MIMO stands for Multiple Input Multiple Output Technology that has worked as the base of Massive MIMO Technology. Massive MIMO is going to be a key enabler in bringing the concept of 5G to reality. MIMO uses multiple antennas for receiving and transmission of the data and in Massive MIMO which is an extension to MIMO a large array of highly directive antennas is used for receiving and transmitting the data.
Massive MIMO provides spatial multiplexing and/or diversity gains. Hence, they are capable of dramatically increasing the spectral efficiency (SE) without extra frequency, power, or time resources. From the perspective of the number of user equipment (UE) supported on the same time-frequency resources.
That makes it a suitable technology for the implementation of 5G which deals with the high frequency and a very large number of users (according to claims and trail run 1 million users per square kilometer). But, a major drawback of current Massive MIMO that it suffers from the near-far problem. Near far the problem is a problem that wireless communication is facing since like the beginning of time. So, what is this near-far problem, it is simply the problem of data rate or difference of the data rate that is experienced by the users. The user which is near the antenna enjoys a higher data rate than the user which is far away. This is due to the reason that farther the single travel more it loses its’ power. This is one of the major disadvantages of traditional cellular design.
Fig 1. Above fig 1. is representing a graph showing that rate difference due to the near-far problem of cellular network design.
The solution to the above problem:
Cell-Free MIMO design is the solution to the above problem.
Cell-Free Massive MIMO is a system where a massive number access points distributed over a large area coherently serve a massive number of user terminals in the same time/frequency band or as is already mention that Cell-Free Massive MIMO is an extension to the current Massive MIMO, it can be defined as system having a large number of individually controllable antennas distributed over a wide area for simultaneously serving a small number of user equipment(UE). In Cell-Free MIMO rather than having a base station (BS) with a large number of antennas to a block or a particular area. We have a distributed set up in which antennas are not fixed over a base station but distributed over the area as individual units connected to a central processing unit (CPU) as shown in fig 2.
Fig 2. Cell-Free Massive MIMO not only solves the near-far problem of the network but also keep the high directivity of the Cellular structure.
Fig 3. Data coverage. Left: cellular network. Right: cell-free Massive MIMO network. SE achieved by UEs at different locations in an area covered by nine APs that are deployed on a regular grid. Note that 8 bit/s/Hz was selected as the maximal SE, which corresponds to uncoded 256-QAM.
It is clear from Fig 3. that Cell-Free MIMO provides a better data rate and coverage than the traditional design MIMO. But this is all about the theoretical implementation there are certain practical hurdles while with the practical implementation of Cell-Free MIMO. Which are stated below:
- There is a requirement of lots of wiring as each antenna is needed to be hardwired to the CPU for proper synchronizing and functioning which is indeed a complicated process also limits the size of the network.
- Another one is that is the complexity of the computation and controlling the network for this type of set up to function properly it is necessary that is signal produce/transmitted by each and every antenna in a given area is properly synchronized with each other so that. They re-enforce each other signals rather than disturbing each other as noise. For this, a high-power Central Processor has required which future hinders its’ practical feasibility.
Image Source & Reference:
Fig 1. & 3
Author: Nomaan Ahamed