Proximity determination using radio devices

1. A radio system comprising a radio receiver and a processing system, wherein:

the radio receiver is configured to detect radio signals transmitted from a radio transmitter on a plurality of frequency channels, and to measure respective signal strengths of the radio signals for each of the plurality of frequency channels; and

the processing system is configured:

to evaluate a measure of statistical dispersion of the respective signal strengths over the plurality of frequency channels, and

to use an average of the respective signal strengths of radio signals in the plurality of frequency channels, in combination with the measure of statistical dispersion to determine information relating to a proximity of the radio transmitter to the radio receiver, said information comprising a separation metric that increases with increasing geometrical distance between the radio transmitter and the radio receiver and decreases monotonically with decreasing statistical dispersion for at least some average signal strengths.

2. The radio system of claim 1 , wherein the processing system is configured to apply a weight to the average of the respective signal strengths, when determining the information relating to the proximity of the radio transmitter to the radio receiver, wherein the weight depends on the measure of statistical dispersion.

3. The radio system of claim 1 , wherein the processing system is configured to determine a separation metric that decreases monotonically with decreasing statistical dispersion for some or all values of average signal strength, and that decreases monotonically with increasing average signal strength for some or all values of statistical dispersion.

4. The radio system of claim 1 , wherein the processing system is configured to evaluate a measure of change in the measure of statistical dispersion, over time, when determining the information relating to the proximity of the radio transmitter to the radio receiver.

5. The radio system of claim 1 , wherein the radio receiver is configured to transmit data to or receive data from the radio transmitter in response to the information relating to the proximity of the radio transmitter to the radio receiver satisfying a proximity criterion.

6. The radio system of claim 5 , wherein said data relates to a payment.

7. The radio system of claim 5 , wherein the radio receiver comprises an antenna that the radio receiver is configured to use both for receiving the aforesaid radio signals on the plurality of frequency channels and for transmitting or receiving said data.

8. The radio system of claim 1 , wherein the measure of statistical dispersion is equal to, or representative of, variance or standard deviation or spread.

9. The radio system of claim 1 , further comprising at least one sensor which is configured to provide additional sensor data, wherein the radio system is configured to utilise said additional sensor data when generating said information relating to proximity.

10. The radio system of claim 9 , wherein the at least one sensor comprises an accelerometer, and the additional sensor data comprises gesture information.

11. The radio system of claim 1 , wherein the radio system comprises the radio transmitter.

12. The radio system of claim 1 , wherein the processing system comprises a processor and a memory storing software instructions for execution on said processor, wherein the processor is at least partially provided as part of the radio receiver or the radio transmitter or a smart card or a remote server, or is included with the radio receiver in a receiver device, or is included with the radio transmitter in a transmitter device.

13. The radio system of claim 1 , wherein:

the radio receiver is configured to detect radio signals on a plurality of frequency channels from a plurality of radio transmitters;

the radio receiver is configured to measure a respective signal strength of each of the radio signals; and

the processing system is configured to evaluate, for each radio transmitter, a measure of statistical dispersion of the respective signal strengths over the respective plurality of frequency channels, and to use the measures of statistical dispersion to identify a closest radio transmitter of the plurality of radio transmitters to the radio receiver.

14. The radio system of claim 1 , further comprising a plurality of radio receivers, wherein:

each of the radio receivers is configured to detect the radio signals from the radio transmitter;

each of the radio receivers is configured to measure respective signal strengths of the radio signals for each of the plurality of frequency channels; and

the processing system is configured to evaluate, for each radio receiver, a measure of statistical dispersion of the respective signal strengths over the plurality of frequency channels, and to use the measures of statistical dispersion to identify a closest radio receiver of the plurality of radio receiver to the radio transmitter.

15. A method of controlling a radio system, the method comprising:

a radio receiver detecting radio signals transmitted from a radio transmitter on a plurality of frequency channels;

measuring respective signal strengths of the radio signals for each of the plurality of frequency channels; and

evaluating a measure of statistical dispersion of the respective signal strengths over the plurality of frequency channels, and using the measure of statistical dispersion to generate information relating to a proximity of the radio transmitter to the radio receiver, said information comprising a separation metric that increases with increasing geometrical distance between the radio transmitter and the radio receiver, such that the separation metric decreases monotonically with decreasing statistical dispersion for at least some average signal strengths.

16. The method of claim 15 , further comprising:

the radio receiver detecting radio signals from a plurality of radio transmitters;

measuring a respective signal strength of each of the radio signals; and

evaluating, for each radio transmitter, a respective measure of statistical dispersion of the signal strengths over the respective plurality of frequency channels, and using the measures of statistical dispersion to identify a closest radio transmitter of the plurality of radio transmitters to the radio receiver.