- Open Access
Advanced technologies for LTE advanced
EURASIP Journal on Wireless Communications and Networking volume 2013, Article number: 25 (2013)
With the explosive growth in mobile data traffic fueled by smartphones and mobileapplications, transition from the 3rd generation (3G) toward 4th generation (4G)cellular networks is progressing at an unprecedented pace. Since the first release(release 8) of the 3rd generation partnership project (3GPP) long-term evolution(LTE) standard in December 2008, LTE has become the de facto standard on the waytoward 4G cellular networks. Accordingly, LTE networks have seen rapid growth since2009 when only two networks were initially launched. The number quickly expandedover the next few years, currently reaching 89 networks across 45 countries . From the users’ side, the number of global LTE subscribers islikely to surge more than five-fold to reach some 50 million in 2012 from 9 millionin 2011, and is expected to exceed 560 million in 2016 .
3GPP has been working on further evolution of the LTE, which is referred to as LTEAdvanced (release 10 and beyond), to develop a true 4G standard. The LTE Advanced istargeted to fulfill or even surpass all the requirements of international mobiletelecommunications-Advanced (IMT-Advanced)  which is an official definition of 4G made by internationaltelecommunication union (ITU) in 2008. These requirements include peak data rates,peak spectral efficiency, cell spectral efficiency, and scalable bandwidth. Thereare a lot of technical challenges for successful standardization of the LTE Advancedthat meets the ITU requirements and supports backward compatibility with the LTE.The key features of the LTE Advanced differentiated from the LTE include support forwider bandwidth, improved uplink performance, better energy efficiency, advancedmultiantenna technology, advanced interference management, and self-organizingnetwork. Accordingly, from the perspective of physical (PHY) and medium accesscontrol (MAC) layers, multihop relay, multicell multi-input multi-output (MIMO),carrier aggregation, and intelligent interference management are challenging areasto be explored. Network aspects to be considered are heterogeneous networks(HetNets), femtocell optimization, and self organizing networks. The aim of thisspecial issue is putting together recent original achievements and developments ofenabling technologies for PHY, MAC, and network layers of the LTE Advanced. Afterextensive reviews, we have selected twelve articles in the relevant areas.
This special issue opens with a comprehensive review article, “Overview ofEnabling Technologies for LTE Advanced” by Thien-Toan Tran, YoanShin, and Oh-Soon Shin. The article provides a detailed view of physical layerfeatures as well as network architecture that qualify the LTE Advanced as a true 4Gtechnology meeting the ITU’s strict definition. Specifically, such features ascarrier aggregation, advanced MIMO, wireless relays, enhanced intercell interferencecoordination (eICIC), and coordinated multipoint (CoMP) transmission/reception arecovered at the right level of detail, which makes this article a good starting pointto understand 4G technology.
The article “Low-Complexity Multiuser MIMO Downlink System based on aSmall-Sized CQI Quantizer” by Jiho Song, Jong-Ho Lee, Seong-CheolKim, and Younglok Kim presents a multiuser multiple-input multiple-output (MU-MIMO)system with random beamforming. The authors develop a user selection algorithmsuitable for random beamforming, and propose an efficient quantizer for channelquality indicator (CQI) to reduce feedback overhead. The small-sized CQI quantizeris designed based on a closed-form expression for the expected SINR of the selectedusers. Numerical results are provided to confirm that the random beamforming withthe proposed CQI quantizer provides higher throughput and better fairness amongusers than conventional schemes under small amount of feedback.
The article “Determinant-Based MIMO Scheduling with Reduced PilotOverhead” by Kyeongjun Ko and Jungwoo Lee proposes a user schedulingscheme with reduced complexity in MU-MIMO cellular systems. In a MU-MIMO cellularsystem where there are many candidate users, it is critical to select a user groupwhich maximizes the overall throughput of the system. However, the optimalscheduling strategy based on exhaustive user selection is computationallyprohibitive when the total number of users is large. In this article, the authorspropose a determinant-based user selection algorithm which reduces the searchcomplexity without much performance degradation. A key contribution of this articleis to use a matrix determinant as a measure of orthogonality as well as channelquality in user selection. In order to reduce the pilot overhead, the authors alsopropose a new pilot scheme with only one set of pilot, which is another keycontribution of this article.
The article “Kalman Interpolation Filter for Channel Estimation of LTEDownlink in High-Mobility Environments” by Xuewu Dai, Wuxiong Zhang,Jing Xu, John M Mitchell, and Yang Yang considers an estimation of fast-fading LTEdownlink channels in high-speed applications of LTE Advanced. In order to adequatelytrack the fast time-varying channel response, an adaptive channel estimation andinterpolation algorithm is essential. In this article, the multi-path fast-fadingchannel is modeled as a tapped-delay, discrete, finite impulse response filter, andthe time-correlation of the channel taps is modeled as an auto regressive (AR)process. Using this AR time-correlation, the authors develop an extended Kalmanfilter to jointly estimate the complex-valued channel frequency response and the ARparameters from the transmission of known pilot symbols. Furthermore, the channelestimates based on the known pilot symbols are interpolated to estimate the unknowndata symbols by using the estimated time-correlation. This article integrates bothchannel estimation based on pilot symbols and interpolation of data symbol into theproposed Kalman interpolation filter.
The article “Low Complexity PAPR Reduction Methods for Carrier-AggregatedMIMO OFDMA and SC-FDMA Systems” by Pochun Yen and Hlaing Minnprovides a solution to the problem of increased peak-to-average power ratio (PAPR)due to carrier aggregation. The authors propose several low-complexity andzero-overhead PAPR reduction methods for carrier aggregated systems based onorthogonal frequency division multiple access (OFDMA) or single-carrier frequencydivision multiple access (SC-FDMA), when MIMO is incorporated. In conjunction withphase-rotation based PAPR reduction methods, a reliable phase rotation detector isalso developed to maintain the receiver performance. The proposed methods are shownto provide substantial PAPR reduction with low complexity and no signalingoverhead.
The article “A Resource Block Organization Strategy for Scheduling inCarrier Aggregated Systems” by Guillermo Galaviz, David HCovarrubias, Angel G Andrade, and Salvador Villarreal tackles resource schedulingproblem in carrier aggregated systems. When each available resource block (RB) ishandled individually, assigning multiple RBs to each user may increase thescheduling delay, affecting the quality of service. The article proposes anefficient scheduling strategy based on a priori organization of available RBs so asto reduce RB assignment delay for carrier aggregated systems. Two different RBorganization algorithms are presented and compared. Numerical results show thatusing the proposed strategy it is possible to reduce the delay required to assignRBs to users without affecting the downlink user capacity as compared toblock-by-block scheduling strategies proposed in literature.
The article “Resource Management Issues for Multi-Carrier Relay-EnhancedSystems” by Jacek Gora and Simone Redana discusses relaying that hasbeen considered in 3GPP as a key feature for enhancing capacity and coverage in LTEAdvanced. Although tens of papers discussed the performance of relay solutions, thispaper proves that there is still much to say about this topic. Indeed, the authorsreviewed the problem of resource allocation in multi-carrier LTE Advanced systemswith a focus on the wireless backhaul as the capacity bottleneck. By comparing thedifferent possible solutions for implementing the wireless backhaul, be they basedon time partitioning, frequency partitioning or a hybrid combination of them, thisarticle offers a precious background for engineers that are interested in relayingin multi-carrier systems.
The article “Optimizing Distance, Transmit Power, and Allocation Time forReliable Multi-hop Relay System” by Pham T Hiep, Fumie Ono, and RyujiKohno addresses optimization problems of multi-hop relay systems to ensureend-to-end channel capacity. First, it tackles individual optimization problems fortransmit power, distance between transceivers, and allocation time. Then, it solvesthe joint optimization problem of these parameters by Markov Chain Monte Carlomethod. The performance of multi-hop relay systems with decode-and-forward method isexamined for each optimization.
The article “Cooperative Relaying in OFDMA Networks Based on the Joint Useof Hierarchical Modulation and Link Adaptation” by Anis Jdidi andTijani Chahed considers an OFDMA network with cooperative relays. For cooperativerelaying, the base station broadcasts a signal which the relay overhears andforwards to the destination. The destination combines the two copies of the receivedsignal from the base station and from the relay to reconstruct the original one. Theauthors make use of hierarchical modulation to send additional information to therelay without the need for extra resource. Moreover, the link adaptation is adoptedto take advantage of the typically good radio conditions between the base stationand relay. They model such a system at the user level including a realistic arrivaland departure setting, and quantify the gains in terms of throughput and blockingprobability.
The article “Uplink Interference Protection and Scheduling for EnergyEfficient OFDMA Networks” by Harald Burchardt, Zubin Bharucha,Gunther Auer, and Harald Haas deals with an uplink interference protection issue inOFDMA systems. One of key challenges for future OFDMA-based networks is inter-cellinterference coordination. With full frequency reuse and small inter-site distances,coping with co-channel interference (CCI) in such networks has become increasinglyimportant. In this article, an uplink interference protection (ULIP) technique tocombat CCI is introduced and investigated. The level of uplink interferenceoriginating from neighboring cells (affecting co-channel mobile stations (MSs) inthe cell of interest) can be effectively controlled by reducing the transmit powerof the interfering MSs. This is done based on the targetsignal-to-noise-plus-interference ratio (SINR) and tolerable interference of thevulnerable link. Bands are prioritized in order to differentiate those(vulnerable/victim) MSs that are to be protected from interference and those(aggressor/interfering MSs) that are required to sacrifice transmission power tofacilitate the protection. Furthermore, MSs are scheduled such that those users withpoorer transmission conditions receive the highest interference protection, thusbalancing the areal SINR distribution and creating a fairer allocation of theavailable resources. In addition to interference protection, the individual powerreductions also serve to decrease the total system uplink power, resulting in agreener system.
The article “A Decentralized Spectrum Allocation and Partitioning Schemefor a Two-Tier Macro-Femtocell Network with Downlink Beamforming” bySunheui Ryoo, Changhee Joo, and Saewoong Bahk deals with interference issue due tofemtocells. Femtocells are expected to be massively deployed in mobile networks,especially with the possibilities offered by the HetNets architecture of LTEAdvanced. However, interference is the major issue in this context and interferencemitigation and avoidance solutions are essential for taking advantage of thepossibilities offered by femtocell deployments. This article discusses spectrumpartitioning and allocation scheme in downlink beamforming systems as an efficientway to reduce interference. The obtained results show that the proposed solution isa promising one, even in decentralized systems. However, there are still furtherinvestigations to perform in this context, especially by taking into account thedynamics introduced by user’s mobility and flow arrivals and departures.
The article “Implementing Opportunistic Spectrum Access inLTE-Advanced” by Vicente Osa, Carlos Herranz, Jose F Monserrat, andXavier Gelabert discusses practical issues when implementing opportunistic spectrumaccess (OSA) in LTE Advanced. Indeed, although theoretical aspects of cognitiveradio have been extensively studied in the literature, very few practicallyimplementable solutions have been proposed. This paper fills this gap for LTEAdvanced systems and proposes the usage of a geo-located data base that retrievesand maintains information about the spatial usage of spectrum in order to allowaccess for secondary users. Using a realistic simulator, this paper shows that thiskind of databases (already used for secondary systems operating within TV whitespaces) is an efficient solution for LTE Advanced spectrum. The editors believe,however, that the road is still long for convincing operators about the opportunityof allowing OSA in their LTE Advanced spectrum, through experimental testbeds andfield trials.
We hope that the articles published in this special issue will serve as goodreferences for readers’ research work and promote further research in theemerging field of LTE Advanced. We would like to take this opportunity to thank allthe authors who have submitted their papers to this special issue and express ourgratitude to all the reviewers who provided valuable feedback to the authors. Theirtimely reviews and valuable comments helped us select the papers as well as improvethe quality of this special issue. Our special thanks are also addressed to thedevoted staff of JWCN, Bernardino McCartney, for his excellent support through theentire editing process, and to the Editor-in-Chief of the journal, Prof. LucVandendorpe, for his valuable comments during the process of defining of the specialissue.
Rapid growth of LTE networks providing growth opportunities for technologysector (Five Star Equities, 3 Aug 2012)
Global LTE subscribers to top 50 mil. this year (Bayanihan, 2012) (accessed 5 Nov, 2012).http://bayanihan.org/2012/08/07/global-lte-subscribers-to-top-50m-this-year-report/
Requirements related to technical performance for IMT-Advanced radiointerface(s), Report ITU-R M.2134 (2008)
The study of O-S Shin and Y Shin was supported in part by the Human ResourcesDevelopment program (No. 20114010203110) of the Korea Institute of EnergyTechnology Evaluation and Planning (KETEP) grant funded by the Korea governmentMinistry of Knowledge Economy, and in part by the Korea CommunicationsCommission (KCC), Korea, under the R&D program supervised by the KoreaCommunications Agency (KCA) (KCA-2012-12-911-01-107).
The authors declare that they have no competing interests.