One of the highlights of 3GPP Release 12 is the introduction of LTE Device-to-Device (D2D) discovery and communication. Release 12 is only the start, but ultimately D2D will create a host of new service opportunities, while also achieving significant performance and efficiency benefits in LTE networks. It does this by enabling mobile devices to discover the presence of other devices in their vicinity and to communicate with them directly, with minimal involvement from the network.
LTE D2D is a major opportunity for mobile network operators, but it is a complex development, which may have far-reaching consequences for mobile network design. D2D will set a precedent for networks relinquishing some of their control of mobile devices and traffic and it will have implications for existing services and future network planning.
Over recent years, proximity-based information and applications have become a major area of interest for the Internet industry, particularly the social networks. The more relevant an application can be made to a user at a particular time, in a particular place and in a particular situation, the more valuable it becomes, and the more likely it is that the user will act on it. This has led to concepts such as “ambient awareness” and “proximate discovery”, in which applications search their local environment for people, information, products or services that are relevant to their users. For example, a keen golfer could be alerted to a special offer at a local golf shop as she passes nearby, a Facebook user could be alerted to the presence of a friend in a local coffee shop, and a theatre fan could be delivered video highlights of a new musical as he walks past the box office.
Major Internet players, including Facebook, Google and Twitter, have acquired or developed location-based components for their online platforms. Current approaches generally rely on an application tracking the location of a user or device (in some cases requiring manual intervention by the user) and cross-referencing the locations of devices and resources in the network to trigger events. This results in solutions that are centralised (requiring extensive processing in the network) and fragmented (each application provider has to gather its own location data). These characteristics seriously limit the mass market potential of these approaches, for both technical reasons (e.g. performance of centralised services and battery drain of replicated location reports) and privacy reasons (e.g. limitations to the information that can be gathered and stored by applications).
LTE D2D could provide a decentralised approach to proximity discovery and device-to-device communication, which is efficient, flexible, dynamic and secure, to enable proximity-based services to flourish. Mobile network operators would then hold a crucial piece of the jigsaw in the delivery of compelling mass market mobile Internet services.
Proximity-based services are not the only motivation for LTE D2D. On the face of it, the capabilities and economics of LTE should be an attractive option for public safety organisations, such as police, fire and ambulance services. Many of these organisations currently use relatively old technology, such as the TETRA system developed in the 1990s, with limited capabilities. The US government has already expressed a desire to move to LTE for future public safety communications. However, a crucial requirement for these users is the ability for mobile devices to communicate directly with each other, even when outside the coverage of a mobile network, or when the mobile network is not operational. Up until now this has not been possible with LTE, but it has been set as one of the objectives of LTE D2D.
It may appear that there is nothing new about local-area device-to-device communication. For example, WiFi and Bluetooth have supported short-range wireless communication of this sort for many years. However, these technologies have shortcomings that would undermine their ability to support mass market deployment of proximity-based services:
- Unlicensed spectrum. WiFi and Bluetooth operate in unlicensed spectrum, without any centralised control of usage or interference. This is not generally a problem when usage densities are low, but it would become a major limitation as proximity-based services proliferate. Throughput, range and reliability would all suffer.
- Manual pairing. WiFi and Bluetooth rely on manual pairing of devices to enable communication between them, which would be a serious stumbling block for autonomous, dynamic proximity-based services.
- Security. The security features of WiFi and Bluetooth are much less robust than those used in public cellular systems. They would not be adequate for major public services and they would be unsuitable for public safety applications.
- Independence from cellular networks. WiFi and Bluetooth operate independently from cellular radio technology such as LTE. Any form of device-to-device discovery based on them would have to run in parallel with cellular radio operation, which would be inefficient and would become a significant drain on device batteries.
Integrating D2D into the LTE-Advanced system offers the prospect of a spectrum-efficient, energy-efficient and secure solution for proximity discovery and device-to-device communication, which would benefit from the LTE eco-system of spectrum, mobile devices and network equipment. It could put mobile network operators at the heart of the emerging market for proximity-based services, as well as satisfying the needs of public safety organisations. Some of the potential benefits of LTE D2D include:
- Radio resource management. Unlike Bluetooth and WiFi, LTE operates in licensed spectrum and the radio resources are carefully managed by the network, to minimise interference and maximise the performance of the system. The same mechanisms can be extended to D2D.
- Performance. Direct communication between nearby devices may be able to achieve even higher throughput and lower latency than communication through an LTE base station. For example, the devices may be closer to each other than either of them is to the nearest base station and a busy base station may be a bottleneck. The network can still exert control over the radio resources used for these connections, to maximise the range, throughput and overall system capacity.
- Spectrum reuse. D2D could enable even tighter reuse of spectrum than can be achieved by LTE small cells, by confining radio transmissions to the point-to-point connection between two devices.
- Network load. Relieving the base stations and other network components of an LTE network of some of their traffic-carrying responsibilities, for example carrying rich media content directly between mobile terminals, will reduce the network load and increase its effective capacity.
- Energy efficiency. Integrating D2D into the LTE system provides the opportunity to achieve energy-efficient device discovery, for example by avoiding the need to scan for other wireless technologies, by synchronising the transmission and reception of discovery signals to minimise their duty cycle and by waking application software only when relevant devices are found in the local area. Meanwhile, direct transmission between nearby devices can be achieved with low transmission power.
- Security. D2D can take advantage of the key generation and distribution mechanisms already available in LTE, to achieve high levels of security.
- Standardisation. Incorporating D2D into the LTE standard will provide a common set of tools for proximity-based services, rather than a disparate set of approaches by different application providers. Public safety organisations can benefit from the worldwide economies of scale achieved by the broader LTE system.
The 3GPP solution for D2D is termed Proximity Services (ProSe) and has two main components, illustrated in the figure below:
- D2D Discovery enables a mobile device to use the LTE radio interface to discover the presence of other D2D-capable devices in its vicinity and, where permitted, to ascertain certain information about them.
- D2D Communication is the facility for D2D mobile devices to use the LTE radio interface to communicate directly with each other, without routing the traffic through the LTE network. The network exerts a light touch by controlling the radio resource allocation and security of the connections.
The aim is to provide D2D services over ranges of up to 500m (dependent on propagation conditions and network loading). For general public services, D2D will be available only when a mobile device is within the coverage of the mobile network, which will allow the network to retain ultimate control over radio resources and security. For public safety applications only, rudimentary D2D capabilities will also be available in the absence of a network.
D2D is a significant departure from the normal mode of operation in LTE (and other cellular networks) and it introduces some interesting design challenges for 3GPP. Here are just a few:
- A choice between LTE uplink and downlink for D2D communication (with implications for interference, capacity, handset complexity and regulation).
- A choice between OFDMA and SC-FDMA for D2D communication (with consequences for handset complexity in particular).
- A choice between static or dynamic allocation of radio resources for D2D discovery and communication.
- Different propagation characteristics for D2D communication (where both ends of the link are low and mobile) compared with traditional cellular networks (where one end of the link is generally high and fixed).
- Co-existence of D2D and normal LTE (e.g. interference caused by a mobile operating in D2D mode adjacent to a mobile operating in normal LTE mode, if one is transmitting and the other is receiving in the same band).
- Division of D2D control functions between the network and mobile devices when in coverage (to achieve efficient operation while maintaining the integrity of radio resource management and security).
- Division of D2D control functions between mobile devices when out of coverage (e.g. uniformly distributed or centralised).
- Reuse of existing LTE features and protocols where possible.
- New features and protocols to accommodate unique aspects of D2D (e.g. resource management, synchronisation, power control, error control, channel measurements).
- Algorithms for choosing between LTE base station communication and LTE D2D communication on a case-by-case basis.
- A choice between synchronous D2D discovery (to conserve battery life) or asynchronous D2D discovery (for flexibility).
- Capacity and frequency of D2D discovery (to achieve a complete and current view of local devices without sacrificing battery life).
- The nature and quantity of information provided by mobile devices during D2D discovery (to satisfy the needs of proximity-based services without compromising battery life or performance).
- Control of privacy and security during D2D discovery and communication.
- Flexible billing options based on a variety of approaches (e.g. per user, per discovery event, per D2D connection, per MB).
While the basic principles of D2D have been established, many of the details are still being studied and developed. Some of these could have far-reaching consequences. For example, the design of the D2D discovery mechanism could make or break D2D. Get it right and D2D will be an essential tool for social networks and other Internet applications. Get it wrong and the service could be unattractive to application providers, unacceptable to users concerned about privacy, or a major drain on device battery life. The design of the D2D communication mode is equally crucial. Get it right and D2D will provide a highly efficient method of off-loading traffic from existing LTE networks, with light but firm control by the network operators. Get it wrong and D2D could have a detrimental effect on existing services, or operators may lose their grip on network traffic.
Not surprisingly, 3GPP is taking a measured approach. It is actively investigating all of the issues mentioned above (and more). However, the specific features introduced in 3GPP Release 12 will be confined to a D2D solution for public safety organisations. Other aspects will be addressed in future releases.
There is no doubt that D2D is a major opportunity for network operators. It has the potential to create a major new revenue stream at the heart of future Internet services, to provide a valuable communication mode for public safety organisations, and to establish a new, efficient mode of network operation. However, D2D also marks a significant shift in network design, in which the network relinquishes some control over the traffic it carries and new modes of radio resource usage impinge on existing users. In the short term the impact of D2D on the network will be relatively small, but D2D sets a precedent and it is not clear where this might lead in the long term. Network operators must seize the opportunity from D2D, while steering its future development in the right direction.