Advantages of Massive MIMO | disadvantages of Massive MIMO
This page covers advantages and disadvantages of Massive MIMO over MIMO. It mentions benefits or advantages of Massive MIMO and drawbacks or disadvantages of Massive MIMO.
What is Massive MIMO?
The Massive MIMO is composed of two terms viz. Massive and MIMO. Massive refers to large number of antennas used in BS (Base Station) antenna array. MIMO refers to multiple spatially separated mobile users which are served by antenna array in same time/frequency resource. Massive MIMO is describes as beamforming with large number of antennas.
In traditional MIMO, spatially separated antennas (in less numbers) are used for transmission/reception of EM waves. It uses two techniques (STBC-Space Time Block Coding and SM-Spatial Multiplexing) to transmit data symbols using antennas.
The figure depicts Baseband and RF modules used in massive MIMO along with antenna array elements. Each RF Transceiver is mapped to two antenna elements in massive MIMO. One antenna element uses vertical polarization and the other uses horizontal polarization. The massive MIMO is also called as mMIMO.
As mentioned, massive MIMO combines radio transceivers or remote radio heads with multiple antenna elements (16/32/64/96) in a single active antenna unit. This increases performance of the system.
Massive MIMO is implemented in two topologies i.e. FDD and TDD. In FDD, different frequencies are used on downlink (DL) and uplink (UL). In TDD, same frequency is used on downlink and uplink. TDD benefits due to channel reciprocity as channel estimation using UL (Uplink) can be utilized for downlink beamforming and hence leads to less overhead.
Single User MIMO vs Multi User MIMO
The antenna arrays are used to serve mobile users in two different ways viz. SU-MIMO (Single User MIMO) and MU-MIMO (Multi User MIMO). The figure is based on wifi6 as per IEEE 802.11ax. The same concept is being used in 4G/LTE and 5G NR (New Radio) technologies.
In SU-MIMO, all the streams of antenna arrays are focused to single user. In MU-MIMO, different streams are focused to different users. Moreover each of these steams can serve more than one mobile subscribers or users with the help of TDD. Massive MIMO uses MU-MIMO technique to increase the spectral efficiency.
Massive MIMO Antenna Array
As shown, in conventional antenna array each column uses 8 to 12 antenna elements to develop vertically fixed directional pattern.
In 64T64R massive MIMO,, there are 64 RF transceivers, each mapped to 2 antenna elements. All 128 antenna elements create antenna pattern enabling both horizontal and vertical beamforming.
Benefits or advantages of Massive MIMO over MIMO
Following are the benefits or advantages of Massive MIMO:
➨It offers higher spectral efficiency and consecutively capacity to support number of users per 5G cell tower or antenna array.
➨It increases average sector throughput (6 times), improves cell edge throughput (3 times) and increases peak sector throughput (8 times).
➨It offers better coverage.
➨It uses beamforming towards desired user of interest and hence reduces interference with its surrounding users. This improves overall performance.
➨It offers higher energy efficiency due to concentration of radiated energy on mobile phone users or UEs.
Drawbacks or disadvantages of Massive MIMO
Following are the drawbacks or disadvantages of Massive MIMO:
➨Massive MIMO units are several times expensive compare to traditional radio units.
➨Massive MIMO antenna designs are more complex and requires more effort and time during assembly line compare to traditional antenna designs.
➨Use of FDD in massive MIMO leads to feedback overhead. This overhead increases with increase in antenna elements. Hence TDD is used due to its channel reciprocity concept.
➨Spacing between antennas is very less and hence requires packing of entire hardware into smaller space. For example, 32T32R requires 64 RF paths with spacing between antennas is approx. 4.2 cm at given frequency of about 3.5 GHz.
➨Due to more RF paths and antennas lot of power is dissipate. This requires inevitable temperature requirements.
➨Massive MIMO requires complex signal processing algorithms at the receiver to address inter user interference.
Conclusion: In order to understand benefits or advantages of Massive MIMO and limitations or challenges of Massive MIMO, it is required to understand comparison between traditional MIMO and mMIMO as well as terminologies of mMIMO.
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5G basic tutorial
5G Frequency Bands
5G millimeter wave tutorial
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