Improvement in SNR with 32-QAM and 128-point FFT in OFDM transmission system
Abstract
High rate data transmission and reception can be successfully implemented if the high rate signal is split into multiple parallel low rate signals and each of them is transmitted on a separate frequency (or sub-carrier). To facilitate separation of the signals at the receiver, the carrier frequencies are spaced sufficiently far apart so that the signal spectra do not overlap. Channel coding plays a very important role in OFDM systems performance. The role of channel coding in conjunction with frequency and time interleaving is to provide a link between bits transmitted on separated carriers of the signal spectrum, in such a way that the information conveyed by faded carriers can be reconstructed in the receiver. Frequency selectivity, currently known to be a disadvantage, is then turned into an advantage that can be called frequency diversity. Using Channel State Information (CSI), channel coding can yield some additional gain. Channel state information is frequency response of the channel or signal-to-noise ratio in each carrier.
Some authors suggested that for SNR value of 26 dB, BER plot reaches zero for 16-QAM over an AWGN channel, while for QPSK scheme, the value of SNR is around 30 dB. So, 16-QAM is better than QPSK for an AWGN channel because it requires less SNR for zero BER. We include that for SNR value of 20 dB, BER plot reaches zero for 32-QAM over an AWGN channel, so 32-QAM is better than QPSK and 16-QAM for an AWGN channel because it requires less SNR for zero BER. The complete model for this OFDM transceiver with 32-QAM and 128 IFFT/FFT points is designed in the Communication Toolbox in MATLAB 7.8.0.
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