Cognitive radio (CR) is a form of wireless communication in which a transceiver can intelligently detect which communication channels are in use and which are not, and instantly move into vacant channels while avoiding occupied ones. This optimizes the use of available radio-frequency (RF) spectrum while minimizing interference to other users.
In its most basic form, CR is a hybrid technology involving software defined radio (SDR) as applied to spread spectrum communications. Possible functions of cognitive radio include the ability of a transceiver to determine its geographic location, identify and authorize its user, encrypt or decrypt signals, sense neighboring wireless devices in operation, and adjust output power and modulation characteristics.
There are two main types of cognitive radio, full cognitive radio and spectrum-sensing cognitive radio. Full cognitive radio takes into account all parameters that a wireless node or network can be aware of. Spectrum-sensing cognitive radio is used to detect channels in the radio frequency spectrum.
The Federal Communications Commission (FCC) ruled in November 2008 that unused portions of the RF spectrum (known as white spaces) be made available for public use. White space devices must include technologies to prevent interference, such as spectrum sensing and geolocation capabilities.
The idea for CR was developed by Joseph Mitola at the Defense Advanced Research Projects Agency (DARPA) in the United States. Full cognitive radio is sometimes known as “Mitola radio.”
What Are Cognitive Radio Networks?
Cognitive (or smart) radio networks like xG’s xMaxsystem 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.
The following graphic shows how a cognitive radio network operates in relation to its environment:
Cognitive vs. Conventional
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.
Cognitive Radios Improve Spectrum Efficiency
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.
Taking Cognitive Radios Further: Interference Mitigation
Most of the research in the cognitive radio field to date has been limited to Dynamic Spectrum Access within the radio device. xG Technology has expanded the application of cognitive techniques beyond DSA in every radio used in the xMax system. xG is leveraging cognitive technology in several other aspects of the radio’s operation and across the entire xMax wireless network.
One of the breakthroughs xG has made that takes its xMax solutions beyond competitive cognitive radios is the addition of sophisticated and patent pending interference mitigation. These interference mitigation techniques allow xMax cognitive radios to increase their dwell time on a channel, even in the presence of interference that would cause traditional radios to fail. This increases the total spectrum bandwidth available for use by the xMax system compared to other radio systems, as well as improving the reliability of the xMax network in harsh RF conditions.
xMax cognitive radio networks are also incorporating MIMO antennas and advanced signal processing algorithms to withstand much higher levels of noise, jamming, and general interference than conventional radios and competing cognitive radio solutions.