Distributed co-operating nodes using time reversal

Abstract

Distributed cooperating nodes of a cluster are used for communications, object location, and other purposes. The nodes can move relative to each other and an intended receiver. The nodes are synchronized and data for transmission from the cluster is distributed to the nodes. The intended receiver sends a sounding signal to the nodes. Each node receives the sounding signal, obtains the channel response between the intended receiver and the node, and time-reverses the channel response. Each node convolves its time-reversed channel response with the data to obtain the node's convolved data. A master node sends a time reference signal to the other nodes. Each node waits a predetermined time following the time reference signal, as determined based on a common time reference. At the expiration of the predetermined time period, the nodes simultaneously transmit their convolved data. The transmissions from the nodes combine coherently in time-space at the intended receiver.

What is claimed is: 
     
         1 . A method of transmitting from a plurality of ad hoc nodes, the method comprising steps of:
 synchronizing each node of the plurality of ad hoc nodes to a common reference of all nodes of the plurality of ad hoc nodes so that said all nodes of the plurality of ad hoc nodes are enabled to transmit synchronously at a predetermined frequency;   receiving at said each node one or more waveforms resulting from a sounding signal;   generating a time-reversed signal at said each node, the step of generating comprising time-reversing the one or more waveforms received at said each node, thereby obtaining a plurality of time-reversed signals, a time-reversed signal of the plurality of time-reversed signals per said each node of the plurality of ad hoc nodes;   sending a trigger signal from a trigger node of the plurality of ad hoc nodes; and   transmitting synchronously at the predetermined frequency the time-reversed signals from said all nodes of the plurality of ad hoc nodes, one of the time-reversed signals per node of the plurality of ad hoc nodes, so that a combination of the plurality of time-reversed signals transmitted by said all nodes is spatially and temporally focused on a subject, the step of transmitting being performed in response to expiration of a predetermined synchronization period following sending of the trigger signal.   
     
     
         2 . A method according to  claim 1 , wherein the step of synchronizing comprises:
 determining at said each node of the plurality of ad hoc nodes, other than a selected node of the plurality of ad hoc nodes, frequency offset of a physical clock of said each node other than the selected node from a physical clock of the selected node, and   determining signal flight times between at least some nodes of the plurality of ad hoc nodes.   
     
     
         3 . A method according to  claim 2 , further comprising measuring the predetermined synchronization period at said each node based on a common time reference of said all nodes. 
     
     
         4 . A method according to  claim 3 , further comprising receiving the trigger signal at said each node of the plurality of ad hoc nodes other than the trigger node. 
     
     
         5 . A method according to  claim 4 , wherein the common time reference is based on the physical clock of the trigger node. 
     
     
         6 . A method according to  claim 5 , wherein the subject is an intended receiver of the time-reversed signals, and every node of the plurality of ad hoc nodes is within radio frequency (RF) Line-of-Sight (LoS) of one or more other nodes of the plurality of ad hoc nodes. 
     
     
         7 . A method according to  claim 6 , wherein
 the sounding signal is emitted by the intended receiver, the sounding signal having a good autocorrelation function approaching an impulse function;   said all nodes of the plurality of ad hoc nodes receive the one or more waveforms resulting from the sounding signal in an identical temporal window.   
     
     
         8 . A method according to  claim 5 , wherein the subject is a first scatterer, the time-reversed signals are intended for an intended receiver, the intended receiver is not the first scatterer, every node of the plurality of ad hoc nodes is within radio frequency (RF) Line-of-Sight (LoS) of one or more other nodes of the plurality of ad hoc nodes, and the intended receiver is not within RF LoS of at least one node of the plurality of ad hoc nodes. 
     
     
         9 . A method according to  claim 8 , further comprising:
 processing the one or more waveforms using Singular Value Decomposition to determine first signatures for launching from said all nodes of the plurality of ad hoc nodes a first transmission temporally and spatially focused on the first scatterer, each first signature corresponding to a different node of the plurality of ad hoc nodes; and   generating the time-reversed signal corresponding to said each node by convolving data intended for the intended receiver with the signature corresponding to said each node;   wherein the sounding signal comprises one or more channel sounding bursts transmitted from the intended receiver, the one or more waveforms comprising reflections of the one or more channel sounding bursts from one or more scatterers, the one or more scatterers comprising the first scatterer.   
     
     
         10 . A method according to  claim 9 , further comprising comparing one or more of the first signatures to a plurality of stored signatures, wherein the stored signatures are stored in a database together with identifications of objects in environment of the plurality of ad hoc nodes corresponding to the stored signatures. 
     
     
         11 . An article of manufacture comprising non-volatile machine-readable storage medium with program code stored in the medium, the program code, when executed by processors of a plurality of ad hoc nodes, configures the plurality of ad hoc nodes to:
 synchronize each node of the plurality of ad hoc nodes to a common reference of all nodes of the plurality of ad hoc nodes so that said all nodes of the plurality of ad hoc nodes are enabled to transmit synchronously at a predetermined frequency;   receive at said each node one or more waveforms resulting from a sounding signal;   generate a time-reversed signal at said each node by time-reversing the one or more waveforms received at said each node, thereby obtaining a plurality of time-reversed signals, a time-reversed signal of the plurality of time-reversed signals per said each node of the plurality of ad hoc nodes;   send a trigger signal from a trigger node of the plurality of ad hoc nodes; and   transmit synchronously at the predetermined frequency the time-reversed signals from said all nodes of the plurality of ad hoc nodes, one of the time-reversed signals per node of the plurality of ad hoc nodes, so that a combination of the plurality of time-reversed signals transmitted by said all nodes is spatially and temporally focused on a subject, the step of transmitting being performed in response to expiration of a predetermined synchronization period following sending of the trigger signal;   wherein said each node of the plurality of ad hoc nodes comprises an antenna, a radio frequency transceiver coupled to the antenna, a local oscillator, and a processor coupled to the transceiver to control operation of the transceiver.   
     
     
         12 . The article of manufacture according to  claim 11 , wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes to synchronize said each node by
 determining at said each node, other than a selected node of the plurality of ad hoc nodes, frequency offset of a physical clock of said each node other than the selected node from a physical clock of the selected node, and   determining signal flight times between at least some nodes of the plurality of ad hoc nodes.   
     
     
         13 . The article of manufacture according to  claim 12 , wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes to measure the predetermined synchronization period at said each node based on a common time reference of said all nodes. 
     
     
         14 . The article of manufacture according to  claim 13 , wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes to receive the trigger signal at said each node of the plurality of ad hoc nodes other than the trigger node. 
     
     
         15 . The article of manufacture according to  claim 14 , wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes so that the common time reference is based on a physical clock of the trigger node. 
     
     
         16 . The article of manufacture according to  claim 15 , wherein the subject is an intended receiver of the time-reversed signals, and every node of the plurality of ad hoc nodes is within radio frequency (RF) Line-of-Sight (LoS) of one or more other nodes of the plurality of ad hoc nodes. 
     
     
         17 . The article of manufacture according to  claim 16 , wherein
 the sounding signal is emitted by the intended receiver;   the sounding signal has a good autocorrelation function approaching an impulse function;   and wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes to receive the one or more waveforms resulting from the sounding signal in an identical temporal window.   
     
     
         18 . The article of manufacture according to  claim 15 , wherein the subject is a first scatterer, the time-reversed signals are intended for an intended receiver, the intended receiver is not the first scatterer, every node of the plurality of ad hoc nodes is within radio frequency (RF) Line-of-Sight (LoS) of one or more other nodes of the plurality of ad hoc nodes, and the intended receiver is not within RF LoS of at least one node of the plurality of ad hoc nodes. 
     
     
         19 . The article of manufacture according to  claim 18 , wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes to:
 process the one or more waveforms using Singular Value Decomposition to determine first signatures for launching from said all nodes of the plurality of ad hoc nodes a first transmission temporally and spatially focused on the first scatterer, each first signature corresponding to a different node of the plurality of ad hoc nodes; and   generate the time-reversed signal corresponding to said each node by convolving data intended for the intended receiver with the signature corresponding to said each node;   wherein the sounding signal comprises one or more channel sounding bursts transmitted from the intended receiver, the one or more waveforms comprising reflections of the one or more channel sounding bursts from one or more scatterers, the one or more scatterers comprising the first scatterer.   
     
     
         20 . The article of manufacture according to  claim 19 , wherein the program code, when executed by the processors of the plurality of ad hoc nodes, further configures the plurality of ad hoc nodes to compare one or more of the first signatures to a plurality of stored signatures, wherein the stored signatures are stored in a database together with identifications of objects in environment of the plurality of ad hoc nodes corresponding to the stored signatures. 
     
     
         21 . A system comprising a plurality of ad hoc nodes, each node of the plurality of ad hoc nodes comprising an antenna, a radio frequency transceiver coupled to the antenna, and a processor coupled to the transceiver to control operation of the transceiver, wherein the plurality of ad hoc nodes is configured to:
 synchronize each node of the plurality of ad hoc nodes to a common reference of all nodes of the plurality of ad hoc nodes so that said all nodes of the plurality of ad hoc nodes are enabled to transmit synchronously at a same predetermined frequency;   receive at said each node one or more waveforms resulting from a sounding signal;   generate a time-reversed signal at said each node by time-reversing the one or more waveforms received at said each node, thereby obtaining a plurality of time-reversed signals, a time-reversed signal of the plurality of time-reversed signals per said each node of the plurality of ad hoc nodes;   send a trigger signal from a trigger node of the plurality of ad hoc nodes; and   transmit synchronously at the same predetermined frequency the time-reversed signals from said all nodes of the plurality of ad hoc nodes, one of the time-reversed signals per node of the plurality of ad hoc nodes, in response to expiration of a predetermined synchronization period following sending of the trigger signal, so that a combination of the plurality of time-reversed signals transmitted by said all nodes is spatially and temporally focused on a subject.