【正文】 symbol i is multiplied by the resulting fading amplitude an,i and rotated by a random phase ?n, advantage of the frequency domain channel model is that the IFFT and FFT operation for OFDM and inverse OFDM can be avoided and the fading operation results in one plexvalued multiplication per discrete multipath channel models introduced in Section can directly be applied to().A further simplification of the channel modeling for multicarrier systems is given by using the socalled uncorrelated fading channel Fading Channel Models for MultiCarrier Systems These channel models are based on the assumption that the fading on adjacent data symbols after inverse OFDM and deinterleaving can be considered as uncorrelated [29].This assumption holds when, ., a frequency and time interleaver with sufficient interleaving depth is fading amplitude an,i is chosen from a distribution p(a)according to the considered cell type and the random phase ?n,I is uniformly distributed in the interval [0,2π].The resulting plexvalued channel fading coefficient is thus generated independently for each subcarrier and OFDM a propagation scenario in a macro cell without LOS, the fading amplitude an,i is generated by a Rayleigh distribution and the channel model is referred to as an uncorrelated Rayleigh fading smaller cells where often a dominant propagation ponent occurs, the fading amplitude is chosen from a Rice advantages of the uncorrelated fading channel models for multicarrier systems are their simple implementation in the frequency domain and the simple reproducibility of the simulation The coherence bandwidth of a mobile radio channel is the bandwidth over which the signal propagation characteristics are correlated and it can be approximated byThe channel is frequencyselective if the signal bandwidth B is larger than the coherence the other hand, if B is smaller than , the channel is frequency nonselective or coherence bandwidth of the channel is of importance for evaluating the performance of spreading and frequency interleaving techniques that try to exploit the inherent frequency diversity Df of the mobile radio the case of multicarrier transmission, frequency diversity is exploited if the separation of subcarriers transmitting the same information exceeds the coherence maximum achievable frequency diversity Df is given by the ratio between the signal bandwidth B and the coherence bandwidth，The coherence time of the channel is the duration over which the channel characteristics can be considered as timeinvariant and can be approximated byIf the duration of the transmitted symbol is larger than the coherence time, the channel is the other hand, if the symbol duration is smaller than , the channel is time nonselective during one symbol coherence time of the channel is of importance for evaluating the performance of coding and interleaving techniques that try to exploit the inherent time diversity DO of the mobile radio diversity can be exploited if the separation between time slots carrying the same information exceeds the coherence number of Ns successive time slots create a time frame of duration maximum time diversity Dt achievable in one time frame is given by the ratio between the duration of a time frame and the coherence time, A system exploiting frequency and time diversity can achieve the overall diversityThe system design should allow one to optimally exploit the available diversity instance, in systems with multicarrier transmission the same information should be transmitted on different subcarriers and in different time slots, achieving uncorrelated faded replicas of the information in both multicarrier systems with flat fading per subchannel and timeinvariance during one symbol cannot exploit diversity and have a poor performance in time and frequency selective fading methods have to be applied to exploit approach is the use of data spreading where each data symbol is spread by a spreading code of length , in bination with interleaving, can achieve performance results which are given forby the closedform solution for the BER for diversity reception in Rayleigh fading channels according to [40]Whererepresents the binatory function，and σ2 is the variance of the soon as the interleaving is not perfect or the diversity offered by the channel is smaller than the spreading code length L, or MCCDMA with multiple access interference is applied,()is a lower L = 1, the performance of an OFDM system without forward error correction(FEC)is obtained, 9which cannot exploit any BER according to()of an OFDM(OFDMA, MCTDMA)system and a multicarrier spread spectrum(MCSS)system with different spreading code lengths L is shown in Figure other diversity techniques are modulation is used for symbol mobile radio channel is modeled as uncorrelated Rayleigh fading channel(see Section ).As these curves show, for large values of L, the performance of MCSS systems approaches that of an AWGN form of achieving diversity in OFDM systems is channel coding by FEC, where the information of each data bit is spread over several code to the diversity gain in fading channels, a coding gain can be obtained due to the selection of appropriate coding and decoding 1基本原理這章描述今日的基本面的無(wú)線通信。顧客可能會(huì)失去對(duì)我們產(chǎn)品的興趣。If you want to show your willingness to cooperate, you’d better go to the pany in 。但日子在繼續(xù)。趁熱吃。However, all of all of us find being kept waiting for more than ten minutes ，你應(yīng)該用雙手遞。The host expects us to arrive at ，你最好不要食言。，慶祝我的生日。，不要開(kāi)窗。If you haven’t yet made up your mind, why not discuss it with your is the responsibility of the parents to make sure that the children receive good 。If you find yourself lost in the street , don’t hesitate to ask for 。However, if we do not have enough courage, the plan may prove to be a plete ，不要事事依靠父母。History is created by 。The best way for young people to understand their parents is to municate with ，不過(guò)得有人接替我的工作。Multimedi