Using the DOCSIS network to backhaul LTE/5G appears as an obvious way to get a cable operator into the mobile market. In fact, the current cable network can be viewed as a massive WiFi backhaul system, because most residential customers are connected to their devices via Wifi. Furthermore, public WiFi hotspots are available from many cable service providers. An LTE/5G deployed in a similar fashion seems like a natural progression for a cable operator.
One of the specific challenges in providing small cell LTE/5G deployments with DOCSIS as a backhaul is clock synchronization. This is a challenge that does not exist with Wifi.
5G/LTE requires clock synchronization. The clock synchronization requirements for Frequency-Division LTE may be somewhat relaxed, but without accurate clock synchronization, Time-Division LTE and advanced modes of 5G operation will not work. An accurate clock is also required for the BWR feature which helps reduce the latency on a DOCSIS mobile backhaul link by aligning the LTE upstream bandwidth scheduler with the DOCSIS upstream bandwidth scheduler.
A common way to synchronize clocks is to connect a GPS receiver to a small cell. This solution has two key issues: 1) the cost of the GPS receiver and 2) more significantly, the cost of installation, especially in apartment installations where an antenna for the GPS may have to be wired up.
How can DOCSIS help? We can deploy a timing protocol, such as IEEE1588, over DOCSIS to achieve clock synchronization. However, because DOCSIS has an asymmetrical upstream and downstream rate, the 1588 protocol “out of the box” would not work well because it assumes symmetry. Cisco helped lead a CableLabs® standardization effort called DTP (DOCSIS Timing Protocol) that solves the problem of transporting 1588 over a cable network. Cisco, in partnership with Intel and Sercomm, is demonstrating the DTP solution at next week’s SCTE Cable-Tec show.
Furthermore, to demonstrate how a wireless system can be built over a DOCSIS backhaul, Cisco has partnered with Sercomm to connect a CBRS small cell to a DOCSIS backhaul. The Sercomm small cell timing is derived from a cable modem based on the Intel® Puma™ 7 SoC, without the need for a GPS.
The full demo is depicted in the picture below:
Achieving clock synchronization over the DOCSIS network is based on work that is standards based and is a critical part in reducing equipment in deployment costs in the integration of mobile services into a cable operator infrastructure.
