Improvement of BER for OFDM using Equalization and Windowing for AWGN and Rayleigh Fading Channels

Abstract: The orthogonal relation between the carriers among different users is well known in OFDM. The overlap of spectrums of various users can be achieved thereby enhancing the spectrum efficiency of the communication system. OFDM is a popular multicarrier modulation technique which is currently being explored for use in the IEEE 802.11e (WiMax) standard. In OFDM, the orthogonality between the closely spaced sub-carriers is important for overcoming the phenomenon of inter-carrier interference (ICI) leading to a reduction in the Bit Error Rate (BER) performance of the system. A technique for shaping the time portion of our measured data so as to minimize the effects of the edges thus reducing the leakage that arises out of the spectrum of FFT is called windowing. The Raised Cosine (RC) window is a period of raised value of cosine function so the negative peaks of the window just touch the nil mark, hence giving its name. In order to protect the transmitted signals against the effect of multipath channels i.e. the inter-symbol interference (ISI), a cyclic prefix is added with the duration which is greater than the delay of multipaths. The phase and amplitude of the received signal also suffers due these effects. As a solution to this, some sub-carriers are sent as a reference in order to determine the role of the multipath channel. Having this estimation, the equalization technique is employed which results in the compensation for the detrimental signal effects created as a result of the multipath channels. Here, a linear equalizer namely Zero Forcing (ZF) Equalization is used in conjunction with Raised Cosine (RC) Windowing approach on traditional Orthogonal Frequency Division Multiplexing (OFDM) system using the modulation technique of 16-QAM modulation at 𝟏𝟎−𝟐.𝟗 Bit Error Rate (BER). The result is that a 5 dB improvement is achieved in the Signal-to-Noise Ratio (SNR) value for Additive White Gaussian Noise (AWGN) Channel as compared to traditional OFDM used in the same channel model. And a 15 dB improvement is achieved in the Signal-to-Noise Ratio (SNR) value for Rayleigh Channel as compared to traditional OFDM for the same channel model at 𝟏𝟎−𝟐.𝟗 Bit Error Rate (BER).

Keywords: OFDM, AWGN, Rayleigh, RC Windowing, Equalization.