Techniques for estimating received signal strength and carrier to interference and noise ratio in OFDM systems

1. A method for determining a received signal strength indicator of a target base station at a subscriber station, comprising:

performing a discrete Fourier transform on a received signal to provide a transformed signal, the transformed signal occupying one or more segments of a channel, the one or more segments each including multiple subcarriers; and

calculating an shift-n cross-correlation based on the transformed signal and a preamble sequence of the target base station to provide a coarse received signal strength indicator for the target base station.

2. The method of claim 1 , wherein the shift-n cross-correlation is a shift-one cross-correlation and the calculating further comprises:

dividing the transformed signal by the preamble sequence of the target base station to obtain a referenced signal; and

calculating the shift-one cross-correlation between the referenced signal and a replica of the referenced signal to provide the coarse received signal strength indicator of the target base station, wherein the replica of the referenced signal corresponds to the reference signal shifted by one.

3. The method of claim 1 , wherein the shift-n cross-correlation is a shift-one cross-correlation and the calculating further comprises:

obtaining a first shift-one autocorrelation vector of the transformed signal by multiplying the transformed signal at one subcarrier with the transformed signal at a next subcarrier;

obtaining a second shift-one autocorrelation vector of the preamble sequence by multiplying an inverse of a preamble symbol at one subcarrier with an inverse of another preamble symbol at a next subcarrier; and

calculating cross-correlation of the first and second shift-one autocorrelation vectors.

4. The method of claim 1 , wherein the calculating further comprises:

averaging values of the coarse received signal strength indicator obtained over a plurality of frames.

5. The method of claim 1 , further comprising:

calculating a shift-one cross-correlation based on the transformed signal and a preamble sequence of an interferer base station to obtain a coarse received signal strength indicator for the interferer base station; and

adjusting the coarse received signal strength indicators of the target and interferer base stations by compensating for bias due to preamble correlation.

6. The method of claim 5 , wherein the adjusting the coarse received signal strength indicators further comprises:

obtaining cross-correlation factors of a first shift-one autocorrelation vector of the preamble sequence of the target base station and a second shift-one autocorrelation vector of the preamble sequence of the interferer base station; and

calculating a bias compensated received signal strength indicator for the target base station based on the coarse received signal strength indicators of the target and interferer base stations and the cross-correlation factors.

7. The method of claim 5 , further comprising:

obtaining coarse received signal strength indicators for a plurality of non-target base stations; and

selecting a set of the non-target base stations based on the coarse received signal strength indicators.

8. A method for determining carrier to interference and noise ratio of a target base station at a subscriber station, comprising:

performing a discrete Fourier transform on a received signal to provide a transformed signal, the transformed signal occupying one or more segments of a channel, the one or more segments each including multiple subcarriers;

calculating a received signal strength indicator for the target base station based on a shift-one cross-correlation of the transformed signal and a preamble sequence of the target base station;

calculating respective received signal strength indicators for one or more interferer base stations on the one or more segments based on a shift-one cross-correlation of the transformed signal and preamble sequences of the one or more interferer base stations;

determining a noise power estimate from a single one of the one or more segments; and

determining the carrier to interference and noise ratio for the target base station based on the received signal strength indicators for the target base station and the one or more interferer base stations and the noise power estimate.

9. The method of claim 8 , wherein the determining a noise power estimate further comprises:

estimating frame-averaged total powers of the received signal on each of the one or more segments over a plurality of frames;

obtaining frame-averaged received signal strength indicators for one or more base stations on a single one of the one or more segments with a lowest estimated average total power; and

subtracting from the frame-averaged total power on the single one of the one or more segments with the lowest estimated average total power a summation of the frame-averaged received signal strength indicators of all of the one or more base stations on the single one of the one or more segments with the lowest estimated average total power to obtain the noise power estimate.

10. The method of claim 9 , wherein the obtaining frame-averaged received signal strength indicators further comprises:

estimating frame-averaged received signal strength indicators for a plurality of base stations; and

selecting one or more strongest base stations from the plurality of base stations based on values of the frame-average received signal strength indicators.

11. The method of claim 8 , wherein the determining a noise power estimate further comprises:

averaging instantaneous noise power estimates obtained over a plurality of frames.

12. The method of claim 8 , wherein the determining a carrier to interference and noise ratio further comprises:

weighting the noise power estimate based on a transmission power boosting factor of the preamble sequence relative to non-preamble regular data subcarriers.

13. The method of claim 8 , wherein the determining a carrier to interference and noise ratio further comprises:

summing the received signal strength indicators of the one or more interferer base stations either on the same segment as the target base station or on all the segments according to a reuse scenario;

determining whether summed interference power is greater than the noise power estimate to determine interference or noise limited scenario;

determining a total interference and noise power estimate based on the determined interference or noise limited scenario; and

computing the carrier to interference and noise ratio for the target base station as a ratio between the received signal strength indicator of the target base station and the total interference and noise power estimate.

14. The method in claim 13 , wherein the determining a total interference and noise power estimate further comprises:

computing a combined noise plus interference power on the segment used by the target base station when the summed interference power is greater than the noise power estimate;

determining, for the interference limited scenario, a total interference power based on the combined noise plus interference power on the segment used by the target base station, the received signal strength indicators of the one or more interferer base stations on the one or more segments, and the noise power estimate; and

determining, for the noise limited scenario, the total interference power based on the received signal strength indicators of the one or more interferer base stations on the one or more segments and the noise power estimate.

15. A communication system, comprising:

a target base station; and

a subscriber station in communication with the target base station, wherein the subscriber station is configured to:

perform a discrete Fourier transform on a received signal to provide a transformed signal, the transformed signal occupying one or more segments of a channel, the one or more segments each including multiple subcarriers; and

calculate an shift-n cross-correlation based on the transformed signal and a preamble sequence of the target base station to provide a coarse received signal strength indicator for the target base station.

16. The communication system of claim 15 , wherein the shift-n cross-correlation is a shift-one cross-correlation and the subscriber station calculates the shift-one cross-correlation by:

dividing the transformed signal by the preamble sequence of the target base station to obtain a referenced signal; and

calculating the shift-one cross-correlation between the referenced signal and a replica of the referenced signal to provide the coarse received signal strength indicator of the target base station, wherein the replica of the referenced signal corresponds to the reference signal shifted by one.

17. The communication system of claim 15 , wherein the shift-n cross-correlation is a shift-one cross-correlation and the subscriber station calculates the shift-one cross-correlation by:

obtaining a first shift-one autocorrelation vector of the transformed signal by multiplying the transformed signal at one subcarrier with the transformed signal at a next subcarrier;

obtaining a second shift-one autocorrelation vector of the preamble sequence by multiplying an inverse of a preamble symbol at one subcarrier with an inverse of another preamble symbol at a next subcarrier; and

calculating cross-correlation of the first and second shift-one autocorrelation vectors.

18. The communication system of claim 15 , wherein the subscriber station calculates the shift-one cross-correlation by:

averaging values of the coarse received signal strength indicator obtained over a plurality of frames.

19. The communication system of claim 15 , wherein the subscriber station is further configured to:

calculate a shift-one cross-correlation based on the transformed signal and a preamble sequence of an interferer base station to obtain a coarse received signal strength indicator for the interferer base station; and

adjust the coarse received signal strength indicators of the target and interferer base stations by compensating for bias due to preamble correlation.

20. The communication system of claim 19 , wherein the subscriber station adjusts the coarse received signal strength indicators by:

obtaining cross-correlation factors of a first shift-one autocorrelation vector of the preamble sequence of the target base station and a second shift-one autocorrelation vector of the preamble sequence of the interferer base station; and

calculating a bias compensated received signal strength indicator for the target base station based on the coarse received signal strength indicators of the target and interferer base stations and the cross-correlation factors.