysis of equal gain combining receiver over independent Weibull fading channels

In this paper, a method for approximating the probability distribution of sum of independent and identical Weibull random variables is adopted to analyze the performance of equal gain combiner (EGC) receiver over non-identical Weibull fading channel (WFC). Our main result is to derive a generalized expression of the probability density function (PDF) of the signal-to-noise ratio (SNR) at the EGC output in the case of non-identical WFC. Based on this PDF, accurate approximation of significant performance criteria, such as outage probability (OP), the amount of fading (AoF), and average symbol/bit error probability (ASEP/ABEP), are derived. In addition, we derived the analytical expressions for channel capacities under various adaptation policies such as optimal rate adaptation (ORA), optimal simultaneous power and rate adaptation (OPRA), channel inversion with fixed rate (CIFR), and truncated channel inversion with fixed rate (TCIFR). The proposed mathematical analysis is complemented by several numerical results and validated using Monte Carlo simulation method.

Power Adaptation for Energy Efficient Bi-directional Relaying with Quality-of-Service Requirement and Individual Peak-Power Limit

This paper addresses an optimal power adaptation (PA) problem of a two-time-slot bi-directional relaying network with a half-duplex amplify-and-forward relay. Unlike the existing studies, our goal is to develop effective PA strategies that can dynamically adjust the transmit-power levels of all the terminals to achieve energy efficiency, while satisfying the individual peak-power limit on each terminal and the quality-of-service (QoS) requirement of the network. By using the instantaneous channel state information (ICSI) and the statistical CSI (SCSI) knowledge, respectively, and with the aid of traffic information, the PA problem is analytically solved, leading to the so-called ICSI and SCSI based PA strategies with closed-form PA solutions for individual transmit-powers at the relay and the two end-terminals. Simulation results have verified the correctness of the derived expressions and confirmed the efficiency of our proposed strategies. It is shown that the proposed PA strategies can significantly reduce the total transmit-power of the network with guaranteed network QoS.

Efficient Analysis of the Ergodic Capacity of Cooperative Non-Regenerative Relay Networks Over Generalized Fading Channels

In this paper, we proposed a novel efficient method of analyzing the ergodic channel capacity of the cooperative amplify-and-forward (CAF) relay system. This is accomplished by employing a very tight approximate moment generating function (MGF) of end-to-end signal-to-noise ratio of 2-hop multi-relay system, which is In this paper, we proposed a novel efficient method of analyzing the ergodic channel capacity of the cooperative amplify-and-forward (CAF) relay system. This is accomplished by employing a very tight approximate moment applicable to myriad of fading environments including mixed and composite fading channels. Three distinct adaptive source transmission policies were considered in our analysis namely: (i) constant power with optimal rate adaptation (ORA); (ii) optimal joint power and rate adaptation (OPRA); and (iii) fixed rate with truncated channel inversion (TCIFR). The proposed frame work based on the novel approximate MGF method is sufficiently general to encapsulate all types of fading environments (especially for the analysis of the mixed fading case)and provides significant advantage to model wireless system for mixed and composite fading channel. In addition to simplifying computation complexity of ergodic capacity for CAF relaying schemes treated in literature, we also derive closed form expressions for the above three adaptive source transmission policies under Nakagami-m fading with i.n.d statistics. The accuracy of our proposed method has been validated with existing MGF expressions that are readily available for specific fading environments in terms of bounds, and via Monte Carlo simulations.

Optimal Power and Modulation Adaptation Policies with Receiver Diversity over Rayleigh Fading Channel

<p>Efficient bandwidth utilization is paramount in wireless communication systems, particulary in fading<br />environments, since fading is one of the major constraints that impair communication in wireless systems.<br />The bandwidth efficiency of a wireless communication system can be enhanced significantly by employing<br />power and modulation adaptation policies with diversity combining gain. In this work, first we examine an<br />analytically-derived solution for Maximum Combining Ratio (MRC) diversity technique for the capacity per<br />unit bandwidth. Then, we design an adaptive transmission system to utilize the diversity combining gain while<br />retaining the target BER by adapting power and constellation size using continuous power, channel inversion<br />with fixed rate and continuous power and disrecte-rate. By considering the effect of diversity combining<br />gain, the designed system yields a reasonable spectral efficiency with respect to target BER that grows as<br />the number of diversity levels increase. Furthermore, the presented results show continuous power and<br />discrete-rate adaptation policy reduces probability of outage unlike its achieved spectral efficiency is close<br />to other selected policies, which ratifies the optimized switching thresholds and makes it best candidate for<br />imperfect channel conditions.</p>

A Low-Complexity Multiuser Adaptive Modulation Scheme for Massive MIMO Systems

A novel low-complexity multiuser adaptive modulation (MAM) scheme is proposed for uplink massive multiple input and multiple output systems with zero-forcing detection, which requires only slow-varying large-scale shadowing information for the users. Closed-form expressions are derived for the average spectral efficiency (ASE) and bit error outage (BEO). In addition, for MAM with joint power control, optimal power adaptation strategy and switching thresholds are obtained. Compared with the conventional fast adaptive modulation, which requires fast-varying small-scale fading information, the proposed MAM scheme achieves similar ASE performance with slight increase in BEO.

A very tight approximate results of MRC receivers over independent Weibull fading channels

In this paper, we study the performance of L-branch maximal-ratio combining (MRC) receivers operating over independent Weibull-fading channels. Our main result is a very tight approximation of the probability density function (PDF) of the signal-to-noise ratio (SNR) at the output of the combiner. Based on this result, accurate approximation of significant performance criteria, such as outage probability and average symbol error rate (ASER) are derived. We also evaluate the average bit error rate (ABER) for several coherent and non-coherent modulation schemes, using a closed-form expression for the moment-generating function (MGF) of the output SNR for MRC receivers. In addition, we derive some analytical expressions for channel capacity under various adaptation policies such as optimal rate adaptation (ORA), Optimal simultaneous power and rate adaptation (OPRA), Channel inversion with fixed rate (CIFR), and Truncated channel inversion with fixed rate (TCIFR). The proposed mathematical analysis is complemented by numerous numerical results, which point out the effects of fading severity on the overall system performance. Computer simulations are also performed to verify the validity and the accuracy of the proposed theoretical approach.

Ergodic Channel Capacity for Rayleigh Fading Channels with Impairments Due to Errors in M-Branch Hybrid Diversity Combining

Novel closed-form expressions are derived for Ergodic channel capacity for fading channels with impairments due to combining errors for M-branch Rayleigh fading with Generalized Diversity Combining. Four different adaptation policies are analyzed for (1) maximal ratio combining (MRC) diversity, (2) spread spectrum (SS) diversity with MRC and Antenna diversity with selection combining (SC), and (3) SS diversity with SC and Antenna diversity with MRC diversity reception cases. Correlation is defined physically as the similarity between the channel coefficients of the pilot and the message. If the pilot and the message are highly correlated, we have a high system performance. Optimal power and rate adaptation policy yields the best spectrum efficiency, while channel inversion with fixed rate policy suffers the highest penalty in comparison to other policies.

Spectral efficiency of dual diversity selection combining schemes under correlated Nakagami-0.5 fading with unequal average received SNR

The spectral efficiency results for different adaptive transmission schemes over correlated diversity branches with unequal average signal to noise ratio (SNR) obtained so far in literature are not applicable for Nakagami-0.5 fading channels. In this paper, we investigate the effect of fade correlation and level of imbalance in the branch average received SNR on the spectral efficiency of Nakagami-0.5 fading channels in conjunction with dual-branch selection combining (SC). This paper derived the expressions for the spectral efficiency over correlated Nakagami-0.5 fading channels with unequal average received SNR. This spectral efficiency is evaluated under different adaptive transmission schemes using dual-branch SC diversity scheme. The corresponding expressions for Nakagami-0.5 fading are considered to be the expressions under worst fading conditions. Finally, numerical results are provided to illustrate the spectral efficiency degradation due to channel correlation and unequal average received SNR between the different combined branches under different adaptive transmission schemes. It has been observed that optimal simultaneous power and rate adaptation (OPRA) scheme provides improved spectral efficiency as compared to truncated channel inversion with fixed rate (TIFR) and optimal rate adaptation with constant transmit power (ORA) schemes under worst case fading scenario. It is very interesting to observe that TIFR scheme is always a better choice over ORA scheme under correlated Nakagami-0.5 fading channels with unequal average received SNR.

Channel capacity of TAS/MRC system with adaptive transmission and channel estimation errors

This correspondence presents the impact of channel estimation error (CEE) on the channel capacity analysis of multiple-input multiple-output (MIMO) system employing transmit antenna selection and maximal ratio combining (TAS/MRC) under adaptive transmission (ADT) techniques over frequency-flat Rayleigh fading channels. In this paper, we evaluated exact closed-form expressions of channel capacity for TAS/MRC system with CEE under different variable power and variable rate adaptation techniques: (i) optimal power and rate adaptation (opra) (ii) constant power with rate adaptation (cpra) (iii) channel inversion with fixed rate (cifr). The channel state information (CSI) consisting of the signal-to-noise ratio (SNR) level is estimated by the decoder and fed back to the transmitter. Further, the ADT schemes can be operated frame by frame, hence permitting for distinct compromises among the attainable capacity and the corresponding implementation complexity. Numerical outcomes illustrate the impact of CEE on the link capacity and outline the performance characteristics among the distinctive adaptation techniques.

Conditional Power and Rate Adaptation for MQAM/OFDM Systems Under CFO With Perfect and Imperfect Channel Estimation Errors

In this paper, a new conditional power and rate adaptation scheme for orthogonal frequency-division multiplexing (OFDM) systems is proposed in the presence of carrier frequency offset (CFO), with perfect and imperfect channel state information (CSI). The conventional adaptive scheme is shown to be a special case of the conditionally adaptive scheme technique that enables the resulting nonconvex optimization problem, which is solved in a feasible way. It leads to a solution for optimal power adaptation that maximizes the spectral efficiency of an OFDM system using $M$ -ary quadrature amplitude modulation (MQAM) under average power and instantaneous bit error rate (BER) constraints. Closed-form expressions for the average spectral efficiency (ASE) of adaptive OFDM systems are derived for perfect and imperfect CSI cases. The theoretical results and computer simulations show that range of the conditional adaptation becomes narrow and the performance of constant power and continuous rate is very close to that of the conditionally adaptive power and continuous rate for higher CFO or high signal-to-noise ratio (SNR) values.

Capacity of Bernoulli-Gaussian Interference Channels in Rayleigh Fading with Full CSI

In this paper, we investigate the channel capacity of a Bernoulli-Gaussian (BG) interference channel in Rayleigh fading when the channel state information (CSI) is known at both the transmitter and receiver via tight lower and upper bounds. Specifically, we first derive an upper bound on the channel capacity assuming a Gaussian-distributed output. Under this assumption, an optimal power adaptation scheme is established and the upper-bound is obtained in closed-form. By assuming a Gaussian-distributed input, we then adopt the derived power adaptation scheme to establish a lower bound on channel capacity. A simple approximation of the instantaneous output entropy using a piecewise-linear curve fitting(PWLCF)-based scheme is then developed, which provides a closed-form estimation of the lower bound with a predetermined error level. Finally, a comparison between the derived upper and lower bounds are made. Both analytical and numerical results show that these two bounds are tight in a wide range of input power levels and they can be used effectively to estimate the channel capacity.

Spectrum efficiency evaluation of LMF channels under various adaptation policies with hybrid diversity schemes

A single-input multiple-output (SIMO) system with lognormal multipath fading (LMF) channel is considered in this paper. The spectrum efficiency (capacity per unit bandwidth) of a LMF channel for various adaptation policies is studied. We obtain the closed-form expressions for various adaptation policies such as (i) optimal power and rate adaptation (OPRA) policy, (ii) channel inversion with fixed rate (CIFR) policy, and (iii) truncated channel inversion with fixed rate (TIFR) policy with hybrid diversity combining techniques like hybrid maximal ratio combining (MRC)-equal gain combining (EGC), hybrid EGC-MRC, and hybrid MRC-selection combining (SC) diversity. Numerical results show that the use of hybrid diversity combining techniques improves the spectrum efficiency over stand-alone diversity combining techniques. OPRA policy that provides the highest spectrum efficiency when compared to other policies is also shown.

Probabilistic Radio Resource Allocation Over CDMA-Based Cognitive Radio Networks

A novel probabilistic radio resource allocation algorithm is proposed for code-division multiple-access cognitive radio networks. Our method incorporates a sensing-based hybrid access- and interference-limited spectrum sharing strategy to maximize the achievable spectral efficiency of cognitive users (CUs) while guaranteeing the quality-of-service (QoS) requirements of licensed services. Specifically, the enhanced performance for CUs is achieved through optimal joint power and rate adaptation to the time-varying multiuser interference and frequency-selective fading channel conditions. We show that this scheme can improve the average spectral efficiency of CUs, while satisfying the peak and average received-interference constraints at the primary receiver.

Optimal Power Control for Underlay Cognitive Radio Systems With Arbitrary Input Distributions

This paper studies optimal power control policies that maximize the achievable rates of underlay cognitive radio systems with arbitrary input distributions under both peak/average transmit power and peak/average interference power constraints for general fading distributions. In particular, optimal power adaptation schemes are formulated and low-complexity optimal power control algorithms are proposed. Additionally, simpler approximations of optimal power control policies in the low-power regime are determined. By considering gamma distributed channel power gains of the interference link between the secondary transmitter and the primary receiver and of the transmission link between the secondary transmitter and the secondary receiver, closed-form expressions for the maximum achievable rate attained with optimal power control in the low-power regime are provided. Through numerical results, the impact of the fading severity of both interference and transmission links and transmit power and interference power constraints on the maximum achievable rate of the cognitive user for different practical constellations and Gaussian signals are investigated.

Spectrum Efficiency Evaluation of SM MC-CDM with ZF USIC Under Different Adaptive Transmission Techniques

In this paper, closed form expressions for capacities per unit bandwidth for Spatially Multiplexed Multicarrier-Code Division Multiplexing with Zero Forcing Unified Successive Interference Cancellation detection are derived for optimal power and rate adaptation, optimal rate adaptation with constant transmit power, channel inversion with fixed rate, and truncated channel inversion adaptation policies, taking into account the effect of perfect channel estimation at the receiver. Optimal power and rate adaptation policy provides the highest capacity over other adaptation policies. Truncated channel inversion policy suffers a large capacity penalty relative to the optimal power adaptation policy. However, the capacity penalty for the truncated channel inversion policy is lower compared to channel inversion with fixed rate policy. Furthermore, we derive analytical results for (1) capacity statistics, (2) Complementary Cumulative Distribution Function, and (3) Probability Density Function.

Spectrum Efficiency Evaluation with Multiuser Scheduling and MRC Antenna Diversity with Effects of Combining Errors for Rayleigh Fading Channels with Feedback

In this paper, we present an analytical framework to jointly investigate MRC diversity and multiuser scheduling under Rayleigh fading channels with the effect of combining errors under a low SNR regime. We derive spectrum efficiency expressions for multiuser scheduling systems with two different cases of multiuser scheduling, (1) Full feedback user scheduling (2) Limited feedback user scheduling with effects of combining errors. A signal to noise ratio (SNR) based user selection scheme is considered. For combining errors, correlation is defined as the magnitude of the complex cross-correlation between the transmission coefficients of the medium associated with pilot and message frequencies. We derive the probability density function and cumulative density function for the above cases. Adaptation policies like (1) Optimal power and rate adaptation policy (OPRA), (2) Optimal rate adaptation policy, (3) Channel inversion with fixed rate policy, (4) Truncated channel inversion with fixed rate policy are employed to improve system performance. It is observed that limited feedback user scheduling along with employment of adaptation policies yields better system performance compared to full feedback user scheduling. Further, OPRA policy yields maximum capacity among the four policies.