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Lte 12 advanced

  1. 1. IEEE Communications Magazine • July 2013154 0163-6804/13/$25.00 © 2013 IEEE INTRODUCTION The deployment of fourth generation (4G) mobile broadband systems based on the Third Generation Partnership Project (3GPP) LTE radio access technology [1] is now ongoing on a broad scale, with a total of 32 million users (August 2012) and expectations of close to two billion users in 2018 [2]. Current commercial LTE deployments are based on 3GPP Release 8/9, the first LTE releases. However, in order to properly respond to increasing operator and end-user expectations, the LTE radio access technology is continuously evolving. The first major step in the evolution of LTE, also referred to as LTE-Advanced or LTE-A, occurred as part of 3GPP Release 10 (Rel-10), finalized in 2010. Rel-10 extended and enhanced the LTE radio access technology in several dimensions, including the possibility for trans- mission bandwidth beyond 20 MHz, improved spectrum flexibility by means of carrier aggrega- tion, enhanced multi-antenna transmission based on an extended and more flexible reference signal structure, and the introduction of relaying func- tionality, that is, the possibility of using LTE radio access not only for the access (network-to- terminal) link but also as a solution for wireless backhauling [3]. Currently, 3GPP is in the concluding stage of LTE Rel-11. In addition to further refining some of the features introduced as part of Rel-10, LTE Rel-11 includes basic functionality for coordinat- ed multipoint (CoMP) transmission/reception as well as enhanced support for heterogeneous deployments (i.e., the deployment of low-power nodes under the coverage of high-power macro nodes). Performance requirements for advanced terminal receivers and for multistandard base sta- tions supporting Global System for Mobile Com- munications (GSM), high-speed packet access (HSPA), and LTE are also part of Rel-11. As the specification of LTE Rel-11 is being finalized, 3GPP is gradually moving its focus toward the next major step in the evolution of LTE. The starting point for the work was a 3GPP workshop, “Release12 and Beyond,” arranged by 3GPP in June 2012. At that work- shop, it was agreed that important aspects to address by the evolution include not only increased capacity and enhanced end-user expe- rience but also improved energy efficiency, reduced cost, better support for diverse data applications, and backhaul enhancements. In addition, LTE needs to embrace new machine- type and short-range communication scenarios that give rise to new traffic types. The major technology areas identified comprise [4]: • Enhanced local area access • Multi-antenna enhancements • Improved support for machine-type commu- nication (MTC) • Direct device-to-device communication The schedule for the upcoming work was also treated, and Rel-12 is tentatively planned to be finalized in June 2014. The purpose of this article is to provide an overview of the major technology areas identi- fied by the workshop on the evolution of LTE (Fig. 1) and discuss some possible technical solu- tions within these areas. However, it is important to understand that work in 3GPP is contribution driven, and additional study items and features may become part of LTE as work progresses. FURTHER ENHANCED LOCAL AREA ACCESS A key tool to improve traffic capacity and extend the achievable data rates of a radio access net- work is a further densification of the network, that is, increasing the number of network nodes and thereby bringing them physically closer to the terminals. In addition to straightforward densification of a macro deployment, network densification can be achieved by the deployment of complementary low-power nodes under the coverage of an existing macro-node layer. In ABSTRACT As the specification of Release 11 of the LTE standards is approaching its completion, 3GPP is gradually moving its focus toward the next major step in the evolution of LTE. The drivers of the LTE evolution include the increasing demand for mobile broadband services and traffic vol- umes as well as emerging usage scenarios involv- ing short-range and machine-type communications. In this article we provide an overview of the key technology areas/compo- nents that are currently considered by 3GPP for Rel-12, including support for further enhanced local area access by tight interaction between the wide area and local area layers, signaling solu- tions for wireless local area network integration, multi-antenna enhancements, improved support for massive MTC, and direct device-to-device communications. ACCEPTED FROM OPEN CALL David Astely, Erik Dahlman, Gabor Fodor, Stefan Parkvall, and Joachim Sachs, Ericsson Research LTE Release 12 and Beyond PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 154
  2. 2. IEEE Communications Magazine • July 2013 155 such a heterogeneous deployment, the low-power nodes provide very high traffic capacity and enhanced service experience (higher end-user throughput) locally (e.g., in indoor and outdoor hotspot positions), while the macro layer pro- vides full area coverage. Thus, the layer with low-power nodes can also be referred to as pro- viding local area access, in contrast to the wide- area-covering macro layer. LTE is already, from its first release, capable of providing high performance in a wide range of scenarios, including both wide area and local area access. However, with most users being sta- tionary or slowly moving, there is an increasing focus on high data rates in (quasi-)stationary sit- uations. Indeed, improved support for local area access was identified as a key area at the Rel-12 workshop, and 3GPP initiated studies in this area in fall 2012. In general, when considering the deployment of a local area layer it is important to under- stand and take into account the differences in terms of characteristics and limitations of such a deployment, compared to a conventional macro- layer deployment. As an example, although low deployment and operational costs, and low ener- gy consumption are important characteristics in general, these aspects should be further empha- sized for local area access deployments. The rea- son is the large number of network nodes in such deployments and the typically low average load/usage per node, underlining the importance of low energy consumption when no users are served by a network node. The large number of nodes also calls for highly automated ways of operating and monitoring the network in order to limit the operational cost. At the same time, in the case when a local area layer is deployed under an overlaid macro layer to which a terminal can always fall back, the reliability and coverage requirements of the local area layer may be relaxed compared to the very high reliability and coverage requirements of a macro layer. In terms of traffic, with very few user termi- nals being active simultaneously within the cov- erage area of each local area node, it can be expected that the traffic dynamics will be large with relatively low average load but high instan- taneous data rates. Finally, compared to a macro layer, it can be expected that within a local area layer, user ter- minals will primarily be stationary or slowly mov- ing. FREQUENCY-SEPARATED LOCAL AREA ACCESS The 3GPP activities on local area access and heterogeneous deployments have, up to and including Rel-11, primarily focused on same-fre- quency operation, that is, when the wide area and local area layers operate on the same carrier frequency. The main reason for this has been the assumption that, especially for operators with a limited spectrum situation, it is not justifiable to split the available spectrum between the layers, reducing the bandwidth and thus also the achiev- able data rates available in each layer. Thus, so far, the features in focus have primarily targeted handling interlayer interference between the dif- ferent layers in a same-frequency deployment. However, in the future additional spectrum will be available primarily at higher frequencies, 3.5 GHz and above, as lower frequencies are already heavily used by cellular as well as non- cellular services. In general, higher frequency bands are, due to propagation characteristics, more challenging to use for wide area deploy- ments. Furthermore, in certain parts of the world, there are regulatory limitations on the output power and outdoor usage of the 3.5 GHz band. With the availability of higher frequency bands, it is, as noted at the 3GPP workshop, more relevant to consider band-separated local area access operating on higher frequency bands with the overlaid macro layer operating on lower cellular bands. Not only does such a frequency- separated local area deployment avoid the inter- Figure 1. Examples of technology areas considered for LTE evolution. Rel-11Rel-10Rel-9Rel-8 Multi-antenna/sites technologies (3D, MIMO, CoMP, etc.) Direct device-to-device communication Network energy efficiency Rel-13Rel-12 MTC enhancements LTELTE Release-12 WS LTE-BLTE-B Further enhanced local area access LTE-ALTE-A PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 155
  3. 3. IEEE Communications Magazine • July 2013156 layer interference issues present in a same-fre- quency deployment, as extensively discussed in Rel-11, but it also provides some additional ben- efits compared to same-frequency operation. The radio frequency (RF) requirements for a wireless node are a trade-off between perfor- mance and cost. Currently, in 3GPP, the RF requirements for a local area access node are in many respects as stringent as their wide area counterparts. One reason for this is that 3GPP has been assuming that local and wide area deployments may share the same frequency band. Stringent RF requirements (e.g., in terms of adjacent channel suppression) are then need- ed to avoid blocking of the local area node as a result of interference from a terminal located close to the local area node but connected to the wide area layer and transmitting at high output power, possibly on a nearby carrier frequency. However, if the additional frequency band is used for local area access only, it may be possi- ble to relax the RF requirements for local area access nodes, opening up a potential reduction in equipment cost. Frequency-separated deployments also allow for different duplex schemes in the wide and local area layers. In general, time-division duplex (TDD) is expected to become more important with an increased interest in local area deploy- ments compared to the situation for wide area deployments to date. For example, an existing wide area frequency-division duplex (FDD) net- work could be complemented by a local area layer using TDD. To better handle the high traf- fic dynamics in a local area scenario, where the number of terminals transmitting to/receiving from a local area access node can be very small, dynamic TDD is beneficial. In dynamic TDD, the network can dynamically use resources for either uplink or downlink transmissions to match the instantaneous traffic situation, which leads to an improvement in end-user performance compared to the conventional static split of resources between uplink and downlink. WIDE AREA/LOCAL AREA INTERACTION: SOFT CELL Traditionally, local area nodes operate indepen- dently of the overlaid macro layer and transmit all the signals associated with a cell, including cell-specific reference signals, synchronization signals, and the full set of system information. Furthermore, a terminal communicates with the same node, macro or local area node, in both uplink and downlink directions. However, in scenarios where basic coverage is already available from the wide area layer, per- formance improvements can be obtained by operating the wide and local area layers in a more integrated manner, as discussed by several companies at the 3GPP workshop. In particular, dual connectivity, where the terminal is simulta- neously connected to two network nodes, can provide multiple benefits in addition to through- put aggregation of two nodes, as discussed below. Uplink-downlink separation: Uplink recep- tion may occur in a network node different from that used for downlink transmissions. Uplink- downlink separation is particularly advantageous in a heterogeneous deployment where the node with the strongest received signal at the terminal (and thus typically used for downlink transmis- sions) may not be the node with the lowest path loss to the terminal (and thus not the best node for uplink reception) as the difference in trans- mission power or load between the wide area and local area nodes can be quite large. Mobility robustness: Vital control plane information such as handover commands can be transmitted from the overlaid wide area layer even if the user data is provided by the local area layer. A terminal being connected only to a single node may lose the connection if it moves outside the coverage area of the local area node before the handover procedure is completed, a problem that is avoided if the overlaid wide area layer is responsible for transmitting handover commands. Alternatively, the handover com- mands can be transmitted from both the source and destination nodes. In Fig. 2, dual connectivity is illustrated where the terminal is connected: • To the wide area layer through an anchor connection used for system information, basic radio resource control (RRC) signal- ing, and possibly low-rate/high-reliability user data • To the local area layer through an assisting connection used for large amounts of high- rate user data Preferably, the terminals should support reception of ultra-lean transmissions with the minimum possible amount of overhead on the assisting connection (i.e., without cell-specific reference signals and system information). Ideal- ly, assisting connection transmissions should only occur in subframes where there is data to be transmitted to a terminal. Not only do ultra-lean transmissions result in a very energy-efficient local area layer, which translates into lower operational cost; it also reduces the interference level. This is a critical enabler for very dense local area deployments as the end-user perfor- mance may otherwise also be interference limit- ed at low to medium loads. Obviously, relaxed RF requirements and dynamic TDD can be applied to the assisting connection. The combi- nation of dual connectivity with ultra-lean trans- missions is sometimes referred to as soft cell. Scheduling of transmissions on the anchor and assisting connections are controlled by the wide and local area nodes, respectively. Thus, as there are separate schedulers for the two carriers and the data on the two connections have no time- critical relation, the latency requirements for the interconnection between the layers are relaxed. Figure 2. Soft cell: dual connectivity to the anchor node and assisting node. System information Low-power node Macro node AssistingAnchor PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 156
  4. 4. IEEE Communications Magazine • July 2013 157 WLAN INTERWORKING Wireless local area networks (WLANs) based on the 802.11 family of standards are already used by many operators for offloading cellular net- works and embracing capacity expansion into license-exempt spectrum bands. The LTE speci- fications provide support for WLAN interwork- ing, including seamless as well as non-seamless mobility, at the core network level. The selection of using WLAN or 3GPP access currently resides in the terminal and is imple- mentation-specific. Existing terminals have no information about the network load and typically connect to the WLAN whenever such a network is found, irrespective of which technology would be preferable from an end-user perspective in a particular scenario. Hence, a terminal today may switch to a WLAN even if it is heavily loaded and the LTE connection would provide a signifi- cantly better end-user experience. Therefore, as part of the Rel-12 work, 3GPP will investigate signaling mechanisms to support radio access network control of the radio access technology used by the terminal. Even tighter interaction between WLAN and LTE could also be envi- sioned in a later release, for example, by using WLAN and LTE as the assisting and anchor connections, respectively, in a soft cell scenario. MULTI-ANTENNA ENHANCEMENTS Different types of multi-antenna transmission techniques have been an integral part of the LTE radio access technology since its first release with further enhancements introduced in later releases. More specifically, Rel-10 extended downlink spatial multiplexing by supporting up to eight layers and introduced uplink spatial multiplexing of up to four layers. An important part of this work was the introduction of a more flexible downlink reference signal structure, paving the way for novel antenna arrangements and new transmission techniques such as CoMP. In Rel-11, the extended reference-signal struc- ture is made available also to layer 1/2 (L1/L2) control signaling through the introduction of the enhanced physical downlink control channel (EPDCCH). For Rel-12, multi-antenna support is expect- ed to be further extended. Two areas discussed at the 3GPP workshop were active-array-antenna systems for 3D beamforming, and massive multi- ple-input multiple-output (MIMO) and CoMP enhancements (Fig. 3). Enhancements to chan- nel state information (CSI) feedback [5], such as an enhanced codebook for four base station antenna ports and finer frequency domain gran- ularity, are also candidates for Rel-12. ARRAY ANTENNA SYSTEMS AND BEAMFORMING Active array antenna systems (AASs), where RF components such as power amplifiers and transceivers are integrated with an array of antennas elements, offer several benefits com- pared to traditional deployments with passive antennas connected to transceivers through feeder cables. Not only are cable losses reduced, leading to improved performance and reduced energy consumption, but also installation is sim- plified and the required equipment space is reduced. Active antennas may also be an enabler for more advanced antenna concepts. Two closely related examples brought up at the 3GPP work- shop are massive MIMO [6], where the number of antennas is substantially larger than in current deployments, and elevation beamforming, where- by, using vertically stacked antenna elements, the elevation domain can be further exploited compared to a conventional sector antenna with a cell-specific feeder network. Several techniques are possible, including dynamic terminal-specific downtilt, multi-user MIMO, and vertical sector- ization. The performances of the different approaches are expected to depend heavily on channel and propagation models as well as on the actual deployment scenarios. Accurate chan- nel modeling is therefore crucial for perfor- mance evaluations, and 3GPP will start by developing detailed channel models, followed by the identification and evaluation of different techniques. Essential to making multi-antenna techniques useful in practice is proper specification of rele- vant RF and electromagnetic compatibility (EMC) requirements. It is therefore important to continue the studies initiated in Rel-11 and to define these requirements as well as the corre- sponding test methodology as part of Rel-12. The spatial dimension is a key aspect of AAS and needs to be accounted for, which increases the complexity of the problem and possibly calls for some limited use of over-the-air (OTA) test- ing. Prior to establishing the relevant perfor- mance requirements, the usefulness of defining more advanced features such as elevation beam- forming or very-large-scale MIMO is limited. COORDINATED MULTIPOINT TRANSMISSION/RECEPTION Coordinated multipoint transmission/reception primarily targets reductions in intercell interfer- ence, but can also be used to improve coverage. So far, both uplink and downlink CoMP have been considered in 3GPP. Uplink CoMP is to a large extent implementation-specific with little need for additional specification work although some smaller enhancements improving the Figure 3. Examples of multi-antenna techniques. Massive MIMOElevation beamforming PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 157
  5. 5. IEEE Communications Magazine • July 2013158 uplink orthogonality is part of Rel-11. Downlink CoMP, on the other hand, requires a larger specification effort, primarily in the area of CSI feedback. For this purpose a multipoint CSI feedback framework was introduced in Rel-11. Due to the many open issues around CoMP, including the question of which schemes to use in which scenarios, it is expected that the work in Rel-12 will continue with regard to multipoint CSI feedback and enhancements to the downlink control channels. Furthermore, CoMP with relaxed backhaul requirements needs to be in focus to broaden the practical applicability of CoMP. LEAN CARRIER Energy efficiency is becoming increasingly important in wireless communication. While energy efficiency on the device side has always been of high importance due do limitations in battery capacity, energy efficiency at the network side is now receiving increased attention for sev- eral reasons: • Energy cost is a significant part of the over- all operational costs, expected to increase in the future with increasing energy prices. • Regulatory/perceptional aspects: Govern- ment-imposed requirements on energy con- sumption is another motivating factor, as is marketing and the general “image” of a product — being “green” is in itself a sales argument. • New deployment scenarios may open up with reduced base station energy consump- tion, such as solar-powered base stations with reasonably sized solar panels in areas with no access to the electrical grid. This is of particular interest for spreading mobile broadband services further in rural areas, especially in the developing world. Network energy efficiency is to a large extent an implementation issue. However, specific fea- tures of the specification may enable enhance- ments in energy efficiency. Especially for high-power macro sites, a substantial part of the energy consumption of the cell site is directly or indirectly due to the power amplifier. Further- more, the energy consumption of currently avail- able power amplifiers is far from proportional to the power amplifier output power. Rather, the power amplifier consumes a non-negligible amount of energy even at low output power (e.g., when only limited control signaling is being transmitted within an “empty” cell). Minimizing the transmission of such “always-on” signals is thus essential as it allows base stations to turn off transmission circuitry when there is no data to transmit. To address these issues, a new carrier type, a lean carrier, is considered for Rel-12 [7]. Part of the design has already taken place in 3GPP with transmission of cell-specific reference signals being removed in four out of five subframes and the EPDCCH being used for control signaling. Not only does this improve network energy effi- ciency, but it also reduces the interference level at low to medium loads, allowing for higher end- user throughput and improving system efficien- cy. In addition to possible further enhancements in this direction, the remaining work to allow for standalone operation in deployments when a regular carrier is not available involves idle mode support and system information distribu- tion. Inherent to the lean carrier is the impact on backward compatibility. Not all signals expected by legacy terminals will be transmitted as part of a lean carrier, implying that only newer termi- nals can access the lean carrier. The deployment of a lean carrier may therefore, in practice, pri- marily be limited to frequency bands not yet used by LTE as there are no legacy terminals operating in these bands. MACHINE-TYPE COMMUNICATION The world is developing toward a networked society, where all kinds of devices interact and share information. As a result, a phenomenal growth of communicating devices and traffic in a variety of fields, such as transport and logistics, smart power grids, and e-health, is expected. It is expected that all devices which benefit from net- work connectivity will eventually become con- nected, and the number of connected devices will by far outnumber the human-centric com- munication devices like smart phones and tablets. To prepare for this scenario, the 3GPP workshop identified machine-type communica- tions (MTC) as an important area for future enhancements. Clearly, there are a large number of very dif- ferent scenarios and use cases for MTC (Fig. 4), and it is difficult, if not impossible, to define a single set of requirements. For example, a remote surveillance camera imposes very differ- ent requirements on the network than a cargo tracking application monitoring the location of a container or an application for remote reading of utility meters. The latter two are typical exam- ples where the challenge does not come from the amount of data transmitted but rather from the massive number of devices communicating and the requirements for low costs and very low power consumption in the terminal. Although LTE is already capable of handling a wide range of MTC scenarios today, several companies at the 3GPP workshop proposed fur- ther enhancements and improvements to even better: • Support low-cost and low-complexity device types to match low performance require- ments (e.g., in peak data rates and delay) of MTC applications • Allow for very low energy consumption for data transfers to ensure long battery life- times • Provide extended coverage options for MTC devices in challenging locations • Handle very large numbers of devices per cell Techniques to address low-cost MTC device types have been studied [8] by 3GPP and form the basis for the MTC enhancements in Rel-12. Of the techniques studied, the possibility for device-side half-duplex operation, reduced peak data rate requirements, and reduced bandwidth operation (less than 20 MHz) are particularly interesting. Additional simplifications may be The world is devel- oping toward a networked society, where all kinds of devices interact and share information. As a result, a phe- nomenal growth of communicating devices and traffic in a variety of fields, such as transport and logistics, smart power grids, and e-health, is expected. PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 158
  6. 6. IEEE Communications Magazine • July 2013 159 achievable if single-antenna operation is allowed (in contrast to the current requirement of two- way receive diversity as a baseline) and the maxi- mum uplink transmit power is reduced. However, it must be ensured that coverage and spectral efficiency are not significantly impacted. In order to enable long battery lifetimes, the energy consumption of every data transfer of a device needs to be reduced to a minimum. For devices with infrequent data transmission, ener- gy consumption can be significantly reduced if the cycles for discontinuous reception (DRX) are significantly extended. This enables a device to make use of extended sleeping times while not transmitting data, which minimizes reading of control channels and mobility-related mea- surements. Furthermore, infrequent transmis- sions of small amounts of user data are typically associated with signaling procedures (e.g., for radio bearer establishment). These signaling pro- cedures are sometimes more expensive from a power consumption perspective than the data transfer itself. Simplifying these procedures for infrequent small data transfers can therefore provide significant benefits. In some use cases MTC devices may be locat- ed in challenging locations where LTE coverage is not available with existing network deploy- ments. This could be the case for, say, some smart meters in the basements of buildings. Options for coverage extensions can be achieved by techniques like enhanced multi-antenna tech- niques (e.g., beamforming), more robust trans- mission modes, and repetition and energy accumulation of signals. Since the number of connected machines is expected to grow to very large numbers, mecha- nisms are needed to handle a large number of devices within a single cell. A load control scheme known as enhanced access barring has been standardized for LTE Rel-11 to avoid RAN overload due to too many spontaneous access attempts. In general, signaling for every connected device can result in very high control plane load. Therefore, lightweight signaling pro- cedures are desired that reduce the signaling load per device caused to the network. Note that a lightweight signaling scheme may at the same time improve device battery consumption as described above. DEVICE-TO-DEVICE COMMUNICATION The recent socio-technological trend in proximi- ty-based applications and services, and the increasing market interest in short-range com- munications have triggered the research and standardization communities to explore the potential of device-to-device (D2D) communica- tions [9–12]. Indeed, proximity-based services, associated requirements, and possible technology enablers are currently being studied by 3GPP [12]. Direct D2D communications, sometimes called direct-mode communications, imply that wireless devices in proximity of each other com- municate in a peer-to-peer fashion without the user plane involvement of a cellular infra- structure [10]. For Rel-12, 3GPP has already started to study the feasibility of proximity ser- vices in commercial as well as public safety (PS) use cases. An example of a PS use case is a first responder officer searching for people in need in a disaster situation. Both of these use cases include both the D2D discovery and network assisted D2D communication phases. As illustrated in Fig. 5, one of the key aspects of D2D communications is the set of spectrum bands in which the communications takes place. Various ad hoc and personal area networking technologies utilizing unlicensed spectrum bands such as the industrial, scientific, and medical (ISM) bands are available for short-range commu- nications, including Bluetooth and WiFi Direct. Although such technologies can operate without any infrastructure assistance (lower left of Fig. 5), future ad hoc LANs operating in unlicensed bands could also benefit from a cellular infrastructure providing node synchronization and assisting secu- rity procedures (lower right). Cellular-controlled short-range communications for cooperative peer- to-peer networking have been shown to improve the performance and energy/resource efficiency of ad hoc networks [10]. Direct D2D communications utilizing licensed (in particular cellular) spectrum has only recent- ly been proposed and studied. In such a scheme, terminals in proximity of each other can exchange information over a direct link rather than transmitting and receiving signals through a cellular base station [11, 12]. Device-to-device communications in cellular spectrum supported by a cellular infrastructure (upper right of Fig. 5) holds the promise of three types of gains pro- vided that radio resources are properly man- aged. The proximity of terminals may allow for extremely high bit rates, low delays, and low power consumption. The reuse gain implies that radio resources may be simultaneously used by cellular as well as D2D links, tightening the reuse factor even of a reuse-1 system. Finally, the hop gain refers to using a single link in the direct D2D mode compared to two hops when the communication is routed via a base station. All these gains can be realized by taking advan- tage of the LTE cellular infrastructure [13]. Figure 4. Machine-type communication. Safety and security ApplicationsEducationTransport 3GPP core network 3GPP RAN EnergyHealthUtilities PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 159
  7. 7. IEEE Communications Magazine • July 2013160 Peer and service discovery is a major issue in D2D communications. Prior to direct communi- cation between two devices, the devices must learn (discover) that they are near each other. Peer discovery without network support is typical- ly time and energy consuming, employing beacon signals and sophisticated scanning and security procedures often involving higher layers and/or interactions with the end user. In network-assisted design, it is a goal to make such peer discovery and pairing procedures faster, more efficient in terms of energy consumption, and more user friendly. Assistance from LTE networks in terms of synchronization, beacon signal configuration, reserving peer discovery resources, and providing identity and security management can help the discovery process of devices irrespective of the subsequent D2D communications taking place in cellular spectrum or using non-cellular technolo- gies, such as Bluetooth or WiFi Direct. Recogniz- ing the potential advantages of cellular network support, the 3GPP study item for Rel-12 explicitly solicits studies on the gains of network controlled discovery and communications [14]. For PS use cases identified for Rel-12, 3GPP is currently studying the feasibility of solutions based on D2D communications in situations in which parts of the network have become unavailable or dysfunctional. In such cases, D2D communications in licensed or dedicated spectrum bands (upper left of Fig. 5) can become a viable technology. CONCLUSIONS This article has provided a high-level overview of some of the major technology areas considered for the further evolution of LTE, including addi- tional features targeting local area access and het- erogeneous deployments, multi-antenna enhancements, energy efficiency enhancements, further improved support for machine-type com- munication, and different aspects of direct device- to-device communication. Clearly, LTE is a very flexible platform, continuously evolving to address new requirements and additional scenarios. REFERENCES [1] E. Dahlman, S. Parkvall, and J. Sköld, 4G – LTE/LTE- Advanced for Mobile Broadband, Academic Press, 2011. [2] Ericsson Mobility Report, 2013; http://www.ericsson. com/res/docs/2013/ericsson-mobility-report-june-2013.pdf [3] C. Zhang, S. Lek Ariyavisitakul, and M. Tao, “LTE- Advanced and 4G Wireless Communications: Part 2,” IEEE Commun. Mag., vol. 50, no. 6, June 2012. [4] 3GPP RWS-120045, “Summary of 3GPP TSG-RAN Work- shop on Release 12 and Onward,” http://3gpp.org/ftp/ workshop/2012-06-11_12_RAN_REL12/Docs/RWS- 120045.zip [5] RP-120413, “Further Downlink MIMO Enhancement for LTE-Advanced,” 3GPP RAN Plenary, Xiamen, China, Feb. 28–Mar. 2, 2012 [6] F. Rusek et al., “Scaling up MIMO: Opportunities and Challenges with Very Large Arrays,” submitted for pub- lication, http://arxiv.org/abs/1201.3210. [7] 3GPP RP-121415, “New Carrier Type for LTE,” http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_57/Do cs/RP-121415.zip. [8] 3GPP TR 36.888, “Study on Provision of Low-Cost Machine-Type Communications (MTC) User Equipments (UEs) Based on LTE.” [9] M. Scott Corson et al., ”Toward Proximity-Aware Internet- working,” IEEE Wireless Commun., Dec. 2010, pp. 26–33. [10] F. H. P. Fitzek, “Cellular Controlled Short-Range Com- munication for Cooperative P2P Networking,” Wireless World Research Forum 17 WG5, 2010. [11] K. Doppler et al., “Device-to-Device Communication as an Underlay to LTE-Advanced Networks,” IEEE Com- mun. Mag., vol. 7, no. 12, 2009, pp. 42–49. [12] G. Fodor et al., ”Design Aspects of Network Assisted D2D Communications,” IEEE Commun. Mag., Mar. 2012, pp. 170–77. [13] M. Belleschi, G. Fodor, and A. Abrardo, “Performance Analysis of a Distributed Resource Allocation Scheme for D2D Communications,” IEEE Wksp. Machine to- Machine Commun., Houston, TX, Dec. 9, 2011. [14] 3GPP TR 22.803, “Feasibility Study for Proximity Ser- vices (ProSe).” BIOGRAPHIES DAVID ASTÉLY [M] received his Ph.D. degree in electrical engineering from the Royal Institute of Technology, Stock- holm, Sweden, in 1999. From 1999 to 2001, he was with Nokia Networks, and since 2001 he has been with Ericsson. He is currently with Ericsson Research as a technical coordi- nator working on research and standardization of future cellular technologies. He received an IEEE Signal Processing Society Best Young Author Paper Award in the area of sig- nal processing for communications in 2000. ERIK DAHLMAN joined Ericsson Research in 1993 and is cur- rently a senior epert in the area of radio access technolo- gies. He has been deeply involved in the development and standardization of 3G radio access technologies (WCDMA/ HSPA) as well as LTE and its evolution. He is part of the Ericsson Research management team working on long- term radio access strategies. He is also co-author of the books 3G Evolution — HSPA and LTE for Mobile Broad- band and 4G — LTE/LTE-Advanced for Mobile Broadband, and, together with Stefan Parkvall, received the Stora Teknikpriset in 2009 for his contributions to the standard- ization of HSPA. He holds a Ph.D. from the Royal Institute of Technology. GABOR FODOR [SM] received a Ph.D. degree in teletraffic the- ory from the Budapest University of Technology and Eco- nomics in 1998. Since then he has been with Ericsson Research, Kista, Sweden. He is currently a master researcher specializing in modeling, performance analysis of, and pro- tocol development for wireless access networks. He has published around 50 papers in reviewed conference pro- ceedings and journals, and holds about 20 patents (grant- ed or pending). He has been one of the Chairs and organizers of the IEEE Broadband Wireless Access Work- shop series since 2007. STEFAN PARKVALL [SM] is currently a principal researcher at Ericsson Research, working in research on future radio access. He is one of the key persons in the devel- opment of HSPA, LTE, and LTE-Advanced radio access. He is a co-author of the popular books 3G Evolution — HSPA and LTE for Mobile Broadband and 4G — LTE/LTE- Advanced for Mobile Broadband. In 2009, he was co- recipient of the prestigious Stora Teknikpriset (Sweden’s major technology award) for his work on HSPA. He received his Ph.D. degree in electrical engineering from the Royal Institute of Technology in 1996. His previous positions include assistant professor in communication theory at the Royal Institute of Technology and a visit- ing researcher at the University of California, San Diego. Figure 5. Direct device-to-device communication scenarios can be categorized in terms of the utilized spectrum resources and the involvement of various network entities such as a cellular base station. Licensed spectrum (D2D-dedicated or cellular, controlled interference) Unlicensed spectrum (unpredictable interference) No network assistance Network assistance PARKVALL LAYOUT_Layout 1 6/26/13 11:32 AM Page 160

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