Channel estimation and compensation techniques for use in frequency division multiplexed systems

• US 20030128656A1
• Filed: 01/07/2002
• Published: 07/10/2003
• Est. Priority Date: 01/07/2002
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Scarpa, Carl
• Granted Patent: US 7,209,433 B2
• Time in Patent Office: Days
• US Class Current: 370/203
• CPC Class Codes: H04L 25/0236 using estimation of the other symbols
• CPC Class Codes: H04L 25/025 using least-mean-square [LMS] method
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

Methods and apparatus for performing channel estimate updates in frequency division multiplexed, e.g., (OFDM), systems are described. After generation of initial channel estimates from received pilots, channel estimates corresponding to individual tones are updated using any one of a plurality of update techniques including, e.g., a constant modulus based method and a reduced constellation decision directed update method. The channel estimate update technique to be used with for an individual tone is selected based on a comparison of a signal noise measurement to one or more thresholds. The channel estimate update technique applied to different tones of the OFDM signal at the same time may vary. Over time, as the level of noise is reduced, the channel estimate update technique will switch from an interpolated pilot method, to a constant modulus algorithm based method, to a reduced constellation decision directed method, to a full constellation decision directed update method.

Claims

1. A method of processing a frequency division multiplexed signal including a plurality of tones, the method comprising:

receiving said frequency division multiplexed signal; and

performing a constant modulus based update operation to update a channel estimate corresponding to at least one tone of the frequency division multiplexed signal.

3. The method of claim 1, further comprising:

performing a reduced constellation decision directed update operation to update said channel estimate.

4. The method of claim 3, further comprising:

performing a full constellation decision directed update operation to update said channel estimate.

5. The method of claim 4, further comprising:

receiving, as part of said frequency division multiplexed signal, a pilot transmitted on said at least one tone; and

using said received pilot to update said channel estimate.

6. The method of claim 1, further comprising:

generating a signal noise measurement value for said at least one tone;

comparing the signal noise measurement value to a first threshold; and

selecting a channel estimate update method, as a function of the comparison of the signal noise measurement value to the first threshold, from a plurality of different channel estimation update methods.

7. The method of claim 6, wherein the plurality of different channel estimation update methods include at least one of a constant modulus based update method and an interpolated pilot value based method.

8. The method of claim 6, wherein the plurality of different channel estimation update methods include at least one of a reduced constellation decision directed update method and a full constellation decision directed update method.

9. The method of claim 6, further comprising:

when said comparison of the signal noise measurement value to the first threshold indicates that the signal noise measurement value does not exceed said first threshold,

comparing the signal noise measurement value to a second threshold; and

wherein the step of selecting a channel estimate update method is also performed as a function of the comparison of the signal noise measurement value to the second threshold.

10. The method of claim 9, wherein a reduced constellation decision directed channel estimate update method is selected when the comparison of the signal noise measurement value to the second threshold indicates that the signal noise measurement value exceeds the second threshold and wherein a full constellation decision directed channel estimate update method is selected when the comparison indicates that the signal noise measurement value is below the second threshold.

11. The method of claim 1, further comprising:

performing a decision directed channel estimate update operation to update a channel estimate corresponding to a second tone of the frequency division multiplexed signal at the same time said constant modulus based update operation is performed.

2. The method of claim 2, further comprising:

using the updated channel estimate to perform a channel compensation operation on a portion of the frequency division multiplexed signal corresponding to said at least one tone.

12. A method of updating channel estimates corresponding to different tones of an orthogonal frequency division multiplexed communications signal, the method comprising, for each of at least two tones of the communications signal, performing the steps of:

generating a signal noise measurement for the tone,

selecting a channel estimate update method for the tone, from a set of at least three different channel estimate update methods, based on a comparison of the generated signal to at least one threshold; and

updating a channel estimate for the tone using the selected channel estimate update method.

13. The method of claim 12, wherein the set of at least three different channel estimate update methods includes a reduced constellation decision directed update method.

14. The method of claim 13, wherein the set of at least three different channel estimate update methods further includes a constant modulus based channel estimate update method.

15. The method of claim 14, wherein the set of at least three different channel estimate update methods further includes a full constellation decision directed update method and an interpolated pilot based channel estimate update method.

16. The method of claim 12, wherein the set of at least three different channel estimate update methods includes a constant modulus based channel estimate update method.

17. The method of claim 16, further comprising the step of using the updated channel estimate generated for each of the tones to perform a channel compensation operation.

18. A method of updating a channel estimate for a carrier signal of an orthogonal frequency division multiplexed communications signal, the method comprising:

receiving the carrier signal; and

performing a reduced constellation decision directed channel estimate update operation, using the received carrier signal, to update said channel estimate.

19. The method of claim 18, further comprising:

after performing said reduced constellation decision directed channel estimate update operation performing a full constellation decision directed channel estimate update operation.

20. The method of claim 19, further comprising:

generating a signal noise measurement;

comparing the signal noise measurement to a threshold; and

using the results of the comparison to determine when to switch from performing said reduced constellation decision directed channel estimate update operation to performing the full constellation decision directed channel estimate update operation.

21. A method of updating a channel estimates for carrier signals of an orthogonal frequency division multiplexed communications signal, the method comprising:

receiving the carrier signals; and

performing a reduced decision directed channel estimate update operation, for at least a first plurality of the received carrier signals of said orthogonal frequency division multiplexed communications signal.

22. The method of claim 21, further comprising:

comparing a signal noise value to a threshold; and

selecting for at least one of said received carrier signals, as a function of said comparison, between performing a decision directed channel estimate update operation and performing a constant modulus based channel estimate update operation.

23. The method of claim 22, wherein a constant modulus based channel estimate update operation is performed for one carrier signal at the same time a reduced decision directed channel estimate update operation is performed for another carrier signal.

24. A method of updating first and second channel estimates corresponding to a first and a second carrier frequency of an orthogonal frequency division multiplexed signal, the method comprising:

generating first and second signal noise measurements for the first and second carrier frequencies, respectively;

independently comparing each of the first and second signal noise measurements to at least one noise threshold to independently select a channel estimate update method to be used to update the first and second channel estimates, respectively the channel estimate update methods including at least one of an amplitude only update method and a reduced constellation decision directed update method.

25. The method of claim 24, wherein the first and second signal noise measurements are signal to noise ratio measurements and where the first threshold is a first signal to noise ratio threshold

26. A receiver apparatus for receiving and processing an orthogonal frequency division multiplexed signal, the apparatus comprising:

a carrier recovery module for performing a carrier recovery operation on the multiplexed signal;

a channel compensation module coupled to the carrier recovery circuit for performing channel compensation operation on each tone of the orthogonal frequency division multiplexed signal;

a signal noise measurement module for generating, for each tone, a signal noise measurement; and

a channel estimate update selection module for selecting between an amplitude only channel estimate update method and an amplitude and phase channel estimate update method, for each individual tone, as a function of the signal noise measurement generated by said signal noise measurement module for the individual tone.

27. The apparatus of claim 26, further comprising:

means for generating an updated channel estimate for each tone of the multiplexed signal as a function of the selected channel estimate update method, coupled to said channel compensation module and said channel estimate update selection module.

28. The apparatus of claim 27, wherein the amplitude only channel estimate update method is a constant modulus based channel estimate update method.

29. The apparatus of claim 28, wherein the amplitude and phase channel estimate update method is a reduced constellation decision directed update method.

30. An apparatus for updating channel estimates in a frequency division multiplexed receiver, the apparatus comprising:

a plurality of channel estimate update modules, said plurality of channel estimate update modules including;

i) a constant modulus channel estimate update module for performing a channel estimate update for a tone of a frequency division multiplexed signal using a constant modulus algorithm; and

ii) a full decision directed channel estimate update module for performing a full decision directed channel estimate update for a tone of a frequency division multiplexed signal; and

a control module for selecting, as a function of a signal measurement, one of said plurality of channel estimate update modules to be used for performing a channel estimate update operation.

31. The apparatus of claim 30,

wherein said signal measurement is a signal noise measurement, the apparatus further comprising a signal noise measurement module coupled to said control module;

and

wherein said plurality of channel estimate update modules further includes a reduced decision directed channel estimate update module.