Long Term Evolution Advanced - An Introduction
Driving the wireless revolution, the cellular industry has been in the midst of a profound paradigm shift. The shift to mobile broadband has resulted in tremendous usage mobile data service but rather a philosophical shift of end-of-user from relying on personal computers to mobile smartphones, tablets and other wireless gadgets. In preparation for this, 3GPP initiated an investigation into E-UTRA in 2004, since then E-UTRA specified the first release of LTE (Rel-8) in March 2009, and evolves to LTE-Advanced to meet ITU-R IMTAdvanced (also called as 4G) requirements; while LTE-Advanced keeps evolving with introducing new features and supporting higher performance and broader services, including multiple market phases: LTE-A, LTE-B, may be followed by LTE-C and so on in the future, as shown in the LTE evolution roadmap in figure below.
While the current 2013 year LTE-A system can support very high peak rate and peak spectral efficiency with up to eight antenna ports transmission and up to 100MHz carrier aggregation, which however is very demanding for terminals and will result in only slow penetration in the coming years, the upcoming LTE-B would focus on the performance improvement to the typical form factor of terminals that can be popular commercially in the coming years. Besides high spectral efficiency for data services, LTE-A had to prove its capability to support a large number of voice calls through Voice over IP (VoIP). Radio resource management for many VoIP calls continuously switching between active and inactive state is challenging. Control channel limitations prevent the dynamic scheduling of every VoIP connection in each 1 ms subframe. Semi-persistent scheduling enables channel quality aware scheduling accounting for control channel limitations.
Despite the increasing demand for packet data based communication, voice communication will always play an important role in wireless networks. Long Term Evolution (LTE) and LTE-Advanced networks are fully IP-based and do not include a circuit switched domain for voice communication as known from GSM and UMTS networks. Packet switching for voice traffic increases efficiency from multiplexing of connections to the same radio resource. The drawback is the limited capability of a packet switched network to guarantee Quality of Service (QoS) requirements. 4G candidate networks had to prove their performance within the IMT-Advanced (IMT-A) evaluation process specified by ITU-R requiring IMT-A systems to support at least 40 voice calls per MHz bandwidth and cell. A cell must therefore be capable to carry user data and control traffic for 200 VoIP calls at 5 MHz system bandwidth. While the required data rate for voice communication is relatively small compared to other applications, voice traffic requires low packet latencies. As the number of VoIP calls in a system increases, packet loss and delay increase until the number of satisfied users drops below a given percentage defining the VoIP capacity.
LTE-B Requirement and Features
For the increasing traffic demand in the wireless communication systems, the general enhancements in LTE-B contains further multi-antenna improvements with 3D-beamforming, potentially a new carrier type specifically aggregated for being backwards compatible, enhanced CoMP, network assisted interference cancellation and mitigation at UE, etc.
Voice over LTE - VoLTE
The Voice over LTE, VoLTE scheme was devised as a result of operators seeking a standardised system for transferring voice traffic over LTE. Originally LTE was seen as a completely IP cellular system just for carrying data, and operators would be able to carry voice either by reverting to 2G /3G systems or by using VoIP.
Operators, however saw the fact that a voice format was not defined as a major omission for the system. It was seen that the lack of standardisation may provide problems with scenarios including roaming. In addition to this, SMS is a key requirement. It is not often realised, that SMS is used to set-up many mobile broadband connections, and a lack of SMS is seen as a show-stopper by many. As mobile operators receive over 80% of their revenues from voice and SMS traffic, it is necessary to have a viable and standardized scheme to provide these services and protect this revenue.
Options for Voice over LTE
When looking at the options for ways of carrying voice over LTE, a number of possible solutions were investigated. A number of alliances were set up to promote different ways of providing the service. A number of systems were prosed as outlined below:
- CSFB, Circuit Switched Fall Back: The circuit switched fallback, CSFB option for providing voice over LTE has been standardised under 3GPP specification 23.272. Essentially LTE CSFB uses a variety of processes and network elements to enable the circuit to fall back to the 2G or 3G connection (GSM, UMTS, CDMA2000 1x) before a circuit switched call is initiated.
- The specification also allows for SMS to be carried as this is essential for very many set-up procedures for cellular telecommunications. To achieve this the handset uses an interface known as SGs which allows messages to be sent over an LTE channel.
- In addition to this CSFB requires modification to elements within the network, in particular the MSCs as well as support, obviously on new devices. MSC modifications are also required for the SMS over SGs facilities. For CSFB, this is required from the initial launch of CSFB in view of the criticality of SMS for many procedures.
- SV-LTE - simultaneous voice LTE: SV-LTE allows to run packet switched LTE services simultaneously with a circuit switched voice service. SV-LTE facility provides the facilities of CSFB at the same time as running a packet switched data service. This is an option that many operators will opt for. However it has the disadvantage that it requires two radios to run at the same time within the handset. This has a serious impact on battery life.
- VoLGA, Voice over LTE via GAN: The VoLGA standard was based on the existing 3GPP Generic Access Network (GAN) standard, and the aim was to enable LTE users to receive a consistent set of voice, SMS (and other circuit-switched) services as they transition between GSM, UMTS and LTE access networks.
- For mobile operators, the aim of VoLGA was to provide a low-cost and low-risk approach for bringing their primary revenue generating services (voice and SMS) onto the new LTE network deployments.
- One Voice / later called Voice over LTE, VoLTE: The Voice over LTE, VoLTE schem for providing voice over an LTE system utilises IMS enabling it to become part of a rich media solution.
3-Dimension Beamforming (3D Beamforming)
The current multiple antenna transmission specification assumed a passive antenna configuration at the Base Station (BS) with a fixed antenna down tilt, which can dynamically control the beam in the azimuth dimension. Thanks to the recent introduction of Active Antenna System (AAS), which benefits from cost reduction in site engineering and lower cable loss compared to conventional passive antenna systems from BS deployment perspective, it is possible to dynamically control the beam in both azimuth and elevation dimension with the two dimensional antenna array structure, i.e. to perform 3D-beamforming. The use of 3D-beamforming provides finer spatial resolution which can further improve the performance of both SU-MIMO and MU-MIMO. AAS with higher order MIMO may also be feasible in the future, when higher frequency bands are available making the antenna size acceptable in the practical installations, as depicted below.
New Carrier Type (NCT)
Flexible spectrum utilization is a clear trend in the wireless industry. Carrier Aggregation technology introduced in LTE Rel-10 enables non-contiguous spectrum utilization and much more bandwidth support through combining the bandwidth from multiple component carriers. It is possible to further improve the spectrum efficiency of some component carriers with flexible bandwidth utilization and on-demand usage of common channels/signals that are part of the Rel-12 NCT work item. One aspect of NCT, as the consequence of best-effort striving for spectrum utilization flexibility, is the issue of non-backward compatibility. The possible loss of backward compatibility could however make NCT expandable to some new features in Rel-12 and beyond, e.g. eMBMS/small cells/MTC/D2D if these features are supported with dedicated carriers. The NCT may also be used in conjunction with carrier aggregation of a legacy LTE carrier. The possibility of using NCT in a standalone mode will however depend on operators' deployment strategies and UE capability limits.
Enhancement for Coordinated Multi -Point communications (eCoMP) CoMP is one of the most important features in LTE-A for improving the cell-edge user experience. LTE-B will evolve CoMP in both ideal and non-ideal backhaul scenarios. In ideal backhaul scenarios, further enhancement such as CSI-RS based RSRP measurement and uplink sounding and power control enhancement will be introduced. In non-ideal backhaul scenarios, schemes will be developed to deal with the limitation of backhaul when using CoMP in order to get higher cell edge throughput and more efficient mobility management.
Network-assisted Interference Mitigation
To further solve the co-channel interference, either from inter-cell or co-scheduled intra-cell users, advanced receivers can be introduced at UE side that benefit from the knowledge about interfering transmissions under possible coordination by the network. The advanced interference cancellation and mitigation may be applied to the traffic channel as well as the control channel and cell-specific reference signals. Evaluation of the gain and feasibility of those diverse advance receivers would provide the guidance for how to balance the performance improvements and the UE implementation complexity as well as the increased signalling overhead in the air-interface.
LTE-Hi: LTE Hotspot Improvement and small cells
In the mobile broadband network, the highest pressure of data traffic growth is in hotspot and indoor area, where the heterogeneous network is needed with the deployment of small cells on top of Macro cells layer. Taking into account the hotspot and indoor scenario characteristics, LTE-Hi (LTE Hotspot improvement) is proposed to optimize the hotspot and indoor transmission, focusing on small cell spectrum efficiency enhancements, efficient operation with traffic adaptation and interference coordination, HetNet mobility enhancement and multi-stream aggregation, as well as the potential HetNet FDD&TDD joint operation.
Small Cell Spectrum Efficiency Enhancements
Hotspot and indoor scenario specific spectrum efficiency optimization are possible by taking advantage of the corresponding channel characteristics. An optimization is to introduce a higher modulation level such as 256 QAM in downlink, taken the advantage of the high probability of better geometry that UEs experience in small cells, due to the shorter distance between UEs and serving cells. Another potential optimization is to reduce the overhead of control signalling, UE specific reference signal and feedback, taking into account the small time and frequency fluctuation/selectivity in hotspot and indoor small cell scenario.
Small Cell Efficient Operation with Traffic Adoption and Interference Coordination
To solve the challenges of the high capacity and low OPEX of the network deployment, there is a potential large-bandwidth small cell dedicated band for hotspot area capacity boosting on top of Macro cell coverage layer. The network operation efficiency and energy efficiency need further enhancements by taking into account the time-varying traffic load and interference management issues. For efficient energy saving and inter-cell interference coordination, traffic adaptive on/off switching and fast cell selection (FCS) can be introduced for small cells. Specifically for the isolated TDD small cell (cluster), it is possible to further adapt the DL/UL configuration flexibly depending on the instantaneous traffic statistics. To facilitate the small cell self configuration, the mechanisms to efficiently discover the small cells and the corresponding configurations is need to monitor the variation of the neighbours and the interference status. In addition, radio interface based synchronization is required to ensure the synchronized operations of the small cell layer as well as between small cells and the macro layer.
HetNet Mobility Enhancement and Multiple Stream Aggregation
The small cell deployment with small coverage poses challenges on the coordination of radio resource utilizations and mobility management across multiple eNBs, esp. for non-ideal backhaul cases. The Macro-assistant mobility management with a new architecture of the Control Plane (CP) at Macro and User Plane (UP) at small cells would be beneficial to avoid high handover failure rate and bad user experience at the edge of small cells and between small cell and Macro cell. Multi-Stream Aggregation (MSA) extends Carrier Aggregation (CA) and CoMP kind of operations to higher layer (MAC and upward), and makes it possible for the UE to receive traffic streams from multiple transmission nodes, even when non-ideal backhaul is involved. MSA can also facilitate the steering of UE traffic to different nodes based on QoS needs, since a UE can be flexibly served by radio resource of multiple nodes. MSA can be applied to Homogeneous or Heterogeneous networks, with intra-frequency or inter-frequency deployment of multiple nodes.
HetNet FDD/TDD Joint Operation
Considering the severe unbalance and fluctuation of downlink/uplink traffic in each hot area and the higher isolation among hot areas in high-frequency band, TDD is well suited for achieving better resource assignment. For those operators that deploy FDD for macro coverage in low-frequency band, the joint operation of TDD and FDD carriers in the heterogeneous network would be a cost efficient way to make full use of spectrum resources. The good commonality between FDD and TDD design in LTE offers the possibility of the efficient joint operation of FDD and TDD heterogeneous networks. For the UEs with the capability of both TDD and FDD access, its throughput especially at cell edge can be improved by aggregation in either physical layer by CA or higher-layer by MSA, depending on the available backhaul. MSA with dual connectivity to both TDD and FDD carriers is applicable to all deployment scenarios with ideal and non-ideal backhaul.
Multi-RAT Operation Enhancement
With more and more deployed RAT layers, which may include GSM, UMTS, LTE, and WiFi, multi-RAT operation becomes a major challenge. Multi-RAT operation includes multi-RAT mobility management, resource allocation and traffic steering.
LTE/HSPA Interworking Enhancement
In the coming years, many operators will have a joint LTE and UMTS network deployment, which requires seamless LTE/HSPA resource management to accommodate seamless multi-RAT operation to accommodate both traditional voice and SMS services, as well as the high-bitrate packet switched data services with most efficient utilization of resources. Potential improvements include the inter-RAT call redirection, connected mode mobility and load balancing between UMTS/HSPA and LTE. More and frequent information exchange would provide reliable mobility and service continuity for user experience and performance consistency across different RAT. An efficient information coordination would lead to reliable mobility, transparent user experience service continuity and performance consistency across a seamless LTE/HSPA single network.
With the significant increase of mobile broad band traffic, more and more operators have started to deploy WiFi to for cellular network capacity boosting. To enable a unified user experience, 3GPP-WiFi interworking has been developed and introduce at core network level, i.e. higher up and more centralized in the system architecture. Further enhancements by LTE-B provide a better cellular network controlled mobility management, including:
- Facilitate the access and authentication, as well as AP discovery mechanisms to reduce the UE power consumption
- Improve the network selection based on the awareness of RAN/AP load, with always access to the best connection between 3GPP and WiFi
Services and Enablers of New Services
LTE network deployment is going to accelerate in the near term globally, with already well supported voice and data services. LTE-B will do some enhancement on the traditional services like congestion control for Smartphone, positioning. In addition, LTE-B/EPC-A system enhancement is required to support new business opportunities, such as MTC, Proximity based services, and group communication services, etc. Further architecture evolution of EPC in a long term may also be required to support a migration of current network to be more open and flexible.
Traditional Service Enhancements
With the high penetration of smart phones, user plane congestion in RAN is expected to occur more frequently and become a serious problem to operator. To reduce the occurrence of congestion and alleviate congestion in a quick and smooth manner, user plane congestion control (UPCON) is introduced, making it possible for RAN to handle the packets of the services with different QoS requirements differently, and also making the core network aware of the RAN congestion status. With the deployment of CoMP, heterogeneous network and multi-antenna arrays, especially AAS with 3D beamforming capability, the traditional positioning mechanisms will not work well, requiring further enhancement on the LTE positioning solutions of Observed Time Difference Of Arrival (OTDOA) and Enhancement-Cell ID (E-CID).
Machine Type Communications (MTC)
The Machine Type Communication (MTC) is identified as one of the biggest challenge for the telecommunication system evolution. 3GPP has been working in LTE-A phase to guarantee the network robustness facing usage by a massive number of MTC devices in the cellular network. While in LTE-B, the focus is more on better support a large variety of MTC application scenarios. For some smartmetering type of MTC services the low cost MTC devices with very low data rate and power consumption may be introduced. To provide a good coverage of such MTC devices that are installed in the basement of residential buildings, enhancements on control and data channels can be introduced to increase the coverage by up to 20dB for the very low bit rate data access. From the network side, it is beneficial to reduce the overhead and signaling surge caused by transmissions of massive MTC connections with frequent transmission of small data packets.
Proximity based Services (ProSe)
To enabling new innovative applications based on LTE network services, 3GPP has started to study Proximity based Services (ProSe) for both commercial and public safety cases in LTE-B. ProSe includes a discovery for ProSe capable terminals and direct device to device (D2D) communication between such terminals. Network support may be needed, specifically when ProSe operates within licensed spectrum. The functionalities of a network based ProSe may address the reliability, security and service authentication of ProSe.
Group Communication Service for LTE (GCSE_LTE)
Group communications as offered by dedicated systems are typically provided as narrow band services and are used, e.g. for critical communications scenarios like public safety. However users of group or critical communications are also interested in using advanced features and services that require broad band support. LTE-B/EPC-A consider under Group Communication Service for LTE supporting or providing such communications, which have to fulfil the demanding performance, capacity and security requirements of comparable dedicated systems, but also add broadband support under the same conditions.