Abstract:
The demand for high speed mobile wireless communication is rapidly growing. Orthogonal Frequency Division Multiplexing (OFDM) technology promises to be a key technique for achieving the high data capacity and better spectral efficiency requirements of wireless communication systems in the near future. However the major cause for signal degradation during transmission of OFDM signal is high peak to average power ratio (PAPR). Although many research activities have been done in recent years but this PAPR problem is still unresolved which leads to power inefficiency, inter-modulation distortion and undesired out-of-band radiation because of the nonlinearity of the High Power Amplifier (HPA). It causes increasing of the Bit Error Rate and high adjacent channel interference, respectively. Therefore, it is desirable to reduce the PAPR of the OFDM signal. However, most of the previously published techniques have additional complexity, some reduce the bandwidth efficiency of the system and some need side information. This thesis aims at contributing to reduce the PAPR of OFDM system efficiently. Firstly, a narrowband pulse (smaller channel bandwidth, high power efficiency and low data rate), Tukey window pulse, is employed in the transmitter section for reducing PAPR of OFDM signal as well as to evaluate their performance such as PAPR, Cross correlation for different number of subcarriers. Furthermore, performance of Kaiser window pulse, another narrowband pulse which has been suggested for PAPR reduction of OFDM system but has not been investigated yet, is also examined in this research. The performance of the employed pulse shaping technique (Tukey window pulse) as well as the suggested pulse shaping method (Kaiser window pulse) are evaluated based on the mathematical model which includes equations for PAPR and cross correlation of each of the above narrowband pulses. These techniques do not require any side information. Moreover, the performance comparison is carried out among all the employed pulse shaping methods including the current pulse shaping approaches as well
as the used method in order to find out the best pulse shaping technique for PAPR reduction in OFDM system efficiently. From the analysis, it is found that among narrowband pulses, Tukey window pulse and broadband pulses (greater channel bandwidth, low power efficiency and high data rate), Square root raised cosine pulse reduce PAPR more significantly than any other pulses as well as the conventional OFDM system. Furthermore Cross correlation function is also examined for both narrowband and broadband pulses since higher correlation between the different samples of the OFDM block reduces the PAPR of the system. Finally, a simulation model is developed to calculate Bit Error Rate (BER) for the Raised cosine pulse, Square root raised cosine pulse (most efficient pulse shaping technique) and the conventional OFDM system. Based on the simulation model, BER performance of OFDM system using pulse shaping method is compared with the conventional OFDM system without pulse shaping method to show the efficiency of the pulse shaping technique for PAPR reduction in OFDM system. Simulation result shows that the Square root raised cosine pulse can reduce the BER more effectively than the conventional system i.e., OFDM system without pulse shaping technique. The results obtained in this research are promising and show the suitability of the pulse shaping technique to reduce PAPR of OFDM system effectively.
