Electronics Project:OFDM PAPR

Implementation of different OFDM PAPR reduction techniques:

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation scheme. It is a special case of multicarrier transmission technology, where a single data stream is transmitted over a number of lower rate subcarriers instead of single carrier system. The main reason to use OFDM is its robustness against the selective fading or narrowband interference, high spectral efficiency and easy implementation. Hence, due to favorable features, it is widely used in modern broadband communication systems. Despite all favorable features in OFDM system for implementation in communication systems, it encounters a noticeable problem of high peak to average power ratio (PAPR). High PAPR becomes huge obstruction to harvest all the features of OFDM system for the implementation of high speed broadband communication systems.

The OFDM signal, which superposes many individual sinusoidal subcarriers, would have high amplitude when these sinusoids are in phase at the inverse fast Fourier transform (IFFT) input, and are thus added constructively to generate large amplitude corresponding to a high PAPR at the IFFT output. When the peak amplitudes of OFDM signals with high PAPR reach or exceed the saturation region of power amplifier at the transmitter and a low noise amplifier at the receiver, the OFDM signals will suffer from nonlinear distortion, spectrum spreading, in band distortion and inter modulation distortion across the OFDM subcarriers. All these demote the bit error rate (BER) at the receiver. One simple solution is to use expensive power amplifiers with large saturation region. However, as high peak amplitudes occur irregularly, these power amplifiers would be inefficient. Besides, high peak are also constrained by design factors such as cost and battery power of electronics. Large PAPR also demands the digital to analog converter (DAC) with enough dynamic range to accommodate the large peak of the OFDM signals. Although, a high precision DAC supports high PAPR with a reasonable amount of quantization noise, but it might be very expensive for a given sampling rate of the system. Whereas, a low precision DAC would be cheaper, but its quantization noise will be significant, and as a result it reduces the signal to noise ratio (SNR) when the dynamic range of DAC is increased to support high PAPR. Furthermore, OFDM signals show Gaussian distribution for large number of subcarriers, which means the peak signals rarely occur and uniform quantization by analog to digital converter (ADC) is not desirable. If clipped, it will introduce in band distortion and out of band radiations (adjacent channel interference) into the communication systems.

Therefore, the best solution is to reduce the PAPR before OFDM signals are transmitted into nonlinear high power amplifier (HPA) and DAC.

This project provides the method which reduces the PAPR of OFDM system and thus makes it suitable for transmission through HPA and DAC.