The Research On Reduction Of Peak-To-Average Power Ratio Of OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in each band. Each subcarrier is orthogonal (independent of each other) with every other subcarrier. It was invented over40year ago, and has been adopted in many technologies such as Asymmetric Digital Subscriber line (ADSL) services, IEEE802.11a/g, IEEE802.16a, Digital Video Broadcast (DVB), Digital Terrestrial Television Audio Broadcast,4G, IEEE802.11n, IEEE802.16, and IEEE802.20.OFDM is a multi-carrier modulation technique that is unlike other modulation techniques. In OFDM, the carriers have substantial overlap. For each single high frequency carrier used, OFDM transmits multiple high data rates signals concurrently using sub carriers. The sub-carriers are orthogonal with each other and hence do not interfere with each other. OFDM is currently the next choice for next generation mobile communications, primarily because it offers high bandwidth efficiency due to overlapping of spectra, its robustness to frequency selective fading channels and its efficient implementation.Despite the advantages of using OFDM, one of its main drawbacks is the high Peak-to-Average Power ratio (PAPR), which could limit transmission efficiency and efficient use of HPA. In this thesis we shall present two new PAPR reduction schemes with improved performance and reasonable complexity. In the first scheme, a modified tone reservation scheme for PAPR reduction using FFT iterations to generate the tones; the reserve tones are designed to both cancel peaks and slightly increase the average power to induce a better PAPR reduction. The tones are generated by means of2FFT operations and the process is sometimes iterated to achieve better PAPR reductions. This scheme achieves a significant PAPR reduction of at least4.6dB when about4%of the carriers are used as reserve tones and with even lesser iterations when simulated in an OFDM system. In the second technique, we combine both tone reservation and SLM (Selected Mapping Technique) to achieve a better PAPR reduction. The design here is such that no extra information is required when implementing SLM.