Patent Publication Number: US-9419703-B2

Title: Techniques and systems for wireless communications

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent document is a continuation of U.S. patent application Ser. No. 13/854,863, filed on Apr. 1, 2013, which is a continuation of U.S. patent application Ser. No. 12/565,639, filed on Sep. 23, 2009, now U.S. Pat. No. 8,411,765, which claims the benefit of priority of U.S. Provisional Patent Application No. 61/104,670, filed on Oct. 10, 2008. The entire contents of the before-mentioned patent applications are incorporated by reference as part of the disclosure of this document. 
    
    
     TECHNICAL FIELD 
     The subject matter described in this document relates to radio frequency wireless communications. 
     BACKGROUND 
     Radio frequency (RF) wireless communications use wireless transfer of RF waves to communicate and transfer information and can be used in a wide range of applications. For example, RF wireless communications can be used to provide interactive broadband access for data and file transfer, GPS services, web surfing, video capture, streaming video, Internet commerce, Internet gaming, and electronic books. Other examples for applications of RF wireless communications include radar, RF imaging, space communications, RF targeting, RF sensor networks, RF surveillance, and various uses of wirelessly directing RF waves from one location to another. Some specific uses of RF wireless communications are static and mobile RFIDs, inventory tracking and control, and security monitoring at security checkpoints such as airports and shipping ports. 
     RF wireless signals can be transmitted between two communication nodes through a transmission channel. The transmission channel includes the electronics and antennas of an RF transmitter at the first node. The actual transmission medium is also part of the transmission channel, including various objects which reflect or scatter the wireless RF signals. Also part of the transmission channel are the electronics and antennas of an RF receiver at the second node. Interactions, such as reflecting and scattering, with any encountered objects which are part of the transmission channel can affect the transmission of the wireless RF signal to the RF receiver and the detection of the received wireless RF signal at the RF receiver. 
     SUMMARY 
     The techniques, apparatus and systems described in this document are based on time reversal of the channel impulse response of an RF pulse in a transmission channel between an RF transmitter and an RF receiver and can be implemented in ways to enhance reception and detection of an RF pulse at the RF receiver against various effects that can adversely affect and complicate the reception and detection of the RF pulse at the RF receiver. 
     A radio frequency (RF) wireless communication system includes a first wireless transceiver at a first location to transmit and receive wireless RF signals. The first wireless transceiver includes a monitor pulse transmitter to wirelessly transmit periodic monitor RF pulses for monitoring a transmission channel in which a wireless RF signal produced by the first wireless transceiver propagates. The first wireless transceiver also includes a signal receiver that receives wireless RF signals for extracting data or information in the received wireless RF signals. 
     In one aspect, an RF system includes a second wireless transceiver at a second location separate from the first location to transmit and receive wireless RF signals in wireless communications with the first wireless transceiver, the second wireless transceiver comprising. The second wireless transceiver includes a monitor pulse receiver to receive and detect the monitor RF pulses from the first wireless transceiver. The second wireless transceiver also includes a signal transmitter that transmits wireless RF signals with RF pulses carrying data or information to be transmitted to the first wireless transceiver. 
     The system also includes a transmission control circuit that processes one or more received monitor RF pulses from the first wireless transceiver to produce a time reversal of an impulse function of the one or more received monitor RF pulses. The transmission control circuit also multiplexes each RF pulse carrying data or information to be transmitted with the time reversed impulse function as part of each wireless RF signal to be transmitted out by the signal transmitter. 
     In another aspect, an RF system for providing RF communications includes a target and a base in communication with the target. The target has a target-transmitter to repeatedly emit RF impulse-pulses and a target-receiver to detect RF data signals. The base includes a base-detector to detect a channel impulse response of the RF impulse-pulses emitted by the target. The base-detector has an analog-to-digital converter to digitize the detected channel impulse response into a detected channel impulse response waveform having a preset number of samples. The base also includes a waveform processing unit. The waveform processing unit includes a temporary memory configured to store the detected channel impulse response waveform and a first memory configured to store and reverse in time domain a previously detected channel impulse response waveform. The waveform processing unit also has a comparator unit to quantify a difference between the channel impulse response waveform in the temporary memory and the channel impulse response waveform in the first memory, and a switch to replace the channel impulse response waveform stored in the first memory with the channel impulse response waveform stored in the temporary memory. The quantified difference is compared against a predefined threshold. The base further includes a multiplexer that adds a copy of the time reversed channel impulse response waveform from the first memory at the beginning of each bit of an RF data stream to produce an output signal, and a transmitter to emit the output signal. 
     In yet another aspect, a method for providing RF wireless communications includes obtaining a channel impulse response for an RF communication system. To obtain the channel impulse response the target repeatedly emits impulse-pulses which are then detected at a base as the channel impulse response associated with the target. Next the detected channel impulse response is digitized into a channel impulse response waveform, and stored in a temporary memory. The method also includes monitoring the obtained channel impulse response. The monitoring includes quantifying a difference between a channel impulse response waveform stored in a first memory and the channel impulse response waveform stored in the temporary memory. When the quantified difference is greater than a predefined threshold, then the channel impulse response waveform stored in the first memory is replaced with the channel impulse response waveform stored in the temporary memory. Then, the target is instructed to emit the impulse-pulses more often. When the quantified difference is less than a predefined threshold the target is instructed to emit the impulse-pulses less often. 
     The method further includes transmitting an RF data stream by using the monitored channel impulse response. The transmitting includes retrieving the stored channel impulse response waveform from the first memory. The waveform is retrieved in last-in-first-out order. Then a signal is prepared for transmission by adding a copy of the retrieved channel impulse response waveform at the beginning of each bit of an RF data stream. The prepared signal is emitted from the base and the RF data stream is detected at the target. 
     In another aspect, an RF wireless communication method includes emitting from a target an impulse-pulse and detecting at a base a channel impulse response associated with the emitted impulse-pulse. The method further includes digitizing the detected channel impulse response into a channel impulse response waveform and storing the channel impulse response waveform in a memory at the base. An RF data stream transmission, using the monitored channel impulse response, includes reversing in time domain the channel impulse response waveform from the memory, then preparing a signal for transmission by adding a copy of the time reversed channel impulse response waveform at the beginning of each bit of an RF data stream, and finally emitting the prepared signal from the base. Next, the method includes detecting the RF data stream at the target. When a parameter of system performance is less than a first threshold another impulse-pulse is emitted from the target. The parameter of system performance can be a bit-to-error-ratio of the detected RF data stream at the target. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other aspects will now be described in detail with reference to the following drawings. 
         FIG. 1  represents an RF wireless communications system including a target and a base. 
         FIG. 2  represents another RF wireless communications system including a target and a base. 
         FIG. 3  is a schematic illustrating an RF wireless transmission between a target and a base. 
         FIG. 4  is a schematic illustrating yet another RF wireless communications system including a target and a base. 
         FIG. 5  is a block diagram of a process for monitoring the quality of an RF wireless transmission between a target and a base. 
         FIG. 6  is another block diagram of the process for monitoring the quality of an RF wireless transmission between a target and a base. 
         FIG. 7  is a schematic diagram illustrating an RF wireless communications system including several targets and a base. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     An RF wireless communication signal can be affected by various effects during transmission, such as multipath scattering and reflections. These effects can cause the RF energy of a short pulse or other coded sequence generated at a first location, e.g., an RF transmitter, to arrive at a second location, e.g., an RF receiver, over a period of time longer than the original pulse duration. 
     The pulse can be emitted from a first antenna at the first location and received by a second antenna at the second location. The RF wave of the pulse can interact with different objects and such interactions cause different portions from the original RF wave to travel along different paths. The portion of the RF wave that travels along the line of sight between the two antennas in a homogeneous medium arrives at the second location before arrival of other portions of the RF wave. Other portions of the RF wave are detected by the antenna at the second location after these portions of the RF wave are scattered or reflected towards the second location by various objects in the transmission channel. Such different portions of the RF wave may have different signal strengths. This presence of different portions of the same RF wave from the original RF pulse with different signal strengths and different arrival times at the second antenna renders the detection of the RF pulse difficult. For example, the addition of contributions from various scattering paths results in a long pulse detected by the second antenna at the second location that may overlap in time with one or more subsequent RF pulses in a sequence of RF pulses generated and transmitted by the first antenna at the first location. The waveform of such a long pulse detected by the second antenna may also be distorted due to different signal strengths of the different portions. In some cases, the detected RF signals at the second antenna may appear completely random and cannot be easily processed to recognize different RF pulses generated by the first antenna. 
     Delays can also result from a signal taking different amounts of time to propagate between the terminals of an antenna and different physical locations on the antenna. These delays are referred to as dispersion to distinguish them from multipath delays. 
     The detected waveform at the second antenna for the short pulse transmitted by the first antenna, however, could be returned through the transmission channel to arrive back at the first location as a short pulse, if each of the different portions of the RF wave received at the second antenna is reversed in time and is directed back to the first antenna along their paths from the first antenna to the second antenna. The antenna at the second location emits the trailing portions of the detected signal first, while the leading portions of the signal are emitted last. Due to the time reversal properties of RF-wave propagation only the respective portions of the RF wave which travel through the transmission channel along the original paths can arrive in phase at the first location. Moreover, because the return paths coincide with the forward paths, all portions of the waveform emitted from the second location arrive at the first location at the same time after applying the delays associated with the time reversal process. Therefore, the antenna at the second location emits a time reversed signal and effectively produces a pulse that is focused in time and space. In this context, the focusing in the time domain consists of shortening the time duration of the pulse emitted from the second location to the time duration of the pulse emitted originally from the first location. The spatial focusing consists of emitting an RF wave from the second location and concentrating a portion of the RF wave energy at the first location. Note that the spatial focusing technique described in this document is accomplished by time reversed emission creating identical conditions as a directional antenna array, but does not require beam shaping elements. 
     If the antenna at the second location emits the time reversed signal repeatedly, a sequence of RF pulses can be detected at the first location. Therefore, time reversal of RF waves can be used to establish RF communications between the antenna at the second location and the antenna at the first location. The antenna at the second location focuses a sequence of pulses at the first location using time reversal emission, thus compensating for noise due to scattering, multipath and dispersive effects. An identical process can be applied to transmit data in the reverse direction. 
     The techniques, apparatus and systems described in this document apply the time reversal of the channel impulse response of the transmission channel through which the RF energy transfers from the first antenna and to the second antenna in transmitting a short RF pulse from the first antenna to the second antenna. The signal received at the second antenna has been transformed from a short RF pulse and represents the impulse response of the transmission channel. This signal received from the first antenna contains multiple portions of the short RF pulse that traveled along different signal paths towards the second antenna. The signal representing the impulse response of the transmission channel is time reversed and transmitted by the second antenna back to the first antenna. Due to the time reversal property, the multiple portions of the transmitted impulse response of the transmission channel retrace the different signal paths back to the first antenna. The multiple portions of the transmitted impulse response of the transmission channel arrive at the first antenna within a pulse duration (of the original short RF pulse) from one another to form the short RF pulse. 
     Once an RF transmission based on time reversal is established, the reception quality can be maintained if no changes occur in the transmission channel. For example, new scattering centers present in the transmission channel and other changes in the transmission channel can alter the scattering paths of the RF wave emitted by the antenna at the second location. Modifications of the transmission channel can also be due to changes in the location or properties of the first or second antennas. 
     As a result, portions of the RF wave emitted at the second location do not return in phase at the first location. The signal-to-noise ratio (SNR) of the pulse reception degrades because part of the energy of the pulses received back at the first location is lost to noise. To avoid degradation of the SNR of the pulse reception at the first location the response of the transmission channel can be monitored and timely updated to reflect the current transmission condition of the transmission channel. The techniques, apparatus and systems described in this document based on the time reversal approach can be implemented to include a monitoring mechanism for monitoring changes in the transmission channel or the location of the end points, and a mechanism for updating the channel impulse response of the transmission channel to account for changes in the transmission channel such as appearance of new scattering objects and disappearance of previously existing scattering objects. 
     Effect of Channel Impulse Response on Data Transmission 
       FIG. 1  illustrates a wireless RF communication system  100  that is subject to multipath scattering and other effects in the transmission channel on the signal reception. The base  110  and the target  120  associated with the communication system  100  in  FIG. 1  are each equipped with a transmitter (TX) and a receiver (RX). The base  110  transmits a data encoded signal  111  to the target  120 . The terms data encoded signal and signal are used interchangeably in the following description. The signal  111  transmitted from the base  110  to the target  120  undergoes scattering from various objects on the line-of-sight and non-line-of-sight paths. The signal received at the target  120  includes interference contributions due to, for example, multipath scattering and dispersion delays. Thus, the signal received at the target  120  has a time duration longer than that of the signal  111  provided at the base  110 . The signal  111  emitted by the transmitter at the base  110  is denoted s(t). The signal detected by the receiver at the target  120  is denoted q(t), and includes two contributions:
 
 q ( t )= s ( t )* h ( t )+ n ( t )  (1)
 
     The first term in EQ. 1 corresponds to the convolution between the signal s(t) and the channel impulse response h(t) of the transmission channel. The second term n(t) represents random noise. Random noise can be caused by, for example, a detection or measurement error. Therefore, the random noise term n(t) in EQ. 1 is independent of the transmission path. 
     The transmission channel includes the entire medium  130  between the base  110  and the target  120 , including areas out of the line of sight from the base  110  to the target  120 . The channel impulse response h(t) depends on the characteristics of the transmission channel. Namely, the number of scattering objects, their placement and orientation with respect the line of sight from the base  110  to the target  120 , can all influence the channel impulse response. Many scattering centers cause many scattering paths, thus lengthening the time duration of the channel impulse response h(t) and implicitly of the received signal q(t). Scattering objects placed far away from the line of sight cause long scattering paths, thus again, lengthening the time duration of h(t) and q(t). On the other hand, if the transmission path does not contain scattering centers and the electronics is optimized, then the channel impulse response due to the path is close to unity, h(t)=1, and the signal q(t) received at the target  120  resembles very well the signal s(t)  111  emitted at the base  110 . In the absence of scattering events, the signal detected by the receiver at the target  120  does not have a longer time duration than the signal  111  emitted by the base  110 . 
     In the wireless RF communication system  100  the transmitter at the base  110  has no knowledge of the channel channel impulse response prior to transmitting RF signals from the base  110  to the target  120 . Therefore, the discrimination and equalization processes needed to extract data from the transmitted signal  111  is performed by the receiver at the target  120 . From the perspective of the target  120 , both the channel impulse response h(t) and the random noise n(t) represent random processes and are difficult to separately account for. 
     Time Reversal of Channel Impulse Response 
     As described earlier, the effects on signal propagation by multipath scattering in the transmission channel  130  can be corrected by time reversal. A pulse submitted from the target  120  is detected by the base  110  after propagation through the transmission channel  130 . The detected pulse experiences the effect of the transmission channel  130 , therefore the base  110  knows the response of the medium to the propagation of an impulse. Thus, the waveform detected by the base  110  is the channel impulse response h(t) characteristic to the transmission channel  130 . For reasons discussed above, if the time reversed channel impulse response h(T-t) is reemitted by the base  110 , the pulse arriving back at the target  120  is approximately the original impulse pulse. Therefore, the time reversed channel impulse response matches and negates the effects of the multipath scattering in the propagation medium  130 . The convolution between the channel impulse response h(t) and the time reversed channel impulse response h(T-t) is given approximately by the impulse (delta) function:
 
 h ( t )* h (− t )=δ( t )  (2)
 
     The signal s(t)  111  is generated by the transmitter at the base  110 . If instead of emitting the signal s(t)  111 , the base  110  emits a signal containing the convolution s(t)*h(T-t), then the signal received at the target  120  is given approximately by
 
 q ( t )=[ s ( t )* h (− t )]* h ( t )+ n ( t )
 
 q ( t )= s ( t )*[ h (− t )* h ( t )]+ n ( t )= s ( t )* g ( t )+ n ( t )  (3)
 
     As suggested by EQs. 2-3, the signal q(t) detected at the target  120  has been filtered by the transmission channel  130 . The signal emitted at the base  110  now contains the time reversed channel impulse response h(T-t), where T represents a time delay to preserve causality of the signal. The time delay T is related to the time it takes the base  202  to receive and reverse in the time domain the signal corresponding to the channel impulse response h(t). As discussed above, the effect of the time reversed channel impulse response h(T-t) is to match and negate the contribution of the channel impulse response h(t). Thus, the effect of the multipath scattering in the transmission channel  130  is cancelled and the signal detected by the receiver at the target  120  is a close replica of the signal s(t)  111  included in the signal emitted by the transmitter at the base  110 . To establish an RF wireless transmission based on time reversal, the time reversed channel impulse response h(T-t) of the transmission channel  130  can be measured and implemented as shown in EQ. 3. 
     Acquisition and Time Reversal of Channel Impulse Response 
     The RF wireless communication system  200  shown schematically in  FIG. 2  can be configured to include circuit elements and devices that detect, digitize and store the channel impulse response h(t). The system  200  includes a base  202  and a target  204  which exchange signals through a transmission channel  130 . For the remainder of this document the terms base and target (or interchangeably remote node) are used to denote the following types system nodes: A base station is a centrally located and high functional communication node. A target may have less functionality than the base station but may be more mobile. 
     The target  204  is equipped, among other things, with a target-TX  210  and a target-RX  220 . The target-TX  210  emits an impulse pulse  260 . The impulse pulse  260  can be a delta pulse, or any other shape of pulse that allows the different paths from the target  204  to the base  202  via different scatterers to be uniquely resolved in the channel impulse response h(t). This impulse pulse  260  passes through the medium  130 , interacting with scattering and multipath elements and arrives at the base  202 . The base  202  includes a base-RX  230 , a base-TX  240 , and a waveform processing unit  250 . The pulse  260  emitted by the target  204  is detected by the base-RX  230  in the form of the channel impulse response h(t) of the transmission channel  130 . 
     The waveform processing unit  250  digitizes the channel impulse response h(t) in a high speed sampling circuit and, in one implementation, the base  202  stores the channel impulse response waveform h(t). The waveform processing unit  250  then reverses in time domain the channel impulse response waveform h(t) to generate a time reversed channel impulse response waveform h(T-t)  270 . The waveform processing unit  250  is described in more detail with respect to  FIG. 4 . In another implementation, in addition to the channel impulse response waveform h(t), the time reversed channel impulse response waveform h(T-t) may also be stored in the waveform processing unit  250 . In yet another implementation, only the time reversed channel impulse response waveform h(T-t)  270  is stored in the waveform processing unit  250 , and not the channel impulse response waveform h(t). In one aspect, the waveform processing unit  250  stores the channel impulse response waveform h(t) in a last-in-first-out (LIFO) buffer. Therefore, the time reversal step is built in as a property of the LIFO buffer: the channel impulse response waveform h(t) goes into the LIFO buffer for storage, and the time reversed channel impulse response waveform h(T-t)  270  comes out of the LIFO buffer upon accessing the buffer. 
     Upon receiving the signal from the target  204 , the base-TX  240  can emit the stored time reversed channel impulse response h(T-t)  270 . Assuming that the medium is linear and reciprocal, the signal emitted at the base  202 , consisting of the time reversed channel impulse response h(T-t)  270 , arrives at the target  204  with all transmission degradations removed, as shown in EQ. 2. The target-RX  220  detects a pulse that resembles the original impulse pulse  260 . As discussed regarding EQ. 2, the original temporal shape of the pulse  260  is preserved upon return to the target  204 . 
     In one exemplary implementation of the communication system  200 , the target-TX  210  and the base-TX  240  are configured to emit analog signals, while the base-RX  230  is configured to receive analog signals. In contrast the target-RX  220  can be configured to receive digital signals. In another exemplary implementation of the communication system  200 , the target-RX  220  can be configured to receive analog signals. 
     In yet another exemplary implementation of the communication system  200  the target-TX  210  and the base-TX  240  are configured to emit digital signals, while the base-RX  230  and the target-RX  220  are configured to receive digital signals. 
     In another aspect, impulse signals can also be transmitted by the base station  202  and data signals can also be transmitted by the target  204 . Such a configuration enables impulse and data signal flow in both directions. This document describes impulse signals transmitted by the target  204  and data signals transmitted by the base station  202  only for illustrative processes. 
     Temporal and spatial focus are obtained for the RF wireless communications system  200 . The pulse  260  emitted by the target  204  as an RF wave is reflected back by the base  202  after time reversal. The reflected and time reversed pulse return precisely at the target  204  location, because only at the target  204  location the waves scattered within the transmission channel  130  add up in phase, both spatially and temporally. 
     Data Transmission from Base to Single Target 
     Once the channel impulse response h(t) of the transmission channel  130  was determined and the time reversed channel impulse response waveform h(T-t) stored by the base  202 , the RF wireless communication system  200  can be configured for data transmission. In this implementation, a data channel is integrated into the base-TX  240  of the RF wireless communication system  200 . 
       FIG. 3  depicts a procedure to integrate the time reversed channel impulse response waveform h(T-t)  270  into the data channel. A data stream s(t)  310 , for example, a digit sequence of 1101001, can be sent from the base  202  to the target  204 . The data stream s(t)  310  intended to arrive at the target  204  is convolved with the time reversed channel impulse response h(T-t)  270 . The time reversed channel impulse response h(T-t)  270  has been stored at the base  202  inside the waveform processing unit  250 . The base-TX  240  transmits a copy of the time reversed channel impulse response h(T-t)  270  for each data bit, as shown in  FIG. 3 . In an exemplary implementation, a one bit is encoded by emission of a copy of the time reversed channel impulse response h(T-t)  270 . A zero bit results when no copy of time reversed channel impulse response h(T-t)  270  at all is sent back, such that there are no waves to align when a zero returns to the target  204 . In another implementation, a zero bit is encoded by emission of a modified copy of the time reversed channel impulse response h(T-t)  270 . 
     Furthermore, ones and zeroes can be coded by different amplitude assignments. Ones and zeroes can also be coded by time, frequency, phase or scale modulation. 
     The temporal length of a bit can have various lengths, depending on the RF communications application. For example, for data rates of order gigabit per second, each bit may be of order 1 ns. The temporal length of the channel impulse response h(t) for a wireless communications system  300  characterized by severe multipath scattering is arbitrary since it depends on the configuration of the physical environment. The time duration of h(t) can be of order 200 ns or more. The sampling frequency is usually two times the maximum frequency of the impulse response. For a 1 nsec impulse and a 1000 nsec channel impulse response h(t), the sampling frequency would typically be set at 2 GS/s resulting in a total of 2000 samples. The base-TX  240  can transmit all or part of the time reversed channel impulse response h(T-t)  270 . In certain implementations, the base-TX  240  can modify the time reversed channel impulse response h(T-t)  270 , by using DSP techniques, prior to transmission to the target  204 . 
     Copies of time reversed channel impulse response h(T-t)  270  are time-multiplexed  320  in a prepared data stream signal based on an appropriate bit boundary synchronization, as illustrated in  FIG. 3 . Because the time reversed channel impulse response h(T-t)  270  is longer than the time between bits, the multiple delayed copies of the time reversed channel impulse response h(T-t)  270  are added before the transmission. In one implementation, the summation is performed digitally before the signal reaches analog circuitry of the base-TX  240 . In another implementation, the summation is performed by analog circuitry of the base-TX  240 . 
     The data stream signal starts at the base  202  as an output signal  370 , prepared as described above, traverses the multipath transmission channel  130  and arrives at the target  204  as a stream of bits  310 ′ corresponding to s(t), for example, 1101001. The respective copy of time reversed channel impulse response h(T-t)  270  corresponding to each bit at the base  202  has been matched and filtered during propagation from the base  202  to the target  204 , as described by EQs.  2  and  3 . The target-RX  220  detects the RF data stream signal s(t)  310 ′, free of the inter-symbol interference caused by the multipath transmission channel  130 . 
     Monitoring Changes of Channel Impulse Response 
     RF communication systems based on time reversal can be operated to exhibit the functionality illustrated in  FIGS. 2-3  when the channel impulse response h(t) of the transmission channel  130  does not change in time. Changes caused by new or modified scattering sources in the transmission channel  130  lead to a decrease in the SNR of the detected signal. As discussed earlier, monitoring the channel impulse response h(t) of the transmission channel  130  allows for timely replacement of the channel impulse response waveform h(t) when new or modified scattering sources are introduced in the transmission path  130 . 
       FIG. 4  illustrates a system to monitor the quality of a time reversal based communication system  400  when changes in channel impulse response are due to dynamic changes of the transmission path. The system  400  is characterized by two parallel signal exchange paths. A data communication path which was described in detail regarding  FIG. 3 . And an impulse-response monitoring path that is presented below. 
     The data communication path of the RF wireless communication system  400  includes the base-TX  240  at the base  202 , and the target-RX  220  at the target  204 . The preparation of the data stream signal convolved with the time reversed channel impulse response waveform is performed by the base-TX  240  as shown in  FIG. 3 . The output signal  370  containing the data stream is emitted by the base-TX  240  and arrives at the target  204  through the transmission channel  130 . As shown in regard to  FIG. 3 , an equalized data stream is received by the target  204  as predicted by EQ. 3. The equalized signal is enhanced with respect to noise and interference. 
     The impulse-response monitoring path of the RF wireless communication system  400  includes a target-TX  210 . An impulse-pulse  260  is emitted by the target-TX  220  towards the base  202  through the transmission channel  130 . The target repeatedly emits impulse-pulses to ensure that the base  210  has an impulse response h(t) that best represents the existing transmission channel  130 . The time interval between consecutively emitted impulse-pulses is short if the transmission channel  130  changes rapidly. The time interval between consecutively emitted impulse-pulses is long if the transmission channel  130  changes slowly. 
     The base  202  is equipped, among other things, with a base-RX  230 , a base-TX  240 , and a waveform processing unit  250 . The impulse-pulse  260  emitted by the target  204  is received by the base-RX  230 . The received signal, representing the channel impulse response h(t) of the transmission channel  130 , is input into the waveform processing unit  250 . 
     The waveform processing unit  250  includes a temporary buffer  410 , a buffer  420 , a comparator unit  430 , and a switch  450 . The buffer  420  can be operated in first-in-first-out (FIFO) mode or in LIFO mode. The temporary buffer  410  can be operated in FIFO mode. In other implementations, the buffers  410  and  420  can be replaced by different regions of digital memory. The channel impulse response h(t) is digitized and the channel impulse response waveform h(t) is placed into the temporary buffer  410 . The buffer  420  stores a previously acquired channel impulse response waveform H(t). The notation convention is that lower case denotes the most recently acquired waveform, while upper case denotes a previously acquired waveform. The channel impulse response waveform H(t) stored in the buffer  420  is being used by the RF wireless communications system  400  to perform time reversal procedures as described in  FIGS. 2-3 . H(t) is also referred to as the channel impulse response waveform of record. 
     Returning to  FIG. 4 , the comparator unit  430  can be used to quantify the differences between the latest detected channel impulse response waveform h(t) and the channel impulse response waveform H(t) of record. Changes between the two waveforms h(t) and H(t) which exceed a predetermined threshold trigger the replacement of the channel impulse response waveform H(t) of record with the most recently acquired channel impulse response waveform h(t). The comparator unit  430  has three terminals. The input  431  receives the channel impulse response waveform h(t) from the temporary buffer  410 . The input  432  receives the channel impulse response waveform H(t) from the buffer  420 . The procedure to quantify the difference between the two waveforms is described in detail with respect to  FIGS. 5-6 . Once the difference between the two waveforms is determined, an up-or-down bit is sent to the output  435  by the comparator unit  430  to activate the switch  450 . 
     If the difference between the latest detected channel impulse response waveform h(t) and the channel impulse response waveform H(t) of record is less than a preset threshold then the switch  450  remains in its default OFF (open) position  455 . Thus, the waveform h(t) remains stored in the temporary buffer  410  and the waveform H(t) remains stored in the buffer  420 . In this situation, the previously acquired channel impulse response waveform H(t) remains the waveform of record. 
     If the difference between the latest detected channel impulse response waveform h(t) and the channel impulse response waveform H(t) of record is larger than a preset threshold then the switch  450  is set to its ON position  451 . Thus, the waveform h(t) stored in the temporary buffer  410  is copied in the buffer  410 . In this situation, the previously acquired channel impulse response waveform H(t) is being replaced with the latest detected channel impulse response waveform h(t). Thus, h(t) becomes the new channel impulse response of waveform of record, H(t)=h(t). 
     In other implementations, other mechanisms can be used for comparing the latest detected channel impulse response waveform h(t), with the previously acquired channel impulse response waveform H(t). Thus, the latest detected channel impulse response waveform h(t) can be selected when h(t) attains a sufficient difference from the previously acquired channel impulse response waveform H(t), according to some criterion derived from a comparison of the two signals. The comparison can be done with fully digital methods in microprocessor and memory or in comparator circuits. 
     The next emitted impulse-pulse  260  by the target-TX  210  is subjected to the same sequence of signal processing steps as the prior impulse-pulse. 
     As discussed earlier, when the channel impulse response h(t) changes slowly, the temporal and spatial focus at the target  204  degrades, therefore the time reversal based RF wireless communications also degrade. 
     In  FIG. 5  a method  500  to monitor the degradation of the channel impulse response is presented as a flow chart diagram. Method  500  is based on components of the monitoring channel described in regard to  FIG. 4 . Returning to  FIG. 5 , the degradation can be corrected, if the target repeatedly emits a pulse  260  which is then stored by the base as the channel impulse response waveform H(t)=h k (t). Index k stands for the k th  instance of the channel impulse response waveform. The base may switch over to retransmitting a new time reversed channel impulse response H(T-t)  270  based on monitoring how the newly acquired channel impulse response waveform h k (t) compares to the channel impulse response waveform H(t) currently in use by the communication system. 
     Step  510  of method  500  shows that the target repeatedly emits impulse pulses which are transmitted to the base. The time interval between consecutive pulses can be set based on need, for example, about 1s in some implementations. In step  520 , the base receives a channel impulse response h(t) and stores the channel impulse response waveform h k (t) in a temporary buffer. In step  530 , the base correlates the newly acquired channel impulse response waveform with a previously acquired channel impulse response waveform H(t)=h k-1 (t). The previously acquired channel impulse response waveform H(t) is stored in a buffer at the base. During conditional step  540 , the correlation coefficient is compared to a reference level (for example 95%). If the correlation coefficient is greater than the reference level then, in step  550 , the base continues to use the older version of the channel impulse response H(t). 
     In step  560 , the base also sends a message to the target to emit the impulse-pulses less often. The target may emit the impulse-pulses less often. 
     Returning to the conditional step  540 , if the correlation coefficient is lower than the reference level, then at step  570  the base switches to use the newly acquired channel impulse response h k (t). Furthermore, the channel impulse response waveform H(t) from the buffer is replaced with the channel impulse response waveform from the temporary buffer, H(t)=hk(t). In yet another implementation, the base also sends a message  580  to the target to emit the impulse-pulses more often. The target may emit the impulse-pulses more often. 
     In another exemplary implementation, to avoid the instabilities related to switching back-and-forth unnecessarily between low and high repetition, the base may perform correlations  530  back to several previously stored channel impulse response waveforms H k (t), H k-1 (t), H k-2 (t), . . . . In one implementation the several previously stored channel impulse response waveforms are stored in additional buffers. In another implementation, the several previously stored channel impulse response waveforms are stored a single large capacity buffer. 
     To correlate two signals h 1 ( t ) and h 2 ( t ) a mathematical algorithm is applied to determine the degree of linear dependence between the signals. Perfect correlation means that (h 1 ( t )−h 1 ( t -Dt))/h 1 ( t -Dt)=(h 2 ( t )−h 2 ( t -Dt))/h 2 ( t -Dt) for all values oft, i.e., the fractional variation occurring in signal h 1 ( t ) is perfectly matched by the same fractional change in signal h 2 ( t ). The algorithm is normalized such that when two signals are identical, the correlation coefficient is ρ=1, when the signals are statistically independent ρ=0, and when the signals behave in the opposite manner, ρ=−1. 
     The correlation coefficient is further calibrated against a metric that characterizes the actual system performance. For example, in terms of a bit error rate (BER) at the base-RX  220 , if the correlation coefficient ρ=85% corresponds to a BER of 10 −15  this may be an acceptable ρ-value for indicating that the signals are identical. In contrast, if the correlation coefficient ρ=95% corresponds to a BER of 10 −2  then the system may be more sensitive to small changes. 
     For the time reversal system illustrated in  FIG. 4 , it is important to detect relatively small changes from perfect correlation. The correlation coefficient can be calibrated against the BER at the target. The log(log) nature of the BER (i.e. a small change in SNR at good SNR) provides sensitivity to the calibrated correlation coefficient. 
     Furthermore, comparison methods, other than correlation of channel impulse responses as described above, may be used in step  530  to determine when to change between the newly acquired channel impulse response and the stored channel impulse response. A metric and a comparison technique that directly relate system performance to information contained in the collected channel impulse response waveforms are presented in the next section. 
     The Signal-to-Interference Ratio (SIR) of a single data pulse convolved with the time reversed channel impulse response H(T-t) and measured at the receiver is a measure of how much the time reversal process realigns the scattered energy (signal) compared to the returning energy that is not properly realigned in time (interference). The channel impulse response waveform currently being used is denoted H n . The SIR of H n  is calculated as 
                       SIR   H     =     1     1   -   H         ⁢     
     ⁢     H   =         ∑   n     ⁢           ⁢       (       H   n     ⁢     H   n       )     2           (       ∑   n     ⁢           ⁢       H   n     ⁢     H   n         )     2                 (   4   )               
The newly captured channel impulse response waveform is denoted h n . Similarly, the SIR of h n  response is
 
     
       
         
           
             
               
                 
                   
                     
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     A hybrid SIR is defined as the SIR obtained by using the existing channel impulse response H, when the channel has actually changed to a new channel impulse response h. This combination is denoted Hh. 
     
       
         
           
             
               
                 
                   
                     
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     The metric of interest, denoted eta, compares the new SIR with the SIR expected from using the old H in the altered channel by calculating 
     
       
         
           
             
               
                 
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     The ratio eta is greater than or equal to one since the best SIR is obtained by using the correct channel impulse response waveform for a given channel. However, there are cases where the old channel impulse response waveform may interact with a substantially different array of scatterers than the new one and produce an increase in SIR. An example of this situation is when the impulse-pulse emitted by the target  204  is too broad to fully resolve all the different scattering delays. This effect tends to reduce the signal component and increase the interference depending on how many of the scattering returns cannot be resolved. Changes in eta reflect changes in system performance, and hence a metric based on eta can stabilize a feedback control loop like the one described in method  500 . 
     Ratio eta is also protected against configurations where high or low correlation may not correlate with a change in the system performance. For moderate channel changes various algorithms can be used to stabilize the feedback loop and to find an optimum update rate. For example, a threshold value of eta th =0.9 for example produces approximately a 1 dB system penalty as the acceptable degradation. If the metric q=eta−eta th &gt;0 the old channel impulse response H(t) is kept and the update rate reduced. If q&lt;0 then the new channel impulse response h(t) is switched and the update repetition increased. This feedback algorithm adjusts the feedback rate to maintain eta−eta th =0. Note that if the base  202  continuously switches to the latest acquired channel impulse response h(t), whether necessary or no, the feedback algorithm cannot not effectively minimize the metric q. 
     In another implementation, the base can update the impulse response at every incoming pulse. 
     Discriminating Between Fast and Slow Changes of Channel Impulse Response 
     Sometimes the base may use the comparison procedure of step  530  described in regard to  FIG. 5  to identify a response where parts of the system are changing and other parts are static. For example, there may be an environment rich in multipath and scattering elements, including a component that is changing rapidly, for example due to cars, aircraft, etc. At the same time, most of the multipath and scattering elements may be changing slowly or not at all. The multipath and scattering elements in the later category may be buildings, trees, fixed antennae and semi-static personnel etc. 
     A decrease in quality of the time reversal RF communication can occur when fast changing scattering elements are present in the transmission channel. The degradation manifests as a reduction in SNR at the receiving target, since not all scattering elements are now contributing to the signal returning to the target in phase. The correlation algorithm  535  described in  FIG. 5  can discriminate between dynamic and static scattering elements. 
     An exemplary implementation of such a method is illustrated schematically in  FIG. 6 . In this implementation, correlations between several previously stored channel impulse response waveforms H k (t), H k-1 (t), H k-2 (t), . . . are being performed in step  610 . A composite correlation coefficient is calculated in step  620  corresponding to a statistically static component of the channel impulse response, and a dynamic component of the channel impulse response. 
     In many cases the channel impulse response may not be completely static or dynamic but consist of a summation of a static component and a dynamic component. If the dynamic component is due to rapidly moving or changing objects, the channel impulse response H(t) may change between the time an impulse response is recorded and the time it is sent back through the channel in a time reversed manner. If there is a static component however, that part will remain constant. In some cases only the component due to the static scatterers is extracted while the dynamic component is removed. This process improves the SIR such that only the pulses that can add to the peak in the returning signal are sent back to the target. There still is degradation in SIR since the returning signal is not perfectly matched to the channel impulse response. 
     On the other hand, for slowly changing channels, the dynamic part which represents the required update to the stored channel impulse response H(t) is included to ensure that the stored channel impulse response H(t) matches the actual channel impulse response h(t). In fact when the feedback control loop  500  mentioned earlier (see EQs. 4-7) is implemented, some dynamic change to the channel impulse response h(t) is necessary in order to keep H(t) updated. In this case the dynamic component is preserved. The signal is processed in order to examine the dynamic component. The dynamic component provides information on how fast the system is changing and what parts of the system are changing. 
     In other systems, like a radar type system, it is the dynamic component that is monitored and static background represents clutter and can be subtracted. Hence there is no simple algorithm that allows to predict when to remove or to use the dynamic component. Therefore any processing algorithm records the dynamic components even if they are subsequently not used. 
     An efficient processing algorithm captures multiple time series of the channel impulse response h k (t) and applies any computationally efficient cross-correlation algorithm designed to highlight only changing components. One example of a processing algorithm is to subtract values of each pair of curves at every sampling instant. The resultant curve only contains contributions from changing components. A new curve can be generated from these residuals or the squared residuals producing a curve with the dynamic elements. The curve containing only the elements that do not change substantially will represent the static components. Other algorithms can be used over multiple curves to get a clearer picture of the statistics of the changes. 
     The dynamic component may reveal important information. For example Doppler shifting is identified as a change in the width of the impulse-pulse emitted by the target  204 , or of the carrier frequency if the impulse is an amplitude modulated carrier pulse. This may reveal the presence of a moving object and allows to infer its velocity. Over time a record can be built of the trends exhibited by the dynamic component and distinguish for example moving objects from simple changes in the reflective properties of objects. 
     The dynamic component of the channel impulse response can also monitor how rapidly a channel is changing. Information on changes in channel impulse response h(t) can be used to anticipate a future state of the system  400 . 
     Returning to step  630  if method  600 , the dynamic component is not separated from the channel impulse response h k (t). In step  640  the composite correlation coefficient is used to decide during conditional step  545  whether to keep the currently stored channel impulse response H(t) or not. Both static and dynamic components may be recorded in step  640  for future use. 
     In another implementation the dynamic component of the channel impulse response h k (t) is removed in step  650 . Only the statistically static component of the channel impulse response contributes  660  to the correlation coefficient used to decide during conditional step  545  whether to keep the currently stored channel impulse response H(t) or not. The SNR at the target  204  increases since energy that does not contribute to signal but only to noise and interference has been removed. Furthermore in step  660 , the base  202  controls the update repetition of the system  400  by the static component only which results in a more stable system. In this implementation, the update repetition is set to maintain only the static component of the channel impulse response. 
     In step  670 , the dynamic component of the channel impulse response can be used  670  to indicate change in the channel impulse response. The dynamic component of the channel impulse response may be used in situations where it is important to monitor for changes in the environment, hence can be used as the basis of an intrusion detection system. The dynamic component of the channel impulse response may be particularly useful for detecting stealth objects which are designed to reflect energy in a direction other than directly back at an emitting source. In which case the only information that can be gathered arises from interaction with scatterers and changes in the scattering signatures. 
     Other Implementations of Monitoring of Channel Impulse Response 
     In another aspect, the target  204  can perform correlation of the returning impulse with the outgoing impulse. Performing the correlation step at the target  204 , enables the target  204  to also recover the information about the transmission channel. Once the data is recovered, the target  204  now has a mechanism for analyzing any dynamic changes in the channel impulse response h(t). Unlike the base-RX  230  which receives impulse pulses at a relatively slow repetition, the target-RX  220  receives the equivalent of probe pulses at the much higher data rate. Hence the target  204  can resolve very small changes in the path channel impulse response h(t) very rapidly. 
     The criterion used by the target to determine the update repetition can either be a direct system performance measure, for example BER, performed at the target or a more complex evaluation of the channel impulse response performed at the base and transmitted to the target by embedding a message in the data or communicating out of band. 
     In another implementation, the system uses neural networking algorithms to learn the relationship between channel impulse response and system performance. 
     In another exemplary implementation, the base  202  may request the target  204  to signal at a higher than necessary rate to allow the base  202  to compare multiple responses in order to evaluate the degradation rate of the channel impulse response. 
     Data Transmission from Base with Multi Element Antenna to Multiple Targets 
     The implementations presented above refer to time reversal based RF wireless communications between a single target  204  and a base  202  equipped with a single element antenna, as illustrated in  FIGS. 2-4 . RF wireless communications involving more than one target and a transmission base with a multi-element antenna can also be implemented. 
     In  FIG. 7 , a number of targets  720 ,  720 ′,  720 ″ can communicate with a base  710  configured to perform RF wireless communications based on time reversal. The base  710  is equipped with a multi-element antenna array  730 - 740 ,  730 ′- 740 ′,  730 ″- 740 ″. In analogy to  FIG. 4 , there exist two signal exchange paths between each target  720  and each element of the antenna array at the base. For example, each target  720  independently transmits a pulse to the base  710 . The pulse  760  from target j  720  (where j=1, . . . , n) passes through the medium  130 , interacts with scattering objects and arrives at a base  710 . 
     In this implementation, each element i  730 - 740 ,  730 ′- 740 ′,  730 ″- 740 ″ (where i=1, . . . , m) of the antenna array records the channel impulse response h ij (t) of the transmission path between target j  720  and element i  730 - 740 . 
     Each element i is coupled with a waveform processing unit  750 . The waveform processing unit  750  digitizes the channel impulse response h ij (t) in a high speed sampling circuit. The sampling circuitry may be collocated at one location within the base  710 . Hence the different distances and delays of the antenna elements can be recorded as part of the appropriate channel impulse H ij (t) response and be removed by the time reverse process. 
     Then, the waveform processing unit  750  time reverses the signal and stores the channel impulse response H ij (t) in the memory, for each antenna element i  730 - 740 . Among other things, the waveform processing unit  750  monitors an m×n matrix of waveforms, where each element of the matrix contains a channel impulse response waveform Hij(t). For example, each element i of the antenna array monitors n waveforms, corresponding to the n targets the element communicates with. 
     The antenna array  730 - 740  retransmits each time reversed channel impulse response H ij (T-t) as an analog signal. As discussed earlier, assuming that the medium  130  is linear and reciprocal, the signal arrives at the target  720  without being affected by the noise caused by the multipath transmission channel  130 . According to the transmission method described in regard to  FIG. 3 , a data stream originating at antenna element i  730 - 740  and intended to arrive at target j  720  is convolved with the time reversed channel impulse response H ij (T-t) prior to emission. 
     In addition to the independent operation mode involving transmission from one target  720  to one element of the antenna array (as described above), the base  710  can operate in a partially coordinated mode (i.e. groupings of antenna elements), or in a fully coordinated mode. While operating in any coordinated mode, the base  710  can effectively focus the signal temporally and spatially at the correct target  720  as long as the channel impulse response H(t) of each target  720  is available at each element of the antenna array at the base  710 . Note that the elements of the antenna array work in a coordinated manner to send a specific stream of data to a target, in order to avoid an overlap of independent or uncoordinated data streams overlapping at the target, as described below. 
     Each of the m elements of the antenna array uses the respective channel impulse response H i1 (t) to simultaneously send data to target j=1. Each channel impulse response H i1 (t) is time reversed with respect to a common synchronized reference time T, i.e. H i1 (T-t i1 ), and then added to each bit of a data stream prepared for transmission to target j=1. Therefore, the emitted signals from each antenna array element i are properly synchronized so that the antenna array spatially and temporally focuses all the signals back to a single target, namely target j=1 in this case. Thus, the time reversal process enables creating a directional antenna with no requirement to steer or aim the antenna array. The time reversed channel impulse response H i2 (T-t i2 ) can also be sent simultaneously but with a different data stream by simply adding at each antenna element i the time reversed channel impulse response H i2 (T-t i2 ) to each bit of the digital stream corresponding to target  2 . Once again, the time reversal process enables that the data indexed for target  1  reaches target  1  and the data indexed for target  2  reaches target  2  and so on. 
     When multiple elements of the antenna array communicate to a specific target, the same feedback technique  500  as described (in  FIG. 5 ) for a single pair can be used. Moreover, a target  720  is instructed to emit impulse-pulses less often only when the threshold-based comparison  540  that triggers a decrease in repetition applies to each element of the antenna array. Alternately, a target  720  is instructed to emit impulse-pulses more often as soon as the threshold-based comparison  540  that triggers an increase in repetition applies to at least one element of the antenna array. 
     An exemplary procedure to determine the order in which the base  710  receives impulse-pulses  760 ,  760 ′,  760 ″, . . . from the respective targets  720 ,  720 ′,  720 ″, . . . is presented below. The base station  710  transitions to silence or a given regular code pattern. This activates each target  720  to send a preprogrammed impulse sequence, for example a sequence of pulses of different pulse widths. The base  710  can detect how many targets  720  are operating. The base  710  can then signal with silence or activation code to silence the targets  720 . Each target  720  is given a default queuing index that instructs the target  720  how long to wait after an activation code. Each target  720  then sends an impulse  760  and waits for an acknowledgment signal from the base  710 . 
     If the target  720  has successfully transmitted an impulse  760 , the acknowledgement signal from the base  710  returns to the target  720  in the form of a narrow pulse. The receipt by the target  720  of the narrow pulse completes the setup of target  720 . The second target  720 ′ is activated and repeats this process. Eventually all targets  720 ,  720 ′,  720 ″, . . . are initialized and the base  710  signals to each to start communications with a regular update. If a target detects a degradation of performance, the target can emit impulse-pulses more often until the reception performance at the target reaches an acceptable performance level. 
     The above technical description refers to the impulse response being generated by a node emitting an ‘impulse’, which is normally understood to be a mathematical “delta” function. This is merely the simplest signal from which the impulse response can be derived. Other signals which can be realized in practice can be used, for example, one pulse or a sequence of pulses where the pulses are the narrowest pulse consistent with a Nyquist receiver, which is a receiver that is designed for optimum signal to noise ratio. These pulses can be optimized for recovery of the signals in a specialized noise environment or simply to enable noise to be averaged from the recovered impulse response. Different shaped pulses may also be employed. The most likely variant of pulse shape from a simple narrow pulse like a Gaussian pulse is a pulse shaped by the transmit filter function and the emitting antenna which may broaden the pulse and differentiate the current fed to the antenna with respect to time. To deconvolve the impulse response from the actual received signal, the node which is responsible for time reversing the impulse response can use the employed pulse shape or sequence, and the receiver filter response. 
     Although a few variations have been described in detail above, other modifications are possible. For example, the logic flow depicted in the accompanying figures and described herein does not require the particular order shown, or sequential order, to achieve desirable results. 
     While this document contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination. 
     Only a few implementations are disclosed. However, variations, enhancements and other implementations can be made based on what is described and illustrated in this document.