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Software defined radio system holds out the hope of full wireless interoperability

Danny Bradbury

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Wireless networks have been a boon for business travellers but users often have to carry multiple devices to get the full benefit. This is because, although laptops, PDAs and smartphones can all send and receive data, they often do so over different networks.

In the days of 2G networks, travellers without a triband phone had to carry multiple units when travelling between GSM networks operating at 900MHz, 1800MHz and 1900 MHz.

Putting different radio systems in a phone to cope with varying frequencies is one thing, but with the proliferation of different wireless standards, things have become more complex. The 2.5G GPRS standard now sits alongside 3G, which is continuing to evolve. 3G network providers are beginning to roll out enhancement technologies such as HSDPA (High-Speed Downlink Packet Access and, in the US, Evdo (Evolution Data Only).

Add other technologies to the mix, such as 802.11a/b/g, 802.11n, Pre-n Mimo and Wimax, and it becomes clear that the number of wireless standards competing for space in the average mobile device is unsustainable.

It took handset suppliers enough time and trouble to engineer three radio frequencies into a mobile phone. Wi-Fi-enabled phones are just starting to appear, six years after the 802.11b specification first became popular. How can mobile hardware suppliers offer end-users the flexibility to support all of the standards without cramming so many circuits into a unit that it becomes unwieldy and power hungry?

Software defined radio (SDR) could be the answer. Traditional radios use hardwired analogue circuits to interpret signals. The circuits enable the radio to operate in a specific range with a specific modulation. In analogue radio, AM and FM are types of modulation, but in modern data networks the type of modulation is the air interface standard that the mobile network uses. Mobile network standards such as GSM and CDMA each use different modulation types.

SDR moves large parts of the analogue circuit into software, which makes the radio programmable. Ideally, a software radio can be updated to cope with new types of signal. For example, a smartphone using software radio would theoretically be able to jump between the different types of modulation used by Wi-Fi, Wimax and 3G.

Software has been creeping into radio for decades, and many mobile handsets already use software to control certain radio functions, said Olli-Pekka Lintula, director of strategic marketing for technology platforms at Nokia. Handset suppliers such as Nokia have been gradually condensing more functions into software using digital signal processors (DSPs), he said.

“To only have one piece of hardware and one antenna, that is the ideal. Whether that is ever reachable is the question. We are driving towards that vision. The word that we would use now is multi-radio, which is already very much software controlled today,” said Lintula.

The SDR Forum, a community of companies promoting the technology, defines five different levels of radio: 

  • Tier zero (hardware radio) has no provision for changing the function of the radio in software 
  • Tier one (software controlled radio) has some control functions in software but cannot change attributes such as modulation or frequency band. A device that used different circuits for different modulations and frequencies and uses software to switch control between them would fall into this category.
  • Tier two (SDR) is where most software radio companies are today. These radios use software to change between different modulation standards, but they might still meet different antennas to perform across different radio frequencies
  • In the future, companies hope to reach tier three (ideal SDR) and tier four (ultimate software radio), at which point a single device without an external antenna would be able to switch between frequencies and air interface types in milliseconds.

This is ultimately where Intel would like the industry to be. The company has already produced a prototype universal communicator handset that can switch between different frequencies.

Such a device would be able to do everything from open your garage door, load up on multimedia content when you were in range of a high-bandwidth network, and even pay wirelessly at toll roads and petrol stations using electronic cash.

Alan Crouch, who manages the communications technology lab at Intel’s headquarters, envisages a device that will turn radio on and off according to what the user is doing, saving power while maintaining the impression that it is on all the time.

One application for SDR is cognitive radio – radio that can adapt to its environment – according to Allan Margulies, executive director of the SDR Forum, who said that cognitive radios can alter their operation to fit usage models.

“Say you want to download a picture in a certain period and it is easier to do that over a GSM network because there is more bandwidth available, if the device makes the decision to use that air interface, it is a cognitive radio,” he said.

One of the biggest benefits of software radio will be the ability for operators to update the radio function in handsets remotely. Ideally, instead of offering a new Wi-Fi-capable mobile phone, they could simply send a software update to a radio handset’s subsystem to make it 802.11b compliant.

If an operator wanted its users to take advantage of the HSDPA upgrade to 3G networks, it could theoretically turn an SDR-capable handset into a 3.5G device by zapping it over the network.

“There is an interest among operators to reduce the necessity to provide a free handset every two years,” said Guenter Weinberger, chief executive of Sandbridge Technologies, which is developing a digital signal processor and compiler for use in handsets that will enable suppliers to introduce SDR functions.

The proposition is a little more complex for mobile device suppliers, who rely on frequent device refreshes for revenue. However, there are advantages for them too, according to Margulies.

“The time to market for phones will be reduced, and logistics issues will be easier to manage. You will not have as many models of phone because they will be based on a standard platform,” he said.

It also makes over-the-air bug fixes more feasible, and SDR will not stop users from buying new phones because of hardware changes such as size and screen quality.

John Chapin, chief technical officer at Vanu, which sells SDR-capable base station modules, said, in the short-term, operators would be able to put multiple air interfaces into a single base station. But in the long term, operators would gain the capability to roll out specialised services on a bespoke basis for corporate users across portions of the network.

“Maybe I am a fleet of taxicabs that wants to do telemetry. I might do a deal with a local operator to have it sent over the network,” he said.

Similarly, it might make it more feasible for the emergency services to use portions of mobile communications operators’ GSM or 3GSM spectrum on an ad hoc basis, in the event of an incident such as last year’s London bombings.

But operators have a long way to go before they can offer corporates such customised services. One of the biggest challenges they face will be handing over voice or data sessions seamlessly between different networks, possibly owned by different people.

Handing over from one base station to another in a network is one thing, but handing a voice call or data session over from a 3G cell to a local wireless network is much more daunting. And do not underestimate the business challenges in terms of billing settlements, said Crouch.

Two working groups are hoping to solve that problem. The IEEE’s 802.21 Group is tackling technical standards for handing over between heterogeneous networks, and the International Roaming Access Protocol Group, which it has been driving, is examining the business side, pulling in operators to help create standard interfaces for roaming and billing purposes.

Still, suppliers and operators face distinct challenges in the move to SDR. One of the biggest issues will be personal area networks. It is not cost effective to implement Bluetooth in SDR, according to Weinberger, because the standard is so well established and Asics (application-specific integrated circuits) are so cheap. The real benefit comes when implementing less widely adopted radio interfaces in software, he said.

One of the best examples of a personal area network technology with limited adoption is ultra wideband, which will offer much faster data transmission rates. However, the faster transmission rates make dedicated Asics necessary, because companies such as Sandbridge cannot process them quickly enough in general-purpose DSPs. Thus, at least for the present, SDR and ultra wideband appear to be incompatible.

“Companies will say the demands are higher, so that is a little bit further out on our horizon,” said Crouch.

"That is a reasonable statement but we cannot ignore it.”

The same goes for the antenna. The antenna is one of three main components in a digital radio, the other two being the radio front end (the amplifier and digital to analogue converters) and the baseband processor, which processes the resulting digital signal. Companies are making great strides in making the latter two components reconfigurable, which enables them to change the air interface, but building a reconfigurable antenna remains a sticking point.

Until that happens, mobile phone companies will be forced to include different antennas in a device to communicate at different frequencies.

Companies such as Skycross are moving in that direction, but the technology is still immature, which is why, for example, Nokia’s N92 smartphone needs multiple antennas to handle cellular and Wi-Fi networks.

Getting SDR into mobile devices will not be like flicking a switch. Instead, the advances will be incremental as companies make the gradual transition from software controlled radio into reconfigurable radio functions and beyond.

It will be one of the bigger drivers for handset and other mobile device functions in the future, and companies focusing on mobile computing for employees would be well advised to follow it closely.


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