我們首先提出了一種叫做OFCDMA (orthogonal frequency CDMA) 的廣義架構來描述用多載波傳輸之CDMA (code division multiple access 分碼存取) 通訊系統，並分析與比較此廣義架構之一些特殊架構在多重路徑的通道 (multipath fading channel) 下各種型態的干擾 (interference) 與傳輸錯誤率受頻率飄移 (frequency offset) 的影響，以提供更多的資訊來設計更好的接收器。基於OFCDMA之類比架構我們發展出一個可程式的(programmable) OFDM-CDMA (正交分頻多工-分碼存取) 數位接收發射器之硬體架構。而只要具備此一硬體架構，靠著軟體的程式就可達成多系統的通訊目標。接下來我們基於此可程式的OFDM-CDMA 數位接收發射器發繼而發展出線性的可程式的OFDM-CDMA多用戶偵測器 (multiuser detector) 與同步器 (multiuser synchronizer) ，設計當中我們同使考慮了時域 (time domain)，頻域 (frequency domain)，空間域 (space domain)，碼域 (code domain) 這些方面的最佳化問題。而關於多用戶同步器的發展我們更提出一套設計訓練數列 (training sequence) 演算法做到只以線性的複雜度達成最佳化的時機偵測(optimum timing estimation)。

A general scheme called orthogonal frequency CDMA (OFCDMA) to describe a CDMA system with orthogonal carrier transmission is first proposed. Three well known multicarrier CDMA schemes, including multicarrier CDMA, multicarrier direct sequence CDMA, and multitone CDMA, can be derived as special cases of the proposed OFCDMA if system parameters are appropriately defined. We proceed a unified analysis of the OFCDMA to systematically study the performance of those three multicarrier CDMA schemes. Multiray multipath fading is assumed in each subchannel and the maximum ratio combing approach is used for detection. Five types of interference occur under this situation. We analyze and compare among the three schemes for each type of interference and discuss their pros and cons. Both qualitative and quantitative results can help design effective OFCDMA receivers in the interference-limited environments.
Combining OFDM and CDMA technologies is attractive for future wireless
broadband communications while software radio realization is a inevitable trend because of its flexibility to diverse multi-system or multi-standard communication environments. Based on the proposed unified framework OFCDMA, we develop a programmable structure for OFDM-CDMA transceivers in spite of three different scenarios to combine OFDM and CDMA. By adjusting system parameters, various system scenarios can be implemented without changing the fundamental hardware and software architecture, which might serve as the foundation for designing software radio.
Next, we develop a programmable OFDM-CDMA multiuser detection structure based on the programmable OFDM-CDMA transceivers architecture. Various system scenarios, such as MC-CDMA, MC-DS-CDMA, and MT-CDMA can be realized by adjusting system parameters in the programmable multiuser detection structure. A unified approach jointly considering multiaccess interference and other interferences in time, space, and frequency domains is used in developing the programmable detection structure. The unified approach not only facilitates designing programmable OFDM-CDMA detection structures but also provides systematic design rules for reduced complexity multiuser detectors. Therefore, we further propose a joint detection strategy to reduce the complexity. It is shown that the proposed programmable detectors based on LMMSE and BLUE are robust against near-far environments due to channel fading effects. The proposed reduced complexity detectors are also shown to be effective in a frequency selective environment. A more effective synchronization is desired for a multiuser detector, which is highly sensitive to estimation errors of timings, carrier phases and amplitudes. Therefore, we develop a programmable multiuser synchronization structure for general
OFDM-CDMA systems. Different synchronization scenarios for various combinations of OFDM and CDMA can be implemented by controlling system parameters. A unified approach jointly considering multiaccess interference and other interferences
in time, space, and frequency domains is used in developing the programmable synchronization structure. We show that for both BLUE and LMMSE estimators, the mean square error of users’ amplitudes and timings approaches zero as noise tends to zero. Simulation results show that BLUE and LMMSE estimators are robust against near-far problems due to channel fading effects.
Following the work in programmable multiuser synchronization, we further consider training sequence design to improve performance. The optimum multiuser
timing estimation based on the maximum a posteiori rule (MAP) has an NP-hard nature. However, by making the submatrices of the effective correlation matrix diagonal with special training sequences, user's signals equivalently become orthogonal.
As a result, the complexity of the optimum solution is reduced to be linear. In this letter, we present a systematical approach to achieve the optimum solution with linear complexity using special training sequences. The existence of those sequences is proved, and algorithms for finding them are proposed for AWGN channels as well as multiray fading channels. Performance analysis shows that our approach further makes a multiuser environment become a single-user one so the performance characteristic is the same as that in a single-user environment. Simulation results show that our approach significantly outperforms the approach with all-1 training sequences.

封面
Abstract
Contents
List of Tables
List of Figures
1 Introduction
1.1 Direct Sequence CDMA Communications
1.2 Multiuser Detections and Synchronizations

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