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Electronic Design

Embedded Timing Solution Synchronizes Small Cells

Symmetricom’s SoftClocks embedded timing and synchronization software provides the technology to sync cells to a common timing standard, which is the key to successfully expanding cellular networks with small cells.

Small cells are gradually becoming a core building block in the wireless infrastructure. As network carriers roll out their Long-Term Evolution (LTE) networks, small cells will cover more subscribers while meeting their needs for wide-bandwidth, high-speed applications like video and games.

Also, small cells won’t replace macro cells, but they will supplement them by filling in the coverage gaps and extending the network coverage. Small cells include residential femto cells as well as traditional carrier pico, micro, and macro cells.

SoftClocks runs on the existing small-cell processor. It’s available in two versions: the SCr for residential femtos and the SCe for enterprise small cells. Both support multiple timing protocols including IEEE 1588 Precision Time Protocol (PTP), Network Time Protocol (NTP), and softGPS.

The timing requirements for implementing LTE in the network are severe. The typical clock precision required is 16 parts per billion (ppb) in the transport network and 50 ppb in the air interface. For time division duplex (TDD) and advanced versions of LTE, the phase requirements are also critical. SoftClock provides the needed level of performance.

In addition, the backhaul method will vary with different types of small cells. Many will use microwave. Others will use fiber. In residential femtos, DSL or cable TV provides the backhaul. With SoftClock, designers can select the best synchronization scheme for each type of cell and backhaul. Then, they can implement a single small-cell design and choose the timing method as required, speeding the design and reducing costs.

The SoftClock portfolio includes several versions. SCr 100 and SCr 200 cover PTP and NTP for residential cells. SCr 200 includes GPS. For the enterprise, SCe 300 and SCe 400 cover PTP and NTP while SCe 400 also handles GPS.

The PTP and NTP versions are software only. The GPS versions require an external GPS receiver module. The software output from the host processor is in the form of a correction signal to the small-cell temperature-controlled crystal oscillator (TCXO).


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