| Traian Andrei, ta8@cec.wustl.edu (A project report written under the guidance of Prof. Raj Jain) | Download |
This paper offers an introduction to the mobile communication standard known as LTE Advanced, depicting the evolution of the standard from its roots and discussing several important technologies that help it evolve to accomplishing the IMT-Advanced requirements. A short history of the LTE standard is offered, along with a discussion of its standards and performance. LTE-Advanced details include brief history of the standard, technical requirements, as well as analysis on the physical layer, resource control, and performance.
LTE, IMT-Advanced, LTE-Advanced, radio standards, physical layer, spectrum flexibility
LTE-Advanced is a 3GPP standard that describes technological advancements to the Long Term Evolution (LTE) a highly flexible radio interface that aims at bridging the gap between 3rd generation and 4th generation (4G) standards – described in IMT-Advanced (International Mobile Telecommunications) [ITUa]; LTE Advanced does meet most of the standards for 4G deployment, though it is often described as 3.9G or pre-4G. However, LTE Advanced is capable of peak download data rates of 1 Gbps, with a wide transmission bandwidth, low C-plane latency, backwards compatibility, increased user throughput and spectrum flexibility.
While work on the LTE standard draws to an end, the direction switches to developing LTE advanced, also referred as 3GPP Release 10. LTE Advanced should be compatible with first release LTE (3GPP Release 8) equipment, and should share frequency bands with first release LTE, thus making it backwards compatible. In 4G, it is estimated that 100 MHz bandwidths will offer data rates of 1 Gbps and while OFDM offers an easy way to increase bandwidth by adding additional subcarriers, the scheduler would have to include a mix of terminals. The 3GPP working groups looking at proposals for the standard have focused mainly on the physical layer; the topics analyzed included relay nodes, scalable system bandwidth exceeding 20 MHz, local area optimization of air interface, flexible spectrum usage, diversity MIMO, etc. Ultimately, standardization is expected to be included in 3GPP Release 10 timeframe. The importance and timeframe of LTE Advanced will mainly depend on the success of LTE itself. LTE Advanced will be fully built on the existing LTE specification Release 10 and not be defined as a new specification series. Major enhancements to LTE were introduced in Release 10 after a correction and improvement phase in Release 9. Since LTE Advanced fulfills most ITU standards for 4G, the 3GPP work plan is similar to the schedule within ITU.
3GPP Long Term Evolution is the name given to the 3GPP standard required to deal with the increasing data throughput requirements of the market. Working groups from 3GPP RAN started to work on standardization for LTE in late 2004. By 2007, all LTE features related to its functionality were finished and by 2008, most protocol and performance specifications were finished and included in Release 8.
3GPP gathered all requirements for LTE in [3GPPd] ; some requirements are written in an absolute fashion – defining concepts from scratch – while others are meant in relation to UTRA nomenclature. In this context, the reference for UTRA baseline is the use of Release 6 HSDPA with 1x1 multi-antenna scheme for the downlink and Release 6 HSUPA with a 1x2 multi-antenna scheme for the uplink. Here are several LTE design parameters [Martin09]:
In order to achieve these goals, LTE made use of a new system architecture combined with enhanced radio access technology. It divided network functions such as modulation, header compression and handover to the radio access network, while others such as charging, mobility management to the core network.
In this section, several concepts related to radio access network are discussed in order to offer better understanding of the technology behind LTE.
This section presents an LTE-Advanced overview starting with a brief history of LTE evolution, a discussion of the most important requirements set by the 3GPP and techical information related to the physical layer, including DL and UL channels, OFDMA/SC-FDMA scheme, and radio interface.
ITU issued an invitation [ITUb] for radio-access technologies beyond IMT-2000 – also referred to as IMT-Advanced. Nevertheless, 3GPP was expecting this event to unfold and had already started a study on LTE-Advanced with the purpose of finding the requirements and technology components so that the evolution of LTE would meet the requirements of IMT-Advanced. The first step was to consider backwards compatibility with the existent version of LTE; this implies that an LTE node would see the LTE-Advanced network as an LTE network. Spectrum compatibility was required for a straightforward, low-cost progression to LTE-Advanced networks, similar to the evolution of WCDMA to HSPA. In addition to these parameters, LTE-Advanced was intended to match or exceed the standards set by the ITU for IMT-Advanced with regards to capacity, data rates and low-cost deployment. [Parkvall09]
The features that ITU had chosen for IMT-Advanced were: a high degree of commonality of functionality worldwide while retaining the flexibility to support a wide range of services and applications in a cost efficient manner; compatibility of services within IMT and with fixed networks; capability of interworking with other radio access systems; high quality mobile services; user equipment suitable for worldwide use; user-friendly applications, services and equipment; worldwide roaming capability; and enhanced peak data rates to support advanced services and applications (100 Mbit/s for high and 1 Gbit/s for low mobility were established as targets for research).
After receiving the Circular Letter [ITUb][Martin09], 3GPP held a workshop with regard to LTE-Advanced matching IMT-Advanced requirements and took several decisions.
The requirements for LTE-Advanced are delineated in TR 36.913 [3GPPe]. The most important requirements are the following [Martin09]:
The physical layer implements OFDMA scheme on the downlink for high spectral efficiency, robustness against frequency-selectivity and multi-path interference. It supports flexible bandwidth deployment, facilitates frequency-domain scheduling and is well suited for MIMO techniques. In the uplink, LTE-Advanced uses SC-FDMA – OFDMA with DFT pre-coding. This implies a common structure of transmission resources compared to the downlink. The addition of the cyclic prefix supports frequency-domain equalization on the transmission. The transmission resource structure basic unit is the physical resource block (PRB). There are 12 subcarriers allocated for 0.5 milliseconds, in pairs, in time domain. The radio interface contains two frame structures to support both FDD and TDD. [3GPPe]
The following points present some of the design features of the downlink and uplink implementation.
The performance of systems such as LTE is already close to the Shannon limit. Thus, in order for LTE-Advanced to manage higher data rates, it must strive for better signal to noise ratios (SNR). Several approaches have been discussed in [Parkvall09] that would improve SNR values. Wider-band transmission and spectrum sharing would meet the high-peak data rate requirement; however, in order to be backwards compatible, spectrum compatibility can be achieved through multiple LTE carrier components. Carrier aggregation is illustrated in Figure 1. Another implementation that would improve data rate is the use of multiple antennas. Technologies such as beam-forming and spatial multiplexing are already incorporated in LTE and are expected to be developed further in LTE-Advanced. Coordinated multi-point (CoMP) transmission is another method of improving system performance. In the downlink, it involves coordination of the transmissions from multiple transmission points depending on how much terminals are aware of transmissions originating from multiple points. Yet another solution would be the inclusion of relays and repeaters on the network node path; thus, the long distances among nodes are eliminated, allowing for higher data rates.
Several simple system tests have been carried out in [Parkvall09]; a CoMP system is simulated; some of the assumptions are listed in table 1 (attached), which are similar to 3GPP simulation case 1 in [3GPPc].
According to [Parkvall09], “Fig. 2 and Fig. 3 show the resulting cell-edge and average active radio link bitrate (R) as a function of the served traffic per cell (T) for the downlink. It is seen that the CoMP system yields significant performance gains. As expected the gain is larger for the system with more coordinated cells. The loss due to using erroneous channel values at the transmitter is evident, but a majority of the gain remains. Fig. 4 and Fig. 5 show the resulting cell-edge and average active radio link bit rate (R) as a function of the served traffic per cell (T) for the uplink. It is seen that the CoMP system yields significant performance gains, and the gains are larger for the system with more coordinated cells. Recall that the transmitted signals in uplink CoMP are generated independently of the channel realizations; hence, from a coordination perspective there is no need to consider channel estimation errors at the transmitter for the uplink. These results are indeed very promising. Note however that several ideal assumption have been made that are challenging to solve, foremost including feedback of estimates of downlink channels (encoding and transmitting with low latency).”
This paper has provided a high-level overview of the LTE-Advanced standard, from it’s evolution as LTE, historical development and technical requirements needed to comply with IMT-Advanced goals, to an overview of the physical layer and performance capabilities.
1. [Parkvall09] Parkvall, Stefan; Astely David, “The Evolution of LTE towards IMT-Advanced”, 2009
2. [3GPPa] 3GPP TS36.300, “Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN); Overall description”
3. [ITUa] Recommendation ITU-R M.1645
4. [3GPPb] 3GPP TR 36.913, “Requirements for Further Advancements for EUTRA” TABLE I. SIMULATION PARAMETERS
5. [ITUb] ITU-R SG5, “Invitation for submission of proposals for candidate radio interface technologies for the terrestrial components of the radio interface(s) for IMT-Advanced and invitation to participate in their subsequent evaluation”, Circular Letter 5/LCCE/2, March 2008
6. [3GPPc] 3GPP TR 25.814, “Physical Layer Aspects for Evolved UTRA “, v.7.0.0.
7. [Martin09] David Martin-Sacristan, Jose F. Monserrat, Jorge Cabrejas-Penuelas, Daniel Calabuig, Salvador Garrigas, and Narcis Cardona, “3GPP LTE and LTE-Advanced”, January 2009.
8. Caire[03] Giuseppe Caire, Shlomo Shamai, “On the Achievable Throughput of a Multiantenna Gaussian Broadcast Channel,” IEEE Trans. Inf. Theory, 2003, 21 (5).
9. [Zhou06] Quan Zhou, Huaiyu Dai, and Hongyuan Zhang, “Joint Tomilson-Harashima Precoding and Scheduling for Multiuser MIMO with Imperfect Feedback,” in proceedings of WNCC 2006.
10. [ITUc], “Guidelines for evaluation of radio interface technologies for IMTAdvanced”, Technical report, July 2008.
11. [3GPPd] 3GPP TR 25.913, “Requirements for Evolved UTRA (EUTRA) and Evolved UTRAN (E-UTRAN),” v.8.0.0, December 2008.
12. [3GPPe] 3GPP TR 36.913 “3GPP; Technical Specification Group Radio Access Network. Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA),” v.9.0.0, December 2009.
(in order of appearance)
LTE – Long Term Evolution
IMT – Internationale Mobile Telecommunications
3GPP - 3rd Generation Partnership Project
3/4 G – 3rd/4th generation
Gbps – gigabits per second
MHz – mega Hertz
Hz – Hertz
MIMO – multiple input multiple output
ITU – international telecommunication unit
RAN – regional area network
UTRA – UMTS Terrestrial Radio Access
OFDM – orthogonal frequency division multiplexing
SC-FDMA – single carrier frequency division multiple access
FFT – fast Fourier transform
IFFT – inverse fast Fourier transform
FDD – frequency division duplex
TDD – time division duplex
QPSK – quadrature phase-shift keying
16 QAM – 16 quadrature amplitude modulation
64 QAM – 64 quadrature amplitude modulation
WCDMA – wideband code division multiple access
DL – download
UL – upload
SIMO – single input multiple output
DFT – discrete Fourier transform
ACK/NACK – positive/negative acknowledgement
CoMP – coordinated multi-point (with regard to transmission)
Last Modified: May 7, 2010