Location of Wideband OFDM Transmitters with Limited Receiver Bandwidth

Abstract

One embodiment takes the form of a system for locating wireless transmitters employing an Orthogonal Frequency Division Multiplexing (OFDM) digital modulation scheme, which comprises transmitting signal components over narrowband frequency channels spanning a wideband channel. The system includes a first receiving system configured to receive a fraction of the signal components transmitted by a first wireless transmitter to be located in a fraction of the narrowband frequency channels, and to process the fraction of the signal components to derive location related measurements. The system further includes at least a second receiving system configured to receive the fraction of the signal components transmitted by the first wireless transmitter, and to process this fraction of the signal components to derive location related measurements.

1 . A location measuring unit (LMU) for use in a system for locating wireless transmitters employing an Orthogonal Frequency Division Multiplexing (OFDM) digital modulation scheme, wherein said OFDM scheme comprises transmitting signal components over a plurality of narrowband frequency sub-channels spanning a wideband channel having a bandwidth of approximately 20 MHz or greater, wherein a high-rate bit stream to be transmitted by a wireless transmitter using said OFDM scheme is split into multiple low-rate bit streams and transmitted in parallel over multiple sub-channels, wherein each low-rate bit stream is transmitted over a narrowband sub-channel by modulating a sub-carrier using a digital modulation scheme, comprising:
 a receiving system configured to receive a fraction of the signal components transmitted by a first wireless transmitter to be located in a fraction of the narrowband sub-channels; and   a processor configured to process said fraction of the signal components to derive location related measurements;   wherein the LMU is configured to cause the receiving system to receive signal components in a plurality of selected narrowband sub-channels, wherein the signal components in the selected narrowband sub-channels are received in parallel.   
     
     
         2 . An LMU as recited in  claim 1 , wherein said fraction of the narrowband frequency sub-channels includes at least one pilot channel in which said first wireless transmitter transmits energy, and wherein the LMU is configured to use signal components in said pilot channel to aid in signal acquisition and demodulation. 
     
     
         3 . An LMU as recited in  claim 2 , wherein the selected sub-channels are determined based upon interference levels. 
     
     
         4 . An LMU as recited in  claim 1 , wherein the receiving system comprises a first frequency conversion circuit configured to convert a first received RF signal component to a first digital baseband OFDM signal; a Fast Fourier Transform (FFT) circuit configured to perform an FFT of the first digital baseband OFDM signal provided by the first frequency conversion circuit; a demodulation circuit configured to produce coded bits based on the output of the FFT circuit; and a first reconstruction circuit configured to reconstruct the first digital baseband OFDM signal based on the coded bits produced by the demodulation circuit. 
     
     
         5 . An LMU as recited in  claim 1 , wherein said location related measurements comprise at least one of the following: time difference of arrival (TDOA) measurements, time of arrival (TOA) measurements, angle of arrival (AOA) measurements, round trip time measurements, and received power measurements. 
     
     
         6 . An LMU as recited in  claim 1 , wherein said fraction of the predefined narrowband frequency channels excludes guard channels in which said first wireless transmitter transmits minimal energy. 
     
     
         7 . An LMU as recited in  claim 1 , further comprising a radio frequency (RF) filter and an intermediate frequency (IF) filter, wherein the receiving system is configured to receive signal components within a bandwidth compatible with said RF and IF filters. 
     
     
         8 . An LMU as recited in  claim 1 , further comprising an analog to digital converter (ADC) characterized by a sample rate, wherein the receiving system is configured to receive signal components within a bandwidth compatible with said sample rate. 
     
     
         9 . An LMU as recited in  claim 8 , wherein said ADC is further characterized by a sample rate after decimation, wherein the receiving system is configured to receive signal components within a bandwidth compatible with said sample rate after decimation. 
     
     
         10 . An LMU as recited in  claim 1 , further comprising memory for storing data representing received signal components, wherein said receiving system is configured to receive signal components within a bandwidth compatible with said available memory. 
     
     
         11 . An LMU as recited in  claim 1 , further comprising a digital signal processor (DSP) characterized by DSP processing throughput, wherein said receiving system is configured to receive signal components within a bandwidth compatible with said DSP processing throughput. 
     
     
         12 . An LMU as recited in  claim 1 , further comprising means for communicating said location related measurements to a processing system for use in computing the location of said first wireless transmitter. 
     
     
         13 . An LMU as recited in  claim 1 , wherein each of the plurality of narrowband subchannels has a bandwidth that is substantially less than 20 MHz. 
     
     
         14 . An LMU as recited in  claim 13 , wherein each of the plurality of narrowband subchannels has a bandwidth that is less than approximately 5 MHz. 
     
     
         15 . An LMU as recited in  claim 13 , wherein each of the plurality of narrowband subchannels has a bandwidth in the range of approximately 3-5 MHz. 
     
     
         16 . A method for use in a system for locating wireless transmitters employing an Orthogonal Frequency Division Multiplexing (OFDM) digital modulation scheme, wherein said OFDM scheme comprises transmitting signal components over a plurality of narrowband sub-channels spanning a wideband channel having a bandwidth of approximately 20 MHz or greater, wherein a high-rate bit stream to be transmitted by a wireless transmitter using said OFDM scheme is split into multiple low-rate bit streams and transmitted in parallel over multiple sub-channels, wherein each low-rate bit stream is transmitted over a narrowband sub-channel by modulating a sub-carrier using a digital modulation scheme, comprising:
 receiving a fraction of the signal components transmitted by a first wireless transmitter to be located in a fraction of the narrowband sub-channels, wherein the receiving includes receiving signal components in a plurality of selected narrowband sub-channels, wherein the signal components in the selected narrowband sub-channels are received in parallel; and   processing said fraction of the signal components to derive location related measurements.   
     
     
         17 . A method as recited in  claim 16 , wherein said fraction of the narrowband sub-channels includes at least one pilot channel in which said first wireless transmitter transmits energy. 
     
     
         18 . A method as recited in  claim 17 , further comprising using signal components in said pilot channel to aid in signal acquisition and demodulation. 
     
     
         19 . A method as recited in  claim 16 , further comprising converting a received RF signal component to a digital baseband OFDM signal; performing a Fast Fourier Transform (FFT) of the digital baseband OFDM signal; producing coded bits based on the output of the FFT; and reconstructing the digital baseband OFDM signal based on the coded bits. 
     
     
         20 . A method as recited in  claim 16 , wherein said location related measurements comprise at least one of the following: time difference of arrival (TDOA) measurements, time of arrival (TOA) measurements, angle of arrival (AOA) measurements, round trip time measurements, and received power measurements. 
     
     
         21 . A method as recited in  claim 16 , wherein said fraction of the predefined narrowband sub-channels excludes guard channels in which said first wireless transmitter transmits minimal energy. 
     
     
         22 . A method as recited in  claim 16 , wherein each of the plurality of narrowband subchannels has a bandwidth that is substantially less than 20 MHz. 
     
     
         23 . A method as recited in  claim 22 , wherein each of the plurality of narrowband subchannels has a bandwidth that is less than approximately 5 MHz. 
     
     
         24 . A method as recited in  claim 22 , wherein each of the plurality of narrowband subchannels has a bandwidth in the range of approximately 3-5 MHz. 
     
     
         25 . A method as recited in  claim 16 , further comprising communicating said location related measurements to a processing system for use in computing the location of said first wireless transmitter.