If we go back to the timeline of Cellular technologies – 3G (WCDMA) took a much longer time in the Network commercialization and to develop the associated ecosystem for e.g. introduction of smartphone was delayed for long years even after WCDMA 3GPP spec was frozen until HSDPA was introduced. One of the major reasons was that the data speeds offered by the technology were not lucrative. But, there was a big leap between 3G and 4G and 4G networks deployment happened very quickly after 3GPP standardization. Majorly there were two primary reasons for this turnaround.
– Higher Data speeds offered by 4G
– Availability of Smartphones with the demand of higher speed
With every new generation of technology the cycle from Standardization to commercialization & adoption is getting reduced and now 5G is considered to be the most accelerated technology so far with the expectations to acquire 500 Mn subscriptions within 3 years of deployment compared to 5 years of LTE.
If we look at the system evolution from 4G EPS, the starting point for 5G deployment will be the Radio Access Network (RAN) with the existing 4G EPC core. This means that 5G core is not instantly needed and 5G will be highly dependent on 4G RAN and Core in the initial phase. This arrangement is also called Non-standalone system which is the basis of all the major deployments happening globally at this point of time. This is also the reason for accelerated 5G deployment i.e. without the introduction of end to end complex new system. Therefore, the 5G rollout in the initial phase are meant only in Radio Access majorly which will offer instant boost to Network capacity and user experience. Non-standalone system is based on Dual connectivity, meaning that UE will be served by both eNB and gNB in both Downlink and Uplink and eNB will act as an anchor carrier – meaning that all the control plane will be with eNB and gNB will only serve the user plane, this is mainly referred to as Option 3x deployment mode. Anchoring by eNB is required at both the ends i.e. towards UE and towards the Network core.
5G Core & Radio Network Motivations and Enablers
There are certainly distinguished key factors of 5G core which will differentiate from EPC and shall support new use cases. We can categorize some of them as follows
– Service Based Architecture: – SBA has the flexibility and can be tailored according to use cases and demands. It will offer lose coupling among different functions which will be based on APIs and one function can easily call out the other pre-defined functions. This will greatly improve the flexibility and encourage open interface making way for easier Inter vendor integration.
– Network Slicing: – Network slicing grabbed a lot of attention and is one of the key features of 5GC that is always talked about. Network slicing is all about creating multiple virtual end to end networks out of the physical network to cater the diverse demand, scenarios and use cases. For e.g. the network can have multiple slices and can cater the demand of both High throughput and low latency when needed, user can subscribe to both the slices simultaneously and can get benefitted.
– QoS Differentiation: – QoS was ensured in 4G also but 5G can offer more refined and more granular or say precise Quality of Service which can add a lot of value to carriers in offering differentiated services
Let’s have a look at the Radio Network motivations for 5G NR where the new concepts have been introduced and on a whole the system is designed to deliver on various aspects of 5G. Some of the key concepts are as follows
– Spectrum: – A very wide range of spectrum has been opened up in 5G, starting from low bands (below 7GHz) and going till mmWave – all of them potentially could be used for deploying 5G NR depending on spectrum availability regionally. This wide range of spectrum options is unique for 5G NR and unlike older cellular technologies. At one extreme where low bands can act as coverage layer, the other extreme can offer higher capacity whereas the mid band can offer both coverage and capacity. This varied spectrum range can offer substantial benefits to carriers and user experience that could go up exponentially.
– Flexible Numerology: – Unlike 4G, there is a significant change in 5G NR physical design and is highly flexible in terms of how the fundamental transmission is handled at the physical layer to address the diverse demands or say to meet diverse envisioned objectives of ITU such as High Throughput, Ultra low latency etc.
– Antenna Techniques: – Antenna techniques such as Beamforming and Massive MIMO would be heavily exploited in 5G with multiple antenna elements. Beamforming on one hand can offer enhanced coverage, Massive MIMO on the other hand can help in maximizing the system capacity or overall cell throughput.
Deployment Strategy, Challenges & Forward Path
In the initial phase of NR deployment, 3.5 GHz (or 28 GHz) would be used for NR and the NR coverage area would be far lesser than that of LTE. NR can provide higher data rate services and LTE can be used to carry control-plane (CP) data to ensure mobility, while NR carries user-plane (UP) data for higher throughput and capacity. In NSA DC, the uplink and downlink services on the UP are carried respectively on LTE and NR. LTE uplink coverage can be used to compensate for the insufficient uplink coverage delivered by NR and thus the low band on LTE and Mid Band or High Band on NR can very much complement each other.
With multiple network entities i.e. eNB, gNB, EPC and 5GC – how they are interworked and interconnected leads to define the various ways in which the network can be deployed. There are multiple options as defined by the standards but Option 3x is the most popular choice globally in the initial phase. The ultimate goal for any Network would be to achieve Option 2 in which the gNB will be connected to 5GC, this is also called Standalone system. The key question that emerges for any deployment strategy is – Would there be direct migration from Option 3 to Option 2? Answer lies in the deployment strategy of carriers. Carriers may have ambitious plans to migrate to Option 2 directly after option 3 and few may have intermediate path as well for e.g. Option 7 (in which Ng-eNB will be anchor and gNB will be secondary connected to 5GC, can also be viewed as replica of Option 3 with the exception of replacing EPC with 5GC).
While the NR deployment can be easily accelerated basis on NSA option 3x, any evolution of cellular technology poses new challenges and 5G NSA is not untouched by some of the issues that we may face. One of the key challenges is to manage Downlink and Uplink connections in Dual Connectivity, for e.g. EN-DC band combination B3+n78 in Uplink, here the Band 3 and Band n78 do not go hand in hand and will be vulnerable to high interference as a result of harmonics issue or Inter Modulation Distortion. There are few solutions to overcome the interference issue, one of the solution is to add RF filtering but the added cost to OEMs and stringent filter requirements do not make this option compelling. The another promising option and much in discussion is “Single Uplink Operation” in which at any instance only one uplink transmission will be done either on LTE or on NR unlike transmitting simultaneously at cell edge where the full device power is needed. Network will anchor and guide the transmissions on Uplink by defining the TDM (Time Division Multiplexing) scheduling pattern and thus UE will keep switching between LTE and NR and the whole power can be allocated to only one uplink transmission at any point of time.
The other key challenge is loss of Uplink coverage and uplink Tx power management caused by Dual Connectivity as the UE Tx power will be distributed between LTE and NR on the two links. The starting point for managing the power is Equal Power Sharing technique in which Max Tx power of UE i.e. 23 dBm will be divided on two links i.e. 20dBm on each link. But, this technique may not be effective as the max power on LTE is fixed now at a lower level and hence there would be UL coverage loss by 3dB. As LTE will be the Master node in NSA carrying the control plane traffic this 3dB loss will impact the coverage and overall system performance.
Another efficient technique is Dynamic Power Sharing (DPS) in which the Dual connectivity will be active with power being dynamically allocated considering the upper limit of UE Tx combined power of 23dBm with more power allocated to LTE and remaining concentrated on NR. This feature is a UE feature and has minimum dependency on Network unlike Single Uplink Operation.
Based on the above techniques, it is recommended to deploy Single uplink operation in case interference is the main concern and Dynamic power sharing in case Uplink throughput at cell edge and coverage are the main concerns. Dynamic power sharing offers 30% better Uplink throughput than SUO at cell edge and the performance is expected to be similar of both the features at the core of the coverage.
Mobility in NSA is again one of the key challenges as the control plane remains with LTE and any handover on NR has to be well coordinated with LTE through tight interworking. Therefore, the overall handover process could be time consuming and there would be significant impact on NR to NR Handover latency and Handover Data interruption time. The NSA NR to NR handover latencies in control plane anchored through LTE procedures can be in the range of 75-300 msec compared to the legacy LTE intra-frequency handover of 60-100 msec. The higher delays comes from the tight interworking between 4G and 5G radio (e.g. LTE handover may require modification of NR cell).
All the above discussed challenges are associated with NSA kind of deployment and these give us enough reasons and motivations to consider Standalone systems which is the ultimate target of almost all the carriers globally considering the motivations offered by 5G Radio Access and 5G Core discussed above.
The transition from Non-Standalone deployment towards Standalone can be done step-wise and may require additional feature to improve the coverage aspects (e.g. Standalone working on low bands).
In order to create a base for NR penetration and to have wider coverage or availability, Dynamic Spectrum sharing (DSS) is going to play a crucial role. The objective of Dynamic Spectrum Sharing is to create space for NR in lower bands of spectrum. Every new technology demands space in the carrier’s existing spectrum chunk for wider deployment, it used to happen in previous generation of technologies also but more in a static way which is also called “Spectrum refarming”. Spectrum refarming usually requires a lot of planning from both technical and commercial aspects and may take even years to create space. Moreover, the complexity increases in emerging markets where there will be diverse user base and requirements. With Dynamic spectrum sharing, the two technologies LTE and NR can co-exist in the same bands and with operation in lower bands – NR will get instant coverage boost and can penetrate in to the network more widely and a carrier can ensure availability of 5G.
There are various ways to implement Dynamic Spectrum Sharing: Either the bandwidth chunks can be dedicated for LTE & NR respectively or it can be shared dynamically- Both Full bandwidth or Partial Bandwidth, depending on the band (e.g. FDD or TDD).
Apart from High Spectrum efficiency and coverage boost there is another aspect of Dynamic Spectrum Sharing which will significantly contribute to evolution and i.e. Voice over NR. Voice has always remained an integral part of any technology offering and to offer seamless coverage for Voice experience – it is important to ensure that the coverage is widely available and Dynamic Spectrum Sharing will just start creating a base for Voice kind of services
Summary: How the 5G Network deployment may unfold?
The starting point of NR deployment will be based on Non standalone systems based on Dual Connectivity mainly in mid bands which will accelerate the NR deployment targeting only eMBB. This will offer a quick capacity boost to existing systems as NR will bring additional bandwidth. It’s important to address some of the key challenges of Dual Connectivity and Interference issues and hence features such as Single Uplink Operation or Dynamic Power Sharing will play a crucial role. In the second phase, it will be highly important to increase the footprint of NR by deploying it in low bands and this is where feature such as Dynamic Spectrum Sharing will play a very important role, to a great extent this will also create a base for VoNR deployment to offer voice seamlessly in Standalone 5G. In the subsequent phases of deployment, it will be important to bring mid/high bands together with low bands through the means of Carrier Aggregation which will ensure both capacity and coverage and will make the overall system ready to migrate from Non standalone to Standalone systems which is the ultimate target of 5G deployment. There are many motivations and advantages offered by 5G core and RAN to move from Non standalone to standalone systems which will enable other use cases as well apart from eMBB which will make the overall migration compelling for carriers.