Research On Adaptive Subcarrier, Bit And Power Allocation Algorithms For OFDM Systems

The main problem in broadband systems is InterSymbol Interference (ISI) due to multipath fading.Orthogonal Frequency Division Multiplexing (OFDM) technique has attracted the attention of people, because it can combat multipath fading and has high band-width efficiency. With more and more requirement on the digital, wideband, personal and mobile communication, OFDM has found its wide applications.It is estimated that OFDM is one of the main techniques in the fourth generation of mobile communication.OFDM systems are of multi-carrier structure .Because of multipath fading and time-variation in radio channel,different subcarrier has different transmission capacity which is time-variant.In a traditional OFDM system which uses the same fixed modulation scheme for all subcarriers,frequency resource is not made the most use of and the bit-error-ratio performance depends on the worst subcarrier.Besides,static TDMA or FDMA is adopted in a traditional multiuser OFDM system.As a result, the best subcarrier for one user may be only used for fractional time or have never been used at all.In order to solve the problems above,we can make use of the multi-carrier structure of OFDM systems,altering the transmit parameters according to the instantaneous status of each subchannel and allocating different number of bits or different amount of power to each subcarrier .In this way,the performance of the system is optimized.While this technique is extended to multiuser OFDM systems,we can allocate subcarriers to users according to the instantaneous status of subchannels,so that the radio resource is allocated to users fairly and effectively.Based on the introduction of the characteristics of radio fading channel, thebasic principle and key techniques of OFDM systems were analyzed in thispaper.And then, adaptive OFDM subcarrier,bit and power allocation algorithmswere analysed and discussed in detail.For single-user adaptive OFDMsystems,Fixd SNR Threshold Bit and Power Allocation Algorithm wasstudied.After the amelioration of the primary algorithm,the throughput of thesystem increased furthest;Besides, a constant throughput and low complexityadaptive bit and power allocation algorithm adapted to the practical applicationswas proposed for the disadvantages of the traditional Greedy Algorithm.What'smore,transmit diversity technique—space-time block coding(STBC) proposedby Alamouti was adopted in the AOFDM system to improve the performance ofthe system,and it was pointed out that adopting transmit diversity technique atthe base station to improve the communication quality was effective andcost-efficient.For the multiuser adaptive OFDM systems,Two-step SubcarrierAllocation Algorithm was introduced,and Hungarian Algorithm was adopted inthe adaptive subcarrier allocation.The simulation result indicates that inconsideration of the performance of efficiency and fairness, the performance ofHungarian Algorithm is excellent.It is adapted to optimize the performance ofthe practical multi-carrier systems.