Channel Coding and Clipping in OFDM for WiMAX using SDR

Recent developments in broadband wireless technology heightened the need for WiMAX which assures high-speed data services. Mobile WiMAX is grounded on orthogonal frequency division multiplexing/orthogonal frequency division multiplexing Access (OFDM/OFDMA) technology which is an increasing important technique in LTE systems. This paper describes the OFDM transceiver implementation using software-defined radio system (SDR). A SDR is a radio communication system where elements have been generally implemented in hardware are rather implemented by software on a personal computer. In this paper, the software part is realized using GNU Radio and the hardware part is implemented using USRP N210. OFDM poses a problem of a Peak to Average Power Ratio (PAPR) or high crest factor. To stave off this problem either High Power Amplifiers (HPAs) with large dynamic range or PAPR reduction techniques are used. The former scheme raises cost of the system, while the latter induces redundancy or distortion. This paper presents a novel architecture (which combines channel coding and clipping) for the PAPR reduction and analyzes various parameters which effects the performance of OFDM such as power spectral density, the crest factor and BER. Channel coding part is framed of three steps randomization, Forward Error Correction (FEC) and interleaving. In clipping, certain threshold limits the amplitude of time domain samples. Without filtering, clipping causes out-of-band radiation. The paper analyzes the out band radiation value (at 2.395 GHz) and PAPR reduction with respect to clipping threshold value. This scheme is preferred because of its lower complexity and hence would be cheaper to implement than conventional reduction techniques. Experimental results prove that the clipping method reduced PAPR significantly as the number of clip and filtering level is increased.