Simulation of OFDM System over Frequency Selective Channel.docx

1、Simulation of OFDM System over Frequency Selective ChannelSimulation 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 c

2、entered 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 wi

3、ll not experience significant ISI. The modulated signals associated with all the subchannels are summed together to form the transmitted signal, given asWith the equivalent baseband signal description, OFDM symbols can be expressed as:Where T is the duration of the 0FDM symbol; is the starting time,

4、N is the number of carriers. is the data symbol assigned to each channelThe subcarrier spectrum and baseband transmitted signal is shown figure.1,figure.2,respectively. Figure.1 subcarrier spectrumFigure.2 baseband signal二、 Demodulation the OFDM signal at receiver三、IFFT Implementation of OFDMWe assu

5、me that, and,so we can get the follow resultSo 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 tra

6、nsform 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 c

7、an 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 PrefixEach sub-data stream rate decreases, reducing the inter-symbol interference (ISI). In addition, inter-symbol int

8、erference can be completely eliminated by adding a guard interval.Figure.4 The Cyclic Prefix四、 MATLAB simulation (1).Firstly, the source generated the bit stream, and then the information sequence is modulated by QAM and mapped into frequency domain signal, the frequency domain signal is subjected t

9、o serial-to-parallel conversion. The parallel data is allocated to 1075 sub-carriers, we add some zero elements to the data in order to be calculated by IFFT.(2).the data is transformed time domain by IFFT,and the in-phase and quadrature components shown in Figure.5 to make the graphics clear , we p

10、lot 20 points.Figure.5 in-phase and quadrature components(3). The latter part of each OFDM symbol is copied to its front to form an OFDM signal having a cyclic prefix. Shown in Figure.4.(4).the time domain data is processed by parallel-to-serial conversion and D/A filter,and then is modulated by the

11、 carriers to be send. And we can get the signal waveform and spectrum of data to be send shown in Figure.6 and Figure.7.Figure.6 the signal waveform to be sendFigure.7 spectrum of signal to be send(5).the transmitted data are transimitted over the wireless channel with additive Gaussian noise, SNR i

12、s set to SNR = 10 dB.(6). The receiving module emulation procedure is opposite to the sending module procedure.the results are shown as follow.Figure.8 receive data componentFigure.9 constellation of QAM五、 Result of simulation analysisThe system simulation shows that the multi-carrier OFDM technolog

13、y has the following characteristics compared with the single-carrier modulation technique:(1).multi-carrier system can effectively combat the inter-symbol interference caused by multipath propagation and greatly simplify the system structure so that the receiver can not use a complex equalizer only

14、by inserting the cyclic prefix method to eliminate inter-symbol interference.(2). OFDM systems can maximize the use of spectral resources due to the orthogonal permutation of the inter-carrier-to-sub-carrier frequency spectrums overlapping each other, which is of great importance in the utilization of increasingly tense spectrum resources.(3). OFDM system can be implemented using discrete Fourier transform algorithm with the digital signal processing technology and large-scale integrated circuit technology to meet the various real-time requirements.