Research On Peak Power Control And Its Optimum Algorithms In OFDM Systems

Multicarrier technology has been one of the remarkable parts in the territory of today's communication research. It can have an impact upon several aspects such as enhancing transmission rate, improving frequency efficiency, overcoming the muilt-path interference and so on. This revitalization of the technique, also known as orthogonal frequency division multiplexing (OFDM), is mainly due to the advancing capabilities of digital signal processors, capability of resisting fading and high transmission rate. OFDM technology has been used in Wireless Local Loop (WLL), Digital Audio Broadcasting (DAB), Wireless Local Area Network (WLAN) etc.. OFDM technology is also easy to combine with space-time code, diversity technique, interference restrain and smart antenna for the greatest extent to enhance physical layer transmission reliability.OFDM technique has many benefits, however, high peak-to-average power ratio (PAPR) has become the vital shortcoming for use. The high PAPR signals will be distorted seriously and decline the system performance when passing through nonlinear channels. On the other hand, because of the high PAPR, the severe linearization for HPA is needed. It will increase the cost of hardware system, or decline the HPA's efficiency. The main purpose of this dissertation is to study the methods and optimum algorithms to reduce the PAPRs of OFDM system and MIMO-OFDM system, and to obtain better PAPR performance and control OFDM system's peak power as a consequence.At first, the base principles of OFDM modem and its PAPR definition have been introduced in this dissertation. At the same time, the statistical characters, measure means and relative parameters of PAPR have been studied carefully. It can be realized to reduce PAPR by using, controlling and modifying these parameters. Continuously, universal PAPR reducing methods have been introduced and their advantages and disadvantages inclusive. And the researches on optimum algorithms for overcoming these disadvantages have become the essential problem in this dissertation. The main research aspects contain improved phase disturbance algorithm, channel coding algorithm and improved pre-aberrance methods.Phase disturbance algorithm can make the PAPR value reduced by destroying the phase coherence; it can reduce the probability of high PAPR. Due to its complexity to compute, the use of phase disturbance algorithm has been restricted. So phase control coding (PCC) is proposed to either reduce PAPR or lessen the compute complexity. PCC method bases on classical partial transmit sequence (PTS) algorithm and then makes 2N→2(?) mapping for OFDM symbols, and do IFFT operation at last. Phase rotating index is represented by the other n-l states, it also calls phase control code. After IFFT, the result data are added to original data symbols. From the final results, the group having the lowest PAPR value will be sent. In addition, an new group partition method has been proposed, which bases on the m-sequence mapping technique. By theoretical demonstration, the new group partition method is excellent than interweave and adjacent partition methods, but is not a patch on the random partition method.Coding algorithm is a novel method to reduce PAPR and it can't bring distortion in the system. In the dissertation, I analyzed the constitution process of Davis's Golay complementary sequences (GCS), and approached the theory foundations of reducing PAPR. To increase the information rate of GCS construction, an improved GCS (IGCS) algorithm is proposed which is constituted by combining Golay code and coding modem. Successively, the PAPR performance with new algorithm has been analyzed. By simulation, IGCS method is able to reach higher information rate than GCS method at the cost of increase PAPR value. Clipping methods are usually used in the condition of large numbers of subcarriers. These methods can be realized easily, but they can bring clipping noise as well as nonlinear distortion. Based on discussing Ochiai clipping and filter algorithm and MMSE receiver construction, I proposed clipping distortion reconstruction iteration algorithm (CDRI). CDRI algorithm utilizes LDPC coding technique, LLR-BP decoding algorithm and MMSE iteration reception to not only reduce PAPR values but also lessen the clipping noise and nonlinear distortion.At the end of this dissertation, the proposed algorithm has different PAPR capability with various kinds of partition methods, and at the same time, new algorithm will amend BER performance.