Adaptive Nc - Research And Implementation Of Key Algorithms In Ofdm System

Orthogonal frequency division multiplexing (OFDM) technology has become oneof the key technologies of the fourth generation mobile communication system for itshigh spectrum efficiency and good ability to anti-multipath fading. As a special case ofOFDM system, Non-Contigous OFDM (NC-OFDM) has all of the characteristics ofOFDM system and can keep the high-speed data transmission in the discontinuousfrequency bands. Adaptive technology can make full use of channel information andselect suitable transmission parameter intelligently, so that it can achieve high spectrumefficiency and improve the performance. Therefore, it is necessary to combine the twotechnologies.In this paper, we mainly completed the research and implementation of the keyalgorithms in the adaptive NC-OFDM system. First we studied two key algorithms ofthe NC-OFDM system: channel estimation algorithm and adaptive algorithm. At thesame time, an improved adaptive algorithm with variable threshold has been presented.Then, with the model based design method, we finished the hardware realization、debugging and testing with Soft Defined Radio Development Platform (SDR DP) whichis developed by the Lyrtech company. The main work of this paper can be divided intofour parts:The first part mainly studied several channel estimation algorithms commonly usedin the NC-OFDM system: estimation based on Least Square(LS)、estimation algorithmbased on Minimum Mean Square Error （MMSE） criterionand transform domainchannel estimation, and then simulation and analysis have been done. The results showthat, LS estimation has the poorest performance; and transform domain channelestimation, as an improved algorithm of the LS estimation, has similar performance toestimation algorithm based on MMSE criterion. However, it is more simply to realize.The second part mainly studied two kinds of common adaptive algorithms of theadaptive modulation and coding algorithms: fixed threshold adaptive algorithm andvariable threshold adaptive algorithm.Fixed threshold adaptive algorithm is simple torealize, but it can only be used in slow fading channel. However, variable threshold adaptive algorithm can be applied not only in slow fading channel, but also can makefull use of channel information and select an appropriate transmission mode, so as toachieve rational use of resources.In addition, with the same bit error ratio, an improvedvariable adaptive threshold algorithm which can accomplish more modes switching hasalso been put forward.The third part finished the hardware implementation of adaptive NC-OFDMtransceiver and receiver system on the SDR Develop Platform using model baseddesign method. In this part, specific design timing diagrams of the key modules havebeen given,such as control of the key signal, Inverse Fast-Fourier-Transformation(IFFT)timing control, the sampling filter group in the transmitter, channel estimation and soon, besides that problems encountered and solution have also been showed.The forth part completed the link test of the adaptive NC-OFDM system. Theresults show that in slow fading channel condition without interference, according to thecurrent channel conditions, this system can select the appropriate coding modulationmode in order to effectively use the channel resources and make a good tradeoffbetween the effectiveness and reliability of the system. If interference exists, this systemcan avoid interference and select no interference frequency band for informationtransmission so as to achieve anti-interference performance. Of course, the gain of theanti-interference performance is at the expense of bandwidth utilization rate. Especiallyin multi-tone interference, the number of available bands is greatly reduced, so that theeffectiveness of the system is also greatly restrained.This paper not only gives the simulation results of the channel estimationalgorithm and adaptive algorithm in the adaptive NC-OFDM system, but also gives theproject implementation with Soft Defined Radio Platform (SDR) and test results. Inshort, it has certain practical value.