Why FR1 over FR2?
As we know that more availability of spectrum data means more network capacity, which leads to faster data rates and better user experiences. With 3GPP release 15 and 5G NR technology, it is going to provide channel bandwidths of up to 100MHz in sub 6 GHz spectrum and up to 400 MHz in mmWave spectrum. The term mmWave refers to a specific part of the radio frequency spectrum between 24GHz and 100GHz. This mmWave spectrum has very short wavelength. Also lower frequency spectrum band cover much greater distances but offer slower data rates, while high frequency bands cover much smaller areas but can carry much more data.
Now here the question arises which band will be used during 5G deployment? Or why initially, FR1 that is sub 6GHz is relevant as compared to FR2?
Very high frequency 5G signals don’t travel very far as well the transition of this FR2 signal is very poor from indoors to outdoors. Although massive MIMO and beam forming ensure that strict line of sight is not a requirement to make use of millimeter wave. A mmWave signal may not be able to penetrate buildings, but it will bounce around them to ensure a decent signal. At the same time indoors, customers will just have to rely more on sub 6GHz and LTE signals. Also mmWave signal strength will degrade somewhat when it rains, which will first result in slightly slower speeds and then potentially connection problems. This degradation will depend on just how hard it’s raining, and other factors like the distance from the cell tower. While connecting at the edge of a mmWave base station range, rain will cause the problems most.
So mmWave is just a small part of bigger 5G spectrum, the Wi-Fi like sub-6GHz and low band spectrum should be there to provide coverage when high frequency signals can’t reach you, providing a backbone that still offers fast data speeds.
On the other hand, sub-6GHz bands are extremely effective in providing coverage and have capacity for a wide range of 5G use cases. That means faster, more uniform data rates both indoors and outdoors for more customers, simultaneously. While mmWave is best in dense urban area and crowded indoor environments, sub-6GHz tackles the part of the spectrum that makes widespread 5G coverage possible.
Another reason to use sub-6GHz over mmWave is that, this band can travel farther and penetrate solid objects like buildings better than a higher frequency spectrum such as mmWave. In fact even a users hand has been shown to block the mmWave signals. In short, mmWave spectrum doesn’t offer the broad coverage that sub-6 GHz spectrum supports. The benefits of mmWave will likely be felt the most in locations where limited coverage isn’t a major concern like, pockets of densely populated urban areas or in crowded indoor locations like sporting events, concerts or airports.
Following are the basic difference in massive MIMO in sub-6GHz and massive MIMO in mmWave:
- In sub-6GHz, beam forming gain gives power savings and better coverage than legacy networks whereas mmWave is not fit for low data rate applications, which will incur significant power overhead.
- In case of URLLC channel hardening improves reliability over legacy networks in sub-6GHz where as in mmWave URLLC is difficult since propagation is unreliable due to blockage.
- Mobility support is great in case of sub-6GHz where as it is very challenging, but theoretically possible in mmWave.
- Spatial multiplexing of tens of UEs is feasible and has been demonstrated in field trials using sub-6GHz but user density is limited if hybrid implementation is used in mmWave.
- Outdoor to indoor communication has high data rate and is reliable in sub-6GHz whereas it is limited due to higher propagation loss in mmWave.
Thus we can conclude that while High frequency band in the mmWave range will provide extreme capacity, the roll out of 5G in sub-6 GHz low band will characterize early commercial phases, which will provide a capacity boost and wide coverage to support most use cases. The below table shows the basic difference between sub-6GHz and mmWave.
|MIMO order||Upto 8×8||Less MIMO order (2×2)|
|Deployment scenario||Macro cell, high user mobility||Small cell, low user mobility|
|Number of simultaneous users||Tens of user, larger coverage area||Few user, smaller coverage area|
|Main benefit||Spatial multiplexing,|
‘null-forming’ for reduced interference
|Beam steering for single user|
|Channel characteristics||Rich multipath propagation||A few propagation path|
|Subcarrier spacing||15,30,60 KHz||60, 120,240 KHz|
|Frequency range||450 Mhz – 6GHz||24 GHz – 52GHz|
|Spectral efficiency||High, due to spatial multiplexing||Lower spectral efficiency (due to high path loss)|