Patent Application
20090023462
The present invention relates generally to determining a location of a mobile station within a wireless network, and more particularly to determining the mobile station's location relative to a position of an antenna beam.
Conventional satellite systems may track non-geostationary satellites from a ground station using a reception antenna beam that traverses a conical scan pattern in a sequential order. Conical scan patterns comprise angular offset directions, e.g., 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. As the ground station transmits a signal, the satellite moves the reception beam in the sequential order around the conical scan pattern according to the offset directions and measures the strength of the signal received at each offset direction. The variations in the measured signal strength for each offset direction have a phase and amplitude relationship to the location of the ground station relative to a nominal center of the reception beam, where the nominal center is equivalent to the center of the scan pattern. Thus, appropriately processing the signal strength measurements yields the direction and amount of offset of the beam center from the true ground station location.
Scanning antenna systems originally operated by mechanically rotating some part of the antenna, such as the feed horn at the focus of a parabolic reflector. Mechanically rotating systems typically require a smooth scanning motion. Digital beamforming provides an alternate solution that generates a scanning beam using a beamforming computer local to the antenna array or remote from the antenna array, as described in the following US patents to Applicant, which are hereby incorporated by reference herein:
Conventional location systems work well for geostationary satellite systems communicating with stationary transmitter devices, e.g., ground stations, because in these cases the signal strength variations mostly result from moving the reception beam according to the conical scan pattern. However, when the satellite communicates with a mobile device, the signal may experience signal strength variations due to other factors, such as distortion caused by slow fading. Such distortion-based signal strength variations degrade the accuracy of the location determination process. Further, the above-discussed conventional systems only allow for the determination of the transmitter location. In some cases, it may be desirable to determine a receiver location. Thus, there remains a need for alternative location determination techniques.
U.S. Pat. No. 6,684,071 to Applicant et al., entitled “Terminal position location using multiple beams” and incorporated herein by reference, provides one alternative technique for finding a location of a receiver. The 071 patent measures the relative signal strengths of signals transmitted in neighboring antenna beams with different centers but overlapping coverage, where the overlapping beams use different communication channels to avoid mutual interference. The receiver cycles around the different communication channels to receive the signals and generate the corresponding signal strength measurements. In some systems, however, the ability to create multiple neighboring beams on different frequencies may be limited by power or spectrum availability. One alternative uses a given amount of power and spectrum to sequentially create one beam at a time. In this case, the strength of the signal at the receiver is measured at different times. As a result, the relative signal strength measurements may be corrupted by fading in the intervening periods. Thus, there remains a need for alternative location determination techniques that are less sensitive to corruption by fading effects.
The present invention determines the location of a mobile station in a wireless network using signals transmitted or received in antenna beams that traverse a scan pattern having a plurality of beam offset directions, referred to herein as beam positions. According to one embodiment, a network device transmits a signal to a mobile station as a position of a transmission antenna beam traverses a scan pattern in a non-sequential order. The mobile station measures a strength of the received signal at a plurality of the beam positions. After processing, which exploits spectral spreading of noise components of the received signal, the location of the mobile station relative to a nominal center of the transmission antenna beam is determined based on the signal strength measurements.
In one exemplary embodiment, a network device receives a signal from a mobile station as a position of a reception antenna beam traverses a scan pattern in a non-sequential order. The network device measures the strength of the received signal at a plurality of the beam positions and spreads a noise component of the signal strength measurements by, for example, reordering the signal strength measurements. After spreading the noise component, the network device determines the location of the mobile station relative to a nominal center of the reception antenna beam based on the reordered signal strength measurements.
Another embodiment uses multi-frequency signals or a signal with multiple subcarriers, (e.g., Orthogonal Frequency Division Multiplexing (OFDM) signals) to determine the location of the mobile station. In this embodiment, a network device generates first and second signals associated with first and second frequencies, respectively. The network device transmits the first and second signals to the mobile station as respective first and second transmission antenna beams execute first and second scan patterns. The first and second scan patterns may comprise, for example, a common scan pattern executed according to first and second orders or at different scanning frequencies. The mobile station measures a strength of the received first and second signals at a plurality of the beam positions of the respective scan patterns. Subsequently, a series of first and second signal strength measurements are jointly processed to determine a combined correlation with the respective scan patterns. The location of the mobile station is determined relative to a nominal center of the transmission antenna beam based on the combined correlation.
According to another exemplary multi-frequency embodiment, the network device receives the first and second signals from the mobile station as respective first and second reception antenna beams execute respective first and second scan patterns, which may comprise a common scan pattern executed in respective first and second orders or at different scanning frequencies. The network device measures a strength of the received first and second signals at a plurality of the beam positions of the respective scan patterns. Subsequently, the network device jointly processes the first and second signal strength measurements to determine a combined correlation with the respective scan patterns. The location of the mobile station relative to a nominal center of the reception antenna beam is then determined based on the combined correlation.
The present invention uses phase and amplitude information derived from a signal associated with an antenna beam traversing a predetermined scan pattern to determine the location of a mobile station relative to a nominal center of the beam, where the nominal center of the beam generally corresponds to the center of the scan pattern. The location determining process of the present invention may be implemented with any wireless network having a network device capable of generating an electronically steerable transmission and/or reception beam 130.
When the network device 110 comprises a satellite, an antenna array comprising a plurality of antenna elements transmits signals to and receive signals from the mobile station 120. The transmitted signal contains repetitive features that can be detected by a receiver, such as a frame with a frame repetition period, timeslots within the frame, and symbols within the time slot. Alternatively, the signal may comprise non-repetitive features, such as a long CDMA spreading code. It will be appreciated that the repetitive and non-repetitive features are synchronized between the network device 110 and the mobile station 120 so that both have a common agreement on “system time” that may be used to time events, such as the instant that a beam 130 moves from a first position to a second position.
The antenna array may comprise any electronically steerable antenna array, e.g., a directly-radiating phased array, a feed array disposed near the focus of a parabolic reflector, etc. The path from the ith antenna element to the jth mobile station 120 may be described by a matrix of complex coefficients Cji, while the path from the jth mobile terminal 120 to the ith antenna element may be described by a matrix of complex coefficients Cij. Thus, a matrix of complex coefficients may be used to define a pointing direction and other characteristics of a beam 130 communicating with the mobile station 120. These matrices are not necessarily reciprocal, as the forward and reverse link channel frequencies may be different.
The antenna array electronically steers the antenna beam 130 to a desired position based on the matrix of complex coefficients. The antenna array may also steer the beam 130 to a plurality of positions associated with a scan pattern, e.g., 0°, 90°,180°, and 270° in the case of a conical scan pattern. In this case, a cyclic set or ring of complex coefficient matrices is generated, where each complex coefficient matrix in the ring corresponds to one beam position in the scan pattern.
The beamforming system 140 may also be used to generate reception antenna beams 130, as shown by
In conventional systems, beam forming system 140 generates a beam 130 that executes a conical scan pattern 150 in a sequential order. This is because, traditionally, the scan pattern was implemented by mechanically wobbling the antenna, which generally required a smooth wobble, such as that created by a constant speed rotation.
When the transmission power is constant, scanning the beam 130 in this manner creates predictable signal strength variations relative to a nominal center of the beam 130. However, signals transmitted or received via antenna beam 130 may experience signal strength variations due to a number of causes other than beam scanning. For example, the measured signal strength may vary due to power control variations at the transmitter or due to noise, such as distortion in the form of slow fading. Such non-beam scanning signal strength variations change the strength of a received signal, which in turn may affect the accuracy of the determined location of the mobile station 120.
The power control variations may be eliminated by fixing the transmission power for a particular frame and/or set of time slots associated with the scan pattern. For example, the beam may be wobbled through its sequence of scanning directions during a period of time having a fixed transmission power, such as during the timeslots of a TDMA frame or during a sequence of TDMA frames over which the power control is held fixed. However, such power control does not address the signal strength variations due to slow fading or other channel noise. To address this problem, one embodiment of the present invention uses a non-sequential traversal of a conical scan pattern 150 to reduce the effect of slow fading of the received signal, and therefore, to reduce the effects of fading, which is a form of multiplicative noise, on the process of determining the location of the mobile station 120.
X=ax
Y=by (1)
In Equation (1), a and b represent constants of proportionality, x represents the cosine component of the measured signal strength variation at the conical scan frequency, and y represents the sine component of the measured signal strength variation at the conical scan frequency. The proportionality constants may be determined by calibration, e.g., by performing measurements with mobile stations or fixed calibration receivers at known locations. When the scan pattern comprises a circular scan pattern, e.g., a conical scan pattern, a and b are the same. However, when the scan pattern is non-circular, e.g., when the scan pattern has an elliptical cross-section, a and b are different. In this case, X aligns with the major axis of the ellipse and Y aligns with the minor axis when a>b, and Y aligns with the major axis of the ellipse and X aligns with the minor axis when a<b. It will be appreciated that the location of the mobile station 120 may also be represented by polar coordinates having an amplitude and phase, where the amplitude directly relates to the distance between the mobile station 120 and the nominal center, and the phase directly relates to the direction or bearing of the mobile station 120 relative to the nominal center. Further, it will be appreciated that the location of a mobile station 120 relative to a nominal center of a terrestrial antenna beam 130 may also be determined based on Equation (1).
One exemplary location processor 160 comprises a measurement unit 162, noise unit 164, and location unit 166, as shown in
In one embodiment, the location unit 166 determines the location of the mobile station 120 by correlating the reordered signal strength measurements with a reference signal. The reference signal, for example, may comprise a sequence of complex samples at angular positions along a complex sinusoid ejkT. The complex samples represent the known beam displacement at regular intervals kT, where k=1, 2, 3, etc., and T represents the time interval for which the beam 130 dwells at a particular position, e.g., during a TDMA time slot or TDMA frame period. The correlation provides signal strength variations having an amplitude and phase component. Location unit 166 uses the amplitude and phase components of the signal strength variations to determine the location of the mobile station 120, where the amplitude yields a distance of the mobile station 120 from the nominal center of the beam 130 and the phase yields an angular direction relative to the nominal center of the beam 130. When the reference signal comprises a narrow-band sine wave complex sinusoid corresponding to a conical scan pattern, the correlation may be obtained by performing a Fourier transform of the reordered signal strength measurements. The fundamental component of the result is then extracted to yield the amplitude and phase indicative of the mobile station's location relative to a nominal center of the beam 130.
Consider the following example, where network device 110 transmits a waveform conforming to the GMR2 standard, which is a derivative of the GSM cellular telephone standard. The GMR2 standard uses 120 ms multiframes, each multiframe comprising 13 TDMA frames, and each TDMA frame comprising 16 timeslots. Frames 1-12 carry traffic to designated mobile stations 120, while frame 13 carries the Slow Associated Control Channel (SACCH) data. In one mode, the 12×16-slot traffic frames may be formatted as 6, 32-slot traffic frames. One mobile station 120 normally listens to one of these 32 slots per frame in each of the 6 frames, and one of the 16 slots in the SACCH frame. At other times, the mobile station 120 may also listen to slots on different frequencies that may be in use in neighboring beams in order to make neighboring beam measurements.
Table 1 shows how a non-sequential order for traversing the conical scan pattern 150 shown in
In Table 1, the frame number refers to the 32-slot frames. The beam position refers to the direction of the offset relative to a nominal center position of the beam 130, and should not be confused with the amount of beam offset. For example, a beam position of 180° refers to a southerly offset relative to the nominal center. The beam offset amount is generally equal for all beam positions, and should be as large as possible consistent with the signal loss due to the pointing error being small enough to be considered negligible, e.g., less than 1 dB. To avoid this signal loss, it may be desirable to implement beam scanning only when determining the location of the mobile station 120. Two SACCH messages occupy one slot in each of eight sequential SACCH frames. Each SACCH message period is 4×120 ms=480 ms, and the pair occupies 960 ms. Thus, a conical scan pattern 150 associated with the SACCH may have 8 beam positions (e.g., 0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°). Table 1 also shows one exemplary non-sequential traversal order for the 8 slots of two SACCH messages. SACCH messages are transmitted at a constant power, which is typically higher than the average power of a power-controlled traffic slot. Therefore, power control-induced variations are not an issue for the SACCH signal. Similarly, traffic slots destined for a particular mobile station 120 are transmitted at a constant power in the 6 (or 12) frames between two SACCH frames, and therefore, power-control induced variations are not an issue if cycling through the conical scan pattern is time-aligned with these slots.
The pattern of Table 1, when reordered as frame number 1, 3, 5, 2, 4, 6 produces a regular pattern of signal strength measurements at 0°, 60°, 120°, 180°, 240°, and 300°, which represent samples of a regular, repetitive sinusoidal cycle. Thus, when reordered, the signal strength variations represented by the signal strength measurements exhibit a sinusoidal characteristic. Likewise, the SACCH pattern when reordered becomes 0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°, which gives a sinusoidal representation of the signal strength variations over two SACCH message periods of 480 ms each, or 1 s in total. While reordering the signal strength measurements has the effect of unscrambling the signal strength variations caused by the non-sequential beam scanning process, reordering randomizes the signal strength variations due to other causes, such as multiplicative noise due to slow fading, which spreads the spectrum of the noise.
The beam positions in Table 1 may be produced by the beamforming system 140 shown in
The network device 110 may alternatively determine the location of the mobile station 120. In one embodiment, the mobile station 120 may transmit the signal strength measurements to the network device 110, either before reordering or after reordering. A location processor 160 in the network device 110 may then reorder the received signal strength measurements (if necessary) and determine the location of the mobile station 120 based on correlations between the reordered signal strength measurements and the scan pattern.
In another embodiment, the network device 110 may include a location processor 160 that determines the location of the mobile station 120 based on a signal received from the mobile station 120. In this embodiment, beamforming system 140 generates a reception antenna beam 130 that traverses the conical scan pattern 150 in the non-sequential order as the mobile station 120 transmits the signal.
The network device 110 generates multiple signals, each having different frequencies. The mobile station 120 receives the signals via respective transmission beams 130 as the position of each of the different transmission beams 130 traverses different conical scan patterns 150 (block 210). In some embodiments, the different conical scan patterns 150 may be derived from a common conical scan pattern 150 executed in different scanning orders (block 210). The mobile station 120 measures the strength of the received signals at two or more of the beam positions for each frequency component (block 220). The mobile station 120 then jointly processes two or more signal strength measurements made on the two or more frequencies to determine a combined correlation (block 250), and determines the location of the mobile station 120 based on the combined correlation.
In a first embodiment, the mobile station 120 jointly processes the signal strength measurements by correlating the signal strength measurements on each frequency with the scan pattern 150 for that frequency, and then adding the correlations obtained for the different frequencies. For example, if the scan pattern 150 for one frequency is a time-order-scrambled conical scan pattern 150, the measurements for that frequency are first re-ordered in sequential order and then Fourier-analyzed to determine the complex value of the fundamental component at that frequency. Fundamental components obtained in this way from the signal strengths measured on each frequency are then complex added to produce a combined correlation, the amplitude and phase of which respectively represent the distance and bearing of the mobile station 120 from the nominal beam center. This embodiment assumes that the nominal beam center is chosen to be the same for each scan pattern 150.
In some implementations, a weighting factor may first be applied to some or all of the fundamental components to account for the possibility that one frequency may yield a more reliable result than another. For example, the DC term from a Fourier analysis represents the mean signal strength for a given frequency. If the mean signal strength is lower for one frequency than for another, the frequency having the lower mean signal strength may be given a proportionally lower weighting factor. Alternatively, Fourier components other than the fundamental component may be used to weight the complex addition appropriately so as to give more weight to less noisy signal strength measurements.
In a second embodiment, different frequency components traverse a common conical scan pattern 150 in different orders. To determine its location, the mobile station 120 jointly processes the signal strength measurements by combining the signal strength measurements having corresponding beam positions. For example, the beam 130 may be “due North” of the nominal beam center at different times for different frequencies. Combining the signal strength measurements for the different frequencies associated with the due North position produces a net signal strength for the due North position. After obtaining net signal strengths in this way for two or more beam positions, the net signal strengths are correlated with the common conical scan pattern 150, for example by Fourier analysis, to obtain a combined correlation with the scan pattern. The amplitude and phase of the combined correlation correspond to distance and bearing of the mobile station 120 from the nominal beam center. It will be appreciated that any signal strength measurements associated with non-sequential scanning orders may be reordered before being combined with other signal strength measurements to spread the multiplicative noise.
A third embodiment may be used when the nominal beam center differs for at least some frequency components. In this case, a location of the mobile station 120 is found by determining the location from the signal strength measurements for each frequency separately and then averaging the determined locations. For example, first and second preliminary locations may be determined based on the signal strength measurements for signals having first and second frequency components using any method described above. The final mobile station location may be determined by averaging the first and second preliminary locations.
It will be appreciated that in each of the three embodiments described above, the location of the mobile station 120 and optionally velocity and higher order derivatives of location may be averaged over several measurements by Kalman filtering, as described in the above-incorporated '071 patent.
The advantage of multi-frequency location determination is most easily understood from the method described under the above-described second embodiment. However, it will be appreciated that all of the above-described embodiments are mathematically equivalent in terms of this advantage. If the times at which the beam 130 is at a given position are different for each frequency component, when the measurements are combined for the same beam position, the net combined signal strength resulting for that beam position will be the sum of signal strengths made over the entire period. Thus, any variation of signal strength over that period will be averaged to the same value for each beam position, at least with the assumption that the signal strength variations due to slow fading are the same for each frequency component. Thus, the effect of non-frequency-selective slow fading is often eliminated completely. If the fading is not the same on each frequency but is uncorrelated, the advantage will be a reduction of the fading error by a factor of the square root of the number of frequency components and by a factor related to the spectral spreading of the multiplicative fading noise when time-order scrambled conical scanning is employed.
Thus, in general, each beam 130 traverses the different scanning patterns 150. The different scanning patterns 150 may comprise any mutually orthogonal scanning patterns 150 that prevent the different beams from being in the same position at the same time. Alternatively or in addition, the different scanning patterns 150 may be generated by executing a common scanning pattern 150 in different scanning orders, with different phase or time offsets, and/or at different scanning frequencies. In one embodiment, shown in
In one embodiment, the mobile station 120 receives the plurality of signals as the beams traverse the conical scan 150. The measurement unit 162 of the location processor 160 makes signal strength measurements of the received signal at two or more of the beam positions, the noise unit 164 reduces the noise, such as noise due to slow fading, in the signals, and the location unit 166 determines the mobile station location based on the noise reduced signal strength measurements. The noise unit 164 may reduce the noise by jointly processing the signal strength measurements corresponding to different frequency components to determine a combined correlation as discussed above, and the location unit 166 may determine the location of the mobile station 120 based on the combined correlation. For example, the location unit 166 may determine the location based on the amplitude and the phase of the combined correlation. The amplitude indicates the distance of between mobile station 120 and the nominal center of beam 130, and the phase indicates the direction of mobile station 120 relative to the nominal center.
To illustrate, consider the following example. Table 2 shows how the mobile station 120 receives each signal in different transmission periods based on the scanning orders shown in
When the fading noise affects all frequencies equally, combining the signal strength measurements corresponding to a particular beam position for each of the beam positions averages out the fading noise, and therefore, improves the accuracy of any subsequent location calculations. In one embodiment, the signal strength measurements may be combined by first extracting the fundamental component of the cyclic variation by, for example, Fourier analysis, and then adding the result for each frequency.
In some cases, it may not be convenient or possible to construct a receiver capable of simultaneously making signal strength measurements on several different frequency channels. This may be addressed by setting the different frequencies to different spectral components of a signal within a single channel. In this scenario, a single channel receiver will suffice for capturing the different signals. An Orthogonal Frequency Division Multiplexing (OFDM) signal represents one exemplary signal that automatically comprises multiple sub-carriers. Thus, an implementation of the above-described multi-frequency process constructs an OFDM signal within a single radio frequency channel. Subjecting the spectral components of the OFDM signal to different beamforming matrices, and changing the matrices in a scanning order that differs in phase, frequency, and/or pattern, as described above, creates a different beam for each of the sub-carriers. This enables the mobile station 120 to measure the signal strength variation of each OFDM sub-carrier. Combining the signal strength measurements of the corresponding sub-carriers of each beam position removes noise or inaccuracies due to flat fading.
In the case of an OFDM communications signal, different frequency components are naturally present in the form of distinct sub-carriers. In non-OFDM cases, the signal may not have specific distinct frequency components, and a conventional receiver for such signals may not normally be constructed to make simultaneous signal strength measurements of several different frequency components. Nevertheless, by choosing, as the different frequency components, the different spectral components of a signal within a single channel, and arranging that the antenna beam directions are wobbled differently for each spectral component, a receiver may be constructed to provide the same advantage for location determination as in the OFDM case. In this scenario, a single channel receiver will suffice for capturing the different spectral components of the signals.
Weighted OFDM sub-carriers targeted to be transmitted by the same antenna element 149 are collected by a multiplexing element 147 in the multiplexer system 146. Multiplexing element 147 may comprise, for example, a plurality of FFT elements, e.g., FFT processors, DFT processors, windowed FFT processors, or any other processor that translates each weighted OFDM spectral component to its respective sub-carrier frequency and combines the signals at all sub-carrier frequencies for a given antenna element 149 to form the OFDM signal for that antenna element 149. This can include up-shaping and down-shaping using cyclic pre-fixes or post-fixes to obtain a desirable spectrum, as is known in the art of pulse-shaped OFDM. By choosing the weighting elements 145 appropriately, each OFDM sub-carrier is beamformed to a different desired beam position according to the conical scan pattern and the corresponding scanning order. The scanning order for each OFDM sub-carrier is preferably different, as discussed above, so that each transmission period is equally represented across the different OFDM sub-carriers. This provides combined signal strengths that are immune to slow fading.
The beamforming system 140 of
An OFDM signal constructed as disclosed above may only need to be transmitted when it is desired to determine the location of a mobile station 120. Reciprocally, the mobile station 120 may transmit an OFDM signal containing a set of distinguishable sub-carriers, and the network device 110 may receive each sub-carrier using a different receiving beam traversing a conical scan pattern 150 in a different scanning order. For example, by reversing the direction of the arrows in
When using a multi-frequency signal, such as an OFDM signal, that contains distinguishable sub-carriers, the antenna directionality for either transmission or reception beams may be varied in a series of time steps that are different for each sub-carrier. Receiver measurements for each sub-carrier and time step may then be collected in memory to form a two-dimensional array of samples. A two-dimensional Fourier transform may be performed on the array of samples. The pattern of antenna directionality variation may be chosen so that its two-dimensional Fourier transform has a single non-zero component. The order of the frequency and time elements of the pattern are preferably order-scrambled in the time dimension, the frequency dimension, or both, and then the signal strength measurements are stored in the two-dimensional array in an unscrambled order. This has the effect of scrambling the order in which errors due to signal variations in time or frequency due to other causes are located in the array. Such errors are then distributed between all Fourier components after the two-dimensional Fourier transform, such that the error is reduced on the component that yields the desired information.
The above described satellite antenna arrays may utilize beamforming signals created in the satellite or at a ground station associated with the satellite. The above-incorporated U.S. patents describe the operation of exemplary antenna arrays located on an orbiting satellite, where the signals from each antenna element may be brought down to the ground station coherently or communicated from the ground station to the satellite-borne antenna array coherently.
While the above describes the invention in terms of satellite antenna beams that traverse a conical scan pattern, it will be appreciated that a terrestrial network device, such as a radio base station, may also be used to generate the electronically steerable antenna beam 130. In this scenario, pairs of vertical collinear antenna arrays may be used to steer the antenna beam 130 in the desired manner. For example, pairs of vertical collinear arrays closely spaced horizontally and connected to phased transmitters and/or diversity receivers may be used to control the azimuthal directivity of a transmission or reception beam. In addition, further pairs with a somewhat larger vertical spacing may be used to control elevation directivity, which translates to moving the beam center along a radial line away from the antenna tower. Thus, such an antenna arrangement may be used to produce a steerable antenna beam 130 for a terrestrial network device 110.
The above provides a method and apparatus for determining a location of a mobile transmitter or a mobile receiver (mobile station 120) using signals that traverse a conical scan pattern in a known order. Such location information may be used to improve communications that require accurate location information.
The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
