With 5G research progressing at a rapid pace, the standardization process has started in 3GPP. As the most prevalent mobile broadband communication technology worldwide, LTE constitutes an essential piece of the 5G puzzle. As such, its upcoming releases (Rel-14 and Rel-15) are intended to meet as many 5G requirements as possible and address the relevant use cases expected in the 5G era.
Authors: Oumer Teyeb, Gustav Wikström, Magnus Stattin, Thomas Cheng, Sebastian Faxér, Hieu Do
Terms and abbreviations
Since its first commercial deployment by TeliaSonera in December 2009 , LTE has become one of the most successful mobile communication technologies worldwide. Currently, there are 537 commercial LTE networks deployed in 170 countries with 1.7 billion subscribers – a number that is expected to rise to a staggering 4.6 billion by 2022 .
In the seven years that have passed since the launch of LTE, major advances have been made in terms of both performance and versatility. For example, LTE Rel-8 introduced a 20MHz bandwidth with peak downlink (DL) data rates of 300Mbps and uplink (UL) data rates of 75Mbps . Minor expansions were made for Rel-9, such as multicast/broadcast services, location-based services and dual layer beamforming. LTE Rel-10, also known as LTE-Advanced, introduced several new features such as carrier aggregation (CA) to provide up to 100MHz bandwidth as well as enhanced support for multi-antennas, heterogeneous deployments and relaying . These features enabled peak data rates in excess of 1Gbps in DL and 500Mbps in UL.
Rel-11 and Rel-12 included enhancements such as the support of machine type communications (MTC), dual connectivity (DC), LTE-WLAN radio interworking, and national security and public safety (NSPS) services including direct device-to-device (D2D) communication . Further advances were made in Rel-13, including spectral efficiency enhancements via Full Dimension multiple-input, multiple-output (FD-MIMO), support for utilizing unlicensed spectrum via Licensed Assisted Access (LAA) and LTE-WLAN aggregation, extended support for MTC through Narrowband Internet of Things (NB-IoT) and enhanced MTC (eMTC), enhanced CA (up to 32 carriers), indoor positioning enhancements, and single-cell-point-to-multipoint (SC-PTM) for broadcast/multicast services .
Since October 2015, 3GPP has used the term LTE-Advanced Pro for Rel-13 and onwards, signifying that LTE has reached a maturity level that not only addresses enhanced functionality/ efficiency but also the support of new use cases.
Global mobile data traffic is expected to grow at a compound annual rate of 45 percent in the coming years, which represents a tenfold increase between 2016 and 2022 . This increase is driven largely by the massive adoption of mobile video streaming. On top of that, the IoT is shifting from vision to reality, and of the 29 billion connected devices it is expected to include by 2022, 18 billion will be IoT (or machine-to-machine) devices . Future 5G networks will need to support these challenging new use cases in a cost and energy efficient manner.
Although the requirements for 5G capabilities are still being finalized both in the ITU  and 3GPP, there is a preliminary agreement regarding the three main use cases the technology must support. As illustrated in Figure 1, they are: enhanced mobile broadband (eMBB), ultra-reliable low latency communications (URLLC) and massive machine type communications (mMTC). eMBB refers to the extended support of conventional MBB through improved peak/average/cell-edge data rates, capacity and coverage. URLLC is a requirement for emerging critical applications such as industrial internet, smart grids, infrastructure protection, remote surgery and intelligent transportation systems (ITSs). Last but certainly not least, mMTC is necessary to support the envisioned 5G IoT scenario with tens of billions of connected devices and sensors.
There are two tracks that make up the 5G radio access roadmap in 3GPP, as illustrated in Figure 2. One is based on the evolution of LTE and the other on New Radio (NR) access. In the LTE-5G track, enhancements will continue to enable it to support as many 5G requirements and use cases as possible. Unlike the LTE-5G track, the NR-5G track is free from backward compatibility requirements and thereby able to introduce more fundamental changes, such as targeting spectrum at high (mm-wave) frequencies. However, NR is being designed in a scalable manner so it could eventually be migrated to frequencies that are currently served by LTE.
While the prospects for the NR-5G track are exciting, the operators that have already made significant investments in LTE do not need to be concerned – a transition from LTE to 5G through 5G plug-ins is the most logical course of action. Both the expectations for LTE Rel-14  – which is scheduled for completion in March 2017 – and the strong ambitions for LTE Rel-15 indicate that the development plans for the LTE-5G track are solid.
The process of making LTE 5G-ready involves a variety of enhancements and new features in Rel-14 and Rel-15. The most significant ones are enhancements to user data rates and system capacity with FD-MIMO, improved support for unlicensed operations, and latency reduction in both control and user planes (UPs). The enhancements in Rel-14 and Rel-15 also aim to provide better support for use cases such as massive MTC, critical communications and ITS.
User data rate and system capacity enhancements
FD-MIMO and unlicensed operations are the two main features in the upcoming releases of LTE that are intended to bring about improved user data rates and system capacity that meet 5G standards.
The MIMO enhancement in 3GPP makes it possible to dynamically adapt transmission both vertically and horizontally by utilizing a steerable two-dimensional antenna array. The concept of FD-MIMO in future LTE releases builds on the channel state information (CSI) feedback mechanisms introduced in LTE Rel-13, in which precoding matrix codebooks support two-dimensional port layouts with up to 16 antenna ports. Non-precoded CSI reference signals (CSIRSs) are transmitted from each antenna and broadcast in the cell, and the precoder is derived by the terminal. LTE Rel-13 also introduced another CSI feedback type with terminal-specific, beamformed CSI-RS, in the same fashion as physical downlink shared channel (PDSCH). In this case, the beamforming direction for each terminal is decided by the base station rather than being derived from terminal feedback.
To enhance both non-precoded and beamformed CSI-RS operation, Rel-14 will introduce several new features, including hybrid non-precoded/beamformed CSI mode with optimized feedback; aperiodic triggering of CSIRS measurements; support for up to 32 antenna ports; spatially rich, advanced CSI feedback; and a semi-open-loop transmission scheme.
Hybrid non-precoded and beamformed CSI mode with optimized feedback will make it possible to intermittently transmit an initial, non-precoded CSI-RS. The terminals can then respond with a desired direction for a second, more frequent, beamformed CSI-RS.
Aperiodic triggering of CSI-RS measurements facilitates CSI-RS resource pooling, enabling the efficient use of measurement resources and the reduction of CSI-RS overhead. As a result, more terminals in the cell will have access to beamformed CSI-RS operation.
Support for 32 antenna ports makes it possible to use feedback-based operation with massive antenna setups, which increases the gains from multi-user MIMO (MU-MIMO).
Spatially rich, advanced CSI feedback will include information about multiple channel propagation paths, so that interference between co-scheduled terminals can be avoided or suppressed. Performance is then comparable to reciprocity-based massive MU-MIMO systems.
The semi open-loop transmission scheme combines full-dimension beamforming and transmit diversity, targeting high-speed terminals where a beam direction is known but short-term CSI changes too quickly.
The anticipated improvement in system capacity and user throughput with Rel-14 FD-MIMO is illustrated in Figure 3 – a 3GPP 3D urban micro scenario featuring 8×4 dual polarized array and non-full-buffer traffic. Performance on the cell edge increases roughly 2.5 times with advanced CSI feedback and support for 32 antenna ports.
LTE operations in unlicensed spectrum
To address ever increasing traffic demands, many network operators are considering complementary use of unlicensed spectrum. LAA was introduced in LTE Rel-13 for DL operation, and it is being enhanced in Rel-14 to support UL. LAA uses CA to combine a licensed band primary cell (PCell) with unlicensed band secondary cells (SCells). The SCells usually have restricted transmission power, however, which results in coverage areas that are smaller than those that PCells are able to provide. In this arrangement, a PCell provides reliable coverage for control messages and high-priority traffic, while the SCells provide a large amount of spectrum and high data rates when available. Figure 4 shows how LAA offers a combination of the main benefits provided by both licensed and unlicensed spectrum.
Several solutions have been incorporated into 3GPP to achieve coexistence with other technologies – such as WLAN – that operate in the same band as LAA. These include dynamic carrier measurement/selection, Listen-Before-Talk protocol, and discontinuous transmission with limited maximum duration. Smart and adaptive traffic management between licensed and unlicensed carriers – and between unlicensed carriers – could also further enhance coexistence. Figure 5 shows the network capacity in an LAA outdoor coexistence scenario where each of two operators deploy four LAA or four WLAN nodes per hotspot . The LAA cells support substantially higher offloading capacity on the same 20MHz channel compared with the WLAN
nodes. This is because the robust LAA physical layer design allows reliable and efficient frequency reuse. In fact, the more efficient LAA network leaves more capacity for the co-channel WLAN.
Further LAA enhancements are expected in LTE Rel-15, most notably UL control information transmission and random access channel support on the unlicensed band SCells. This would make it possible to offload more traffic from the licensed band PCells and allow for further deployment as well as enabling use cases such as fiber connected remote radio heads.
Another potential enhancement in LTE Rel-15 is dual connectivity between licensed band main evolved node B (eNB) and unlicensed band secondary eNB. This would further broaden deployment possibilities by allowing aggregation between network nodes that are not connected via low-latency backhaul. Finally, Rel-15 may enable more deployment options and scenarios, such as standalone and mMTC operations in unlicensed spectrum.
Another important aspect of LTE enhancement is the implementation of latency reduction techniques for the user and control planes (UPs and CPs). Latency reduction not only contributes to data rate enhancements but also enables new use cases such as critical communication and ITS.
User plane latency reduction
Implementing fast UL access is the first step toward reducing UP latency. As specified in Rel-14, fast UL access makes it possible to configure a terminal with an uplink grant available in each millisecond, to be used only when there is uplink data to transmit. Using the current scheduling request (SR) based access, the terminal must transmit a request, wait for a grant, and then wait to use the grant. A comparison of fast UL access with SR access is illustrated in the a and b tracks of Figure 6. The pre-configured grant in fast UL access minimizes the waiting time, which reduces the average radio access delay for uplink data by more than half.
The other latency reduction step consists of two enhancements that are both targeted for specification in Rel-15. The first is reduced processing time: making the terminal respond to downlink data and uplink grants in three milliseconds instead of four. The second is the introduction of shorter transmission time intervals (TTIs): speeding up the whole chain of waiting for a transmit opportunity, scheduling and preparing for a transmission, transmitting the data, and ultimately processing the received data and sending feedback.
With a short TTI, as illustrated in the c track of Figure 6, transmissions can be made with a shorter duration (as little as one-seventh of the length of a normal LTE TTI). Each of these short transmissions can be scheduled separately with a new DL in-band control channel, with feedback sent in a new UL control channel. The scheduling and feedback are sent in adjacent subframes for the shortest transmission time, resulting in a total radio access one-way transmission delay of about 0.5ms, including data processing time.
Figure 7 illustrates the gains in round-trip time (RTT) made by employing short TTI and fast UL access. From simulations, improvements have also been observed in the throughput for File Transfer Protocol (FTP) download by up to 70 percent: an effect caused by a faster TCP bitrate ramp-up thanks to the shorter RTT of data and response.
LTE state transitions involve significant signaling: going from RRC_IDLE to RRC_CONNECTED comprises 9 transmissions over the air interface. Two options for signaling reduction were introduced in Rel-13: RRC connection suspend/ resume for use with UP based data transfer over data radio bearers (DRBs) and data over non-access stratum (DoNAS) for CP-based data transfer over the signaling radio bearer (SRB).
The suspend/resume feature allows the data connection to be suspended temporarily and the context to be stored in the RAN and core network (CN) during RRC_IDLE. At the next transition to RRC_CONNECTED, the connection is resumed with the stored context, significantly reducing the signaling to four or five transmissions. The DoNAS feature achieves a similar reduction of signaling by omitting access stratum (AS) security and by transferring data over the CP instead of establishing traditional UP radio bearers.
To accommodate the ever increasing number of devices, small and/or infrequent data volumes and stricter delay requirements, Rel-14 and Rel- 15 aim for further reduction of signaling between terminals and network nodes (RAN and CN).
In Rel-14, the suspend/resume feature is being improved by reducing the signaling between the base station (BS) and the CN. In Rel-13, the BS-CN connection was released together with the air interface connection. In Rel-14, the BS-CN connection can be kept when the BS-terminal connection is suspended. The RAN takes over the responsibility of paging the terminal upon the arrival of DL data, for example.
Two additional control plane latency reduction improvements are expected in Rel-14 or Rel-15. The first is an enhancement that would enable earlier data transmission by making it possible to multiplex UP radio bearer data with connection resume signaling. The second is known as release assistance indication, which would allow the terminal to indicate that it has no more UL data and that it does not anticipate DL data, thereby enabling early transition to RRC_IDLE.
New use cases for 5G
A number of improvements in LTE Rel-14 and Rel- 15 are designed to provide improved support for use cases such as massive MTC, critical communications and ITS.
Massive machine type communications
LTE MTC and NB-IoT were developed to address mMTC use cases . They offer similar improvements with regard to coverage enhancement, battery life, signaling efficiency and scalability, but address slightly different demands in terms of flexibility and performance. As shown in Figure 8, LTE MTC is more capable of supporting higher data rates and both intra-RAT and inter-RAT connected mode mobility. With the new LTE MTC Category M1 (Cat-M1) and NB-IoT, which were specified in 3GPP Rel-13, it is anticipated that modem cost can be drastically reduced compared with Rel-8 Cat-1 devices. Cost will vary depending on features, options and implementation. Modem cost reductions are expected to be in the order of 75-80 percent for Cat-M1  and even more for NB-IoT with its further reduced feature set.
LTE Rel-14 aims to further enhance LTE MTC and NB-IoT by improving performance and addressing more use cases. Higher data rates and efficiency will be achieved in Rel-14 by allowing larger chunks of data to be carried in each transmission and increasing the number of hybrid automatic repeat request (HARQ) processes to enable parallel outstanding transmissions while waiting for feedback. Larger channel bandwidth for LTE MTC (up to 5MHz) enhances support for voice and audio streaming as well as other applications and scenarios. NB-IoT enhancements for random access and paging increase the versatility of non-anchor carriers.
Rel-14 will further enable positioning applications (in which knowledge of device location is critical) by supporting enhanced reference signals that take into account the smaller NB-IoT/LTE MTC bandwidth. Enhancements to connected mode mobility will improve service continuity. Multicast transmission will make the delivery of the same content to multiple devices more efficient, optimizing use cases such as firmware upgrades and synchronous control of things like streetlights, for example. Support for the lower NB-IoT power class of 14dBm will enable the use of smaller batteries and support devices with a small form factor.
Voice coverage for LTE MTC will be improved in Rel-14 by increasing VoLTE coverage for half-duplex FDD/TDD through techniques that reduce DL repetitions, new repetition factors, and adjusted scheduling delays. MTC devices and use cases will also benefit from the signaling reduction enhancements in LTE Rel-14.
mMTC use cases will also benefit from a few other enhancements in LTE Rel-15, including:
- latency improvements resulting from the multiplexing of user data with connection resume signaling
- efficiency improvements resulting from enhanced access/load control in idle and connected modes
- battery life improvements resulting from relaxed DL monitoring requirements in idle mode
- improved support for additional use cases such as wearables.
Use cases such as power grid surveillance, safety-critical remote control, and critical manufacturing operations require both low latency and high reliability above the current HARQ level (see Figure 9). In order for LTE to meet these 5G requirements, there is an aim for two improvements to be made for Rel-15: reliable short TTI operation and reliable 1ms operation.
By building on the short TTI and fast UL features, the packet error rate can be reduced to a 10-5 level through a combination of robust coding of control and data messages, diversity, and automatic repetitions without feedback. Since the processing is kept on a short timescale, the entire chain of transmissions can be delivered within 1ms with the combined reliability of multiple trials. (The target is small cells, such as factories and offices.) In addition, wide-area coverage with relaxed latency but extreme reliability can also be targeted by automatic repetitions of robustly coded 1ms transmissions with enhanced feedback.
Intelligent transportation systems
The use of ICT to enable safer and more efficient transportation systems is known as ITS. 3GPP has been developing a solution for vehicle-to-everything (V2X) communications for Rel-14, addressing the connection between vehicles (vehicle-to-vehicle or V2V), vehicle-to-network (V2N), vehicle-to-infrastructure (V2I), and vehicle-to-pedestrian (V2P), as illustrated in Figure 10.
LTE-based ITS benefits from the coverage of the existing networks and the centralized security. However, new ITS use cases are demanding in terms of latency and system capacity. Therefore, the direct D2D interface, known as sidelink (SL), and the LTE cellular air interface are being enhanced in Rel-14 to support these requirements. For example, increased pilot symbol density will make it possible to optimize the SL for quickly changing propagation conditions and severe frequency shifts at the receiver due to high relative speed (up to 500km/h) and higher carrier frequency (up to 6GHz).
Improved radio resource management is another important enhancement to support ITS applications. This is based on a sensing-based resource selection protocol, where each device autonomously learns how other devices use the radio resources and predicts their future behavior, taking advantage of the quasi-periodic nature of the ITS messages.
Rel-14 supports the usage of geographical location information to enable centralized resource allocation in the eNB or to autonomously select a resource within a configured radio resource pool. It also supports Multimedia Broadcast/Multicast Service (MBMS) protocols that are optimized for low latency and coverage, and efficient delivery of V2X messages. Finally, the expected enhancements will provide fair and efficient coexistence with non-3GPP ITS technologies such as dedicated short range communications (DSRC).
Figure 11 shows a numerical comparison of the capability of different technologies for broadcasting V2V messages. In typical scenarios (urban and highway), the solutions based on LTE (SL with centralized resource allocation and cellular multicast) perform significantly better than the one based on DSRC.
LTE is well positioned to deliver on all the most important 5G requirements, including user data rate and system capacity enhancements with FD-MIMO, improved support for unlicensed operations, and latency reduction in both user plane and signaling. The improvements planned in Rel-14 and Rel-15 will not only ensure that LTE will provide better support for massive MTC and ITS; they will also enable LTE to address new use cases such as critical communications. In light of this, we are confident that LTE will continue to play a major role in mobile communications for many years to come.
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