Inter cell interference issues greater than expected

Technical considerations dominated much of the discussion at LTE World Summit this week, with Chinese equipment vendor ZTE claiming that inter cell interference is much higher than first thought and proposed solutions built into the LTE specification might not be sufficient remedy.

Speaking to telecoms.com about technical issues operators will face in the future, Lei Xue, director Europe CTO group at ZTE, said that 3G networks are already congested and 4G networks will be congested by 2017.

Of more concern, Lei said, is the discovery that interference on those networks is actually much higher than was first thought. The big question now is whether operators can rely on the CoMP standard to solve the problem.

CoMP, or Coordinated Multipoint Transmission/Reception, is an interference management technology first specified in LTE Release 11 and expected to be a major component of LTE Advanced. It forms part of what SK Telecom today claimed was the world’s first commercial launch of LTE Advanced.

By coordinating and combining signals from multiple antennas, CoMP helps deliver a more consistent user experience for users on the cell edge or moving into new cells and instigating a handover. It can essentially turn the signal interference at the cell edge into a useful signal and help operators optimise their network.

A secondary part of the interference issue identified by ZTE is that the CoMP specification relies on significant backhaul assets being in place. But those assets are seldom available in real world environments.

In response the company has developed a scheduling tool called Cloud Radio to help operators make the most of the huge investments they’ve made in fibre and backhaul in general.

Based on SDR (software defined radio) it uses existing backhaul assets to assist in interference control. The ‘cloud’ element is a reference to the solution’s elasticity – the ability to create a so called ‘super cell’ dynamically and give improved experience at the cell edge.

“CoMP required originally that operators had fibre installed,” said Lei, “but Cloud Radio means they can make use of existing DSL rollouts.”

The company claims that there is a big gap between standard backhaul found in situ and the requirements of CoMP that were set down to solve the problem. With insufficient inter-site coordination set out in live LTE environments, there is a risk of poor quality user experience.

So the implementation of a Cloud Scheduler, Lei said, should help cells coordinate with each other in order to provide better performance. The enhanced cell cluster (super cell) would actually follow the user as they move around the network, replacing the static clusters which often suffer with degradation of experience at the cell edge where there is most interference, or as the user is handing over to a new cell cluster.

Not only does ZTE claim the system can provide a better experience on the cell edge, but it can also double cell edge capacity and reduce handover frequency by 50 per cent.