This weekend at Hamvention (Dayton, Ohio) Andy, Mike and Steve from SDRplay will be showing an early version of Diversity reception running in SDRuno with an RSPduo.
Above: New diversity reception controls in SDRuno demo shown in Dayton
The RSPduo gives users the ability to implement receiver diversity as it contains two independent tuners that are phase and time synchronous. Depending upon the usage case and reception conditions, diversity can give substantial improvements in reception performance, particularly in a mobile multipath fading environment. There are many different types of diversity, but they all basically rely upon de-correlating the signal between two antennas. This de-correlation can be achieved through different polarisations or spatial separation.
It’s important to be aware, that the key to getting decent gains from diversity lies in having the correct antenna setup and if you get it wrong, you can actually make things worse! That said, in some circumstances,the improvements can also be dramatic.
Here’s a bit more about what’s going on. In a multipath fading environment, signal reflections can combine in anti-phase in a way that results in the signal being cancelled. This is called a deep fade. If we have two antennas that are separated by ¼ wavelength at the frequency of interest, the spatial separation can help ensure that whilst one antenna is in a deep fade, the other may not be. As a consequence, either by simply selecting the antenna with the best signal or by combining the two signals it is possible to make the receiver considerably more resilient to fading.
Similarly if you consider the same antenna setup but this time without multipath effects. You will end up with roughly the same level of signal power on each antenna. By adjusting the phase of one signal before combining the two paths, it is possible to ensure that the signals add in-phase and thus constructively giving a 6 dB increase in the signal power. However, the noise generated by each tuner is uncorrelated and so when the noise power is combined, the increase is only 3 dB. Therefore is it possible to see a net 3 dB improvement in the SNR as a result of combination of the two paths, which is equivalent to reducing the noise figure of a single receiver by 3 dB. If the amplitudes are not exactly equal , then as well as adjusting the phase, you would also adjust the amplitude of one path to ensure that they were equal before combination. This principle of trimming the gain and phase prior to combining the two paths to give the best possible SNR is called Maximum Ratio Combination (MRC) diversity.
It is important to understand that MRC diversity is not guaranteed to give better performance that what may be received from a signal antenna. For example, if one antenna is in a deep fade, combining the two signals will result in a degraded SNR when compared to simply selecting the antenna with the strongest signal. That is simply because, combining will not boost the signal power, but will boost the noise power. However, in a mobile multipath fading environment, MRC diversity will substantially improve receiver resilience to fading.
Under certain circumstances, the same principle can be used to cancel interference without substantially degrading the strength of the signal of interest. In this case the antenna positions are adjusted so that the interfering signal arrives at the two antennas with the same signal strength and in phase. If the signal of interest is arriving from a direction that is orthogonal to the source of interference, then the antennas will see the wanted signal of equal strength, but with a 90 degree phase shift. If the two paths are now subtracted (added with a 180 degree phase shift), then the interfering signal will cancel, but the de-correlated signal of interest will combine, thus resulting in a substantial improvement in the SNR.
One other useful application for such a setup is that given a known separation between two antennas, being able to measure the phase difference between the signal at each antenna, simple trigonometry allows for an estimation of the direction from which the signal has come. This approach is sometimes referred to as ‘time of arrival direction finding’. This technique will only work well with signals that are substantially free from multi-path.
The software release roadmap for SDRuno has been updated to include V1.32 in the near future as a special release of the diversity software for RSPduo owners. This is the updated sequence of releases planned by the SDRplay software development team:
1.32: Diversity (RSPduo only)
Plugin support (via documented API)
Remote client support
VFO IQ and audio recording
Automatic LO system (user just sets VFO)
Better support for multiple VRX use (improved settings support)
Improved INI file support (backup/restore)
Improved service support (restart/recovery)
2.0: Cross platform support (Windows, Linux/x86, MacOS)
All versions will also include bug fixes and user feedback (where possible)
It is not possible to commit the exact dates of these releases but these are the priority for our software development team.