Power Adaptation Policy Research Articles

Capacity Analysis of Channel Inversion Using Space-Time Block Coding with Transmit Power Control

In this paper, we analyze channel inversion with transmit diversity in terms of capacity and bit error rates in correlated fading. We obtain the Shannon capacity of Rayleigh fading channel under channel inversion using spatially correlated dual branch orthogonal space-time block coding, in comparison with the optimal power and rate adaptation policy, and provide a simple fixed-rate power adaptation policy to invert the Rayleigh fading into additive white Gaussian noise channel, and obtain the bit error rates using binary phase-shift keying modulation with/without channel inversion. It is found that the correlation generally causes a loss in capacity, which becomes more distinct with channel inversion relative to the optimal power and rate adaptation policy, especially when power correlation is greater than 0.5, and hinders otherwise possible full inversion. Transmit power truncation introduces power gain over full inversion, which manifests itself as an increase in capacity and a decrease in bit error probability. This gain increases with decreasing truncation level of the allowed peak transmit power. Inverting with truncation, a gain of about 19 dB in signal-to-noise ratio is obtained over classical single-antenna transmission without inversion. This gain may be further increased by a combination of truncation and correlation levels, as the correlation surprisingly turns to advantage when the normalized permitted peak-power is below 2 dB. However, the gain is normally accompanied by lower throughput or bandwidth expansion for a fixed data rate due to outage.

Energy-Efficient Power Control in Fading Channels With Markovian Sources and QoS Constraints

In this paper, energy-efficient power adaptation policies in fading channels are analyzed when data arrivals are modeled as Markovian processes (namely, discrete Markov, Markov fluid, and discrete and fluid Markov modulated Poisson processes) and statistical quality of service (QoS) constraints are imposed on buffer overflow probabilities. In the analysis, both transmission and circuit power consumptions are considered. After formulating energy efficiency (EE) as maximum throughput normalized by the total power consumption, optimal power control policies that maximize EE are obtained for different source models. The impact of source randomness on EE is determined. Optimal power control schemes maximizing the throughput under EE or average power constraints are also investigated. With this, tradeoff between throughput and EE is studied. Finally, the analysis is extended to multichannel scenarios. Overall, the influence of source statistics, QoS constraints, and number of subchannels on the optimal power control policies, throughput, and EE performance is identified.

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>

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.

Open-Loop Power Adaptation in Nanosensor Networks for Chemical Reactors

Due to their extremely small form factor, wireless nanosensor networks (WNSNs) have the potential to monitor and control chemical processes right at the molecule level lifting the process efficiency to a level not possible with conventional methods. However, sensor networking within a chemical reactor is challenging due to its time-varying chemical composition, which creates a time-varying radio channel. The nanosensors therefore need to continuously adapt their transmission power according to the chemical composition while maintaining a low overall power budget. We show that this problem can be modeled as a Markov decision process (MDP). However, the MDP solution requires the sensors to know the composition of the reactor at each time instance, which is difficult to realize due to computation and communication constraints at the nanoscale. We therefore choose to use open-loop methods for power adaptation, which allows nanosensors to dynamically adjust their powers based on a policy derived entirely from offline analysis of the expected channel variation over time. Using extensive simulations, we evaluate the performance of the proposed open-loop power adaptation method for improving the efficiency of a widely deployed gas-to-liquid conversion process known as Fischer-Tropsch (FT) synthesis. We find that the proposed method performs close to optimal achieving a three fold improvement in FT efficiency with only a 100 femto watt power consumption on average for the nanosensor communications. In terms of maximum achievable process efficiency, our method outperforms the previously proposed open-loop power adaptation policy by 30% and the nonadaptive power allocation by 52%.

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.

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.

Variable-Rate, Variable-Power Network-Coded-QAM/PSK for Bi-Directional Relaying Over Fading Channels

Network coded modulation (NCM) holds the promise of significantly improving the efficiency of two-way wireless relaying. In this contribution, we propose near instantaneously adaptive variable-rate, variable-power QAM/PSK for NC-aided decode-and-forward two-way relaying (DF-TWR) to maximize the average throughput. The proposed scheme is optimized subject to both average-power and bit-error-ratio (BER) constraints. Based on the BER bounds, we investigate a discrete-rate adaptation scheme, relying on a pair of solutions proposed for maximizing the spectral efficiency of the network. We then derive a closed-form solution based power adaptation policy for a continuous-rate scheme and quantify the signal-to-noise ratio (SNR) loss imposed by NC-QAM. Our simulation results demonstrate that the proposed discrete adaptive NC-QAM/PSK schemes are capable of attaining a higher spectral efficiency than their fixed-power counterparts.

Spectrum Efficiency Evaluation with Diversity Combining for Fading and Branch Correlation Impairments

In this paper, closed-form expressions for capacities per unit bandwidth for fading channels with impairments due to Branch Correlation are derived for optimal power and rate adaptation, constant transmit power, channel inversion with fixed rate, and truncated channel inversion policies for maximal ratio combining diversity reception case. Closed-form expressions for system spectrum efficiency when employing different adaptation policies are derived. Analytical results show accurately that optimal power and rate adaptation policy provides the highest capacity over other adaptation policies. In the case of errors due to branch correlation, optimal power and rate adaptation policy provides the best results. All adaptation policies suffer no improvement in channel capacity as the branch correlation is increased. This fact is verified using various plots for different policies. With increase in branch correlation, capacity gains are significantly larger for optimal power and rate adaptation policy as compared to the other policies. The outage probability for branch correlation is also derived and analyzed using plots for the same.

System performance of Rayleigh fading channel in the presence of adjacent channel interference

In this paper, the authors study the system performance of adaptive transmission techniques in the presence of adjacent channel interference (ACI). The authors obtain closed-form expressions for system capacity when employing different adaptive transmission policies with and without diversity schemes. The simulation results of spectrum efficiency in a WiMAX network employing 256 orthogonal frequency division multiplexing subcarriers are obtained for the analytical expressions derived for selection combining diversity under various adaptation policies. Analytical results show that the optimal power and rate adaptation policy provides the highest spectrum efficiency. Both analytical and simulation results are found to be in close agreement with each other, which shows that the obtained analytical results works well with the scenario of ACI. Also, maximal ratio combining scheme yields the highest spectrum efficiency when subjected to ACI.

Antenna Selection with Power Adaptation in Interference-Constrained Cognitive Radios

The performance of an underlay cognitive radio (CR) system, which can transmit when the primary is on, is curtailed by tight constraints on the interference it can cause to the primary receiver. Transmit antenna selection (AS) improves the performance of underlay CR by exploiting spatial diversity but with less hardware. However, the selected antenna and its transmit power now both depend on the channel gains to the secondary and primary receivers. We develop a novel Chernoff-bound based optimal AS and power adaptation (CBBOASPA) policy that minimizes an upper bound on the symbol error probability (SEP) at the secondary receiver, subject to constraints on the average transmit power and the average interference to the primary. The optimal antenna and its power are presented in an insightful closed form in terms of the channel gains. We then analyze the SEP of CBBOASPA. Extensive benchmarking shows that the SEP of CBBOASPA for both MPSK and MQAM is one to two orders of magnitude lower than several ad hoc AS policies and even optimal AS with on-off power control.

Capacity of correlative nakagami-m fading channels under adaptive transmission and maximal-ratio combining diversity technique

The study of channel capacity evaluation in conjunction with maximal ratio diversity-combining (MRC) is presented in this paper. Analysis of the capacity in correlative Nakagami-m fading channels is observed. Using the proposed fading model, the power and rate adaptation, constant transmit power, channel inversion with fixed rate and truncated channel inversion adaptation policies are analyzed. Our results show that the power and rate adaptation policy, being only slightly higher than capacity of constant transmit power policy, provides the highest capacity over the other adaptation policies. The results also show that truncated channel inversion adaptation policy is better alternative compared to complete channel inversion policy for all values of fading severity, diversity order and correlation coefficient.

SEP-Optimal Transmit Power Policy for Peak Power and Interference Outage Probability Constrained Underlay Cognitive Radios

In underlay cognitive radio (CR), a secondary user (SU) can transmit concurrently with a primary user (PU) provided that it does not cause excessive interference at the primary receiver (PRx). The interference constraint fundamentally changes how the SU transmits, and makes link adaptation in underlay CR systems different from that in conventional wireless systems. In this paper, we develop a novel, symbol error probability (SEP)-optimal transmit power adaptation policy for an underlay CR system that is subject to two practically motivated constraints, namely, a peak transmit power constraint and an interference outage probability constraint. For the optimal policy, we derive its SEP and a tight upper bound for MPSK and MQAM constellations when the links from the secondary transmitter (STx) to its receiver and to the PRx follow the versatile Nakagami-m fading model. We also characterize the impact of imperfectly estimating the STx-PRx link on the SEP and the interference. Extensive simulation results are presented to validate the analysis and evaluate the impact of the constraints, fading parameters, and imperfect estimates.

Novel Closed-Form Expressions for Ergodic Capacity of MIMO-OSFBC-OFDM Systems Over Channel Adaptation Policies

In this work, closed-form expressions for capacities per unit bandwidth for MIMO-OFDM systems employing Orthogonal Space-Frequency Block Coding over multipath frequency-selective fading channels are derived for adaptation policies like optimal power and rate adaptation, optimal rate adaptation with constant transmit power, channel inversion with fixed rate, and truncated channel inversion polices. A Signal-to-Noise Ratio based user selection scheme is considered. Optimal power and rate adaptation policy provides the highest capacity over other adaptation policies. Capacity penalty is the highest for optimal rate adaptation with constant transmit power policy, while the performance of channel inversion with fixed rate policy and truncated channel inversion policy lie between that of OPRA and ORA policies.

Cross-Layer Optimization of Two-Way Relaying for Statistical QoS Guarantees

Two-way relaying promises considerable improvements on spectral efficiency in wireless relay networks. While most existing works focus on physical layer approaches to exploit its capacity gain, the benefits of two-way relaying on upper layers are much less investigated. In this paper, we study the cross-layer design and optimization for delay quality-of-service (QoS) provisioning in two-way relay systems. Our goal is to find the optimal transmission policy to maximize the weighted sum throughput of the two users in the physical layer while guaranteeing the individual statistical delay-QoS requirement for each user in the datalink layer. This statistical delay-QoS requirement is characterized by the QoS exponent. By integrating the concept of effective capacity, the cross-layer optimization problem is equivalent to a weighted sum effective capacity maximization problem. We derive the jointly optimal power and rate adaptation policies for both three-phase and two-phase two-way relay protocols. Numerical results show that the proposed adaptive transmission policies can efficiently provide QoS guarantees and improve the performance. In addition, the throughput gain obtained by the considered three-phase and two-phase protocols over direct transmission is significant when the delay-QoS requirements are loose, but the gain diminishes at tight delay requirements. It is also found that, in the two-phase protocol, the relay node should be placed closer to the source with more stringent delay requirement.

Capacity analysis of Rayleigh fading channels in low signal‐to‐noise ratio regime for maximal ratio combining diversity because of combining errors

In this research work, the authors derive closed-form expressions for capacity per unit bandwidth (spectrum efficiency) of correlated Rayleigh fading channels under maximal ratio combining diversity at low signal-to-noise ratios (SNRs). Spectrum efficiency expressions are derived for M diversity branches under adaptation policies, such as: (i) optimal power and rate adaptation (OPRA) policy, (ii) optimal rate adaptation (ORA) policy and (iii) truncated channel inversion with fixed rate (TIFR) policy. Even at low SNRs, if space diversity is exercised using a single-input multiple-output system, spectrum efficiency achieved is higher compared with that achieved when signals are uncorrelated and have no diversity. This forms the focal point of this study. Analytical results show accurately that OPRA policy provides highest capacity over other adaptation policies. The spectrum efficiency for the three policies and outage probability for the low SNR case are derived, plotted and analysed in detail in this work.

Performance Analysis of MC-CDMA Systems Under Nakagami Hoyt Fading

Based on an alternative expression for Q-function, a simple bit error rate expression is derived in this paper for multicarrier code division multiple access systems with maximal ratio combining in correlated Nakagami-q channels. Furthermore, in this paper, we derive bounds on the probability of error and ergodic capacity of spatially multiplexed MC-CDM systems with zero forcing unified successive interference cancellation technique. Closed-form expressions for Capacities per unit bandwidth and Outage probability using optimal power and rate adaptation policy are derived and plotted. Asymptotic approximations and upper bounds on spectrum efficiency are also derived and plotted. Numerical results for Symbol Error Rate are also derived and plotted using MATLAB.

Spectrum Efficiency of WiMAX Networks in the presence of Interference with Diversity Combining

Worldwide Interoperability for Microwave Access) is a promising technology which can offer high speed voice, video and data services upto the requirements at the customer's end. The objectives of this paper is to evaluate the Performance evaluation of a WiMAX system under various diversity schemes (Selection combining, Maximal ratio combining and Equal gain combining), employing different adaptive transmission policies, such as Optimal power and rate adaptation policy, Optimal rate adaptation with constant transmit power policy, Channel inversion with fixed rate policy, and Truncated channel inversion policy, subjected to co-channel interference and adjacent channel interference. WiMAX system incorporates OFDM with 256 sub-carriers with QPSK modulation as the transmission scheme. Simulated results of the estimated spectrum efficiency show that the implementation of Optimal power and rate adaptation policy under Selection combining is highly effective to combat co- channel interference and adjacent channel interference in the WiMAX communication system. Keywords-channel interference; Adjacent channel interference; Optimal power and rate adaptation policy; Optimal rate adaptation with constant transmit power policy; Channel inversion with fixed rate policy; Truncated channel inversion with fixed rate policy.

Spectrum efficiency of Fixed WiMAX OFDM network in the presence of co-channel interference with diversity combining

WiMAX (Worldwide Interoperability for Microwave Access) is a promising technology which can offer high speed voice, video and data services upto the requirements at the customer's end. The objectives of this paper is the Performance evaluation of a WiMAX system under various diversity schemes (Selection combining, Maximal ratio combining and Equal gain combining), employing different adaptive transmission policies, such as Optimal power and rate adaptation policy, Optimal rate adaptation with constant transmit power policy, Channel inversion with fixed rate policy, and Truncated channel inversion policy, subjected to co-channel interference. The WiMAX system incorporates OFDM with BPSK modulation as the transmission scheme. Simulated results of the estimated spectrum efficiency shows that the implementation of Optimal power and rate adaptation policy is highly effective to combat co-channel interference in the WiMAX communication system, under Selection combining.