Device and method for reducing the self interference signal in a full-duplex communication system

Abstract

A device for reducing a self-interference contribution in a full-duplex wireless communication system configured to transmit a transmission signal and modulated by a baseband signal, and configured to receive a reception signal containing a self-interference contribution corresponding to the transmission signal, the reduction device comprising a first reduction module, configured to take a replica of the transmission signal, and configured to generate a first reduction signal, the device further comprising: a second reduction module, arranged so as to be able to take a replica of the baseband signal, and capable of generating a second reduction signal that is a function of the temporal derivative of the baseband signal, a subtractor, linked to the first reduction module and to the second reduction module, and configured to subtract from the reception signal the first reduction signal and the second reduction signal.

Claims

1. A method for reducing at least one self-interference contribution in a full-duplex wireless communication system, configured to transmit a transmission signal with a transmission carrier and modulated by a baseband signal, and configured to receive a reception signal containing at least one self-interference contribution corresponding to the transmission signal, said reduction method comprising: at least one first reduction step, in which a first reduction module takes a replica of the transmission signal and generates a first reduction signal, a second reduction step, in which a second reduction module generates a second reduction signal that is a function of the temporal derivative of the replica of the baseband signal, a subtraction step in which the first reduction signal and the second reduction signal are subtracted from the reception signal, and a calibration step prior to said first and second reduction steps and prior to said subtraction step, said calibration step comprising the following substeps: a first substep of determination of a first complex gain of the first reduction module, which minimizes a residue signal corresponding to the difference between the self-interference contribution included in the reception signal and corresponding to the transmission signal, and the first reduction signal; then a second substep of determination of a second complex gain of the second reduction module which minimizes the difference between the residue signal and the second reduction signal, wherein the calibration step further comprises a substep of determination of a digital delay minimizing the difference between the residue signal and the second reduction signal, said substep of determination of a digital delay being performed after the first substep of determination of a first complex gain and after the second substep of determination of a second complex gain.

2. The method according to claim 1, a temporal derivative of said replica of the baseband signal mixed with the transmission carrier being supplied to the second reduction module.

3. The method according to claim 2, said second reduction module applying a second complex gain to the temporal derivative of the replica of the baseband signal mixed with the transmission carrier, the second complex gain being determined so as to generate a destructive interference between, on the one hand, a residue of the destructive interference between the at least one self-interference contribution included in the reception signal and the first reduction signal, and, on the other hand, the second reduction signal.

4. The method according to claim 1, a digital delay being applied to said replica of the baseband signal before said replica is supplied to the second reduction module.

5. The method according to claim 1, said first reduction module applying a first complex gain to a replica of the transmission signal in order to supply the first reduction signal, the first complex gain being determined so as to generate a destructive interference between the self-interference contribution included in the reception signal and the first reduction signal.

6. The method according to claim 1, said substep of determination of the second complex gain further comprising the determination of a digital delay minimizing the difference between the residue signal and the second reduction signal.

7. The method according to claim 1, said calibration step being performed periodically or in case of a change of the environment of the full-duplex wireless communication system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features, details and advantages of the invention will emerge on reading the description given with reference to the attached drawings given by way of example and which represent, respectively:

(2) FIG. 1A: a full-duplex system with a single antenna of the prior art (already described);

(3) FIG. 1B: the impulse response illustrating the two self-interference paths of FIG. 1A (already described);

(4) FIG. 2A: a diagram of the prior art of self-interference reduction (already described);

(5) FIG. 2B: a simulation of the paths represented in FIG. 2A (already described);

(6) FIG. 3: an architecture of the reduction device according to the invention;

(7) FIG. 4: a schematic illustration of the method according to the invention;

(8) FIG. 5: an adaptation of the reduction device according to the invention to communications of MIMO type.

DETAILED DESCRIPTION

(9) Consider FIG. 3, which schematically illustrates an architecture of the reduction device according to the invention, of a communication device COM. A baseband transmission signal TX.sub.BB comprises a component I (phase component) and a component Q (quadrature component), which can be amplified by a baseband amplifier BB.sub.AMP. One example of signals of the components I and Q is shown at the bottom of FIG. 2B. In the case of a single-carrier modulation QPSK, each component I and Q can take two states. The component I is mixed by a mixer Mix with a transmission carrier signal LO.sub.TX, and the component Q is mixed by the mixer Mix with the same transmission carrier signal LO.sub.TX (phase-shifted by 90 by the phase-shifting module PST. The sum of these two mixed signals is then amplified by an amplifier PA, and the amplified signal, designated TX.sub.OUT, is sent to an antenna interface (not represented). The reception signal RX.sub.IN contains a self-interference contribution corresponding to the transmission signal TX.sub.OUT, that the device according to the invention aims to erase. The reduction of the self-interference contribution aims to send to the low-noise amplifier LNA only the useful reception signal, that is to say without any self-interference contribution, to then be demodulated in a reception chain known to the person skilled in the art, notably comprising a mixer M.sub.RX and a phase-shifting module PSR, and finally have a baseband reception signal BB.sub.OUT. It should be noted that the phase-shifting modules PST and PSR are not necessary for all the modulations, in particular for the non-complex modulations (for example the single-carrier modulation BPSK).

(10) A replica of the transmission signal TX.sub.OUT is taken and is brought to a first reduction module B.sub.CC. The first reduction module B.sub.CC applies a complex gain G.sub.CC* to the transmission signal TX.sub.OUT, in order to supply a first reduction signal SIG.sub.CC. The complex gain G.sub.CC* of the first reduction module B.sub.CC and applied to the transmission signal TX.sub.OUT can be determined periodically, and/or as a function of the change of environment, during a so-called calibration step, described later.

(11) The subtractor SUB subtracts the first reduction signal SIG.sub.CC from the reception signal RX.sub.IN. The subtraction of the two signals is, in reality, an addition of the signals by destructive interference, the complex gain G.sub.CC* of the first reduction module Bcc being selected for the first reduction signal SIG.sub.CC to be in phase opposition relative to the reception signal RX.sub.IN, with identical amplitude.

(12) As mentioned previously, in case of variation of the envelope of the transmission signal TX.sub.OUT, the reduction of the self-interference contribution is not perfect, allowing a residual signal RES to appear. The generation of a second reduction signal SIG.sub.ENV, corresponding to the residual signal RES, would then make it possible, by subtracting the second reduction signal SIG.sub.ENV from the residual signal RES, to recover only the useful reception signal, stripped of any self-interference contribution.

(13) Now consider that the transmission signal can be written in the form:
x(t)=A(t).Math.e.sup.j.sup.c.sup.t

(14) in which A(t) denotes the envelope of the baseband signal TX.sub.BB, and in which .sub.c denotes the pulsing of the transmission carrier LO.sub.TX.

(15) The reception signal RX.sub.IN comprising the self-interference contribution corresponds to the transmission signal TX.sub.OUT, but with a real gain G.sub.INT representing the attenuation introduced by the passage through the antenna interface, and a delay t.sub.INT corresponding in particular to the paths in the antenna interface (see FIG. 1A).

(16) The reception signal RX.sub.IN can therefore be written:
RX.sub.IN(t)=G.sub.INT.Math.A(tt.sub.INT).Math.e.sup.j.sup.c.sup.(tt.sup.INT)

(17) The delay corresponding to the passage through the antenna interface can be broken down into a multiple of the period of the carrier plus a residual delay less than this period. The envelope varies slowly relative to the carrier, so it is therefore possible to approximate the total delay by the multiple of the period of the carrier, the residue being able to be seen as a phase term. The reception signal RX.sub.IN can therefore be written:
RX.sub.IN(t)=G.sub.INT.Math.A(tt.sub.INT).Math.e.sup.j.sup.c.sup.t+.sup.INT

(18) Likewise, the first reduction signal SIG.sub.CC deriving from the first reduction module B.sub.CC, can be written:
SIG.sub.CC(t)=G.sub.CC.Math.A(tt.sub.CC).Math.e.sup.j.sup.c.sup.(tt.sup.cc)

(19) in which G.sub.CC designates the modulus of the complex gain applied to the replica of the transmission signal TX.sub.OUT, and t.sub.CC designates the delay, corresponding to a phase-shift.

(20) By considering the delay applied by the first reduction module B.sub.CC as being a phase-shift, the reception signal SIG.sub.CC can therefore be written:
SIG.sub.CC(t)=G.sub.CC.Math.A(tt.sub.CC).Math.e.sup.j.sup.c.sup.t+.sup.cc

(21) The residual signal RES is the difference between the reception signal RX.sub.IN and the first reduction signal SIG.sub.CC:
RES=RX.sub.INSIG.sub.CC
RES=G.sub.INT.Math.A(tt.sub.INT).Math.e.sup.j.sup.c.sup.t+.sup.INTG.sub.CC.Math.A(tt.sub.CC).Math.e.sup.j.sup.c.sup.t+.sup.cc

(22) The phase .sub.CC and the gain G.sub.CC being aligned respectively on the phase .sub.INT and the gain G.sub.INT of the antenna interface, in a step of calibration of the phase and of the gain, the residue can be written as a function of an aligned phase and of an aligned gain G:
RES=G.Math.[A(tt.sub.INT)A(tt.sub.CC)].Math.e.sup.j.sup.c.sup.t+

(23) The envelope signal A(t) can be likened, over short intervals, to a linear function, as the changes of envelope in FIG. 2B illustrate. Thus, A(t) can be written:
A(t)=.Math.t+

(24) in which and designate the parameters of a linear function.

(25) Consequently, the residue can be written:
RES=G.Math.[(.Math.t.Math.t.sub.INT)(.Math.t.Math.t.sub.CC)].Math.e.sup.j.sup.c.sup.t+
RES=G.Math..Math.(t.sub.CCt.sub.INT).Math.e.sup.j.sup.c.sup.t+

(26) RES=G.Math.A(t.sub.CCt.sub.INT).Math.e.sup.j.sup.c.sup.t+, in which A(t) designates the temporal derivative of A(t).

(27) In other words, the inventors have established that the residue can be seen as being the carrier signal modulated by the temporal derivative of the baseband input signal. Thus, by subtracting from the residue signal RES the baseband transmission signal, derived and delayed, and to which a complex gain G.sub.ENV* is applied whose parameters are defined during a calibration step, it is possible to recover, in the reception chain, the useful reception signal, stripped of any self-interference contribution.

(28) The delay to be applied to the derivative of the envelope signal corresponds to t.sub.INT-t.sub.CC. This delay can be applied to the already derived signal, but that would require applying a consequential analogue delay, with bulky delay lines. A preferred solution is to apply a digital delay to the envelope signal, the digital delay being able to be generated very simply, using, for example, a digital memory, then to derive this already delayed signal.

(29) In order to derive the envelope signal, referring to FIG. 3, a temporal derivative (/t), performed for example using a differentiator circuit, is applied to a replica of the components I and Q of the baseband transmission signal TX.sub.BB, before these replicas are mixed, using a mixer M.sub.ENV, with the same carrier as the transmission signal TX.sub.OUT. The use of the same carrier allows the second reduction signal SIG.sub.ENV to be entirely synchronized on the transmission signal TX.sub.OUT.

(30) FIG. 4 illustrates a reduction method implemented by a circuit of the type of FIG. 3. It comprises a first reduction step E1, together with a second reduction step E2, then a step of subtraction E3 of the signals deriving from the first reduction step E1 and from the second reduction step E2. Periodically, or in case of a change of the environment of the full-duplex communication system, a calibration step E0 is performed, in order to correctly parameterize the first reduction module B.sub.CC and the second reduction module B.sub.ENV.

(31) The calibration allows the reduction device to take account of its environment, and to follow the modifications of the environment, for example, the reflection sources, likely to generate several self-interference contributions. The calibration consists in selecting a complex gain, namely a gain in amplitude and a phase-shift, that the first reduction module B.sub.CC and the second reduction module B.sub.ENV will apply with constant parameters between two calibration procedures.

(32) The calibration step E0 is done in at least two distinct substeps. The first substep SE1 consists, initially, in determining the gain and phase parameters of the first reduction module B.sub.CC, by transmitting a given transmission signal TX.sub.OUT. The determination of the gain and phase parameters can be performed for example by varying each of the two parameters in steps and by selecting the pair of parameters which minimizes a power of a difference signal between the reception signal RX.sub.IN and the first reduction signal SIG.sub.CC, said power being able to be measured using a power detector arranged in the reception chain.

(33) Once these parameters of the first reduction module B.sub.CC are identified, the second substep SE2 consists in determining the complex gain parameters G.sub.ENV* of the second reduction module B.sub.ENV, as well as the digital delay to be applied to the replica of the baseband transmission signal TX.sub.BB before deriving it. Like the determination of the parameters of the complex gain G.sub.CC*, the determination of the parameters of the complex gain G.sub.ENV* can be performed for example by varying each of the two gain and phase parameters in steps and by selecting the pair of parameters which minimizes a power at the output of the subtractor. During this second substep SE2, an optimal digital delay is determined, in order to minimize, as in the substeps SE1 and SE2, the output power of the subtractor SUB. As an alternative, the optimal digital delay can be determined during a substep SE3, distinct from the substep SE2, which notably has the advantage of being able to separate, in the simulation of the method according to the invention, the observation of the optimization of the parameters of the complex gain G.sub.ENV* and of the digital delay.

(34) The present invention has been presented in the case of a system with a single antenna interface INT. It could be applied also to multiple-antenna systems of MIMO (Multiple Input Multiple Output) type. FIG. 6 illustrates an adaptation to MIMO communications of the reduction device according to the invention. In the context of full-duplex MIMO communications, each antenna would then have two levels of interferences to be distinguished, namely its own self-interference contribution, and the interferences from the other antennas of the MIMO system, whose level is lower. These two types of interferences could be processed by the reduction method according to the invention.

(35) The description refers only to a single self-interference contribution to be erased. However, as the processing is linear, it can be generalized to the reduction of several self-interference contributions.