一種只需部分通道相位資訊的新型空間預編碼的效能評估與晶片設計

Abstract

正交時空編碼 (OSTBC) 能提供全多樣性 (full diversity)，全速率 (full rate) 及 複數資料 (complex symbols) 傳輸的編碼方式只適用於兩隻傳輸天線架構中。而實現 正交時空編碼，傳輸端需要額外記憶體以存取連續傳輸資料供編碼及解碼用。此存取動 作會造成信號延遲。當正交時空編碼用在正交頻率多工系統 (OFDM) 時，延遲會變得更 嚴重，因為子載波 (subcarriers) 數通常很大。而針對束波集成 (beamforming) 技 術，接收端需要計算通道矩陣的特徵分解並回傳資訊。回傳的資訊會隨傳輸及接收天線 數增加而增加。當束波集成被用在正交頻率多工系統時，回傳的資訊量常會很大而限制 此技術在通道變化快速環境下的使用。此外，所有子載波的特徵分解對接收端而言需要 很大的計算量。而且，傳輸端各個子載波需要做矩陣乘法。這些缺點可能限制束波集成 技術在可攜式裝置，如手機的運用. 在此計畫中，我們提出一種新型的空間預編碼架構。此架構不僅可達成全多樣性，全速 率及複數資料傳輸，而且還能提供預編碼增益 (precoding gain)。在兩隻傳輸天線架 構中，此架構只需一位元迴授資訊其效能即可勝過 Alamouti 架構。此架構可以用在任 意傳輸天線數。若傳輸天線數為 N，我們發現 (N-1) 位元即可使此架構提供全多樣性 以及預編碼增益且預編碼增益會隨傳輸天線數增加而增加。此架構不需在傳收端提供額 外記憶體也不會造成信號延遲。此提出的預編碼架構可直接應用在使用多輸入多輸出技 術的規格中，例如，IEEE 802.11n 以及 IEEE 802.16 系列規格中. 在此一研究中，我們將對此新型架構的效能及複雜度與正交時空編碼以及束波集成技 術做一比較。此外，我們將以晶片實現此一技術。

In this proposal, we propose a novel precoding scheme for MIMO (multiple input and multiple output) systems to overcome the drawbacks of orthogonal space-time block code (OSTBC) and beamforming techniques. For OSTBC, the coding scheme that achieves full diversity order for complex symbols with full rate exists only for two transmit antennas, i.e. Alamouti scheme. Also, to implement OSTBCs, we need to store consecutive symbols in both transmitter and receiver sides for encoding and decoding purposes, which requires extra memory and leads to latency. The memory and latency increase as the OSTBCs applied to OFDM (orthogonal frequency domain multiplexing) systems, where the number of subchannels can be up to several thousand, e.g. up to 2048 in standards of IEEE 802.16 family. For beamforming, the receiver needs to compute the eigendecomposition of MIMO channel. Moreover, it requires feedback to send back the eigendecomposed information. The amount of feedback increases as the numbers of transmit and receive antennas increase. When beamforming is used in OFDM systems, the feedback amount is in general very large and hence may somewhat be impractical when the channel environment varies rapidly. Moreover, the eigendecompositions for all subchannels lead to high computational burden in the receiver. Furthermore, the transmitter and receiver need to perform matrix multiplications based on the eigendecomposition. All those drawbacks may limit beamforming techniques be used in portable devices such as mobile phone, where simple transceiver design is more appreciated. In this proposal, we propose a novel spatial precoding scheme that can provide additional coding gain as well as achieve full diversity order for complex symbols with full rate. In systems with 2 transmit antennas, the proposed precoding scheme can outperform the Alamouti scheme with only 1-bit feedback information due to the additional precoding gain. Moreover, unlike orthogonal space-time block code, this precoding scheme can be used for arbitrary number N of the transmit antennas. We found that using (N-1)-bit feedback information, the proposed scheme can achieve a full diversity order while also provide additional precoding gain in systems with N transmit antennas. The precoding gain increases as N increases. Unlike orthogonal space-time block codes, this precoding scheme does not need extra memory to store consecutive symbols and hence does not result in latency in both transmitter and receiver sides. Furthermore, we propose a fast algorithm to compute the feedback information for precoding to avoid the use of exhaustive search. The proposed spatial precoding scheme and the algorithm to generate precoding codewords can be directly used in standards that use MIMO techniques such as IEEE 802.11n and IEEE 802.16 family. In this research, we will investigate the performance and complexity of the proposed precoding scheme and that of OSTBC and beamforming. Also, we will show how to use the proposed techniques in IEEE 802.11n and IEEE 802.16 family. Finally, we will implement the proposed architecture with IC.