Simulation of OFDM System over Frequency Selective Channel.docx

Simulation of OFDM System over Frequency Selective Channel 一、 Orthogonal Frequency Division Multiplexing (OFDM) The simplest form of OFDM divides the data stream into multiple substreams to be transmitted over different orthogonal subchannels centered at different subcarrier frequencies. The number of substreams is chosen to make the symbol time on each substream much greater than the delay spread of the channel or, equivalently, to make the substream bandwidth less than the channel coherence bandwidth. This insures that the substreams will not experience significant ISI. The modulated signals associated with all the subchannels are summed together to form the transmitted signal, given as With the equivalent baseband signal description, OFDM symbols can be expressed as: Where T is the duration of the 0FDM symbol; is the starting time,N is the number of carriers. is the data symbol assigned to each channel The subcarrier spectrum and baseband transmitted signal is shown figure.1,figure.2,respectively. Figure.1 subcarrier spectrum Figure.2 baseband signal 二、 Demodulation the OFDM signal at receiver 三、IFFT Implementation of OFDM We assume that , and ,so we can get the follow result So OFDM complex equivalent baseband signal can be implemented using discrete inverse Fourier transform(IDFT).we can get the transmitted symbol using discrete Fourier transform(DFT) at receiver. From the above analysis,we can get that the OFDM system can transform the symbol to be transmited at frequency domain into data symbol at the time domain using N points IDFT. After the carrier modulation, the OFDM symbol is sent to the channel. The received signal is subjected to coherent demodulation at receiver. In practice, OFDM modulation and demodulation can be realized by fast Fourier transform (FFT / IFFT). The OFDM implementation of multicarrier modulation is shown in Figure.3. Figure.3 OFDM with IFFT/FFT Implementation 三、 The Cyclic Prefix Each sub-data stream rate decreases, reducing th