[MM S01 E02] Massive MIMO

In my previous article, I explained that 5G is addressing all communications requirements you can named. So in case your requirement is to download "Game of Thrones" all eight seasons in few minutes, then you really need supper download speed. Guess what , 5G can do it easily in eMBB slice (enhanced MBB). Current commercially developed 5G networks can easily cross 1 Gbps Download throughput. The Magic is done by Massive MIMO technique used in NR or 5G air interface.

What is Massive MIMO?

Simply , Missive MIMO = MIMO+Beam-forming

But there is a big confusion about massive MIMO when I discuss with most experts.

"Massive MIMO increased the throughput in down link of NR?"

Actually, Massive part of MIMO is not for achieving high peak throughput. Multi-layer MIMO and 100 Mhz bandwidth of NR increased the throughput. Massive part came into picture due to beam-forming and beam-forming improved the coverage by reducing interference (focused beam ) and energy efficiency of RF radiation signal.

Current NR implementations are still using four layer MIMO ( Same as later LTE Networks) or 4*4 MIMO. If we do a simple math we can easily figure-out how NR achieving over 1Gbps throughput.

As we know, LTE with 20 Mhz channel BW can achieve 300 Mbps in 4*4 MIMO mode (theoretical throughput). So 100 Mhz channel BW in NR can achieve five times of LTE theoretical throughput which is 1.5 Gbps. ( This is rough calculation for comparison purpose only, most accurate calculation can be done via below link

Please note that above simple throughput calculation is based on 64QAM which is the typical modulation supported by LTE. But NR supported 256QAM also. I have to mention here that Massive MIMO beam-forming is supporting to achieve require SNR for 256QAM due to directed beam.

Beam-forming - It is a concept of constructing a more directive signal towards user. So how to do that? There are two ways to create more directive signals:

1. Using parabolic shape reflectors behind the dipole - This method is used in Microwave links in point to point communication.
2. Using antenna array and transmit same signal in same phase or in different phases from each antenna (More antennas in the array will result more narrow beam) - This method is used in beam-forming in 5G.

Below is a demonstrated animation to explain beam-forming. In above, I mention about phase. here is the deal, in below animation, you can see same signal is transmitted, but not same time (look closely), this is called different phase signals. Now look at the arrow direction, you can see that signals are kind of collecting on that direction.

This is called constructive interference in theory which is creating a strong signal in that direction due to same phase signals are summing up to create a strong signal. So the main lobe of the antenna signal is in direction of the arrow showing in the animation. What if the signal is transmitted without any phase different from all antennas? Then main lobe will direct to the direction of dotted line in the animation. So it is very clear that by changing the phase of transmitted signal in a antenna array can achieve a fully directive beam to user direction. One more very important aspect is that , gain of main lobe will be N times of transmitted signal (N=Number of antennas in the array).

So from above animation, it is very clear how the main lobe can be directed to the user direction using phase. But how the main lobe become very narrow with more antennas in the array ? In Theory, it is because of Distracted Interference, means signals will be cancels each other when they are in opposite phase, so more antennas transmit signal will be result more signal cancellation with same phase, so finally very narrow beam will be resulted.

Note: Above animation is taken from commons.wikimedia.org

Below video is explaining beam-forming nicely. Thanks Dr.Emil for cool video.

Thanks,

Hassan QADI