Coherent Applications 2017-04-24T09:59:24+00:00

Radiolocation and Earth exploration

In radar systems, energy from a pulse or other signal is radiated and a radio wave returned in response to the radiated signal is analyzed to determine location or other information of targets from which the radiation is reflected. Accordingly, radar detection equipment must provide accurate information of the returned radio wave. A radar system may have a coherent or a non-coherent receiver. In a receiver using coherent detection, local oscillation is arranged to be synchronous with the signal produced by the transmitter that is radiated by the radar system antenna. Coherent radar detection utilizes both the magnitude and phase of the returned signal to extract target information. More information can be found here.

Bandwidth aggregation (for continuous/non-continuous spectrum)

The fusionimage blog describes, how to combine the bandwidth of 2 RTL-SDR dongles in gnuradio. The author describes his method as follows: “I simply took two RTL-SDR dongles at their max. bandwidth of 2.4 MHz, resampled the signals to 4.8 MHz, then shifted the first signal down by 1MHz, the other one 1 MHz up, added them together, divided the combined signal by 2 to lower the noise floor again and finally feed it into a FFT plot”. The result shows the combined spectrum. Moreover, the author states that “this isn’t any useful to decode or demodulate any signals, because of phase and clock misalignment.”

In contrary to the described method, the coherent receiver has the common clock, so that the combined signal could be used to decode or demodulate the signals. Please note, that all signals have to be synchronised before addition (see for more details).

2 channels

FFT plot of 2 connected RTL-SDR dongles for the bandwidth aggregation in gnuradioFFT plot of the RTL-SDR sources

4 channels

gnu radio 4 receivers schema for band aggregationFFT Result of 4 receivers in gnu radio

Signal Decoding Improvement

Coherent averaging method (also known as time-synchronous averaging) is used to increase the signal-to-noise radio (SNR). The technique is very well described in the book of Richard G. Lyons “Understanding Digital Signal Processing”, chapter 11. The achievable results can be summarized as follows.

The SNR gain coefficient equals:

SNRcoh = SQRT(N)

or as as logarithm value:

SNR cohgain (dB) = 20 * log10 (SNRcoh)=20*log10 (SQRT (N)) = 10 * log10 (N)

where N is a number of receivers.

You can make all types of the coherent averaging experiments using our coherent receivers.

Piotr Krysik has developed a GNU Radio block “Multi-RTL”. Moreover, the user Fabio writes in the RTL-SDR blog following comment: “Using multiple receivers could be interesting also to perform noise reduction. Combining samples over the same band could reduce the Gaussian Noise. Filtering out the signal from a reference receiver with the RF input terminated with a dummy load could be used to reduce system noise. Filtering out the signal from a reference receiver with reference antennas could be used to reduce other site specific noise. Would be nice to have such alternatives explored in a dynamic way, where you can trade sensitivity and bandwidth on the run.” Indeed, that can be very interesting experiments that we could make in the future.

The resulted sound was without delays or reverberations.

GNU Radio Schema

gnu radio schema using multi-rtl sources

Channel delays estimation using Multi-RTL

channel delay calculation in gnu radio using multi_rtl

No antenna connection

no antenna connection to the RTL-SDR receivers

Antenna connected to the 1st receiver

the antenna is connected to the first RTL-SDR receiver

Antenna connected to the 2nd receiver

the antenna is connected to the second RTL2832U receiver

Antennas connected to the both single channel receivers

both antennas are connected to RTL-SDR receivers

Synchronized receiving on different frequencies, e.g. for frequency hopping decoding

Passive radars

Radio telescope (high sensitivity wide-band pulsar detection) or scientific applications requiring phase coherent receivers

Direction finding (pseudo-Doppler and Watson-Watt technique) and Drone detection

Beamforming in Software Defined Radio

Very nice introduction to the beamforming techniques using more sophisticated receivers.