Improved Rate-Maximization-Based Adapitive Resource Allocation For Multiuser OFDM Systems

1. IntroductionThe growing demand for wireless multimedia services requires reliable and high-rate data communications over a wireless channel. But under the current mobile environment, conventional modulation techniques hardly meet the requirements of high-speed communications for limited spectrum resources and inter-symbol interference. However, orthogonal frequency division multiplexing (OFDM) can effectively increase the transmission rate for its ability to improve spectrum efficiency and combat multi-path. It is a promising technology in wide band wireless systems. Moreover, dynamic resource allocation techniques can greatly improve the system spectrum efficiency by adaptive subcarrier and transmit power allocation according to the instantaneous channel state information (CSI). In frequency selective fading channel, the combination of OFDM and dynamic allocation can utilize the merits of both technologies and is attracting more and more interests.For multiuser OFDM systems, two classes of resource allocation schemes exist: 1) fixed resource allocation; 2) dynamic resource allocation. Two classes of optimization techniques have been proposed in dynamic multiuser OFDM allocation, namely: 1) margin adaptive (MA); 2) rate adaptive (RA).In this paper, we focus on rate adaptive dynamic resource allocation. In rate adaptive resource allocation, subcarrier and transmit power allocation are performed to maximize the overall data rate while achieving proportional fairness amongst users under a total power and BER constraints. Though the sum capacity is distributed proportionally amongst users in [24], during performing power distribution among users, solving the set of functions provides the optimal power allocation scheme. The equations are, in general, nonlinear. Iterative methods, such as the Newton-Raphson, can be used to obtain the solution, with a certain amount of computational effort. After the above consideration, this paper improves subcarrier allocation method and proposes a simple power allocation algorithm. This algorithm reduces the computational cost of the system and increase system performance. Meantime, it assures proportional fairness amongst users.2. Improved adaptive resource allocation algorithmThis paper proposes a simple adaptive allocation algorithm. It is a low-complexity suboptimal algorithm that the subcarrier allocation and the power allocation are independently conducted. First, under the assumption of an equal power distribution, the subcarrier allocation algorithm based on RA rule is performed for each user. Second, don't execute the water-filling allocation algorithm completely,otherwise only comfirm the standard multiuser water-filling gate value and allocate the transmission power and the bit to each subcarrier. Under the limiting factors of BER and the total power, maximize the sum capacity and overcome the complexity of the former methods.3. Simulation results and performance analysisSimulation results show that the proposed algorithm compared to the former methods,the proposed method in this paper can firstly satisfy differently required data rate and secondly the sum capacity is distributed more fairly among users. Thirdly,it decreases the complexity of calculation.Finally,the result of simulation indicates that the system performance of the proposed algorithm is better than before. Notice that this capacity gain is from subcarrier allocation. Because every user chooses the most efficient subcarrier which makes the total transmitted bit increased, the sum capacity is increased accordingly.