Distance estimation

1. A method, comprising:

transforming an orthogonal frequency division multiplexed signal, received at a receiver, in order to determine data from sub-carriers of the received orthogonal frequency division multiplexed signal, wherein the orthogonal frequency division multiplexed signal comprises phase shift keying symbols;

comparing data determined from a first sub-carrier of the orthogonal frequency division multiplexed signal with first reference data to determine a first phase difference between a phase of the first sub-carrier when transmitted and a phase of the first sub-carrier when received;

comparing data determined from a second sub-carrier of the orthogonal frequency division multiplexed signal with second reference data to determine a second phase difference between a phase of the second sub-carrier when transmitted and a phase of the second sub-carrier when received; and

using the determined first phase difference and the determined second phase difference to determine an average distance between the receiver and an origin of the orthogonal frequency division multiplexed signal,

wherein the average distance is an average of a first distance for the first sub-carrier of the orthogonal frequency division multiplexed signal and a second distance for the second sub-carrier of the orthogonal frequency division multiplexed signal.

2. The method as claimed in claim 1 , wherein the average distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using a multiple of a period of time and a correction to the multiple of the period of time.

3. The method as claimed in claim 2 , wherein the receiver is a transceiver, and the method further comprises the transceiver transmitting a further orthogonal frequency division multiplexed signal to a further transceiver, and, subsequently, the further transceiver transmitting the orthogonal frequency division multiplexed signal to the transceiver wherein the time taken by the further transceiver between receiving the further orthogonal frequency division multiplexed signal and transmitting the signal to the transceiver is a multiple of the period of time.

4. The method as claimed in claim 3 , wherein the average distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using a multiple of the period of time, the time taken by the further transceiver and the correction to the multiple of the period of time.

5. The method as claimed in claim 2 , wherein the determined first and second phase differences are used to estimate the correction to the multiple of the period of time.

6. The method as claimed in claim 1 , wherein the data determined from the first sub-carrier is a first symbol and the first reference data is determined by comparing the first symbol with a plurality of possible symbols to estimate which of the plurality of possible symbols is a best match for the first symbol; and wherein the first reference data and the first symbol are the same if no phase difference is determined.

7. The method as claimed in claim 1 , wherein the orthogonal frequency division multiplexed signal is transformed from the time domain to the frequency domain.

8. The method as claimed in claim 1 , wherein the receiver removes a multi-path interference effect from the orthogonal frequency division multiplexed signal.

9. The method as claimed in claim 1 , wherein the receiver is a transceiver, and the method further comprises the transceiver transmitting a further orthogonal frequency division multiplexed signal to a further transceiver, and, subsequently, the further transceiver transmitting the orthogonal frequency division multiplexed signal to the transceiver.

10. The method as claimed in claim 9 , wherein a multi-path interference effect is removed from the further orthogonal frequency division multiplexed signal by the further transceiver and subsequently the further orthogonal frequency division multiplexed signal is re-transmitted to the transceiver as the orthogonal frequency division multiplexed signal.

11. An apparatus, comprising:

transformation circuitry configured to transform an orthogonal frequency division multiplexed signal, received at a receiver, in order to determine data from sub-carriers of the received orthogonal frequency division multiplexed signal, wherein the orthogonal frequency division multiplexed signal comprises phase shift keying symbols;

a comparator configured to compare the data determined from a first sub-carrier of the orthogonal frequency division multiplexed signal with first reference data to determine a first phase difference between a phase of the first sub-carrier when transmitted and a phase of the first sub-carrier when received, and configured to compare the data determined from a second sub-carrier of the orthogonal frequency division multiplexed signal with second reference data to determine a second phase difference between a phase of the second sub-carrier when transmitted and a phase of the second sub-carrier when received; and

estimation circuitry configured to use the determined first phase difference and the determined second phase difference to determine an average distance between the apparatus and an origin of the orthogonal frequency division multiplexed signal,

wherein the average distance is an average of a first distance for the first sub-carrier of the orthogonal frequency division multiplexed signal and a second distance for the second sub-carrier of the orthogonal frequency division multiplexed signal.

12. The apparatus as claimed in claim 11 , wherein the estimation circuitry is configured to determine the average distance by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using a multiple of a period of time and a correction to the multiple of the period of time.

13. The apparatus as claimed in claim 12 , wherein the estimation circuitry is configured to use the determined first and second phase differences to estimate the correction to the multiple of the period of time.

14. The apparatus as claimed in claim 11 , wherein the data determined from the first sub-carrier is a first symbol; and wherein the comparator is configured to determine the first reference data by comparing the first symbol with a plurality of possible symbols to estimate which of the plurality of possible symbols is a best match for the first symbol; and wherein the first reference data and the first symbol are the same if no phase difference is determined.

15. The apparatus as claimed in claim 11 , wherein the transformer is configured to transform the orthogonal frequency division multiplexed signal from the time domain to the frequency domain.

16. A non-transitory computer-readable storage medium storing a computer program comprising computer program instructions that, when executed by at least one processor, cause an apparatus to perform at least the following:

transforming an orthogonal frequency division multiplexed signal, received at a receiver, in order to determine data from the sub carriers of the received orthogonal frequency division multiplexed signal, wherein the orthogonal frequency division multiplexed signal comprises phase shift keying symbols;

comparing the data determined from a first sub-carrier of the orthogonal frequency division multiplexed signal with first reference data to determine a first phase difference between a phase of the first sub-carrier when transmitted and a phase of the first sub-carrier when received;

comparing the data determined from a second sub-carrier of the orthogonal frequency division multiplexed signal with second reference data to determine a second phase difference between a phase of the second sub-carrier when transmitted and a phase of the second sub-carrier when received; and

using the determined first phase difference and the determined second phase difference to determine an average distance between the receiver and an origin of the orthogonal frequency division multiplexed signal,

wherein the average distance is an average of a first distance for the first sub-carrier of the orthogonal frequency division multiplexed signal and a second distance for the second sub-carrier of the orthogonal frequency division multiplexed signal.

17. The non-transitory computer-readable storage medium as claimed in claim 16 , wherein the average distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using a multiple of a period of time and a correction to the multiple of the period of time; and the determined first and second phase differences are used to estimate the correction to the multiple of the period of time.