Towards full cognitive radio
Wireless Spectrum is a valuable resource. Any doubt about that statement was dispelled by the AWS-3 spectrum auction, which is reported to have netted a total of $44.9B in bids. This huge amount of money licenses the cellular providers to use specific bands of spectrum in fixed geographical locations so that they can support their customers.
The concept of fixed and predefined frequencies and modulation techniques dates back to the earliest days of radio. It is enshrined in our frequency allocations and has worked well for the last century.
Unfortunately, today the demand for spectrum is outstripping the finite supply. Different techniques are being used to alleviate the crunch. As an example, more efficient modulation such as orthogonal frequency-division multiple (OFDM) is being adopted. With OFDM and efficient digital compression schemes, a degree of rationalization has been applied to terrestrial television channels in many countries. The analog transmissions have been replaced by digital services with greatly reduced bandwidths and guard band requirements. The result is that some spectrum has been freed up for other applications.
Even with these efforts, there is insufficient spectrum available to meet the ballooning needs of services such as cellular. Consequently, the planning and research for the next generation cellular system is driving towards frequencies up to 60 GHz and beyond. Operation at these millimeter wavelengths will be far from easy and the range will be measured in feet (or meters) rather than the current cells where the wireless footprint covers whole districts.
This technical legacy has resulted in a significant underutilization of many parts of the radio spectrum. Some reports suggest that at any one time only 10% of the available spectrum is being used.
Cognitive Radio (CR) is a concept that promises to offer much higher utilization of the available spectrum. Briefly, a CR-enabled wireless would adapt to the current environment and use resources that are not being used. This simplified definition provides an insight into why CR has not already been adopted.
The ideal CR system would be able to monitor a very wide range of frequencies to identify which are being used and which are free at that moment in time.
It is obviously impractical to have a bank of receivers individually tuned to scan an entire frequency band. However, the latest semiconductor technology makes it possible to use a programmable receiver that can be swept across a range of frequencies. The LMS7002M from Lime Microsystems, for example, can be tuned anywhere from 100 kHz to 3800 MHz. It features dual transceivers that are fully programmable on the fly. The gain and bandwidth are also programmable, and each channel includes Received Signal Strength Indicators (RSSI) in both the RF and base band domains. This allows the CR to scan RF frequencies of interest and then “home in” on any potential free channel. The CR can boost the gain and narrow the bandwidth to obviate the possibility of mistakenly jumping onto an occupied frequency.
For two or more CR-enabled radios to establish a communication channel, they must have a method of agreeing what frequency, power levels, and modulation scheme to adopt (Figure 1). One technique is to use a base station to pass messages between the participants. The base station need only be used to exchange connection data, and then the CR radios switch to the agreed settings.
Figure 1. CR sets can continuously scan the spectrum