This guest post was written by Alon Geva, Timing & Synchronization Expert, CTO Office at RAD
Delivering sub-microsecond time accuracy to cellular base stations is one of the major challenges facing cellular providers as they deploy their new LTE networks, creating unique challenges in the backhaul segment. This is exacerbated by LTE-A’s stringent synchronization requirements and, eventually, by 5G, now on the horizon.
Before the debut of 4G, the standard way to deliver a clock reference was to install Global Navigation Satellite System (GNSS) access at every cell site. A GNSS receiver is usually referred to as a Primary Reference Time Clock (PRTC).
The limitations of GNSS
This approach, however, is impractical in the 4G environment, since its network architecture is different. The most prominent change is that 4G accommodates large numbers of small cells to deliver higher capacities and data speeds. Stationing a GNSS antenna at every 4G cell site will be problematic due to cost. Apart from the unprecedented volume of antennas that would have to be bought, installed and maintained, the rapidly falling price of small cells will accelerate their deployment only further. Beyond all this, of course, is that every antenna requires an unobstructed sky view, a major problem for small cells, which, in many cases, are installed indoors, on building walls and closed spaces such as shopping malls, basements and traffic tunnels.
But that’s just the beginning.
The scariest problem of all is GNSS jamming and spoofing. Being a passive radio technology element, a GNSS receiver can easily be jammed using a $5 piece of equipment bought on eBay, disrupting the base station and even cause it to crash temporarily. The problem is worse in metropolitan areas with a dense concentration of cellular base stations, as well as moving vehicles. In principle, GNSS jammers, used in cars to block speed or log recordings, could occasionally disrupt base stations as they pass by.
So what’s the alternative?
With the transition to 4G, the IEEE 1588-2008 (Precision Time Protocol Version 2, or simply PTPv2) standard for precision timing synchronization over packet networks is becoming the accepted approach for cellular network synchronization. IEEE 1588 time distribution requires a stable primary Grandmaster that counts time units based on a given standard timescale (e.g., UTC – Coordinated Universal Time), a mechanism for measuring delay between the primary counter and a client requiring the time information (e.g., the slave inside the base station).
Backhaul synchronization distribution for 3G networks was mainly frequency-driven, with a central Grandmaster servicing hundreds or even thousands of base stations over a non-supporting backhaul. For 4G networks, stringent frequency and phase synchronization require PTP on-path support from (almost) every network element between the cell sites, something that would mandate costly modification of existing network elements. In many cases, such as when the mobile service provider is outsourcing its backhaul transport or leasing backhaul segments from third-party transport providers, this approach is simply not feasible.
A strategy to overcome this obstacle, one that is gradually gaining momentum, is to distribute a relatively large number of smaller PRTC/PTP Grandmasters in the access network, each servicing up to a few dozen PTP slaves. This results in shorter time distribution chains and dramatically cuts the number of intermediate network elements that need to be enhanced with on-path PTP support. To do that, more PRTCs/PTP Grandmasters are required. As a result, the cost of the distributed PTP Grandmaster equipment dramatically increases the cost of the overall solution.
The answer: miniaturize the Grandmaster
That obstacle, however, is easily overcome through miniaturization. A PTP Grandmaster with a built-in GNSS receiver can be packaged into a finger-sized SFP that plugs into an SFP port on any existing third-party network device. This solution seamlessly instantly upgrades the network with a fully featured PTP Grandmaster, a PRTC and various redundancy options that sustain operations in the event of a GNSS failure. As an added benefit, no additional space is required at the POP for an additional PTP Grandmaster box, and power consumption effectively remains unchanged.
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