Enhancing data efficiency in OFDM (IEEE 802.11a) using equalization (11A)
Reddy, S. Gangadhar
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IEEE 802.11a OFDM signal passed through a dispersive channel introduces ISI & ICI. To avoid ISI & ICI, cyclic prefix (guard interval) is introduced between the OFDM symbols. But the cyclic prefix contributes about 20% (0.8us/4us) of the total symbol duration. Transmission of OFDM symbols without cyclic prefix increases the data efficiency, but introduces ICI & ISI. The ISI & ICI can be removed by equalization at the receiver. Spill over in the frequency domain is same as ICI & ISI. This spill over can be observed at the unused carriers also (carriers numbered from 0-6, 33, 59-64 are unused in IEEE 802.11a). Equalization is done in the frequency domain by making use of spill over in unused carriers. A relationship is brought between the length of the channel impulse response and the number of unused carriers for perfect equalization in the absence or very little AWGN-channel noise. To avoid the noise amplification caused by Zero Forcing equalization and to satisfy the relationship derived, a methodology using Time domain equalizer (TEQ) is proposed and simulation results are presented supporting it. Alternatively, computational efficient algorithm for indoor environment is suggested by transmitting data in unused carriers with the combination of used carriers. By this method, ICI & ISI is removed at the transmitter itself and it even doesn’t amplify the noise.
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