Transmitting and receiving on the same radio frequency, at the same time, has been demonstrated by the UK's University of Bristol in a new YouTube video.
The system being used is running 900MHz and 1800MHz with exactly the same equipment.
There are a number of companies working on this technology, called full duplex radio, a technology traditionally viewed as being extremely difficult to implement – if not impossible.
However, what has now seemingly made it possible are improvements in processing, which allows the noise of the transmitted signal to be subtracted from the received signal.
One of the first patents on full duplex was filed by UK-based electronics, defence and telecoms company Plessey in 1980, for a combat radio repeater using the tech. This was called "Groundsat" and was used by the British Army. There was also a patent filed by Bristol University in the 1990s.
Bristol's Professor Mark Beach explains the system, above
In theory, full duplex does not affect the propagation, although one expert we spoke to said that there could be limitations from second-order effects.
Leo Laughlin, an assistant at the University of Bristol's Faculty of Engineering, told The Register: "Full duplex does not effect the propagation in any fundamental way; what you have going 'over the air' are the two signals (uplink and downlink) following the exact same propagation path, but traveling in opposite directions."
"However, achieving full duplex in a practical cellular system may only be practical in smaller cells (although perhaps no smaller than today's microcells)," he added. "This is because to achieve any throughput gain from full duplex you must suppress the interference to an acceptably low level, and this level depends on the difference between the transmit and receive signal powers."
He said that in a smaller cell, where signals don't need to travel as far, "the transmit signal powers are lower, and therefore the power of the self-interference caused by the transmission is also lower [and] this reduces the amount of interference cancellation required, thereby making full duplex easier to achieve."
"Because of this," added Laughlin, "I think full duplex is likely to be a technology which is deployed to increase capacity and data rates in high density small cell areas of the network, and is not likely to be used for wireless access in macrocells," although it could be used for backhaul in both types of cell.
In theory it should be happy at the frequencies above 6GHz, which are being touted for 5G, as full duplex is frequency agnostic. Furthermore, the smaller antenna sizes and the use of beam-forming could not only enable microwave operation, but could also be used to help avoid self-interference.
But this is theory and it's not yet been tried; the team acknowledges there is lots more work to be done in this space. There is already a proprietary point-to-point system which gets more than a gigabit of data through a carrier 28MHz wide.