Method and system for selecting cyclic prefix length based on signal quality reports

• US 8,422,577 B1
• Filed: 03/25/2011
• Issued: 04/16/2013
• Est. Priority Date: 03/25/2011
• Status: Active Grant

• Current Assignee: Sprint Spectrum LP (Deutsche Telekom AG)
• Original Assignee: Sprint Spectrum LP (Deutsche Telekom AG)
• Inventors: Shetty, Manoj, Vargantwar, Sachin R., Oroskar, Siddharth S., Rai, Deveshkumar N.
• Primary Examiner(s): ODOM, CURTIS B
• Application Number:
  US13/072,420
• Time in Patent Office: 753 Days
• Field of Search: 375/260, 375/267, 375/299, 370203-211
• US Class Current: 375/260
• CPC Class Codes: H04L 1/0006 by adapting the transmission format
• CPC Class Codes: H04L 1/0026 Transmission of channel quality indication
• CPC Class Codes: H04L 27/2607 Cyclic extensions
• CPC Class Codes: H04L 27/2646 using feedback from receiver for adjusting OFDM transmission parameters, e.g. transmission timing or guard interval length

Abstract

During a first time interval, an access point transmits orthogonal frequency division multiplexing (OFDM) signals using a first cyclic prefix length. The access point receives a plurality of signal reports, such as Channel Quality Indicators (CQIs), from user devices being served by the access point. Based on the signal reports, the access point determines a distribution of signal qualities experienced by the user devices. The access point selects a second cyclic prefix length based, at least in part, on the distribution of signal qualities. The access point transmits OFDM signals using the second cyclic prefix length during a second time interval.

Claims

1. A method of adjusting a length of a cyclic prefix used in orthogonal frequency division multiplexing (OFDM) communications, comprising:

transmitting, through an access point, OFDM signals using a first cyclic prefix length;

receiving a first plurality of signal quality reports from user devices being served by said access point, wherein each of said signal quality reports indicates a reported signal quality value;

determining a distribution of reported signal quality values based on said first plurality of signal quality reports;

selecting a second cyclic prefix length based, at least in part, on said first plurality of signal quality reports, wherein selecting said second cyclic prefix length based, at least in part, on said signal quality reports comprises selecting said second cyclic prefix length based, at least in part, on said distribution of reported signal quality values; and

transmitting, through said access point, OFDM signals using said second cyclic prefix length.

2. The method of claim 1, wherein said distribution of reported signal quality values includes a plurality of signal quality values, Q_i, ranging from a lowest signal quality value, Q_l, to a highest signal quality value, Q_M.

3. The method of claim 2, wherein said distribution of reported signal quality values includes for each signal quality value, Q_i, a corresponding number of reports, R_i, of that signal quality value.

4. The method of claim 3, wherein said distribution of reported signal quality values includes a total number of reports, N, wherein N=Σ

R_i.

5. The method of claim 4, wherein selecting said second cyclic prefix length based, at least in part, on said distribution of reported signal quality values comprises:

determining a mean signal quality value in said distribution of reported signal quality values; and

selecting said second cyclic prefix length based, at least in part, on said mean signal quality value.

6. The method of claim 4, wherein selecting said second cyclic prefix length based, at least in part, on said distribution of reported signal quality values comprises:

determining a threshold signal quality value, Q_T, such that the number of reports with signal quality values ranging from Q_T to Q_M in the distribution is at least a predetermined percentage of said total number of reports; and

selecting said second cyclic prefix length based, at least in part, on said threshold signal quality value, Q_T.

7. The method of claim 1, wherein said signal quality reports comprise Channel Quality Indicators (CQIs).

8. The method of claim 1, wherein said first cyclic prefix length is used during a first time interval and said second cyclic prefix length is used during a second time interval, and wherein at least a portion of said first plurality of signal quality reports is received during said first time interval.

9. The method of claim 8, further comprising:

receiving a second plurality of signal quality reports, wherein at least a portion of said second plurality of signal quality reports is received during said second time interval; and

selecting a third cyclic prefix length based, at least in part, on said second plurality of signal quality reports; and

transmitting, through said access point, OFDM signals using said third cyclic prefix length, wherein said third cyclic prefix is used during a third time interval.

10. The method of claim 1, wherein said second cyclic prefix length differs from said first cyclic prefix length.

11. A method of adjusting a length of a cyclic prefix used in orthogonal frequency division multiplexing (OFDM) communications, comprising:

during a first time interval, an access point transmitting OFDM signals using a first cyclic prefix length;

said access point receiving a plurality of signal quality reports from a plurality of user devices being served by said access point, wherein each signal quality report indicates one of M preset signal quality values ranging from Q_l to Q_M;

for each signal quality value Q_i from Q_l to Q_M, determining a number of times, R_i, that said signal quality value, Q_i, was indicated in said plurality of signal quality reports, to provide a distribution of signal quality values;

selecting a second cyclic prefix length based, at least in part, on said distribution of signal quality values; and

during a second time interval, said access point transmitting OFDM signals using said second cyclic prefix length.

12. The method of claim 11, wherein said plurality of signal quality reports is received during said first time interval.

13. The method of claim 11, wherein selecting said second cyclic prefix length based, at least in part, on said distribution of signal quality values comprises:

referring to a mapping between at least a subset of said M preset signal quality values and a plurality of preset cyclic prefix lengths; and

selecting said second cyclic prefix length from among said preset cyclic prefix lengths, such that said second cyclic prefix length is adequate, according to said mapping, for at least a predetermined percentage of signal quality values in said distribution of signal quality values.

14. The method of claim 11, wherein said signal quality reports comprise Channel Quality Indicators (CQIs), wherein M=16, and wherein said M preset signal quality values range from 0 to 15.

15. The method of claim 11, wherein said second cyclic prefix length differs from said first cyclic prefix length.

16. A system, comprising:

a transmitter for transmitting OFDM signals to user devices, wherein said OFDM signals include a cyclic prefix with a cyclic prefix length;

a receiver for receiving signal quality reports from said user devices; and

a controller coupled to said transmitter and said receiver, wherein said controller is configured to adjust said cyclic prefix length periodically based, at least in part, on said signal quality reports by a process comprising;

(i) controlling said transmitter to use a first cyclic prefix length during a first time interval;

(ii) determining a distribution of signal qualities experienced by said user devices based on signal quality reports received by said receiver;

(iii) selecting a second cyclic prefix length based, at least in part, on said distribution; and

(iv) controlling said transmitter to use said second cyclic prefix length during a second time interval.

17. The system of claim 16, wherein said second cyclic prefix length differs from said first cyclic prefix length.

18. The system of claim 16, wherein said transmitter, receiver, and controller are in a base station of a wireless telecommunications network.

19. The system of claim 16, wherein said signal quality reports comprise Channel Quality Indicators (CQIs).