Abstract:
Orthogonal Frequency Division Multiplexing (OFDM) has received widespread attention in recent years and has been proposed or adopted in a number of communication systems, such as wireless local area network (WLAN), digital audio broadcasting (DAB) and digital video broadcasting (DVB). It has been considered as one of the strong standard candidates for the next generation mobile radio communication systems. Multiplexing a serial data symbol stream into a large number of orthogonal candidates makes the OFDM signal spectral bandwidth efficient. It has been shown that performance of OFDM systems in frequency selective fading channels is better than that of single carrier modulation system.
The main implementation disadvantage of OFDM is the possibility of high peak-to-average power ratio (PAPR). As OFDM signal has number of independently modulated single-carriers, these can give large PAP ratio when added up coherently. High PAPR brings on disadvantages like signal distortion in the non-linear range of high power amplifier (HPA), degradation of bit error rate (BER) and increased complexity of analog-to-digital (A/D) and digital-to-analog (D/A) convertors. Various techniques have been proposed for reducing the PAPR in OFDM systems. The deterministic schemes such as clipping, peak windowing limit the PAPR below a threshold level, but result in in-band distortion and out-of-band radiation. A more attractive alternative are the scrambling and redundancy based techniques that include coding, selective mapping (SLM) and partial transmit sequences (PTS). They can reduce PAPR without BER degradation but with a small penalty of reduced transmission rate or increased average power due to introduction of redundancy. This work overviews the most popular redundancy based techniques and suggests hybrid systems combining scrambling with error control. The emphasis has been on the design of coded OFDM systems (COFDM) incorporating Turbo & LDPC codes which employ scrambling to not only give a lower PAPR but also an improved BER.
