# Patent

## Title
Circuits and methods for spatio-spectral interference mitigation

## Abstract

A plurality of receivers each having: an RF amplifier having an RFAMP input coupled to an antenna and having an RFAMP output; a capacitor having a CAP first side connected to the RFAMP output and a CAP second side; a passive mixer coupled to the RFAMP input, the CAP second side, and the output of a local oscillator phase shifter; an operational transconductor amplifier having an OTA input connected to the CAP second side and having an OTA output; a feedback resistor connected between the OTA input and the OTA output; a baseband transconductor having a BBGM input connected to the OTA input and a BBGM output; a cancelling transconductor having a CANCELLER output connected to the BBGM output and having a CANCELLER input; and an attenuator between the OTA output and the CANCELLER input, wherein the OTA output of each of the plurality of receivers are connected together.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent Applications Nos. 62/288,852, filed Jan. 29, 2016, and 62/452,280, filed Jan. 30, 2017, each of which is hereby incorporated by reference herein in its entirety.


STATEMENT REGARDING GOVERNMENT FUNDED RESEARCH
This invention was made with government support under FA8650-14-1-7414 awarded by DARPA. The government has certain rights in the invention.


BACKGROUND
Multiple-antenna techniques, often referred to as multiple-input-multiple-output (MIMO) techniques, enable communication with increased spectral efficiency, either through linear beamforming which enhances signal-to-noise ratio (SNR), spatial multiplexing which enhances data rate through multiple parallel streams, or spatial diversity which enhances link reliability.
MIMO systems that leverage multiplexing or diversity gains typically require space-time array signal processing that can practically only be implemented in the digital domain, and thus require digitization in each antenna signal path. A digital array also enables powerful digital array calibration. However, since the spatial processing is only performed in the digital domain, RF/analog spatial filtering is forsaken, leaving the RF/analog circuits and the data converters exposed to all the spatial content that is present at the antenna aperture, desired or not. In the presence of strong interference signals, such an architecture requires high instantaneous dynamic range from the RF/analog circuits and the data converters, leading to high power consumption and cost.
Accordingly, new circuits and methods for spatio-spectral interference mitigation are desirable.
SUMMARY
Circuits and methods for spatio-spectral interference mitigation are provided.
In some embodiments, circuits for mitigating spatio-spectral interference in a multi-input, multi-output receiver are provided, the circuits comprising: a plurality of receivers, each comprising: a local oscillator generator having a plurality of phase outputs, including a first phase output and a second phase output; an antenna having an output; an RF amplifier having an input coupled to the output of the antenna and having an output; a first switch having a control input coupled to the first phase output, having a first side connected to the input of the RF amplifier, and having a second side; a first capacitor having a first side connected to the second side of the first switch and having a second side connected to the output of the RF amplifier; a second switch having a control input coupled to the second phase output, having a first side connected to the input of the RF amplifier, and having a second side; a second capacitor having a first side connected to the second side of the second switch and having a second side connected to the output of the RF amplifier; a first operational transconductance amplifier having an input connected to the second side of the first switch and having an output; a first feedback resistor having a first side connected to the input of the first operational transconductance amplifier and having a second side connected to the output of the first operational transconductance amplifier; a second operational transconductance amplifier having an input connected to the second side of the second switch and having an output; a second feedback resistor having a first side connected to the input of the second operational transconductance amplifier and having a second side connected to the output of the second operational transconductance amplifier; a first transconductor having an input connected to the input of the first operational transconductance amplifier and having an output; a second transconductor having an input connected to the input of the second operational transconductance amplifier and having an output; a first attenuator having an input connected to the output of the first operational transconductance amplifier and having an output; a third transconductor having an input connected to the output of the first attenuator and having an output coupled to the output of the first transconductor; a second attenuator having an input connected to the output of the second operational transconductance amplifier and having an output; and a fourth transconductor having an input connected to the output of the second attenuator and having an output coupled to the output of the second transconductor, wherein the outputs of the first operational transconductance amplifier in each of the plurality of receivers are connected together, and wherein the outputs of the second operational transconductance amplifier in each of the plurality of receivers are connected together.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an example of a schematic of a circuit for canceling spatial and spectral blockers in accordance with some embodiments.
FIG. 2 is an example of a schematic of an integrated circuit having four single-element receivers for canceling spatial and spectral blockers in accordance with some embodiments.
FIG. 3 is an example of a schematic of an example of a single-element receiver of the circuit of FIG. 2 in accordance with some embodiments.
FIG. 4 is an example of a local oscillator path of a single-element receiver of the circuit of FIG. 3 in accordance with some embodiments.
FIG. 5 is an example of a schematic of an operational transconductance amplifier that can be used in the circuit of FIG. 1 in accordance with some embodiments.
FIG. 6 is an example of a schematic of a transconductor that can be used in the circuit of FIG. 1 in accordance with some embodiments.



DETAILED DESCRIPTION
Circuits and methods for spatio-spectral interference mitigation are provided.
Turning to FIG. 1, an example 100 of a circuit for reducing the impact of spatial blockers and spectral blockers on desired signals in a MIMO receiver in accordance with some embodiments is illustrated.
As shown, circuit 100 includes N antennas 102, 104, and 106, N RF amplifiers 108, 110, and 112, N passive mixers and capacitors 114, 116, and 118, N phase shifters 120, 122, and 124, N operational transconductance amplifiers (OTAs) 126, 128, and 130, N feedback resistors RFB 132, 134, and 136, N baseband transconductors 138, 140, and 142, N attenuators 144, 146, and 148, N cancellation transconductors 150, 152, and 154, N analog-to-digital converters (ADCs) 164, 166, and 168, and a digital beamforming mechanism 170.
Each path from left to right in FIG. 1 from an antenna to an ADC forms an element of circuit 100. As shown, the signals at the outputs of OTAs 126, 128, and 130 are connected together at node VBF.
N antennas 102, 104, and 106 can be implemented and arranged in any suitable manner, and any suitable number of antennas can be implemented, in some embodiments. For example, in some embodiments, the N antennas can be arranged in a uniform linear array with a λ/2 antenna spacing, where λ is wavelength in free space at the local oscillator (LO) frequency input to the mixer.
N RF amplifiers 108, 110, and 112 can be implemented in any suitable manner, and any suitable number of RF amplifiers can be implemented, in some embodiments. For example, in some embodiments, each RF amplifier can be implemented using a low noise amplifier (LNA).
N passive mixers and capacitors 114, 116, and 118 can be implemented in any suitable manner, and any suitable number of passive mixers and capacitors can be implemented, in some embodiments. For example, in some embodiments, the passive mixers can be implemented using switching transistors and capacitors can be implemented using a Metal-Insulator-Metal structure with a capacitance of 10 pF.
N phase shifters 120, 122, and 124 can be implemented in any suitable manner, and any suitable number of phase shifters can be implemented, in some embodiments. For example, in some embodiments, the phase shifters can be Cartesian phase shifters that are used in each LO path for notch steering. More particular examples of the phase shifters that can be used in some embodiments are shown in, and described in connection with, FIG. 4.
N operational transconductance amplifiers (OTAs) 126, 128, and 130 can be implemented in any suitable manner, and any suitable number of OTAs can be implemented, in some embodiments.
N feedback resistors RFB 132, 134, and 136 can be implemented in any suitable manner, and any suitable number of feedback resistors can be implemented, in some embodiments. For example, in some embodiments, feedback resistors can be 3.5 k ohm.
Together, the pairs of OTAs 126, 128, and 130 and feedback resistors RFB 132, 134, and 136, respectively, in each receiver path or element form a gyrator.
N baseband transconductors gm 138, 140, and 142 can be implemented in any suitable manner, and any suitable number of transconductors can be implemented, in some embodiments.
N attenuators 144, 146, and 148 can be implemented in any suitable manner, and any suitable number of attenuators can be implemented, in some embodiments. For example, in some embodiments the attenuators can be 7-bit digital attenuators that are controlled digitally by off-chip control signals from a hardware processor (not shown).
N cancellation transconductors gm 150, 152, and 154 can be implemented in any suitable manner, and any suitable number of transconductors can be implemented, in some embodiments. For example, in some embodiments, these cancellation transconductors can be identical to the baseband transconductors.
N analog-to-digital converters (ADCs) 164, 166, and 168 can be implemented in any suitable manner, and any suitable number of ADCs can be implemented, in some embodiments.
Digital beamforming mechanism 170 can be implemented in any suitable manner, and any suitable number of digital beamforming mechanisms can be implemented, in some embodiments.
During operation, when signals are equal in magnitude and in phase as they arrive from the broadside direction (θ=0°, or perpendicular to a line of MIMO antennas), the impedance seen at each signal input (RBB1,2 . . . N in FIG. 1) is low due to the high gain of OTAs 126, 128, and 130. When signals arrive at end-fire incidence (θ=+−/90°, or parallel to a line of MIMO antennas), the impedance seen at each signal input (RBB1,2 . . . N in FIG. 1) is equal to the resistor RFB due to the AC ground at VBF.
This impedance profile can be translated to RF at the antenna so that incidence-angle-dependent input reflection coefficients are provided that are reflective at broadside incidence angles, and matched to 50Ω at off-broadside incidence angles. Thus, voltages from signals incident from broadside are suppressed at the inputs of antennas 102, 104, and 106, outputs of RF amplifiers 108, 110, and 112, and at the inputs of baseband transconductors gm 138, 140, and 142.
In some embodiments, the notch formed at broadside incidence angles can be steered in any direction by phase-shifting the LO signals of mixers 114, 116, and 118 relative to each other using phase shifters 120, 122, and 124.
In addition, in some embodiments, tunable spectral filtering can be provided by tuning the LO signal frequency in any suitable manner, such as using a separate hardware processor.
In some embodiments, the attenuators and the cancellation transconductors can be used to perform feedforward cancellation of residual interference and distortion. In order to do so, a beam formed in the notch direction at node VBF is used as a replica of blockers in that direction. Next, the variable attenuators scale the replica to match the residue. The attenuators can be controlled in any suitable manner, such as using a separate hardware processor. Then, cancellation transconductors generate a residue current. Finally, the residue current is subtracted from the current generated by the baseband transconductors at nodes 156, 160, and 162 to thereby cancel the residual interference and distortion.
Turning to FIG. 2, an example 200 of an integrated circuit (IC) having four single-element receivers based on FIG. 1 in accordance with some embodiments is shown. In some embodiments, these single-element receivers can operate over 0.1 to 1.7 GHz. In some embodiments, IC 200 can be implemented in any suitable technology, such as 65 nm CMOS.
As illustrated in FIG. 2, IC 200 can include four single-element receivers 202, 204, 206, and 208. These receivers can be implemented in any suitable manner, and although only four receivers are shown in FIG. 2, any suitable number of receivers can be used. For example, in some embodiments, each of receivers 202, 204, 206, and 208 can be implemented using example a receiver like receiver 300 of FIG. 3.
As shown in FIG. 2, the signal VBF can be connected to VBF of other ICs 200 so that more that more than four single-element receivers can be connected together.
As shown in FIG. 3, receiver 300 includes an RF amplifier 302, four passive mixers 304, 306, 308, and 310, four capacitors 312, 314, 316, and 318, two operational transconductance amplifiers (OTAs) 320 and 322, four feedback resistors 324, 326, 328, and 330, two baseband transconductors 332 and 334, two attenuators 336 and 338, two cancellation transconductors 340 and 342, two variable gain amplifiers 344 and 346, two buffers 348 and 350, a divide-by-two local oscillator (LO) divider 352, a phase shifter 354, and a 25% duty cycle generator 356.
RF amplifier 302 can be implemented in any suitable manner, and any suitable number of RF amplifiers can be implemented, in some embodiments. For example, in some embodiments, RF amplifier can be implemented using a low noise amplifier (LNA).
Passive mixers 304, 306, 308, and 310 can be implemented in any suitable manner, and any suitable number of passive mixers can be implemented, in some embodiments. For example, in some embodiments, the passive mixers can be implemented using switching transistors.
Capacitors 312, 314, 316, and 318 can be implemented in any suitable manner, and any suitable number of capacitors can be implemented, in some embodiments. For example, in some embodiments, capacitors can be implemented using Metal-Insulator-Metal structures with a capacitance of 10 pF.
Operational transconductance amplifiers (OTAs) 320 and 322 can be implemented in any suitable manner, and any suitable number of OTAs can be implemented, in some embodiments. For example, in some embodiments, OTAs 320 and 322 can be implemented as OTA 500 as illustrated in FIG. 5.
Feedback resistors 324, 326, 328, and 330 can be implemented in any suitable manner, and any suitable number of feedback resistors can be implemented, in some embodiments. For example, in some embodiments, feedback resistors can be 3.5 k ohm.
Together, the sets of OTAs 320 and 322 and feedback resistors RFB 324, 326, 328, and 330 in each receiver path or element form a gyrator.
Baseband transconductors 332 and 334 can be implemented in any suitable manner, and any suitable number of baseband transconductors can be implemented, in some embodiments. For example, in some embodiments, transconductors 332 and 334 can be implemented as transconductor 600 as illustrated in FIG. 6.
Attenuators 336 and 338 can be implemented in any suitable manner, and any suitable number of attenuators can be implemented, in some embodiments. For example, in some embodiments, the attenuators can be 7-bit digital attenuators that are controlled digitally with control signals from a hardware processor, and can be implemented in transconductor 600 as illustrated in FIG. 6.
Cancellation transconductors 340 and 342 can be implemented in any suitable manner, and any suitable number of cancellation transconductors can be implemented, in some embodiments. For example, in some embodiments, these cancellation OTAs can be identical to the baseband transconductors.
Variable gain amplifiers (VGAs) 344 and 346 can be implemented in any suitable manner, and any suitable number of VGAs can be implemented, in some embodiments. For example, in some embodiments, the VGAs can have 30 dB range of gain control.
Buffers 348 and 350 can be implemented in any suitable manner, and any suitable number of buffers can be implemented, in some embodiments. For example, in some embodiments, the buffers can be 50 ohm buffers.
Divide-by-two local oscillator (LO) divider 352 can be implemented in any suitable manner, in some embodiments.
Phase shifter 354 can be implemented in any suitable manner, and any suitable number of phase shifters can be implemented, in some embodiments. For example, in some embodiments, the phase shifter can be implemented as described below in connection with FIG. 4.
25% duty cycle generator 356 can be implemented in any suitable manner, in some embodiments.
Turning to FIG. 4, examples of circuits implementing divide-by-two local oscillator (LO) divider 352, phase shifter 354, and 25% duty cycle generator 356 of FIG. 3 in accordance with some embodiments are shown.
As illustrated, FIG. 4 shows a divide-by-two circuit 402 that is driven by a local oscillator (LO) signal (that is twice the desired LO signal) and that drives resistors 404 and variable capacitors 406 to produce signals I-Path+, I-Path−, Q-Path+, and Q-Path−. Together the resistors and variable capacitors provide a local oscillator slew rate control filter that partially removes harmonic component.
As also illustrated, an I-path phase shifter 408 and a Q-path phase shifter 410 can each be formed from banks of parallel, selectable transconductance cells 412 and 414 and resistors 416 and 418. The banks can have any suitable number, such as six, bits of control. In some embodiments, the cells 412 can each have the same transconductance value and the cells 414 can each of the same transconductance value. Phase shifter 408 is driven by signals I-Path+ and I-Path− and produces signals IPS+ and IPS−. Phase shifter 410 is driven by signals Q-Path+ and Q-Path− and produces signals QPS+ and QPS−.
As further illustrated, the signals IPS+, IPS−, QPS+, and QPS− are provided to a 25% duty-cycle generator 420 that generators signals φ1, φ2, φ3, φ4, which are 25% duty cycle versions of IPS+, IPS−, QPS+, and QPS−, respectively, so that signals φ1, φ2, φ3, φ4 do not overlap. Signals φ1, φ2, φ3, φ4 can then be used to control switches 304, 306, 308, and 310 (FIG. 3) of the passive mixers.
Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of embodiment of the invention can be made without departing from the spirit and scope of the invention, which is limited only by the claims that follow. Features of the disclosed embodiments can be combined and rearranged in various ways.

## Claims

1. A circuit for mitigating spatio-spectral interference in a multi-input, multi-output receiver, comprising:
a plurality of receivers, each comprising:
a local oscillator generator having a plurality of phase outputs, including a first phase output and a second phase output;
an antenna having an output;
an RF amplifier having an input coupled to the output of the antenna and having an output;
a first switch having a control input coupled to the first phase output, having a first side connected to the input of the RF amplifier, and having a second side;
a first capacitor having a first side connected to the second side of the first switch and having a second side connected to the output of the RF amplifier;
a second switch having a control input coupled to the second phase output, having a first side connected to the input of the RF amplifier, and having a second side;
a second capacitor having a first side connected to the second side of the second switch and having a second side connected to the output of the RF amplifier;
a first operational transconductance amplifier having an input connected to the second side of the first switch and having an output;
a first feedback resistor having a first side connected to the input of the first operational transconductance amplifier and having a second side connected to the output of the first operational transconductance amplifier;
a second operational transconductance amplifier having an input connected to the second side of the second switch and having an output;
a second feedback resistor having a first side connected to the input of the second operational transconductance amplifier and having a second side connected to the output of the second operational transconductance amplifier;
a first transconductor having an input connected to the input of the first operational transconductance amplifier and having an output;
a second transconductor having an input connected to the input of the second operational transconductance amplifier and having an output;
a first attenuator having an input connected to the output of the first operational transconductance amplifier and having an output;
a third transconductor having an input connected to the output of the first attenuator and having an output coupled to the output of the first transconductor;
a second attenuator having an input connected to the output of the second operational transconductance amplifier and having an output; and
a fourth transconductor having an input connected to the output of the second attenuator and having an output coupled to the output of the second transconductor,
wherein the outputs of the first operational transconductance amplifier in each of the plurality of receivers are connected together, and
wherein the outputs of the second operational transconductance amplifier in each of the plurality of receivers are connected together.