Delivering on the promises of 4G LTE

With commercial LTE deployments well underway around the world, many network operators are finding themselves running three generations of technology at the same time. Not only is this situation resulting in many operational headaches due to the management of separate 2G, 3G and 4G LTE networks but also because the traffic running across those networks is becoming increasingly unbalanced.

Telecoms.com recently spoke to Mr Lei Xue, Director, Europe CTO Group, at ZTE, to get his thoughts on how operators can tackle these scenarios while making best use of their existing network assets.

At the recent LTE World Summit in Amsterdam, network engineers of the world were discussing technical implementations of LTE technology and its impact on existing and legacy network installations. At the event ZTE raised an interesting point, claiming that inter cell interference in commercial deployments is much higher than first thought and proposed solutions built into the LTE specification might not be sufficient remedy.

The LTE Asia conference is taking place on the 18^th-19^th September 2013 at the Suntec, Singapore. Click here to download a brochure for the event.

“In a current network you have 2G, 3G and LTE and the common situation is that network traffic is unbalanced. This is the first challenge to solve,” says Lei. “3G networks are already congested and 4G networks will probably be congested by 2017, so we find that radio performance is quite limited, especially due to interference. Moreover we find that standards do not allow for a lot of things LTE promised it could do.”

It’s well known that engineers talk about LTE downlink speed of 150Mbps in the lab but in the field with interference considerations, operators will more likely experience a downlink of 30Mbps. The problem is that is not what LTE promised. Then you have impacts on the user experience, which is often quite different at the cell edge where you might have very low signal to noise ratio due to heavy interference, compared to the cell centre.

The obvious solution to dealing with these issues is for an operator to sink more investment into backhaul and fibre resources and to rely on solutions such as CoMP (Coordinated Multipoint Transmission/Reception) built into the LTE standard. Yet Lei raises the point that with 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 techniques like CoMP to solve the problem. Rather, he says that ZTE set about designing Cloud Radio to tackle these particular challenges.

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.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.According to the 3GPP: “In CoMP operation, multiple points coordinate with each other in such a way that the transmission signals from/to other points do not incur serious interference or even can be exploited as a meaningful signal.”

But the issue identified by ZTE is that the CoMP specification relies on significant backhaul assets being in place and those assets are seldom available in real world environments. The specification for CoMP relies largely on fibre backhaul being available but Cloud Radio means operators can use existing backhaul solutions such as IP-RAN, Microwave and PTN etc.

Cloud Radio helps operators make the most of the huge investments they’ve made in fibre and legacy backhaul and means they don’t have to worry about investing a huge amount more. Based on SDR (software defined radio) it uses existing backhaul assets to assist in interference control and because it is a software upgrade, the vendor can keep innovating and customers can benefit from fresh updates.

The ‘cloud’ element is a reference to the solution’s elasticity – the ability to deploy innovative technology such as a ‘super cell’ dynamically and give improved user experience at the cell edge. Essentially, helping operators deliver on the promises of 4G LTE.

In the Cloud Radio solution, the cloud scheduling part has very low bandwidth requirement, so it can be deployed with any IP backhaul. While cloud coordination need some additional transmission bandwidth (from several Mbps to hundreds of Mbps) to exchange the coordinative data among multiple cells/sites, that means the suitable transmission conditions for cloud coordination could be IP-RAN, Microwave, PTN etc. But that’s still much lower than the bandwidth requirement of a standard CoMP solution.

“CoMP required originally that operators had fibre installed,” says Lei, “but Cloud Radio means they can make use of existing backhaul resources. If you have an IP-RAN/Microwave/PTN backhaul then that’s good and you don’t need to upgrade. So Cloud Radio helps you to improve your cell performance and the imbalance between technologies. With insufficient inter-site coordination set out in live LTE environments, there is a risk of poor quality user experience.”

Cloud Radio
In an LTE network, base stations are deployed more densely than in 2G and 3G networks. Considering Shannon’s Theory, the capacity of a single link has approximated its theoretical limit. To improve performance, operators need to improve system capacity by using a heterogeneous network (HetNet) topology. To relieve the strain on radio spectrum, intra-frequency networking has been introduced into LTE, but this increases interference between cells. Interference can greatly reduce performance at the cell edge and affect resource utilisation and cell-edge user experience. Cloud radio is a set of network optimisation techniques that eliminate inter-cell interference in time, space, and frequency.

So the implementation of a Cloud Scheduler as part of the Cloud Radio proposal helps cells coordinate with each other in order to provide better performance. An 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.

One on the many innovative features in Cloud Radio is super cell, which delivers a very smooth experience and reduces handover. When a user hands-over normally the network has to allocate resources to fulfil the request, which can be severely affected by signal strength, so the network can end up dedicating more resource to the user. A super cell aggregates several cells into one reducing handover and creates a dedicated radio resource for a user but minimises the impact of that resource.

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. In fact, Lei claims that with fibre in place, Cloud Radio can deliver a 380 per cent improvement at the cell edge and without fibre this drops to a still impressive 280 per cent improvement.

The technology is an extension of ZTE’s previously introduced Cloud RAN solution which enabled one BBU to support 1,000 cells while Cloud Radio allows operators to pool backhaul resource on demand. The whole focus is about getting more out of what operators already have available without further significant investment, so the carriers can focus on delivering to end users what 4G has promised.