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Sunday, August 14, 2011

LTE: WHERE TO GO IN THE FUTURE?


The evolution of LTE does not end with LTE release 10. Rather LTE will continue to evolve into release 11, release 12, and so on, with each new release bringing additional capabilities and further enhanced system performance into the LTE radio-access technology. Not only will the additional capabilities provide better performance in existing applications, they may also open up for, or even be motivated by, new application areas. Examples hereof are home automation, smart transportation, security, and e-books, but the list is continuously growing as additional applications benefiting from mobile connectivity are emerging. Similarly, new ways of deploying cellular networks, for example more extensive use of massive beam-forming or ubiquitous access to optical fibers in the backhaul, may call for enhancements in the radio interface. some possible areas for the future evolution of LTE may include the following;

Advanced Multi-Cell Coordination

Coordinated Multi-Point transmission and reception (CoMP) refers to a wide range of different techniques with the common denominator being the dynamic coordination of transmission and/ or reception at multiple geographically separated sites with the aim to enhance system performance and end-user service quality. Many different coordination schemes of very different characteristics fall under the joint umbrella of CoMP, ranging from dynamic inter-cell scheduling coordination to joint transmission/reception at multiple sites. In the former case, CoMP can, to a large extent, be seen as an extension of the inter-cell interference coordination that is already part of LTE.

Joint reception means that the signals received at multiple sites are jointly processed for enhanced reception performance. Maximum-ratio combining and interference-rejection combining are examples of schemes that can be used to combine the uplink transmission received at multiple points. This is, in many respects, similar to softer handover used within a site, for example in WCDMA/HSPA based systems, but extended to multiple sites. Joint transmission implies that data is transmitted to a mobile terminal jointly from several sites, thereby not only reducing the interference but also increasing the received power. The transmission from the sites can also take the instantaneous channel conditions at the different terminals into account to enhance the received signal strength, while at the same time reducing the interference between different transmissions.
In general, both joint reception and transmission pose high requirements on low latency in the communication between the network node involved in the joint processing and the different antennas involved in the reception/transmission. Hence, in practice, it can be expected that the different sites may be connected in the form of a centralized RAN (C-RAN) deployment for example.

Network Energy Efficiency

Low energy consumption for mobile terminals has been an important requirement ever since the emergence of hand-held terminals roughly 20 years ago. The driving force has been the reduction in battery size and improved battery time. Today, reduced energy consumption also in the radio-access network is receiving increased attention for several reasons:
- The cost of energy is a far from negligible part of the overall operational cost for the operator. Thus, reduced energy consumption is one component in the everlasting quest for reduced operating costs for network operators.

- In some rural areas, it may not even be possible to connect the base station to the electrical grid. With sufficiently low energy consumption, reasonably sized solar panels could be used as power source, instead of the diesel generators commonly used today.

- In today’s world, where energy consumption and the related climate impact is seen as one of the great challenges for the future, the mobile industry should lead the way for reduced energy consumption.

Regarding the third point, it is important to bear in mind though that the entire ICT (Information and Communication Technologies) sector today contributes roughly 2% of the overall world energy consumption and the contribution of the mobile-communication sector is just a fraction of that. At the same time, the emergence of mobile broadband communication opens up tremendous opportunities for reducing the global energy consumption in general by, for example, virtual meetings replacing traveling to physical meetings. Still, low energy consumption in mobile networks is important. In particular, taking into account large future traffic increases, where traffic growth of several hundred times up to perhaps 1000 times can be expected in a longer-term perspective, low energy consumption of the mobile networks will most likely be as important a performance metric as capacity, data rates and latency, and must be treated as such.

Reducing the energy consumption of mobile networks is, to a large extent, an implementation issue. However, it is important to ensure that the basic principles of the radio-access technology allows for low energy consumption. One key characteristic is that the energy consumption should scale with the use of the radio resources, not only on average but also on a short-term basis. That is, during points in time where there is no traffic, the energy consumption should be reduced to an absolute minimum, for example by avoiding all but absolutely necessary transmissions. LTE already provides several tools that can be used for this. Nevertheless, the future evolution of LTE should further strive for minimizing transmission of signals strictly not needed. This will be even more important as the network becomes more and more dense with more and more network nodes. In such network deployments, which may typically be heterogeneous networks, pico cells may often be deployed to provide high data rates, rather then being needed for capacity reasons. Thus, the load per cell may be relatively low and each pico cell may often be more or less “empty”, further stressing the importance of very low energy consumption when idle.

Machine-Type Communication

Similar to earlier mobile-communication systems, LTE has been designed with data services in mind and much effort has been put into developing techniques for providing high data rates and low latencies for services such as file downloading and web browsing. However, with the increased availability of mobile broadband, connectivity has also become a realistic option for machine-type communication. Machine-type communication spans a wide range of applications, from massive deployment of low-cost battery-powered sensors to remote-controlled utility meters, to surveillance cameras. Many of these applications can be handled by LTE already; communicating with a surveillance camera, for example, is not significantly different from uploading a file, in which case high data rates are paramount. However, other applications may not require transmission of large amounts of data or low latency, but rather pose challenges in terms of a vast amount of devices connecting to the network.

According to some sources, 50 billion connections, most of them machine-type communications, can e expected in the year 2020. Handling such a large number of devices is likely to be a challenge mainly for the core network, but improvements in the area of connection setup and powerefficient handling of control signaling in the radio-access network may be of interest.

New ways of Using Spectrum
Spectrum has been, is, and will continue to be a scarce resource for the mobile-communication industry. In particular, in light of the continuous increase in the data rates requested and the corresponding need for wider bandwidths, it is expected that spectrum will remain a scarce resource. Historically, up until now, the mobile industry has relied on spectrum dedicated for mobile communication and licensed to a certain operator. This will also clearly be the main track for the future. However, in situations where licensed spectrum is not available, other possibilities for increasing the spectrum availability are of interest. This could include the use of unlicensed spectrum, or secondary spectrum primarily used for other communication services, as a complement to operation in the licensed spectrum. Broadcast spectrum not used (in some areas) is often referred to as “white space”. Related to this is the concept of cognitive radio – that is, radio-access-related functionality related to “smart” selection of spectrum usage. However, the applicability of cognitive radio to cellular communication is a relatively new area and further studies are required to assess the feasibility and impact of such usage.

Direct Device-to-Device Communication
One possible longer-term evolution path would be to extend the LTE radio-access technology with support for direct device-to-device communication – that is, make it possible for two mobile terminals to communicate directly with each other without going via the network. This has been studied in academia for some years, for example as part of the European research project WINNER . Scenarios where the possibility for direct device-to-device communication could be of interest include situations where no network infrastructure is available, for example national security and public safety (NSPS), and situations where network infrastructure is available but when communication directly between the terminals could be more efficient, either in terms of requiring a smaller amount of radio resources for the same communication quality or allowing for improved communication quality. In the latter case, one could envision different degrees of network interaction in the device-to-device communication. However, this area is still in its infancy and scenarios where deviceto- device communication offers benefits and how to interact with infrastructure-based communication need to be better understood.

Related to device-to-device communication is the possibility to also use LTE as a radio-access technology in the home, for example a digital camera connecting to the TV. Although such connectivity is possible to some extent today by using (semi-proprietary) technology, basing this type of communication on LTE could be attractive. When outside the home, the devices can communicate with the cellular LTE network, but while at home they can seamlessly connect to the local LTE network covering the home. Furthermore, for devices that are to be used at home only, operation without a SIM card and interaction with an external core network should be possible. Adding this type of functionality to LTE is most likely a relatively simple task.

The list of technology areas above should be seen as examples. Some of these technologies will most likely be part of future releases of LTE, while others may not happen at all. There may also be other technologies, not listed above and maybe yet to be discovered, that could be of interest for LTE evolution. What is clear though is that LTE is a very flexible platform that can evolve in different directions to meet the future needs of wireless communication. Given the size of the LTE ecosystem, such evolution is a very attractive path for future wireless communication.

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