在現代高移動性通訊中，嚴重的都普勒頻移會引發載波間干擾（ICI），大幅削弱傳統 OFDM 系統的效能。為此，正交時頻空間（OTFS）技術透過將符號映射至延遲-都普勒域，將劇烈的時變衰落轉化為穩定且稀疏的二維通道，以獲取全分集增益。同時，結合低密度奇偶檢查碼（LDPC）優異的糾錯能力與軟資訊處理特性，能進一步大幅提升系統整體的傳輸可靠度。
然而，OTFS 複雜的二維干擾使傳統偵測器面臨複雜度瓶頸，且單向分離的「偵測-解碼」架構無法有效反饋真實干擾狀態。為突破限制，本論文提出專為 OTFS 設計的低複雜度改進式梯度搜尋演算法（IGSA）。該演算法以 LMMSE 作為初始估計，利用差分度量構建離散梯度，沿最優方向進行解空間探索，能以極低運算成本鎖定可靠估計值。
為進一步提升系統效能，本研究提出結合 IGSA 與 LDPC 的並列聯合接收機架構。該架構透過雙向並列疊代交換軟性資訊（Soft Information），並利用解碼器回傳的外部資訊（Extrinsic Information）執行軟干擾消除（Soft-PIC），有效降低殘餘干擾並動態引導 IGSA 逃離局部最佳解。模擬證實，在高速移動環境下，此架構在維持低運算複雜度的同時，展現出顯著優於傳統分離式設計的位元錯誤率（BER）表現，為未來通訊系統提供了極具競爭力的解決方案。

Orthogonal Time Frequency Space (OTFS) modulation is a promising technology for 6G high-mobility networks, effectively addressing the doubly-selective fading challenges of traditional OFDM systems. However, the severe two-dimensional inter-symbol interference (2D-ISI) in the delay-Doppler domain significantly complicates receiver design. Existing detection algorithms, including linear methods, Message Passing Algorithms (MPA), and conventional Gradient Search Algorithms (GSA), struggle with either high computational complexity, severe error floors caused by short cycles, or the inability to output soft information for channel decoding.
To overcome these limitations, this thesis proposes a low-complexity joint iterative receiver named the Improved Gradient Search Algorithm-Parallel Joint (IGSA-PJ). The IGSA frontend introduces an intelligent pruning mechanism and local recursive updates to drastically reduce the search complexity from a polynomial O(D^2) to a linear O(D). Furthermore, a Soft-PIC mechanism is developed to convert the hard decisions into reliable soft log-likelihood ratios (LLRs). By integrating interleaving mechanisms and dynamic trust weights, the proposed architecture establishes a robust bi-directional soft information exchange between the physical layer detector and the Low-Density Parity-Check (LDPC) decoder.
Simulation results under a 300 km/h Extended Vehicular A (EVA) multipath channel demonstrate the superiority of the proposed IGSA-PJ receiver. Compared to conventional MPA-PJ architectures, the IGSA-PJ successfully avoids divergence and completely eliminates the error floor phenomenon, maintaining a consistent bit-error-rate (BER) descent. Ultimately, this architecture achieves excellent detection performance with significantly reduced computational costs, providing a highly practical physical layer solution for future high-mobility communications.

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