分散式無線多輸出入多媒體通訊系統---子計畫四：分散式無線多輸出入通訊系統之先進上行傳輸技術

Abstract

不論在通道容量(channel capacity)或是鏈路可靠性(link reliability)上，多天線系統皆提供具有前瞻性的增益表現，並且絲毫沒有犧牲頻譜效率。當互相獨立的平行通道在傳輸端與接收端之間形成，空間分集多樣性(spatial diversity)被充分利用，多輸入輸出(MIMO)系統的理論通道容量增益已被證明會隨著傳輸與接收天線的數量呈線性成長。更甚者，在一個多用戶(multiuser)環境，如果充分利用多用戶分集多樣性(multiuser diversity)，將能達成更高的頻譜效率(spectrum efficiency)。 正交分頻多重進接(OFDMA)這類的技術常常搭配MIMO系統來「平坦化」或「對角線化」(flatten or diagonalize)頻率選擇性(frequency-selective)通道，因為在通道中每個使用者被分配到不同的頻帶。如此一來，透過這類技術，等化通道過程可以被簡化。然而，此類型技術需要傳輸端注入循環週期字首(cyclic prefix)當作防衛區間(guard interval)，浪費了寶貴的頻寬。在眾多的系統標準中，例如IEEE 802.16e，上行傳輸需要一段很長的循環字首來防衛通道延遲(channel delay spread)與不同用戶間不同步所造成的符號/區塊間干擾(ISI/IBI)與載體間干擾(ICI)。然而，某些嚴苛傳輸環境可能導致通道延遲時間超過循環字首的最大長度，進一步導致ISI與ICI。例如在都市環境中，每個基地台蜂巢狀覆蓋區的邊緣。還有當不同使用者之間的傳輸延遲(propagation delay)很明顯時，ICI會存在於接收到的信號之中。為了維持高頻譜效率，可以使用很多點數的快速傅立葉轉換(FFT)。但是，這樣帶來的缺點包括：增加傳送端硬體複雜度、減少載體間隔、讓系統更容易受頻率偏差與震盪器雜訊影響。而且，較高數目的子載體(sub-carrier)會增加鋒對均功率值(peak-to-average-power ratio, PAPR)，必須使用高度線性的功率放大器。針對這方面的改進，日前被提出來的單載波分頻多重進接(SC-FDMA)已經用於3GPP LTE的上行傳輸。這造就一種新穎的混合OFDM-SC解決方案：移動終端使用SC-FDMA於上行傳輸，基地台使用OFDMA於下行傳輸。為了進一步減少傳送端的能量消耗，尾隨零(trailing-zero, TZ)可以用來取代循環字首。吾人欲針對防衛區間長度不足的情形（故造成接收端IBI），進行等化技術的研究，特別是用於reduced-TZ MIMO-OFDM系統與reduced-TZ MIMO-SC系統。 目前，WiMAX802.16e標準提出使用OFDMA於上行與下行傳輸。如前言所提，可以想見未來系統中OFDM與SC可能會同時存在，讓傳送端得以利用許多區塊傳輸單載波系統(block based SC system)的優勢。但在多用戶情境中，共同通道干擾(co-channel interference, CCI)將會大幅弱化系統表現。CCI可能來自單一或多個蜂巢單位(cell)的使用者。在OFDMA中，蜂巢式內部干擾可能在時間與/或頻率同步錯誤時發生。 當已知欲求信號方向與陣列幾何形式(array manifold)，適應性天線陣列技術(adaptive antenna array techniques)傳統上常被用來壓抑CCI。很不幸地，在典型無線通訊系統中，由於多路徑反射的緣故，接收端會收到許多份傳送訊號，讓陣列幾何形式難以定義。為了支援WiMAX與SC-FDMA系統（例如3GPP LTE）的上行傳輸，我們提議研究MIMO-OFDMA與MIMO-SC-FDMA盲目波束成形器(blind beamformer)於未知/無法測量的陣列幾何形式，以便抑制上行傳輸必然發生的CCI。 總結來說，我們提議依此時程在以下領域進行研究： 第一年： • 調查已知的上行傳輸通道等化技術，針對以下系統： o 循環字首正交分頻多工(CP OFDM) o 尾隨零正交分頻多工(TZ OFDM) o 循環字首單載波(CP SC) o 尾隨零單載波(TZ SC) • 與子計畫3共同合作提供不足防衛區間(insufficient guard interval)上行傳輸的分析與建議。 • 調查已知的上行傳輸共同通道干擾抑制演算法(CCI suppression algorithms)。 • 建構可以衡量這些演算法的模擬平台。 第二年： • 設計針對不足尾隨零多輸出入正交分頻多工系統(reduced-TZ MIMO-OFDM)上行傳輸的演算法。 • 設計針對不足尾隨零多輸出入單載波系統(reduced-TZ MIMO-SC)上行傳輸的演算法。 第三年： • 設計多輸出入正交分頻多重進接(MIMO-OFDMA)系統上行傳輸的盲目空間與時間-空間波束成形器(blind spatial and spatial-temporal beamformer)演算法來抑制CCI。 • 設計多輸出入單載波分頻多重進接(MIMO-SC-FDMA)系統上行傳輸的盲目空間與時間-空間波束成形器演算法來抑制CCI。 • 系統整合，評估與展示。

link reliability without any impact on spectral efficiency. It has been proven that the theoretical gain in multiple-input multiple-output (MIMO) capacity grows linearly with the number of transmit and receive antenna. This is true when spatial diversity is fully exploited such that independent parallel channels between the transmitter and receiver can be formed. When used in a multiuser environment, higher spectrum efficiency can be achieved by further exploiting multiuser diversity. Techniques such as orthogonal frequency division multiple access (OFDMA) are used in conjunction with MIMO to flatten or diagonalize frequency-selective channels, where users are assigned different frequency bands. This is done such that the equalization process can be simplified. However, this requires transmission of a long guard interval in the form of cyclic prefix, which wastes valuable bandwidth. In systems such as IEEE 802.16e, a long cyclic prefix is used in the uplink to guard against ISI/IBI and ICI caused by the channel delay spread and asynchronicity between different users. However, in certain harsh propagation environments such as cell edges of clustered urban environments, the channel delay spread can be longer than the maximum length of the cyclic prefix, thus inducing ISI and ICI. Moreover, when the propagation delay differences among different users are significant, ICI will exist in the received signal. In order to sustain high spectral efficiency, a large FFT size can be used. However, this increases the complexity of the transmitter and reduces the intercarrier spacing of the subcarriers which subsequently makes the system more susceptible to frequency offset and oscillator phase noise. Also a higher number of subcarriers will increase the peak-to-average-power ratio (PAPR), demanding the use of highly linear and consequently inefficient power amplifiers. To mitigate this impairment, single carrier – frequency division multiple access (SC-FDMA) has recently been proposed for uplink transmission for 3GPP LTE. This results in a mixed OFDM-single carrier (SC) solution, where the mobile terminal uses SC-FDMA for uplink transmission while the basestation utilizes OFDMA in the downlink. To further reduce power consumption at the transmitter, trailing-zero (TZ) can be used to in place of cyclic prefix. We like to investigate equalization techniques for the scenario where the length of the guard interval is not sufficient thereby inducing IBI at the receiver. Specifically, we like to investigate the design of novel equalization algorithms for uplink transmission for reduced-TZ MIMO-OFDM and reduced-TZ MIMO-SC system.Currently, WiMAX 802.16e standard has proposed to use OFDMA for multiple access for both uplink and downlink. As alluded earlier, it is envisioned that OFDM and SC will coexist in other systems such that downlink transmission will utilize OFDMA while uplink transmission will utilize SC-FDMA, so that the transmitter can leverage the many advantages inherent in block based SC system. However, in a multiuser scenario, co-channel interference (CCI) can greatly deteriorate system performance. Sources of the CCI can be from both intra- and intercell users. In the case of OFDMA, intracell interference can occur when synchronization error in time and/or frequency causes a loss of orthogonality between the subcarriers. Adaptive antenna array techniques have traditionally been used to suppress CCI given that the direction-of-arrival of the signal of interest is estimated correctly and the array manifold is well known. Unfortunately, in typical wireless communication systems, the receiver receives multiple copies of the transmitted signal, thus making the array manifold to be poorly defined. In order to support uplink transmission for WiMAX system and SC-FDMA based system such as 3GPP LTE, we propose to investigate MIMO-OFDMA and MIMO-SC-FDMA blind beamformer for uncalibrated/unknown array manifold in order to suppress CCI in the uplink transmission. In summary, we propose to make contributions in the following areas: 1st year: • Survey existing channel equalization techniques for uplink transmission for o CP OFDM system. o TZ OFDM system. o CP SC system. o TZ SC system. • Work with subproject 3 to provide analysis and recommendations for uplink transmission when the length of the guard interval is insufficient. • Survey existing co-channel interference suppression algorithms for uplink transmission scheme. • Construction of simulation platform for evaluation of these algorithms. 2nd year: • Design algorithm for reduced-TZ MIMO-OFDM for uplink transmission. • Design algorithm for reduced-TZ MIMO-SC system for uplink transmission. 3rd year: • Design algorithm for blind spatial and spatial-temporal beamformer for MIMO-OFDMA system to suppress co-channel interference cancellation for uplink transmission. • Design algorithm for blind spatial and spatial-temporal beamformer for MIMO-SC-FDMA system to suppress co-channel interference cancellation for uplink transmission. • System integration, evaluation and demo.