| Summary: | 碩士 === 逢甲大學 === 通訊工程所 === 97 === Multicarrier modulation (MCM) systems have been widely used in both wireline and wireless communications recently, e.g., the digtal subscribe line (DSL), wireless local area networks (WLANs), and digital video broadcasting (DAB). Orthogonal frequency-division multiplexing (OFDM) is a special form of multicarrier transmission where a single high speed fata stream is transmitted over a number of lower-rate subcarriers.
For multiuser OFDM communications, orthogonal frequency division multiple access (OFDMA) inherits from OFDM the weakness of being sensitive to the carrier frequenct offset (CFO) effect between the transmitter and the receiver. CFO causes the loss of orthogonality among subcarriers and introduces intercarrier interference (ICI). When any user has a nonzero CFO in the OFDMA uplink, the CFO not only causes the performance drgradation of this particular user but also leads to multiple access interference (MAI) for other users. In the uplink transmission, it is difficult to guarantee that every user transmits his/her signal simutaneously. This will leads to timing mismatch among users. In order to separate multiple users’ signals and suppress MAI, the advanced signal processing algorithms have to be applied at the uplink receiver to achieve the correct estimation of each user’s CFO.
In this research, the minimum variance distortionless response (MVDR) algorithm is proposed to estimate the carrier frequency offset (CFO) of the downlink OFCDM (code aided OFDMA) system, which is a highly spectrally eddicient system. Since subcarrier reuse sheme is used for multiple users using multiple access (MA) code, the proposed method is expected to greatly mitigate the effects caused by the multi-access interference (MAI) intridused by CFO, and timing mismatch among users, by taking the advantage of CDMA systems using MA code to enhance bit error rate (BER) performance. The method of the pilot symboaided modulation scheme is used to compensate for the fluctuation of amplitude and phase due to fading.
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