# Patent

## Title
MIMO/transmit diversity channel estimation

## Abstract

A receive station of the present invention despreads a received signal over multiple successive symbol periods of an extended measurement period to determine individual channel estimates over each symbol period of an extended period for multiple pilot signals. In one example, the extended measurement period comprises two extended periods, each of which comprise two symbol periods. As a result, the present invention provides channel estimates sufficient for both slow and fast fading conditions. The receive station determines a combined channel estimate for the pilot signals over each symbol period of an extended period by despreading the received signal over each symbol period of an extended measurement period using a common spreading sequence. Based on the combined channel estimates, the receive station determines individual channel estimates for each pilot signal over each symbol period of the extended period.

## Background
RELATED APPLICATIONS
This application is a continuation-in-part application of co-pending U.S. application Ser. No. 11/955,526, filed on 13 Dec. 2007, which is incorporated herein by reference.


BACKGROUND
The present invention relates generally to determining channel estimates for a MIMO communication system, and more particularly to determining such channel estimates in slow and fast fading conditions.
Release 7 of the UMTS standard uses a common 256-chip spreading sequence to generate an extended 512-chip spreading sequence for pilot signals generated by respective transmitters in a Multiple-input, Multiple-Output (MIMO) communication system. To distinguish between the transmitters, the polarity of the common spreading sequence in the extended spreading sequence is alternated every 256 chips for one transmitter relative to the other transmitter. This produces two mutually orthogonal 512-chip spreading sequences for the pilot signals.
The receiver may despread the received signal using the extended 512-chip spreading sequences to determine the channel estimates for each pilot signal. This process provides one channel estimate for every 512 chips of each pilot signal. For a pilot signal having ten symbol periods per slot, where each symbol period contains 256 chips, this process provides five channel estimates per slot for each pilot signal. Conventional systems average the five channel estimates over the slot to gain a slot-specific channel estimate with reduced noise and/or interference.
The slot-specific channel estimates resulting from the above-described process typically have sufficient accuracy for slow fading conditions, but not for fast fading conditions. Thus, there remains a need for determining accurate channel estimates under fast fading conditions in MIMO communication systems.
SUMMARY
The present invention despreads a received signal over multiple symbol periods to determine channel estimates for multiple pilot signals that are sufficient for both slow and fast fading conditions. More particularly, a wireless receive station according to the present invention receives a signal comprising multiple pilot signals from multiple different transmit antennas of a transmit station in a MIMO communication system, where the pilot signals are spread over an extended period spanning multiple symbol periods. A despreader determines a combined channel estimate for each symbol period of an extended measurement period spanning multiple ones of the extended periods by despreading the received signal over each symbol period of the extended measurement period using a common spreading sequence. Based on the combined channel estimates, a signal processor determines individual channel estimates for each pilot signal over each symbol period of the extended period. For example, the signal processor may evaluate a system of linear equations based on the combined channel estimates to determine the individual channel estimates.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of an exemplary MIMO communication system.
FIGS. 2A and 2B show exemplary extended spreading sequences for two pilot signals transmitted by two antennas.
FIG. 3 shows a block diagram of a receive station according to one exemplary embodiment of the present invention.
FIG. 4 shows a process according to one exemplary embodiment of the present invention.
FIG. 5 shows exemplary extended spreading sequences for four pilot signals transmitted by four antennas.



DETAILED DESCRIPTION
FIG. 1 shows a block diagram of a MIMO communication system 10. Communication system 10 comprises a transmit station 20 and a receive station 30. The transmit station 20 may comprise a base station and the receive station 30 may comprise a wireless communication device, or vice versa. The transmit station 20 transmits wireless signals from multiple transmit antennas 22 to multiple receive antennas 32 of the receive station 30. It will be appreciated that while FIG. 1 only shows transmitting wireless signals from two transmit antennas 22a, 22b to two receive antennas 32a, 32b, MIMO communication system 10 may transmit wireless signals from any number of transmit antennas 22 to any number of receive antennas 32. It will also be appreciated that the present invention applies to Multiple-Input, Single-Output (MISO) communication systems that implement transmit diversity.
The transmit station 20 may transmit pilot signals from each antenna 22 to provide a receive station 30 with known data. The receive station 30 may use the received pilot signals to determine channel estimates for the wireless communication channels between each transmit and receive antenna 22, 32. The receive station 30 may use the determined channel estimates, for example, to improve signal quality and suppress interference. To enable the receive station 30 to distinguish channel estimates associated with different pilot signals, the transmit station 20 transmits mutually orthogonal pilot signals from different transmit antennas 22. For simplicity, the following discussion focuses on two pilot signals s1, s2 generated by the transmit station 20 using two mutually orthogonal spreading sequences, transmitted by respective transmit antennas 22a, 22b, and received by one receive antenna 32a. 
For UMTS, the transmit station 20 generates the orthogonal pilot signals using an extended 512-chip spreading sequence derived from a common 256-chip spreading sequence SScomm. The polarity of the common spreading sequence may be alternated every 256 chips of an extended 512-chip period for one transmit antenna 22a relative to the other transmit antenna 22b. This produces two mutually orthogonal extended spreading sequences. The transmit station 20 uses the extended spreading sequences to generate the pilot signals s1 and s2.
FIGS. 2A and 2B show exemplary extended spreading sequences 40, 42 used to generate pilot signals for transmission from different transmit antennas 22. For this example, SScomm spans one symbol period 44, and the extended spreading sequences span an extended period 46 comprising two symbol periods 44. To ensure mutual orthogonality between the transmitted pilot signals, one extended spreading sequence alternates the polarity of the common spreading sequence SScomm over the extended period 46 relative to the other spreading sequence. For example, the extended spreading sequence 40 for the first transmit antenna 22a (FIG. 2A) may comprise two aggregated copies of SScomm having a common polarity, while the extended spreading sequence 42 for the second transmit antenna 22b (FIG. 2B) may comprise two aggregated copies of SScomm having opposite polarity. The resulting spreading sequences 40, 42 are mutually orthogonal over the extended period 46. Using these mutually orthogonal spreading sequences, the transmit station 20 may generate mutually orthogonal pilot signals s1, s2 for transmission by different transmit antennas 22a, 22b to the receive station 30.
The receive station 30 receives the pilot signals as a combined signal at one receive antenna 32a, and determines channel estimates based on the received signal. For example, receive station 30 may determine channel estimates for the channel corresponding to s1 and for the channel corresponding to s2 by despreading the received signal using the extended 512-chip spreading sequences. This process provides one channel estimate for every 512 chips of each pilot signal. For a pilot signal having ten symbol periods per slot, where each symbol period contains 256 chips, this process provides five channel estimates per slot for each pilot signal. Such channel estimates are typically sufficient under slow fading conditions. However, under fast fading conditions, such channel estimates do not have sufficient accuracy. Indeed, under fast fading conditions, where the channel coefficients experience a significant change over the extended period 46, the extended spreading sequences do not provide orthogonal estimates. For fast fading conditions, it is desirable to have individual channel estimates over each symbol period 44 of each pilot signal. However, because the pilot signals are not mutually orthogonal over the symbol period 44, despreading the received signal over one symbol period 44 does not provide the desired individual channel estimates.
The present invention addresses this problem by despreading the received signal over an extended measurement period 48 spanning multiple extended periods 46 to determine individual channel estimates over each symbol period 44 for each pilot signal. For the example of FIGS. 2A and 2B, the extended measurement period 48 spans two extended periods 46, each of which include two symbol periods 44.
FIG. 3 shows an exemplary receive station 30 for implementing the present invention, while FIG. 4 shows an exemplary process 100 implemented by the receive station 30. Receive station 30 comprises a receiver front end 34 and a channel estimation unit comprising a despreader 36 and a signal processor 38. The receiver front end 34 receives a wireless signal comprising the pilot signals s1 and s2 (block 110), and filters, downconverts, and amplifies the received signal. The despreader 36 despreads the received signal output by the receiver front end 34 using the common spreading sequence to determine a set of despread pilot signals over each symbol period 44 of the extended measurement period 48 (block 120). The signal processor 38 processes the set of despread pilot signals to determine combined channel estimates over each symbol period 44. Signal processor 38 subsequently processes the combined channel estimates to determine individual channel estimates over each symbol period 44 of the extended period 46 for each pilot signal (block 130). This process may be repeated to obtain channel estimates for symbol periods 44 of additional extended periods 46 of the received signal.
The despreader 36 may despread the received signal over the extended period 46 using the mutually orthogonal spreading sequences discussed above, and the signal processor 38 may process the resulting despread pilot signals to determine average channel estimates over the extended period 46 for the pilot signals. For example, the received signal may be despread over an extended period 46 using the extended spreading sequence associated with the first transmit antenna 22a (see FIG. 2A) to determine an average channel estimate c1 for s1 over the extended period 46. The received signal may also be despread over the extended period 46 using the extended spreading sequence associated with the second transmit antenna 22b (see FIG. 2B) to determined an average channel estimate c2 for s2 over the extended period 46. Equation (1) shows the resulting average channel estimates c1, c2.




  
    
      
        
          
            
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In Equation (1), Δ1 and Δ2 represent the dynamic change in the channel estimates between the symbol periods 44 for respective pilot signals s1, s2. The channel estimates in Equation (1) represent average channel estimates for the extended period 46. As discussed above, the average channel estimates do not provide the actual channel estimate (c1 or c2) for a first symbol period 44a of the extended period 46. Further, neither estimate provides the actual channel estimate (c1+Δ1 or c2+Δ2) for a subsequent symbol period 44b of the extended period 46.

To address this problem, the despreader 36 of the present invention despreads the received signal over each symbol period 44 of an extended measurement period 48 using the common spreading sequence SScomm, and the signal processor 38 processes the resulting despread pilot signals to generate the combined channel estimates c1,comb, c2,comb, c3,comb, and c4,comb given by Equation (2).

c1,comb=c1+c2 

c2,comb=c1+Δ1−c2−Δ2 

c3,comb=c1+2Δ1+c2+2Δ2 

c4,comb=c1+3Δ1−c2−3Δ2  (2)

More particularly, the received signal is despread over a first symbol period 44a of the extended measurement period 48 using SScomm to generate c1,comm, and is despread over subsequent symbol periods 44b, 44c, 44d of the extended measurement period 48 using SScomm to generate c2,comb, c3,comb, and c4,comb, respectively. The signal processor 38 processes the combined channel estimates c1,comb, c2,comb, c3,comb and c4,comb to solve for the four unknowns: c1, c2, Δ1, and Δ2. In so doing, the signal processor 38 determines individual channel estimates over each symbol period 44 of the extended period 46 for both s1 and s2.

Signal processor 38 may process the combined channel estimates c1,comb, c2,comb, c3,comb, and c4,comb using a system of linear equations to solve for the four unknowns c1, c2, Δ1, and Δ2. Equations (1) and (2) may be collectively written in matrix form as:




  
    
      
        
          
            
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Equation (3) may be rewritten as:





  
    
      
        
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Signal processor 38 may determine the individual channel estimates over each symbol period 44 of the extended period 46 by solving Equation (4) using the combined channel estimates. More particularly, signal processor 38 uses the results from Equation (4) to determine c1, c1+Δ1, c1+2Δ1, and c1+3Δ1 for consecutive symbol periods 44a, 44b, 44c, 44d of the extended measurement period 48 of the pilot signal associated with the first transmit antenna 22a, and to determine c2, c2+Δ2, c2+2Δ2, and c2+3Δ2 for consecutive symbol periods 44a, 44b, 44c, 44d of the extended measurement period 48 of the pilot signal associated with the second transmit antenna 22b. It will be appreciated that in evaluating Equation (4), signal processor 38 also determines the dynamic change (Δ1 and Δ2) in the channel estimates from one symbol period 44 to the next for the respective pilot signals. The signal processor 38 may further process and/or evaluate Δ1 and Δ2 to determine a fading rate for the pilot signals. For example, larger Δ1 values with respect to c1 indicate a faster fading channel for s1 than smaller values.

The present invention provides a despreader 36 and signal processor 38 that determine individual channel estimates over each symbol period 44 of an extended period 46 for multiple pilot signals when mutually orthogonal extended spreading sequences are used to generate the pilot signals. This enables the receive station 30 to determine a symbol-to-symbol sliding average of the channel estimates for each pilot signal. Such symbol-to-symbol sliding averages are more accurate, and therefore, more appropriate for fast fading conditions than the above-discussed slot-specific channel estimates.
The above describes the invention in terms of an extended period 46 spanning two 256-chip symbol periods 44. However, it will be appreciated that the present invention may be used with any extended period 46 that spans any number of multiple symbol periods 44 where mutually orthogonal spreading sequences are used over the extended period. For example, when four transmit antennas 22 transmit to one receive antenna 32, the extended period 46 may span four 256-chip symbol periods 44, as shown by FIG. 5. For this example, despreader 36 despreads the received signal output by the receiver front end 34 using the common spreading sequence to determine a set of despread pilot signals over each symbol period 44a, 44b, . . . , 44h of an extended measurement period 48 spanning eight symbol periods 44. The signal processor 38 processes the set of despread pilot signals to determine combined channel estimates (c1,comb, c2,comb, . . . , c8,comb) over each symbol period 44a, 44b, . . . , 44h. Signal processor 38 subsequently processes the combined channel estimates to determine c1, c2, c3, and c4, and Δ1, Δ2, Δ3, and Δ4 that are used to determine the individual channel estimates over each symbol period 44a, 44b, 44c, 44d of the extended period 46.
The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

## Claims

1. A method of determining channel estimates for pilot signals transmitted from different transmit antennas in a MIMO communication system wherein said pilot signals are spread over an extended period spanning multiple symbol periods, the method comprising:
despreading the received signal over each symbol period of an extended measurement period spanning multiple ones of the extended periods using a common spreading sequence to obtain combined channel estimates for the pilot signals over each symbol period of the extended measurement period; and
determining individual channel estimates for the pilot signals over each symbol period of the extended period based on the combined channel estimates by evaluating a system of linear equations based on the combined channel estimates, wherein evaluating the system of linear equations comprises evaluating:


c1,comb=c1+c2 



c2,comb=c1+Δ1−c2−Δ2 



c3,comb=c1+2Δ1+c2+2Δ2 



c4,comb=c1+3Δ1−c2−3Δ2,

where c1,comb, c2,comb, c3,comb, and c4,comb represent the combined channel estimates over different symbol periods of the extended measurement period, c1 and c2 represent the individual channel estimates over one symbol period for a first and a second pilot signals, respectively, and Δ1 and Δ2 represent channel estimate differences between symbol periods for the first and the second pilot signals, respectively.

2. The method of claim 1 further comprising determining a fading rate associated with the pilot signals based on the combined channel estimates.

3. The method of claim 1 wherein mutually orthogonal extended spreading sequences comprise first and second extended spreading sequences generated based on the common spreading sequence for respective first and second pilot signals and spanning the extended period.

4. The method of claim 3 wherein the first extended spreading sequence is generated based on multiple copies of the common spreading sequence having a common polarity for each symbol period of the extended period, and wherein the second extended spreading sequence is generated based on multiple copies of the common spreading sequence having an alternating polarity for each symbol period of the extended period.

5. The method of claim 1 wherein mutually orthogonal extended spreading sequences each span two successive symbol periods, wherein the common spreading sequence spans one symbol period, and wherein the extended measurement period spans four successive symbol periods.

6. A receive station in a MIMO communication system, the receive station comprising:
a receiver front end to receive a signal comprising a combination of multiple pilot signals from different transmit antennas in the MIMO communication system wherein said pilot signals are spread over an extended period spanning multiple symbol periods;
a despreader configured to:
despread the received signal over each symbol period of extended measurement period spanning multiple ones of the extended periods using a common spreading sequence to obtain a set of despread pilot signals over each symbol period of the extended measurement period;

a signal processor configured to:
determine combined channel estimates for the pilot signals over each symbol period of the extended measurement period based on the set of despread signals; and
determine individual channel estimates for the pilot signals over each symbol period of the extended period based on the combined channel estimates

wherein mutually orthogonal extended spreading sequences comprise a first and a second extended spreading sequences generated based on the common spreading sequence for respective first and second pilot signals and spanning the extended period, wherein the first extended spreading sequence comprises multiple copies of the common spreading sequence having a common polarity for each symbol period of the extended period, and wherein the second extended spreading sequence comprises multiple copies of the common spreading sequence having an alternating polarity for each symbol period of the extended period.

7. The receive station of claim 6 wherein the signal processor is further configured to determine a fading rate associated with the pilot signals based on the combined channel estimates.

8. The receive station of claim 6 wherein the mutually orthogonal extended spreading sequences each span two successive symbol periods, wherein the common spreading sequence spans one symbol period, and wherein the extended measurement period spans four successive symbol periods.

9. The receive station of claim 6 wherein the signal processor determines the individual channel estimates by evaluating a system of linear equations based on the combined channel estimates.

10. The receive station of claim 6 wherein the receive station comprises a wireless communication device.

11. A channel estimation unit in a receive station of a MIMO communication system, wherein the receive station receives a signal comprising a combination of multiple pilot signals from different transmit antennas in the MIMO communication system, wherein said pilot signals are spread over an extended period spanning multiple symbol periods, said channel estimation unit comprising:
a despreader configured to:
despread the received signal over each symbol period of an extended measurement period spanning multiple ones of the extended periods using a common spreading sequence to obtain a set of despread pilot signals over each symbol period of the extended measurement period;

a signal processor configured to:
determine combined channel estimates for the pilot signals over each symbol period of the extended measurement period based on the set of despread signals; and
determine individual channel estimates for the pilot signals over each symbol period of the extended period based on the combined channel estimates by evaluating a system of linear equations based on the combined channel estimates,
wherein the signal processor evaluates the system of linear equations by evaluating


c1,comb=c1+c2 



c2,comb=c1+Δ1−c2−Δ2 



c3,comb=c1+2Δ1+c2+2Δ2 



c4,comb=c1+3Δ1−c2−3Δ2,


 where c1,comb, c2,combc3,comb, and c4,comb represent the combined channel estimates over different symbol periods of the extended measurement period, c1 and c2 represent the individual channel estimates over one symbol period for a first and a second pilot signals, respectively, and Δ1 and Δ2 represent channel estimate differences between symbol periods for the first and second pilot signals, respectively.

12. The channel estimation unit of claim 11 wherein the signal processor is further configured to determine a fading rate associated with the pilot signals based on the combined channel estimates.

13. The channel estimation unit of claim 11 wherein mutually orthogonal extended spreading sequences comprise first and second extended spreading sequences generated based on the common spreading sequence for respective first and second pilot signals and spanning the extended period.

14. A method of determining channel estimates for pilot signals transmitted from different transmit antennas in a MIMO communication system wherein said pilot signals are spread over an extended period spanning multiple symbol periods, the method comprising:
despreading the received signal over each symbol period of an extended measurement period spanning multiple ones of the extended periods using a common spreading sequence to obtain combined channel estimates for the pilot signals over each symbol period of the extended measurement period; and
determining individual channel estimates for the pilot signals over each symbol period of the extended period based on the combined channel estimates;
wherein mutually orthogonal extended spreading sequences comprise a first and a second extended spreading sequences generated based on the common spreading sequence for respective first and second pilot signals and spanning the extended period, wherein the first extended spreading sequence is generated based on multiple copies of the common spreading sequence having a common polarity for each symbol period of the extended period, and wherein the second extended spreading sequence is generated based on multiple copies of the common spreading sequence having an alternating polarity for each symbol period of the extended period.