Performance Enhancement of OFDM Systems using Pulse Shaping

The demand of wireless broadband multimedia communication is growing exponentially. To support high data rate multimedia applications on wireless fading channels a very careful choice of modulation technique is required. Since Orthogonal Frequency Division Multiplexing (OFDM) communication systems basic principle of splitting the high speed data stream into number of lower rate data streams, that are transmitted simultaneously over a number of subcarriers [1-2], they are first choice for almost all the fourth generation (4G) [3-4] and fifth generation (5G) wireless communication systems. OFDM is thus widely used in wireless and mobile communication standards such as IEEE802.11, Worldwide Interoperability for Microwave Access (IEEE802.16-WiMAX), Long Term Evolution (LTE) and LTE-Advanced (LTE-A). The main benefit of using multi-carrier based OFDM system is that the wireless channel becomes time-invariant during one OFDM symbol and that the frequency selective fading per subcarrier becomes nearly flat. The OFDM systems also provide high spectral efficiency due to high number of orthogonal sub-carriers implemented in discrete domain by Inverse Fast Fourier Transform (IFFT) technique. Also, individual subcarriers can adapt different modulation schemes depending on channel condition on each subcarrier. Although, OFDM systems incorporate immunity to multipath fading and provide high spectral efficiency, there are two major problems. One issue is OFDM system’s sensitivity to frequency offset and other is high peak to average power ratio (PAPR) of transmitted signal [12]. The OFDM system’s sensitivity against carrier frequency offset introduces intercarrier interference (ICI) and high PAPR causes out-of-band radiation. In this thesis work, first a new transmitter side frequency domain pulse shape is proposed to reduce average ICI power which is called New Windowing Function (NWF) and it is found 5 that the newly proposed NWF pulse shape provides better ICI, Signal to Interference Ratio (SIR) and bit error rate (BER) performance, as compared to all existing transmitter side pulse shapes methods in literature [64-69]. Then, we analyzed the peak to average power ratio (PAPR) performance of the proposed NWF pulse shape for 64-subcarrier OFDM systems which as per best of our knowledge no other work of ICI reduction using transmitter side pulse shaping has reported till date. The PAPR analysis shows that the proposed pulse shape performs far better as compared to pulse shapes reported in literature [67-69]. In the second phase of this work, a novel transmitter side Modified Raised Cosine Power (MRCP) pulse shape has been proposed to enhance the performance of OFDM system by reducing both ICI and PAPR. A detailed performance analysis w.r. t. ICI, SNR, BER and complementary cumulative distribution function (CCDF) of the PAPR versus threshold PAPR (PAPR0) has been done. In the third phase of the thesis work, Improved Phase Modified Sinc (IPMS) Pulse shape is proposed to reduce ICI and PAPR to further enhanced performance of OFDM systems. Although, this pulse shape outperforms our earlier proposed pulse shapes as well as all other transmitter side pulse shapes proposed so far in literature [64-69] with respect to ICI, SINR, and PAPR performances. However, BER performance of IPMS pulse is either comparable or less that the recently proposed pulses. In the final phase of the work, we have proposed Modified Better Than Raised Cosine Power (MBTRCP) Pulse which has got comparable ICI, SNR and PAPR performances as compared to IPMS pulse but at the same time performs far better than IPMS pulse in term of BER.