Introduction to Massive Mimo

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By/ Eng. Ahmed Nasser Ahmed
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•Massive MIMO Definition
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•Multiuser VS Single user
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•Massive MIMO Potential
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•TDD or FDD
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•Massive MIMO Limitation
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•Massive MIMO Research Problem
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•Massive MIMO channel
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•Massive MIMO Coding
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Massive MIMO (also known as “Large-Scale Antenna Systems”, “Very Large MIMO”, “Hyper MIMO”, “Full- Dimension MIMO” and “ARGOS”) [1]
is a new research field, where base stations are equipped with a very large number of antennas as compared to previously considered systems[2].
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SU-MIMO
the multiplexing gain may disappear when the signal power is low, relative to interference and noise, or in propagation
environments with dominating line-of-sight or insufficient scatters.
SU-MIMO systems also require complex and expensive multiple-antenna terminals.
Practical size limitations on terminals also limit the number of antennas that can be used and thereby the achievable multiplexing gains. [3]
MU-MIMO
It is shown that all the effects of uncorrelated noise and fast fading disappear, as does the intra- cell interference.
The only remaining impediment is the inter-cell interference due to pilot contamination [3]
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1.Massive MIMO can increase the capacity 10 times or more:
The capacity increase results from the aggressive spatial multiplexing used in massive MIMO. [1]
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2) Massive MIMO increases data rate[1]:
because the more antennas, the more independent data streams can be sent out and the more terminals can be served simultaneously.
Each terminal can be given the whole bandwidth
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3) Massive MIMO can be built with inexpensive, low- power components [1]:
With massive MIMO, expensive, ultra-linear 50 Watt amplifiers used in conventional systems are replaced by hundreds of low-cost amplifiers with output power in the milli-Watt range
Massive MIMO reduces the constraints on accuracy and linearity of each individual amplifier and RF chain
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4) Improved energy efficiency[1]:
because the base station can focus its emitted energy into the spatial directions where it knows that the terminals are located
5) Reduce interference:
because the base station can purposely avoid transmitting into directions where spreading interference would be harmful.
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6) Massive MIMO enables a significant reduction of latency on the air interface[1]:
Massive MIMO relies on the law of large numbers and beamforming in order to avoid fading dips, so that fading no longer limits latency.
7) Massive MIMO simplifies the multiple-access layer[1]:
the channel hardens so that frequency-domain, scheduling no longer pays off.
Each terminal can be given the whole bandwidth, which renders most of the physical-layer control signaling redundant.
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8) Massive MIMO increases the robustness tointentional jamming[1]:
Massive MIMO offers many excess degrees of freedom that can be used to cancel signals from intentional jammers.
If massive MIMO is implemented by using uplink pilots for channel estimation, then smart jammers could cause harmful interference with modest transmission power. However, more clever implementations using joint channel estimation and decoding should be able to substantially diminish that problem.
9) These degrees of freedom can be used for hardware- friendly signal shaping[1]:
A massive MIMO system has a large surplus of degrees of freedom. For example, with 200 antennas serving 20 terminals, 180 degrees of freedom are unused.
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We need a pilot signal for Channel state information (CSI) estimation , but there are two problems :
First, optimal downlink pilots should be mutually orthogonal between the antennas. This means that the amount of time frequency resources 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 responses that each terminal must estimate is also proportional to the number of base station antennas. Hence, the uplink resources needed to inform the base station about the channel responses would be up to a hundred times larger than in conventional systems [1] [4].
The solution is to operate in TDD mode, and rely on reciprocity between the uplink and downlink channels [1]
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Main goals:
Decrease of the multiuser interference.
Increase in the achievable sum-rates.
Types:
Linear precoding.
Block diagonalizationprecoding.
Tomlinson-Harashimaprecoding.
Vector perturbation precoding.
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Main goal:
Is to separate the data streams of the users at the receiver
Types:-
Optimal Maximum Likelihood detector
Linear detectors
Decision feedback detectors
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1) Channel Reciprocity
the hardware chains in the base station and terminal transceivers may not be reciprocal between the uplink and the downlink [1]
2) Pilot Contamination
Pilot contamination is encountered only when analyzing a multi-cell MIMO system with training, and is lost when narrowing focus to a single-cell setting. Pilot contamination occurs when the channel estimate at the base station in one cell ! The authors were supported in part by the DoDand the ARO Young Investigator Program (YIP). becomes polluted by users from other cells. The use of non orthogonaltraining sequences causes this contamination [4]
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3) Non-CSI@TX operation
-Before a link has been established with a terminal, the base station has no way of knowing the channel response to the terminal. This means that no array beamforming gain can be harnessed. In this case, probably some form of space-time block coding is optimal.
-Once the terminal has been contacted and sent a pilot, the base station can learn the channel response and operate in coherent MU-MIMO beamforming mode, reaping the power gains offered by having a very large array [1].
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4) Distance between users and base station [2]:
closely located with line of-sight (LOS) conditions:it is implied that the spatial separation of the users is particularly difficult, The LOS condition may cause high correlation between the channels to different users.
co-located with NLOS:we still have the users closely spaced but with NLOS to the base station antenna arrays. The NLOS condition with rich scattering should improve the situation by providing more “favorable” propagation and thus allowing better spatial separation of the users
well separated with LOS: the increased separation of users should help to improve the performance, the spatial fingerprints of the four users are significantly different, which indicates a favorable user decorrelationsituation for the large arrays [2].
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1) Fast and distributed, coherent signal processing:
-Massive MIMO arrays generate vast amounts of baseband data that need be processed in real time
2) The challenge of low-cost hardware:
3) Internal power consumption the total power consumed must be considered:
-that includes the cost of baseband signal processing. Much research must be invested into highly parallel
4) Cost of reciprocity calibration:
5) Pilot contamination:
-Design of innovative training schemes
-Design of precodersand resource allocation algorithms that can deal with pilot contamination.
6)Iterative detection and decoding
-Reducing the number of iterations
-Improving the exchange of soft information.
7) used multiple antenna at receivers
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[1] Larsson, Erik G., et al. "Massive MIMO for next generation wireless systems."arXivpreprint arXiv:1304.6690(2013).
[2] Gao, Xiang, et al. "Massive MIMO in real propagation environments."arXivpreprint arXiv:1403.3376(2014).
[3] Gao, Xiang, et al. "Linear pre-coding performance in measured very-large MIMO channels."Vehicular Technology Conference (VTC Fall), 2011 IEEE. IEEE, 2011.
[4] Jose, Jubin, et al. "Pilot contamination problem in multi- cell TDD systems."InformationTheory, 2009. ISIT 2009. IEEE International Symposium on. IEEE, 2009.
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Thank You
Contact me:
Web site:
www.ahmed_nasser_eng.staff.scuegypt.edu.eg
Email:
ahmed.nasserahmed@gmail.com
Ahmed.nasser@eng.suez.edu.eg
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Introduction to Massive Mimo

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