[[alternative]]Low Complexity MIMO System Using Combined Block Coding in Space, Time, and Frequency for High Speed Data Communications in Multipath Fading Channels

計畫編號：NSC96-2221-E032-003研究期間：200708~200807研究經費：516,000[[abstract]]無線通道含藏的多路徑衰退(multipath fading)效應，讓不斷朝向高速率的通訊需求帶來巨大 的挑戰。在所有對抗多路徑衰退效應的通訊技術中，多天線技術為俱有極佳之頻譜使用效率 (spectral efficiency)的方法，當然，配合目前極成熟的天線製作與訊號處理技術，讓多發射-多 接收天線系統成為一有效且確實可行之提升通道容量的方法。 除了高速率通訊需求，低複雜度設計隱含低成本的商業化考量。我們注意到最早由Alamouti 與Tarokh et al.提出之正交時空區塊碼(orthogonal space-time block code)，利用正交的特性讓 原本多輸入-多輸出通道等效成多個單輸入-單輸出通道，則接收端之最大似然解碼(maximum likelihood decoding)不僅保有原本低錯誤率的特性，也大幅降低其計算複雜度。然則，正交時空 區塊碼為針對頻率平坦(frequency-flat) quasi-static 通道所設計， 對於頻率選擇 (frequency-selective) 通道的最熱門通訊系統為架構在正交分頻多工(orthogonal frequency division multiplexing)之下，因為正交分頻多工能將頻率選擇通道等效成多個頻率平坦通道，如 此簡化了接收端設計。 本計畫為有系統的提出俱低錯誤率(高速率)、低複雜度的多天線區塊碼的設計與分析。從頻率 平坦通道的時空區塊碼，至頻率選擇通道正交分頻多工之空頻(space-frequency)區塊碼，到最後為 將時間相關(time correlation)都考慮進去的時空頻碼(space-time-frequency)區塊碼。每一階段 的主題都鎖定在能將多輸入-多輸出通道等效成多個單輸入-單輸出通道的低複雜度區塊碼。這樣的 議題在quasi-static 時空區塊碼的文獻很多，但在fast-fading 時空區塊碼就很少，而正交分頻多 工之下的空頻區塊碼又更少。至於正交分頻多工之下且考慮時間相關的時空頻碼，就目前所能找到 的文獻中，尚未見有發表於此範疇，這也是本計畫的特點。 從我們初步的研究中，已得到一些樂觀且肯定的結果，透露出本計畫的可行性。特別要強調， 我們研究的編碼設計並未限定在正交碼，對於所提出的區塊碼，不僅使用Monte-Carlo 電腦模擬做 驗證，我們將推導在Rayleigh fading 下之exact closed-form 錯誤率公式，並且通道的功率分佈 為任意的，並非如大多數文獻只限於相同、獨立Rayleigh fading 下的正交碼。[[sponsorship]]行政院國家科學委員

Signal detection for non-orthogonal space-time block coding over time-selective fading channels

In the case of non-quasi-static (i.e., time-selective fast fading) channels, which do exist in practice, the performance of the existing NO-STBC detectors can suffer from an irreducible error floor. To this end, this letter proposes a zero-forcing-based signal detector, which is not only computationally simple but also highly effective in mitigating the impact of channel variation on system performance

Performance Analysis of MIMO-STBC Systems with Higher Coding Rate Using Adaptive Semiblind Channel Estimation Scheme

Semiblind channel estimation method provides the best trade-off in terms of bandwidth overhead, computational complexity and latency. The result after using multiple input multiple output (MIMO) systems shows higher data rate and longer transmit range without any requirement for additional bandwidth or transmit power. This paper presents the detailed analysis of diversity coding techniques using MIMO antenna systems. Different space time block codes (STBCs) schemes have been explored and analyzed with the proposed higher code rate. STBCs with higher code rates have been simulated for different modulation schemes using MATLAB environment and the simulated results have been compared in the semiblind environment which shows the improvement even in highly correlated antenna arrays and is found very close to the condition when channel state information (CSI) is known to the channel

Signal detection for orthogonal space-time block coding over time-selective fading channels: A PIC approach for the G(i) systems

One major assumption in all orthogonal space-time block coding (O-STBC) schemes is that the channel remains static over the entire length of the codeword. However, time selective fading channels do exist, and in such case the conventional O-STBC detectors can suffer from a large error floor in the high signal-to-noise ratio (SNR) cases. This paper addresses such an issue by introducing a parallel interference cancellation (PIC) based detector for the G(i) coded systems (i = 3 and 4)

Coded DS-CDMA Systems with Iterative Channel Estimation and no Pilot Symbols

In this paper, we describe direct-sequence code-division multiple-access (DS-CDMA) systems with quadriphase-shift keying in which channel estimation, coherent demodulation, and decoding are iteratively performed without the use of any training or pilot symbols. An expectation-maximization channel-estimation algorithm for the fading amplitude, phase, and the interference power spectral density (PSD) due to the combined interference and thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate codes. After initial estimates of the fading amplitude, phase, and interference PSD are obtained from the received symbols, subsequent values of these parameters are iteratively updated by using the soft feedback from the channel decoder. The updated estimates are combined with the received symbols and iteratively passed to the decoder. The elimination of pilot symbols simplifies the system design and allows either an enhanced information throughput, an improved bit error rate, or greater spectral efficiency. The interference-PSD estimation enables DS-CDMA systems to significantly suppress interference.Comment: To appear, IEEE Transactions on Wireless Communication

Digital watermark technology in security applications

With the rising emphasis on security and the number of fraud related crimes around the world, authorities are looking for new technologies to tighten security of identity. Among many modern electronic technologies, digital watermarking has unique advantages to enhance the document authenticity. At the current status of the development, digital watermarking technologies are not as matured as other competing technologies to support identity authentication systems. This work presents improvements in performance of two classes of digital watermarking techniques and investigates the issue of watermark synchronisation. Optimal performance can be obtained if the spreading sequences are designed to be orthogonal to the cover vector. In this thesis, two classes of orthogonalisation methods that generate binary sequences quasi-orthogonal to the cover vector are presented. One method, namely "Sorting and Cancelling" generates sequences that have a high level of orthogonality to the cover vector. The Hadamard Matrix based orthogonalisation method, namely "Hadamard Matrix Search" is able to realise overlapped embedding, thus the watermarking capacity and image fidelity can be improved compared to using short watermark sequences. The results are compared with traditional pseudo-randomly generated binary sequences. The advantages of both classes of orthogonalisation inethods are significant. Another watermarking method that is introduced in the thesis is based on writing-on-dirty-paper theory. The method is presented with biorthogonal codes that have the best robustness. The advantage and trade-offs of using biorthogonal codes with this watermark coding methods are analysed comprehensively. The comparisons between orthogonal and non-orthogonal codes that are used in this watermarking method are also made. It is found that fidelity and robustness are contradictory and it is not possible to optimise them simultaneously. Comparisons are also made between all proposed methods. The comparisons are focused on three major performance criteria, fidelity, capacity and robustness. aom two different viewpoints, conclusions are not the same. For fidelity-centric viewpoint, the dirty-paper coding methods using biorthogonal codes has very strong advantage to preserve image fidelity and the advantage of capacity performance is also significant. However, from the power ratio point of view, the orthogonalisation methods demonstrate significant advantage on capacity and robustness. The conclusions are contradictory but together, they summarise the performance generated by different design considerations. The synchronisation of watermark is firstly provided by high contrast frames around the watermarked image. The edge detection filters are used to detect the high contrast borders of the captured image. By scanning the pixels from the border to the centre, the locations of detected edges are stored. The optimal linear regression algorithm is used to estimate the watermarked image frames. Estimation of the regression function provides rotation angle as the slope of the rotated frames. The scaling is corrected by re-sampling the upright image to the original size. A theoretically studied method that is able to synchronise captured image to sub-pixel level accuracy is also presented. By using invariant transforms and the "symmetric phase only matched filter" the captured image can be corrected accurately to original geometric size. The method uses repeating watermarks to form an array in the spatial domain of the watermarked image and the the array that the locations of its elements can reveal information of rotation, translation and scaling with two filtering processes

Space time coding in MIMO systems

Multiple-input multiple-output (MIMO) antenna technology is promising for high-speed wireless communications without increasing the transmission band- width. Space time coding (STC) is a scheme that employs multiple antennas to increase transmission rate or to improve transmission quality. STC is used widely in mobile cellular networks, wireless local area networks (WLAN) and wireless metropolitan area networks (WMAN). However, there are still many unsolved or partially solved issues in STC. In this thesis, I propose a new STC design from cyclic design. I then propose a systematic method to design quasi-orthogonal space time block codes (QOSTBC) for an arbitrary number of transmit antennas, and derive the optimal constellation rotation angles to achieve full diversity. I also propose an analytical method to derive the exact error probabilities of orthogonal space time block codes (OSTBC). In order to improve the error performance, I introduce an adaptive power allocation scheme for OSTBC. Combining STC with continuous phase modulation (CPM) is an attractive solution for mobile commu- nications for which power is limited. Thus, I apply OSTBC to binary CPM with modulation index h = 0.5, and develop a simplified receiver for such scheme. Finally, I present a decoding method to reduce the complexity of QOSTBC without degrading its error performance