大型天線陣列系統的秩值與複合通道估計及閉迴路傳收機之設計

Abstract

我們探討一個裝有大型天線陣列基地台(BS)來服務多個單天線用戶(UE)的多輸入多輸出(MIMO)分時多工系統。此系統藉由上行(UL)的領航信號(pilot)來估計包含大尺度衰減係數(LSFC)及小尺度衰減係數(SSFC)的通道狀態資訊(CSI)。雖然在MU-MIMO或是分散式MIMO系統之運作，LSFC資訊是不可或缺的，然而有關MIMO通道估計之研究往往被假設為已知或是被忽略。我們利用大型天線陣列之通道硬化(channel-hardening)並能同時收到大量空間樣本的特性，在不需SSFC資訊的前提下能有效的利用相對少量的領航符元精準地估計LSFC。 至於SSFC的估計，我們利用降秩(rank-reduced, RR)通道模型來完成。由於這種方法之降秩效應需選擇適當的基底並有準確的秩值估計，後者又需先知道通道的空間相關矩陣。針對這三項議題我們首先分析了最佳先設(predetermined)基底的選擇，證明兩種常用的基底之近優性(near-optimality)。接著我們探討秩值對SSFC估計的性能影響、設計一套秩值決定的演算法，最後並發展了估計空間相關矩陣的演算法。這些成果乃是以我們對SSFC估計法的均方誤差(mean squared error, MSE)性能的詳細分析為基礎。我們結合了大、小尺度衰減係數的估計並證明在接收信號之入射角度擴散(AS)不大時，還可利用適當的RR模型來一併估計平均的接收角度(AoA)。比起使用不含AoA資訊的模型之通道估計法，這種方法不但可降低MSE而且所估得的角度資訊可用來形成下傳鏈路的波束。 最後，我們推導了上行領航信號的設計方式，並且在含括前述的複合通道估計器後，提出了分別適用於分時多工和分頻多工模式的閉迴路傳收機(closed-loop transceiver)設計流程與細部演算法。由電腦實驗結果可以看出我們的複合通道估計器、秩值決定及空間相關矩陣估計等演算法在大型天線陣列系統中的優異表現。

We consider a multiuser (MU) multiple-input multiple-output (MIMO) time-division duplexing (TDD) system in which the base station (BS) is equipped with a large number of antennas for communicating with single-antenna mobile users. In such a system the BS has to estimate the channel state information (CSI) that includes large-scale fading coe?cients (LSFCs) and small-scale fading coe?cients (SSFCs) by uplink pilots. Although information about the former FCs are indispensable in a MU-MIMO or distributed MIMO system, they are usually ignored or assumed perfectly known when treating the MIMO CSI estimation problem. We take advantage of the large spatial samples of a massive MIMO BS to derive accurate LSFC estimates in the absence of SSFC information and with a training overhead no larger than that required by conventional LSFC estimators. With estimated LSFCs, SSFCs are then obtained using a rank-reduced (RR) channel model. We analyze the mean squared error (MSE) performance of the proposed composite channel estimator and prove that the separable angle-of-arrival (AoA) information provided by the RR model is bene?cial for enhancing the estimator's performance, especially when the mean angle spread of the uplink signal is not too large. To fully exploit the RR channel model, we have to select a proper basis and determine associated rank (modeling order). We solve these two issues by developing a rank-determination algorithm based on two popular bases and verify the near-optimality through computer simulations. We discuss uplink pilot design and suggest closed-loop transceiver design flows for both TDD and FDD modes using the estimated AoA for downlink beamforming and the LSFC information for power allocation. Some computer experiment results are provided to validate the e?ciencies of the proposed estimators and the rank determination method.