Cognitive Radio Networking
What are Cognitive Radio Networks?
Cognitive (smart) radios are able to identify spectrum (i.e. radio frequencies) that is not being used, and to 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. Depending on the radio technology being employed, the detection and decision process can take a few seconds, or in the case of xMax, it samples, detects and determines if interference has reached unacceptable levels up to 33 times a second.
However, one of the breakthroughs xG has made that take its xMax solutions beyond competitive cognitive radios is the addition of sophisticated and patent pending interference mitigation. This enables xMax to increase its dwell time on a channel, even in the presence of interference that would cause traditional cognitive radios to abandon the channel and search for cleaner spectrum. This increases the total spectrum bandwidth available for use by the xMax system (compared to other radio systems), as well as increasing the reliability of the xMax network in harsh RF conditions. More on xG’s interference mitigation technology can be found here.
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 it's real-world tuned algorithms reduce signaling overhead and optimizes throughput and quality.
For example, below is a screen capture from a spectrum analyzer reading taken from one of the xMax network towers in Ft Lauderdale. 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.
Figure 1 – Looking at congested spectrum in the frequency domain
Most radios would see this as an unacceptable level of interference. However, xMax is able to divide these frequencies into very small time segments (30 milliseconds) and find gaps where it can send a burst of signal while the spectrum is quiet.
The next figure shows what this same interference looks like to xMax. Since xMax can look for usable frequencies 33 times per second (i.e. in the time domain) it can find gaps where it can transmit its highly efficient and short 30 millisecond signal.
Figure 2 - A small section the same interference examined in the time domain
xMax divides the 900 MHz spectrum block shown in figure 1 into 18 channels – giving it 18 opportunities (windows) every 30 milliseconds to find available spectrum.
In short, the xMax cognitive radio network sees windows of opportunity where other radios see walls of interference.
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” (use by cognitive radios).
Below is a chart compiled by Shared Spectrum Company. It shows that the utilization of most frequency bands, as measured in seven separate cities and locations throughout the US, is actually quite low. Think of this chart as a time lapse picture of Figure 2 above for a much longer time, over many more frequency bands.
Spectrum occupancy over seven locations
From this we can see that allowing cognitive radios to use these frequencies opportunistically would free up significant wireless bandwidth and substantially improve overall spectrum utilization.
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.
