Long-Term Evolution [LTE]
The 3GPP long-term evolution [LTE] is the step towards the radio air-interface evolution for 3G technology to deliver “Mobile Broadband”. It is being defined and standardized by the 3GPP to functionally evolve the radio access technology and enhance the performance of 3G technologies to meet user-expectations over long-term i.e, 10 years and beyond. LTE targets to achieve this by improving the 3G coverage, system capacity, data rates and spectrum efficiency. It also aims at reducing the latency and enhance other radio performance parameters while reducing user and operator costs. The above LTE requirements would be fulfilled by the use of new multiple access schemes on the air interface: OFDMA (Orthogonal Frequency Division Multiple Access) in downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) in uplink. Furthermore, Multiple-Input and Multiple Output(MIMO) antenna schemes are used to achieve higher bit-rates. The first section of the article presents the evolution of the 3GPP-LTE, while the second section lists the physical performance targets as defined by the standards. The section that follows presents the technical building blocks and the architecture of the LTE system. The article concludes by discussing the economic target defined by the standards and the current status of the LTE system.
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The large-scale deployment of the Wide-band Code Division Multiplexing (W-CDMA) or the 3G technology across the globe prompted the 3GPP to take steps towards the evolution of the 3G air interface. The High-Speed Downlink Packet Access (HSDPA) was introduced in 3GPP Release 5 to increase the performance of the downlink while High-Speed Uplink Packet Access (HSUPA) was introduced in 3GPP Release 6 to enhance the uplink data rates. HSPA+(High-Speed Packet Access Plus) is being introduced in release 7 to enhance performance of HSPA based radio networks in terms of spectrum efficiency, peak data rate and latency, and exploit the full potential of WCDMA. The characteristics of HSPA+ such as the use of the downlink MIMO (Multiple Input Multiple Output), higher order modulation for uplink and downlink, improvements of layer 2 protocols, continuous packet connectivity and enhanced uplink meet immediate and mid-term needs of the end-users. However the operator and end-users expectations are growing rapidly and alternative competitive access technologies are emerging continuously. To ensure long-term competitiveness of 3G technology, the 3GPP included the “Evolved UTRA and UTRAN “work item in 2004. The aim of the work item is to investigate the means of achieving enhanced service provisioning by improving data rates, capacity, spectrum-efficiency, and latency thereby providing optimum support for packet-switched services.
Physical air-interface Performance Requirements of the 3GPP Long –term Evolution [LTE]:
The requirements for the design of the 3GPP LTE system is prescribed in the 3GPP specification 3GPP TR 25.913 and is summarized as follows:
- Providing significantly higher data rates compared to the existing technology such as the HSDPA and enhanced uplink, with target peak data rates up to 100 Mb/s for the downlink and up to 50 Mb/s for the uplink.
- The capability to provide three to four times higher average throughput and two to three times higher cell-edge throughput when compared to systems based on HSDPA and enhanced uplink as standardized in 3GPP Release 6.
- Increased spectral efficiency upto four-folds compared to 3G technology.
- Improved architecture and signalling to significantly reduce control and user plane latency, with a target of less than 10 ms user plane RAN round-trip time (RTT) and less than 100 ms channel setup delay.
- Support scalable bandwidths of 5, 10, 15, 20 MHz and including bandwidths smaller than 5 MHz for more flexibility. In order to protect the investments already made by the operators, updates and modifications to the existing radio network architecture is being proposed. This involves a smooth migration into other frequency bands, including those currently used for second-generation (2G) cellular technologies such as GSM and IS-95.
- Support for operation in paired (Frequency Division Duplex / FDD mode) and unpaired spectrum (Time Division Duplex / TDD mode) is possible.
- Support for end-to-end Quality of Service
- Support for inter-working between the existing UTRAN/GERAN and other non-3GPP systems. The handover delay between them to be less than 300 milliseconds for real-time services and less than 500 milliseconds for non-real-time services.
- An enhanced Multimedia Broadcast Multicast Service(E-MBMS) shall be supported.
- Reduced capital and operational expense shall be ensured.
- Optimized support for low mobile speeds (0-10 mph) as well as support for high mobile speeds (10 -30 mph).
LTE System Building blocks:
The following technological building blocks enable to meet the LTE system requirements as prescribed by the 3GPP:
Radio Interface Technology:
- In order to meet the requirements of higher data rates, a new radio transmission technology called the Orthogonal Frequency Division Multiplexing (OFDM) has been selected for the downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) for the uplink. In an OFDM system, the available spectrum is divided into multiple carriers, called sub-carriers, which are orthogonal to each other. Each of these sub-carriers is independently modulated by a low rate data stream. Different bandwidths are realized by varying the number of subcarriers used for transmission, while the subcarrier spacing remains unchanged. In this way operation in spectrum allocations of 1.25, 2.5, 5,10, 15, and 20 MHz is supported. OFDM enables transmission adaptation in frequency domain in E-UTRA. OFDM has several benefits including its robustness against multipath fading and its efficient receiver architecture. It is used in WLAN, WiMAX and broadcast technologies.
- In order to achieve higher throughputs and increased spectral efficiency so as to meet the coverage, capacity and data rate requirements, Multiple Input Multiple Output (MIMO) antenna solutions are used by the LTE systems. MIMO refers to the use of multiple antennas at the transmitter and the receiver side. MIMO beamforming could be used to increase coverage and/or capacity, and spatial multiplexing, sometimes referred to as MIMO, can be used to increase data rates by transmitting multiple parallel streams to a single user.
- In order to meet the improved latency requirement, it was required to reduce the number of network nodes involved in data processing and transport. A flatter architecture as prescribed by the standards would lead to improved latency and transmission delay. Figure below depicts a simplified LTE system architecture and it consist of two types of network nodes one at user plane and the other at the control plane.
- Evolved NodeB(eNodeB): It is the enhanced BTS that provides the LTE air interface and performs radio resource management for the enhanced LTE radio interface
- Access Gateway(AGW): It provides the termination of LTE bearer and acts as the mobility anchor point and packet date network gateway for user plane.
- SAE is a study within 3GPP targeting at the evolution of the overall system architecture. The focus of this work is on the packet-switched domain with the assumption that voice services are supported in this domain. This study envisions of an all-IP network  and the support of heterogeneous access networks in terms of mobility and service continuity.
LTE Economic Targets – Benefits to Operators and End-users:
- Performance and capacity of LTE systems as discussed in the earlier sections shall facilitate the provisioning of high-quality multimedia-rich applications. While the users are catered with innovative services, the operators generate revenue from alternate avenues other than from voice.
- Avoidance of complicated architectures and unnecessary interfaces, reuse of existing system and spectrum, efficient operations and management along with the optimized performance by the radio technologies yield an overall reduction cost per bit. This benefit the end-users to access services at a low cost and operators benefit from low OPEX and CAPEX.
Current Status and Future of LTE:
For LTE that offers high-performance radio interface, it requires a high-performance core network inorder to experience commercial success. Impact on the overall network architecture including the core network is being investigated in the context of 3GPP System Architecture Evolution (SAE). It aims at optimizing the core network for packet-switched services and including the IP multimedia subsystem that supports all access technologies. The combined evolution of LTE and SAE forms the basis for the 3GPP release-8. As of today[3GPP website], 3GPP has approved to freeze the functional requirements of LTE as well as SAE as part of release-8.
There is proof of substantial industrial commitment towards LTE deployment in form of contributions and intellectual inputs to the 3GPP LTE specification groups. Also, many recent press announcements from vendors and operators indicate the same.
 3GPP TS25.855, “High Speed Downlink Packet Access;Overall UTRAN Description”, version 5.0.0.
 3GPP TS25.999,”High Speed Packet Access Evolution, Frequency Division Duplex”, version 6.1.0
 3GPP TS 25.913; Requirements for E-UTRA and E-UTRAN(Release 7)
 3GPP, RP-040461, ”Proposed Study Item on Evolved UTRA and UTRAN“, www.3gpp.org.
 H. Ekström et al., “Technical Solutions for the 3G Long-term Evolution”, IEEE Communications Magazine, March 2006.
 E. Dahlman et al. “The 3G Long-Term Evolution – Radio Interface Concepts and Performance Evaluation”, Proceedings of the VTC 2006 Spring.
 3GPP TS 22.978; All-IP Network (AIPN) feasibility study (Release 7)
 3GPP TS 23.882;” 3GPP system architecture evolution (SAE): Report on technical options and conclusions”, Release 7.1.9
 3GPP TS 36.201;” Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description”, Release 8.0.1