Analysis of Signal Distortion Techniques in PPDU Frame based 4G Systems

Abstract

High data rate with high bandwidth efficiency and low power consumption are some of the requirements that the modern wireless communication systems 3G and 4G have to meet for offering new high class services to be distributed to the customers. In wireless communication, signals are mainly damaged by frequency selective fading and multipath delay spreading. This leads to a high probability of errors at the receiver end. Hence single carrier mobile communication systems do not perform well for high speed data rate transmission with maximum throughput at limited bandwidth. To tackle these problems channel coding and adaptive equalization have been used. But due to delay of these techniques and high cost of hardware implementation, it is very difficult to use these techniques in systems which are operating at high bit rates. The solution to all these problems is to use a multi-carrier system; Orthogonal Frequency Division Multiplexing (OFDM) is an example of it. The attraction of OFDM is mainly due to how the system handles the multipath interference at the receiver. Multipath generates two effects: frequency selective fading and Inter-Symbol Interference (ISI). The "flatness" perceived by a narrow-band channel overcomes the frequency selective fading, and modulating at a very low symbol rate, which makes the symbols much longer than the Channel Impulse Response (CIR), diminishes the ISI. Using powerful error correcting codes together with time and frequency interleaving yields even more robustness against frequency selective fading and the insertion of an extra guard interval between consecutive OFDM symbols can reduce the effects of ISI even more. Thus, an equalizer in the receiver is not necessary. With so much large advantages of OFDM, there are some limitations and complications in practical systems too. The two main drawbacks of OFDM system are: the large dynamic range of the amplifier also referred as Peak to Average Power Ratio (PAPR) and its sensitivity to frequency errors. PAPR results in inter-modulation among the sub-carriers and undesired out-of-band power radiations. It is impossible to keep the out-of-band power under the limit, if power amplifiers are not operated with large back-offs. So it considerably reduces the efficiency of the High Power Amplifier (HPA) at the transmitter side. Hence, the reliable reduction of PAPR is important for the practical implementation of OFDM system. In this thesis the improvement in PAPR of OFDM system is discussed. We have examined many PAPR reduction techniques and compared them and get an optimum result in finding best technique. The success of these techniques in terms of histogram, PAPR-reduction, Bit Error Rate (BER) and average power variation are evaluated. We have done many experiments on clipping techniques and companding techniques and finally it has been found that CC technique has good PAPR reduction performance.