Digital Receiver Architectures for OFDMA Systems Using Polyphase Filter Banks

Orthogonal frequency division multiplexing (OFDM) systems are very sensitive to phase noise and frequency dispersion. Time and frequency synchronization are very important for the proper signal recovery in the receiver. OFDMA is the extension of OFDM modulation to multi-user application and environments. The difficulties are multiplied if we consider an OFDMA system, where the sub carriers are shared between different users. The receiver has task to establish relationship between channel bandwidth, channel separation and channel sample rate. The receiver also chooses optimal sampling instants and to align the subcarrier frequencies of an OFDM symbol that contain signals form multiple users. The proposed schemes are applicable to adjacent subcarrier permutation OFDMA systems where users are separated in bands of adjacent subcarries. This paper presents the use of Polyphase filter bank in the receiver to simplify the frequency (and timing) synchronization tasks. By using polyphase filter bank the OFDMA synchronization problem is split in to single user OFDM task in very efficient manner. Further in order to achieve low processing load, techniques of multirate signal processing are adopted. MATLAB simulations of 10-, and 50-channel resampling receivers using polyphase filter bank are performed. The computational work load for Polyphase structure is determined and some measures for the reduction of the computational burden are presented.