Studying Virable Rate Space-time Block Coding In OFDM System

Mobile communication means communication between mobile objects or between mobile and still objects .For example ,communication among human ,car and ship .Mobile communication system includes wireless telephone ,wireless calling ,land cellular and satellite mobile communication .Mobile communication technique and market make a rapid progress during 21st century ,new modulation ,signal transmitting and coding technique are not only theoretical studying theme ,but also a standard technique ,and will be widely used.We must use new modulation ,signal transmitting and channel coding technique in order to ensure securing all kinds of channel data in modern digital communication system .Space-time coding is one of the latest encodings that combat detrimental effects of fading channels in 3 Generation system. There are lots of institutes studying space-time coding .Space-time coding is a new channel encoding technique putting encoding ,modulation ,transmitting diversty and receiving diversity technique together,which will improve channel transmitting performance . Space-time coding has excerllent performance from recent research which can achieve higher diversity and coding gains,enhance system capacity and anti-jamming ,anti-multichannel performance ,and provide high rate ,high quality transmitting data without losing bandwidth compared with not using space-time coding .Nowadays space-time coding is mostly based on non-frequency choosing frequency fading channel, however lots of wireless communication system belongs to frequency-choosing fading channel ,that means channel is not flatness ,then system performance will be sharp descending,which will heavily restrict space-time coding using in future width band mobile communication . Putting space-time coding and OFDM together was mentioned in 1996,which drew a widely attention in the communication fields. Combining space-time coding with OFDM makes frequency-choosing channel into some parataxis non-frequency choosing son channels,such systems not only own diversity and encoding gains ,but also simple receiver equalizers of OFDM system .This article based on space-time coding and OFDM technique proposes variable rate space-time block coding using with OFDM system. Traditional space-time coding provides invariable coding rate ,however variable rate space-time block coding provides lots of coding rates more than 1 . This article focuses on the followings :1. Analysing variable rate space-time coding matrix ,including encoding rate formula, the space-time area formula and PLe (i),the minimum value of the BERs associated with the variable rate space-time block codes having code rate RL (i). First we will introduce the three formulas :(1) variable rate space-time block codes encoding rate formula: Where L is encoding matrix slot, k iis the amount of the space-time codes, qi is the codes.(2) the formula of space-time area nL Where l i and pi are line amount and row amount of Gi ,then k i is decided by the formula.See the following figure,where L = 6, n=4,k1 =4, (3) formula of choosing optimum rateVariable rate space-time block codes can provide several code rate,but these rates are not all optimum , so we choose the optimum using the following formula : Whereκn,L is the amount of the vector ( k1 ,k2,...,kn) which composed of slot L , n transmitting antennas space-time block codes .VectorΝn,L is the amount of space-time block codes rates ,slot L and n transmitting antennas.2. Studying variable rate space-time block codes in OFDM system Variable rate space-time block codes proposed is about flatness degrading channel .Actually widthband transmitting system is multipathway frequency choosing fading channel. Because OFDM can perfectly combat multipathway disturbing,change frequency choosing fading channel to flatness fading channel ,combing OFDM with variable rate space-time block codes,put the advantage of space-time block coding into OFDM system can promote the system performance .(1) First the article shows two transmitting atennas,see the following figure: Then we can get three rates:1,1.5,2,there is a better transmitting rate compared with traditional space-time block coding .(2) The situation of transmitting and receiving with two transmitting antennas Encoding signal output by spur track transmitted to IDFT convertor of antenna 1,began IDFT convertion , then transmitted by antenna 1 after append cyc prefixion . Encoding signal output by spur track transmitted to IDFT convertor of antenna 2,began IDFT convertion , then transmitted by antenna 2 after append cyc prefixion .Receiving OFDM signal gets rid of the cyc prefixion,bengan DFT convertor,transfer serial two signal of all son carrier to space-time decoder .Decoder will began space-time decoding according to channel estimating value .All decoder output signal began parallel to in series with two signals to get decoder output series.(3) Transmitting and receiving signal We can get transmitting signal formula according to transmitting theory and get the same results as two signals in fading Rayleigh channel STBC-OFDM system in VRSTBC-OFDM system .This N transmitting ,M receiving antennas system can get NM receiving diversity and can get the same diversity results as maximum rate combination system.(4) Simulation with 2,3 and 4 transmitting antennas in VRSTBC-OFDM system3. Comparing VRSTBC-OFDM to other STC methods(1)Comparing with one OFDM transmitter ,STBC-OFDM transmitting diversity and two transmitting antennasTraditional OFDM technique makes signal series simply parallel to in series ,but STBC encodes the signals ,strongly promotes encoding efficiency and then use STBC in OFDM system ,so the performance must be better than traditional OFDM technique(2)Comparing VRSTBC-OFDM to STBC-OFDM The same condition : 512 subchannels with one OFDM , protecting interval 40μs , corresponding CP includes 32 samplings ,that overcomes the ISI . We must assume further : A, every receiving antenna has the same signal power B, every multichannel is independent with Rayleigh C, every receiver can get perfect CSIThe former assumption is perfect ,so the simulation is great in the best situation .Why do we give such assumption ? We can compare VRSTBC-OFDM to STBC-OFDM in this assumption .There are some point which need to improve further in the thesis such as how to combine other encoding method to all kinds of STBC .