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Signal Processing for DS/CDMA and OFDM-CDMA Systems: Interference Suppression, Timing Recovery and Space-Time Processing
|關鍵字:||CDMA, OFDM, Interference, Timing Recovery, Space-Time Processing|
分碼多重接取, 正交分頻多重接取,干擾, 時序回復, 空時訊號處理
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Code-division multiple access (CDMA) has been claimed to offer large capacity improvements over time-division multiple access (TDMA) and frequency-division multiple access (FDMA) in cellular radio systems. Unlike the other two multiple access techniques, the capacity of CDMA tends to be limited by multiple access interference (MAI) and the near-far problem rather than noise. In a multiuser environment, orthogonality among spreading codes is severely distorted due to multipath delay spread, such that the system capacity will be limited by the incurred MAI. Effective interference reduction will increase system capacity, which means interference suppression techniques are necessary in improving the overall system performance. Spread spectrum communication needs to synchronize the spreading waveforms in transmitting and receiving ends. If the two waveforms are out of synchronization by a little chip time, insufficient signal energy will reach the receiver, the performance of data demodulation is thus degraded. Besides, the use of antenna arrays for diversity techniques can enhance the link quality and spectral efficiency, and further apply to multirate schemes. Another challenge is the combination of orthogonal frequency division multiplexing (OFDM) and CDMA: the OFDM-CDMA technique; we classify it to multicarrier code-division multiple access (MC-CDMA) and multicarrier direct-sequence code-division multiple access (MC-DS-CDMA). The above mentioned issues will be discussed in detail in this thesis. Firstly, we consider the problem of simultaneous MAI-rejection, intersymbol interference (ISI) suppression and data detection. This adaptive receiver is based on cost functions evaluated at the code despreader''s and data detector''s output. It is a bit-rate adaptive receiver and is simple and is designed in such a way that the despreader output statistic would converge to desired property. The filter weights are updated only once every bit interval. Therefore, the computational complexity for our receivers is lower than that of the conventional chip-decision methods. Secondly, the blind generalized sidelobe canceller (GSC) -based receiver is introduced by dividing the signal space into two subspaces, a constraint subspace and an orthogonal subspace. In general, the GSC is used to cancel the sidelobe to highlight the array manifold of signal of interest in spatial domain. We extend the spatial concept of the GSC Filter to temporal domain and then refer to blind adaptive filter which can be converted to an unconstrained minimization problem by using GSC to eliminate MAI. In direct-sequence (DS) spread spectrum (SS) systems, the conventional delay lock loop (DLL) has been considered for pseudo-noise (PN) code tracking, even though the DLL in not designed for operation when channel fading is present. In our proposed receiver, the adaptive filter is used to estimate the desired signal and suppress the multipath and MAI effects based on the received signals. Simultaneously, we use the adaptive algorithm to estimate the code delay through the same error signal. In addition to above process, the noncoherent receiver employs the decision-feedback differential detection to recover the MDPSK signal. At last, the symbol-based adaptive antenna receivers are employed in multirate CDMA systems. The weight updated equations of the adaptive antennas are related to the spreading code vector when smart antennas are used in CDMA systems, and we adjust the despreading code in time domain. We call this new scheme Wiener code filter, which behaves like the Wiener filter. Wiener filter in the adaptive antenna that has the ability to be against interference in nature. Wiener code filter of adaptive antenna not only has the ability to against interferences in time domain but also has the function of despreader in CDMA multiuser environment. We can regard it as the structure of combining the Wiener filter and despreader. The OFDM-CDMA, which combines CDMA with OFDM, has become an attractive alternative to DS-CDMA for future high-data-rate wireless communication systems. OFDM-CDMA systems can be categorized mainly into two types. One conveys all elements of each transmittted sequence over a number of subcarriers simultaneously, and the other is a scheme of parallel transmission of narrowband DS waveforms. In this thesis, the former system is specifically refered to as MC-CDMA and the latter one as MC-DS-CDMA. We will concentrate on the construction of adaptive equalization for MC-CDMA and MC-DS-CDMA communication systems over a dispersive channel.
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