WO2016117733A1

WO2016117733A1 - Multi-antenna transmission device and reception device

Info

Publication number: WO2016117733A1
Application number: PCT/KR2015/000758
Authority: WO
Inventors: 조동호; 김민호; 박대희; 서혜인
Original assignee: 한국과학기술원
Priority date: 2015-01-23
Filing date: 2015-01-23
Publication date: 2016-07-28

Abstract

The present invention is configured to convert an in-phase signal and a quadrature signal to pulse signals, which have different duty cycles, by means of pulse width modulation or switches in a single RF MIMO system so as to use the pulse signals for the transmission of an input signal, thus can be implemented in a simple structure, and can use a relatively wide signal domain and thus support a high transmission capacity more stably.

Description

Multi-antenna Transmitter and Receiver

TECHNICAL FIELD The present invention relates to a multi input multi output (MIMO) system, and more particularly, to an inphase signal and quadrature using pulse width modulation or a switch in a single radio frequency (RF) MIMO system. (quadrature) By converting the signal into pulse signals having different duty cycles and using them for the transmission of the input signal, a simple structure can be realized, and the relatively wide signal domain can be used to make it more stable. The present invention relates to a multi-antenna transmission apparatus and a reception apparatus capable of supporting high transmission capacity.

In general, a MIMO system is a technology for transmitting signals by installing two or more antennas in a transmitter and a receiver of a communication system, and transmits different signals within the same band for each antenna. It is a system that can increase.

On the other hand, in the conventional communication system to implement a multiple input multiple output (MIMO) system as described above to configure a transceiver by configuring a plurality of antennas and multiple RF lines. For example, to construct a 4x4 MIMO system, a communication system may be configured using four or more antennas and four RF lines.

At this time, each RF line has a modem for modulating / demodulating a signal, a high performance digital to analog converter (DAC) for converting a digital signal into an analog signal, An oscillator for generating a carrier frequency, an amplifier for amplifying a signal, and the like are included.

As such, since various devices are used for the RF line, the RF line is bulky and expensive. In particular, in N x N MIMO systems, as N increases, the number of RF lines must increase, thereby increasing the price and size of the transceiver.

1 illustrates a configuration method of MIMO in an existing communication system. Referring to FIG. 1 above, a signal s (t) transmitted in a conventional communication system may be represented by a phasor composed of a magnitude and azimuth as shown in Equation 1 below. It is represented by two independent parameters s1 and s2. That is, after generating two signals by controlling two independent parameters in one RF, it is added again and radiated to the antenna. Therefore, N RFs are required to transmit N signals.

Equation 1

As one of the conventional methods for overcoming these disadvantages, N signals can be obtained by arranging several parasitic antennas around one active antenna to adjust the loading impedance of each parasitic antenna. An antenna system using a single RF for transmitting has been proposed.

2 illustrates a schematic configuration of a single RF MIMO communication system using the conventional load impedance regulation described above.

In the conventional system as shown in FIG. 2, when the active antenna 200 and the parasitic antenna 202 are disposed at close distances, mutual coupling occurs, and thus, a mutual coupling matrix measured in advance. The current flowing through each antenna is controlled by adjusting the load impedance applied to the parasitic antenna through the matrix).

However, when implementing a single RF MIME system using load impedance control as described above, in order to increase antenna power efficiency, a lossless element must be configured to generate load impedance. The load impedance is limited to being able to construct only imaginary signals. In addition, since the signal is controlled based on the mutual coupling relationship, there is a problem that all signal domains cannot be represented.

Equation 2 below represents the current I flowing through each antenna in the system of FIG.

Equation 2

Referring to Equation 2 above, I denotes an N x 1 vector representing the current flowing through the antenna, V denotes an N x 1 vector representing the voltage flowing through the antenna, and Z denotes an N x N antenna mutual coupling matrix. matrix, X is an N x 1 diagonal matrix representing the load impedance. Accordingly, there is a problem in that a single RF MIMO system with a load impedance control method has a limitation in implementing performances corresponding to the MIMO system having respective RF circuits for each antenna.

Therefore, in the present invention, a simple structure can be realized by converting an in-phase signal and a quadrature signal into pulse signals having different duty cycles using pulse width modulation or a switch in a single RF MIMO system, and using them for transmission of an input signal. In addition, a relatively wide signal domain can be used to provide a multi-antenna transmission apparatus and a receiving apparatus that can support a higher transmission capacity more stably.

The above-described present invention is a multi-antenna transmitter, comprising: a modem generating n signals for wireless transmission, and a first duty cycle or n negative pulse signals for n positive pulse signals corresponding to the n signals. A base band unit for determining a second duty cycle and a pulse signal corresponding to the first and second duty cycles are generated and outputted, and an inverse transform is performed on a pulse signal corresponding to the second duty cycle among the generated pulse signals. And a pulse width modulator for outputting the negative pulse signal and the positive pulse signal or the negative pulse signal output through the pulse width modulator with a reference signal distributed for each of the plurality of antennas. It includes a mixer that outputs to the antenna.

The pulse width modulator may output the negative pulse signal to the pulse signal generated by the second duty cycle through an inverter selectively provided at an output terminal of the pulse width modulator.

The pulse width modulator may generate the pulse signal such that the positive pulse signal and the negative pulse signal are included in one symbol period.

The mixer may adjust the transmission period of the reference signal according to first and second duty cycles of the positive pulse signal or the negative pulse signal applied through the pulse width modulator.

The mixer may be arranged on a plurality of antenna lines connecting the pulse width modulator and the respective antennas, respectively.

The transmitting device may further include an RF resonator for generating the reference signal having a preset carrier frequency, an amplifier for amplifying the reference signal with an amplitude of a predetermined magnitude, and the amplified reference signal to the respective antennas. It characterized in that it comprises a power divider to apply.

In addition, the present invention provides a multi-antenna transmission apparatus, comprising a modem for generating n signals for wireless transmission, and a first duty cycle or n negative pulse signals for n positive pulse signals corresponding to the n signals. A base band unit for determining a second duty cycle, a switch unit positioned on each antenna line connected to a plurality of antennas, and selectively applying a reference signal distributed to the plurality of antennas to each antenna through a switching operation; And a switch triggering unit configured to control the switching operation of the switch unit by using the information of the first and second duty cycles so that the reference signal is applied to the respective antennas.

The switch unit may include a first terminal to which the reference signal distributed to the plurality of antennas is applied, a second terminal connected to each antenna line through an inverter, and the same terminal connected to the second terminal through a rear end of the inverter. And a third terminal connected to the antenna line and a fourth terminal connected to the ground to selectively switch the first terminal to any one of the second terminal to the fourth terminal.

The switch triggering unit controls the switching operation of the switch unit by using information of the first and second duty cycles, and controls the first terminal to be connected to a third terminal on the switch unit during the first duty cycle. And controlling the first terminal to be connected to the second terminal in the second duty cycle section, and controlling the first terminal to be connected to the fourth terminal in the section without the positive or negative pulse signal. .

The switch triggering unit may control the switching operation of the switch unit according to the first duty cycle or the second duty cycle to adjust the transmission period of the reference signal applied to each antenna.

The inverter may invert the reference signal and output the converted reference signal during the switching operation according to the second duty cycle.

The switch unit may be disposed on a plurality of antenna lines connecting the respective antennas.

The switch unit may have an impedance equal to the impedance applied to each of the antennas as a load impedance, and impedance matching may be performed by the load impedance when the switch unit is turned off.

The transmitting apparatus may include an RF resonator generating a reference signal having a preset carrier frequency, and an amplifier for amplifying the reference signal with an amplitude having a predetermined magnitude and applying the same to the switch unit.

In addition, the present invention is a multi-antenna receiver, a plurality of antennas for receiving a plurality of different signals transmitted wirelessly through a communication network, and provided on an antenna line connected to each of the plurality of antennas are positive pulse signal of the signals A first sampler for sampling the first sampled signal and outputting a first sampled signal, a second sampler for sampling the negative pulse signal among the signals, and outputting a second sampled signal, and an FFT for the first sampled signal and the second sampled signal And an FFT unit configured to detect a amplitude of a carrier frequency of the signal in a frequency domain by performing a transform.

The first and second sampling signals may be characterized by including transmission period information on the carrier frequency of each signal.

According to the present invention, it is possible to implement a simple structure by converting an in-phase signal and a quadrature signal into pulse signals having different duty cycles using pulse width modulation or a switch in a single RF MIMO system, and using them for transmission of an input signal. There is an advantage. In addition, it is possible to use a relatively wide signal domain has the advantage of supporting a higher transmission capacity more stably.

In addition, in particular, since the MIMO system can be configured using only a single RF in the transmitting apparatus, there is an advantage that the transmitting apparatus can be miniaturized and low-cost production is possible.

1 is a diagram illustrating an MIMO configuration method in a conventional communication system;

2 is a schematic configuration diagram of a single RF MIMO communication system using conventional load impedance control;

3A and 3B illustrate the same signal in the time domain and the frequency domain for a repeated direct function according to an embodiment of the present invention;

4 is a conceptual diagram of pulse width modulation according to an embodiment of the present invention;

5 is a detailed circuit diagram of a multi-antenna transmitter using pulse width modulation according to an embodiment of the present invention;

6 is a detailed circuit diagram of a multi-antenna transmission apparatus using a switch according to another embodiment of the present invention;

7 is a detailed circuit diagram of a multiple antenna receiver according to an embodiment of the present invention;

8 is a conceptual diagram illustrating a pulse signal sampling according to an embodiment of the present invention.

Hereinafter, with reference to the accompanying drawings will be described in detail the operating principle of the present invention. In the following description of the present invention, when it is determined that a detailed description of a known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the specification.

First of all, in the present invention, a rectangular function (hereinafter, referred to as a 'direct function') is not an inphase signal or quadrature signal such as [Equation 1] used in conventional RF in constructing a signal. Is generated periodically to form a pulse signal.

Here, the direct function may be expressed as g (t) as shown in Equation 3 below, and if the Fourier transform is performed in the frequency domain, the Equation 4 is expressed as Equation 4. In this equation, n denotes the n-th signal, denotes a duty function of a pulse function, that is, a duty cycle of a pulse signal, and denotes a period of the direct function.

Equation 3

Equation 4

Referring to

Equations

3 and 4 above, the direct function is expressed in the form of a sinc function added in the frequency domain, where q = 0 is a carrier frequency, so q = Only the amplitude at zero is considered.

That is, at q = 0, the amplitude of the carrier frequency is linearly proportional to. Therefore, the amplitude of the frequency signal corresponding to the carrier frequency in the frequency domain can be controlled by adjusting the duty function, that is, the duty cycle of the pulse.

3A and 3B show the same signal in the time domain and the frequency domain for the repeated direct function according to an embodiment of the present invention.

3A and 3B, when a symbol time is 50ns and two pulses have a duty cycle of 70%, an amplitude determined by the duty cycle appears in the frequency domain. At this time, since the carrier frequency is not multiplied, the corresponding amplitude appears at f = 0, which is a value at q = 0.

On the other hand, the method of applying the signal described above is called pulse width modulation, pulse width modulation as described above can be extended as shown in FIG.

That is, referring to FIG. 4, if the pulse signal can have a value of -1 as well as 0 and 1, the duty cycle τ ₂ of the inverted direct function as well as the duty cycle τ _{1 of the} direct function. You can also adjust. At this time, a positive pulse signal (+ pulse) and a negative pulse signal (-pulse) exist simultaneously for one symbol duration. In this case, letting the period for the positive pulse signal, the period for the negative pulse signal, and the ratio of the two periods can vary depending on the situation.

Hereinafter, the structure of two transmitters and the structure of one receiver for applying the above-described pulse width modulation to a MIMO system using multiple antennas will be described in detail.

5 illustrates a detailed circuit configuration of a multi-antenna transmitter using pulse width modulation according to an exemplary embodiment of the present invention. The RF resonator 500, the amplifier 502, the power divider 504, the modem 508, The base band unit 510 may include a pulse width modulator 512, a mixer 516, and the like.

Hereinafter, an operation of each component of the multi-antenna transmitter of the present invention will be described in detail with reference to FIG. 5.

First, the RF resonator 500 generates a reference signal having a preset carrier frequency, and the generated reference signal is an amplifier 502 connected to a rear end of the RF resonator 500. Can be applied as

The amplifier 502 amplifies the reference signal generated from the RF resonator 500 to an amplitude of a preset magnitude.

The power divider 504 distributes the reference signal amplified from the amplifier 502 in correspondence to the number of antennas provided in the transmitting apparatus, and through each antenna line connected to the plurality of antennas 506, respectively. Is applied to the antenna.

The modem 508 generates n signals to be wirelessly transmitted corresponding to the number of antennas in a communication system using multiple antennas such as a MIMO system. In this case, the signal generated by the modem 508 may be an in-phase signal and a quadrature signal.

When the base band chip 510 receives the in-phase signal and the quadrature signal generated by the modem 508, the base band chip 510 performs pulse width modulation to remove the positive band signal. 1 determines a second duty cycle (τ _n-) for the pulse signal of the duty cycle (τ _{n +)} and negative (negative).

The pulse width modulation unit 512 generates and outputs a pulse signal corresponding to the first and second duty cycles received from the base band unit 510. At this time, the pulse width modulator 512 outputs the pulse signal corresponding to the first duty cycle as a positive pulse signal without inverse transformation, and performs the inverse transformation on the pulse signal corresponding to the second duty cycle to perform a negative pulse. Output as a signal.

In this case, an element such as an inverter 514 may be used for the inverse conversion as described above, but is not limited thereto. In addition, although elements such as the inverter 514 are alternately installed in the output terminal of the pulse width modulator 512 in FIG. 5, the present invention is not limited thereto.

The mixer 516 mixes a positive pulse signal or a negative pulse signal output through the pulse width modulator 512 with reference signals distributed from the power divider 504 to each of the plurality of antennas 506. Output to each antenna. In this case, the mixer 516 may be disposed on the plurality of antenna lines 518 connecting the pulse width modulator 512 and each antenna.

That is, each of the n antenna lines 518 includes a positive pulse signal / negative pulse signal output from the pulse width modulator 512, an RF resonator 500, an amplifier 502, a power divider 504, and the like. A sine wave signal of the carrier frequency output from the containing single RF circuit is multiplied by the mixer 516 and radiated through each antenna.

6 illustrates a detailed circuit configuration of a multi-antenna transmission apparatus using a switch according to another exemplary embodiment of the present invention. The RF resonator 600, the amplifier 602, the modem 604, the base band unit 606, and the switch are shown. It may include a triggering unit 608, a switch unit 610 and the like.

At this time, in the structure of the single RF MIMO transmitter using the switch as shown in FIG. 6, the result of the operation, that is, the signal applied to the antenna becomes the same as compared with the transmitter using the pulse width modulation shown in FIG. Implement the operation using a non-switch. That is, as in the structure using pulse width modulation, the base band unit 606 calculates the duty cycle corresponding to each signal, which is provided by the switch triggering unit 608 on the respective antenna lines at a suitable time. May be generated as a positive pulse signal or a negative pulse signal.

Hereinafter, an operation of each component of the multi-antenna transmitter of the present invention will be described in detail with reference to FIG. 6.

First, the RF resonator 600 generates a reference signal having a preset carrier frequency, and the generated reference signal is an amplifier 602 connected to a rear end of the RF resonator 600. Can be applied as

The amplifier 602 amplifies a reference signal generated from the RF resonator 600 to an amplitude of a predetermined magnitude as described above, and the amplified reference signal is connected to each antenna line connected to the plurality of antennas 622. 620 is applied to each antenna.

The modem 604 generates n signals to be wirelessly transmitted corresponding to the number of antennas in a communication system using multiple antennas such as a MIMO system. In this case, the signal generated by the modem 604 may be an in-phase signal and a quadrature signal.

When the baseband unit 606 receives the in-phase signal and the quadrature signal generated by the modem 604, the base band unit 606 performs pulse width modulation to perform a first duty cycle with respect to a positive pulse signal. _{n +} ) and a second duty cycle τ _n− for the negative pulse signal.

The switch unit 610 is switched under the control of the switch triggering unit 608 to selectively turn on / off between a plurality of terminals existing therein.

That is, the switch unit 610 may be provided with a first terminal 612, a second terminal 616, a third terminal 614, a fourth terminal 617, and the like as illustrated in FIG. 6, and the switch According to the control of the triggering unit 608, the first terminal 612 to which the reference signal is applied is selectively switched to any one of the second terminal 616 to the fourth terminal 619 to correspond to the carrier frequency of the reference signal. A sine wave signal can be applied to each antenna 622 corresponding to the first duty cycle or the second duty cycle.

In this case, the first terminal 612 may be a terminal to which a reference signal of a carrier frequency output from a single RF circuit including the RF resonator 600 and the amplifier 602 is applied. In addition, the second terminal 616 is a terminal connected to each antenna line 620 may be provided with a device such as an inverter 618 for inverse conversion of the signal. In addition, the third terminal 614 is a terminal connected to the same antenna line 620 to which the second terminal 616 is connected, or at the rear end of the inverter 618 located at the second terminal 616 side and the antenna line 620. The terminal is connected to radiate to the antenna 622 as it is for a signal that does not need inverse transformation.

In addition, the fourth terminal 619 is a terminal connected to the ground and is a terminal connected when the switch unit 610 is turned off. In this case, an impedance of the antenna 622 is applied to the fourth terminal 619. The same impedance as may be connected. That is, when the switch 611 of the switch unit 610 is connected to the fourth terminal 619 to become an open circuit, the impedance facing the antenna 622 in the RF circuit is variable, which may cause a problem. have. Therefore, when the switch 611 is connected to the fourth terminal 619 by connecting an impedance equal to the impedance of the antenna 622 to the fourth terminal 619 to the ground, the RF circuit faces the antenna 622. Make sure the impedance is always constant. As a result, impedance matching can be stable, thereby preventing distortion of a reference signal from a single RF circuit.

The switch triggering unit 608 controls the switching operation of the switch unit 610 using the information of the first and second duty cycles.

In this case, in the switch operation control as described above, the switch triggering unit 608 controls the first terminal 612 on the switch unit 610 to be connected to the third terminal 614 during the first duty cycle. In the second duty cycle section, the first terminal 612 may be controlled to be connected to the second terminal 616. In addition, the first terminal 612 may be controlled to be connected to the fourth terminal 619 in a period where there is no positive or negative pulse signal. Accordingly, the sine wave reference signal of the carrier frequency output from a single RF circuit including the RF resonator 600, the amplifier 602, and the like through the control of the switch unit 610 by the switch triggering unit 608. Is radiated to each antenna 622.

At this time, the switch triggering unit 608 controls the switching operation of the switch unit 610 such that the positive and negative pulse signals corresponding to the first and second duty cycles are included in one symbol period. In addition, the switch triggering unit 608 may control the switching operation of the switch unit 610 according to the first or second duty cycle to adjust the transmission period of the reference signal applied to each antenna 622.

On the other hand, when the pulse width modulation is used in the configuration of the transmitter as shown in Figs. 5 and 6 as described above, it is necessary to detect the amplitude at the carrier frequency on the receiver side, and also to distinguish the positive pulse signal from the negative pulse signal. There is.

FIG. 7 illustrates a configuration of a multi-antenna receiver for detecting an amplitude at a carrier frequency and distinguishing a positive pulse signal from a negative pulse signal according to an embodiment of the present invention. , A second sampler 704, an FFT unit 706, and the like.

Hereinafter, referring to FIG. 7, the operation of each component of the multi-antenna receiving apparatus of the present invention will be described in detail.

First, a plurality of different signals that are wirelessly transmitted through a communication network may be received from a plurality of antennas 700 provided in the receiving apparatus.

When a signal is received through the plurality of antennas 700 as described above, the first sampler 702 samples the positive pulse signal among the received signals and outputs the first sampling signal, and the second sampler. 704 may output a second sampling signal by sampling a negative pulse signal among the received signals. In this case, the first sampler 702 and the second sampler 704 may be provided on an antenna line connected to each of the plurality of antennas 700 as shown in FIG. 7.

The fast fourier transform (FFT) unit 706 performs an FFT transform on the first and second sampling signals generated by the first sampler 702 and the second sampler 704 to perform a frequency domain. To display. This makes it possible to detect the amplitude of the carrier frequency of the received signal.

That is, the positive pulse signal and the negative pulse signal received by the plurality of antennas are respectively illustrated in FIG. 8 through the first sampler 702 as the positive sampler and the second sampler 704 as the negative sampler. As in the positive or negative pulse signal can be distinguished. In this case, the signal sampled by the first sampler 702 is information on the duty cycle τ _{n +} of the positive pulse signal, and the signal sampled by the second sampler 704 is the duty cycle τ _n− of the negative pulse signal. You can get information about). Subsequently, each sampled value can be displayed in the frequency domain in the time domain through the FFT, and thus it is possible to detect the amplitude of the carrier frequency. Demodulation can be performed.

As described above, according to the present invention, a simple RF MIMO system uses a pulse width modulation or switch to convert an in-phase signal and a quadrature signal into a pulse signal having different duty cycles, thereby simplifying the transmission of the input signal. It can be implemented in a structure, and can use a relatively wide signal domain to support high transmission capacity more stably.

Meanwhile, in the above description of the present invention, specific embodiments have been described, but various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the invention should be determined by the claims rather than by the described embodiments.

Claims

A modem for generating n signals for wireless transmission;

A base band unit for determining a first duty cycle for n positive pulse signals corresponding to the n signals or a second duty cycle for n negative pulse signals;

A pulse that generates and outputs a pulse signal corresponding to the first and second duty cycles, and inversely converts a pulse signal corresponding to the second duty cycle among the generated pulse signals to output the negative pulse signal. Width modulator,

A mixer for mixing the positive pulse signal or the negative pulse signal output through the pulse width modulator with a reference signal distributed for each of a plurality of antennas and outputting the same to each antenna

Multi-antenna transmitter comprising a.
The method of claim 1,

The pulse width modulator,

And outputting the pulse signal generated by the second duty cycle as the negative pulse signal through an inverter selectively provided at an output terminal of the pulse width modulator.
The method of claim 1,

The pulse width modulator,

And generating the pulse signal such that the positive pulse signal and the negative pulse signal are included in one symbol period.
The method of claim 1,

The mixer,

And controlling the transmission period of the reference signal according to the first and second duty cycles of the positive pulse signal or the negative pulse signal applied through the pulse width modulator.
The method of claim 1,

The mixer,

And a plurality of antenna lines respectively connected to the pulse width modulator and the respective antennas.
The method of claim 1,

The transmitting device,

An RF resonator for generating the reference signal having a preset carrier frequency;

An amplifier for amplifying the reference signal with an amplitude having a predetermined magnitude;

A power divider for distributing and applying the amplified reference signals to the respective antennas

Multi-antenna transmission apparatus comprising a.
A modem for generating n signals for wireless transmission;

A base band unit for determining a first duty cycle for n positive pulse signals corresponding to the n signals or a second duty cycle for n negative pulse signals;

A switch unit positioned on each antenna line connected to a plurality of antennas and selectively applying a reference signal distributed to the plurality of antennas to each antenna through a switching operation;

Switch triggering unit for controlling the switching operation of the switch unit using the information of the first and second duty cycle to apply the reference signal to each antenna

Multi-antenna transmitter comprising a.
The method of claim 7, wherein

The switch unit,

A first terminal to which the reference signal distributed to the plurality of antennas is applied, a second terminal connected to each antenna line through an inverter, and a second terminal connected to the same antenna line connected to the second terminal through a rear end of the inverter And a fourth terminal connected to the third terminal and the ground to selectively switch the first terminal to any one of the second terminal to the fourth terminal.
The method of claim 8,

The switch triggering unit,

The switching operation of the switch unit is controlled by using the information of the first and second duty cycles, and during the first duty cycle, the switch unit controls the first terminal to be connected to a third terminal, and the second duty cycle section. The multi-antenna transmission apparatus controls the first terminal to be connected to the second terminal, and controls the first terminal to be connected to the fourth terminal in a section in which there is no positive or negative pulse signal.
The method of claim 7, wherein

The switch triggering unit,

And controlling the switching operation of the switch unit according to the first duty cycle or the second duty cycle to adjust the transmission period of the reference signal applied to each antenna.
The method of claim 8,

The inverter,

And inverting the reference signal and outputting the reference signal during the switching operation according to the second duty cycle.
The method of claim 7, wherein

The switch unit,

And a plurality of antenna lines arranged on the plurality of antenna lines connecting the respective antennas.
The method of claim 8,

The switch unit,

And a load impedance having the same impedance as that applied to each of the antennas, and performing impedance matching by the load impedance when the switch unit is turned off.