One of the major themes of 3GPP Release 12 (due for completion in 2014) is enhanced support for small cells in LTE. With mobile network traffic expected to increase dramatically over the coming years, the ability to reuse spectrum many times in dense usage areas will make small cells crucial.
In a previous post on 3GPP Standards Releases for LTE I identified a number of features related to the support of small cells, including the home eNodeB in Release 9, relay nodes and interference coordination in Release 10, and coordinated multipoint operation in Release 11. However, there has been no shortage of ideas to enhance the support of small cells in 3GPP Release 12 and beyond. The general aim is to use small cells to extend the capacity, coverage and service capability of LTE in a manner that is energy- and cost-efficient. This requires careful and potentially dynamic co-ordination of the activities of large and small cells, for example to balance their use of system resources and to manage the interference between them.
In this post I want to talk about one particular aspect of enhanced small cells, which was brought to my attention recently by my ex-colleague Sunil Chotai (also known as “Zed”). The idea is known variously as “soft cells”, “phantom cells” and “amorphous cells” and could transform the way multimedia content is delivered in densely populated areas. One example application is the distribution of video content at sports stadia or concert venues.
The principle of soft cells is to provide a mobile terminal with simultaneous logical connections (although not necessarily physical connections) to both a macrocell and a picocell, in an arrangement described as “wide-area assisted local-area access”, as illustrated in the figure below.
The macrocell provides an “anchor carrier”, which gives the mobile terminal a robust wide-area signalling connection, carrying system information and basic radio resource control (and potentially low rate or high reliability user data). The picocell provides a “booster carrier”, which supports highly efficient delivery of user traffic within its local area. The anchor carrier ensures that a mobile terminal has a reliable signalling connection as it moves around a network coverage area, but traffic can be offloaded to a local booster carrier if a user requires a period of intense communication.
The solution is particularly effective when the booster carriers can be deployed in a different frequency band to the anchor carriers, because this relaxes the interference problems between the layers. However, it can also be used where the macrocells and picocells are deployed in the same band. Picocells are particularly suited to new spectrum at higher frequencies, where range is not so important.
A number of features are required to enable the soft cell concept in LTE. The mobile terminal has to be able to support multiple transport channels and MAC entities, along with scheduling of the activities related to the anchor and booster carriers. It must also be possible for the mobile terminal to discover and synchronise to the booster carriers, while maintaining a signalling relationship with an anchor carrier. Also the operation of the booster carriers can be optimised to their traffic-related role, by minimising overheads, controlling interference and increasing battery efficiency.
Enhanced small cell developments offer the prospect of substantial capacity gains, but they will not be available until Release 12 and will they require new mobile terminals to enter the market before they have a significant impact. Therefore it will be sometime beyond 2015 before their full benefit is brought to bear.
