| 1. IntroductionOrthogonal Frequency Division Multiplexing (OFDM) is a widely used multi-carrier modulation scheme. It divides the multipath fading channel into a number of parallel frequency dependent flat fading channels. It can support high speed data service and has strong ability of combating with multipath interference. Simultaneously, OFDM can dynamically allocate system resources (such as bit, power, etc) according to subchannel's fading and multipath delay characteristics. The MIMO system uses multiple antennas at both the transmitter and the receiver to transmit signal in the same frequency band at the same time, which can significantly increase the channel capacity linearly with the number of antenna elements without extra bandwidth and extra power. To a certain extent, MIMO can overcome multipath fading while it is powerless for frequency selecting fading. So the technique combined with MIMO and OFDM is becoming a hot issue in communication.Generally, resource allocation can be divided into two categories: (1) Fixed resource allocation schemes, such as TDMA and FDMA, assign an independent dimension, e.g. time slot or subchannel to each user. It is rigid regardless of the current channel condition. (2) Dynamic resource allocation schemes, allocate a dimension adaptively to the users based on their channel gains. Two classes of optimization techniques have been proposed in the dynamic multiuser OFDM literatures: margin adaptive (MA) and rate adaptive (RA). The objective of MA is to achieve the minimum overall transmit power under the constraints on the users'data rate or bit error rate. While the objective of RA is to maximize each user's capacity with a total transmit power constraint. This paper mainly studies the MA optimization technique in MIMO-OFDM system.2. MA optimization technique in OFDM systemA subcarrier, bit and power allocation method is proposed in this paper, which is based on the users'QoS. (A) Not considering the rate request between users. We first discuss the subcarrier allocation. Under optimal theory, each subcarrier should be assigned to the user who has the best channel gain for the subcarrier. In the next bit allocation, a genetic algorithm is introduced to solve the problem through selection, crossover and mutation and series of other operations. (B) Considering the rate request between users. Under this situation, algorithm should balance the rate request of each user. Subcarriers allocation has two-step processes. In the initial process, subcarriers are assigned to each user according to its rate request. In the optimization process, genetic algorithm is used. In order to obtain a better performance, the algorithm using an adaptive portfolio modulation scheme and also some adjustments are done for the genetic algorithm parameters. Once the subcarriers have been allocated, the single-user bit, power allocation methods, such as greedy method, can be used.3. MA optimization technique in MIMO-OFDM systemAssuming the information of the channel state is known, a MOMO-OFDM system can be translated into several independent OFDM systems by the application of EVD. According to the beam forming technique, each subcarrier's largest eigen value is used as a allocation reference. The simulation results show that the method in this paper can be applied to multi-user MIMO-OFDM system.4. Simulation and ConclusionThe simulation of the proposed resource allocation algorithm on computer with MATLAB is performed. Simulation results show that the proposed algorithm has much better performance compared to the fixed subcarrier allocation TDMA and FDMA. Compared with [23] and [24], system performance of the proposed algorithm is better than that of [23]. In addition, the simulation results also show that the method in this paper can be applied to multi-user MIMO-OFDM system. |
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