寬頻合作式無線多輸出入通訊系統---子計畫四：合作式多輸入輸出上行傳輸之干擾消除技術研究(I)

Abstract

下一世代的無線通訊系統，例如IEEE 802.16m或3GPP Long Term Evolution (3GPP LTE)，必須提供給高速移動終端用戶更高的頻譜效率以及連結可靠度。以IEEE 802.16m系統來說，基地台(Base Station, BS)與終端用戶的連結必須能在120-350 km/hr的高速維持15.0 bps/Hz的頻譜效率。然而，太長的通道延遲效應(channel delay spread)、快速衰弱(fast fading)通道與巢內外(intra/inter-cell)干擾會嚴重降低系統性能表現。近來有兩種新興的傳輸技術可以解決上述問題：網路多輸出入(Network MIMO)與協力中繼(cooperative relay)。可以想見的是，結合這些現代科技將能夠大幅提昇表現增益。即使終端用戶在巢邊(cell edge)或是只具備單一天線，也能提供可靠、無干擾與高速的服務。我們將稱這種技術為合作式多輸出入(Cooperative MIMO)，並且探討與Cooperative MIMO系統等化以及干擾抑制相關的議題。 MIMO系統的分析設計中，吾人常假設MIMO通道具有平坦衰弱(flat fading)性質，這種性質在多載波調變使用下成立。例如正交分頻多工(OFDM)可以將頻率選擇(frequency selective)通道轉化成多個平坦衰弱通道。然而，此類型技術需要傳輸端注入循環週期字首(cyclic prefix, CP)當作防衛區間(guard interval, GI)，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)與載波間干擾(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、減少載波間隔、讓系統更容易受頻率偏差與震盪器雜訊影響。而且，較高數目的子載波(subcarrier)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(Single Carrier, SC)調變已經用於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。我們提議研究在GI不足情況下的等化技術以增進頻譜效率與解決不同用戶間的同步問題，因為此時接收機會有ISI, IBI以及ICI(針對OFDM系統)。明確而具體來說，我們想將目前成果，針對單輸出入(SISO)與單輸入多輸出(SIMO)OFDM系統GI不足的等化技術，延伸至MIMO-OFDM、SISO-SC、SIMO-SC與MIMO-SC系統，以達成在準靜止(quasi-static)與快速衰弱通道中的上行傳輸。更甚者，我們將開始研究利用空間-頻率區塊編碼(space-frequency block coding, SFBC)與分散式空間-時間-頻率區塊編碼(space-time-frequency block coding, STFBC)的等化技術來改進RS、BS之間的頻譜效率。 除了通道引起的干擾之外，共同通道干擾（co-channel interference, CCI）也是降低系統準確性的常見原因。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，造成子載波之間正交性的喪失。只要採用較小的傳輸區塊，此點可以輕易地解決。然而，要抑制不同cell間用戶不同步的干擾並不是這麼容易。已發表文獻中有提到，系統表現可以經由BS間相互合作傳輸而大幅提昇，此法被定名為網路多輸出入（Network MIMO）。缺點是，這種技術需要所有BS之間的高速連結。更甚者，消除干擾需要準確的通道狀態資訊(channel state information, CSI)。在不嚴重犧牲系統表現的前提之下，我們提議探討BS不需要完整CSI的干擾消除技術。我們將採用疊加訓練序列(superimposed training sequence)來設計空間與空間-時間合作式波束成形器消除OFDMA、SC-FDMA系統中CCI。 總歸來說，我們提議在下列幾方面做出貢獻： 第一年： • 針對reduced GI MIMO-OFDM系統上行傳輸設計演算法，此法必須具備在quasi-static與快速衰弱通道中抑制CCI能力。 • 針對reduced GI SISO-SC, SIMO-SC與MIMO-SC系統上行傳輸設計演算法，此法必須具備在quasi-static與快速衰弱通道中抑制CCI能力。 • 初步探討使用分散式SFBC與分散式STFBC來支持RS與BS間的上行通訊。 • 與子計畫三合作，針對GI不足情況的上行傳輸，提供分析與建議。 第二年： • 針對合作式MIMO-OFDMA系統上行傳輸設計CCI抑制演算法，以空間與空間-時間波束成形器的型態呈現。 • 針對合作式MIMO-SC-FDMA系統上行傳輸設計CCI抑制演算法，以空間與空間-時間波束成形器的型態呈現。 • 系統整合，評估與展示。

English Abstract Title: Interference Cancellation Techniques for Cooperative MIMO Uplink Transmission Principle Investigator: Carrson C. Fung Sponsor: National Communication Program Keywords: Uplink transmission, cooperative multiple-input multiple-output, equalizer, interference cancellation, OFDMA, SC-FDMA For next generation wireless communication systems, such as IEEE 802.16m or 3GPP Long Term Evolution (3GPP LTE), higher spectrum efficiency and link reliability need to be provided for highly mobile terminals. For example, for IEEE 802.16m, connection between the basestation and mobile terminal needs to be maintained at high vehicular speed of 120-350 km/hr with a target spectrum efficiency of 15.0 bps/Hz. Unfortunately, long channel delay spread, fast fading channel, intra- and intercell interference can dramatically degrade performance. Two new transmission schemes, Network MIMO and cooperative relay, have recently been proposed to circumvent these problems. It is envisioned that tremendous performance gain can be achieved by combining these complementary technologies in order to provide reliable, interference-free, high data rate service to mobile terminals, even if they are located near cell edges or are only equipped with a single antenna. We shall call this scheme Cooperative MIMO. In this work, we proposed to investigate issues pertaining to equalization and interference suppression in the context of Cooperative MIMO systems. Analysis and design of MIMO systems often assumed that the MIMO channel is flat-fading, which is valid when multicarrier modulation techniques such as OFDM are utilized to transform a frequency-selective fading channel into multiple flat fading channels. However, this requires transmission of a long guard interval, usually 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 modulation 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 can be used to in place of cyclic prefix. To improve spectral efficiency and deal with the problem of asynchronicity among different users, we like to investigate equalization techniques for the scenario where the length of the guard interval is not sufficient, thereby inducing ISI, IBI and ICI (for OFDM only) at the receiver. Specifically, we like to extend our current work on reduced guard interval equalization for SISO- and SIMO-OFDM systems to MIMO-OFDM, SISO-SC, SIMO-SC and MIMO-SC systems for the uplink under quasi-static and fast fading channels. Furthermore, we also to like to begin a study on equalization techniques using distributed space-frequency block coding (SFBC) and distributed space-time-frequency block coding (STFBC) in order to improve spectral efficiency between relays and basestations. Besides channel induced interference, it is well known that co-channel interference (CCI) can greatly deteriorate system performance. Sources of CCI can be from 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. This can easily be mitigated by using smaller block size. It is more difficult, however, to suppress intercell interference due to asynchronous nature of the interferers. It has been shown in literature that system performance can greatly be enhanced by coordinating transmission among basestations, which is known as Network MIMO. This, unfortunately, requires a high-speed backhaul to be connected among all cooperating basestations. Furthermore, accurate CSI is essential in canceling out the interferers. We proposed to investigate other interference cancellation techniques that do not require complete CSI to be shared amongst all the basestations without making much sacrifices in performance. We proposed to exploit superimposed training sequence to design spatial and spatial-temporal cooperative beamformers for OFDMA and SC-FDMA systems to suppress CCI. In summary, we propose to make contributions in the following areas: 1st year: • Design algorithm for reduced guard interval MIMO-OFDM system for uplink transmission with co-channel interference suppression ability for quasi-static and fast fading channels. • Design algorithm for reduced guard interval SISO-SC, SIMO-SC and MIMO-SC systems for uplink transmission with co-channel interference suppression ability for quasi-static and fast fading channels. • Initiate investigation on using distributed SFBC and distributed STFBC to support uplink communication between relay stations and basestations. • Work with subproject 3 to provide analysis and recommendations for uplink transmission when the length of the guard interval is insufficient. 2nd year: • Design algorithm for spatial and spatial-temporal cooperative beamformer for Cooperative MIMO-OFDMA system to suppress co-channel interference for uplink transmission. • Design algorithm for spatial and spatial-temporal cooperative beamformer for Cooperative MIMO-SC-FDMA system to suppress co-channel interference for uplink transmission. • System integration, evaluation and demo.