Patent Application Titled "Apparatus and Method for Estimating Doppler Shift in Underwater Communication" Published Online (USPTO 20160192307)

By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors KIM, Seung-Guen (Daejeon, KR); LIM, Yong-Kon (Daejeon, KR); KIM, Sea-Moon (Daejeon, KR); PARK, Jong-Won (Daejeon, KR); YUN, Changho (Sejong-Si, KR), filed on December 28, 2015, was made available online on July 7, 2016.

No assignee for this patent application has been made.

Reporters obtained the following quote from the background information supplied by the inventors: "The present disclosure generally relates to an apparatus and method for estimating a Doppler shift for underwater communication and, more particularly, to an apparatus and method for estimating a Doppler shift for underwater communication, which may estimate a Doppler shift by computing correlations between output values of a matched filter and using a time gap between points at which the correlations have maximum values.

"In terrestrial and underwater radio communications, a Doppler shift occurs by relative movements of communication devices. The Doppler shift increases relative to an increase in approaching or receding speed of a relative distance between the transmitter and receiver according to the Doppler effect.

"For example, a wavelength of light changes according to movement of an object, i.e., it becomes longer as the object moves farther away and becomes shorter as the object moves closer.

"While a Doppler shift occurs, respective frequency components of a signal undergo different Doppler shifts and the length of the signal becomes long or short relative to an amount of the Doppler shift. To quantitatively represent the Doppler shift, a relative Doppler shift, .DELTA., is used, which may be expressed in the following equation 1:

".DELTA.=v/c (1)

"where, v is a relative difference in moving speed between the transmitter and the receiver, and c is the velocity of waves in the medium. Due to the Doppler shift, distortions may occur in the received signal on the time axis r(t) and the frequency axis f.sup.D, which may be expressed in the following equations 2 and 3, respectively:

"r(t)=s((1+.DELTA.)t) (2)

"f.sup.D=f(1+.DELTA.) (3)

"where s(t) is a signal for transmission, r(t) is a Doppler-shifted received signal, f is one of frequency components of the signal for transmission of a bandwidth, and f.sup.D is a frequency component resulting from a Doppler shift of the frequency component f.

"In terrestrial radio communications using radio waves, the speed of waves c is defined to be approximately 3.times.10.sup.8 m/s, which is the same as the speed of light. Generally, in terrestrial radio communications, signals having a few kHz to tens of MHz of bandwidth are transmitted at a carrier frequency of hundreds of MHz to a few GHz. For example, in the case of Automatic Identification System (AIS) used in the safe operation of vessels, communication data is transmitted at a frequency around 160 MHz of carrier frequency on a channel having a bandwidth of 25 kHz. Furthermore, assuming that the relative moving speed between the transmitter and receiver is about 15 m/s (or 54 km/h), the relative Doppler shift .DELTA. has a very small value. Since packet signals typically used in radio communications are about tens of ms long, an amount of increase or decrease in signal length resulting from the Doppler shift is very small in one packet length, and thus the distortions occurring on the time axis can often be ignored. However, the carrier frequency used for terrestrial communications is about within a few MHz to a few GHz, and the signal may be distorted due to distortions occurring on the frequency axis from the Doppler shift. A frequency band of signals used for terrestrial communications is a narrow band having a relatively small ratio of carrier frequency to bandwidth, which is often less than a few percentage of the carrier frequency. Accordingly, all frequency components in the signal band may be approximated to the carrier frequency to be subject to the same Doppler shift. In terrestrial narrow band communication, such a Doppler shift is approximately considered as a carrier frequency error, which is estimated and compensated using a frequency-shift synchronization method.

"Since a velocity of waves in a medium that transmits information in underwater communication using sound waves is about 1500 m/s, and the velocity of waves in a medium in terrestrial communication is about 3.times.10.sup.8 m/s, a Doppler shift in underwater communication using sound waves may appear to be 2.times.10.sup.6 times greater than in terrestrial radio communication that uses radio waves at the same relative difference in moving speed between the transmitter and the receiver.

"In underwater communication using sound waves, a carrier frequency is about from a few kHz to tens of kHz, and the usage bandwidth of the signal becomes up to tens of percentage of the carrier frequency, that is, the underwater communication signal is a wideband signal. In underwater communication, the wideband signal may lead to occurrence of different frequency shifts at the same Doppler shift for respective frequency components of the signal, and may thus be more appropriately approximated with a change in length of a received packet signal due to the Doppler shift in the time domain. The Doppler shifted received packet signal is expressed in the equation 2.

"FIG. 1 shows a structure of a packet signal for underwater communication. Referring to FIG. 1, in conventional underwater communication using wideband signals, in order to estimate a Doppler shift, known signals 10 and 30 are transmitted by being placed at either ends of packet data 20 with a known length, and the receiver uses a method for estimating the Doppler shift based on a time gap T.sub.RX (between two signal points where peak values of outputs of matched filtering of the known signals from a matched filter appear, a time gap T.sub.TX between the two known signals, and a relationship with the Doppler shift, as expressed in equation 4.

"Equation 4 is as follows:

".DELTA. ^ = T RX - T TX T TX ( 4 ) ##EQU00001##

"where, T.sub.TX is a gap between signals transmitted to estimate a Doppler shift, and T.sub.RX (is a time gap between two points that represent peak values of outputs of a matched filter in the receiver, which may be greater or smaller than T.sub.TX according to the Doppler shift.

"In underwater communication using sound waves, as previously described, the known signals 10 and 30, known to be robust to Doppler shift, are placed at either ends of the packet data 20, or placed with a predetermined interval, and the receiver uses a method for estimating a Doppler shift by measuring a time gap between two peak values of outputs of a matched filter.

"The method may be suitably used when there is an Additive White Gaussian Noise (AWGN) channel or when one path among multipaths has a much larger energy than others. However, underwater channel conditions are characterized in that there may be similar size multipath components, that the multipath components with a sparse distribution arrive at the receiver end, and that the multipath components are changed in size over time, that is, time-varying channel response. In the time-varying multipath underwater channel condition, if there are multipath components having similar magnitude, peak values of the outputs of the matched filter corresponding to the front and rear parts of the packet may be determined based on respective different paths due to the variation of the magnitude of the multipath components.

"FIG. 2 shows an instance of occurrence of an error in estimation of a Doppler shift using conventional estimator when there are similar size multipaths. In the case that there are two multipaths with similar sizes in the reception of a signal having a structure of a packet signal 100 of FIG. 2, when a known signal 1 110 and a known signal 2 130 are received, the sizes of the respective multipaths and a difference in arrival time are represented in FIG. 2 by solid and dashed arrows for the two multipaths, respectively. As can be seen from FIG. 2, the path represented by the solid line has the largest size in receiving the known signal 1 110, and the path represented by the dashed line has the largest size in receiving the known signal 2 130. In this case, using the aforementioned method for estimating a Doppler shift commonly used in underwater communication causes an error in estimation of the Doppler shift. In other words, an error occurs in measurement of the time gap between the two peak values, leading to an error in estimation of the Doppler shift."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an apparatus and method for estimating a Doppler shift for underwater communication to prevent occurrence of an error in measuring a time gap between two peak values of outputs of a matched filter corresponding to front and rear parts of a packet in a case that the two peak values are determined by different paths.

"In order to accomplish the above object, the present invention provides an apparatus for estimating a Doppler shift for underwater communication.

"The apparatus includes: a multipath delay profile determiner for detecting a correlation between a known signal 1 and a received packet signal, time (T.sub.10) at a point having maximum energy of the correlation, and a multipath delay profile at the time (T.sub.10); a correlation calculator for calculating a correlation between the multipath delay profile and a correlation between a known signal 2 and the received packet signal; a maximum value detector for detecting time (T.sub.20) at a point having maximum energy of the correlation calculated by the correlation calculator; and a Doppler shift estimator for determining a Doppler shift using a difference between the time (T.sub.10) and the time (T.sub.20) at points having respective maximum energy.

"The apparatus may further include: a matched filter for receiving the packet signal and performing matched filtering on the received packet signal; and a second known signal range calculator for calculating a range in which the known signal 2 exists.

"The multipath delay profile determiner is configured to calculate the correlation of the received packet signal and the known signal 1 with respect to time (T.sub.1) corresponding to a starting point of the received packet signal, as in the following equation:

".chi..sub.sr(t.sub.1)=.intg..sub.0.sup.Ts*(.tau.)r(t.sub.1+.tau.)d.tau., for

"T.sub.1-T.sub.window-low.ltoreq.t.sub.1<T.sub.1+T.sub.window-high, calculate accumulated energy of correlations over Tch time in a window zone as in the following equation:

"E.sub.sr(t.sub.11)=.intg..sub.0.sup.T.sup.ch|.chi..sub.sr(t.sub.11+.tau.- )|.sup.2d.tau., for

"T.sub.1-T.sub.window-low.ltoreq.t.sub.11<T.sub.1+T.sub.window-high-T.s- ub.ch, and determine matched filtered data corresponding to a point at which the accumulated energy of correlations in the window zone has the maximum value as the multipath delay profile.

"The second signal range calculator is configured to represent the range (T.sub.X) in which the known signal 2 exists as in the following equation:

"T.sub.X=)T.sub.2-T.sub.2a,T.sub.2+T.sub.2a, where

"T.sub.2=T.sub.10+T.sub.TX, (-T.sub.2a, T.sub.2a) is a range of changes in arrival time of a first signal and represented by T.sub.2a=T.sub.TX.times..DELTA..sub.max, .DELTA..sub.max denotes a maximum amount of a relative Doppler shift, and T.sub.TX denotes a time gap between the two known signals.

"The matched filter is configured to calculate the correlation .chi..sub.sr (t.sub.2) of the known signal 2 and the received packet signal as in the following equation:

".chi..sub.sr(t.sub.2)=.intg..sub.0.sup.Ts*(.tau.)r(t.sub.2+.tau.)d.tau. for

"T.sub.2-T.sub.2a-T.sub..alpha..ltoreq.t.sub.2<T.sub.2+T.sub.2a+T.sub..- beta., where, T.sub..alpha. and T.sub..beta. have values of zero or more, which are determined according to the margin of detection range for estimation of a Doppler shift in the stage of system design, wherein the correlation calculator is configured to calculate a correlation between a correlation for (.chi..sub.1(t)=.chi..sub.sr(T.sub.10+t) for 0.ltoreq.t.ltoreq.T.sub.ch in a window zone and the calculated correlation (.chi..sub.sr(t.sub.2)) as in the following equation:

".chi..sub..chi..chi.(t.sub.2)=.intg..sub.0.sup.T.sup.ch.chi..sub.t*(.tau- .).chi..sub.sr(t.sub.2+.tau.)d.tau. for

"T.sub.2-T.sub.2a-T.sub..alpha..ltoreq.t.sub.2<T.sub.2+T.sub.2a+T.sub..- beta., and wherein the maximum value detector is configured to detect the time (T.sub.20) of a point at which energy of the correlation between multipath delay profile determiner for the known signal 1 and the correlation between the known signal 2 and the received packet signal .chi..sub.sr(t.sub.2) has the maximum value, as in the following equation:

"T.sub.20=max.sub.t.sub.2|.chi..sub..chi..chi.(t.sub.2)|.sup.2.

"The Doppler shift estimator may be configured to obtain the Doppler shift in the following equation:

".DELTA. ^ = T 2 O - T 1 O T TX , ##EQU00002##

"where T.sub.TX refers to a time gap between two known signals.

"The apparatus may further include an energy calculator for receiving a packet signal and calculating energy of the matched filtered packet signal; a comparator for comparing the energy of the packet signal calculated by the energy calculator with a threshold; and a packet signal start detector for analyzing the comparison result from the comparator to determine that the packet signal begins at the point if the energy is greater than the threshold, and sending information about the point at which the packet signal begins to the multipath delay profile determiner.

"In order to accomplish the above object, the present invention also provides a method for estimating a Doppler shift for underwater communication.

"The method includes a first detection process of detecting a correlation between a known signal 1 and a received packet signal, time (T.sub.10) at a point having maximum energy of the correlation, and a multipath delay profile; a second detection process of detecting time (T.sub.20) of a point having maximum energy of a correlation between a correlation between a known signal 2 and the received packet signal and the multipath delay profile; and a Doppler shift estimation process of determining a Doppler shift using the time (T.sub.10) and the time (T.sub.20).

"The first detection process may include determining whether it is a starting point of the received packet signal; performing matched filtering on the received packet signal if it is determined that it is a starting point of the packet signal (first matched filtering process); and detecting maximum energy of correlations in a window zone using the matched filtered data, detecting corresponding time (T.sub.10), and determining matched filtered data corresponding to a point at which the matched filtering result has a maximum value, as a multipath delay profile.

"The second detection process may include performing matched filtering on the received packet signal if a starting point of the known signal 2 has come (second matched filtering process); and calculating maximum energy of a correlation of an output of the second matched filtering and the multipath delay profile, and setting time corresponding to the maximum energy of the correlation as time (T.sub.20) having maximum energy.

"Determining matched filtered data corresponding to a point at which the matched filtering result has the maximum value, as a multipath delay profile may include calculating the correlation of the received packet signal and the known signal 1 with respect to time (T.sub.1) corresponding to a starting point of the received packet signal, as in the following equation:

".chi..sub.sr(t.sub.1)=.intg..sub.0.sup.Ts*(.tau.)r(t.sub.1+.tau.)d.tau. for

"T.sub.1-T.sub.window-low.ltoreq.t.sub.1<T.sub.1+T.sub.window-high, calculating accumulated energy of correlations over Tch time in a window zone as in the following equation:

"E(t.sub.11)=.intg..sub.0.sup.T.sup.ch|.chi..sub.sr(t.sub.11+.tau.)|.sup.- 2d.tau. for

"T.sub.1-T.sub.window-low.ltoreq.t.sub.11<T.sub.1+T.sub.window-high-T.s- ub.ch, and determining matched filtered data corresponding to a point, at which the accumulated energy of correlations in the window zone has a maximum value, as the multipath delay profile.

"The second detection process may include calculating the correlation .chi..sub.sr(t.sub.2) of the known signal 2 and the received packet signal as in the following equation:

".chi..sub.sr(t.sub.2)=.intg..sub.0.sup.Ts*(.tau.)r(t.sub.2+.tau.)d.tau. for

"T.sub.2-T.sub.2a-T.sub..alpha..ltoreq.t.sub.2<T.sub.2+T.sub.2a+T.sub..- beta., and where, T.sub.2=T.sub.10+T.sub.TX, (-T.sub.2a, T.sub.2a) is a range of changes in arrival time of a first signal and represented by T.sub.2a=T.sub.TX.times..DELTA..sub.max, .DELTA..sub.max denotes a maximum amount of a relative Doppler shift, T.sub..alpha. and T.sub..beta. have values of zero or more, which are determined according to the margin of a detection range for estimation of a Doppler shift in the stage of system design, wherein a correlation between the correlation (.chi..sub.1(t)=.chi..sub.sr(T.sub.10+t) for 0.ltoreq.t<T.sub.ch) of a window zone and the calculated correlation (.chi..sub.sr(t.sub.2)) is calculated as in the following equation:

".chi..sub..chi..chi.(t.sub.2)=.intg..sub.0.sup.T.sup.ch.chi..sub.t*(.tau- .).chi..sub.sr(t.sub.2+.tau.)d.tau. for

"T.sub.2-T.sub.2a-T.sub..alpha..ltoreq.t.sub.2<T.sub.2+T.sub.2a+T.sub..- beta., and wherein the time (T.sub.20) of a point, at which the energy of the correlation between multipath delay profile determiner for the known signal 1 and the correlation between the known signal 2 and the received packet signal .chi..sub.sr(t.sub.2) has the maximum value, is detected as in the following equation:

"T.sub.20=max.sub.t.sub.2|.chi..sub..chi..chi.(t.sub.2)|.sup.2.

"The Doppler shift estimation process may include obtaining the Doppler shift as in the following equation:

".DELTA. ^ = T 2 O - T 1 O T TX , ##EQU00003##

"After the first matched filtering process, the method may further include: calculating energy of a matched filtered packet signal in the first matched filtering process; determining whether the calculated energy of the packet signal is equal to or greater than a threshold; and determining that the packet signal begins at a point from which the energy of the packet signal is equal to or greater than the threshold, if the energy of the packet signal is equal to or greater than the threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

"The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

"FIG. 1 shows a structure of a packet signal for estimating a Doppler shift for underwater communication;

"FIG. 2 shows an instance of an error occurring in estimation of a Doppler shift when there are multipaths in similar size;

"FIG. 3 is a block diagram schematically illustrating a receiver for receiving a packet signal for estimating a Doppler shift, according to an embodiment of the present disclosure;

"FIG. 4 is a flowchart illustrating a process of estimating a Doppler shift, according to an embodiment of the present disclosure;

"FIG. 5 is a block diagram of a receiver for receiving a packet signal for estimating a Doppler shift, according to another embodiment of the present disclosure;

"FIG. 6 is a flowchart illustrating operation 5410 of FIG. 4 for determining a multipath delay profile, according to an embodiment of the present disclosure; and

"FIG. 7 is a flowchart illustrating an operation of FIG. 5 for detecting a starting point of a packet signal, according to an embodiment of the present disclosure."

For more information, see this patent application: KIM, Seung-Guen; LIM, Yong-Kon; KIM, Sea-Moon; PARK, Jong-Won; YUN, Changho. Apparatus and Method for Estimating Doppler Shift in Underwater Communication. Filed December 28, 2015 and posted July 7, 2016. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=264&p=6&f=G&l=50&d=PG01&S1=20160630.PD.&OS=PD/20160630&RS=PD/20160630

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