Cognitive radios have recently emerged as a new paradigm to build intelligent and spectrum-aware radio systems. The current approach with cognitive radios is to create spectrum awareness so that radio transceivers can move on to free or unused spectrum bands. The Federal Communications Commission (FCC), which is the entity that regulates spectrum usage in the U.S., is considering opening some radio and television frequencies for this sort of use.
Additionally, specific projects by the Defense Advanced Research Projects Agency (DARPA) for example, aim at extending this paradigm by defining spectrum usage policy languages (XG-project) The Institute of Electrical & Electronics Engineers (IEEE) has started work on the necessary standardization towards the rapid deployment of spectrum agile systems.
To maximize the benefit from spectrum agility, a system should be able to use very wide spectrum areas. Of course, wide spectrum is intended to mean that cognitive radio should be able to use spectrum from a large selection of bands, while the transmission itself might be narrowband.
A problem arising with cognitive radio lies in that cost-effectively scanning a wideband radio-spectrum in (near) real-time fashion is far from easy. One possible approach would involve using smart antennas with extremely wideband A/D-converters, and the supporting radio frequency circuitry. Technology adapted to support this approach is not available at the moment. Even roadmaps indicate that technology possibly made available in the future will entail a power-consumption budget quite unlikely to lead to commercially viable approaches. A possible exception may be certain military systems where very high power budgets are available.
Another approach, based on the use of several narrowband receivers, is to inefficient, with increased costs and power-consumption. Wideband scanning of frequencies (over several GHz) by using, e.g., an OFDM (Orthogonal Frequency Division Multiplexing) narrowband receiver will not lead to a scanning time fast enough, and could also be very expensive in terms of a power budget.
In fact, the prior art includes a variety of circuits and arrangements adapted to operate on very wide frequency ranges. For instance, U.S. Pat. No. 3,662,316 describes a pulse receiver for detecting short baseband electromagnetic pulses employing a dispersion-less, broadband transmission line antenna. The arrangement includes a circuit operating with a biased semiconductor diode located within the transmission line for instantaneously detecting substantially the total energy of the baseband pulse, and providing a corresponding output adapted for application in conventional utilization circuits.
As a further example, U.S. Pat. No. 5,345,471 describes an ultra-wideband (UWB) receiver utilizing a strobed input line with a sampler connected to an amplifier. In a differential configuration, +/−UWB inputs are connected to separate antennas or two halves of a dipole antenna. The two input lines include samplers which are commonly strobed by a gating pulse with a very low duty cycle. In a single ended configuration, only a single strobed input line and sampler is utilized. The samplers integrate, or average, up to 10,000 pulses to achieve high sensitivity and good rejection of uncorrelated signals.
By way of still a further example, U.S. Pat. No. 5,677,927 describes an impulse radio communications system using one or more subcarriers to communicate information from an impulse radio transmitter to an impulse radio receiver. The impulse radio communication system is an ultra-wideband (UWB) time domain system. The use of subcarriers provides impulse radio transmissions added channelization, smoothing and fidelity. Subcarriers of different frequencies or waveforms can be used to add channelization of impulse radio signals. Thus, an impulse radio link can communicate on many independent channels simultaneously by employing different subcarriers for each channel.
The impulse radio uses modulated subcarriers for time positioning a periodic timing signal or a coded timing signal. Alternatively, the coded timing signal can be summed or mixed with the modulated subcarriers and the resultant signal is used to time modulate the periodic timing signal. Direct digital modulation of data is another form of subcarrier modulation for impulse radio signals. Direct digital modulation can be used alone to time modulate the periodic timing signal or the direct digitally modulated periodic timing signal can be further modulated with one or more modulated subcarrier signals. Linearization of a time modulator permits the impulse radio transmitter and receiver to generate time delays having the necessary accuracy for impulse radio communications.