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M ASSIVE MIMO FOR N EXT G ENERATION W IRELESS S YSTEMS Erik G.Larsson,ISY ,Linköping University,Sweden Fredrik Tufvesson,Lund University,Sweden Ove Edfors,Lund University,Sweden Thomas L.Marzetta,Bell Labs,Alcatel-Lucent,USA April 25,2013Abstract Multi-ur MIMO offers big advantages over conventional point-to-point MIMO:it works with cheap single-antenna terminals,a rich scattering environment is not required,and resource allocation is simplified becau every active terminal utilizes all of the time-frequency bins.However,multi-ur MIMO,as originally envisioned with roughly equal numbers of rvice-antennas and terminals and FDD operation,is not a scalable technology.Massive MIMO (also known as “Large-Scale Antenna Systems”,“Very Large MIMO”,“Hyper MIMO”)makes a clean break with current practice through the u of a large excess of rvice-antennas over active terminals and TDD operation.Extra antennas help by fo-cusing energy into ever-smaller regions of space to bring huge improvements in throughp
ut and radiated energy efficiency.Other benefits of massive MIMO include the extensive u of inexpensive low-power components,reduced latency,simplification of the MAC layer,and robustness to intentional jamming.The anticipated throughputs depend on the propaga-tion environment providing asymptotically orthogonal channels to the terminals,but so far experiments have not disclod any limitations in this regard.While massive MIMO ren-ders many traditional rearch problems irrelevant,it uncovers entirely new problems that urgently need attention:the challenge of making many low-cost low-precision components that work effectively together,acquisition and synchronization for newly-joined terminals,the exploitation of extra degrees of freedom provided by the excess of rvice-antennas,reducing internal power consumption to achieve total energy efficiency reductions,and finding new deployment scenarios.
1Background:Multi-Ur MIMO Maturing
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MIMO technology
•incread data rate,becau the more antennas,the more independent data streams can be nt out and the more terminals can be rved simultaneously;
•enhanced reliability,becau the more antennas the more distinct paths that the radio signal can pro
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•improved energy efficiency,becau the ba station can focus its emitted energy into the spatial directions where it knows that the terminals are located;and
•reduced interference becau the ba station can purpoly avoid transmitting into direc-tions where spreading interference would be harmful.
All improvements cannot be achieved simultaneously,and there are requirements on the propa-gation conditions,but the four above bullets are the general benefits.MU-MIMO technology for wireless communications in its conventional form is maturing,and incorporated into recent and evolving wireless broadband standards like4G LTE and LTE-Advanced(LTE-A).The more antennas the ba station(or terminals)are equipped with,the better performance in all the above four respects.However,the number of antennas ud today is modest.The most modern standard,LTE-Advanced,allows for up to8antenna ports at the ba station and equipment being built today has much fewer antennas than that.
2Going Large:Massive MIMO
Massive MIMO
Figure1:Some possible antenna configurations and deployment scenarios for a massive MIMO ba station.
feed them back to the ba station.This will not be feasible in massive MIMO systems,at least not when operating in a high-mobility environment,for two reasons.First,optimal downlink pilots should be mutually orthogonal between the antennas.This means that the amount of time-frequency resourc
es needed for downlink pilots scales as the number of antennas,so a massive MIMO system would require up to a hundred times more such resources than a conventional system.Second,the number of channel respons that each terminal must estimate is also proportional to the number of ba station antennas.Hence,the uplink resources needed to inform the ba station about the channel respons would be up to a hundred times larger than in conventional systems.The solution is to operate in time-division duplex(TDD)mode,and rely on reciprocity between the uplink and downlink channels.
While the concepts of massive MIMO have been mostly theoretical so far,and in particular stimulated much rearch in random matrix theory and related mathematics,basic testbeds are becoming available[2]and initial channel measurements have been performed[3,4].
3The Potential of Massive MIMO
Massive MIMO technology relies on pha-coherent but computationally very simple process-ing of signals from all the antennas at the ba station.Some specific benefits of a massive MU-MIMO system are:
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•Massive MIMO can increa the capacity10times or more and simultaneously,improve the radiated
energy-efficiency in the order of100times.
The capacity increa results from the aggressive spatial multiplexing ud in massive MIMO.The fundamental principle that makes the dramatic increa in energy efficiency possible is that with large number of antennas,energy can be focud with extreme sharp-ness into small regions in space,e Fig.2.The underlying physics is coherent superpo-sition of wavefronts.By appropriately shaping the signals nt out by the antennas,the ba station can make sure that all wave fronts collectively emitted by all antennas add up constructively at the locations of the intended terminals,but destructively(randomly) almost everywhere el.Interference between terminals can be suppresd even further by ,zero-forcing(ZF).This,however,comes at the cost of more transmitted power,as illustrated in Fig.2.
More quantitatively,Fig.3depicts the fundamental tradeoff between the energy efficiency in terms of bits transmitted per Joule of energy spent,and spectral efficiency in terms of bits transmitted per unit of radio spectrum consumed.Thefigure illustrates the relation for the uplink,from the terminals to the ba station(the downlink performance is similar). Thefigure shows the tradeoff for three cas:
–a reference system with one single antenna rving a single terminal(purple),
–a system with100antennas rving a single terminal using conventional beamform-ing(green)
–a massive MIMO system with100antennas simultaneously rving multiple(about 40here)terminals(red,using maximum-ratio combining;and blue,using zero-forcing).
The attractiveness of maximum-ratio combining(MRC)compared with zero-forcing(ZF) is not only its computational simplicity—multiplication of the received signals by the conjugate channel respons,but also that it can be performed in a distributed fashion, independently at each antenna unit.The prediction in Fig.3is bad on an information-theoretic analysis that takes into account intracell interference,as well as the bandwidth and energy cost of using pilots to acquire channel state information in a high-mobility environment[5].With the MRC receiver,we operate in the nearly noi-limited regime of information theory.This means providing each terminal with a rate of about1bit per complex dimension(1bps/Hz).In a massive MIMO system,when using MRC and when operating in the“green”regime,that is,scaling down the power as much as possible without riously affecting the overall spectral efficiency,multiur interference and ef-fects from hardware imperfections mostly drown in the thermal noi.The reason that the overall spectral efficiency still can be10times higher than in conventional MIMO is that many tens of terminals are rved simultaneously,in the same time-frequency resource. When operating in the1bit/dimension/terminal r
egime,there is also some evidence that
Figure 2:Relative field strength around a target terminal in a scattering environment of size 800λ×80
0λ,when the ba station is placed 1600λto the left.Average field strengths are calculated over 10000random placements of 400scatterers,when two different linear precoders are ud:a)matched-filtered (MRT)precoders and b)ZF precoders.Left:pudo-color plots of average field strengths,with target ur positions at the center (⋆),and four other urs nearby (◦).Right:average field strengths as surface plots,allowing an alternate view of the spatial focusing.win7怎么升级到win10系统
intersymbol interference can be treated as additional thermal noi [6],hence offering a way of disposing with OFDM as a means of combatting intersymbol interference.
•Massive MIMO can be built with inexpensive,low-power components.
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Massive MIMO is a game-changing technology both with regard to theory,systems and implementation.With massive MIMO,expensive,ultra-linear 50Watt amplifiers ud in conventional systems are replaced by hundreds of low-cost amplifiers with output power in the milli-Watt range.The contrast to classical array designs,which u few antennas fed from high-power amplifiers,is significant.Several expensive and bulky items,such as large coaxial cables,can be eliminated altogether.(The typical coaxial cables ud for tower-mounted ba stations today are more than four centimeters in diameter!)
