Despite the massive proliferation of mobile 3G/4G networks, indoor coverage still remains patchy especially at the edge of the macro cell. It becomes a customer satisfaction issue if the call gets dropped as soon as the subscriber walks into his/her home or small office/home office (SOHO). To overcome the issue, mobile operators are deploying small base stations inside homes or SOHOs, called femtocells. Analysts project that femtocells will comprise more than 80% of all small cell shipments in the coming years.
A femtocell is a low-power wireless base station or access point for cellular access indoor in areas with limited or no coverage. The access point operates in a licensed band and uses the customer's DSL or DOCSIS/cable broadband ISP connection modem to backhaul the traffic to the mobile network. Femtocell is a plug and play device that can be automatically detected by the ISP, it registers and authenticates to the cellular core network, self-upgrades and provides location capabilities as well.
Since femtocells are deployed in poor coverage zones and the operator has no control over where they are deployed inside the house, synchronization is a critical challenge. In-building signal penetration, attenuation and multi path propagation make the use of GNSS for synchronization difficult. The access point must synchronize with the operator's core network without manual intervention.
GNSS, macro sniffing (ability to derive synchronization by listening to the cellular network) and wired backhaul timing protocols such as Network Time Protocol (NTP) and Precision Time Protocol (PTP) also known as IEEE 1588 -2008 v2 are some of the methods that can be used to synchronize the femtocell. GPS is the most mature and widely used and also includes GLONASS and Galileo. However GPS reception is insufficient in most indoor applications and is subject to in building attenuation as well as multipath delays. In some situations GPS may be vulnerable to other attacks such as jamming, spoofing and solar events.
Macro sniffing in an indoor environment is also subject to the same nuances of unfavorable RF characteristics of in-building coverage, as does the GPS.
Home backhaul (ISP) based packet timing and synchronization solutions are the best options to provide reliable synchronization to the femtocell. Though highly stable oven controlled crystal oscillators (OCXO) can meet or exceed the frequency accuracy requirements in femtocells, such oscillators are not a viable option due to their high cost. Given their relatively low costs, femtocells require sophisticated mechanisms to discipline a low-cost oscillator such as a voltage controlled temperature compensated crystal oscillator (VCTCXO).
Qulsar's Managed Timing Module (M64) provides a best of breed synchronization solution for femtocell applications.
Managed Timing Engine Module (M64)
The Timing Module is a IEEE 1588 PTP slave that can be integrated into the femtocell electronics. It is an extremely low power solution based on a low cost oscillator. It takes in IEEE 1588 PTP as input and delivers 1PPS, frequency 5/10/20/25 Mhz and Time of the Day (TOD) as output.
With additional electronics, the Timing Module also supports GNSS based synchronization. To support both GNSS and PTP with holdover, it is recommended to use the Managed Timing Board.
Managed Timing Engine Board