The sales pitch for full duplex is a powerful one: these new radios could instantly double the capacity of today’s wireless networks by transmitting and receiving signals on the same frequency, at the same time. That promise has made network engineers eager to deploy it in cellular base stations and mobile devices ever since the technology began to pick up steam around 2007.
But in reality, transmitting and receiving messages at the same time on the same frequency has an unfortunate side effect. It causes twice as much interference as performing each function in turn or on separate bands. So while full duplex radios can dramatically improve spectrum efficiency, the resulting interference means more connections would be lost if a network were constructed wholly of them.
Knowing this, a team of researchers from New York University in the U.S. and Ireland’s Trinity College is studying how wireless engineers can get the best of both worlds by blending full and half duplex radios into the same network. They say that doing so can will allow designers to balance spectrum efficiency and top-notch coverage for the area they wish to serve.
Sanjay Goyal, a coauthor and PhD student at New York University, says service providers will inevitably confront this tradeoff when they construct future base stations featuirng full duplex technology. It might make more sense to include more full duplex components in a crowded area, for example where bandwidth demand is skyhigh. Rural communities, meanwhile, should probably use fewer full duplex cells in order to maximize coverage.
Goyal and his colleagues demonstrated the concept by toying with the ratio of half and full duplex base stations in a model network. Their simulation included 1,000 base stations in a square kilometer, which is similar in structure to many dense networks found in cities.
They adjusted the ratio of full and half duplex cells (low-powered radio nodes with a shorter range than traditional cell towers) in their simulated network and recorded the results in coverage and spectrum efficiency. All the handheld devices in their model were assumed to be half duplex because full duplex radios are too expensive and require more power than is considered practical for smartphones and tablets.
Their results showed that wireless engineers could tweak the mix of full and half duplex cells in a network to prioritize greater efficiency or better coverage to suit their purposes. Including more full duplex cells delivered higher spectrum efficiency at the cost of a few more dropped calls. On the other hand, featuring more half duplex units improved coverage at the expense of overall efficiency.
Goyal hopes wireless engineers will find this result useful and perhaps even freeing as they plot tomorrow’s wireless networks. “If you don't need spectrum efficiency gain, don't use full duplex,” he says. “But if you want spectrum efficiency gain and you want coverage, you need to tune the number of full duplex base stations according to that.” Goyal presented the group’s work on Thursday at the IEEE International Conference on Communications in Kuala Lumpur, Malaysia.
The team also tested whether it was possible within a full duplex system to enable a faster connection only for those users trying to download data versus those wishing to send it. After all, the average network has about five times as much traffic in downloads than uploads. Indeed, by lowering the transmission power of nearby devices and thereby eliminating interference, the group showed that recipients’ devices enjoyed speedier downloads than they would otherwise.