Cognitive (or smart) radio networks are an innovative approach to wireless engineering in which radios are designed with an unprecedented level of intelligence and agility. This advanced technology enables radio devices to use spectrum (i.e., radio frequencies) in entirely new and sophisticated ways. Cognitive radios have the ability to monitor, sense, and detect the conditions of their operating environment, and dynamically reconfigure their own characteristics to best match those conditions.
Using complex calculations, xMax cognitive radios can identify potential impairments to communications quality, like interference, path loss, shadowing and multipath fading. They can then adjust their transmitting parameters, such as power output, frequency, and modulation to ensure an optimized communications experience for users.
Fig:Cognitive Radio Network
Conventional, or “dumb” radios, have been designed with the assumption that they were operating in a spectrum band that was free of interference. As a result, there was no requirement to endow these radios with the ability to dynamically change parameters, channels or spectrum bands in response to interference. Not surprisingly, these radios required pristine, dedicated (i.e., licensed) spectrum to operate.
By contrast, xMax cognitive radios have been engineered from the ground up to function in challenging conditions. Unlike their traditional counterparts, they can view their environment in great detail to identify spectrum that is not being used, and quickly tune to that frequency to transmit and/or receive signals. They also have the ability to instantly find other spectrum if interference is detected on the frequencies being used. In the case of xMax, it samples, detects and determines if interference has reached unacceptable levels up to 33 times a second.
The following image illustrates how xMax cognitive radios operate differently from conventional radios. It shows screen captures of spectrum analyzer readings taken from an xMax network tower in Ft Lauderdale, FL. The frequencies being measured are in the unlicensed 900 MHz ISM band. Because this spectrum is unlicensed (i.e., free of charge for anyone to use) it is used by hundreds, if not thousands of radios in the local area for applications like cordless phones, baby monitors, commercial video security systems, etc.
The figure at the left shows how a conventional radio would view this—as an environment having an unacceptable level of interference for communicating. The figure at the right shows what this same interference looks like to xMax. xMax is able to divide these frequencies into very small time segments (33 milliseconds) and find usable gaps where it can transmit its short and highly efficient signals—at moments when the spectrum is quiet.
xMax divides the 900 MHz spectrum block shown into 18 channels—giving it 18 opportunities (windows) every 33 milliseconds to find available spectrum.
In short, the xMax cognitive radio network sees windows of opportunity where other radios see walls of interference.
To reduce “thrashing” and unnecessary channel switching due to temporary and very short-lived interference phenomenon, or degraded network conditions (that do not cause a noticeable impact to performance or quality), actual channel and handovers decisions are made by trending multiple samples and measurements. The system only switches from its current channel when extreme levels of interference exceed its built-in interference mitigation capabilities. This enables xMax to use frequencies and find available bandwidth where other radios can only see static, yet its real-world tuned algorithms reduce signaling overhead and optimize throughput and quality.
The ability of xMax cognitive radios to make real-time autonomous decisions and dynamically change frequencies (referred to as dynamic spectrum access, or DSA) allows them to intelligently share spectrum and extract more bandwidth—which improves overall spectrum efficiency. It achieves this by “opportunistic use” of shared frequencies like unlicensed spectrum.
xMax cognitive radio technology was designed to be “frequency agnostic.” That is, its cognitive “Identify and Utilize” spectrum sensing technology can be used to power radios in any frequency band. This is beneficial since the FCC and wireless regulatory bodies around the world are in the process of opening up new spectrum, as well as reclassifying existing spectrum, to be made available for opportunistic use by cognitive radios.
This would allow new market entrants, utilities, public safety, enterprise and even existing wireless operators to offer new services, additional bandwidth and higher capacity without requiring these entities to purchase expensive and scarce wireless spectrum.
