Next generation SDR interference reduction should be tackled by Spatial Interference Filtering Techniques. Here is why and how.
Simply put, next generation SDR interference reduction should be achieved by spatial filtering, often called beamforming. In this series, I will describe spatial filtering and how it can address some of our biggest challenges to radio reception at medium and high frequencies.
At present, and for the foreseeable future, our biggest challenge to radio reception is local RFI and the increasing man-made noise floor. In particular, radio frequency interference spewed out by consumer equipment and switched-mode power supplies destroys our listening interests. I have described these challenges in my RFI Survival Guide.
Many of us have implemented external spatial filtering using the so called “noise cancelers” such as ANC-4 and MFJ1026. We also use similar phasing devices to create antenna patterns in certain directions. But these are outboard analog devices that have many limitations and few modern features. It’s time to incorporate spatial filtering into our SDR receivers.
Most of our SDR radios already do an excellent job of temporal and spectral filtering. These are time-based approaches that include noise blanking and reduction, as well as notch and bandwidth filtering. But when the buzz from a nearby plasma television or SMPS shows up on your frequency, your fancy modern radio has no tools to eliminate it.
For a taste of what lies ahead, watch this video demonstration of spatial filtering from G7CNF.
Next Generation SDR Interference Reduction – SIFT
Just for fun, I am calling the new solution SIFT, standing for Spatial Interference Filtering Techniques. It is time for makers of mid-range SDR to incorporate these basic requirements, as follows.
- SDR receivers should contain at least two separate channels. In effect, they should all be dual receivers with separate antenna inputs for each receiver, and separate IQ data outputs.
- Dual receivers will share synchronized frequency translation and sampling clocks. Data output will also be synchronized. Thus, we will have coherent receivers capable of spatial filtering or beamforming.
- Data should be time stamped, likely using VITA Transport Layer (VITA 49) standard, which also contains context meta-data.
- Supporting control and demodulating software will support diversity reception, beamforming and anti-phase interference cancellation.
There are presently a few hardware and software solutions that can achieve the SIFT requirements. We will describe these shortly. Next up, though, we describe the theory behind beamforming.
Spatial filtering has been around for decades. It shows up pretty much everywhere except the SDR used by hams and shortwave listeners. Let’s change this.