3.3.4.1 Site investigation
To implement source localisation, the site investigation is an important step, which can make sure the SDRs can receive the signal source with reasonable SNR. It can be implemented by using one SDR to receive the signal of interest, followed by the time-frequency analysis of the received signal. Figure 3.21 shows the received signal in the time domain and the frequency domain. In the time domain, the waveform of the received signal can be observed. More importantly, from the signal spectrum in the frequency domain, the signal SNR can be obtained. If on a site where the SNR
0 1 2 3 4 5 6 7 x 10−3 −1 0 1 Time, s Normalized amplitude
The signal in the time domain
Time, s
Frequency, Hz
Amplitude spectrogram of the signal
1 2 3 4 5 6 7 x 10−3 0 2 4 6 8 10 12 x 106 Magnitude, dB −120 −100 −80 −60 −40 −20 0 2 4 6 8 10 12 x 106 −200 −150 −100 −50 Frequency, Hz Magnitude, dB Amplitude spectrum of the signal
Figure 3.21: The time-frequency of the received signal
of the received signal is low or the source signal is submerged into the noise floor, it is very likely that the TDOA measurements will be very noisy or even wrong. In this situation, the site to take measurements needs to be changed. In addition, the bandwidth of the signal source can also be observed from the signal spectrum analysis so that we can check whether the parameters of the low pass filter are set properly.
Here are some examples to show why the site investigation is a must step before taking measurements and implementing source localisation. For the FM signal at 106.3 MHz, we found it can be received with reasonable SNR at all the investigated sites. However, when we want to receive the FM signal at 88 MHz from a different transmitter, we found that it can only be received in some regions that are relatively close to the signal emitter because the transmission power of that radio station is low. Even in the areas where the signal can be received properly, we found that the radio station of FM 88 MHz only works at certain hours during the day, while the radio station of 106.3 MHz works the whole day. For the reception of TV signal, at one site, signal from 5 TV stations can be received, their carrier frequency are 177.5 MHz, 184.5 MHz, 191.5 MHz, 219.5 MHz and 226.5 MHz. The first three TV signal have better signal reception quality than the latter two. However, at another site, only the last two TV signals at 219.5 MHz and 226.5 MHz can be detected. Therefore, we have to use the signal from one of the last two TV stations to implement the source localisation.
3.3.4.2 Signal acquisition and localisation
The signal processing diagram of one SDR-based receiver is shown in Figure 3.22, the UHD Source block manages the interaction between a USRP and its host PC. After
UHD Source Low Pass
Filter File Sink
USRP
Host PC
Figure 3.22: Processing diagram of SDR receiver
−1 −0.5 0 0.5 1 x 107 10−4 10−2 100 102 104
The spectrum of received signal by one SDR using 25 MHz sampling rate
Frequency (Hz) Amplitude (dB) −1 −0.5 0 0.5 1 x 105 10−2 10−1 100 101 102 Frequency (Hz) Amplitude (dB)
Figure 3.23: The use of LPF to extract signal of interest
being processed by the UHD Source block, the received baseband signal is passed to a Low Pass Filter (LPF), where the unwanted signal is attenuated. After the LPF, the received samples are passed to a File Sink block to be stored in the local host PC and uploaded to a central server.
The low pass filter plays an important role in obtaining accurate TDOA measure- ments. Since a high sampling rate is used to improve the time resolution of TDOA measurements, the unwanted interference signal can also be received under the sam- pling rate of 25M sps. The waveform of the received signal in the frequency domain is shown in Figure 3.23, where only the part included in the red box is the signal of interest. One can also see that the amplitude of some of the unwanted signals is even larger than the signal of interest. By zooming in the figure to the frequency of the signal of interest, the spectrum of the target signal can be observed. The aim of implementing a low pass filter is to remove all the unwanted signal and only keep the signal of interest.
Set up the system
- Identify the signal source of interest using frequency scan. - Deploy SDRs spatially.
- Set up interest connection and communication between SDRs and their host PCs.
System monitoring and remote control
- Check whether multiple SDRs are synchronized. - Set the trigger time of signal reception after they are
synchronized.
- After signals are received by SDRs, they are uploaded automatically to a central server.
localisation
- At the central server, collect received signal and obtain TDOA measurements through cross correlation.
- Estimate the source location using localisation algorithms.
Figure 3.24: TDOA-based passive source localisation procedure
The work procedure of the passive source localisation system is shown in Fig- ure 3.24. In practical source localisation, SDRs are deployed spatially in a large outdoor area to overhear the signal source of interest. The choice of the sites where the SDRs are deployed is determined by the previous site investigation based on the time-frequency analysis. A central server PC is used to monitor and control multiple SDRs remotely. After the synchronization is achieved through on-board GPSDOs, a specific time to trigger the event of SDR reception in the future is set for all SDRs. When the time arrives, SDRs begin to receive the signal, store the samples on the host computers and then upload to the central server PC automatically. At the cen- tral server, the TDOA measurements are calculated and then the location of the signal source is estimated. The whole procedure can be conducted many rounds to obtain multiple location estimates.