Tuning algorithm for multi-tap signal cancellation circuit

A self-interference signal cancellation circuit includes a transmitter for transmitting a transmit signal, a plurality of signal paths, a controller, and a receiver for receiving a signal. Each signal path includes a delay element and a variable attenuator having attenuation levels set by the controller. A combiner generates an output signal by combining outputs of the signal paths. The circuit computes a matrix based on first and second output signals associated with first and second attenuation levels. The controller concurrently varies the attenuation level of each signal path so that a product of the matrix and the attenuation levels of the signal paths is substantially equal to the received signal. The circuit may iteratively compute the matrix using different transmit signal frequencies or with an FFT. The controller iteratively varies the attenuation level of the attenuators until a sum of the product and the received signal satisfies a predefined condition.

FIELD OF THE INVENTION

The present invention relates to wireless communication, and more particularly to a full duplex wireless communication system.

BACKGROUND OF THE INVENTION

A wireless system often operates in a half-duplex mode to either transmit or receive data at any given time. A device operating in a full-duplex mode may simultaneously transmit and receive data. However, the simultaneous transmission and reception of data are carried out over different frequencies. For example, a full-duplex cell phone uses a first frequency for transmission and a second frequency for reception. As is well known, using the same frequency for simultaneous transmission and reception in a conventional wireless system results in significant amount of self-interference at the receiver, thereby, rendering the system ineffective in receiving the desired signal.

BRIEF SUMMARY OF THE INVENTION

A circuit, in accordance with one embodiment of the present invention, includes a transmitter for transmitting a transmit signal at a selected frequency, a plurality of signal paths, a controller unit, and a receiver for receiving a signal. Each signal path includes a delay element and a variable attenuator having attenuation levels that can be set by the controller unit. A combiner generates an output signal by combining outputs of all the signal paths. The circuit computes a matrix based on first and second output signals associated with first and second attenuation levels. The controller circuit concurrently varies the attenuation level of each signal path so that a product of the matrix and the attenuation levels of the signal paths is substantially equal to the received signal.

In one embodiment, the circuit further includes, at least one antenna for receiving a signal. In one embodiment, each of the signal paths is adapted to receive a sample of a transmit signal and generate a delayed and weighted sample of the transmit signal. In one embodiment, the circuit further includes a plurality of passive couplers adapted to change a phase of one or more delayed and weighted signal paths in relation to the transmit signal, and a coupler adapted to subtract the result of the product from the receive signal. The circuit can iteratively compute the matrix using different transmit signal frequencies or by taking a frequency domain representation of the response to a wideband transmit signal, such as taking the fast Fourier transform.

In one embodiment, the circuit computes the matrix by applying a wideband transmit signal to the signal paths and computing the Fourier transform of the transmit signal and receive signal. The matrix may be a complex M×N matrix, where M is the number of frequency bins in the Fourier transform and N is the number of signal paths. The frequency domain representation of the received signal may be a complex M×1 column vector, and the attenuation level of each signal path is an element of a real N×1 column vector. In one embodiment, the product of the matrix and the attenuation levels of the signal paths is a complex M×1 column vector that is substantially equal to the received signal.

In one embodiment, the circuit further includes a circulator having a first port coupled to an antenna, a second port coupled to a transmit line of the circuit, and a third port coupled to a receive line of the circuit. In one embodiment, the second port of the circulator is disconnected from the transmit line while the attenuation levels are set, output signals are obtained for the computation of the matrix. In one embodiment, the third port of the circulator is connected to the receive line and all attenuators are set to the maximum attenuation level while the signal is received by the receiver.

In one embodiment, the controller unit iteratively varies the attenuation level of each variable attenuator until a sum of the product and the received signal satisfies a predefined condition.

Embodiments of the present invention also provide a method for performing a self-interference signal cancellation in a full-duplex wireless communication system with a cancellation circuit. The method includes supplying a sample of a transmit signal at a first frequency by a transmitter to the cancellation circuit. The cancellation circuit includes a plurality of parallel signal paths, each of the signal paths includes a delay element and a variable attenuator. The method further includes setting a first attenuation level to one or more of the variable attenuators, obtaining a first output signal of the cancellation circuit, setting a second attenuation level to the one or more of the variable attenuators, and obtaining a second output signal of the cancellation circuit. The method also includes generating a matrix based on the first and second output signals, receiving a signal at a receiver, and varying an attenuation level of each of the variable attenuators so that a product of the matrix and the attenuation levels is substantially equal to the received signal.

The method, in accordance with one embodiment of the present invention, further includes, supplying the sample of the transmit signal at a second frequency by the transmitter to the cancellation circuit, setting the first attenuation level to the one or more of the variable attenuators, obtaining a third output signal of the cancellation circuit, setting the second attenuation level to the one or more of the variable attenuators, obtaining a fourth output signal of the cancellation circuit, and expanding the matrix based on the third and fourth output signals. The method can iteratively repeat the above steps with a third, a fourth frequencies to expand the matrix.

In one embodiment, the matrix can be expanded to an M×N matrix, where M is the number of transmit signal frequencies and N is the number of signal paths of the cancellation circuit.

In one embodiment, the first attenuation level is the maximum attenuation level and the second attenuation level is the minimum attenuation level. In one embodiment, the first and second attenuation levels are applied to the variable attenuators of all of the signal paths.

In one embodiment, the method further includes iteratively varying the attenuation level of each variable attenuator until a sum of the product and the received signal satisfied a predefined condition.

The following description, together with the accompanying drawings, will provide a better understanding of the nature and advantages of the claimed invention.

DETAILED DESCRIPTION OF THE INVENTION

It is understood that the term “vector” is a synonym for signal. The terms “complex vector” and “real vector” are synonyms for complex signal and real signal, respectively. A vector can be an analog signal including an analog component I(f), Q(f), or a digital signal including a digital component I(n), Q(n). A complex vector include a complex-valued signal I+jQ. A column vector is an m×1 matrix, i.e., a matrix consisting of a single column of m elements. A m×n matrix is a matrix consisting of m rows and n columns.

FIG. 1is a simplified block diagram of a full-duplex wireless communication device100, in accordance with one embodiment of the present invention. Wireless communication device100, which may be a cellular phone, a base station, an access point or the like, is configured to transmit data/signals via transmit antenna A11and receive data/signals via a receive antenna A12. Wireless communication device (herein alternatively referred to as device)100is also shown, as including, a transmit front-end201, a signal splitter210, a receive front end301, a signal combiner310, and a self-interference cancellation circuit390. Device100may be compatible and operate in conformity with one or more communication standards such as WiFi™, Bluetooth®, GSM EDGE Radio Access Network (“GERAN”), Universal Terrestrial Radio Access Network (“UTRAN”), Evolved Universal Terrestrial Radio Access Network (“E-UTRAN”), Long-Term Evolution (LTE), and the like.

Transmit front-end201is adapted to process and generate transmit signal A. Signal splitter210splits the transmit signal and delivers a portion (sample) of this signal, i.e., signal B, to self-interference cancellation circuit390. The remaining portion of the transmit signal, which is relatively large (e.g., 85% of the transmit signal) is delivered to transmit antenna A11. Because the transmit and receive antenna A11and A12operate in substantially the same frequency band, signal IN received by receive antenna A12includes the desired signal as well as a portion of the transmitted signal OUT. The transmitted signal component that is received by antenna A12is an undesirable signal and is referred to hereinafter as the self-interference signal. Self-interference cancellation circuit390operates to reconstruct the self-interference signal, which is subsequently subtracted from the received signal IN. To achieve this, self-interference cancellation circuit390generates a multitude of weighted and delayed samples of the transmit signal, and combine these signals to generate signal C that is representative of the self-interference signal. Signal combiner310is adapted to subtract the signal it receives from self-interference cancellation circuit390from the signal it receives from antenna A12, thereby to deliver the resulting signal D to receive front-end300. Accordingly, the self-interference component of the signal received by receive front-end300is substantially degraded. In one embodiment, self-cancellation circuit390may cancel, e.g., 20-25 dB of the self-interference signal.

FIG. 2is a simplified block diagram of a full-duplex wireless communication device (hereinafter alternatively referred to as device)200, in accordance with another embodiment of the present invention. Device200is similar to device100except that device200has a single antenna A21used for both transmission and reception of signals. Device200also includes a circulator350that provides isolation between its ports. Circulator350is adapted to concurrently deliver the transmit signal and the receive signal to and from antenna A21. In one exemplary embodiment, circulator350provides approximately 15 dB of isolation between the transmit and receive paths, thereby reducing the self-interference on the receive port by approximately 15 dB.

FIG. 3is a simplified block diagram of a full-duplex wireless communication device (hereinafter alternatively referred to as device)300, in accordance with one exemplary embodiment of the present invention. Device300is shown as including a transmitter front end201, a receiver front end301, a transmit/receive antenna A31, a circulator350, and a self-interference cancellation circuit390as is also disposed in devices100and200shown inFIGS. 1 and 2, respectively. Transmitter front end201receives a complex digital signal I(n)+jQ(n), converts the complex digital signal into an analog signal I(f)+jQ(f), and frequency translates the analog signal to a RF transmit signal205. Coupler210receives a sample of transmit signal205and in response delivers a through signal212to circulator350, and a coupled signal214to splitter215. Self-interference signal cancellation circuit390is adapted to reconstruct the self-interference signal314from the sample of the transmit signal214. The reconstructed self-interference signal314is subtracted from received signal218by coupler310thereby to recover the signal of interest305, also referred to as the desired signal. The desired signal305is delivered to receiver front end301for further processing.

AlthoughFIG. 3is shown as having 8 taps51,52,53,54,55,56,57,58, it is understood that a signal cancellation circuit in accordance with embodiments of the present invention may have any number of taps. Signal cancellation circuit390is adapted to enable full duplex wireless communication by canceling or minimizing the self-interference signal received by receiver301and caused by signal transmission at transmitter201.

Signal cancellation circuit390is adapted to receive a sample of the transmitted signal205(self-interference signal) transmitted by transmitter201, reconstruct the self-interference signal from the received samples, and cancel the reconstructed signal from received signal305thereby to recover the signal of interest, also referred to as the desired signal. Signal cancellation circuit390is described in detail in US Application No. 61/736,726, the content of which is incorporated herein by reference in its entirety.

In the following, for simplicity, the same reference number may be used to identify both the path through which a signal travels, as well as to the signal which travels through that path. For example, reference numeral25may be used to refer to the signal so identified inFIG. 3, or alternatively to the path through which this signal travels. Furthermore, in the following, the terms splitter, coupler, or combiner are alternatively used to refer to an element adapted to split/divide a signal to generate more signals and/or couple/combine a multitude of signals to generate one or more signals. Such a component is also alternatively referred to herein as splitter/coupler. Furthermore, using couplers for signal distribution and combination is only one example. It is understood that other isolating components or a pair of antennas may be used instead of a circulator.

Signal cancellation circuit390is shown as including, eight variable attenuators41,42,43,44,45,46,47,48each adapted to attenuate the signal it receives from its associated delay element in accordance with a different attenuation signal ATT1-ATT8. Accordingly, signals130,125,115,105,135,145,155,160supplied respectively by variable attenuators41,42,43,44,45,46,47,48(alternatively and collectively referred to herein using reference number4) are time-delayed, weighted signal samples that are used to reconstruct the self-interference component of the transmitted signal.

In accordance with the present invention, the attenuation signals ATT1-ATT8applied to variable attenuators41,42,43,44,45,46,47,48are selected so as to maximize the matching between signals314and218—respectively supplied by coupler315and circulator350—in order to achieve maximum cancellation. To achieve this, the cancellation paths (8 in the exemplary embodiment shown inFIG. 3) are first characterized for a number of different frequencies. One such characterization includes generating a complex response plot (also known as polar plot) of the cancellation circuit after disconnecting the self-interference path, i.e., disconnecting path218from coupler310. As is well known, a complex plot shows the real and imaginary parts of a signal on x-axis and y-axis respectively. One may obtain the complex response plot by dividing the frequency domain representation of the received signal by the frequency domain representation of the transmitted signal, commonly known as the channel response. Alternatively, the received signal may be used without such division if the same transmit signal is used across all measurements.

In one embodiment, the attenuation signals applied to the attenuators are initially set to a maximum value. Next, signal314generated by the cancellation circuit at the output of coupler315is measured at a first frequency f1, thus defining a first starting point, as described further below. Such a measurement is repeated N times at N different frequencies to generate N starting points each associated with a different one of the N frequencies.FIG. 4shows a plot400of the N responses. Exemplary points s1, s2, and s3respectively show the response of the cancellation circuit at the output of coupler315at frequencies f1, f2and f3. The response at N frequency points can also be measured by transmitting a single wideband signal and measuring the N-point frequency domain representation of the response.

If the level of attenuation applied to any of the attenuators is varied from a maximum value to a minimum value at any given frequency, the response of the cancellation circuit is observed as following a substantially linear path on the complex plane. For example, assume that at frequency fond with the maximum amount of attenuation applied to attenuator41, the starting point (response of the cancellation circuit with the circulator path disconnected) is represented by s1. If the attenuation level ATT1is subsequently changed from the maximum attention level to a lower attenuation level, the response of the cancellation circuit at frequency f1is observed as traversing a linear path from point s1to point p11defined by:
imag{p11}=imag{s1}+m11*(real{p11}−real{s1})  (1)
Where p11represents the cancellation circuit's response at frequency f1, and m11represents the slope of the line connecting points s1and p11, as shown inFIG. 4.

It is also determined that the complex output response changes linearly with

10ATTi20,
where ATTirepresents the attenuation level applied to attenuator4iin dB. For a frequency fj, the complex output response may thus be defined as:

Assume that the attenuation levels ATT1and ATT2are applied to attenuators41and42while the frequency is maintained at f1. The superposition principle is then used to determine the system's response, as shown below:

Likewise, assume that the attenuation levels ATT1and ATT2are applied to attenuators41and42while the frequency is maintained at f2. The superposition principle is then used to determine the system's response, as shown below:

Assuming that the cancellation circuit responses are measured at three different frequencies, namely f1, f2, and f3, and the attenuation levels, ATT1-ATT8are varied concurrently during each sub-frequency, the response of the cancellation circuit may be represented by:
Pf=M*ATT  (5)

In expression (5), Pfis a complex column vector defined by:

[Pf⁢⁢1Pf⁢⁢2Pf⁢⁢3]
S is a complex column vector defined by:

[s1s2s3]
M is a complex matrix of size 3×8 defined by:

[m11⁢m21⁢⁢…⁢⁢m71⁢m81m12⁢m22⁢⁢…⁢⁢m72⁢m82m13⁢m23⁢⁢…⁢⁢m73⁢m83]
And ATT is a real column vector defined by:

Next, the complex response of the self-interference path at these three frequencies f1, f2, and f3is measured, i.e., after connecting the circulator back to coupler310. Assume the complex interference measurement is defined by:

This self-interference may be measured, for example, after applying maximum attenuation levels to the attenuators. Variable I thus represents the superposition of the actual self-interference with the attenuators receiving the attenuation levels leading to cancellation response at point S, as described above. The actual self-interference is thus defined by (I−S).

Therefore, the solution to the equation:
(I−S)+(S+M*ATT)=0
which is the same as
I+M*ATT=0  (6)
provides the optimum attenuation levels.

The attenuation values obtained from equation (6) are on the continuous real line and thus are rounded to the nearest attenuation values if step attenuators are used. This quantization step may lead to a less-than-optimum self-interference signal cancellation and a residual signal. In another embodiment, discrete algebra may be used to account for quantization of the attenuation levels to achieve optimal solution for the attenuation levels.

The above description applies to tuning the cancellation circuit initially. The same principle may also apply to adapting the cancellation circuit to channel changes. Any channel change may lead to non-ideal conditions leading to a small residual signal.

Assume the residual interference signal is represented by Iresidual. Residual attenuation levels ATTresidualmay thus be obtained by solving the following equation:
Iresidual+M*ATTresidual=0

The residual attenuation values ATTresidualmay be subsequently used to compute new attenuation levels iteratively. Accordingly, this process is iteratively carried out until a predefined condition is satisfied.

The above embodiments of the present invention are illustrative and not limitative. For example, while the above description is applicable to a method of tuning a cancellation circuit, it is understood that in other embodiments, it is possible to measure Pfand S without disconnecting the self-interference path, in which case both Pfand S are offset by the self-interference path, thus resulting in achieving the same matrix M. Embodiments of the present invention are not limited by the number of taps used in the signal cancellation circuit. Nor is the invention limited by the number of frequencies used to measure responses to optimize the attenuation values. Embodiments of the present invention are not limited by the type of delay element, attenuator, passive coupler, splitter, combiner, or the like, used in the cancellation circuit. Although the above description is provided with reference to a multi-tap feed-forward cancellation circuit, it is understood that the above descriptions of the present invention are equally applicable to a multi-tap feedback cancellation circuit.

FIG. 5shows a flowchart of a method500for canceling or reducing the self-interference signal at a receiver of a communication device, in accordance with one embodiment of the present invention. To achieve this, a sample of the transmit signal at a first frequency is supplied to the cancellation circuit at510. Thereafter, the controller sets a first attenuation level to one or more variable attenuators at520. A first output signal is obtained at the output of the cancellation circuit by combining output signals from all the signal paths at530. In an embodiment, the first attenuation level may be applied to all attenuators. Next, the controller set a second attenuation level to the one or more cancellation attenuators at540. In an embodiment, the second attenuation level may be applied to all attenuators. In an embodiment, the first attenuation may be the maximum attenuation level and the second attenuation level may be the minimum attenuation level. Thereafter, a second output signal is obtained at the output of the cancellation circuit by combining output signals from all the signal paths at550. Note that transmit signal is a complex signal, thus, the output signals are also complex signals. Next, at560the communication device generates a matrix based on the first and second output signals that are obtained with the corresponding first and second attenuation levels. For example, the device may compute the matrix by solving Equation (1) or determine the matrix graphically using the plot ofFIG. 4. The device may iteratively repeats the above steps510to540to generate and expand the matrix by supplying a sample of the transmit signal at a second frequency, at a third frequency, etc. In an embodiment, the matrix is determined or computed with the receiver being disconnected from the antenna or from the circulator. Next, a signal is received at the receiver at570. The controller concurrently varies the attenuation level of each signal path so that the product of the matrix and the attenuation levels of the signal paths is substantially equal to the received signal at580. In an embodiment, these thus obtained attenuation levels are applied to the signal paths, whose outputs are combined by combiner315to generate a signal representative of the self-interference signal. The reconstructed signal is subsequently subtracted from the received signal to cancel or reduce the self-interference signal at the receiver.

The above embodiments of the present invention are illustrative and not limitative. Embodiments of the present invention are not limited by the number of taps used in the signal cancellation circuit. Embodiments of the present invention are not limited by the type of delay element, attenuator, passive coupler, splitter, combiner, amplifier, or the like, used in the cancellation circuit. Embodiments of the present invention are not limited by the number of antennas used in a full-duplex wireless communication device. Embodiments of the present invention are not limited by the frequency of transmission or reception of the signal. Embodiment of the present invention are not limited by the type or number of substrates, semiconductor or otherwise, used to from a full-duplex wireless communication device. Other additions, subtractions, or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.