Channel tracking in an OFDM wireless receiver

• US 7,821,915 B2
• Filed: 05/30/2008
• Issued: 10/26/2010
• Est. Priority Date: 08/12/2002
• Status: Expired due to Term

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Dankoski, Paul C. P., Hart, Brian, Perahia, Eldad, Pignatelli, David J.
• Primary Examiner(s): Trost, IV; William
• Assistant Examiner(s): Kasparek; Kyle C
• Application Number:
  US12/130,368
• Publication Number:
  US 20080232497A1
• Time in Patent Office: 879 Days
• Field of Search: 370/208, 370/209, 370/210, 370/480, 375/235
• US Class Current: 370/208
• CPC Class Codes: H04L 25/0228 with direct estimation from sounding signals
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

An apparatus and a method to track the channel response for subcarriers in an OFDM receiver for a packet of information that includes a known transmitted part. The receiver has a signal-to-tone transformer to determine subcarriers corresponding to a received packet. The method stores a function of an initial estimate of the channel response for a subcarrier, and accepts a pre-decision constellation point value that is a post channel correction using a first estimate of the channel response. The method includes: makes a decision using the pre-decision constellation point value, re-modulates the decision to form a post-decision constellation point value, and forms a complex valued product of the function of the first estimate for the subcarrier and the complex-valued ratio of the pre-decision and post-decision values. This complex valued product forms a measure of the current channel estimate to use for updating the stored quantity.

Claims

1. An apparatus comprising:

a receiver configured to wirelessly receive a packet of information that includes a known transmitted part and to digitize the received signal, the known transmitted part having subcarriers with known values;

a signal-to-tone transformer to determine the modulated subcarriers of a received signal corresponding to the received packet;

a channel storage unit coupled to the signal-to-tone transformer including;

an initial channel calculator to calculate a function of the initial channel response of subcarriers from the known part of the received packet;

a memory subsystem configured to at a first time store a function of an initial channel response corresponding to the known part of the received packet, and at later times to store additionally or instead updates to or of previous version or versions of the channel response;

a channel corrector coupled to the channel storage unit and to the signal-to-tone transformer configured to correct the modulated subcarriers of the received signal for the respective channel using the contents of the memory subsystem;

the channel corrector configured to form channel corrected constellation values for the received signal;

a decision and channel drift circuit coupled to the channel corrector to form post-decision constellation values corresponding to the channel corrected constellation values and to form a measure of the channel drift from the post decision constellation values and the pre-decision constellation values; and

an updating circuit coupled to the channel storage unit to update the memory subsystem with an updated value determined from previous contents of the memory subsystem and an update, the update determined from the memory subsystem contents and the measure of the channel drift between symbols,

such that drift of the channel response is tracked.

2. An apparatus as recited in claim 1, wherein the updating circuit includes a weighting and summing circuit coupled to the memory subsystem to update the memory subsystem with a weighted sum of the pre-update function in the memory subsystem and an update, the update determined from the pre-update function and the measure of the channel drift between symbols, such that the memory subsystem contents track channel drift from symbol to symbol, the tracking according to the relative weightings of the weighted sum.

3. An apparatus as recited in claim 1, wherein the function of the initial channel response initially stored in the memory subsystem for a subcarrier is the initial channel response for the subcarrier multiplied by the known value of the subcarrier for the known transmitted part, and wherein the apparatus is also configured to factor out the known transmitted part prior to or concurrent with the correcting for the respective channel.

4. An apparatus as recited in claim 1, wherein the function of the channel response stored in the memory subsystem for a subcarrier is post factoring by the known value of the subcarrier for the known part, such that a factoring of the known part is included prior to the input from the signal-to-tone transformer to the memory subsystem.

5. An apparatus as recited in claim 1, wherein the signal-to-tone transformer produces subcarriers in rectangular coordinates, and the memory subsystem stores the function of the channel response in rectangular coordinates, OR

wherein the signal-to-tone transformer produces subcarriers in rectangular coordinates, and wherein the memory subsystem stores the function of the channel response in polar coordinates.

6. An apparatus as recited in claim 1, wherein the decision circuit forms a hard decision.

7. An apparatus as recited in claim 1, wherein the decision circuit produces re-encoded decoded decisions.

8. An apparatus as recited in claim 1, wherein the subcarriers of the packet include pilot tones, the apparatus further comprising a pilot correction circuit between the channel corrector and the decision and channel drift circuit, the pilot correction circuit to correct for one or both of channel drift and timing errors.

9. An apparatus as recited in claim 1, further comprising a circuit configured to determine a measure of the signal quality of a predetermines part of a packet received by the receiver, and wherein the updating by the update takes into account the measure of the error vector magnitude.

10. An apparatus as recited in claim 1, further comprising a circuit configured to determine a measure of the error vector magnitude of a predetermines part of a packet received by the receiver,

wherein the updating by the update takes into account the measure of the error vector magnitude.

11. An apparatus as recited in claim 1, wherein the known part of the packet includes a first known part of at least one known symbol and a second known part of at least one known symbol, and wherein the initial channel calculator initially stores a function of the channel response based on received signals corresponding to the first part, and wherein initial channel calculator and memory are arranged to average the functions of the channel responses corresponding to the first and second parts, such that after the signals corresponding to the second known part are received, the function of the initial channel response in the memory is the average of the functions of the channel responses corresponding to the first and second parts.

12. An apparatus as recited in claim 11, wherein the function of the channel response stored in the memory subsystem for a subcarrier is post factoring by the known value of the subcarrier for the known symbol, the apparatus further comprising a factoring unit between the signal-to-tone transformer and the memory subsystem to factor by the known subcarrier value.

13. An apparatus as recited in claim 12, further comprising a channel smoother coupled to the memory to smooth the channel responses to take into account correlation between channel responses of adjacent subcarriers, the apparatus further including a factoring circuit to factor out the known subcarriers values of the known part.

14. An apparatus as recited in claim 11, further comprising a channel drift change detector coupled to the channel drift circuit to detect the change in channel drift, such that the relative weighting is a function of the detected change in channel drift.

15. An apparatus as recited in claim 14, wherein the memory is updated only if the detected change in channel drift exceeds a change threshold.

16. A method of operating an apparatus comprising a wireless receiver and a signal-to-tone transformer, the method comprising:

the wireless receiver wirelessly receiving a signal corresponding to a packet of information, the packet being an OFDM packet that includes a known transmitted part having subcarriers with known values;

transforming the received signal corresponding to the packet using the signal-to-tone transformer carrying out a signal-to-tone transformation to determining the subcarriers of the received signal corresponding to the packet;

calculating a function of a channel response of subcarriers from the known part of the packet;

storing the function of the channel response;

channel correcting the modulated subcarriers of the received signal for the channel using the stored initial or updated function of the channel response to form channel-corrected constellation values for the received signal;

forming post-decision values corresponding to the channel corrected constellation values; and

forming a measure of the channel drift from the post decision values and the channel-corrected constellation values,

wherein initially the function of an initial value of the channel response is stored and, as more packets are received, updates to the function or updated values of the function of the channel response are stores, the difference between a pre-update stored function of the channel response and updated values of the function of the channel response being dependent on the pre-update stored function of the channel response and on the measure of the channel drift between symbols,

such that the stored function of the channel response tracks the drift of the channel response as more packets are received.

17. A method as recited in claim 16, further comprising forming a weighted sum of the pre-update stored function of the channel response and an update, the update determined from the pre-update stored function of the channel response and the measure of the channel drift between symbols.

18. A method as recited in claim 16, wherein the forming the post-decision values forms hard demodulated decisions.

19. A method as recited in claim 16, wherein the forming the decision produces re-encoded decoded decisions.

20. An apparatus comprising:

means for wirelessly receiving a signal corresponding to a packet of information, the packet being an OFDM packet that includes a known transmitted part having subcarriers with known values;

means for signal-to-tone transforming the received packet to determining the subcarriers of a signal corresponding to the received packet;

means for calculating a function of a channel response of subcarriers from the known part of the received packet;

means for storing configured to store the function of the channel response;

means for channel correcting the modulated subcarriers of the received signal for the channel using the stored initial or updated function of the channel response to form channel-corrected constellation values for the received signal;

means for forming a decision to determine post-decision values corresponding to the channel corrected constellation values; and

means for forming a measure of the channel drift from the post decision values and the channel-corrected constellation values,

wherein initially the function of an initial value of the channel response is stored and, as more packets are received, updates to the function or updated values of the function of the channel response are stores, the difference between a pre-update stored function of the channel response and updated values of the function of the channel response being dependent on the pre-update stored function of the channel response and on the measure of the channel drift between symbols,

such that the stored function of the channel response tracks the drift of the channel response as more packets are received.