Symbol alignment method

• US 6,353,636 B1
• Filed: 12/23/1998
• Issued: 03/05/2002
• Est. Priority Date: 12/23/1998
• Status: Expired due to Term

• Current Assignee: RPX Clearinghouse LLC (RPX Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Humphrey, Leslie Derek, Wallace, Andrew David, Tate, Christopher Neville
• Primary Examiner(s): Pham, Chi
• Assistant Examiner(s): PHU, PHUONG M
• Application Number:
  US09/219,005
• Time in Patent Office: 1,168 Days
• Field of Search: 375/260, 375/229, 375/331, 375/354, 375/355, 375/230, 370/503
• US Class Current: 375/260
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2679 Decision-aided

Abstract

A method, and related apparatus, for determining symbol alignment in a discrete multi-tone communications system comprising the steps of: receiving a signal on each of at least one selected carrier frequency; for each of a plurality of receiver window alignments with respect to said signals, deriving a measure of received signal power level on each said selected carrier frequency; for each said receiver window alignment and selected carrier frequency deriving a measure of spectral leakage on at least one adjacent carrier frequency; selecting a receiver window alignment responsive to said derived measures.

Claims

1. A method of determining symbol alignment in a discrete multi-tone communications system comprising the steps of:

receiving a signal on each of at least one selected carrier frequency;

for each of a plurality of receiver window alignments with respect to said signals, deriving a measure of received signal power level on each said selected carrier frequency;

for each said receiver window alignment and selected carrier frequency deriving a measure of spectral leakage on at least one adjacent carrier frequency;

selecting a receiver window alignment responsive to said derived measures.

2. A method according to claim 1 wherein said at least one adjacent carrier frequency is a silent carrier frequency.

3. A method according to claim 1 wherein silent carrier power levels are at least in the order of 30 dB lower than active carrier power levels.

4. A method according to claim 1 wherein said selection of receiver window alignment is responsive to the difference between the received signal power level on said selected carrier frequency and a multiple of said received signal power level on said adjacent carrier frequencies.

5. A method according to claim 4 wherein said multiple is in the order of 50.

6. A method according to claim 1 wherein each said measure of received signal power levels is a time average of a plurality of measures of received signal power levels.

7. A method according to claim 1 wherein said selected receiver window alignment is one of said set of receiver window alignments.

8. A method according to claim 1 wherein said selected receiver alignment window is interpolated between alignments in said set of receiver window alignments.

9. A method according to claim 1 wherein said carrier frequencies are in a frequency band 0-10 MHz.

10. A method according to claim 1 wherein successive selected carrier frequencies are separated by a plurality of silent carrier frequencies.

11. A method according to claim 1 wherein selected carrier frequencies are distributed across a frequency band utilised by said discrete multi-tone communications system.

12. A method according to claim 2 wherein the step of selecting a receiver window alignment responsive to said derived measures comprises the steps of:

identifying which receiver window alignment gives rise to the largest difference;

searching backwards and forwards to identify receiver window alignments at which said differences drop below a given fraction of said largest difference;

selecting an optimal receiver window alignment between said receiver window alignments at which said differences drop below a given fraction of said largest difference.

13. A method according to claim 12 wherein said receiver window alignment between said receiver window alignments is chosen to be approximately mid-way between said receiver window alignments at which said differences drop below a given fraction of said largest difference.

14. A method according to claim 12 wherein said fraction is approximately nine tenths.

15. A method according to claim 1 wherein said signal forms part of a TDD communications channel comprising a succession of frames, each frame comprising a receive region and each comprising a succession of DMT symbols.

16. A method according to claim 15 wherein said step of deriving measures of received signal power level on each of said selected carrier frequencies is performed on a single symbol in each said receive region.

17. A method of according to claim 1 wherein the step of comprising the steps of selecting a receiver window alignment responsive to said derived measures comprises the steps of:

selecting a narrower range of receiver window alignments responsive to said derived measures;

selecting, within said narrower range, a second range of receiver window alignments with respect to said signals;

for each said receiver window alignment in said second range deriving a further measure of received signal power level on each said selected carrier frequency;

for each said receiver window alignment in said second range and selected carrier frequency deriving a further measure of spectral leakage on at least one adjacent carrier frequency;

selecting a receiver window alignment responsive to said further derived measures.

22. A telecommunications network comprising a signal receiver according to claim 16.

18. A signal receiver arranged to receive discrete multi-tone signals and comprising:

a signal input arranged to receive a signal on each of at least one selected carrier frequency;

a processor arranged to select a plurality of receiver window alignments with respect to said signals, for each said receiver window alignment to derive a measure of received signal power level on each said selected carrier frequency, for each said receiver window alignment and selected carrier frequency to derive a measure of spectral leakage on at least one adjacent carrier frequency and to select a receiver window alignment responsive to said derived measures.

19. A discrete multi-tone system comprising:

a receiver according to claim 18;

a transmitter having a signal output and arranged to provide said signals and to remain silent on said adjacent carrier frequencies; and

a transmission medium arranged to convey discrete multi-tone signals from said output port to said input port.

20. A TDD communications system comprising a signal receiver according to claim 18.

21. A VDSL discrete multi-tone modem comprising a signal receiver according to claim 18.

23. A program for a computer on a machine readable medium arranged to perform the steps of:

receiving a signal on each of at least one carrier frequency;

applying each of a plurality of receiver window alignments to said signals;

deriving a measure of each said receiver window alignment as applied to each said signal;

for each said receiver window alignment deriving a measure of spectral leakage on a carrier frequency adjacent at least one of said at least one carrier frequencies;

selecting a receiver window alignment responsive to said derived measures.