Using WPT as a New Method Instead of FFT for ‌Improving the Performance of OFDM Modulation

Orthogonal frequency division multiplexing (OFDM) is used in order to provide immunity against very hostile multipath channels in many modern communication systems.. The OFDM technique divides the total available frequency bandwidth into several narrow bands. In conventional OFDM, FFT algorithm is used to provide orthogonal subcarriers. Intersymbol interference (ISI) and intercarrier interference (ICI) impairements are caused by time domain rectangular windowed sine and cosine basis functions. FFT-OFDM is a very popular multi–carrier modulation (MCM) technique. It has some interesting features such as low complex modulation/demodulation implementation, simple and fast frequency domain channel estimation/ equalization. Also, by transmitting data over different parallel frequencies, FFT-OFDM has spectrum efficiency due to overlapped sub-channels and immunity against fading channels. Unfortunately, FFT-OFDM has serious drawbacks i.e. high sensitivity to ISI and ICI which caused by time domain rectangular windowed sine and cosine basis functions and their high level side lobes in frequency domain. For this purpose, cyclic prefixes (CP) are added at the beginning of the OFDM symbols and this causes bandwidth and power inefficiencies. In order to provide more efficient MCM technique, besides preserving the advantages of conventional FFT-OFDM, discrete wavelet modulation (DWM) and wavelet packet modulation (WPM) have been introduced in recent years. Therefore, it is possible to use time domain equalization (TEQ) or overlap frequency domain equalization (overlap FEQ) to reduce the interferences effectively in the absence of CP. Although TEQ techniques are more complicate than FEQ in conventional OFDM, WPT-OFDM has bandwidth and power enhanced efficiencies and this makes it so appropriate for digital communication systems. In recent years, several studies have been done on the wavelet theory, wavelet and WPM modulation in comparison with FFT-OFDM. Because of the good performance of WPT, a number of studies are still on the performance of WPT in hostile channels with more details. Also, there are a number of studies about various kinds of FEQ and TEQ such as zero force (ZF) and minimum mean square error (MMSE) in the peresence of AWGN and some fading channels. These researches also contain the comparison of FEQ for FFT-OFDM and overlap FEQ for WPT-OFDM. Todays, 3GPP standard is spread in different domains like 3G, 4G and LTE-A technologhies. In this paper, all the parameters are chosen according to 3GPP standards. For demonstrating the benefits of discrete WPT, two OFDM modulation schemes, i.e. FFT-OFDM and WPT-OFDM with two applied channels i.e. 6-tap rural area (RA6) and 6-tap typical urban (TU6) channels are considered. The performance of  two systems are investigated by the measure of bit error rate (BER) in different SNRs(dB). Also, Wavelet families i.e. Haar, Daubechies6 , Symlet5 and Coiflet5 are compared with FFT in OFDM system with QPSK, 16-QAM and 64-QAM constellation mappings. In the receiver side, FEQ is used in FFT-OFDM and overlap FEQ is used in WPT-OFDM to equalize multipath fading channels. This is a comprehensive comparison between FFT-OFDM and WPT-OFDM with different constellations, a number of wavelet families, different equalizer with two applied channels in order to implement a real environment. The simulation results demonstrate performance improvement of the system using WPT-OFDM scheme. In order to evaluate the performance of these two OFDM techniques, the required SNRs for reaching BER =10-3 are extracted and compared for both systems. It was observed that one can obtain better performance by using Haar wavelets as orthogonal basis function rather than FFT in OFDM modulation.  We achieved better performance by using Haar wavelets rather than FFT in OFDM modulation. As a result, WPT-OFDM can be applied , with better performance, in different OFDM-based applied technologhies such as DAB( Digital Audio Broadcast), WiMAX( worldwide Interoperability for Microwave Access), DVB( Digital Video Broadcast).