Design of Improved Coding Schemes with Low Peak to Average Power Ratio for OFDM System

Abstract

Orthogonal frequency division multiplexing (OFDM) is an exceptionally attractive practice in the field of wireless communications because of the spectrum efficiency and channel robustness. It is modulation technique in which a high-rate bit stream is split into a large number of parallel low rate data sub-streams and the sub-streams are utilized for modulating the number of orthogonal sub-carriers with the help of Fourier transform processing. OFDM attains robustness against multipath fading due to extended symbol interval for every sub-carrier and against the inter-symbol interference (ISI) due to introduction of a guard interval in each OFDM symbol. Therefore, OFDM has been accepted in many modern communication applications, like wireless local area networks (WLANs) and digital video & audio broadcast. In spite of numerous plus points, a foremost disadvantage of OFDM is the high peak to average power ratio (PAPR). In view of the fact that OFDM signal consists of numerous independently modulated sub-carriers, which results in high PAPR when summed-up coherently. This necessitates the use of power amplifier (linear) with the huge dynamic range. High PAPR increases the dynamic range of analog-to-digital converter (ADC), also it reduces the efficiency of linear power amplifier. Consequently, discovering methods to reduce transmitted signal’s PAPR in the OFDM systems is becoming an extremely active field of research in the communication communities. Numerous procedures have been suggested in the literature for reducing the PAPR. These procedures may be classified into the signal mixing methods & signal distortion methods. Signal mixing methods are the various techniques which suggest how to mix up the codes for reducing the PAPR. Scrambling of signals is achieved by various available coding techniques. Shapiro Rudin series, Golay complementary series, Barker codes, M sequences may be applied proficiently for diminishing the PAPR. On other hand, with increase in carrier count, operating cost related to comprehensive investigation of best code will increase exponentially. Additional realistic signal scrambling solution methods are partial transmit sequences (PTS), selective level mapping (SLM), and block codes. In this research, various significant features have been described and mathematical investigation of various techniques has been presented to decrease the PAPR, which are having a potential to offer considerable drop in PAPR at the price of decrease in data rate, amplification in transmit signal power, increase in bit error rate (BER), and computational complexity . Fundamental prerequisite of practical PAPR reducing techniques include compatibility with the family of existing modulation techniques, elevated spectral effectiveness and low complexity. There are lot of aspects to be taken into consideration before a precise PAPR reducing method is selected. These factors consist of PAPR reducing capability, decrease in BER at the receiver end, no loss in data rate, reduction in computational complexity and so on. There is no definite PAPR reducing method which is the appropriate solution. Instead the PAPR reducing method ought to be cautiously selected according to different requirements of system. OFDM signal obtained using Golay sequences has the advantage of low PAPR values. It has superior error-correcting capability as well. Signals using quadrature-amplitude-modulation (QAM) techniques are normally implemented in OFDM system, 16 QAM OFDM series having very low PAPR is created with 2 quaternary-phase-shift-keying (QPSK) Golay series (complementary). The results are extended to 256 QAM series. In this research, a technique has been proposed to make 16 QAM and 256 QAM series by combining QPSK sequences. If these QPSK series are Golay series resulting from Reed–Muller codes, then the obtained QAM series has low PAPR which is appropriate for OFDM systems. The recommended method reduces the PAPR and the simulation results illustrate that the peak power is reduced by approximately 1dB and by approximately 4dB as compared to other methods.Simulation results demonstrate that the proposed solution can accomplish the bit-error-rate requirements of WiMAX. In this study, reduction of PAPR in OFDM systems intended for QAM constellation has been considered, particularly for 16-QAM and 256 QAM. The techniques proposed in this investigation can be applied for producing OFDM series having low PAPR for 1024-QAM constellations. When transmitting signals, one of the most important issues is to keep the transmission errors as low as possible. Or in other words, to obtain a reliable transmission link, the bit-error-rate (BER) should be kept within certain limits. However, the probability of transmission errors strongly depends on the signal-to-noise ratio (SNR) of the transmitted signals. Here, the power amplifier plays a key role in the sender part: the more power, the higher the SNR, the lower the probability of transmission errors. One should take care to keep the peak-to-average power ratio (PAPR) of the transmitted signal low in order not to push the power amplifier into its nonlinear operation region. Classical techniques use clipping or backing-off the input signal to reduce the PAPR of the transmitted signal. However, these techniques have a negative influence on the SNR and hence on the BER. In this work we present a new approach to reduce the BER of transmitted signals in order to reduce the PAPR. The presented technique introduces sequences into the data. The idea behind introducing sequences is to add redundancy in order to obtain a more reliable communication link. Reducing the BER of OFDM signals can not only be a way to reduce the PAPR of a transmission link, but since it prevents the power amplifier to go into compression, it also provides a technique to enhance the efficiency of the power amplifier. The performance of OFDM is tested for M-QAM using MATLAB/SIMULINK toolbox. It is observed that sequences play an important role to improve the efficiency of the system. After analysis of sequences in M-QAM system we observed that BER is reduced in comparison to a system without sequences. Various sequences were analyzed to improve the QOS of OFDM system and it can be seen that the system behaves efficiently with Golay sequence as compared to other sequences. The WiMAX (Worldwide interoperability for microwave access) physical layer is based on OFDM, a method that presents high-quality confrontation to multipath, and permits WiMAX to function in non-line of sight (NLOS) circumstances. The technique proposed may be used to reduce the PAPR of WiMAX OFDM system. IEEE 802.16, which is the standard related to wireless broadband guarantees large bandwidth over transmission in the long range. This standard states the combined interface of medium access control (MAC) and physical layer of broadband wireless access together with the air interface. BER performance of WiMAX physical layer based wireless communication system using the convolutional coding and various QAM modulation schemes have been analysed. The BER curves are used to evaluate the performance of various modulation and coding scheme used. These results offer a complete indication of the WiMAX systems using OFDM physical layer under different circumstances of modulation and various input sequences. It has been observed that sequences play a significant role to increase the effectiveness of system. After the investigation of sequences in M-QAM system, it has been observed that BER reduces in contrast to system without sequences. A variety of sequences were analysed to enhance the system performance and it may be observed that the system performs better with Golay sequence as compared to any other sequence. Decrease in BER of WiMAX system not only decreases the PAPR, it also improves the efficiency of the system, because it avoids the power amplifier to go into compression.