An Efficient Adaptive Modulation Scheme for Wireless OFDM Systems

An adaptive modulation scheme is presented for multiuser orthogonal frequency‐division multiplexing systems. The aim of the scheme is to minimize the total transmit power with a constraint on the transmission rate for users, assuming knowledge of the instantaneous channel gains for all users using a combined bit‐loading and subcarrier allocation algorithm. The subcarrier allocation algorithm identifies the appropriate assignment of subcarriers to the users, while the bit‐loading algorithm determines the number of bits given to each subcarrier. The proposed bit‐loading algorithm is derived from the geometric progression of the additional transmission power required by the subcarriers and the arithmetic‐geometric means inequality. This algorithm has a simple procedure and low computational complexity. A heuristic approach is also used for the subcarrier allocation algorithm, providing a trade‐off between complexity and performance. Numerical results demonstrate that the proposed algorithms provide comparable performance with existing algorithms with low computational cost.     

Use of linear programming for dynamic subcarrier and bit allocation in multiuser OFDM

An adaptive subcarrier allocation and an adaptive modulation for multiuser orthogonal frequency-division multiplexing (OFDM) are considered. The optimal subcarrier and bit allocation problems, which are previously formulated as nonlinear optimizations, are reformulated into and solved by integer programming (IP). A suboptimal approach that performs subcarrier allocation and bit loading separately is proposed. It is shown that the subcarrier allocation in this approach can be optimized by the linear-programming (LP) relaxation of IP, while the bit loading can be performed in a manner similar to a single-user OFDM. In addition, a heuristic method for solving the LP problem is presented. The LP-based suboptimal and heuristic algorithms are considerably simpler to implement than the optimal IP, plus their performances are close to those of the optimal approach.     

Adaptive Subband Bit and Power Loading Algorithm and its Application to OFDM Based IEEE 802 .16e

1IntroductionRecently,alarge amount of research has beenfocusedonthe high data rate transmissiontechniques over wire-less channels . One of the main requirements on thesetransmissiontechniques is the ability to combat the In-ter-Symbol Interference (ISI) ,a major problemin highdata rate transmission over multipath fading channels .OFDMis one of the promising solutions to this prob-lem[1 ~3 ,12],andrecently IEEE802 .16e[7]alsosuggest-ed using OFDM structure as its physical layer . Thecom…     

OFDM-UWB系统基于用户速率的多用户动态资源分配算法

在原有动态资源分配算法基础上,提出了一种基于用户速率需求的动态资源分配算法。该算法在满足用户数据速率需求和服务质量要求(QoS)的前提下,以用户公平性为原则,分步执行子载波和比特分配来降低系统总的发射功率。首先,通过比较不同子载波对用户速率的影响,引入速率影响因子,对子载波进行分配;然后为每个用户子载波分配比特,并根据用户速率需求进行比特调整。为了进一步降低系统的复杂度,提出了一种通过子载波分组来完成子载波比特分配的方法。仿真结果表明,该算法能够降低系统功耗、误码率和系统复杂度。     

A Fast Subcarrier, Bit, and Power Allocation Algorithm for Multiuser OFDM-Based Systems

In this paper, we propose a real-time subcarrier, bit, and power allocation algorithm for orthogonal frequency-division multiplexing-based multiuser communication systems in downlink transmission. Assuming that base stations know the channel gains of all subcarriers of all users, the proposed loading algorithm tries to minimize the required transmit power while satisfying the rate requirement and data error rate constraint of each user. The novel algorithm simultaneously determines subcarrier, bit, and power allocation by enhancing the suboptimal algorithm by Wong while having the same computational complexity. The proposed scheme offers better performance in terms of transmit power than that of Wong , as demonstrated in the simulation results, whereas the performance of the scheme in Wong was close to that of the optimal solution.     

Bit loading with BER-constraint for multicarrier systems

We present discrete adaptive bit loading algorithms for multicarrier systems with uniform (nonadaptive) power allocation operating in a frequency selective fading environment. The algorithms try to maximize the overall throughput of the system while guaranteeing that the mean bit error rate (BER) remains below a prescribed threshold. We also study the impact of imperfect subcarrier signal-to-noise ratio information on throughput performance. Results show that the proposed algorithms have approximately the same throughput and mean BER as the optimal allocation while having a significantly lower computational complexity relative to other algorithms with near-optimal allocations. Moreover, when compared with algorithms that employ approximations to water filling, the computational complexity is comparable while the overall throughput is closer to the optimum.     

电力线通信系统中的子载波组分配

研究电力线通信系统在各种约束下的速率自适应子载波分配模型,提出一种基于层次分析法的动态子载波组分配算法。为了对比还提出以一种不含决策者偏好信息和比特功率比最大化的逐子载波分配算法。在典型电力线信道环境下还仿真以一个子载波为一组的子载波组分配算法,结果表明一子载波一组的子载波组分配算法性能最优,但其复杂度最大,而动态子载波组分配算法的性能与逐子载波分配对比算法的性能相当,且接近于一子载波一组的分配算法,复杂度大大减少。     

Power allocation for OFDM using adaptive beamforming over wireless networks

The performance of a multiuser wireless network using orthogonal frequency-division modulation (OFDM), combined with power control and adaptive beamforming for uplink transmission is presented here. A network-wide adaptive power control algorithm is used to achieve the desired signal-to-interference-and-noise-ratio at each OFDM subcarrier and increase the power efficiency of the network. As a result, we can achieve a better overall error probability for a fixed total transmit power. With the assumption of fixed-modulation for all subcarriers, transmit powers and beamforming weight vectors at each subcarrier are updated jointly, using an iterative algorithm that converges to the optimal solution for the entire network. Unlike most of the loading algorithms, this approach considers fixed bit allocation and optimizes the power allocation and reduces the interference for the entire network, rather than a single transmitter. We also propose joint time-domain beamforming and power control to reduce the complexity resulting from the number of beamformers and fast Fourier transformed blocks. The proposed algorithm is also extended to COFDM and we show that it improves the performance of those systems.     

Rate adaptive resource allocation for OFDM downlink transmission

In this paper, we present a multiuser rate adaptive resource allocation for OFDM downlink transmission. This new algorithm assign one bit at a time to user that has the minimum total power in the subcarrier that requires the least additional power. Simulation results show that the proposed algorithm achieves a higher capacity than previous existing algorithms and distributed the overall capacity more fairly among users.     

一种多用户MIMO/OFDM下行链路保障QoS的自适应传输机制

在用户QoS参数限制下,该文提出一种在多用户MIMO/OFDM下行链路使系统总速率最大的资源分配机制。基站应用空分多址接入,使得每个子载波可支持多个用户,应用线性预编码方法抵消用户间的干扰,提出保障QoS的自适应功率、比特分配方案。该文并提出两种可应用到实际系统的低复杂度的比特加载和比特去除算法,仿真结果表明,该两种方法性能可非常接近最优遍历算法。     

Loading Algorithm for Multicarrier Spatial Diversity Systems with Antenna Selection

In this paper, a novel loading algorithm consisting of four variants is proposed for a multicarrier transceiver employing multiple antennas configured for spatial diversity. The primary objective of the proposed algorithm is to increase the overall throughput while ensuring the mean bit error rate (BER) is below a specified limit. To achieve this, spatial diversity is employed to improve the subcarrier signal-to-noise ratio (SNR) values. Simultaneously, (uniform or non-uniform) bit allocation, which is a function of subcarrier SNR, is performed to increase throughput. To reduce power consumption, spatial processing complexity, and hardware costs, antenna subset selection is also performed by the proposed algorithm to choose a set of active transmit/receive antennas. The results show that combining bit allocation with spatial diversity (employing antenna subset selection) can yield substantial throughput increases.     

Low complexity adaptive power and subcarrier allocation for OFDMA

The problem of low complexity sub-optimal power and subcarrier allocation for OFDMA systems is addressed in the paper. We propose a heuristic non-iterative method that is an extension of the ordered subcarrier selection algorithm recently proposed for a single user case to the OFDMA systems. The suggested method improves the average BER-performance of the system and it also provides fairness between the users in terms of the bit rates. Both the theoretical analysis and simulation results confirm efficiency of the proposed technique. Based on the proven lemma about inversion of power gains, we strengthen the suggested method by low complexity power loading that consists in equalization of the received signal-to-noise ratios of each user     

A sub-optimal joint subcarrier and power allocation algorithm for multiuser OFDM

This paper investigates subcarrier and power allocation in multiuser OFDM. The aim is to maximize the overall rate while achieving proportional fairness amongst users under a total power constraint. Achieving the optimal solution is computationally demanding thereby necessitating the use of sub-optimal techniques. Existing sub-optimal techniques either use fixed power allocation and perform only subcarrier allocation or handle subcarrier and power allocation separately. In this paper, we propose an algorithm that performs joint subcarrier and power allocation. Simulation results are shown to compare the performance of the proposed algorithm with that of existing algorithms.     

Adaptive subcarrier allocation and bit loading for voice/data transmission in multiuser OFDM systems

A new algorithm of adaptive subcarrier allocation and bit loading (A‐SABL) is proposed for simultaneous voice and data transmission in multiuser OFDM systems. The algorithm takes advantage of the frequency diversity and the voice/data transmission requirements to dynamically assign the number of subcarriers and bits/per symbol on each subcarrier for each user in a single cell. Due to the strict delay requirement of voice service, the subcarriers with low channel gains are assigned for voice transmission with a small number of bits per symbol to guarantee its required bit‐error‐rate (BER) and transmission rate. Based on the remaining subcarriers with high channel gains and the transmission power, the throughput of data transmission is then maximized by loading as many bits as possible on each subcarrier to achieve the required transmission bit rate and BER. Theoretical analysis and simulation on the proposed algorithm show that a better performance is obtained than previously reported schemes. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance evaluation of multicast MISO–OFDM systems

In this paper, we analyze the performance of multicast orthogonal frequency division multiplexing (OFDM) systems with single and multiple transmit antennas. We show that the resource allocation that includes the subcarrier allocation, bit loading, and the precoding vector selection in the multiple-input single-output (MISO) case is a difficult optimization problem. Consequently, we propose suboptimal algorithms based on the maximization of the sum data rate and the maximization of the minimum user data rate criteria. For practical application, we consider a complete transmission chain by combining powerful erasure codes with the proposed algorithms. Using this scheme, we guarantee that each user receives the same amount of information to decode the same data. Simulation results show that, for both single-input single-output (SISO)–OFDM and MISO–OFDM cases, the proposed multicast OFDM systems achieve gains over the worst user case algorithm.     

On the use of NSGA-II for multi-objective resource allocation in MIMO-OFDMA systems

This paper investigates the problem of dynamic subcarrier and bit allocation in downlink of Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiple Access (OFDMA) Systems. Using Singular Value Decomposition, the MIMO fading channel of each subcarrier is transformed into an equivalent bank of parallel Single Input Single Output sub-channels. To achieve the capacity bound, one must solve a multiuser subcarrier allocation and the optimal bit allocation jointly. To alleviate the computational complexity of joint subcarrier and bit allocation, several suboptimal solutions have been proposed. These suboptimal solutions handle subcarrier and bits individually. We propose the use of Non-dominated Sorting Genetic Algorithm (NSGA)-II, which is a multi-objective Genetic Algorithm, for joint allocation of bits and subcarriers, in the downlink of MIMO-OFDMA system. NSGA-II is intended for optimization problems involving multiple conflicting objectives. Here the two conflicting objectives are Rate Maximization and Transmit Power Minimization. The simulation results indicate remarkable improvement in terms of convergence over previous approaches involving Evolutionary algorithms. At the same time capacity achieved by the proposed algorithm is found to be comparable with that of previous algorithms.     

Joint subcarrier and power allocation in uplink OFDMA systems

In this letter, we focus on joint subcarrier and power allocation in the uplink of an OFDMA system. Our goal is to maximize the rate-sum capacity in the uplink. For the purpose, we formulate an optimization problem subject to subcarrier and power constraints and draw necessary conditions for optimality, from which we derive joint subcarrier and power allocation algorithms. Simulation results show that our proposed scheme enhances the system capacity, providing almost near optimal solutions with low computational burden.     

组播OFDM系统中基于子载波成对调整的功率分配算法

 研究了在总功率的约束下,组播OFDM系统中最大化组播速率的功率分配问题. 首先引入贪婪算法求解该问题以降低复杂度,但贪婪算法只执行预设的固定次数分配后即停止,其截止条件与功率分配过程中的状态无关,无法有效地提高系统性能;进而提出了两种基于子载波成对调整的功率分配算法,在初始功率分配的基础上选择子载波对交换功率,将功率重新分配给能够使组播速率提高得最多的子载波,并只在无法进一步提高组播速率时才停止调整. 研究和仿真结果表明,所提出的两种调整算法能够有效地提高组播速率,性能接近根据凸函数优化方法求得的最优解,且显著降低了算法的复杂度.     

Cooperative Subcarrier Allocation and Power Allocation in the Downlink of an Amplify-and-Forward OFDM Relaying System

The increasing use of relays in wireless communication systems is a driving force to explore innovative techniques that can improve the quality of service as well as enhance the coverage in wireless systems. In this article, we propose a two-step approach for subcarrier allocation and power allocation in the downlink of an OFDM-based amplify-and forward relaying system. In the first step, the total power is allocated to mobile users and relays based on the optimum power ratio derived for the single-relay system. In the second step, subcarriers are assigned to the users based on cooperative subcarrier allocation algorithms. Compared to the non-cooperative algorithm with flat power allocation, the total throughput is shown to improve by applying the two-step cooperative subcarrier allocation and power allocation algorithms.     

An extension to the ordered subcarrier selection algorithm (OSSA)

In this letter, we propose an extension to the ordered subcarrier selection algorithm (OSSA) for orthogonal frequency division multiplexing (OFDM) systems. The result is a simple algorithm for minimizing the bit error rate of the OFDM system at a fixed throughput. The proposed algorithm employs multiple modulations (non-uniform bit loading) within an OFDM symbol. However, unlike existing bit loading algorithms that have a very high computational complexity, the proposed algorithm is based only on the ordered statistics of the subcarrier gains and is consequently very simple. After ordering the subcarriers based on their gains, progressively higher order modulations are used with increasing gains. The key aspect here that greatly simplifies the algorithm is that the modulation used on a subcarrier depends only on the position of its gain in the ordered set and not on the actual values of the gains. We show an analytical approach for determining the parameters of the algorithm.