Variable-power Adaptive Modulation Research Articles

Performance of orthogonal STBC-MIMO with variable-power adaptive modulation and delayed feedback in Nakagami fading channels

A variable-power (VP) adaptive modulation (AM) scheme with space-time block code (STBC) is developed in MIMO systems, and the corresponding performance is investigated in Nakagami fading channels. The optimum switching thresholds are derived so that the spectrum efficiency (SE) under an average power and a target bit error rate (BER) constraint is maximized. By the switching thresholds, average BER and SE are further derived. As a result, closed-form expressions are obtained. Besides, the exact and approximate BER expressions of VP AM and constant-power (CP) AM systems with delayed feedback are respectively derived. Simulation results show that the theoretical SE and BER can match the corresponding simulation well. The results indicate that VP-AM with STBC provide higher spectrum efficiency than the CP counterparts, and has slighter bite error rate performance degradation when the normalized delay is less than 0.01.

Performance Analysis of Variable-Power Adaptive Modulation With Outdated Feedback for Space–Time Coded MIMO Systems

The performance analysis of a space-time coded multiple-input-multiple-output (MIMO) system that employs different variable-power adaptive-modulation (VP-AM) strategies with outdated channel-state information (CSI) over Rayleigh fading channels is presented. The optimal design of VP-AM with power control (PC) in time (VP-AM-TIME) is first developed. The optimum switching thresholds of VP-AM-TIME for attaining maximum spectrum efficiency (SE) subject to a target bit error rate (BER) and an average power constraint are derived. A closed-form expression of the optimal temporal PC in terms of the Lambert function is obtained. A suboptimal VP-AM-TIME is also developed to further simplify the algorithm. The existence and uniqueness of the Lagrange multiplier that was used in the constrained optimization for the optimal and suboptimal VP-AM-TIME are investigated. By using the switching thresholds, we obtain closed-form expressions of SE and average BER. Then, a VP-AM scheme with joint spatial-temporal power control (VP-AM-JOINT) is developed. The joint spatial-temporal PC problem can be solved by transforming it into an inner-outer optimization problem. Theoretical analysis and simulation results show that the proposed optimal VP-AM-TIME can obtain the same performance as the existing optimal VP-AM-TIME but with much lower computational complexity, and the new suboptimal VP-AM-TIME outperforms the existing suboptimal scheme and obtains a performance closer to the optimal scheme. The results also show that the new VP-AM-JOINT scheme is always superior to the optimal VP-AM-TIME and provides considerable gains in SE over a wide range of feedback delays, although the two schemes have almost the same performance when the feedback delay becomes large.

Performance of variable-power adaptive modulation with space–time block coding and imperfect channel state information over Rician fading channels

The performance analysis of a space–time block coded multiple input multiple output system with variable-power (VP) adaptive modulation (AM) over Rician fading channels for imperfect channel state information is presented. The optimum switching thresholds for attaining maximum spectrum efficiency (SE) subject to a target bit error rate (BER) and an average power constraint are derived. In the derivation, a very tight BER expression of quadrature amplitude modulation (QAM) is used to develop a new power adaptation scheme that can fulfil the target BER even at low signal-to-noise ratio (SNR) and achieve higher SE than the scheme using the BER upper bound commonly adopted in the literature. The existence and uniqueness of the Lagrange multiplier used in the constrained optimisation are investigated. It is shown that the Lagrange multiplier is unique when it exists. By using the switching thresholds, the authors obtain closed-form expressions of SE and average BER of the system. Computer simulation shows that the theoretical analysis is in good agreement with simulation. The results show that the proposed VP-AM scheme provides better SE than the constant-power counterparts and the VP-AM scheme using the commonly used BER bound.

Performance of variable-power adaptive MQAM with transmit antenna selection and delayed feedback in Nakagami Fading channel

In this paper, the performance of MIMO systems with variable-power (VP) adaptive MQAM and transmit antenna selection (TAS) over Nakagami-m fading channel is presented. The optimum switching thresholds for attaining maximum spectrum efficiency (SE) subject to a target bit error rate (BER) and an average power constraint are derived. It is shown that the Lagrange multiplier in the constrained SE optimization will be unique if the existence condition for MIMO system with TAS is satisfied. A practical iterative algorithm based on Newton's method for finding the Lagrange multiplier is proposed. By using the switching thresholds, closed-form expressions of the SE and average BER are obtained. Besides, BER expressions with delayed feedback are derived for VP and constant-power (CP) systems, respectively. With these expressions, the impact of feedback delay on the BER performance is effectively assessed. Computer simulation for SE and BER shows that the theoretical analysis and simulation are in good agreement. The results show that the VP adaptive modulation (AM) system with TAS provides better SE than the CP counterpart and VP AM system with space-time coding, and has slight BER performance degradation for the normalized time delay less than 0.01.

Performance analysis of variable-power adaptive modulation with antenna selection over Rayleigh fading channels

The performance of multi-input multi-output (MIMO) systems with variable-power (VP) adaptive modulation (AM) and antenna selection (AS) over Rayleigh fading channels for both perfect and imperfect channel state information (CSI) is presented. In this study, the optimum fading gain switching thresholds for attaining maximum spectrum efficiency (SE) subject to a target bit error rate (BER) and an average power constraint are derived. It is shown that the Lagrange multiplier in the constrained SE optimisation is unique if the existence conditions for MIMO systems with transmit AS and receive maximal-ratio combining under perfect or imperfect CSI are satisfied. By using the switching thresholds, we obtain closed-form expressions of the SE and average BER for VP and constant-power (CP) systems. Simulation results for SE and BER are in good agreement with the theoretical analysis. The results show that the VP-AM system using AS with the derived VP control scheme provides better SE than the CP counterpart and the VP-AM system with space-time coding and can fulfil the target BER for different signal-to-noise ratios (SNRs).

Performance analysis of variable-power adaptive modulation in space-time block coded MIMO diversity systems

In this paper, the performance analysis of multi-antenna MIMO systems with variable-power (VP) adaptive modulation and space-time coding is presented. The fading gain value is partitioned into a number of regions by which we decide the transmission power according to the region the fading gain falls in. The optimum fading gain region boundaries (switching thresholds) for maximizing spectrum efficiency (SE) under a target BER and average power constraint are derived. It is shown that the Lagrange multiplier in the constrained SE optimization will be unique if the existence condition for multi-antenna systems is satisfied, and is unique for single antenna systems. A practical iterative calculation method based on Steepest Descent Algorithm for finding the Lagrange multiplier is proposed. With the switching thresholds, the closed-form expressions of the SE and average BER, respectively for VP system and constant-power (CP) system are achieved. Simulation results for SE and average BER are in good agreement with the theory analysis. The results show that VP adaptive modulation with space-time coding provides better SE than the counterpart with CP under a target BER and average power constraint.

Performance of Variable-Power Adaptive Modulation With Space–Time Coding and Imperfect CSI in MIMO Systems

The performance analysis of multi-input-multi-output (MIMO) systems with M-ary quadrature amplitude modulation (MQAM) and a space-time block code (STBC) over flat Rayleigh fading channels for imperfect channel state information (CSI) is presented. In this paper, the optimum fading gain switching thresholds for attaining maximum spectrum efficiency (SE) subject to a target bit-error rate (BER) and an average power constraint are derived. It is shown that the Lagrange multiplier in the constrained SE optimization does exist and is unique for imperfect CSI and for single-input-single-output (SISO) systems under perfect CSI. On the other hand, the Lagrange multiplier will be unique if the existence condition for MIMO under perfect CSI is satisfied. Numerical evaluation shows that the variable-power (VP) adaptive modulation (AM) with STBC provides better SE than its constant-power (CP) counterpart.

Quality-of-Service Driven Power and Rate Adaptation over Wireless Links

We propose a quality-of-service (QoS) driven power and rate adaptation scheme over wireless links in mobile wireless networks. Specifically, our proposed scheme aims at maximizing the system throughput subject to a given delay QoS constraint. First, we derive an optimal adaptation policy by integrating information theory with the concept of effective capacity for a block fading channel model. Our analyses reveal an important fact that there exists a fundamental tradeoff between throughput and QoS provisioning. In particular, when the QoS constraint becomes loose, the optimal power-control policy converges to the well-known water-filling scheme, where Shannon (ergodic) capacity can be achieved. On the other hand, when the QoS constraint gets stringent, the optimal policy converges to the total channel inversion scheme under which the system operates at a constant rate. Inspired by the above observations, we then consider a more practical scenario where variable-power adaptive modulation is employed over both block fading and Markov correlated fading channels. In both cases, we derive the associated power and rate adaptation policies. The obtained results suggest that the channel correlation has a significant impact on QoS-driven power and rate adaptations. The higher the correlation is, the faster the power-control policy converges to the total channel inversion when the QoS constraint becomes more stringent. Finally, we conduct simulations to verify that the adaptation policy proposed for Markov channel models can also be applied to the more general channel models.