5G - NSA vs SA Made Easy
India's biggest challenge is to unlock the full potential of 5G by facilitating the deployment of SA networks. Can it? Yes. Provided it frees up the 900 MHz band by driving BSNL's 4G in the 800 MHz.
I am sure the readers might have heard and read about these two 5G deployment modes. NSA is a 5G network in Non-Standalone Mode, and the SA on the other hand is a 5G network in Sandalone Mode. I plan to use this note as an opportunity to explain the concepts as simply as possible so that readers can understand the real challenge that we face in India from the point of view of deploying fully capable robust 5G networks. But before we dive into details, let’s first review the basics.
5G Non-Standalone (NSA)
In this mode, the foundation of the 5G network is the 4G underlay. All 5G devices need to first lock in the 4G network for them to be able to get access to 5G services. In order words, no access to 5G is possible without the underlay 4G. This is how the network looks pictorially.
Note, in the NSA mode, the 5G network is ONLY used for capacity, i.e. driving data speeds, and 4G is used to expand network coverage - especially indoors. Coverage enhancement is possible due to better propagation characteristics of low-frequency bands, which in this case is the 900 MHz band. Both 4G and 5G use the “common core network”, which is nothing but the legacy 4G core.
5G NSA Limitations
Handsets are incapable of dynamically selecting different slices simultaneously
The biggest limitation of the 5G NSA comes from the requirement for all its traffic to be processed by the common 4G core. Such a configuration prevents the applications hosted on handsets from dynamically selecting network slices individually. This means it is not possible for the handset to distinguish the slices at the application level. In order words, if the handset has to select a slice, then the traffic from all its applications will have no option but to get steered through the same (selected slice).
For example, let’s say a banking application requires a network slice such that its traffic is not routed into the internet, instead, the same is required to be steered directly to the bank’s server to ensure a high level of security. The 5G NSA mode simply can’t execute this requirement. If it did, then all the traffic emanating from the handset will get routed to the Bank’s Server - making the other apps useless.
Each layer of the 5G Band acts independently by itself for Handshaking
When the BTS transmits information using a specific band then it is the duty of the handset to respond back to let the BTS know that it has received the signal. This is called handshaking and it takes place at the physical layer. In the case of the NSA mode, the handset sends this acknowledgment using the same band in which it originally received the data.
For example, let’s say the transmission from the BTS has taken place in the 3.5 GHz band, then the BTS will be expecting a response back from the handset in the 3.5 GHz band itself. The handset cannot use any other band to send this acknowledgment signal.
Why is this even a problem? The reason - the coverage of the 3.5 GHz band is dependent on the RF link design of this acknowledgment signal. If the BTS receives no acknowledgment from the handset in the 3.5 GHz band (whose coverage is limited), then the lower spectrum bands (with better coverage), let’s say 1.8 GHz, can’t be used to send data back to the BTS. Doing so can improve the overall coverage of the 5G network. The following diagram explains, how the underlay 1800/2100 MHz band is being used to stretch the coverage of 3.5 GHz by a little.
Fortunately, the RF link budget of the 3.5 GHz Ack signal is designed such that it is just a shade better than the normal traffic signal in the same band - driving the overall coverage of the network beyond the capability of the 3.5 GHz band. In such a configuration the upload traffic from the handset can now use the lower 1.8/2.1 GHz bands to send traffic back to the BTS.
Indoor coverage is dependent on 4G
This is also a huge disadvantage of the NSA mode. Had the 5G spectrum at the low-frequency band been available in the first place there was no need for deploying the 5G network in this mode. Hence, for all practical purposes for indoor coverage one is dependent on 4G, as the high-frequency bands (> 1 GHz) aren’t that good at getting inside the buildings.
In other words, unless we have spectrum in the sub-GHz band we simply can’t deploy a robust 5G SA network.
5G Standalone (SA)
Unlike the NSA mode, the SA mode uses a separate core for routing and processing 5G traffic. Hence, all the disadvantages of 5G NSA can get overcome once SA is deployed. The network is capable of creating multiple slices and the handsets are capable of selecting all of them dynamically at the granularity of the application hosted on the device. The following diagram explains.
Here all three spectrum bands are acting in unison to drive traffic into the 5G core which is totally separated out from the 4G core.
5G SA Advantages
Each application on the device can have its own slice
Not only that, these slices can be configured for a specific SLA (Service Level Agreement - speed, latency, availability, etc). Also, access to these slices can be available even when one is roaming outside his/her operator’s network provided the slices so chosen are from those supported and standardized by GSMA on the handset. Typically GSMA has standardized 8 slices with predefined SLAs.
The network is integrated from the point of view of Handshaking
This means that there is no limitation of receiving acknowledgment from only the band which was used to receive incoming traffic - expanding the coverage of the high-frequency bands significantly. In order words, one can continue to receive high-speed incoming traffic from the downlink path of the 3.5 GHz band, as long as the 3.5 GHz RF downlink budget permits such a reception- the probability of which is very high due to the BTS’s high power tx capability compared to that of the handset.
5G is possible Indoors and can be used to drive voice
This is due to the fact that 700 MHz has better in-building coverage compared to the high-frequency bands.
5G SA Limitations
Low-Frequency Bands Needs Reasonable Capacity
The 5G SA is highly dependent upon the low spectrum bands for in-building coverage and reasonable cell edge speeds. Now low quantum of spectrum in this band will make the network highly unsymmetric. In order words, the upload speeds of the network inside the building or at cell edges will in no way match the download speeds of the 5G network- causing huge frustration in consumers’ experience.
Note, that the 3.5 GHz has 100 MHz of spectrum vs only 10 MHz in the 700 MHz band deployed in India in a practical network. Therefore upload speeds in a typical 5G SA network in India will be extremely low compared to the download speeds.
Difficult to Combine Frequencies of different bands at Sub GHz level
Now if one tries to combine the 700 MHz band with 850 MHz with an objective to drive the capacity of his Sub GHz spectrum then it will hit some technical bottlenecks. The reason is due to the proximity of these bands in the frequency domain, high cut-off filters will be needed to separate out 700 MHz signals from the 850 MHz - making the device complex. The complexity will expand further as you will also need to combine the signals emanating from the 3.5 GHz band as well - turning the device into the 3 CA (Carrier Aggregation) configuration. We all know - the more the CA, the more will be the device costs due to the need for an additional RF chain for supporting the new band for the purpose of aggregation.
5G Issues In India
India seems to have tied its hands and feet both when it comes to enabling a robust 5G SA network capable of all the flexibility and speeds. Why? All the spectrum in the 700 MHz band has been given to PSUs with only 5 MHz left for the operators to take in the future. Now the SA player (RJIO) on the ground already has 10 MHz of 700, now with another 5 MHz his tally will expand to 15 MHz (nothing more than this is possible). Ideally, 20 MHz in 700 would have been optimal, but 15 is not that far and should be fine.
But what about the other two operators, Bharti and VI? Looks like that for them the path toward the 700 MHz band is closed forever. The 900 MHz is occupied by 4G, and BSNL’s 2G. Anything less than 10 MHz is NOT good enough for deploying 5G SA, and that too when you need a parking spot for your 4G 900 MHz customers in the interim till they all migrate to 5G. You can’t move them (4G customers in 900 MHz) to the high-frequency bands (1800/2100 MHz), as it will destroy their user experience. Therefore, you have no choice but to keep them in the 900 MHz band. Hence, Bharti and VI have no option but to gun for the 600 MHz band. Here, fortunately, 40 MHz is available. But unfortunately, this band has no - ecosystem and will take a long time to develop (maybe more than 5 years). So Bharti and VI will have to continue to live with NSA for a long time to come.
For more details read my earlier note - “How to Improve the Quality of 5G Services in India”
Now, unfortunately, BSNL is going in the reverse direction. They have 10 MHz of spectrum in the 700 MHz band. Instead of using this band for 5G, they are deploying 4G there. Which is neither here nor there. The ecosystem of devices for 4G in the 700 MHz band is extremely poor. Hence, BSNL will not be successful in getting 4G subscribers, and that will mean that it (BSNL) will have to continue to support 2G for a long time, thereby blocking the precious 900 MHz band with 2G services and preventing Bharti and VI to use it for deploying the 5G SA network in there. Later, when BSNL realises their mistake they will end up uprooting all their 4G investments in the 700 MHz band and deploy 5G there. This also means they will have to keep the 900 MHz band locked to support their 2G subscribers for a long time.
Read my earlier note - BSNL’s Revival Plan — Falling Just Short of the Finish Line
The 900 MHz band details are available here.
Possible Solution
Immediately give BSNL 800 MHz band. See the 800 MHz band plan here. A lot of spectrum is available here to help BSNL do 4G - all of it was offered in the last auction and there were no takers. Also, the ecosystem of 4G in 800 MHz is pretty strong. BSNL can move all their 2G 900 MHz customers into 800 MHz. This will enable Bharti and VI to a path toward 5G SA. At least a 10 MHz spectrum will be available for Bharti for a 5G SA deployment in the 900 MHz band. This will be a win-win for all.
Conclusion
India needs 5G SA networks for it to unlock the full potential of 5G. Only one operator having that capability is simply not good enough. Others too need to be empowered. Doing so is the responsibility of the Govt and the regulator. Else 5G in India will make little impact and the current imbalance in capabilities might also disturb the competitive dynamics of the market - NOT good for India and its consumers.