Sprint: 3G to 4G/LTE Network Transformation

Challenge: Expedite 4G/LTE data introduction and enable cost-effective solutions for all other services over 3G infrastructure, using Enhanced Circuit Switched Fallback (eCSFB).

Background: The employer end customer is a national Tier 1 wireless service provider deploying 4G/LTE in its network throughout the continental USA. Voice and ancillary services are a must for LTE users. However, the initial deployment support data only over LTE. Furthermore, LTE coverage is geographically limited in the first rollouts. A dual radio smartphone cuts into profitability margin. The challenge was to provide a solution using an economical handset option that supports LTE wireless broadband data service, while voice and ancillary services are provided through the existing 3G 1xRTT Circuit Switched infrastructure.

Solution: Our SME frontend development was key to that aspect of the overall success of the eCSFB program. Over the course of 10 months, he was the key player in many multi-vendor (RAN, EPC, and UE) discussions where he was shaping the technical requirements, guiding the vendors while referring to the 3GPP and 3GPP2 specifications. He eventually wrote a standards-grade technical specification that was used by different organizations within Sprint and its vendors as the eCSFB reference. The specification included the detailed Solution Architecture, end-to-end call scenarios, capacity and performance budgets to support mobile data, initiated and terminated calls, SMS, Multimedia message, Location Based Services, E911, as well as CMAS over 4G-3G access.

In this solution, the LTE network broadcasts its support to 1x circuit switched fall back in System Information Block 8 (SIB-8). Dual mode, single radio 1xRTT/LTE devices capable of eCSFB attach to the LTE network and pre-register in the 1xRTT domain. Whenever the User Equipment needs to originate or terminate a voice call, while LTE attached, it will be directed to “fall back” to the existing 3G CDMA radio network, using a handover procedure—UHDM, ECAM or Release with Redirection—to connect the voice services. If the UE is engaged in an LTE data session, it will be suspended for the duration of the voice call. At the conclusion of the call, the device reselects to LTE domain and resumes the data session on hold.

For other Circuit Switched services such as SMS, Network Initiated (NI) coarse Location Base Services (LBS), registrations and pages, eCSFB allows UEs to remain on LTE network while tunneling the services from the 3G network over the S102 interface between the 1x Interworking Solution (IWS) and LTE Mobility Management Entity (MME). NI precise LBS requires to fall back to 1x for the UE to establish an IS-801 session with the Position Determination Entity (PDE) over the Circuit Switched traffic channel.

Services such as Communications Assistance for Law Enforcement Act (CALEA) and Commercial Mobile Alert System (CMAS) are supported natively on 1x or LTE, thus, not requiring fall back from LTE to 1x. Applications such as QChat, Hotspots, and Phone as a Modem (PAM) have access to Application Programming Interface (API) to enable them to ignore incoming paging to avoid session interruption. In RAN Sharing scenarios, users have the capability to fall back to 1x services where Customer’s 3G coverage is available