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Phase Ready?

As you think about the evolution of your network, don't limit your thoughts to the frequency stability you need now. We want to help ensure that network roll-outs today will still be relevant come the explosion of HetNets and Small Cells.

GPS Timing - coming to the edge of a Network near you?

This may seem a strange title for many readers - if you have a CDMA network you've been working with GPS at cell sites for many years. Here in Europe however we have tended in the past to use the E1 used for backhaul to deliver the sync quality we need to deliver 15ppb at the air interface. We've also more recently delivered this frequency performance from the core of Carrier Ethernet networks using PTP and SyncE.
Should your GPS have a problem, the holdover oscillator (lets say an OCXO) kicks in and gives you time to get out to site and make a fix. Likewise the PTP client devices usually use OCXO for holdover, and although the time to fix is usually shorter than is the case for GPS there is still adequate time to mobilise for a fix.

So why consider GPS at the edge now? Well we're probably a year or so away from having to roll out TD-LTE, or LTE-A features, that require adjacent cells (Macro or Small Cells) to be within +/-1.5us of each other. Packet based timing from the core is still being tested and standardised; microwave radio manufacturers have a real challenge to make this work well (and they're doing great work to get there);and you may not have end to end control of the backhaul network in any case. These issues may or may not be performance affecting enough to put a brake on considering PTP supported by SyncE for phase, but you'd be a fool not to consider the alternatives.

So we examine GPS at the edge; its been done successfully for years, right? Trouble is the game has changed - its got a lot harder. We're all OK with your GPS timing solution locked and running as it should, but what about failure? As in frequency networks the holdover technology kicks in - but OCXO will only hold the +/-1.5us for probably four HOURS, not four DAYS! Have you got the budget, or indeed available power in the cabinet, to use OCXO? TCXO performance continues to improve in leaps and bounds so perhaps using the latest TCXO could give you three hours to mobilise. How is Ops going to handle this? Will you need vehicles parked at strategic motorway junctions waiting for equipment to fail?
On a practical level where can you put the antenna? Have you mast space and the personnel required to roll this out at thousands of locations? Can you hide the antenna in the cabinet? Will it see enough of the sky to work there? Could you roll out Assisted GPS (A-GPS) to help here? If you deploy antennas at low level how long before they're vandalised? Does your network design allow for other cells to provide backup sync? If your timing is embedded in the eNodeB do you have to swap out a whole box to solve a GPS or sync issue?

These issues are real and need addressing, but none of them are necessarily show stoppers to delivering a robust network with adequate phase performance. The trick is to know what the issues are and to address them head on. Don't hope things won't go wrong because they will. Examine the cases of Edge GPS and Phase from the Core from a position that with the right planning and testing they will work.

Make sure you and your network are Phase Ready!
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