LTE-Advanced ? Moving Beyond 4G

Bernard Breton
Sep. 6 2012

By Bernard Breton, Chief Marketing and Strategy Officer, InfoVista

One of the driving forces behind the creation of the LTE Release 10 specifications was the desire to fulfill the requirements set out by ITU for IMT-Advanced in order to acquire the 4G label. Considering the increased peak data rates as the primary target, carrier aggregation is clearly the most significant improvement as it allows for much greater theoretical data rates and lower latencies, simply by leveraging all possible non-contiguous carriers allocated to a wireless operator. While reducing the need for contiguous spectrum, carrier aggregation also provides interesting side effects that are of great benefit in HetNet deployments. For example, with cross-carrier scheduling, the resources of secondary carriers can be scheduled on the PDCCH of the primary carrier, which reduces the interference to the control channel. Similarly, there is no need to perform inter-frequency handovers between secondary carriers as long as the primary carrier provides coverage. Along the same lines, advanced multiple antenna capabilities bring the spectral efficiency of LTE-Advanced way beyond the ITU requirements. On the downlink, 8×8 MIMO is achieved with the introduction of additional transmission modes that rely on the extensive use of UE-specific reference signals. On the uplink. a maximum of four spatial layers is now available to devices with four transmit antennas (rather large devices such as laptops and tablets), complimented by Multi-User MIMO (MU-MIMO) where two terminals can each transmit up to two spatial layers.

It is, however, unquestionable that appealing peak data rate values are not sufficient and alone will not improve the cell edge user experience and solve the capacity crunch, for which the commonly adopted solution is based on the introduction of small cells. But LTE Release 10 is not only about adding small cells to the network. It also comes with a set of interference management techniques that make heterogeneous networks a safe and reliable solution. Amongst these techniques, eICIC (Enhanced Inter-Cell Interference Coordination) consists of time-domain partitioning that mitigates the interference between the macro layer, which provides the large area coverage umbrella, and the pico layer, which fills coverage holes and absorbs traffic hotspots.

More fundamental changes to cellular communications have also been introduced and can be classified under the generic term of cooperative communications. This includes support for relay nodes as well as a number of multi-cell cooperation techniques known as CoMP (Coordinated Multiple Point transmission and reception). In all cases, backward compatibility is maintained so that LTE Release 8 users are not impacted. This is done by configuring various subframes as MBSFN subframes to Release 8 terminals.

In addition to meeting the accessory peak data rate requirements of IMT-Advanced, LTE Release 10 also focuses on a far more important issue; that of network capacity and lays the ground work for further enhancements in Release 11. The LTE specifications not only provide a means to achieve high end-user requirements but also provide a way to deal with the potential side effects, in particular the increased signaling overhead and interference. The LTE specification updates are tightly aligned with good network planning and optimization practices, but the gains introduced by new technologies are dependent upon a change in the network design and optimization approaches. For example, Mentum Planet's analysis, simulations, and optimization tools all reflect the impact of Release 10 functionality on data rates achieved by compliant devices, accounting for carrier aggregation and advancements in multiple antenna transmissions, but also acknowledging the increased overhead due to, for example, UE-specific reference signals. In terms of capacity planning, it is critical that network loads be accurately estimated. With functionality such as multi-user MIMO, CoMP and SON in general, this task can only be performed by the most advanced simulation algorithms. This makes careful network planning more important than ever.

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