Massive MIMO | M-MIMO basics | Massive MIMO advantages

This page covers M-MIMO (Massive MIMO) basics. It mentions Massive MIMO advantages and Massive MIMO (M-MIMO) disadvantages. M-MIMO operational aspects and applications are also covered.

The term MIMO stands for Multiple Input Multiple Output. It refers to multiple antennas used for transmission and reception of electro-magnetic waves. There are two techniques by which complex data symbols are transmitted using multiple antennas viz. STBC (Space Time Block Coding) and SM (Spatial Multiplexing).

In STBC, data symbols are transmitted at time instant t=t1 and later modified copies of same data symbols which are transmitted at t1 are re-transmitted at t=t2. Using this concept errors can be minimized to greater extent but at the expense of redundant data symbols. Redundant data symbols help in correcting errors and this eliminates the need of re-transmission. In SM, different data symbols are transmitted at all the time instants and no redundant copies are used for transmission. This increases data rates. SM relies on pilot subcarriers for channel estimation and equalization. Pilot subcarriers are embedded in between data symbols before transmission of symbols. There systems such as mobile wimax where in both STBC and SM are employed to have benefits of both coverage and capacity improvements.

In general channel estimation can either be done at BS or MS. In Massive MIMO, BS estimates channel response based on pilot subcarriers received from MSs (Mobile Subscribers).
Refer Channel Estimation and Equalization Matlab code.

Massive MIMO Antenna Array Configurations

In MIMO, there are two concepts based on how Base station antennas are used to serve Mobile subscribers or users of the cellular service or service provider network. They are classified as single user MIMO and multi-user MIMO.

In Single user MIMO, All the streams from Base station antennas are focused to one single user. In Multi user MIMO, different streams produced using combination of different antennas are focused to different users or subscribers. Moreover one stream can serve more than one users or subscribers. Massive MIMO uses multi user MIMO concept. This is depicted in the figure-1 above. As shown different antenna configurations such as spherical, cylindrical, distributed, linear, rectangular etc. are used in massive MIMO based wireless cellular systems.

In Massive MIMO concept, large number of antennas (usually 32 to 64) are used at base station simultaneously to serve tens of users or mobile subscribers (MSs) in the same time-frequency grid. M-MIMO has many benefits over conventional MIMO system. The difference between Massive MIMO and conventional MIMO can be understood by studying operation of both the systems along with their merits (advantages) and de-merits (disadvantages).

Massive MIMO can also be known by other names such as "Large Scale Antenna Systems", "Hyper MIMO", "Very Large MIMO", "ARGOS" and "Full Dimension MIMO".

Massive MIMO Advantages

Following are the Advantages of Massive MIMO (M-MIMO) system:
• High spectrum efficiency due to large multiplexing gain as well as antenna array gain.
• High energy efficiency due to concentration of radiated energy on MS/UE.
• High reliability due to large diversity gain.
• Weak inter-user interference and enhanced physical security due to orthogonal MS channels and extremely narrower beam.
• Simple scheduling scheme.
• Robustness to individual element failure due to large number of antenna array elements.
• Massive MIMO can be developed using low power and in-expensive components.
• It enables large reduction in latency on air interface.

Massive MIMO Disadvantages

Following are the disadvantages of Massive MIMO (M-MIMO) system:
• Pilot contamination due to limited orthogonal pilot subcarriers as of bounded coherent interval and bandwidth.
• High signal processing complexity due to utilization of large number of antennas and multiplexing of UEs (or Mobile subscribers).
• Sensitive to beam alignment, as extremely narrower beam is used which is sensitive to movement of MS or swaying of antenna array.
• Channel reciprocity assumption is used in TDD mode of Massive MIMO.

Massive MIMO (M-MIMO) Applications

Following are the applications of Massive MIMO (M-MIMO):
• 4G LTE
• LTE Advanced
• Advanced WLAN versions e.g. 802.11ac, 802.11ad


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