Analysis & The Settling Schemes Of Doppler Effect In Mobile Transmission Of Video Signals

In modern mobile transmission systems, the transmission of video signalsappears to be a more and more important part. When the receiver moves at a highspeed, the Doppler Effect is much more salient and the time variance of thewireless channel is very fast. The data stream of video signals is large, and strictreal time transmission is required. Also, bit errors rate must be low. Thus, themobile wireless channel must be of high-data-rate, low-error-rate and this tend tobe a challenge to the transmission systems.In this paper, the property of video-signal-transmission is analyzed. Theproblems of video-signal-transmission in the mobile wireless channels are putforward.The mobile wireless channel is the most complex in variant channels. In wirelesscommunications environments, a signal transmitted from the transmitter receivesthe receiver through many different paths, subjected to shadow effect, DopplerEffect, fading effect and or so. With the high moving of receiver, the DopplerEffect is prominent and the received signals undergo a course of fast fading. Thetime-varying of the channel is enlarged, having much more bit errors, and thebandwidth is limited.This paper analyzed the transmission of radio waves in mobile wirelesscommunication environments and the problems associated with the receivingprocess. The Doppler Effect, the multipath effect and their combination isdiscussed in detail. Their mathematical formulations are presented. Several figuresplotted by Matlab programs give an intuitionistic expression. According to thesimulation results, it can be seen that the energy of received signals varies inrandom. The amplitude and phase of received signals also go through a randomvariation. The received signals are much distorted.The maximum Doppler frequency shift is also called Doppler spread, and it is aimportant parameter of wireless channels. This paper analyzes the emerging ofDoppler frequency shift in physics and gives the general calculation formula ofmax Doppler frequency shift.Dealing with the prominent Doppler frequency shift, prevenient studies are mostfocused on several aspects. By exact channel estimation, one can compensated theDoppler Effect at the receiver, called adaptive receiving. This method possesseslittle robust properties. Using channel coding technology, the bit-error-rate can belowed, thus the Doppler Effect could be resisted. The latter is the new tendency ofthe technology development. But the coding and decoding process is complex incalaulation and is tend to introduce delay to the communication system.Error Control Technology (ECT) had became a key technique in videocommunication field . This technique could effectively improve the visual qualityof decoded video information at receiver. In the paper, the coding technologyusually used for video transmission in communication system is introduced.The Orthogonal Frequency Division Multiplexing (OFDM) is a kind of digitalmodulation with multicarriers. It has many advantages compared with othermodulation method, such as higher spectrum efficiency, stronger ability of resistingmultipath fading effect and much simplier implementation in communicationsystems. This paper introduces the basic principle OFDM and concisely presentssome measures to overcome multipath fading.Turbo code is the most advanced coding technology nowadays. Itserror-correcting performance is very closely to the Shannon limit. The theory ofturbo code and the basic structures of its encoder and decoder usually adopted areintroduced. The application of Turbo code in CDMA and WCDMA systems is alsoincluded.The system of OFDM combined with coding technology also called COFDM(Coding Orthogonal Frequency Ddivision Multiplexing), and it can enhance theability to lower the effect of multipath fading. Combining OFDM with Turbocoding technology, the COFDM system can present both advantages of OFDM andTurbo code. Choosing the structure parameters, the interleaving and the structuresof encoder and decoder applicable to OFDM system, searching for new method forfast calculation, are the possible way to overcome the prominent multipath DopplerEffect in mobile video transmission.