Enhanced Circuit Switched Fallback (eCSFB)

The 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 role outs. A dual radio smart phone 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.

CeleniumSM consultant was subcontracted through one of the Customer’s RAN vendor as an embedded resource serving the Customer’s interest. Our SME was the frontend interface to all Customer’s RAN OEMs, and user equipment chipset manufacturer. The solution was developed to support LTE access around Macro Cells and early stage investigations on the interaction of Small Cells in eCSFB deployments was researched.


In this engagement, an eCSFB Feature Requirement Document was developed as a vehicle for vendor management. Customer expectations were set in partnership with OEMs, and a buy-in was obtained from project stakeholders and vendors of access and core network, as well as handset manufacturer.

Transaction Flows were developed to provide executive-level Use Cases of services. The corresponding Call Scenarios consolidated the 3GPP, 3GPP2 and legacy CDMA detailed message exchange of each service provided to the users.

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 a 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 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 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