The Need for SRVCC in VoLTE

The use of Single Radio Voice Call Continuity (SRVCC) in early deployments of VoLTE networks is critical to maintaining the current customer expectations of the voice call experience. As LTE networks often lag earlier technology networks in coverage, software reliability and general network optimization, SRVCC is the vital gateway to transparently maintaining the customer experience. Operators will continue to enhance the LTE networks coverage and reliability to the standard of current legacy networks, however the ability to rely on the 3G/2G voice networks for backup is critical. Not only for fallback at call setup (CSFB or Circuit Switch Fallback), but the ability to hand down to the legacy networks mid-call as conditions degrade.

In early VoLTE deployments, SRVCC has experienced numerous core network, radio access network (RAN), and handset implementation issues that have it often ingrained in engineering teams as a KPI issue and choose not to utilize it at all. Handover between any network type are generally lower percentage handovers due to the complexity of inter-systems. VoLTE hand down to WCDMA/GSM is even more complex with packet voice having to also be converted to circuit switched voice on the network core. It is this complexity that is continually having to have all the bugs worked out. With the thought of not utilizing SRVCC in the minds of engineering teams, it is often the responsibility of OEMs and national engineering organizations to provide the confidence that SRVCC has improved and meets the requirements of the business.

The issues with SRVCC have typically moved from system bugs to when SRVCC should be triggered. Operators attempt to balance between performing SRVCC at the last possible moment and maintain as many call on the LTE network by minimizing the usage of SRVCC versus doing it early on before quality or coverage degrades too far to allow the best change of SRVCC success. To add to the issues of SRVCC, most were triggered solely on the LTE RSRP levels. Without utilizing quality triggers, SRVCC triggers too early in rural areas, and often too late in interference limited areas due to high cell overlap or capacity demands driving up the noise floor. As quality is consider in the event triggering both on the current serving conditions as well as the new network being handed down to, the correct timing for the various types of RF environments will be accounted for when the device truly needs to change networks. Quality driven triggers with adequate timer settings will be the key to a consistently positive customer experience.

Balancing engineering goals with customer experience will come to the forefront of internal network discussions. With engineering metrics often highlighting negatively the time subscribers do not spent on LTE, engineers will favor keeping calls unnecessarily on LTE in poor quality conditions. In conjunction with timer extensions often utilized, customers will likely end the calls due to continued poor quality rather than drop making the decision to stay on LTE seem positive statistically although the negative experience was actually experienced by the user.

Without SRVCC, operators will need to quickly spend extensive capital into extensive LTE network densification and optimization to maintain the current customer experience on their legacy voice networks. The benefits of high definition voice on VoLTE will more than be offset by poor quality and dropped calls by the users. Similar to IRAT being the band-aid for initial WCDMA networks, SRVCC is vital to the success of VoLTE networks and the network efficiency gained by LTE deployments and spectrum consolidation. Operators will need to be both diligent in finding and resolving issues with SRVCC in partnership with their OEMs as well as continue to have faith in the specification, without it, VoLTE networks will be a downgrade to the current customer experience.