Study On The Application Of Time-domain Equalization Technology Based On OFDM System

1. IntroductionAlong with the exaltation of people's request to communication quantity, theworld wide correlative organizations are all concentrating on the research anddevelopment of the new generation technology. By using parallel transmission andguard interval (GI), multi-carrier modulation (MCM) has been given muchattention for its ability of mitigating inter-symbol interference (ISI) caused byfrequency-selective channels and multi-path fading distortion. As a valid MCMmode, Orthogonal Frequency Division Multiplexing (OFDM) has been consideredto be one of the core technologies in 4G-communication system. With theadvantage of high utilization efficiency on system frequency with low cost, OFDMis good at overcoming ISI and be extended on network structure easily, which canprovide high quality service than actual communication technology and bettersolution for wireless communication realization.Under serious dispersion, high-fading channel or some network mode,although OFDM itself has used the diversity characteristic of multi-path channel,long time delay will be brought. If channel impulse response is beyond the range ofGI, the appearance of ISI and inter-channel interference (ICI) are inevitable. ThenTime-domain equalizer (TEQ) must be introduced, which will be designed forshortening channel impulse response to combat ISI and ICI.Based on the research of OFDM basic theories and the simulation of severalcurrently used TEQ algorithms, the dissertation has mainly researched theapplication of time domain equalization technology in OFDM system, which aimedto analyze the performance and applicability of TEQ under different OFDMcondition by Matlab simulation.2. Study and Realization on Time Domain Equalization technologyThe ultimate goal of TEQ designed is to combat ISI and ICI as soon aspossible, reduce the bit error rate (BER) of the system, achieve the biggest channelcapacity, get maximum bit ratio. Many kinds of algorithm criterions come forth, forinstance: Minimum Mean Square Error (MMSE), Maximum shortening Signal toNoise Ratio (MMSNR), Maximum Bite Ratio (MBR) etc. Some feasible TEQalgorithms derive from the above criterions have been proposed.Based on the analyze and simulation results for some commonly usedalgorithm, the dissertation studied and realized an algorithm based on MMSEcriterion, which minimizes the mean square error between the output of the virtualpath-shortening impulse response (SIR) and the output of a physical path-targetimpulse response (TIR). In forming the model to design the TEQ, a reducedconvolution matrix is exploited to lower the computational complexity.This algorithm has a comparatively reduced computational complexity forintroducing LDLT that needs one more substitution step to calculate the last resultdecomposition instead of LU decomposition, which economize half of thecomputational load. Therefore, although LDLT algorithm is used to factorize factor,(and )the LU decomposition's forward and backward substitutions are used tocalculate the optimum TEQ coefficients. So it could be practically used in the highrate OFDM systems. Through the calculation, the computation complexity isreduced by 25 times. Compared to others TEQ algorithms, it is very attractive forpractical use for its simplicity. Mathematical analysis and simulation results areprovided to show the validity and the advantages of the ameliorative algorithm, theworse channel is, the better TEQ effect is.3. Establishment of OFMD simulation systemBased on research of OFDM system composing and parameter choice, twokinds of OFDM system simulation flat-CPOFDM (Cyclic Prefix OFDM) andZPOFDM (Zero Padding OFDM) are implemented. The difference is GI exceedingmode. The CPOFDM system become main application for its ability of mitigatingmulti-path interferes by inserting CP. The ZPOFDM system also is considered asanother approach of restraining ISI due to inserting zero intervals. As show fromthe simulation result, under the premise of approximate system capability, theZPOFDM system efficiency raised. Some simulations for one of the mainapplicable OFDM system—2k DVB-T and single antenna and single multi-pathfrequency-selective fading channels which are fit for the environment in ourcountry have been carried on. Besides, the contrast of system performance indifferent fading environment has been analyzed. The conclusion of the simulationsshows that the performance of the single frequency environment in longermulti-channel time delay is better than that of the single antenna environment.4. Application Research of TEQ technique based on different OFDMconditionsIn DVB-T system, time delay is usually so long that it can result in longchannel impulse response, therefore TEQ is introduced. In this dissertation, underthe flat channel and the frequency-selective channel, fore-and-aft TEQ systemperformance is simulated, and the result indicate that in frequency choice channelTEQ effect is better than in flat channel. Three kinds of signals: text, voice andimage are also transmitted for performance comparison by simulation experiment,and the result proves the validity of TEQ.5. ConclusionBased on study of OFDM and many TEQ algorithms, the dissertationaccomplishes a low computational load algorithm, simulates and compares theperformance of fore-and-aft TEQ system among different OFDM conditions. Theresult shows that in longer time delay or more serious fading multi-path, TEQ ispracticable and valid. According to this dissertation, certain reference andsignificance is provided for the research on TEQ application and optimizingOFDM system.