Apparatus and method for correcting doppler shift in mobile communication systems

[0001] This invention relates to communication systems, and more particularly to a system and method for removing the Doppler shift in the frequency of a signal transmitted from a mobile platform, and/or removing the Doppler shift from a signal received by a mobile receiving platform.

[0002] In a mobile communication network, Doppler shift occurs when the velocity vector of a transmitting mobile platform differs from the velocity of a receiving mobile platform. For example, when two platforms are stationary with respect to each other (or with respect to a common reference frame) and are communicating with each other, the frequency of the signal received by the receiving platform from the transmitting platform will be the same as the frequency transmitted by the transmitting platform. In this case no Doppler frequency shift exists since the distance between the two platforms remain constant. When the distance between the two platforms is reducing with time, the frequency of the signal received by the receiving platform from the transmitting platform will be higher than the frequency transmitted by the transmitting platform due to the Doppler affect. When the distance between the two platforms is increasing with time, the frequency of the signal received by the receiving platform from the transmitting platform will be lower than the frequency transmitted by the transmitting platform due to the Doppler affect.

[0003] It is noted that distance between two platforms can also increase or decrease with time though one of the two platforms is stationary with respect to a common reference frame. This relative distance variation between the two platforms with respect to time will result in a Doppler frequency shift that needs to be accounted for by the receiving platform regardless whether it is the moving platform or the stationary platform with respect to a common reference frame.

[0004] For a nominal frequency of f₀, the actual frequency at the receiver is f₀+Δf, where Δf is the Doppler shift. To accommodate this variation in the received frequency of an electromagnetic wave signal, previously developed systems call for the receiver electronics to accept a wider frequency bandwidth than the nominal frequency bandwidth of the signal. This increases the amount of noise entering the receiver, thereby reducing the signal-to-noise ratio. In addition, the variation in frequency means that the system must use larger guard bands, i.e., unused bands of frequency between each link's nominal frequency and the frequencies of other links. This arrangement wastes bandwidth. Furthermore, because the incoming frequency is not precisely known, the receiving modem must scan over a range of frequencies before it can lock onto the carrier. This reduces the time available for data to be received, especially in time division multiple access (TDMA) systems where the modem must resynchronize at the beginning of every time slot. The above described Doppler shift applies to acoustic signals having a frequency of possibly less than 1 Hz to hundreds of KHz, as well as to electromagnetic wave signals. The Doppler shift has the negative effect of increasing the time to establish a two-way communication link due to longer modem synchronization times, the drawback of necessitating the extra bandwidth needed for guard bands, and serves to increase noise present with the received signal as a result of the use of the guard bands.

[0005] The present invention is directed to a system and method for compensating for the Doppler shift effect on a signal being transmitted from a transmitting platform to a receiving platform, where at least one of the two platforms is a mobile platform.

[0006] In one preferred implementation the system of the present invention involves the use of a transmitter having an associated processor. The transmitter is located on a mobile platform. A receiver having its own processor is disposed on a separate platform. Signals, for example electromagnetic wave signals, are transmitted from the transmitting mobile platform to the receiving mobile platform. The transmitting mobile platform determines its velocity vector relative to a reference frame that is common to all platforms in the system, as well as a unit vector that represents the direction of the receiving platform relative to the transmitting platform. The processor associated with the transmitter uses the velocity vector and the unit vector to calculate a Doppler shift that would be experienced by the receiving platform if the receiving platform were stationary with respect to a common reference. The processor then alters the transmitted frequency of the signal from the transmitter as necessary to cancel the Doppler shift that the receiving platform would experience.

[0007] In another preferred implementation, the receiving platform operates to cancel the Doppler shift on a signal that it is receiving from a transmitting platform. In this embodiment the receiver determines a vector of velocity relative to the common reference frame, as well as a unit vector in the direction from itself to the transmitting platform. A processor associated with the receiver uses this information to determine a Doppler shift that would be experienced by the receiving platform when receiving the signal from the transmitting platform, if the transmitting platform was stationary. The process to adjust the frequency of the receiver is needed to cancel the Doppler shift in the receive signal that is attributable to movement of the receiving platform. In another alternative preferred embodiment, both the receiving and transmitting platforms perform the above-mentioned Doppler shift determinations. The transmitting platform removes the Doppler shift that would be imparted to the transmitted signal as a result of motion of the transmitting platform, while the receiving platform similarly determines and removes the Doppler shift component that is attributable to its own motion, relative to the transmitting platform.

[0008] Another alternative preferred embodiment is designed for two-way communications where each platform includes both a receiver and a transmitter that perform the above-mentioned Doppler shift determinations. The transmitter on each platform removes the Doppler shift that would be imparted to the transmitted signal (and experienced by the other mobile platform), as a result of its own motion, while the receiver on each platform similarly determines and removes the Doppler shift component that is attributable to its own motion, relative to the other platform.

[0009] The present invention is not limited to electromagnetic wave signals but can be applied to acoustic signals, including sonar signals, as well as optical signals including laser signals. Furthermore, the present invention can be used with electromagnetic wave or light signals transmitted through the atmosphere or between spacecrafts, or to electromagnetic wave or acoustic signals between two underwater vessels communicating with each other while submerged underwater. The present invention is equally applicable to situations where one platform is stationary and the other one is mobile, or where both platforms are mobile.

[0010] The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments.

[0016] The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0017] Referring to FIG. 1, there is shown a system 10 that includes a preferred implementation of the present invention. The system 10 generally includes a transmitting mobile platform 12 having a transmitter 14 carried thereon, and a receiving mobile platform 16 having receiver 18 located thereon. Assuming for the moment that receiving platform 16 is stationary or moving toward mobile platform 12, then electromagnetic wave signals transmitted from transmitter 14 of transmitting platform 12, as represented by signal line 20, would be increasing slightly in frequency since platform 12 is moving towards platform 16. This increase in frequency represents the Doppler shift that the receiving platform 16 “sees.” If receiving platform 16 is moving towards transmitting platform 12, then an additional Doppler shift component will be experienced by receiving platform 16 due to its own movement.

[0018] With brief reference to FIG. 2, the transmitter 14 on the transmitting platform 12 is in communication with a processor 22. An antenna 24 is used for transmitting electromagnetic wave signals. The receiving platform 16 similarly includes a processor 26 which is in communication with the receiver 18. An antenna 28 is used for receiving electromagnetic wave signals from the receiver 18. Antennas 28 and 24 may be located on an exterior surface of each mobile platform 12 and 16 or at a suitable location internal to a fuselage of each mobile platform 12 and 16. Furthermore, while the transmitting platform 12 and the receiving platform 16 are each illustrated as aircraft, it will be appreciated that the present invention can be implemented with any mobile platform such as trains, busses, ships, satellites, spacecrafts, missiles, submarines, torpedoes, other airborne vehicles, other undersea vessels, other land vehicles, or other space vehicles. Furthermore, each mobile platform could have a bi-directional transceiver for both transmitting and receiving electromagnetic wave signals. In this instance, as will be explained in greater detail in the following paragraphs, each mobile platform will determine the necessary Doppler shift corrections needing to be applied, on both transmit and receive operations, as a result of its own motion.

[0019] Referring further to FIG. 1, the transmitting platform 14, to remove the Doppler effect that will be experienced by the receiving platform 16 due to the motion of the transmitting platform 12, must determine 1) its own velocity relative to a common, pre-established reference frame, and 2) a direction to the receiving platform 16 relative to the common reference frame. If the receiving platform 16 is a mobile platform such as illustrated in FIG. 1, then the receiving platform also must determine its velocity relative to the same (i.e., common, pre-established) reference frame, and also its direction relative to the transmitting platform 12 relative to the same common, pre-established reference frame. The scenario with both platforms 12 and 16 moving towards each other will be assumed for the purpose of the following description.

[0020] The total Doppler shift of the electromagnetic wave signal 20 received at the receiver 18 is due to two velocities: the velocity of the transmitting platform 12, as well as the velocity of the receiving platform 16. Assuming for the moment that platform 16 is stationary, the received frequency at receiver 18 can be represented by the following equation:
Δf_T=f₀r_TR·v_T/c
where f_T is the frequency emitted by the transmitter; “c” is the speed of light; “·” represents a vector dot product; where Δf_T is the frequency shift due to the transmitting platform's motion; and where f₀ represents the carrier frequency of the transmitted signal 20 for the stationary platform.

[0021] With the present invention, each mobile platform 12, 16 performs local adjustments to eliminate the part of the frequency shift that is due to its own motion. For example, if the transmitting platform 12 and the receiving platform 16 were each stationary, the transmitting platform 12 would transmit at f₀ and the receiving platform 16 would receive the electromagnetic wave signal 20 at f₀. However, since the transmitting platform 12 is moving relative to the receiving platform 16, the transmitting platform 12 first determines its velocity vector v_T relative to an agreed upon common reference frame, for example, Local Earth Coordinates or common stellar coordinates. In this regard, it will be appreciated that a common coordinate frame of reference for position and velocity in a three dimensional coordinate system will be necessary in most instances. Three dimensional coordinate systems can be obtained by the aircraft's on-board navigation system which provides longitude, latitude, altitude, direction, and velocity with respect to Local Earth Coordinates. This can be provided by multiple earth stations or multiple GPS satellites (GPS constellation) to correlate a position and determine a velocity. This can be accomplished for a spacecraft by referencing to multiple heavenly bodies (sun, planets, stars, constellations, etc.) to triangulate a position and determine a velocity. There are numerous other common coordinate navigation systems that can be used to determine position and velocity such as Long Range Radio Aid to Navigation (LORAN), VHF Omnidirectional Range navigation system (VOR), etc.

[0022] The transmitting platform 12 must then determine a unit vector r_TR in the direction from itself to the receiving platform 16 with respect to the common reference frame. The precise mechanism or system for determining the unit vector r_TR depends on the system in which the invention is used. For example, and with further reference to FIG. 1, if the satellite 30 is employed with the system 10, then each platform 12 and 16 can continuously report its position to other platforms operating in a given region via the satellite's 30 transponder. This would be assuming that the satellite is a geosynchronous satellite whose location is constant relative to a position on Earth. Other means of establishing a common reference frame could involve having each transmitting platform continuously track the direction of its link partners using synchronous beam cloning, using a signal strength indication in combination with pointing a directional antenna, or other suitable means.

[0023] Referring further to FIG. 1, since the transmitting platform 12 knows its approximate position relative to the satellite 30, and since the transmitting platform is able to retrieve approximate position information on the position of the receiving platform 16 via the satellite 30, a unit vector 32 (r_TR) can be determined. Unit vector 32 represents the direction from the transmitting platform 12 to the receiving platform 16. Similarly, the receiving platform 16 determines a unit vector 34 that represents the direction from it to the transmitting platform 12. The unit vector 34 (r_RT) is also determined using position information of the position of the transmitting platform 12 that is obtained via the satellite 30. Receiving platform 16 obtains its own position information via any suitable means, such as its inertial navigation system or from satellite 30.

[0024] The processor 22 on the transmitting platform 12 determines the Doppler frequency shift (Δf_T) that a stationary receiver in direction r_TR would observe. The processor 22 adjusts the frequency at which transmitter 14 transmits, thus changing its emitted frequency from f₀ to (f₀−Δf_T). This cancels the Doppler shift due to the transmitting platform's 12 motion so the actual Doppler shift experienced by a stationary receiver in direction r_TR would be zero.

[0025] The receiving platform 16 performs similar steps. The processor 26 initially obtains a velocity of the receiving platform 16 (v_R) using the same reference frame as the transmitting platform 12 (i.e., the common reference frame). The receiving platform 16 also determines the unit vector r_RT 34 representing the direction from itself to the transmitting platform 12. The processor 26 computes the Doppler frequency shift (Δf_R) that the receiver would observe for a signal transmitted at frequency f₀ from a stationary transmitter along the unit vector 34 (i.e., r_RT). For electromagnetic wave signals, this frequency shift can be given by the equation:
Δf_r=f₀r_RT·v_r/c
where “v_r” is the velocity of the receiving platform.

[0026] The processor 26 then adjusts the frequency setting of receiver 18, changing its nominal frequency from f₀ to f₀+Δf_R. Given that the transmitting platform 12 has removed the frequency shift component (i.e., Δf_T) due to the transmitting platform's 12 motion, the frequency (f_R) of the signal arriving at the receiving platform 16 can be given by the equation:

[0027] $\begin{array}{c}{f}_R=f_0+\Delta f_r+\Delta f_T-\Delta f_T\\ =f_0+\Delta f_R.\end{array}$

[0028] The term f_R thus represents exactly the frequency at which the receiving platform's receiver 18 expects to receive the electromagnetic wave signal 20. This substantially eliminates the problems that arise from frequency mismatch due to the Doppler shift of the signal 20.

[0029] In practice, the remaining frequency mismatches after the above-described Doppler shift corrections are applied are typically about 1.5-2.0 orders of magnitude smaller than the Doppler mismatch remaining when using many previous developed correction systems. Many remaining frequency mismatches are largely due to measurement errors in speed and direction, and the time lag between these measurements. Additionally, errors in frequency adjustment (i.e. imprecise voltages used to adjust frequency, or imprecise voltage-to-frequency shift coefficients) also can affect the degree of frequency mismatch reduction achieved with the present invention. However, for equipment typically used on aircraft, these errors are typically on the order of one percent to a few percent that of the Doppler shifts encountered without the benefit of the present invention. Thus, the noise entering the receiver, the frequency scan time and the guard bandwidth can all be reduced significantly compared to what is needed with previous correction systems that do not achieve the significant degree of Doppler mismatch reduction that the present invention achieves.

[0030] With brief reference to FIGS. 3A and 3B, a simplified flowchart setting forth the basic steps of one preferred implementation of the present invention 10 will be described. This implementation assumes that both the transmitting platform 12 and the receiving platform 16 are moving relative to the common reference frame. At operation 50, transmitting platform 12 obtains a velocity vector (v_T) representing its speed, relative to the common reference frame. At operation 52, the transmitting platform 12 next obtains the unit vector (r_TR) for the direction from it to the receiving platform 16. At operation 54, the processor 22 determines, from the velocity vector (v_T) and the unit vector (r_TR), the Doppler shift (Δf_T) affecting the signal 20 being transmitted as a result of the motion of the transmitting platform 12. In operation 56, the processor 22 then controls the transmitter 14 in a manner so that the frequency of the signal 20 transmitted is adjusted as needed to cancel out the Doppler shift (Δf_T) that will be experienced by the receiving platform 16. These operations are periodically repeated to adjust for changes in v_T and r_TR. The update rate can be as low as 10 ms for fast aircraft or missiles for which the transmitting platform 12 is relatively close to the receiving platform 16 (i.e., quick dynamic shifts in Doppler). The update rate can be several seconds or longer for submarines with slow maneuvers while receiving ultra low frequency signals (slow dynamic shifts in Doppler). The update rate will depend on how fast the Doppler shift changes. Doppler shift dynamics depend on 1) how fast the relative velocity changes between the receiver and the transmitter and 2) the relative difference in platform velocity with respect to the signal velocity.

[0031] At operation 58, the receiving platform 16 obtains velocity vector (v_r) information relative to the common reference frame. At operation 60, the receiving platform 60 obtains unit vector (r_RT) for the direction from it to the transmitting platform 12. At operation 62, the receiving platform 16 determines the Doppler shift (Δf_T) affecting signal 20 as a result of its own motion, and assuming that transmitting platform 12 is stationary. Finally, at step 64, the processor 26 adjusts the receive frequency of receiver 18 to cancel out the Doppler shift that is attributable to the motion of the receiving platform 16. These operations are periodically repeated to adjust for changes in v_R and r_RT.

[0032] For a two-way communications link between platform 12 and platform 16, there would be a receiver and a transmitter on each platform. The receiver and transmitter on platform 12 will have a common platform velocity vector (v_T12=v_R12) and a common direction vector (r_T12R16=r_R12T16). The resulting transmitter and receiver Doppler frequency adjustments made on platform 12 will be equal and opposite (Δf_T12=−Δf_R12). Platform 16 would also have a receiver and a transmitter that will have a common platform velocity vector (v_T16=v_R16) and a common direction vector (r_T16R12=r_R16T12). The resulting transmitter and receiver Doppler frequency adjustments made on platform 16 will be equal and opposite (Δf_T16=−Δf_R16). Thus, each mobile platform would be performing a Doppler shift correction during both transmitting and receiving operations.

[0033] The formula for Doppler shift of sound waves varies depending on whether the source is moving, the observer is moving, or both. If the observer (receiver) is moving, the formula for frequency f′ that the receiver hears is:
f′=(v+v_observer)/v  Formula 1
where v is the speed of sound in the medium, v_observer is the speed of the observer (receiver) toward the source, and f is the frequency in the absence of any Doppler shift. Note that v_observer is negative if the observer (receiver) is moving away from the source. If the source (transmitter) is moving, the formula for frequency f′ that a stationary receiver hears is:
f′=fv/(v−v_{stationary receiver})  Formula 2

[0034] where v and f are the same as before and v_{stationary receiver} is the speed of the transmitter toward the receiver.

The two formulae combine when both transmitter and receiver are in motion:
f′=f(v+v_observer)/(v−v_{stationary receiver}).

[0035] For the present system and method, the receiver would use the Formula 1 above to correct the incoming frequency and the transmitter would use the Formula 2 to correct the outgoing frequency.

[0036] The present invention thus offers a means to reduce the waste of transmitter to receiver frequency synchronization time and bandwidth that is currently caused by Doppler shift in mobile communication systems. This waste is most significant in systems that use fast moving nodes like aircraft, missiles and spacecraft. The Doppler shift has less impact on slow moving systems such as automobiles, vans, trucks or land vehicles, and watercraft. With such slow moving vehicles, the signal velocity is very high compared to the speed of the vehicle as is typically the case when electromagnetic radio frequencies or light frequencies are used. Doppler shift can be significant for relatively slow moving nodes if the signal velocity is comparatively slow, as can be the case when acoustic signals are used.

[0037] While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.