It's no secret why broadcasters and wireless carriers both want to move more digital content over the air (and in some cases are joining forces to do so). For broadcasters, mobile represents a new market for digital content for which they have already paid. For carriers, digital content attracts more customers, sells more minutes and generates higher spend per customer.

Doug Lowrie

Single frequency networks (SFNs) can help. It's a technology both digital broadcasters and carriers should consider (or reconsider) now that the technology is truly ready for prime time.

Theoretically, a SFN looks like an efficient, robust and economical way to reach subscribers. In a SFN, multiple transmitters over a wide area send exactly the same digital information on exactly the same frequency and at exactly the same instant. The technology is now specified within the Digital Video Broadcast-Terrestrial (DVB-T) and Digital Video Broadcast-Handheld (DVB-H) standards.

Several potential advantages immediately come to mind. SFNs could help wipe out dead spots, for example, by allowing operators to place transmitters inside buildings or closer to urban canyons. Furthermore, only a single infrastructure would be needed to reach both mobile and fixed customers, saving possibly huge expenses in deployment and operations. And since SFNs, by definition, only use one frequency that means licensing is faster and less expensive than if licensing multiple frequencies.

SFN CHALLENGES
Despite such apparently compelling benefits, SFNs have been slow to catch on. That's probably because SFNs have traditionally been difficult to implement.  One obvious challenge: The signals from multiple transmitters must be highly synchronized. Transmitters' coverage areas overlap, so many subscribers receive signals from more than one transmitter. If signals from different transmitters are out of sync, fading occurs. That's when waveforms subtract amplitude from each other because they are out of alignment at the receiver. Even if waveforms align perfectly, the data carried on those waveforms can still be scrambled if the bits themselves are out of sync. Finally, the signals assigned to adjacent subcarriers may interfere with each other if their subcarrier frequencies are allowed to drift and overlap, which could happen if the timing sources on which those frequencies are based start to drift.

These three issues – fading, data scrambling and subcarrier interference – have been well-known to network designers for years. What's also been well-known is that all three issues could be solved simultaneously if there were a  way to promulgate highly synchronized time and frequency references everywhere those references were needed. Well, now there is.

SFN operators can now specify as much timing precision as they need – wherever they need it – in forms they can distribute throughout their network infrastructures. Technology providers have responded to operators' timing requirements with products that offer high levels of accuracy, redundancy, phase noise reduction – plus other features, including hot swappable technology and remote management – to ensure reliable SFN operation.

Fundamentally, these solutions depend on the accuracy of two timing sources – GPS and quartz or rubidium holdover clocks. But both GPS and holdover clocks have also been around for years. So what's different now? And why should network operators care?

What's different now is that scientists have figured out how to combine the respective strengths of GPS and holdover clocks so as to effectively cancel out their respective weaknesses. The inherent strength of GPS is its accuracy as averaged over long periods – yet on a second-to-second basis GPS accuracy can vary widely due to atmospheric conditions and other factors. The strength of a holdover clock, however, is just the opposite. On a second-to-second basis (or even on a day-to-day basis) holdover clocks can be extremely accurate. That's why in timing applications their traditional role has been to maintain accurate time for short periods if the GPS signal is lost.

But raw GPS time – even if "held over" – may not be accurate enough for SFN networks due to its short-term variability. If it were, operators might have long ago addressed SFN timing challenges simply by installing GPS receivers at the headend and at transmit sites. They have not. What's new is the creation of hybrid timing sources that offer the short-term sub-microsecond accuracy of crystal or rubidium clocks combined with the long-term sub-microsecond stability of GPS.

That's good news for wireless operators – or broadcasters thinking about adding wireless to their existing infrastructure. The SFN timing challenge is reduced to a single all-in-one solution they can just plug in wherever sub-microsecond synchronization is required. In fact, adding that synchronization to a SFN is no more expensive or complex than adding it to traditional telecom or cable networks. But SFNs offer advantages these other networks don't — advantages operators can now enjoy for much less cost and risk.

Lowrie is a product marketing manager for Symmetricom's Timing, Test & Measurement Division.
He can be reached at DLowrie@Symmetricom.com.