Phase domain modulation method dependent on spatial position

A phase domain modulation method dependent on a spatial position is provided. The method mainly includes the following steps: a transmitter and a receiver perform time synchronization to obtain a synchronization time; the transmitter performs a phase domain precoding operation on an original signal to obtain a phase domain pre-coded signal; the receiver receives the phase domain pre-coded signal, obtains a phase domain initial reception signal, and performs a phase domain matching operation on the phase domain initial reception signal to obtain an estimation of the original signal.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is the national phase entry of International Application No. PCT/CN2021/090981, filed on Apr. 29, 2021, which is based upon and claims priority to Chinese Patent Application No. 202011235645.5, filed on Nov. 9, 2020, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention belongs to the technical field of telecommunications, and in particular, relates to a phase domain modulation method dependent on a spatial position.

BACKGROUND

Traditional anti-interception and anti-deception methods depend on encryption and authentication technologies above the network layer. However, with the improvement of computing power, upper-layer encryption and authentication technologies are facing severe challenges. For example, it is difficult to manage, distribute and maintain secret keys. Long secret keys cause high computing overhead and a waste of resources. The improvement of eavesdropping capability imposes great threat on the upper-layer encryption methods based on computational complexity. In order to deal with these problems, the physical-layer secure communication is proposed over the world. The security gate is moved forward, and the randomness (interference, noise, etc.) of the physical-layer is used to get rid of the dependence on the long secret keys. However, most of the existing physical-layer secure communication technologies depend on the reciprocity of a wireless channel, but the reciprocity of the channel is difficult to be strictly met. Although the existing spatial physical-layer security technologies, such as spatial beamforming and direction modulation, can get rid of the limitation of channel reciprocity, only the security performance in the angle domain can be provided. If the eavesdropper and the legitimate receiver are located at the same direction angle, no security is ensured.

SUMMARY

An objective of the present invention is to overcome the defects mentioned above, and provide a phase domain modulation method dependent on a spatial position.

The technical problems of the present invention are solved as follows:a phase domain modulation method dependent on a spatial position is based on a transmitter, a receiver and a plurality of channel resources, where the transmitter is configured to process and transmit an original signal, the receiver is configured to recover the original signal, the channel resources are used by the transmitter and the receiver, and the channel resources include time-domain, frequency-domain, space-domain and code-domain resources.

The method of the present invention includes the following steps:S1: performing time synchronization by the transmitter and the receiver to obtain a synchronization time;S2: performing, by the transmitter, a phase domain precoding operation on the original signal to obtain a phase domain pre-coded signal, and transmitting, by the transmitter, the phase domain pre-coded signal to the receiver using the plurality of channel resources; andS3: receiving, by the receiver, the phase domain pre-coded signal to obtain a phase domain initial reception signal, and performing a phase domain matching operation on the phase domain initial reception signal to obtain an estimation of the original signal.

In S2, the phase domain precoding operation includes the following steps:S2-1: generating, by the transmitter, a high-dimensional precoding signal β(t+Δτ) according to the synchronization time t and a transmission delay Δτ to the receiver:

β⁡(t+Δτ)=[β1(t+Δτ)β2⁢(t+Δτ)⋮βN⁢(t+Δτ)]where βj(t+Δτ) represents the jthdimension of the high-dimensional precoding signal, j=1, 2, . . . , N, N represents the number of dimensions of the high-dimensional precoding signal, and N does not exceed the number of the channel resources,

βj(t+Δ⁢τ)=δ+∏Tp=1Ap,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp(t+Δ⁢τ)]where T represents the number of phase domain precoding layers, T≥1, 1≤p≤T, nprepresents an index of the pthlayer of phase domain precoding branches, 1≤np≤Np, Nprepresents the number of the pthlayer of phase domain precoding branches,

N1×N2×…×NT=N,nT+∑p=1T-1[(nP-1)⁢∏Tl=p+1Nl]=j,Δ⁢fp
represents a frequency increment of the pthlayer Ap,nprepresents an amplitude of a precoding signal on the npthbranch in the pthlayer of phase domain precoding branch, and δ is a normal number agreed by the transmitter and the receiver in advance and has a value meeting

δ+∏Tp=1Ap,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp((t+Δ⁢τ)]>0;S2-2: performing phase domain high-dimensional mapping on a phase ∠s0(t) of the original signal to obtain a high-dimensional original phase signal ∠s(t):

∠⁢s⁡(t)=[∠⁢s1(t)∠⁢s2⁢(t)⋮∠⁢sN⁢(t)],∑j=1N∠sj(t)=∠⁢s0(t)⁢mod⁡(2⁢π)where the number of dimensions of the high-dimensional original phase signal is N, Sj(t) represents the jthdimension of the high-dimensional original signal, and mod is a remainder function; andS2-3: processing the high-dimensional original phase signal according to the high-dimensional precoding signal to obtain a phase domain pre-coded signal ξ(t):

In S3, a specific process of the phase domain matching operation is as follows:

s^0′(t)=exp⁢{j[γ1⁢(t)γ2⁢(t)…γN(t)][∠⁢ξ^1(t)∠⁢ξ^2⁢(t)⋮∠⁢ξ^N⁢(t)]}=exp[j⁢∑j=1Nγj(t)⁢∠⁢ξ^j(t)where {circumflex over (ξ)}(t)=[{circumflex over (ξ)}1(t) {circumflex over (ξ)}2(t) . . . {circumflex over (ξ)}N(t)]Trepresents a phase domain initial reception signal, a superscript T represents transposition, [∠{circumflex over (ξ)}1(t) ∠{circumflex over (ξ)}2(t) . . . ∠{circumflex over (ξ)}N(t)]Trepresents a phase of the phase domain initial reception signal, ∠ represents a phase taking operation, γj(t) represents a matched signal corresponding to βj(t+Δτ) and has a value meeting

γ1(t)⁢{δ+∏Tp=1Ap,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp⁢t]}=1⁢mod⁡(2⁢π),
and ŝ′0(t) represents an estimation of the original signal.

In S2-2, the phase domain high-dimensional mapping adopts the following methods:a first method:

∠⁢sj(t)=1N⁢∠⁢s0(t)+θj(t),
where θj(t) is a jthphase domain random offset signal and meets that [θ1(t) θ2(t) . . . θN(t)]Tis located in a solution space of an equation

Further, the transmitter adopts a narrow-beam antenna to be pointed to the receiver.

A phase domain modulation method dependent on a spatial position is based on a transmitter, a receiver and a plurality of channel resources, where the transmitter is configured to process and transmit an original signal, the receiver is configured to recover the original signal, the channel resources are used by the transmitter and the receiver, and the channel resources include time-domain, frequency-domain, space-domain and code-domain resources.

The method of the present invention includes the following steps:S1: performing time synchronization by the transmitter and the receiver to obtain a synchronization time;S2: performing, by the transmitter, a phase domain precoding operation on the original signal to obtain a phase domain pre-coded signal, performing, by the transmitter, an I/Q domain precoding operation on the phase domain pre-coded signal to obtain an I/Q domain pre-coded signal, and transmitting, by the transmitter, the I/Q domain pre-coded signal to the receiver using the channel resources; andS3: receiving, by the receiver, the I/Q domain pre-coded signal to obtain an I/Q domain initial reception signal, performing an I/Q domain matching operation on the I/Q domain initial reception signal to obtain an I/Q domain matched signal, and performing, by the receiver, a phase domain matching operation on the I/Q domain matched signal to obtain an estimation of the original signal.

In S2, the phase domain precoding operation includes the following steps:S2-1: generating, by the transmitter, a phase domain high-dimensional precoding signal β(t+Δτ) according to the synchronization time t and a transmission delay Δτ to the receiver:

β⁡(t+Δ⁢τ)=[β1(t+Δ⁢τ)β2⁢(t+Δ⁢τ)⋮βN⁢(t+Δ⁢τ)]where βj(t+Δτ) represents the jthdimension of the phase domain high-dimensional precoding signal, j=1, 2, . . . , N, N represents the number of dimensions of the high-dimensional precoding signal, and N does not exceed the number of the channel resources,

βj(t+Δ⁢τ)=δ+∏Tp=1Ap,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp(t+Δ⁢τ)]where T represents the number of phase domain precoding layers, T≥1, 1≤p≤t, nprepresents an index of the pthlayer of phase domain precoding branches, 1≤np≤Np, Nprepresents the number of the pthlayer of phase domain precoding branches,

N1×N2×…×NT=N,nT+∑p=1T-1[(nP-1)⁢∏Tl=p+1Nl]=j,
Δfprepresents a frequency increment of the pthlayer, Δp,nprepresents an amplitude of a precoding signal on the npthbranch in the pthlayer of phase domain precoding branch, and δ is a normal number agreed by the transmitter and the receiver in advance and has a value meeting

δ+∏Tp=1Ap,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp((t+Δ⁢τ)]>0;S2-2: performing phase domain high-dimensional mapping on a phase ∠s0(t) of the original signal to obtain a high-dimensional original phase signal ∠s(t):

∠⁢s⁡(t)=[∠⁢s1(t)∠⁢s2⁢(t)⋮∠⁢sN⁢(t)],∑j=1N∠⁢sj(t)=∠⁢s0(t)⁢mod⁢(2⁢π)where the number of dimensions of the high-dimensional original phase signal is N, sj(t) represents the jthdimension of the high-dimensional original signal, and mod is a remainder function; andS2-3: processing the high-dimensional original phase signal according to the high-dimensional precoding signal to obtain a phase domain pre-coded signal ξ(t):

The I/Q domain precoding operation includes the following steps:S2-4: generating, by the transmitter, an I/Q domain high-dimensional precoding signal α(t+Δτ) according to the synchronization time t and a transmission delay Δδ to the receiver:

α⁡(t+Δ⁢τ)=[α1(t+Δ⁢τ)α2⁢(t+Δ⁢τ)⋮αM⁢(t+Δ⁢τ)]where α1(t+Δτ) represents the ithdimension of the I/Q domain high-dimensional precoding signal, i=1, 2, . . . , M, M represents the number of dimensions of the high-dimensional precoding signal, and M×N does not exceed the number of the channel resources,

M1×M2×…×ML=M,kL+∑m=1L-1[(km-1)⁢∏l=m+1LMi]=i,
and Δfmrepresents a frequency increment of the mthlayer; andS2-5: performing I/Q domain high-dimensional mapping on the jthdimension ξj(t) of the phase domain pre-coded signal ξ(t) to obtain a jthhigh-dimensional signal ξj(t):

ξj(t)=[ξj,1(t)ξj,2⁢(t)⋮ξj,M⁢(t)],∑k=1Mξj,k(t)=ξj(t)where the number of dimensions of the jthhigh-dimensional signal is M, and ξj,k(t) is the kthdimension of the jthhigh-dimensional signal ξj(t); andS2-6: processing the jthhigh-dimensional signal according to the I/Q domain high-dimensional precoding signal to obtain a jthI/Q domain pre-coded signal xj(t):

xj(t)=[xj,1(t)xj,2⁢(t)⋮xj,M⁢(t)]=[ξj,1(t)⁢α1(t+Δ⁢τ)ξj,2⁢(t)⁢α2⁢(t+Δ⁢τ)⋮ξj,M⁢(t)⁢αM⁢(t+Δ⁢τ)]where xj,k(t) represents the kthdimension of the jthI/Q domain pre-coded signal, andcombining, by the transmitter, the jthI/Q domain pre-coded signal into an I/Q domain pre-coded signal:

In S3, a specific process of the I/Q domain matching operation is as follows:

where {circumflex over (x)}(t) represents an I/Q domain initial reception signal,

αk*(t)=∏m=1Lexp[j⁢2⁢π⁡(km-1)⁢Δ⁢fm⁢t],
* represents conjugation, {circumflex over (ξ)}i(t) represents the ithdimension of an I/Q matched signal, and the finally obtained I/Q domain matched signal is as follows:

ξ^(t)=[ξ^1(t)ξ^2⁢(t)⋮ξ^N⁢(t)]a specific process of the phase domain matching operation is as follows:

γj(t)⁢{δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp⁢t]}=1⁢mod⁡(2⁢π),
and ŝ′0(t) represents an estimation of the original signal.

In S2-2, the phase domain high-dimensional mapping adopts the following methods:a first method:

∠⁢sj(t)=1N⁢∠⁢s0(t)+θj(t),
where θj(t) is a jthphase domain random offset signal and meets that [θ1(t) θ2(t) . . . θN(t)]Tis located in a solution space of an equation

In S2-5, the I/Q domain high-dimensional mapping adopts the following methods:a first method:

ξj,i(t)=1M⁢ξj(t)+ni(t),
where ni(t) is an ithI/Q domain random offset signal and meets that [n1(t) n2(t) . . . nM(t)]Tis located in a solution space of an equation

Further, the transmitter adopts a narrow-beam antenna to be pointed to the receiver.

The beneficial effects of the present invention are:

According to the phase domain signal processing method of the present invention, secure communication is achieved without reducing the power efficiency of a system. The dependence of a physical-layer secure communication system on channel state information can be overcome, and the security in the distance domain is realized, so that only the legitimate receiver at an expected distance can receive a correct signal while the receiver at other distances cannot receive the correct signal.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is further described with reference to the accompanying drawings and embodiments.

This embodiment provides a phase domain modulation method dependent on a spatial position, which is based on a transmitter, a receiver and a plurality of channel resources, where the transmitter is configured to process and transmit an original signal, the receiver is configured to recover the original signal, the channel resources are used by the transmitter and the receiver, and the channel resources include time-domain, frequency-domain, space-domain and code-domain resources.

The method in this embodiment includes the following steps:S1: time synchronization is performed by the transmitter and the receiver to obtain a synchronization time;S2: the transmitter performs a phase domain precoding operation on the original signal to obtain a phase domain pre-coded signal, and the transmitter transmits the phase domain pre-coded signal to the receiver using the plurality of channel resources; andS3: the receiver receives the phase domain pre-coded signal to obtain a phase domain initial reception signal, and a phase domain matching operation is performed on the phase domain initial reception signal to obtain an estimation of the original signal.

In S2, the phase domain precoding operation includes the following steps:S2-1: the transmitter generates a high-dimensional precoding signal β(t+Δτ) according to the synchronization time t and a transmission delay Δτ to the receiver:

β⁡(t+Δ⁢τ)=[β1(t+Δ⁢τ)β2⁢(t+Δ⁢τ)⋮βN⁢(t+Δ⁢τ)]where βj(t=Δτ) represents the jthdimension of the high-dimensional precoding signal, j=1, 2, . . . , N, N represents the number of dimensions of the high-dimensional precoding signal, and N does not exceed the number of the channel resources,

βj(t+Δτ)=δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp(t+Δτ)]where T represents the number of phase domain precoding layers, T≥1, 1≤p≤T, nprepresents an index of the pthlayer of phase domain precoding branches, 1≤np≤Np, Nprepresents the number of the pthlayer of phase domain precoding branches,

N1×N2×…×NT=N,nT+∑p=1T-1[(np-1)⁢∏Tl=p+1Nl]=j,
Δfprepresents a frequency increment of the pthlayer, Ap,nprepresents an amplitude of a precoding signal on the npthbranch in the pthlayer of phase domain precoding branch and has a value determined in advance, and δ is a normal number agreed by the transmitter and the receiver in advance and has a value meeting

δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp(t+Δτ)]>0;S2-2: phase domain high-dimensional mapping is performed on a phase ∠s0(t) of the original signal to obtain a high-dimensional original phase signal ∠s(t):

where the number of dimensions of the high-dimensional original phase signal is N, sj(t) represents the jthdimension of the high-dimensional original signal, and mod is a remainder function; andS2-3: the high-dimensional original phase signal is processed according to the high-dimensional precoding signal to obtain a phase domain pre-coded signal ξ(t):

where j=√{square root over (−1)}, and ξj(t) represents the jthdimension of the phase domain pre-coded signal.

In S3, a specific process of the phase domain matching operation is as follows:

where {circumflex over (ξ)}(t)=[{circumflex over (ξ)}1(t) {circumflex over (ξ)}2(t) . . . {circumflex over (ξ)}N(t)]Trepresents a phase domain initial reception signal, a superscript T represents transposition, [∠{circumflex over (ξ)}1(t) ∠{circumflex over (ξ)}2(t) . . . ∠{circumflex over (ξ)}N(t)]Trepresents a phase of the phase domain initial reception signal, ∠ represents a phase taking operation, γj(t) represents a matched signal corresponding to βj(t+Δτ) and has a value meeting

γl(t)⁢{δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp⁢t]}=1⁢mod⁡(2⁢π),
and s′0(t) represents an estimation of the original signal.)

In S2-2, the phase domain high-dimensional mapping adopts the following methods:a first method:

∠⁢sj(t)=1N⁢∠⁢s0(t)+θj(t),
where θj(t) is a jthphase domain random offset signal and meets that [θ1(t) θ2(t) . . . θN(t)]Tis located in a solution space of an equation

Further, the transmitter adopts a narrow-beam antenna to be pointed to the receiver.

This embodiment provides a spatial position-dependent modulation method, which is based on a transmitter, a receiver and a plurality of channel resources, where the transmitter is configured to process and transmit an original signal, the receiver is configured to recover the original signal, the channel resources are used by the transmitter and the receiver, and the channel resources include time-domain, frequency-domain, space-domain and code-domain resources.

The schematic flowcharts of the method in this embodiment are shown inFIGS.1A and1B, including the following steps:S1: time synchronization is performed by the transmitter and the receiver to obtain a synchronization time;S2: the transmitter performs a phase domain precoding operation on the original signal to obtain a phase domain pre-coded signal, the transmitter performs an I/Q domain precoding operation on the phase domain pre-coded signal to obtain an I/Q domain pre-coded signal, and the transmitter transmits the I/Q domain pre-coded signal to the receiver using the channel resources; andS3: the receiver receives the I/Q domain pre-coded signal to obtain an I/Q domain initial reception signal, an I/Q domain matching operation is performed on the I/Q domain initial reception signal to obtain an I/Q domain matched signal, and the receiver performs a phase domain matching operation on the I/Q domain matched signal to obtain an estimation of the original signal.

In S2, the phase domain precoding operation includes the following steps:S2-1: the transmitter generates a phase domain high-dimensional precoding signal β(t+Δτ) according to the synchronization time t and a transmission delay Δτ to the receiver:

β⁡(t+Δ⁢τ)=[β1(t+Δ⁢τ)β2⁢(t+Δ⁢τ)⋮βN⁢(t+Δ⁢τ)]where βj(t+Δτ) represents the jthdimension of the phase domain high-dimensional precoding signal, j=1, 2, . . . , N, N represents the number of dimensions of the high-dimensional precoding signal, and N does not exceed the number of the channel resources,

βj(t+Δ⁢τ)=δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp(t+Δτ)]where T represents the number of phase domain precoding layers, T≥1, 1≤p≤T, nprepresents an index of the pthlayer of phase domain precoding branches, 1≤np≤Np, Nprepresents the number of the pthlayer of phase domain precoding branches,

N1×N2×…×NT=N,nT+∑p=1T-1[(np-1)⁢∏i=p+1TNl]=j,
Δfprepresents a frequency increment of the pthlayer, Ap,nprepresents an amplitude of a precoding signal on the npthbranch in the pthlayer of phase domain precoding branch and has a value determined in advance, and δ is a normal number agreed by the transmitter and the receiver in advance and has a value meeting

δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp(t+Δ⁢τ)]>0.S2-2: phase domain high-dimensional mapping is performed on a phase ∠s0(t) of the original signal to obtain a high-dimensional original phase signal ∠s(t):

∠⁢s⁡(t)=[∠⁢s1(t)∠⁢s2⁢(t)⋮∠⁢sN⁢(t)],∑j=1N∠⁢sj(t)=∠⁢s0(t)⁢mod⁡(2⁢π)where the number of dimensions of the high-dimensional original phase signal is N, sj(t) represents the jthdimension of the high-dimensional original signal, and mod is a remainder function.

In S2-2, the phase domain high-dimensional mapping adopts the following methods:a first method:

∠⁢sj(t)=1N⁢∠⁢s0(t)+θj(t),
where θj(t) is a jthphase domain random offset signal and meets that [θ1(t) θ2(t) . . . θN(t)]Tis located in a solution space of an equation

∑j=1Nθj(t)=0.S2-3: the high-dimensional original phase signal is processed according to the high-dimensional precoding signal to obtain a phase domain pre-coded signal ξ(t):

ξ⁡(t)=[ξ1(t)ξ2⁢(t)⋮ξN⁢(t)]=[exp[j⁢∠⁢s1(t)⁢β1(t+Δ⁢τ)exp[j⁢∠⁢s2(t)⁢β2(t+Δ⁢τ)⋮exp[j⁢∠⁢sN(t)⁢βN(t+Δ⁢τ)]where ξj(t) represents the jthdimension of the phase domain pre-coded signal.The I/Q domain precoding operation includes the following steps:S2-4: the transmitter generates an I/Q domain high-dimensional precoding signal α(t+Δτ) according to the synchronization time t and a transmission delay Δτ to the receiver:

α⁡(t+Δ⁢τ)=[α1(t+Δ⁢τ)α2⁢(t+Δ⁢τ)⋮αM⁢(t+Δ⁢τ)]where αi(t+Δτ) a represents the ithdimension of the I/Q domain high-dimensional precoding signal, i=1, 2, . . . , M, M represents the number of dimensions of the high-dimensional precoding signal, and M×N does not exceed the number of the channel resources,

M1×M2×…×ML=M,kL+∑m=1L-1[(km-1)⁢∏l=m+1LMl]=i,
and Δfmrepresents a frequency increment of the mthlayer.

S2-5: I/Q domain high-dimensional mapping is performed on the jthdimension of ξj(t) of the phase domain pre-coded signal ξ(t) to obtain a jthhigh-dimensional signal ξj(t):

ξj(t)=[ξj,1(t)ξj,2⁢(t)⋮ξj,M⁢(t)],∑k=1Mξj,k(t)=ξj(t)where the number of dimensions of the jthhigh-dimensional signal is M, and ξj,k(t) is the kthdimension of the jthhigh-dimensional signal ξj(t).

In S2-5, the I/Q domain high-dimensional mapping adopts the following methods:a first method:

ξj,i(t)=1M⁢ξj(t)+ni(t),
where ni(t) is an ithI/Q domain random offset signal and meets that [n1(t) n2(t) . . . nM(t)]Tis located in a solution space of an equation

∑i=1Mni(t)=0.S2-6: the jthhigh-dimensional signal is processed according to the I/Q domain high-dimensional precoding signal to obtain a jthI/Q domain pre-coded signal xj(t):

xj(t)=[xj,1(t)xj,2⁢(t)⋮xj,M⁢(t)]=[ξj,1(t)⁢α1(t+Δτ)ξj,2⁢(t)⁢α2⁢(t+Δτ)⋮ξj,M⁢(t)⁢αM⁢(t+Δτ)]where xj,k(t) represents the kthdimension of the jthI/Q domain pre-coded signal; andthe transmitter combines the jthI/Q domain pre-coded signal into an I/Q domain pre-coded signal:

In S3, a specific process of the I/Q domain matching operation is as follows:

αk*(t)=∏m=1Lexp[j⁢2⁢π⁡(km-1)⁢Δ⁢fm⁢t],
* represents conjugation, {circumflex over (ξ)}1(t) represents the ithdimension of an I/Q matched signal, and the finally obtained I/Q domain matched signal is as follows:

A specific process of the phase domain matching operation is as follows:

γj(t)⁢{δ+∏p=1TAp,np⁢cos[2⁢π⁡(np-1)⁢Δ⁢fp⁢t]}=1⁢mod⁡(2⁢π),
and ŝ′0(t) represents an estimation of the original signal.

Further, the transmitter adopts a narrow-beam antenna to be pointed to the receiver.