Characterizing multipath delays in antenna array and synthetic aperture radar systems
Multiple radio transmissions are processed to determine, for each of a number of directions of arrival of the radio transmissions, a most direct direction of arrival, for example, to distinguish a direct path from a reflected path from the target. In some examples, the radio transmissions include multiple frequency components, and channel characteristics at different frequencies are compared to determine the direct path.
1. A method comprising:
receiving a plurality of radio transmissions from a target device, each radio transmission of the plurality of radio transmissions being received at a plurality of antennas at known locations, wherein each radio transmission comprises a plurality of components at different transmission frequencies;
processing the received radio transmissions to determine, for each of a plurality of directions of arrival of the radio transmissions, a characterization of a channel from the target device, the characterization of the channel including characterizations of the channel at the different transmission frequencies of the components of the radio transmissions, wherein at least one of the directions of arrival corresponds to a reflected path from the target device to the locations of the antenna; and
determining a most direct direction of arrival from the plurality of directions of arrival, including computing for each direction of arrival a quantity representing a path length from the target using the characterization of the channel for the direction of arrival at the different transmission frequencies, and comparing said computed quantities to determine the most direct direction;
wherein receiving and processing the plurality of radio transmissions from a target device at the plurality of antennas at known locations includes applying a Synthetic Aperture Radar (SAR) approach.
2. The method of claim 1 wherein computing the quantity representing a path length comprises computing a quantity representing a relative path length relative to one or more other directions of arrival.
3. The method of claim 2 wherein comparing said computed quantities to determine the most direct direction comprises selecting the direction of arrival with the smallest relative path length of the plurality of directions of arrival.
4. The method of claim 1 further comprising determining the plurality of directions of arrival by processing the received radio transmissions and identifying directions of arrival with local maximum energy over a range of directions of arrival.
5. The method of claim 1 wherein the radio transmissions comprise frequency division multiplexed encodings of transmission data.
6. The method of claim 5 wherein the radio transmissions comprise Orthogonal Frequency Division Multiplexed (OFDM) encodings of transmission data.
7. The method of claim 5 wherein processing the received radio transmissions, for each of the plurality of directions of arrival, includes determining a phase response of a channel from the target device to a location of the plurality of antennas at each of the different transmission frequencies.
8. The method of claim 7 wherein determining the quantity representing the path length includes determining a quantity characterizing a relationship between the phase response and the frequency at each of the different transmission frequencies.
9. The method of claim 8 wherein the quantity characterizing a relationship between the phase response and the frequency comprises a quantity characterizing rate of change of phase response as a function of frequency.
10. The method of claim 1 wherein receiving the plurality of radio transmissions from a target device at the plurality of antennas at known locations includes, for each radio transmission receiving said radio transmission at an array of fixed antennas comprising a first antenna and a plurality of other antennas all at fixed locations.
11. The method of claim 10 wherein processing the received radio transmissions to determine the characterization of the channel from the target device for each of the plurality of directions of arrival includes combining channel estimates from the target each antenna of the array of fixed antennas.
12. The method of claim 1 wherein receiving the plurality of radio transmissions from a target device at the plurality of antennas at known locations includes, for each radio transmission receiving said radio transmission at a first antenna at a fixed location, and at a second moving antenna.
13. The method of claim 11 wherein processing the received radio transmissions to determine the characterization of the channel from the target device for each of the plurality of directions of arrival includes combining channel estimates from the target to the first antenna and to the second moving antenna at each of the different transmission frequencies.
14. The method of claim 1 wherein the target device comprises at least one of a wide area cellular communication device and a wireless local area network device.
15. A localization system comprising:
a plurality of antenna;
a receiver coupled to the plurality of antennas for receiving radio signals acquired at the plurality of antennas, the radio signals including a plurality of radio transmissions from a target radio device;
a signal processor coupled to the receiver configured to process the plurality radio transmissions from the target radio device to determine a most direct direction of arrival, wherein each radio transmission comprises a plurality of components at different transmission frequencies, the signal processor being configured to
processing the received radio transmissions to determine, for each of a plurality of directions of arrival of the radio transmissions, a characterization of a channel from the target device, the characterization of the channel including characterizations of the channel at the different transmission frequencies of the components of the radio transmissions, wherein at least one of the directions of arrival corresponds to a reflected path from the target device to the locations of the antenna, and
determining a most direct direction of arrival from the plurality of directions of arrival, including computing for each direction of arrival a quantity representing a path length from the target using the characterization of the channel for the direction of arrival at the different transmission frequencies, and comparing said computed quantities to determine the most direct direction;
wherein receiving and processing the plurality of radio transmissions from the target device at the plurality of antennas at known locations includes applying a Synthetic Aperture Radar (SAR) approach.
16. A localization system configured to perform all the steps of claim 1 .
17. Software, embodied on a non-transitory machine-readable medium, comprising instructions for causing a processor of a localization system to perform all the steps of claim 1 .