xG handsets
Basestation
If you’ve been following the development of wireless over the years, you’ve witnessed the transition to software-defined radio (SDR) technology in most big systems, like cellular. You also have probably heard of an SDR enhancement called cognitive radio, or radios that are smart enough to decide where and when to transmit, as well as what modulation to use. Such radios are agile enough to adjust their frequency and power to the available spectrum, interference, and other conditions and change their characteristics to optimize the connection.
“Cognitive radio has always been talked about as ‘the next big thing’ for as long as I can remember. However, practical and cost-effective cognitive radio networks have finally arrived. We are now able to build and deliver game-changing commercial and tactical cellular systems using cognitive technology today,” says Rick Rotondo of xG Technology, which has built a complete cognitive radio system with amazing performance. This shows that cognitive radio could be at least one solution to some of the pressing problems in wireless, especially the spectrum shortage.
The company has built a carrier-class cognitive radio system in the unlicensed 900-MHz industrial, scientific, and medical (ISM) band (902 to 928 MHz). While carriers like AT&T and Verizon paid billions for 20 MHz of spectrum in the most recent 700-MHz auction, xG uses the free and available 26 MHz of spectrum in the ISM band. But wait. Isn’t that spectrum just as bad or worse than the 2.4-GHz band as far as interference goes? Yes, all sorts of services use it, such as telemetry, remote control, and video surveillance.
But cognitive radio makes this spectrum suitable for voice and data calls. Cognitive radios listen to the band in use to determine where the potential interference is and then switch to an unoccupied frequency. The xG handsets (Fig. 1) actually scan the band 33 times per second looking for the interference and identifying the clear spots where a good link can be formed. Then they notify the basestation (Fig. 2), causing the frequency to change as needed to keep a clean connection.
The company’s xMax system adheres to the Federal Communications Commission’s (FCC) part 15 rules for the 902- to 928-MHz spectrum. Radios, and the basestations, can have up to 1 W. The system divides the spectrum into eighteen 1.44-MHz channels and uses robust binary phase shift keying (BPSK) modulation. Access is time division multiple access (TDMA), and each channel can handle up to 12 voice calls. A data rate capability of up to 1.5 Mbits/s is also planned for each channel.
As for real-world systems, xG has one major carrier-grade network in full operation with another contracted for deployment by the U.S. Army in February 2011. The deployed network is in Ft. Lauderdale, Fla., while the other new system is being readied for Fort Bliss in Texas. The company is confident that the two networks will conclusively prove the method workable and even superior in some aspects to existing cellular networks.
Furthermore, the potential is great because despite the fact that most cellular systems are switching to Long-Term Evolution (LTE), the xMax system can operate robustly in crowded unlicensed spectrum and LTE cannot. Potential applications include rural cellular, the Smart Grid and utilities, public safety, white space applications, and certainly the forthcoming national broadband system. The lack of cost-effective licensed spectrum is proving to be a major impediment in these sectors. Cognitive radio could be an alternative you may not know you had.