US20100284447A1 - Frequency domain feedback channel estimation for an interference cancellation repeater including sampling of non causal taps - Google Patents

Frequency domain feedback channel estimation for an interference cancellation repeater including sampling of non causal taps Download PDF

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US20100284447A1
US20100284447A1 US12/776,957 US77695710A US2010284447A1 US 20100284447 A1 US20100284447 A1 US 20100284447A1 US 77695710 A US77695710 A US 77695710A US 2010284447 A1 US2010284447 A1 US 2010284447A1
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Prior art keywords
channel
signal
taps
channel estimate
samples
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Dhananjay Ashok Gore
Michael Mao Wang
Gwendolyn Denise Barriac
James Arthur Proctor, Jr.
Tao Tian
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Qualcomm Inc
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Qualcomm Inc
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Priority to US12/776,957 priority Critical patent/US20100284447A1/en
Priority to TW099114996A priority patent/TW201134127A/zh
Priority to PCT/US2010/034424 priority patent/WO2010132483A2/fr
Assigned to QUALCOMM INCORPORATED reassignment QUALCOMM INCORPORATED ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: PROCTOR, JAMES ARTHUR, JR., BARRIAC, GWENDOLYN DENISE, GORE, DHANANJAY ASHOK, TIAN, TAO, WANG, MICHAEL MAO
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/022—Channel estimation of frequency response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/155—Ground-based stations
    • H04B7/15564—Relay station antennae loop interference reduction
    • H04B7/15585—Relay station antennae loop interference reduction by interference cancellation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/0224—Channel estimation using sounding signals
    • H04L25/0226—Channel estimation using sounding signals sounding signals per se

Definitions

  • This disclosure generally relates to repeaters in wireless communication systems.
  • Wireless communication systems and techniques have become an important part of the way we communicate. However, providing coverage can be a significant challenge to wireless service providers. One way to extend coverage is to deploy repeaters.
  • a repeater is a device that receives a signal, amplifies the signal, and transmits the amplified signal.
  • FIG. 1 shows a basic diagram of a repeater 110 , in the context of a cellular telephone system.
  • Repeater 110 includes a donor antenna 115 as an example network interface to network infrastructure such as a base station 125 .
  • Repeater 110 also includes a server antenna 120 (also referred to as a “coverage antenna”) as a mobile interface to mobile device 130 .
  • server antenna 120 also referred to as a “coverage antenna”
  • donor antenna 115 is in communication with base station 125
  • server antenna 120 is in communication with mobile devices 130 .
  • signals from base station 125 are amplified using forward link circuitry 135
  • signals from mobile device 130 are amplified using reverse link circuitry 140 .
  • Many configurations may be used for forward link circuitry 135 and reverse link circuitry 140 .
  • repeaters there are many types of repeaters. In some repeaters, both the network and mobile interfaces are wireless; while in others, a wired network interface is used. Some repeaters receive signals with a first carrier frequency and transmit amplified signals with a second different carrier frequency, while others receive and transmit signals using the same carrier frequency. For “same frequency” repeaters, one particular challenge is managing the feedback that occurs since some of the transmitted signal can leak back to the receive circuitry and be amplified and transmitted again.
  • the repeater manages feedback using a number of techniques; for example, the repeater is configured to provide physical isolation between the two antennae, filters are used, or other techniques may be employed.
  • a method for estimating a feedback channel for a wireless repeater in a wireless communication system where the wireless repeater has a first antenna and a second antenna to receive a receive signal and transmit an amplified signal and the receive signal is a sum of a remote signal to be repeated and a feedback signal resulting from the feedback channel between the first and second antenna of the wireless repeater includes using blocks of N samples of the amplified signal as a pilot signal, using blocks of N samples of the receive signal, estimating the feedback channel between the first antenna and the second antenna using frequency domain channel estimation using samples of the pilot signal and samples of the receive signal, generating a time domain feedback channel estimate from the frequency domain channel estimation, the time domain feedback channel estimate having N taps; and retaining M1 causal taps and M2 non-causal taps in the time domain feedback channel estimate, the M1 causal taps being the first M1 taps of the channel estimate and the M2 non-causal taps
  • the time domain channel estimate is truncated by setting channel taps in the time domain channel estimate outside of the M1 causal taps and the M2 non-causal taps to zero.
  • a machine-readable medium includes instructions, which, when executed by a machine, cause the machine to perform operations.
  • the instructions includes using blocks of N samples of an amplified signal as a pilot signal, using blocks of N samples of a receive signal, estimating the feedback channel between a first antenna and a second antenna of a wireless repeater using frequency domain channel estimation and using samples of the pilot signal and samples of the receive signal, generating a time domain feedback channel estimate from the frequency domain channel estimation, the time domain feedback channel estimate having N taps, and retaining M1 causal taps and M2 non-causal taps in the time domain feedback channel estimate where the M1 causal taps are the first M1 taps of the channel estimate and the M2 non-causal taps are the last M2 taps of the channel estimate.
  • a computer-readable medium including program code stored thereon includes program code to use blocks of N samples of an amplified signal as a pilot signal, program code to use blocks of N samples of a receive signal, program code to estimate the feedback channel between a first antenna and a second antenna of a wireless repeater using frequency domain channel estimation and using samples of the pilot signal and samples of the receive signal, program code to generate a time domain feedback channel estimate from the frequency domain channel estimation, the time domain feedback channel estimate having N taps, and program code to retain M1 causal taps and M2 non-causal taps in the time domain feedback channel estimate where the M1 causal taps are the first M1 taps of the channel estimate and the M2 non-causal taps are the last M2 taps of the channel estimate.
  • a wireless repeater having a first antenna and a second antenna to receive a receive signal and transmit an amplified signal where the receive signal is a sum of a remote signal to be repeated and a feedback signal resulting from a feedback channel between the first antenna and the second antenna includes a channel estimation module configured to estimate the feedback channel between the first antenna and the second antenna using frequency domain channel estimation and using a signal indicative of the amplified signal as a pilot signal and using blocks of N samples of the pilot signal and blocks of N samples of the corresponding receive signal, to generate a time domain feedback channel estimate from the frequency domain channel estimation, and to retain M1 causal taps and M2 non-causal taps in the time domain feedback channel estimate, the M1 causal taps being the first M1 taps of the channel estimate and the M2 non-causal taps being the last M2 taps of the channel estimate.
  • the wireless repeater may include a delay element configured to introduce a delay to samples of the amplified signal to align the largest
  • FIG. 1 is a simplified diagram of a repeater according to the prior art.
  • FIG. 2 shows a diagram of a repeater environment according to some embodiments of the current disclosure.
  • FIG. 3 is a block diagram of a repeater implementing interference cancellation according to one embodiment of the present invention.
  • FIG. 4 is a flowchart illustrating the channel estimation method for cancelling multiplicative error terms which can be implemented in the repeater of FIG. 3 according to one embodiment of the present invention.
  • FIG. 5 is a flowchart illustrating the channel estimate method for cancelling the ICI/ISI error terms which can be implemented in the repeater of FIG. 3 according to one embodiment of the present invention.
  • FIG. 6 is a detail block diagram of a repeater in which the channel estimation method for sampling both causal and non-causal taps can be implemented according to one embodiment of the present invention.
  • FIG. 7 illustrates the relationship between the receive signal, the pilot signal and the feedback channel.
  • FIG. 8 illustrates the channel response of the channel estimate without the delay adjustment to left shift the pilot signal samples.
  • FIG. 9 illustrates the channel response of the channel estimate after the delay adjustment to left shift the pilot signal samples.
  • FIG. 10 illustrates the channel response of the channel estimate showing the causal taps and the non-causal taps to be used by the channel estimation method according to one embodiment of the present invention.
  • FIG. 11 is a flowchart illustrating the channel estimation method for keeping both the causal and the non-causal taps of the feedback channel estimate according to one embodiment of the present invention.
  • FIG. 12 is a flowchart illustrating the channel estimation pruning method in a wireless repeater according to one embodiment of the present invention.
  • FIG. 13 is a flowchart illustrating the channel estimation pruning method in a wireless repeater according to an alternate embodiment of the present invention.
  • Prior art repeaters such as those described above may provide significant advantages for cellular telephone or similar networks.
  • existing repeater configurations may not be suitable for some applications.
  • existing repeater configurations may not be suitable for indoor coverage applications (e.g., repeating signals for a residence or business environment) which may be more difficult to obtain the desired isolation between the repeater's antennas.
  • the target is to achieve as high a gain as reasonable while maintaining a stable feedback loop (loop gain less than unity).
  • increasing the repeater gain renders isolation more difficult due to the increased signal leaking back into the donor antenna.
  • loop stability demands require that the signal leaking back into the donor antenna from the coverage antenna be much lower than the remote signal (the signal to be repeated), whether due to spatial isolation or other techniques such as digital cancellation.
  • the maximum achievable signal to interference/noise ratio (SINR) at the output of the repeater is then the same as the SINR of the remote signal at the input to the repeater.
  • SINR signal to interference/noise ratio
  • systems and techniques herein provide for wireless repeaters with improved isolation between the repeaters' donor antenna (“the receiving antenna” for the example of a forward link transmission) and the coverage antenna (“the transmitting antenna” for forward link transmissions). Furthermore, in some embodiments, systems and techniques herein provide for a unique repeater design employing interference cancellation or echo cancellation to significantly improve the isolation. In some embodiments, the interference cancellation and echo cancellation are realized using improved channel estimation techniques provided herein for accurate estimation of the channel. Effective echo cancellation requires very accurate channel estimation of the leakage channel. In general, the more accurate the channel estimate, the higher the cancellation and hence the higher the effective isolation.
  • interference cancellation or “echo cancellation” refers to techniques that reduce or eliminate the amount of leakage signal between repeater antennas; that is, “interference cancellation” refers to cancellation of an estimated leakage signal, which provides for partial or complete cancellation of the actual leakage signal.
  • FIG. 2 shows a diagram of an operating environment 200 for a repeater 210 according to embodiments of the current disclosure.
  • the example of FIG. 2 illustrates forward link transmissions; i.e., a remote signal 140 from a base station 225 is intended for a mobile device 230 .
  • a repeater such as repeater 210 , may be used in environment 200 if an un-repeated signal along the path 227 between base station 225 and mobile device 230 would not provide sufficient signal for effective voice and/or data communications received at mobile device 230 .
  • Repeater 210 with a gain G and a delay ⁇ is configured to repeat a signal received from base station 225 on a donor antenna 215 to mobile device 230 using a server antenna 220 .
  • Repeater 210 includes forward link circuitry for amplifying and transmitting signals received from the base station 225 to mobile device 230 through donor antenna 215 and server antenna 220 . Repeater 210 may also include reverse link circuitry for amplifying and transmitting signals from mobile device 230 back to base station 225 .
  • the gain G would be large, the inherent delay ⁇ of the repeater would be small, the input SINR would be maintained at the output of repeater 210 (this can be of particular importance for data traffic support), and only desired carriers would be amplified.
  • the gain of repeater 210 is limited by the isolation between donor antenna 215 and server antenna 220 . If the gain is too large, the repeater can become unstable due to signal leakage.
  • Signal leakage refers to the phenomenon where a portion of the signal that is transmitted from one antenna (in FIG. 2 , server antenna 220 ) is received by the other antenna (in FIG. 2 , donor antenna 215 ), as shown by the feedback path 222 in FIG. 2 . In other words, signal leakage is a result of the transmitted signal not being totally blocked by antenna isolation between the receiving and transmitting antennas.
  • the repeater would amplify this feedback signal, also referred to as the leakage signal, as part of its normal operation, and the amplified feedback signal would again be transmitted by server antenna 220 .
  • the repeated transmission of the amplified feedback signal due to signal leakage and high repeater gain can lead to repeater instability.
  • signal processing in repeater 210 has an inherent non-negligible delay A.
  • the output SINR of the repeater is dependent on RF non-linearities and other signal processing.
  • the desired carriers can vary depending on the operating environment or market in which the repeater is deployed. It is not always possible to provide a repeater that amplifies only the desired carriers.
  • a repeater suitable for indoor coverage e.g., business, residential, or similar use
  • the repeater has an active gain of about 70 dB or greater which is an example of a sufficient gain for coverage in a moderately sized residence.
  • the repeater has a loop gain of less than one for stability (loop gain being referred to as the gain of the feedback loop between the transmitting antenna and the receiving antenna) and a sufficient amount of margin for stability and low output noise floor.
  • the repeater has a total isolation of greater than 80 dB.
  • the repeater employs interference/echo cancellation to achieve a high level of active isolation, which is significantly more challenging than the requirements of available repeaters.
  • Some techniques of the current disclosure utilize channel estimation to enable the required level of echo cancellation.
  • the feedback channel the channel between the antennas
  • the residual error, post echo cancellation can be sufficiently below the remote signal to realize the desired loop gain margin for stability.
  • the communication system in which the repeater of the present invention can be deployed includes various wireless communication networks based on infrared, radio, and/or microwave technology.
  • Such networks can include, for example, a wireless wide area network (WWAN), a wireless local area network (WLAN), a wireless personal area network (WPAN), and so on.
  • WWAN wireless wide area network
  • WLAN wireless local area network
  • WPAN wireless personal area network
  • network and “system” are often used interchangeably.
  • a WWAN may be a Code Division Multiple Access (CDMA) network, a Time Division Multiple Access (TDMA) network, a Frequency Division Multiple Access (FDMA) network, an Orthogonal Frequency Division Multiple Access (OFDMA) network, a Single-Carrier Frequency Division Multiple Access (SC-FDMA) network, a Long Term Evolution (LTE) network, a WiMAX (IEEE 802.16) network and so on.
  • CDMA network may implement one or more radio access technologies (RATs) such as cdma2000, Wideband-CDMA (W-CDMA), and so on.
  • Cdma2000 includes IS-95, IS-2000, and IS-856 standards.
  • a TDMA network may implement Global System for Mobile Communications (GSM), Digital Advanced Mobile Phone System (D-AMPS), or some other RAT.
  • GSM and W-CDMA are described in documents from a consortium named “3rd Generation Partnership Project” (3GPP).
  • Cdma2000 is described in documents from a consortium named “3rd Generation Partnership Project 2” (3GPP2).
  • 3GPP and 3GPP2 documents are publicly available.
  • a WLAN may be an IEEE 802.11x network
  • a WPAN may be a Bluetooth network, an IEEE 802.15x, or some other type of network.
  • the techniques may also be implemented in conjunction with any combination of WWAN, WLAN and/or WPAN.
  • the pilot signal used for channel estimation is the amplified signal being transmitted to the mobile device (downlink) or the base station (uplink).
  • the amplified signal leaks back from the transmit antenna to the receive antenna, resulting in a feedback signal being added to the remote signal at the repeater input.
  • the feedback signal is estimated and then cancelled out. This increases the effective isolation between the donor and coverage antennas. If the estimate of the feedback channel is sufficiently accurate, the feedback signal can be almost completely subtracted out from the repeater input signal. The more accurate the channel estimate, the more amplification of the output signal the repeater can sustain while maintaining the required isolation for stability. In other words, the accuracy of the repeater's channel estimate and the repeater's achievable gain are directly related.
  • an echo cancellation repeater implements channel estimation in the frequency domain.
  • Frequency domain channel estimation provides particular advantages such as reduced complexity and increased robustness.
  • frequency domain channel estimation through the use of FFT-IFFT type processing typically relies on a cyclic prefix in the pilot signal for maintaining orthogonality.
  • the problem with applying frequency domain channel estimation in repeater applications is that the “pilot signal” is really just the signal to be transmitted (or the amplified signal) and there is no inserted cyclic prefix in the “pilot” signal to ensure orthogonality of the different “frequency bins”.
  • Systems and methods of the present invention enable the use of frequency domain channel estimation in an echo cancellation repeater in the absence of a cyclic prefix in the pilot signal which is the transmitted signal.
  • Channel estimation performance degradation as a result of the lack of a cyclic prefix in the pilot signal is mitigated through the estimation and cancellation of the additive errors and multiplicative errors.
  • Systems and methods for improving the accuracy of frequency domain channel estimation in an echo cancellation repeater through symbols pruning, sampling of non-causal taps and other techniques are also described.
  • Frequency domain channel estimation in a repeater using FFT-IFFT type processing on a pilot signal without cyclic prefix presents challenges in terms of additive and multiplicative noise errors, such as ICI and ISI error terms.
  • the ICI and ISI error terms arise as a result of the convolution becoming linear rather than circular due to the lack of cyclic prefix and because the pervious symbol leaks into the new symbol when there is no cyclic prefix.
  • h is the perfect channel estimate of the feedback channel for a given channel tap in a repeater
  • the actual channel estimate for the given channel tap as a result of the lack of cyclic prefix becomes: ⁇ *h+m+z, where m represents random noise, z represents the additive components of the ICI and ISI error terms, and ⁇ represents the multiplicative component of the ICI error term.
  • the multiplicative error component ⁇ is very close to 1 but can affect accuracy in the estimation is a deviates from the value of 1. More specifically, the multiplicative noise error introduces bias to the channel estimate.
  • the use of frequency domain channel estimation in the absence of a cyclic prefix in the pilot signal introduces multiplicative error that degrade the accuracy of the channel estimation.
  • the multiplicative error “a” is given for each channel tap in the time domain.
  • the multiplicative error is especially a problem when small block size is used for the FFT processing.
  • a method to estimate and correct for the multiplicative error terms due to the use of frequency domain channel estimation in the absence of a cyclic prefix in the pilot signal (the amplified or transmit signal) is described. More specifically, the multiplicative error terms are manifested as scaling factors on the channel estimate.
  • a channel estimation method performs scaling of a time domain channel estimate obtained from a frequency domain channel estimation process in the absence of a cyclic prefix in the pilot signal to correct the multiplicative error terms.
  • the scaling operation applies a set of scalar multiplication factors (“scaling factors” or “correction scaling factors”) to the time domain channel estimate obtained from the frequency domain channel estimation process.
  • scaling factors depend only on the tap index and the block size used for the frequency domain channel estimation process.
  • the frequency domain channel estimation process uses FFTs (Fast Fourier Transforms) to transform time domain signals into frequency domain signals, and obtains an estimate of the channel in the frequency domain by processing these frequency domain signals.
  • the channel estimate in time domain obtained as a result of taking the IFFT (Inverse Fast Fourier Transforms) of the frequency domain channel estimate is a scaled replica of the channel being estimated (also referred to as the “original channel”), where each tap has a different scaling factor.
  • the time domain channel estimate obtained from the IFFT operation is scaled using scaling factors that are function of only the tap index and the FFT block size or block length.
  • the set of scalar multiplication factors is derived as follows.
  • the input signal x is often referred to as the pilot signal for channel estimation.
  • the input signal x of the channel estimation signal model is the echo cancelled transmit signal or the amplified signal of the repeater.
  • the output signal y of the channel estimation signal model is the receive signal at the repeater.
  • the receive signal y at the repeater is the pilot signal x (transmit signal) transmitted through the feedback channel h plus noise z.
  • the feedback channel h may include one or more channel coefficients, associated with one or more delay paths in the channel.
  • the feedback channel h may include multiple delay paths.
  • the ICI error term can be split up into an additive error component and a multiplicative error component.
  • the multiplicative error component ⁇ n of the ICI error term for a given tap n is given as:
  • n is the tap index (0 to N ⁇ 1) and N is the block size of the FFT.
  • multiplicative error terms result and can be estimated to be
  • n is the tap index and N is the FFT block size.
  • the multiplicative error terms are cancelled out by scaling the time domain channel estimate using a set of scalar multiplication factors given as
  • a wireless repeater incorporates a channel estimation module configured to perform frequency domain channel estimation using the channel estimation method described above for cancelling multiplicative errors in the channel estimate.
  • FIG. 3 is a block diagram of a repeater implementing interference cancellation according to one embodiment of the present invention.
  • a repeater 250 receives an input signal on a donor antenna 270 for downlink communications.
  • Repeater 250 also transmit an output signal on a server antenna 275 for downlink communications.
  • Repeater 250 includes an echo canceller 252 for implementing cancellation of the feedback signal.
  • Repeater 250 amplifies the echo cancelled transmit signal at a gain stage 254 providing a variable gain G.
  • Repeater 250 also includes a channel estimation module 260 for estimating the feedback channel 268 (denoted as “h”) between antennas 275 and 270 .
  • Channel estimation module 260 provides a channel estimate ⁇ to echo canceller 252 to enable the estimation and cancellation of the undesired feedback signal.
  • repeater 250 receives the receive signal y being the sum of a remote signal to be amplified and a feedback signal (or leakage signal) being the version of the transmitted signal that leaks back from the transmit antenna into the receive antenna.
  • the transmit signal x or a signal indicative of the transmit signal x
  • the remote signal is treated as noise.
  • Repeater 250 being an echo cancellation repeater, operates to estimate the feedback signal in order to cancel out the undesired feedback signal component in the receive signal.
  • channel estimation module 260 generates a feedback channel estimate ⁇ for echo canceller 252 .
  • Echo canceller 252 generates a feedback signal estimate based on the feedback channel estimate.
  • Echo canceller 252 subtracts the feedback signal estimate from the receive signal y to generate the echo cancelled transmit signal x. As long as the feedback signal estimate is accurate, the undesired feedback signal is removed from the receive signal and echo cancellation is realized.
  • the echo cancelled transmit signal x is coupled to a variable gain stage 254 providing a gain of G to the echo cancelled transmit signal before transmission on antenna 275 .
  • FIG. 3 illustrates only elements that are relevant to operation of the channel estimation method of the present invention.
  • Repeater 250 may include other elements not shown in FIG. 3 but known in the art to realize the complete repeater operation.
  • the channel estimation module 260 performs frequency domain channel estimation.
  • the results of the frequency domain channel estimation is converted to a time domain channel estimate (such as through IFFT) that may include multiplicative errors due to the lack of a cyclic prefix in the pilot signal.
  • Channel estimation module 260 operates to scale the time domain channel estimate obtained from the frequency domain channel estimation process using a set of scalar multiplication factors (“scaling factors” or “correction scaling factors”) where the scalar multiplication factors are function of only the tap index and the FFT block size, as described above.
  • the scaled time domain channel estimate ⁇ can then be used by echo canceller 252 for echo cancellation.
  • the scalar multiplication factors have the effect of improving the accuracy of the channel estimation by canceling the multiplicative error terms resulted from the frequency domain channel estimation on a pilot signal in the absence of a cyclic prefix.
  • FIG. 4 is a flowchart illustrating the channel estimation method for cancelling multiplicative error terms which can be implemented in the repeater of FIG. 3 according to one embodiment of the present invention.
  • channel estimation method 300 starts by receiving the receive signal y and the transmit signal x at the channel estimation module (step 302 ).
  • the channel estimation module performs frequency domain channel estimation (step 304 ).
  • the channel estimation module performs Fast Fourier Transform (FFT) on N-sample blocks of the transmit signal as the pilot signal and N-sample blocks of the receive signal.
  • FFT Fast Fourier Transform
  • frequency domain pilot signal blocks and frequency domain receive signal blocks are generated from blocks of N samples of the pilot signal and the receive signal.
  • a channel estimate in the frequency domain is obtained from these frequency domain samples.
  • the channel estimation method obtains a time domain channel estimate from the frequency domain channel estimate, such as by performing Inverse Fast Fourier Transform (IFFT) operations (Step 306 ).
  • IFFT Inverse Fast Fourier Transform
  • the IFFT is performed on a signal generated by one or more signal processing operations on the frequency domain pilot blocks and the frequency domain receive signal blocks.
  • the channel estimation method scales the time domain channel estimate using a set of correction scaling factors (step 308 ).
  • the correction scaling factors are function of only the tap index and the FFT block size. The resulting scaled channel estimate can then be used for echo cancellation in the repeater.
  • the scaling of the time domain channel estimate is applied after the IFFT operation. In other embodiments, the scaling of the time domain channel estimate can also be carried out after averaging, or other signal processing operations such as truncation or thresholding operations.
  • channel estimation module 260 may be implemented in software, hardware, firmware, or some combination, and may be configured to perform the functions described in the embodiments described herein.
  • channel estimation module 260 may comprise memory and one or more processors that may be dedicated to the channel estimation module 260 or be shared with other features or modules of the repeater.
  • N-sample blocks indicative of the transmit signal may be stored in memory, as well as N-sample blocks indicative of the receive signal.
  • the processor may access instructions to read the stored data and perform FFT operations on the data to generate frequency domain pilot signal blocks and frequency domain receive signal blocks.
  • the processor may access instructions to generate a frequency domain channel estimate from the frequency domain pilot and receive signal blocks.
  • the processor may access instructions to perform an inverse fast Fourier transform on the frequency domain channel estimate to generate a time domain channel estimate.
  • the processor may access information indicative of one or more scaling factors to scale the time domain channel estimate, while in some embodiments scaling is performed prior to the IFFT operations.
  • the FFT and IFFT operations may be performed using hardware such as a digital signal processor (DSP).
  • DSP digital signal processor
  • the use of frequency domain channel estimation in the absence of a cyclic prefix introduces errors such as inter-carrier interference (ICI) and inter-symbol interference (ISI) that degrade the accuracy of the channel estimation. More specifically, the ICI/ISI errors arise as a result of the difference between a linear convolution and a circular convolution in the frequency domain channel estimation process due to the lack of a cyclic prefix in the pilot signal. If there is no cyclic prefix, the previous symbol will leak into the current one, causing inter-symbol interference. According to embodiments of the present invention, a method to estimate the ICI and ISI error terms in the frequency domain channel estimate and remove the error terms from the channel estimate is provided to improve the accuracy of the channel estimation.
  • ICI inter-carrier interference
  • ISI inter-symbol interference
  • the channel estimation method for canceling ICI/ISI errors can be implemented in an echo cancellation repeater such as repeater 250 of FIG. 3 .
  • the channel estimation module 260 can implement the channel estimation method of the present invention for canceling ICI/ISI errors due to the use of frequency domain channel estimation in the absence of a cyclic prefix in the pilot signal.
  • symbols of the pilot signal (the transmit signal), as well as symbols of the receive signal, are grouped in blocks of length N, where N is the size of the FFT being performed on both the pilot symbols and the receive symbols.
  • the receive symbols are assumed to be the linear convolution of the pilot symbols and the channel, plus noise. Because there is no cyclic prefix, the pilot symbols of the previous block leak into the current receive symbols block, causing ISI (inter-symbol interference). Similarly, ICI (inter-carrier-interference) is also created on account of the lack of a cyclic prefix. In the absence of a cyclic prefix, the receive symbols are just a linear convolution of the pilot symbols and the channel. If there was a cyclic prefix in the pilot symbols, the received symbols would be the circular convolution of the current block of pilot symbols and the channel. The difference between circular and linear convolution results in ICI error terms.
  • H(f) denotes the perfect feedback channel estimate
  • the feedback channel estimate ⁇ (f) for a particular frequency if computed only using one block of pilot signal and one block of receive signal, is given as:
  • the ICI error terms are reconstructed using the most recent channel estimate, and introduced to the current receive symbol block so that the receive symbols block approximately equals to the circular convolution of the current pilot symbols block and the feedback channel.
  • the ISI terms are reconstructed using the most recent channel estimate and introduced to the current receive symbols block to minimize the leakage from the previous block of pilot symbols.
  • the removal of the ISI terms also renders the receive symbols block approximately equals to the circular convolution of the current pilot symbols block and the feedback channel. In this manner, ICI and ISI error terms in the channel estimate are cancelled out and a more accurate channel estimate is obtained.
  • the ICI error term and the ISI error term are reconstructed as follows.
  • the channel estimation process estimates the channel h using the receive samples y(n) assuming that the transmit (pilot) samples x(n) are known.
  • the receive samples y(n) are divided into blocks of length N and the i-th block of the receive samples is expressed as:
  • a linear convolution matrix L i for the i-th block of the pilot samples x(n) is given as:
  • a circular convolution matrix C i for the i-th block of the pilot samples x(n) is given as:
  • the receive sample block y 1 can be written in matrix-vector notation using the linear convolution matrix L 1 as follows:
  • the receive sample block y i can be expressed using the circular convolution matrix and the linear convolution matrix as follows:
  • the first term in the above expression is the ideal circular convolution of the pilot samples with the feedback signal.
  • the second term in the above expression represents the ICI/ISI error terms resulted from the difference between the circular convolution and linear convolution. Since both the linear convolution matrix L i and the circular convolution matrix C are based on the pilot samples and are known, the ICI and ISI error terms can be computed using the latest channel estimate h. The computed error terms can then be subtracted from the receive sample block to remove the ICI/ISI errors and the channel estimate can be computed using the corrected receive sample block to yield a more accurate channel estimate.
  • the ICI and ISI error terms are eliminated by subtracting the circular convolution matrix from the linear convolution matrix first (L i ⁇ C i ) multiplying by the latest channel estimate h and then subtracting the difference (L i ⁇ C 1 )* ⁇ from the receive sample block y i .
  • the error is subtracted in the time domain. The channel would then be estimated using frequency domain processing.
  • X i , H and Z i are the DFT of x i , h and z i , respectively.
  • the ICI and ISI error terms are subtracted from the receive sample block.
  • the subtraction can take place in the time domain or in frequency domain. Assuming ⁇ tilde over (h) ⁇ is the latest channel estimate or the most recent channel estimate, the latest channel estimate can be used to calculate an updated channel estimate as follows:
  • the error in the channel estimate is cancelled.
  • the result of introducing the error correction term in the receive sample block is to make the receive sample block look more like the result of a circular convolution.
  • the error term is subtracted in the frequency domain.
  • the frequency domain channel estimate ⁇ is given as:
  • Y C i denotes the corrected Y i being (Y i ⁇ F(L i ⁇ C i ) ⁇ tilde over (h) ⁇ ).
  • the error term on the channel estimation can be expressed as: diag(X i ) ⁇ 1 *F(L i ⁇ C i )* ⁇ tilde over (h) ⁇ .
  • This error term on the channel estimate can be subtracted out directly from the channel estimate.
  • the computed error correction term is processed through the channel estimation process and then the processed channel estimate error term is subtracted from the computed channel estimate.
  • the time domain channel estimate is formed by taking the inverse DFT of H and is given as:
  • the frequency domain channel estimation process is performed using a Discrete Fourier Transform (DFT).
  • the frequency domain channel estimate process can be performed using a Fast Fourier Transform (FFT).
  • DFT Discrete Fourier Transform
  • FFT Fast Fourier Transform
  • FIG. 5 is a flowchart illustrating the channel estimate method for cancelling the ICI/ISI error terms which can be implemented in the repeater of FIG. 3 according to one embodiment of the present invention.
  • channel estimation method 350 starts by receiving blocks of N samples of the transmit signal (the pilot signal) and the receive signal (step 352 ). Then, using the block of N pilot samples and a current channel estimate or the most recent channel estimate, method 350 estimates an error correction term.
  • method 350 computes a linear convolution matrix L i (step 354 ) and also computes a circular convolution matrix C i (step 356 ). Then, the error correction term is computed using the linear and circular convolution matrices and the most recent channel estimate (step 358 ).
  • the error correction term is (L i ⁇ C i ) ⁇ tilde over (h) ⁇ in the time domain and F(L i ⁇ C i ) ⁇ tilde over (h) ⁇ in the frequency domain where ⁇ tilde over (h) ⁇ denotes the latest or most recent channel estimate and F is a frequency domain transform.
  • the error correction term is subtracted from the receive sample block (step 360 ). The subtraction can be carried out in the time domain or in the frequency domain.
  • An updated channel estimate is then computed using the corrected receive sample block and the pilot sample block in frequency domain (step 362 ). In some embodiments, the updated channel estimate is computed using the current block of corrected receive samples and pilot samples as well as previous blocks of similarly corrected receive samples and their corresponding pilot samples. The most recent channel estimate is then replaced with the updated channel estimate (step 364 ).
  • the channel estimation method for ICI/ISI error terms reconstruction and removal is repeated on the next block of N samples of the transmit signal and the receive signal using the latest or most recent channel estimate to compute the error correction term to improve the channel estimate (step 366 ). In each repetition, the ICI/ISI error correction term is computed using the latest channel estimate and then removed from the receive samples to provide an improved channel estimate.
  • the channel estimation method for ICI/ISI error terms reconstruction and removal is iterated on the current block of N samples of the transmit signal and the receive signal for a fixed number of iterations (step 368 ). In each iteration, the ICI/ISI error correction term is computed using the latest channel estimate and then removed from the receive samples to provide an improved channel estimate.
  • step 368 illustrates an alternate embodiment of the channel estimation which is optional and may be omitted if iteration on the same sample blocks is not desired.
  • channel estimation module 260 in repeater 250 may be used to implement the channel estimate method of the present invention for cancelling the ICI/ISI error terms.
  • Channel estimation module 260 may be implemented in software, hardware, firmware, or some combination, and may be configured to perform the functions described in the embodiments described herein.
  • channel estimation module 260 may comprise memory and one or more processors that may be dedicated to the channel estimation module 260 or be shared with other features or modules of the repeater.
  • N-sample blocks indicative of the transmit signal may be stored in memory, as well as N-sample blocks indicative of the receive signal.
  • the processor may access instructions to read the stored data and compute the linear convolution matrix and compute the circular convolution matrix.
  • the processor may further access instructions to compute the error correction term and to subtract the error correction term from the receive samples.
  • the processor may access instructions to generate updated channel estimates from the corrected receive sample block and to replace the most recent channel estimate with the updated channel estimate.
  • the processor may access instructions to iterate the computation using the current receive samples and pilot samples for a fixed number of iterations.
  • the processor may access instructions to repeat the computation using the next block of receive and pilot samples.
  • the convolution computation and channel estimate computation operations may be performed using hardware such as a digital signal processor (DSP).
  • DSP digital signal processor
  • the channel estimation method of the present invention provides particular advantages by mitigating the channel estimation performance loss due to the lack of a cyclic prefix. Improvement in the channel estimation, in the context of a repeater, allows a significant improvement in the amount of gain achievable. Furthermore, even for a given, fixed gain in the repeater, when the channel estimate improves, the output SNR (signal-to-noise ratio) increases (where output SNR is a measure of the noise introduced by the repeater), meaning that the fidelity of the repeater improves.
  • SNR signal-to-noise ratio
  • frequency domain channel estimation in the absence of a cyclic prefix in the pilot signal introduces ICI and ISI error terms that degrade the accuracy of the channel estimation. More specifically, the ICI/ISI error terms arise as a result of the difference between the linear convolution and the circular convolution due to the lack of a cyclic prefix in the pilot signal.
  • a channel estimation method operates to capture both the causal and the non-causal taps of the channel in generating the channel estimate. By capturing the non-causal taps as well as the causal taps of the channel, a more accurate channel estimate can be obtained.
  • a channel estimation method for mitigating the channel estimation performance loss due to the lack of a cyclic prefix in the pilot signal shifts the largest channel tap in the channel estimate to the reference time “0” for channel estimation. However, shifting the channel tap in this manner causes all the channel taps that occur before the largest tap to become “non-causal” taps. That is, part of the feedback channel to be estimated becomes non-causal.
  • the channel estimation method of the present invention operates to capture both the causal and the non-causal taps of the channel in generating the channel estimate.
  • discarding the non-causal part of the channel may result in instability well before the repeater has reached full gain.
  • a ⁇ 20 dB output SNR gain can be achieved when the repeater reaches full gain.
  • FIG. 6 is a detail block diagram of a repeater in which the channel estimation method for sampling both causal and non-causal taps can be implemented according to one embodiment of the present invention.
  • a repeater 400 receives a remote signal S(t) on a donor antenna 415 which is coupled to receive circuitry including a transceiver front end circuit 416 and a receive filter 443 .
  • the receive signal samples r[k] from the transceiver front end circuit 416 are coupled to receive filter (Rx filter) 443 and then to an echo canceller including a summer 444 for echo cancellation.
  • the echo cancelled receive signal r′[k] is coupled to a delay element 446 to introduce a desired amount of delay D 1 to decorrelate the transmit signal from the remote signal.
  • Delay element 446 can be a fixed delay or a variable delay element as shown in FIG. 6 . In other embodiments, delay element 446 can be provided before the echo canceller.
  • the delayed echo cancelled signal is coupled to transmit circuitry including a transmit filter (Tx filter) 448 , a gain stage 449 applying a gain of G and a transceiver front end circuit 418 .
  • Gain stage 449 generate the transmit signal samples y[k] which are provided to transceiver front end circuit 418 .
  • Repeater 400 transmits the transmit signal Y(t) generated by transceiver front end circuit 418 on a coverage antenna 420 .
  • a gain control block 480 controls the variable gain of gain stage 449 and a channel estimation block 450 performs channel estimation of the feedback channel.
  • the transmit signal samples y[k] are used as the pilot signal for the gain control block 480 and the channel estimation block 450 .
  • the channel estimation block 450 implements frequency domain channel estimation, such as FFT-IFFT type processing or DFT-IDFT type processing.
  • Channel estimation block 450 also receives the receive signal samples r[k] and perform channel estimation to generate a feedback channel estimate ⁇ . More specifically, channel estimation block 450 generates the feedback channel estimate ⁇ using at least N samples of the pilot signal and N samples of the receive signal r[k].
  • the feedback channel estimate ⁇ is provided to a feedback signal estimation block 452 which, together with the transmit signal y[k], computes a feedback signal estimate ⁇ circumflex over (l) ⁇ [k].
  • the feedback signal estimate computation should also go through the same filtering to generate the corresponding feedback signal estimate.
  • the canceller 444 is positioned after the receive filter, therefore, the feedback signal estimate ⁇ circumflex over (l) ⁇ [k] is computed as the convolution of the feedback channel estimate with the transmit signal y[k] and also with the receive filter (Rx filter 443 ).
  • the feedback signal estimate ⁇ circumflex over (l) ⁇ [k] is provided to summer 444 to be subtracted from the receive signal r[k] for realizing echo cancellation.
  • the pilot samples y[k] are shifted so that the largest channel tap is moved to a reference time “0”.
  • a variable delay element 454 introduces a delay D 2 to the pilot samples before the pilot samples are provided to the channel estimation block 450 .
  • the adjustable delay D 2 operates to delay the pilot signal in time and the desired amount of delay D 2 is introduced so that the largest channel tap is aligned with the reference time 0 .
  • samples of the pilot signal (the transmit signal), as well as samples of the receive signal, are grouped in blocks of length N, where N is the size of the frequency transform (such as FFT).
  • FIG. 7 illustrates the relationship between the receive signal, the pilot signal and the feedback channel. More specifically, the receive signal is the pilot signal convolve with the feedback channel h.
  • the channel estimation method applies a variable delay, such as through variable delay element 454 , to each block of N pilot samples.
  • the delayed pilot samples are then sent with the receive samples to the channel estimation block to estimate the feedback channel.
  • FIG. 8 illustrates the channel response of the channel estimate without the delay adjustment to left shift the pilot signal samples.
  • FIG. 9 illustrates the channel response of the channel estimate after the delay adjustment to left shift the pilot signal samples. Referring to FIG. 8 , without any delay adjustment, the largest channel tap does not align with reference time 0 . However, referring to FIG. 9 , after applying an appropriate amount of delay D 2 , the largest channel tap of the channel response is now shifted to the left to be aligned with reference time 0 .
  • FIG. 10 illustrates the channel response of the channel estimate showing the causal taps and the non-causal taps to be used by the channel estimation method according to one embodiment of the present invention.
  • the largest channel tap is now at reference time 0 .
  • there is some signal energy present at the end of the FFT block as shown by the enlarged graph in FIG. 10 .
  • the signal energy at the end of the FFT block is indicative of the channel taps that become non-causal due to the left shifting of the pilot samples.
  • the channel estimation method retain both the causal taps and the non-causal taps in the time domain feedback channel estimate.
  • the channel estimate is of a length N.
  • M1 causal taps and M2 non-causal taps are retained in the channel estimate.
  • the causal taps are taken from sample 0 to M1, where M1 is assumed to be much less than N since the delay spread of the feedback channel for the repeater is typically small.
  • the non-causal taps are taken from sample N ⁇ M2 to N, where M2 is assumed to be much less than N for the same reason as M1.
  • the total channel estimate (both the causal and the non-causal taps M1+M2) is less than N taps and only M1+M2 taps of the channel estimate are kept.
  • the receive filter Rx filter 443 lies after the received samples are tapped for channel estimation and before the summer 444 of the echo canceller. Because the actual echo cancellation happens after the receive filter 443 , for that reason, in one embodiment, the pilot signal is convolved with the total channel estimate ⁇ and with a filter, such as the receive filter, to generate the feedback signal estimate ⁇ circumflex over (l) ⁇ [k] for echo cancellation. That is, the total feedback channel estimate ⁇ is convolved with the receive filter to generate a “composite channel estimate” given as ⁇ *RxFilter. The composite channel estimate is then convolved with the pilot samples y[k] to generate the feedback signal estimate. The feedback signal estimate thus generated is then supplied to summer 444 to be subtracted from the receive signal r[k] to realize echo cancellation.
  • a filter such as the receive filter
  • the pilot signal needs to be convolved with both the channel estimate as well as another filter representative of the receive signal filtering, such as the receive filter. This can be done by convolving the pilot signal with a composite channel estimate comprised of the channel estimate as well as the other filter. If the other filter has a delay of D which is greater than M2, the number of non causal taps, and the other filter is circularly convolved with the channel estimate, then after keeping the first M1+L ⁇ 1 taps where L is the length of the filter, both the non-causal and causal taps are included in the composite channel estimate.
  • FIG. 11 is a flowchart illustrating the channel estimation method for keeping both the causal and the non-causal taps of the feedback channel estimate according to one embodiment of the present invention.
  • channel estimation method 500 starts by receiving blocks of N samples of the amplified signal as a pilot signal and receiving blocks of N samples of the receive signal (step 502 ). Then, method 500 introduces a delay to the samples of the pilot signal to align the largest channel tap of the feedback channel to a first reference time (step 504 ). Method 500 then estimates the feedback channel between the first antenna and the second antenna using frequency domain channel estimation using samples of the pilot signal and samples of the receive signal (step 506 ). Method 500 generates a time domain feedback channel estimate from the frequency domain channel estimation.
  • the time domain feedback channel estimate having N taps (step 508 ).
  • method 500 retains M1 causal taps and M2 non-causal taps in the time domain feedback channel estimate (step 510 ).
  • the M1 causal taps are the first M1 taps of the channel estimate and the M2 non-causal taps are the last M2 taps of the channel estimate.
  • M1+M2 is less than N and only M1+M2 taps of the channel estimate are kept.
  • no delay is introduced to the pilot samples and the channel estimation method operates in the same manner to capture both the causal taps and the non-causal taps of the channel estimate.
  • delay D 2 in repeater 400 may be set to zero and step 504 in method 500 is optional.
  • Channel estimation module 260 in repeater 250 may be used to implement the channel estimate method of the present invention for keeping both the causal and the non-causal taps of the feedback channel estimate.
  • Channel estimation module 260 may be implemented in software, hardware, firmware, or some combination, and may be configured to perform the functions described in the embodiments described herein.
  • channel estimation module 260 may comprise memory and one or more processors that may be dedicated to the channel estimation module 260 or be shared with other features or modules of the repeater.
  • N-sample blocks indicative of the transmit signal may be stored in memory, as well as N-sample blocks indicative of the receive signal.
  • the processor may access instructions to read the stored data and introduce the necessary delay.
  • the processor may access instructions to perform FFT operations on the data to generate frequency domain pilot signal blocks and frequency domain receive signal blocks.
  • the processor may access instructions to generate a frequency domain channel estimate from the frequency domain pilot and receive signal blocks.
  • the processor may access instructions to perform an inverse fast Fourier transform on the frequency domain channel estimate to generate a time domain channel estimate.
  • the processor may access instructions to retain causal taps and non-causal taps in the time domain channel estimate.
  • the FFT and IFFT operations may be performed using hardware such as a digital signal processor (DSP).
  • DSP digital signal processor
  • the M1 causal taps and the M2 non-causal taps of the time domain channel estimate are scaled using the set of correction scaling factors described above with reference to FIG. 4 for cancelling multiplicative error in the channel estimate.
  • the correction scaling factors are function of only the tap index and the FFT block size. The resulting scaled channel estimate can then be used for echo cancellation in the repeater.
  • the feedback channel estimate is equal to the FFT size N, for example 1024.
  • the feedback channel duration is limited. For instance, in repeater applications, the feedback channel is usually on the order of tens of samples, such as 64 samples.
  • Channel estimation algorithm based on pilots introduces noise in the channel estimate. The noise is strong where the pilot is weak, that is, out-of-band. The noise also permeates to all samples of the time-domain channel estimation.
  • the receive filter (RxFilter) is sometimes used to reduce channel estimation noise by suppressing the noise at a stopband.
  • a channel estimation method in a repeater applies truncation to the time-domain channel estimate to zero out channel taps that do not contain the feedback channel so as to reduce channel estimation noise. More specifically, the method uses a priori information about the channel length to determine in which taps the channel lies. Channel taps that are outside of the channel are zeroed out to reduce noise.
  • the channel estimate is truncated by zeroing out the channel taps that are outside of the channel. For instance, in one embodiment, for a channel estimate ⁇ having a size N, the truncation is applied to keep the first M taps of the channel and to zero out channel taps M+1 to N.
  • the truncated channel estimate is thus given as:
  • channel estimate truncation is applied to preserve both the causal and non-causal taps of the channel estimate.
  • the pilot samples are left shifted to align the largest channel tap at reference time 0 , part of the channel becomes non-causal and the non-causal channel needs to be accounted for in the channel estimation process.
  • both the causal and the non-causal are used for the purpose of channel estimation.
  • the channel estimate is truncated by zeroing out the channel taps that are outside of the causal and the non-causal channels. For instance, the truncation is applied to keep the first M1 taps and the N ⁇ M2 to N taps of the channel while zeroing out channel taps M1+1 to N ⁇ M2 ⁇ 1.
  • the truncated channel estimate is thus given as:
  • ⁇ [ ⁇ 0 , ⁇ 1 , ⁇ 2 ,L ⁇ M 1,0L0, ⁇ N ⁇ M 2, ⁇ N ⁇ M 2+1,L ⁇ N ⁇ 1 ],
  • M1 is greater than or equal to the causal channel length L and M2 is greater than or equal to the non-causal channel length P.
  • M1 is greater than or equal to the causal channel length L and M2 is greater than or equal to the non-causal channel length P.
  • channel estimate noise is reduced to improve the accuracy of the channel estimation.
  • a method for improving the stability of a repeater in the presence of large signal dynamics implements channel estimation pruning.
  • the power of the remote signal is monitored to detect sudden changes in the receive power.
  • the channel estimation pruning method operates to discard channel estimates or pilot and receives samples associated with large change in the signal dynamics of the remote signal. In this manner, the overcall channel estimate will not be corrupted due to sudden changes in the receive power.
  • FIG. 12 is a flowchart illustrating the channel estimation pruning method in a wireless repeater according to one embodiment of the present invention.
  • the channel estimation pruning method of the present invention can be implemented in an echo cancellation repeater, such as the repeater shown in FIG. 3 or FIG. 6 .
  • the channel estimation pruning method is implemented in a channel estimate module in the channel estimation block ( 260 or 450 ) of the repeaters in FIG. 3 and FIG. 6 . More specifically, the channel estimation block estimates the feedback channel using blocks of incoming pilot samples (x in FIG. 3 ; or delayed y[k] in FIG. 6 ) and blocks of incoming receive samples (y in FIG. 3 ; or delayed r[k] in FIG. 6 ).
  • a final channel estimate is computed using the blocks of incoming pilot samples and the blocks of incoming receive samples. The final channel estimate is then used by the echo canceller to cancel out the feedback signal.
  • a channel estimate is computed using a block of incoming pilot samples and a block of incoming receive samples and multiple channel estimates thus computed are then averaged to generate the final channel estimate.
  • the channel estimate computed for each individual pilot sample block and the corresponding receive sample block is referred to as a “sub channel estimate”.
  • a channel estimation pruning method 600 implemented in a channel estimation algorithm operates to discard samples or blocks of N samples of the pilot signal and the receive signal used for the channel estimation after a large jump in remote signal power has been detected, N being the size of the FFT used for frequency domain channel estimation or the size of the feedback channel estimate.
  • Method 600 starts by receiving the incoming receive signal at a repeater which includes the remote signal (step 602 ).
  • the receive signal is the sum of the remote signal and the feedback signal.
  • a signal in the feedback loop of the repeater is used as the pilot signal for channel estimation.
  • the pilot signal is the echo cancelled transmit signal or the amplified signal of the repeater.
  • the pilot signal and the receive signal, or blocks of pilot signal samples and blocks of receive signal samples, are provided to the channel estimation block for performing channel estimation (step 603 ).
  • swings in the power level of the remote signal are detected to determine if there are large power swings in the remote signal (step 604 ).
  • the swings in the power level of the remote signal can be detected directly from the receive signal or it can be detected indirectly through measurements of other signals having a power level response corresponding to the remote signal.
  • swings in the remote signal is detected by using an FIR (finite impulse response) or IIR (infinite impulse response) filter. The detection is implemented by monitoring the power differential between samples of the receive signal spaced a given time apart to determine if a sudden change in the power level has occurred.
  • method 600 When no large power swings are detected in the gain control input signal (step 606 ), method 600 returns to step 602 to continue to receive the incoming receive signal and continue to estimate the feedback channel based on the incoming samples of the pilot signal and the receive signal. However, when a large power swing, such as a power swing of 9 dB or greater, is detected (step 606 ), samples or blocks of N samples of the pilot signal and the corresponding receive signal are discarded (step 608 ). A final channel estimate is computed using one or more of the undiscarded blocks of the pilot and receive signals (step 610 ). Method 600 then returns to step 602 to continue to receive incoming receive signal. In this manner, samples of or blocks of N samples of the pilot signal and receive signal associated with swings in the remote signal power are discarded before the pilot signal samples and receive signal samples corrupt the channel estimates.
  • a large power swing such as a power swing of 9 dB or greater
  • method 600 discards both the current block of N samples and one or more previous blocks of N samples of the pilot and receive signals in response to a large power swing being detected in the remote signal.
  • the reason to discard both the current and the previous samples of the pilot and receive signals is that there is always some delay between the time a change in remote signal power occurs, and the time the repeater can detect that change. Therefore, it is possible that some of the pilot signal samples and the receive signal samples are already corrupted by the power swing. Accordingly, in channel estimation pruning method 600 , both the current and the previous pilot and receive signal samples are discarded when a large power swing is detected so that no corrupted samples are retained and used for computed the final channel estimate.
  • swings in the remote signal can be detected using the receive signal, or the cancelled signal at any point along the main path through the repeater.
  • FIG. 13 is a flowchart illustrating the channel estimation pruning method in a wireless repeater according to an alternate embodiment of the present invention.
  • the channel estimation pruning method operates to discard sub channel estimates after a large jump in the power signal of the remote signal has been detected.
  • Method 700 starts by receiving the incoming receive signal at a repeater which includes the remote signal (step 702 ).
  • the pilot signal and the receive signal, or blocks of pilot signal samples and blocks of receive signal samples, are provided to the channel estimation block for performing channel estimation (step 703 ).
  • Sub channel estimate for each incoming block of pilot signal samples is thus computed (step 704 ).
  • Each sub-channel estimate is computed using a block of N samples of the pilot signal and a block of N samples of the receive signal.
  • step 705 swings in the power level of the remote signal are detected to determine if there are large power swings in the remote signal.
  • step 706 When no large power swings are detected in the gain control input signal (step 706 ), method 700 returns to step 702 to continue to receive the incoming receive signal and continue to estimate the feedback channel based on the incoming samples of the pilot signal and the receive signal.
  • step 708 when a large power swing, such as a power swing of 9 dB or greater, is detected (step 706 ), one or more sub channel estimates are discarded (step 708 ).
  • a final channel estimate is computed using a single undiscarded sub channel estimate or by averaging two or more undiscarded sub channel estimates (step 710 ).
  • Method 700 then returns to step 702 to continue to receive incoming receive signal.
  • both the current and one or more previous sub channel estimates are discarded when a large power swing is detected.
  • the final channel estimate used should be the last good final channel estimate, that is, the latest final channel estimate that does not use the sub channel estimates being discarded.
  • the reason to discard both the current and the previous channel estimates is that there is always some delay between the time a change in remote signal power occurs, and the time the repeater can detect that change. Therefore, it is possible that a bad sub channel estimate has already occurred by the time the channel estimation pruning method 700 detects the sudden change in receive power. Repeater instability can occur if only the current sub channel estimate is discarded.
  • both the current and the one or more previous sub channel estimates are discarded when a large power swing is detected and the last good final channel estimate is used to continue averaging of the channel estimates.
  • method 700 discards all of the sub channel estimates that may be corrupted by the power swing so that no corrupted sub channel estimate is retained.
  • Channel estimation module 260 in repeater 250 may be used to implement the channel estimation pruning method of the present invention.
  • Channel estimation module 260 may be implemented in software, hardware, firmware, or some combination, and may be configured to perform the functions described in the embodiments described herein.
  • channel estimation module 260 may comprise memory and one or more processors that may be dedicated to the channel estimation module 260 or be shared with other features or modules of the repeater.
  • N-sample blocks indicative of the transmit signal may be stored in memory, as well as N-sample blocks indicative of the receive signal.
  • the processor may access instructions to read the stored data and perform channel estimation on the pilot signal blocks and receive signal blocks.
  • the processor may access instructions to detect changes in the power level of the remote signal.
  • the processor may access instructions to discard samples or blocks of samples of the pilot and receive signals, while in some embodiments, one or more sub channel estimates may be discarded.
  • the various operations may be performed using hardware such as a digital signal processor (DSP).
  • DSP digital signal processor
  • the advantage of applying power detection in an interference cancellation repeater is that stability is maintained regardless of the signal dynamics of the remote signal. If there were no power detection to deal with such signal dynamics, the repeater could potentially become unstable.
  • the channel estimation pruning method of the present invention ensures that an on-frequency repeater can run robustly in the presence of large scale signal dynamics of the remote signal. More specifically, the channel estimation pruning method of the present invention ensures that channel estimates do not become corrupted due to undesired signal dynamics so that channel estimation and echo cancellation can run robustly in the presence of changing signal dynamics of the remote signal.
  • Computer-readable media includes computer storage media.
  • a storage media may be any available media that can be accessed by a computer.
  • such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer.
  • Disk and disc includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.
  • control logic used herein applies to software (in which functionality is implemented by instructions stored on a machine-readable medium to be executed using a processor), hardware (in which functionality is implemented using circuitry (such as logic gates), where the circuitry is configured to provide particular output for particular input, and firmware (in which functionality is implemented using re-programmable circuitry), and also applies to combinations of one or more of software, hardware, and firmware.
  • the methodologies may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein.
  • Any machine readable medium tangibly embodying instructions may be used in implementing the methodologies described herein.
  • software codes may be stored in a memory, for example the memory of mobile station or a repeater, and executed by a processor, for example the microprocessor of modem.
  • Memory may be implemented within the processor or external to the processor.
  • memory refers to any type of long term, short term, volatile, nonvolatile, or other memory and is not to be limited to any particular type of memory or number of memories, or type of media upon which memory is stored.

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PCT/US2010/034424 WO2010132483A2 (fr) 2009-05-11 2010-05-11 Estimation de canal de contre-réaction dans le domaine des fréquence pour un répéteur à suppression d'interférence comprenant l'échantillonnage de prises non causales

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EP2430807A2 (fr) 2012-03-21
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US8611227B2 (en) 2013-12-17
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KR101385646B1 (ko) 2014-04-16

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