A few months ago I wrote a summary of facts and figures on HSPA+, LTE and LTE-Advanced, based on the 3GPP Release 11 standard. With Release 12 due to be finalised in the middle of 2014, I thought I would write about some of the developments in the pipeline in the 3GPP work programme. The expected freeze date for Release 12 is September 2014, so its features are likely to start appearing in equipment around the start of 2016.
The nature of the 3GPP work programme is such that it has to strike a balance between the development of features that provide incremental improvements in the next release of the standard, while also doing the groundwork for more substantial changes that will deliver major steps forward in the future. Preparing for the major leaps has never been more important, as diverse multimedia devices and services transform the nature of mobile networks, requiring order of magnitude increases in capacity over the coming years, along with improvements in user data rates, coverage, quality and energy efficiency.
The 3GPP work programme is a substantial undertaking. For example, the latest “Overview of 3GPP Release 12”, published at the end of 2013, contains 289 pages and outlines the work on nearly 200 features and studies. Full details can be found on the 3GPP web site, but for the purposes of this post I have picked a few notable areas of work related to the LTE radio access network, summarised in the figure below. Some of these will lead to specific features in 3GPP Release 12, while others will pave the way for developments in future releases.
The figure includes a small selection of many developments related to small cells, which are crucially important to the evolution of mobile networks because of their potential to enable massive reuse of spectrum. This will be necessary to achieve the dramatic increase in network capacity that will be required over the next decade, with some estimates suggesting a need for as much as a 1000× uplift. The introduction of large numbers of low cost, low power, small cells into a network creates a host of new challenges (and some opportunities) for network architecture, mobility management, network planning and interference management. For example:
- One of the benefits of small cells is that the close proximity of a mobile device to its base station can provide higher signal-to-noise ratios than would normally be expected with macrocells. This provides the opportunity to introduce higher order modulation, such as 256 QAM for the downlink, in situations where there is minimal interference and low mobility. This could double the throughput compared with 128 QAM in Release 11.
- In a previous post on enhanced small cells I discussed the idea of “anchor” and “booster” carriers, whereby a mobile terminal can be connected simultaneously to a macrocell (which provides a robust, wide-area signalling connection) and a small cell (which provides efficient delivery of user traffic in its local coverage area at a particular point in time). 3GPP has received many contributions on this subject (referred to variously as soft cells, phantom cells, wide-area assisted local-area access, etc) and it is developing the concept under the banner of dual connectivity.
- Increasing the number and variety of cells in a mobile network makes it more difficult for mobile terminals (and base stations themselves) to maintain an accurate picture of the base stations around them at any given time. 3GPP is developing new, efficient mechanisms for mobile stations to discover the small cells available to them without draining their batteries excessively. One example of this is in relation to on-off small cells. Large numbers of small cells in a network inevitably leads to increased radio interference and power consumption. This can be mitigated by allowing base stations to switch off (rather like eco-friendly cars) when they are not required. However, this requires new techniques to enable mobile terminals to discover the availability of these intermittent cells in an efficient way.
- To minimise the cost of deploying large numbers of small cells, it is necessary to incorporate mechanisms for their base stations to configure themselves automatically. One example of this is the challenge of distributing synchronisation to all of the cells, where 3GPP is introducing the means for small cells to derive their synchronisation from the radio interface of surrounding base stations.
Another significant area of study for 3GPP is the further evolution of Multiple Input Multiple Output (MIMO) antennas. In my previous post on LTE MIMO theory and practice, I explained that 2×2 downlink MIMO is already widely deployed in LTE networks and that 3GPP Release 11 allows for up to 8×8 MIMO in the downlink and 4×4 MIMO in the uplink. However, over recent years there has been extensive research into new MIMO concepts, including elevation beamforming and massive MIMO (also referred to as Full Dimension MIMO or FD-MIMO). 3GPP is developing 3D radio channel models that will help in the development of both of these concepts:
- Elevation beamforming uses active antenna arrays at base stations to direct antenna beams in the vertical dimension as well as in the horizontal plane. This creates new possibilities, including splitting cells vertically and potentially distinguishing between individual mobile terminals based on their height. These techniques can be particularly useful in urban environments, where mobile users may be distributed in tall buildings, by enabling network operators to increase the number of users supported in a cell and/or to improve the quality of service provided.
- The principle of massive MIMO is to use large numbers (potentially hundreds) of base station antenna elements to create very narrow beams of radiation, which maximise the signal-to-noise ratios experienced by individual mobile terminals. This technique will be crucial as mobile systems move into higher frequency ranges, in order to compensate for the greater radio signal path loss experienced at those frequencies.
Previously I have explained that carrier aggregation allows LTE to exploit the capacity of multiple carriers, without the need for the carriers to be contiguous, or even in the same frequency band. Ultimately LTE will be able to support the aggregation of 5×20MHz carriers, which will have substantial benefits for system capacity and user throughput. However, the Release 11 implementation is limited to two downlink carriers. Release 12 will extend this to three downlink carriers and will also provide the facility for aggregation of two uplink carriers (although downlink aggregation will be limited to two carriers in this case). A further development in Release 12 will be a framework for the aggregation of FDD and TDD carriers, to provide network operators with greater flexibility in how they exploit their spectrum allocations.
The Release 12 work programme is also addressing a number of new and improved service capabilities:
- Machine-Type Communications (MTC) is becoming an increasingly important aspect of mobile communications, with expectations that there will be a proliferation of simple, low-cost, low-power devices using mobile networks to send and receive data in the coming years. To support this requirement, 3GPP is working on a range of features to allow devices of this sort to benefit from LTE services, even where conditions may be challenging. For example, new categories of low-cost terminals could support low data rates by means of a single receive antenna, narrowband data channel and half duplex operation. It may be possible for these terminals to operate on the margins of LTE coverage, for example deep inside buildings, by means of data repetition and receiver developments.
- Already a number of short range wireless technologies, such as Bluetooth and NFC, allow mobile devices to communicate directly with each other for various reasons, including electronic payments, advertising and sharing information. 3GPP is investigating methods for LTE itself to be used by mobile devices to discover and communicate directly with other local LTE devices, within and possibly beyond LTE network coverage, for both public safety and non-public safety applications.
- A year ago I wrote about Verizon Wireless’s plans to launch an LTE Multimedia Broadcast Multicast Service (eMBMS). eMBMS was first included in Release 9 as a means for LTE to deliver multimedia services, such as broadcast TV, to some or all LTE users in an area. This may be a valuable service for LTE network operators, but it needs refinements to unlock its potential fully. Release 12 will introduce the capability to measure the performance of eMBMS multicast/broadcast services, to support network planning and optimisation and thereby delivery the highest quality of service.
The specific features to be included in Release 12 are still under consideration and will not be finalised until the middle of 2014. However, a number of these are already well developed and can be expected to appear in the forthcoming release. Among those mentioned above, the carrier aggregation enhancements, MTC developments and eMBMS measurement can all be expected to feature. Look out for the full set later this year.