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Massive MIMO in 5G

The wireless research community has embarked on a journey to build the technology of tomorrow which will deliver remarkable enhancement in network throughput and capacity, improvements in spectral efficiency, shortened end-to-end latency, heightened reliability and more. These enhancements are driven by the key performance requirements which are defined by the International Telecommunications Union (ITU). The enhancements in the performance for IMT-2020 (5G) over IMT-Advanced are momentous with a 20Ximprovement in peak data rate from 1 Gb/s to 20 Gb/s. Similarly, the user experienced data rate increases 10X from 10 Mb/s to 100 Mb/s and latency is reduced by a factor of 10 from 10 ms down to 1 ms.

Currently the 4G base stations consist of a dozen of ports for antennas which handle all cellular traffic: eight for transmitters and four for receivers. However, 5G base stations can support almost a hundred ports, meaning many more antennas can fit on a single array. This kind of capability means a base station can send and receive signals from many users at once thus increasing the capacity of mobile networks by a factor of 22 or greater. This is the massive MIMO (multiple-input multiple-output) technology or sometimes referred to as Large Scale MIMO. It results in a channel response that is quasi-orthogonal and has the potential to yield huge gains in spectral efficiency. This would enable devices to serve with the same time and frequency resources within a given cell. The figure below shows a comparison of a typical 4G MIMO cell to that of a 5G massive MIMO cell. These massive MIMO base stations would be able to serve far more devices as anticipated by the 5G IoT use case and others.


Figure 1.MIMO Generation

The key technological characteristics of Massive MIMO are:
1 Fully digital processing: superior performance and better energy efficiency.
2 The reliance on reciprocity of propagation and TDD operation: enables downlink channels to be estimated from uplink pilots and obviating the need for prior or structural knowledge of the propagation channel.
3 Computationally inexpensive pre-coding/decoding algorithms.
4 Array gain: in a closed-loop link budget enhancement proportional to the number of base station antennas.
5 Channel hardening: effectively removes the effects of fast fading.
6 The provision of uniformly good quality of service to all terminals in a cell
7 Autonomous operation of the base stations: with no sharing of payload data or channel state information with other cells, there is no requirements of accurate time synchronization.
8 The possibility to reduce accuracy and resolution of transceiver frontends and the digital processing and number representations in computations.


Figure 2.Massive MIMO exploitation of large antenna arrays into spatially multiplex many terminals.

Massive MIMO looks very promising in theory; however, it is yet to be tested in large-scale field trials to prove its viability for widespread commercial deployment. Installing such huge number of antennas to handle cellular traffic can also cause more interference if those signals cross. Thus, 5G stations must merge other new technologies, such as millimetre waves, small cells, full duplex, and beam forming.

With these 5G technologies, engineers hope to build the wireless network that future smart phone user, VR gamers, and autonomous cars will rely on every day. Researchers and companies have set great expectations for 5G by promising ultra-low latency and record breaking data speeds for consumers. If they can solve the remaining challenges, and figure out how to make all these systems work together, ultrafast 5G service could reach consumers in the next five years.

In India, Airtel announced the deployment of Massive (MIMO) on 26th September 2017,claiming it to be the first 5G-capable deployment in India. The company is starting the first round of deployment in Bengaluru and Kolkata and will then expand to other parts of the country soon, the company said in a press statement.

References:
https://spectrum.ieee.org/video/telecom/wireless/5g-bytes-massive-mimo-explained
http://www.ni.com/white-paper/52382/en/
https://5g.ieee.org/tech-focus/march-2017/massive-mimo-for-5g
http://gadgets.ndtv.com/telecom/news/airtel-5g-india-massive-mimo-deployment-4g-speeds-increase-bengaluru-kolkata-1755370

-Neela Sakhardande (2016-18)

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