12 June 2012
Developing a test regime for white space communications devices
The digital switchover is making a range of spectrum available that is being vacated by analogue tv transmissions. Now industry is looking to exploit the bandwidth in innovative ways.
Analogue tv, along with other users, such as fm radio stations and radio microphones, occupied spectrum ranging from 50MHz to beyond 800MHz, particularly in the US. Some of this spectrum will remain reserved for existing users, but the area from 470MHz to 700MHz is being targeted by a rapidly developing community looking to create white space communications systems. While pioneering work is being done by companies such as Neul, the work of developing standards for white space products and services is being driven by the Weightless Special Interest Group (SIG).
The Weightless standard basically addresses M2M communications and just happens to use white space, said William Webb, chief technology officer for Neul, who is leading the Weightless SIG's efforts. "There are other white space standards, but Weightless doesn't see them as competitive; they just happen to use the same area of the spectrum."
He said Weightless currently has six sub groups – PHY, MAC, security, applications, regulatory and test. "The test sub group will have to devise tests against each of the other group's outputs. In particular, the output from the PHY, MAC and security sub groups will form the main block of what the test group will have to get its teeth into."
Bringing new communications technology into service in a new area of the spectrum is going to involve some challenges, said Webb. "One thing we determined fairly quickly is that the test requirements for white space communications will fall into two groups. One area is the tests needed to ensure the terminals will operate with the network. We need to ensure they won't upset system functionality and don't take more capacity than they should. These are what could be seen as classic communications tests. But there's also the whole area of regulatory testing."
In Webb's opinion, most tests in the first group will be similar to those already being performed by mobile test organisations. "Broadly, that's what Weightless is," Webb continued. "Devices talk to the network, so there will need to be checks on whether the device sends the right protocols backwards and forwards, whether there's the right level of sensitivity and whether the power output is correct. Other tests will include whether the device responds appropriately to commands."
These tests will be familiar to those already developing communications products, but Webb believes the regulatory aspects of test will prove more challenging. "They are a bit of an unknown at the moment," he conceded.
One of the problems with handling the regulatory issues is that only the US has published a final test and certification process for white space. "The UK has produced an early draft," he noted, "but nothing more and, as far as we know, there has been no output from any other country."
Where white space communication differs from, say, Bluetooth is that it will use licensed spectrum. "That means there will be a higher threshold than normal in order to protect other users," Webb said. "It's not like gsm, which will only interfere with other gsm calls. White space communications may interfere with tv transmissions." Even though analogue tv transmissions have all but ceased, digital terrestrial tv is still transmitted in the white space spectrum and has to be immune from interference. "If there is interference," Wood said, "viewers are likely to see screen freezes and blocky artefacts."
Webb believes regulators will insist on tight, detailed regulations in order to ensure devices have very small out of band emissions. In that way, if a white band link is operating on a frequency adjacent to a tv channel, the data being transmitted won't spill over into the tv link.
With the US authorities being ahead in terms of developing certification processes, the Federal Communications Commission is looking to conduct tests on all radios, examining their output masks and making sure they fit the regulations.
Because of this international disparity, Webb believes there may well be a two stage process. "Firstly, Weightless certification against a profile, then certification by each country," he suggested. "It could become a bit burdensome as the number of companies licensing white space communications grows, but I think most countries will look at the Weightless testing process and say that will be sufficient for their purposes."
And that's what happens at the moment with communications approaches such as Bluetooth and Wi-Fi. "National regulators don't test these devices themselves," Webb continued, "they accept that the test specification is suitable."
At the highest level, white space test will be a matter of taking a device through the process of checking that it does what it should, said Webb. "But there may be two steps here. Firstly, there'll be tests of radio emissions, with a more rigorous second stage of making sure there is no interference."
Test and certification have been placed together intentionally in a single sub group, due to their close interrelationship. The sub group's work commenced with determining strategies for testing and has since moved on to define the actual test and certification processes for Weightless network and terminal equipment.
"It's taking the specification and determining what tests need to be done in order to ensure the devices comply," Webb commented. "It is written in the specification that white space communications should have a bandwidth of 8MHz for Europe and 6kHz for US based applications. The test group will say 'the bandwidth mask needs to be verified' and there will be rf measurements needed to ensure devices comply."
The work in developing the test and certification strategies is proceeding in advance of the Weightless specification being issued. "The Weightless spec is currently at version 0.7," Webb said. "We're planning quarterly plenary meetings to update the spec and each meeting will add 0.1 to the version number. By the time we get to version 1.0 – which should be Q1 2013 – the spec should be stable."
But, necessarily, the test sub group is working a little behind the other groups in order to take their outputs and weave them into the test requirements. "They are probably working three months behind," Webb said, "so the test spec should be available in Q2 2013."
With the Weightless spec still some way from being finalised, it's no surprise that there are no devices available commercially. Neul has started development of a white space chip set and is well into the process, Webb said. "It will be taping this design out shortly. But in doing so, it's running the risk that the standard will evolve in a way which it doesn't expect and that it will be left with a non compliant chip. That would be a problem.
"Neul is comfortable that the PHY part of the draft spec is stable and finished, but there may be changes to the MAC element. If there are, these will probably be handled with upgraded firmware."
He said Neul's chip set is likely to be sampling in Q1 2013, with volume in Q2 2013. "That will fit nicely with the availability of the test spec," he concluded.