【導(dǎo)讀】隨著移動通信的快速發(fā)展，用戶數(shù)量與用戶需求的急劇攀升，使得移動通信成為了通信界最具有市場潛力的技術(shù)。而無線通信與有線通信比較時暴露出來的弱點(diǎn)，如移動信道環(huán)境惡劣、用戶使用突發(fā)性大等，使得提高無線通信的容量和質(zhì)量需要諸多突破點(diǎn)。所以短缺的頻譜資源刺激著人們向提高頻譜利用率的方向努力。自二十世紀(jì)九十年代MIMO技術(shù)提出以來，由于在提高信道容量方面取得的突出成果，逐漸被無線通信領(lǐng)域的多個標(biāo)準(zhǔn)所應(yīng)用，成為了在提供可靠無線傳輸鏈路方面一個重要突破。MIMO技術(shù)是由多天線分集技術(shù)和空間編碼技術(shù)相結(jié)合而產(chǎn)生的，它的一個很重要的特點(diǎn)就是：不再是對抗多徑效應(yīng)，而是巧妙地利用它。其后通過理論推導(dǎo)與軟件仿真對MIMO通信系統(tǒng)的信道容量進(jìn)行了研究，總結(jié)了各因素對信道容量的影響。

　　

【正文】 MOFrom Wikipedia, the free encyclopediaIn radio, multipleinput and multipleoutput, or MIMO (pronounced mymoh by some and memoh by others), is the use of multiple antennas at both the transmitter and receiver to improve munication performance. It is one of several forms of smart antenna technology.MIMO technology has attracted attention in wireless munications, because it offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the same total transmit power over the antennas to achieve an array gain that improves the spectral efficiency (more bits per second per hertz of bandwidth) and/or to achieve a diversity gain that improves the link reliability (reduced fading). Because of these properties, MIMO is an important part of modern wireless munication standards such as IEEE (WiFi), 4G, 3GPP Long Term Evolution, WiMAX and HSPA+.History of MIMOFirst conceptsThe earliest ideas in this field go back to work by AR Kaye and DA George (1970), Branderburg and Wyner (1974)[1] and W. van Etten (1975, 1976). Jack Winters and Jack Winters at Bell Laboratories published several papers on beamforming related applications in 1984 and 1986.[2]PrincipleThe multiuser MIMO concept of spacedivision multiple access (SDMA) was proposed by Richard Roy and Bj246。rn Ottersten, researchers at ArrayComm, in 1991. Their US patent (No. 5515378 issued in 1996[3]) emphasizes an array of receiving antennas at the base station and plurality of remote users.Arogyaswami Paulraj and Thomas Kailath proposed the concept of spatial multiplexing (SM) using MIMO in 1993. Their US patent (No. 5,345,599 issued in 1994[4]) emphasized wireless broadcast munications applications and splitting a highrate signal into several lowrate signals.In 1996, Greg Raleigh, Gerard J. Foschini, and Emre Telatar refined new approaches to MIMO technology, considering a configuration where multiple transmit antennas are colocated at one transmitter to improve the link throughput effectively.[5][6][7]Bell Labs was the first to demonstrate a laboratory prototype of spatial multiplexing in 1998, where spatial multiplexing is a principal technology to improve the performance of MIMO munication systems.[8]Wireless standardsIn the mercial area, Iospan Wireless Inc. developed the first mercial system in 2001 that used MIMO with orthogonal frequencydivision multiple access technology (MIMOOFDMA). Iospan technology supported both diversity coding and spatial multiplexing. In 2005, Airgo Networks had developed an IEEE precursor implementation based on their patents on MIMO. Following that in 2006, several panies (including at least Broad, Intel, and Marvell) fielded a MIMOOFDM solution based on a prestandard for WiFi standard. Also in 2006, several panies (Beceem Communications, Samsung, Run Technologies, etc.) had developed MIMOOFDMA based solutions for IEEE WiMAX broadband mobile standard. All uping 4G systems will also employ MIMO technology. Several research groups have demonstrated over 1?Gbit/s prototypes.Functions of MIMOMIMO can be subdivided into three main categories, precoding, spatial multiplexing or SM, and diversity coding.Precoding is multistream beamforming, in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In (singlestream) beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase and gain weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect. In lineofsight propagation, beamforming results in a well defined directional pattern. However, conventional beams are not a good analogy in cellular networks, which are mainly characterized by multipath propagation. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding with multiple streams is often beneficial. Note that precoding requires knowledge of channel state information (CSI) at the transmitter and the receiver.Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures and the receiver has accurate CSI, it can separate these streams into (almost) parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signaltonoise ratios (SNR). The maximum number of spatial streams is limited by the lesser of the number of antennas at the transmitter or receiver. Spatial multiplexing can be used without CSI at the transmitter, but can be bined with precoding if CSI is available. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as spacedivision multiple access or multiuser MIMO, in which case CSI is required at the transmitter.[9] The scheduling of receivers with different spatial signatures allows good separability.Diversity Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods, a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called spacetime coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain