MIMO技术的简介

TELE 9754 Coding and InformationTheoryResearch Workshop ReportAbstract—Mobile wireless communication has become one of the most important aspects of our daily life. The continuously increasing usage has imposed great pressure upon telecommunication system where the availability of channel capacity and spectral resources are limited. Multiple Input Multiple Output (MIMO) is considered as one of the possible solutions to the above problem and has attracted considerable attention among researchers and engineers in the field of mobile communication due to the great advantages it exhibits. In recent years, MIMO technology has been developed into more sophisticated forms and utilized in some common communication devices around us. This report is intended to provide readers with a brief review of the historical and technological developments of MIMO, and its applications.I. INTRODUCTIONOur wireless communication systems have undergone remarkable developments and progresses in the past 20 years, from 1G to 4G and the upcoming 5G. Such systems have provided our life with significant conveniences which were otherwise impossible and unachievable before the 1980s. However, under the condition of limited bandwidth resources and channel capacity, the developing communication scheme is unable to meet the fast growing demand from users of mobile devices. In other words, our communication system has somewhat attained its bottleneck and needs some new technology to enhance its performance. On the other hand, MIMO equipped with modern efficient signal processing techniques and processing hardware demonstrates prominent characteristics that could be taken to mitigate the above problems. MIMO can be defined, in simple terms, as a system which consists of multiple antennas at both the transmitter and receiver sides [6]. A systematic diagram of MIMO is illustrated by Figure 1.Figure 1. Systematic diagram of a MIMO systemThe underlying fact which enables MIMO to attract intense attention is that it could exploit the advantages of beamforming gain, spatial diversity and spatial multiplexing to enhance the performance of a communication system without extra consumption of spectral resources.The content of this report is organized in six separate sections. Section II offers readers a set of abbreviations used throughout the report. Section III illustrates the historical developments and milestones of MIMO from theory to implementations. Section IV introduces, in general sense, how MIMO functions and achieves the aforementioned advantages. Section V categorizes MIMO into various classes based on the properties it composes and some comparisons among them would be made. Section VI provides some examples of application of MIMO in modern communication scheme. Finally, a brief conclusion will be drawn in Section VI. Additional information can be found by referring to the Appendix section.II. TABLE OF ABBREVIATIONSThe following table (Table 1) lists a set of commonlyA BRIEF REVIEW ON MIMO TECHNOLOGY AND ITS APPLICATIONSLikai Ma z3326280used abbreviations to which will be referred in the following sections of this report. Table 1. Table of abbreviations III. HISTORICAL DEVELOPMENT OF MIMO [1] The history of MIMO can be dated back several decades ago. Although the idea of MIMO was not proposed until the 1970s, antenna arrays, also known as smart antennas (illustrated in Figure 2) had been developed to take the advantage of diversity and enhance wireless transmission and reception in analogue communications. CLASSIFICATION OF MIMO Figure 2. An example of antenna array. The idea of MIMO was first conceived in the 1970s in Bell Laboratory, which was inspired by the desire to overcome the problem of bandwidth limitation and interference in transmission cables. Such idea was too difficult to be realized and had remained in the form of theory for a long period of time, due to the limitation that the processing hardware and signal processing algorithms available at that stage was unable to support MIMO signal processing. Nevertheless, the theory of MIMO had continued to be enriched by some of the early researchers ’, including A.R Kaye, D.A George, Branderburg, Wyner and W. Van. Etten. In the late 1980s, MIMO theory had further been developed by Jack Salz and Jack Winters whose work centralizedaround the idea of beamforming.The concept of SM was proposed in 1993 by Arogyaswami Paulraj and Thomas. In 1996, Greg Raleigh and Gerard J. Foschini further developed the approaches towards MIMO using co-located antennas at the transmitter. Significant breakthrough in practical application of MIMO did not take place until the late 1990s. In 1998, SM was first demonstrated in the formFigure 2.Timeline of development of MIMO.of prototype in Bell Lab. Since then, the development of MIMO had been accelerated and some products with such technology integrated started to be available commercially. In 2002, Iospan Wireless Inc. launch the first commercial product with MIMO embedded, which was a milestone in the real application of this technology. Later, in 2005, the first standard of WLAN (IEEE 802.11n), also commonly known as Wi-Fi, with MIMO-OFDM was produced by Airgo Networks and has become more and more popular since then. The more detailed historical development of MIMO is depicted as a timeline and can be found in Figure 2.IV. HOW DOES MIMO WORKThe underlying principle of MIMO is that signals transmitted and received at both the transmitter and receiver sides combine together so that either parallel data sub-streams are formed or SNR is improved [3]. The benefits that MIMO exploits are known as beamforming, spatial diversity and spatial multiplexing.Figure 3.Smith chart showing the technique of beamformingBeamforming is achieved by focusing energy in some desired angular direction through appropriate choice of antenna parameters [1, 2]. The Smith chart in Figure 3 illustrates the idea of beamforming where the main lob is pointing at a particular angular direction while the side lobes are significantly suppressed. When the channel between the transmitter and receiver are located within the range of LOS, MIMO can be configured to exploit the advantages of beamforming so that the antenna gains combine constructively and thereby an enhanced receiving power and SNR are attained in the link.When multiple copies of a signal are transmitted from the transmitter, they may subject to non-idealities in the communication channel, for example fading, reflection and refraction, to different extents. Multiple replicas of the signal incoming from different directions can be analyzed by employing some sophisticated DSP algorithms to recover the original transmitted signal if those signals are highly uncorrelated. Such technique is referred as spatial diversity [2]. In general, the more the extent of uncorrelation, the better the effect of spatial diversity. MIMO could also take the advantages of spatial diversity to improve the quality of the received signal (ie, increased SNR) and hence to provide a more reliable communication link.Figure 4. The MIMO channel capacity increases almost linearly with the number of transmitting or receiving antennas [5]In a fading channel, particularly Rayleigh fading with CSI known to the receiver, MIMO could form a number of parallel and independent sub-channels through which a code word can be divided into a number of pieces and transmitted separately [4, 5]. In other words, a higher transmission rate (channel capacity) could be achieved. In theoretical sense, the channel capacity increases approximately with the number of transmitting or receiving antennas, as depicted in Figure 4. This discovery has a tremendous implicationuponcommunication system, that higher information exchange rate can be achieved without consuming extra bandwidth, by introducing additional antennas at the transmitter and receiver sides. The benefits exploited by MIMO are summarized in the following table (Table 2).Table 2. Summary table of MIMO techniquesIn general, beamforming, spatial diversity and spatial multiplexing are three rivaling techniques that engineers should make appropriate decisions on what could be sacrificed in order to gain more advantages from the others. The inter-relations among these techniques are depicted in Figure 5 [2].Figure 5. Inter-relations among three MIMO techniquesAlthough they are rivaling factors, they are not necessarily mutually exclusive, meaning that by making appropriate decisions on to what extent those are used, one can design a communication scheme which employs a combination of those techniques such that certain degrees of advantages of them can be involved. Such decision should be based solely upon the specific engineering problem to be solved. V. V ARIOUS TYPES OF MIMOA MIMO system can be divided into different classes according to some specific criterion. A MIMO system is commonly classified according to the criterions that whether multiple users are able to be served simultaneously. The classifications is shown as in Figure 6.Figure 6. Classification of MIMOIn the case of multiple users, a MIMO system is referred as SU-MIMO if only a single user among them is served at a time. In contrary, the term MU-MIMO is defined for the case where multiple users can be served in parallel. The following figure (Figure 7) depicts a comparison between SU-MIMO and MU-MIMO.Figure 7. Comparison between SU-MIMO and MU-MIMO [7](A) SU-MIMO SYSTEM [7, 8]In SU-MIMO, the time-frequency resources are allocated entirely to a single user in a given communication session. If SM is employed, multiple sub-streams can then be created to scale up the channel capacity by the order dictated by the minimum of transmitting or receiving antennas. Different users can be served through the use of TDMA or FDMA.One can see that since the order of increases in channel capacity in SU-MIMO is limited by the transmitter or receiver side which consists of the smallest number of antennas, the improvements in channel throughput may be very limited, particularly for cellular communication networks. In other words, the user end would likely be the constraint on the enhancements of channel capacity. The number of antennas that can be integrated to the users’ mobile devices, such as mobile phones, is very limited, mainly due to limitations like portability and space availability.(B)MU-MIMO SYSTEM [7, 8]MU-MIMO can be considered as an extension to the theory of SU-MIMO. In a MU-MIMO system, multiple users can be served in parallel with the same time-frequency resources available. By exploiting the advantages of SM, the channel throughput for MU-MIMO can then be enhanced by the number of transmit antennas with sufficient number of users, namely a similar scaling principle carried by the case of SU-MIMO.As oppose to SU-MIMO, MU-MIMO better exploits the multiplexing gain provided by SM, which is achieved by allocating different users to different sub-channels. Different users can not only be served by employing TDMA or FDMA (in SU-MIMO), but also by means of SDMA. Therefore, MU-MIMO has more advantages over SU-MIMO in terms of time, frequency and spatial allocations.VI. APPLICATION OF MIMO IN MODERNCOMMUNICATION SCHEME [3]As the developments in both powerful signal processing hardware and more sophisticated MIMO models have become available in recent years, the application of MIMO in our modern communication systems have been made possible as oppose to the past, mainly by the ITU and 3GPP.Some of the common communication systems, including the 3G/4G network, Wi-Fi (IEEE 802.11n) and WiMAX have already integrated some MIMO technologies to a certain extent where various forms of MIMO have been deployed and different advantages are exploited. The use of MIMO technology in modern communication systems can be depicted by the following figure (Figure 8).Figure 8.Application of MIMO in modern communication systems.The current CDMA2000 standard, one of the 3G standards (WCDMA, CDMA2000 and TD-SCDMA) has adopted transmit diversity, while the WCDMA-based UMTS has also enabled implementation of transmit diversity and beamforming at base stations. Furthermore, the 3GPP LTE employs SU-MIMO with SM and STC. The more advanced version, so called 3GPP LTE-Advanced further extends from what has been designed in LTE and has involved MU-MIMO and multi-cell MIMO.In IEEE802.16 standard (also commonly known as WiMAX), MIMO-OFDMA, a technique that utilizes OFDM modulation scheme in combination with multiple antennas, has been deployed.IEEE802.11n or Wi-Fi is another commonly used communication standard and has implemented several MIMO technologies to enhance its data through put, channel capacity and overall performance. The techniques employed by Wi-Fi are mainly antenna selection, STC and beam forming. The following table (Table 3) provides a summary for the different MIMO technologies used in those communication schemes and their performance (data rate).Table 3. Summary of MIMO technologies in modern communication systems and their overall performances.VII. CONCLUSIONIn conclusion, the historical developments, classification and current applications associated with MIMO technologies have been outlined and reviewed in this report. It can be seen that MIMO has a great deal of advantages over other traditional communication technologies. MIMO can also be used in conjunction with other existing techniques including digital modulation (OFDM in particular), coding (STC, DPC and etc) and multiple access (TDMA and FDMA) in order to derive more powerful and efficient communication schemes and provide users with better communication quality. Although there still exits some compelling problems regarding the wide application of MIMO, one can see that such technology will be more extensively integrated in our future generation wireless communication systems.REFERENCE[1] Raut, Pravin W., and S. L. Badjate. "MIMO-Future Wireless Communication."[2] Sibille, Alain, Claude Oestges, and Alberto Zanella. MIMO: from theory to implementation. Academic Press, 2010.[3] Clerckx, Bruno, and Claude Oestges. MIMO Wireless Networks: Channels, Techniques and Standards for Multi-antenna, Multi-user and Multi-cell Systems. Academic Press, 2013.[4] Holter, Bengt. "On the capacity of the MIMO channel: A tutorial introduction."Proc. IEEE Norwegian Symposium on Signal Processing. 2001.[5] Liang, Yang Wen. "Ergodic and Outage Capacity of Narrowband MIMO Gaussian Channels." Dept. of Electrical and computer Engg. University of British Columbia, V ancouver, British Columbia (2005).[6] Telatar, Emre. "Capacity of Multi‐antenna Gaussian Channels." European transactions on telecommunications 10.6 (1999): 585-595.[7] Bauch, Gerhard, and Guido Dietl. "Multi-user MIMO for achieving IMT-Advanced requirements." Telecommunications, 2008. ICT 2008. International Conference on. IEEE, 2008.[8] Li, Qinghua, et al. "MIMO techniques in WiMAX andLTE: a feature overview."Communications Magazine, IEEE 48.5 (2010): 86-92.。