Tham khảo tài liệu 'wimax, new developments 2011 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Design and Implementation WiMAX Transceiver on Multicore Platform 155 x n -L YX .X -JNt i - NCP n N -1 1 where X k is the modulated data on the kth subcarrier of one OFDM symbol N is the number of subcarriers and NCP is the length of cyclic prefix. The impulse response of multi-path channel can be approximately denoted as L h T t a1 t S T -y 2 1 1 where L is the total number of paths and a1 and T1 are the complex gain and time delay of the lth path. It supposes that the signals are transmitted over a quasi-static multipath fading channel that is to say the channel varies much slowly and the fading coefficients can be assumed to be constant during the OFDM block 5 . Assuming perfect time and frequency synchronization the model of received signal at the BS after removal of the CP can be written as 1 N-1 y n -1 Y H k X k e 2mk N z n 0 n N-1 3 where H k is the channel frequency response at the kth subcarrier and z n is the additive white complex Gaussian noise AWCGN . Algorithm Selections For the transmitter of WiMAX PHY the algorithm of each module such as FEC coding modulation map constellation etc. is mature relatively. Thus we focus on the discussion of algorithm selection for receiver block in this section. . Synchronization Timing and frequency synchronization are two important tasks needed to be performed by the receiver. Through the timing and frequency offset estimation and correction the effects of ISI inter symbol interference and ICI inter-carrier interference can be reduced. In the presence of symbol timing offset STO and carrier frequency offset CFO equation 3 should be modified as follows -J N-1 j2zk n-0 j2asn y n -j_ Y H k X k e N e z n -4Ni 0 0 n N-1 where 0 is the normalized STO with respect to the sample duration and s denotes the normalized CFO as a fraction of the intercarrier spacing. A number of approaches to estimate timing and frequency offset in OFDM systems have been presented in the literature. Some operate in the time domain 6 7 .
