4G LTE-Advanced Technology Overview
4G LTE refers to the evolved version of LTE that is being developed by 3GPP to meet or exceed the requirements of the International Telecommunication Union (ITU) for a true fourth generation radio-communication standard known as IMT-Advanced. 4G LTE, whose project name is LTE-Advanced, is being specified initially in Release 10 of the 3GPP standard, with a functional freeze targeted for March 2011. The 4G LTE standard will continue to be developed in subsequent releases. In October 2009, the 3GPP Partners formally submitted LTE-Advanced to the ITU as a candidate for 4G IMT-Advanced1. The certified technology specifications for IMT-Advanced are expected to be published in early 20112.
Key ITU requirements for IMT-Advanced that 4G LTE will support include the following3:
- A high degree of common functionality worldwide while retaining the flexibility to support a wide range of local services and applications in a cost-efficient manner
- Compatibility of services within IMT and with fixed networks
- Capability for interworking with other radio systems
- High quality mobile services
- User equipment suitable for worldwide use
- User-friendly applications, services, and equipment
- Worldwide roaming capability
- Enhanced peak data rates to support advanced mobile services and applications (100 Mbps for high mobility and 1 Gbps for low mobility)
3GPP cites an additional reason to align 4G LTE with IMT-Advanced: IMT-conformant systems will be candidates for any new spectrum bands identified by the WRC-07 (World Radiocommunication Conference). 4
3GPP Technical Report (TR) 36.913, "Requirements for Further Advancements for E-UTRA (LTE-Advanced)," describes the requirements for further advancement of the LTE E-UTRA (air interface) and E-UTRAN (air interface network).5 These requirements are based on those of the ITU for IMT-Advanced as well as on 3GPP operators' own requirements for advancing LTE. Major technical considerations for 4G LTE development include:
- Continual improvement to the LTE radio technology and architecture
- Scenarios and performance requirements for interworking with legacy radio access technologies
- Backward compatibility of LTE-Advanced with LTE. An LTE terminal should be able to work in an LTE-Advanced network and vice versa. Any exceptions will be considered by 3GPP.
- Account taken of recent WRC-07 decisions for new IMT spectrum as well as existing frequency bands to ensure that LTE-Advanced geographically accommodates available spectrum for channel allocations above 20 MHz. Also, requirements must recognize those parts of the world in which wideband channels are not available.
System performance requirements
The system performance requirements for 4G-LTE will in most cases exceed those of IMT-Advanced. The 1 Gbps peak data rate required by the ITU will be achieved in 4G LTE using 4x4 MIMO and transmission bandwidth wider than approximately 70 MHz. In terms of spectral efficiency, today's LTE (Release 8) satisfies the IMT-Advanced requirement for the downlink, but the bps/Hz must be doubled in LTE-Advanced to meet the 4G requirement.
Table 1 compares the spectral efficiency targets for LTE, LTE-Advanced, and IMT-Advanced. Note that the peak rates for LTE-Advanced are substantially higher than the IMT-Advanced requirements, which highlights a desire to drive up peak performance in 4G LTE, although targets for average performance are closer to ITU requirements. However, TR 36.913 states that targets for average spectral efficiency and for cell-edge user throughput efficiency should be given higher priority than targets for peak spectral efficiency and other features such as VoIP capacity.5 Thus 4G LTE work will be focused on the challenges of raising average and cell-edge performance.
Table 1. Performance targets for LTE, Advanced-LTE, and IMT-Advanced6
The flexibility of spectrum is another important consideration. Actual available spectra differ by region and country. 3GPP is studying the feasibility of various deployment scenarios for spectrum allocation.
3GPP solution proposals
Proposed solutions for achieving LTE-Advanced performance targets are defined in 3GPP TR 36.814, "Further Advancements for E-UTRA Physical Layer Aspects."7 The following features are supported in LTE-Advanced proposals:
- Carrier and spectrum aggregation--The lack of contiguous spectrum for wider transmission bandwidths (to 100 MHz) forces the use of carrier aggregation to meet peak data rate and spectrum flexibility requirements. Aggregation of contiguous and non-contiguous component carriers is allowed.
- Enhanced uplink multiple access--The addition of N-times DFT-spread OFDM (also known as "clustered SC-FDMA") will satisfy increased data rate requirements while maintaining backward-compatibility with LTE.
- Higher order MIMO transmission--Up to 8x8 MIMO in the downlink and 4x4 MIMO in the uplink is used to reach peak data rates. Beamforming with spatial multiplexing is being considered to increase data rates, coverage, and capacity.
- Coordinated multipoint (CoMP) transmission and reception--This MIMO variant is intended to improve performance for high data rates, cell‐edge throughput, and system throughput. Is being studied for Release 11.
- Relaying--In-channel relays receive, amplify, and retransmit downlink and uplink signals to improve coverage. More advanced relaying enables the use of some subframes in a channel to carry backhaul traffic. The main use cases for relays are to improve urban or indoor throughput, to add dead zone coverage, or to extend coverage in rural areas.
Other proposals related to 4G LTE address the support needs of an increasingly heterogeneous network that combines macro-, micro-, pico-, and femtocells, along with repeaters and relay nodes. Work is ongoing to develop advanced methods of radio resource management including new self-optimizing network (SON) features. The 4G LTE specifications also continue to focus on the use of femtocells and home base stations (eNBs) as a means of improving network efficiencies and reducing infrastructure costs.
Industry-supported field trials are already demonstrating the viability of many of the technical concepts in LTE-Advanced, and 3GPP's submission to the ITU included a self-evaluation of its proposals concluding that LTE-Advanced meets all IMT-Advanced requirements for being officially certified as 4G. Nevertheless, the timing of 4G LTE deployment is difficult to predict and will likely be dependent on industry demand and the success of today's Release 8 and 9 LTE rollouts.
Design and test challenges
As an evolution of LTE, LTE-Advanced will pose similar challenges to design and test engineers. The LTE standard is new and quite complex, with multiple channel bandwidths, different transmission schemes for the downlink and uplink, both frequency and time domain duplexing (FDD and TDD) transmission modes, and use of MIMO antenna techniques. LTE and LTE-Advanced will likely have to co-exist with 2G and 3G cellular systems for some time, so interworking necessities and potential interference remain important issues. In a difficult radio environment, LTE sets the bar for performance targets very high, and LTE-Advanced raises it even higher.
Some new challenges are anticipated with the solutions proposals for LTE-Advanced. For example, carrier aggregation will undoubtedly pose major difficulties for the UE, which must handle multiple simultaneous transmit and receive chains. The addition of simultaneous non-contiguous transmitters create a highly challenging radio environment in terms of spur management and self-blocking. Simultaneous transmit or receive with mandatory MIMO support will add significantly to the challenge of antenna design.
The introduction of clustered SC-FDMA in the uplink allows frequency selective scheduling within a component carrier for better link performance, and the PUCCH and PUSCH can be scheduled together to reduce latency. However, clustered SC-FDMA increases peak to average power ratio (PAR) by several dB, adding to transmitter linearity issues. Simultaneous PUCCH and PUSCH also increase PAR. Both features create multi-carrier signals within the channel bandwidth and increase the opportunity for in-channel and adjacent channel spur generation. Test tools will need to be enhanced with capability for signal generation and analysis of multicarrier signals in 4G power amplifiers.
Higher order MIMO will increase the need for simultaneous transceivers in a manner similar to carrier aggregation. However, MIMO has an additional challenge in that the number of antennas will multiply, and the MIMO antennas will have to be de-correlated. It will be especially difficult to design multiband, MIMO antennas with good de-correlation to operate in the small space of a 4G UE. Conducted testing of higher order MIMO terminals will no longer be usable for predicting actual radiated performance in an operational network. A study item in Release 9 of the 3GPP standard is looking at MIMO over the air testing as an alternative.
From the UE perspective, relaying is completely transparent. In this case the challenge is all on the network side. For the system to work, the link budget from the relay node to the macro eNB must be good, which implies line of site positioning. The main operational challenge in getting relaying to work will be in the management of the UE. The UE must be instructed to hand over to a relay node that is within range and release the relay node when the UE goes out of range. If this process is not well managed, the performance of the cell could actually go down, not up as intended.
These are just a few of the challenges that 4G LTE will present wireless design and test engineers. As the 4G specifications are published and the certification process moves ahead, so too will test vendors have to increase the capability of their products and invent ingenious new ways to verify the performance of the evolving 4G systems.
1 Press release, "3GPP Partners propose IMT-Advanced radio," Geneva, October 8, 2009.
2 3GPP Americas, "Mobile Broadband Innovation Path to 4G: Release 9, Release 10 and Beyond," www.3Gamericas.org
3 ITU-R M [IMT-TECH], "Requirements related to technical performance for IMT-Advanced radio interface(s)," August 2008.
4 3GPP web site, 3GPP - LTE-Advanced. 3GPP - LTE-Advanced
5 3GPP TR 36.913 V9.0.0 (2009-12), www.3gpp.org/ftp/Specs/archive/36_series/36.913
6 Rumney, Moray. LTE and the Evolution to 4G Wireless: Design and Measurement Challenges, p.416. 2009, Keysight Technologies Publication.
7 3GPP TR 36.814, www.3gpp.org/ftp/Specs/archive/36_series/36.814/