Hindawi Publishing Corporation

EURASIP Journal on Wireless Communications and Networking Volume 2009, Article ID 354089, 10 pages doi:10.1155/2009/354089

Research Article

On theWay towards Fourth-GenerationMobile:3GPP LTE and LTE-Advanced

DavidMart´ ın-Sacrist ´ an, Jose F.Monserrat, Jorge Cabrejas-Pe˜ nuelas, Daniel Calabuig, Salvador Garrigas, and Narcıs Cardona

iTEAM Research Institute, Universidad Politecnica de Valencia, Camino de Vera S/N, 46022 Valencia, Spain

Long-TermEvolution (LTE) is the new standard recently speciﬁed by the 3GPP on the way towards fourth-generationmobile. This paper presents the main technical features of this standard as well as its performance in terms of peak bit rate and average cell throughput, among others. LTE entails a big technological improvement as compared with the previous 3G standard. However, this paper also demonstrates that LTE performance does not fulﬁl the technical requirements established by ITU-R to classify one radio access technology as a member of the IMT-Advanced family of standards. Thus, this paper describes the procedure followed by the 3GPP to address these challenging requirements. Through the design and optimization of new radio access techniques and a further evolution of the system, the 3GPP is laying down the foundations of the future LTE-Advanced standard, the 3GPP candidate for 4G. This paper oﬀers a brief insight into these technological trends.

1. Introduction

In the last years, technology evolution in mobile communi- cations is mainly motivated by three relevant agents: (1) the market globalization and liberalization and the increasing competence among vendors and operators coming from this new framework, (2) the popularization of IEEE 802 wireless technologies within the mobile communications sector and, ﬁnally, (3) the exponential increase in the demand for advanced telecommunication services.

Concerning the last item, the envisaged applications to be supported by current and future cellular systems include Voice over IP (VoIP), videoconference, push-to-talk over cellular (PoC), multimedia messaging, multiplayer games, audio and video streaming, content download of ring tones, video clips, Virtual Private Network (VPN) connections, web browsing, email access, File Transfer Protocol (FTP). All these applications can be classiﬁed in several ways based on the Quality of Service (QoS) treatment that they require. Some of themare real-time and delay-sensitive, like voice and videoconference, while some others require integrity, high data-rate, and are sensitive to latency (like VPN and FTP).

The simultaneous support of applications with diﬀerent QoS requirements is one of the most important challenges that cellular systems are facing. At the same time, the spectrum scarcity makes that new wideband cellular systems are designed with very high spectral eciency.

It is precisely that this increasing market demand and its enormous economic beneﬁts, together with the new chal- lenges that come with the requirements in higher spectral eﬃciency and services aggregation, raised the need to allocate new frequency channels to mobile communications systems. That is why the ITU-R WP 8F started in October 2005 the deﬁnition of the future Fourth-GenerationMobile (4G), also known as International Mobile Telecommunications (IMTs) Advanced, following the same model of global standard- ization used with the Third Generation, IMT-2000. The objective of this initiative is to specify a set of requirements in terms of transmission capacity and quality of service, in such a way that if a certain technology fulﬁls all these requirements it is included by the ITU in the IMT-Advanced set of standards. This inclusion ﬁrstly endorses technologies and motivates operators to invest in them, but furthermore it allows these standards to make use of the frequency bands.

The race towards IMT-Advanced was oﬃcially started in March 2008, when a Circular Letter was distributed asking for the submission of new technology proposals [1]. Previous to this oﬃcial call, the 3rd Partnership Project (3GPP) established the Long Term Evolution (LTE) standardization activity as an ongoing task to build up a framework for the evolution of the 3GPP radio technologies, concretely UMTS, towards 4G. The 3GPP divided this work into two phases: the former concerns the completion of the ﬁrst LTE standard (Release 8), whereas the latter intends to adapt LTE to the requirements of 4G through the speciﬁcation of a new technology called LTE-Advanced (Release 9 and 10). Following this plan, in December 2008 3GPP approved the speciﬁcations of LTE Release 8 which encompasses the Evolved UTRAN (E-UTRAN) and the Evolved Packet Core (EPC). Otherwise, the LTE-Advanced Study Item was launched in May 2008, expecting its completion in October 2009 according to the ITU-R schedule for the IMT-Advanced process. In the meantime, research community has been called for the performance assessment of the deﬁnitive LTE Release 8 standard.

This paper describes the main characteristics of LTE Release 8 and evaluates LTE link level performance consid- ering a transmission chain fully compliant with LTE Release 8 and including realisticHARQand turbo-decoding. Besides, the capacity of LTE systems is analyzed in terms ofmaximum achievable throughput and cell capacity distribution in a conventional scenario. These studies allow having a rough idea on the beneﬁts and capabilities of the new standard. Finally, this paper oﬀers an overview of the current research trends followed by 3GPP in the deﬁnition process of LTE- Advanced thus foreseeing the main characteristics of next generation mobile.

2. LTE

3GPP Long Term Evolution is the name given to the new standard developed by 3GPP to cope with the increasing throughput requirements of the market. LTE is the next step in the evolution of 2G and 3G systems and also in the provisioning of quality levels similar to those of current wired networks.

2.1 LTE Requirements.

3GPP collected in [8] the require- ments that an evolved UTRAN should meet. Some of the requirements are deﬁned in an absolute manner while other requirements are deﬁned in relation to UTRA performance. It is worth to mention that for the UTRA baseline it is considered the use of Release 6 HSDPA with a 1 × 1 multiantenna scheme for the downlink and Release 6HSUPA with a 1 × 2 multiantenna scheme in uplink. For the sake of comparison, in LTE it is considered transmission using up to 2 × 2 antennas in downlink and up to 1 × 2 antennas in uplink.

Among others, LTE design targets are the following.

The system should support peak data rates of 100Mbps in downlink and 50Mbps in uplink within a 20MHz bandwidth or, equivalently, spectral eﬃ- ciency values of 5 bps/Hz and 2.5 bps/Hz, respec- tively. Baseline considers 2 antennas in UE for downlink and 1 antenna in UE for uplink
Downlink and uplink user throughput per MHz at the 5% point of the CDF, 2 to 3 times Release 6 HSPA.
Downlink averaged user throughput per MHz, 3 to 4 times Release 6 HSDPA. Uplink averaged user throughput per MHz, 2 to 3 times Release 6 Enhanced Uplink.
Spectrum eﬃciency 3 to 4 times Release 6 HSDPA in downlink and 2 to 3 times Release 6 HSUPA in uplink, in a loaded network.
Mobility up to 350 km/h.
Spectrum ﬂexibility, seamless coexistence with previ- ous technologies and reduced complexity and cost of the overall system.

2.2 LTE Release 8 Technical Overview.

Tomeet these require- ments, a combination of a new system architecture together with an enhanced radio access technology was incorporated in the speciﬁcations.

2.2.1. Architecture.

There are diﬀerent types of functions in a cellular network. Based on them, network can be split into two parts: a radio access network part and a core network part. Functions like modulation, header compression and handover belong to the access network, whereas other functions like charging or mobility management are part of the core network. In case of LTE, the radio access network is E-UTRAN and the core network EPC.

Radio Access Network

he radio access network of LTE is called E-UTRAN and one of its main features is that all services, including real-time, will be supported over shared packet channels. This approach will achieve increased spectral eciency which will turn into higher systemcapacity with respect to current UMTS and HSPA. An important consequence of using packet access for all services is the better integration among all multimedia services and among wireless and ﬁxed services..

The main philosophy behind LTE is minimizing the number of nodes. Therefore the developers opted for a single-node architecture. The new base station is more complicated than the Node B inWCDMA/HSPA radio access networks, and is consequently called eNB (Enhanced Node B). The eNBs have all necessary functionalities for LTE radio access network including the functions related to radio resource management.

Core Network.

The new core network is a radical evolution of the one of third generation systems and it only covers the packet-switched domain. Therefore it has a new name: Evolved Packet Core.

Following the same philosophy as for the E-UTRAN, the number of nodes is reduced. EPC divides user data ﬂows into the control and the data planes. A speciﬁc node is deﬁned for each plane plus the generic gateway that connects the LTE network to the internet and other systems. The EPC comprises several functional entities.

The MME (Mobility Management Entity): is respon- sible for the control plane functions related to subscriber and session management.
The Serving Gateway: is the anchor point of the packet data interface towards E-UTRAN. Moreover, it acts as the routing node towards other 3GPP technologies.
The PDN Gateway (Packet Data Network): is the termination point for sessions towards the external packet data network. It is also the router to the Internet.
The PCRF (Policy and Charging Rules Function): controls the tariﬀ making and the IP Multimedia Subsystem (IMS) conﬁguration of each user..

The overall structure of LTE is shown in Figure 1..

Figure 1: LTE Release 8 architecture.

2.2.2. Radio Access Fundamentals.

The most important technologies included in the new radio access network are Orthogonal Frequency Division Multiplexing (OFDM), multidimensional (time, frequency) dynamic resource allo- cation and link adaptation, Multiple Input Multiple Output (MIMO) transmission, turbo coding and hybrid Automatic Repeat reQuest (ARQ) with soft combining. These technolo- gies are shortly explained in the following paragraphs..

OFDM. Orthogonal Frequency Division Multiplexing is a kind of multicarrier transmission technique with a relatively large number of subcarriers. OFDM oﬀers a lot of advan- tages. First of all, by using a multiple carrier transmission technique, the symbol time can be made substantially longer

than the channel delay spread, which reduces signiﬁcantl or even removes the intersymbol interference (ISI). In othe words, OFDM provides a high robustness against frequenc selective fading. Secondly, due to its speciﬁc structure OFDM allows for low-complexity implementation by mean of Fast Fourier Transform (FFT) processing. Thirdly, th access to the frequency domain (OFDMA) implies a hig degree of freedomto the scheduler. Finally, it oﬀers spectrum ﬂexibility which facilitates a smooth evolution from alread existing radio access technologies to LTE.

ПIn the FDD mode of LTE each OFDM symbol is trans- mitted over subcarriers of 15 or 7.5 kHz. One subframe lasts 1ms, divided in two 0.5ms slots, and contains several consecutive OFDM symbols (14 and 12 for the 15 and 7.5 kHz modes, resp.)

In the uplink, Single Carrier Frequency DivisionMultiple Access (SC-FDMA) is used rather than OFDM. SC-FDMA is also known as DFT-spread OFDM modulation. Basically SC-FDMA is identical to OFDM unless an initial FFT is applied before the OFDM modulation. The objective of such modiﬁcation is to reduce the peak to average power ratio, thus decreasing the power consumption in the user terminals.

Multidimensional Dynamic Resource Allocation and Link Adaptation. In LTE, both uplink and downlink transmission schemes can assign smaller, nonoverlapping frequency bands to the dierent users, oﬀering frequency division multi- ple access (FDMA). This assignment can be dynamically adjusted in time and is referred to as scheduling. Accordingly, the LTE resources can be represented as a time-frequency grid. The minor element of this grid is called resource element and consists of one subcarrier during an OFDM symbol. However, the minor LTE resource allocation unit is the resource block that consists of 12 subcarriers during one slot.

MIMO. One of the most important means to achieve the high data rate objectives for LTE is multiple antenna transmission. In LTE downlink it is supported one, two or four transmit antennas in the eNB and one, two or four receive antennas in the UE.Multiple antennas can be used in diﬀerent ways: to obtain additional transmit/receive diversity or to get spatial multiplexing increasing the data rate by creating several parallel channels if conditions allow to. Nevertheless, in LTE uplink although one, two or four receive antennas are allowed in the eNB, only one transmitting antenna is allowed in the UE. Therefore, multiple antennas can be only used to obtain receive diversity.

Turbo Coding. In order to correct bit errors, introduced by channel variations and noise, channel coding is utilized. In case of the LTE downlink shared channel (DL-SCH) a turbo encoder with rate 1/3 is used, followed by a rate matching to adapt the coding rate to the desired level. In each subframe of 1ms, one or two (withmulticodewordMIMO) codewords can be coded and transmitted.

Hybrid ARQ with Soft Combining.Hybrid ARQ with soft combining is a technique that deals with the retransmission of data in case of errors. In an ARQ scheme, the receiver uses an error-detecting code to check if the received packet contains errors or not. The transmitter is informed by a NACK or ACK respectively. In case of a NACK, the packet is retransmitted.

A combination of forward error correction (FEC) and ARQ is known as hybrid ARQ. Most practical hybrid ARQ schemes are built around a CRC code for error detection and a turbocode for error correction, as it is the case of LTE.

In hybrid ARQ with soft combining, the erroneously received packet is stored in a buﬀer and later combined with the retransmission(s) to obtain a single packet that is more reliable than its constituents. In LTE full incremental redun- dancy (IR) is applied, which means that the retransmitted packets are typically not identical with the ﬁrst transmission but carry complementary information.

Diferent methods can be used to assess the performance of a mobile technology. Each method is best suited for a particular kind of performance assessment. For instance, analytical methods or inspections are valid to evaluate peak data rates or peak spectral effciencies. However, a deeper performance analysis requires the usage of simulation. Simulators are usually divided in two classes: link level simulators and system level simulators. Link level simulators are used to emulate the transmission of information from a unique transmitter to a unique receiver modeling the physical layer with high precision. They include models for coding/decoding,MIMO processing, scrambling, modulation, channel, channel estimation and equalization, and so forth. System level simulators emulate the operation of a network with a number of cells and several users per cell. In this kind of simulators, higher level functions are included for call admission control, scheduling, power control, and so forth, while link to system level models is used to facilitate the emulation of each radio link. This section presents some results obtained from both types of simulators.

In the course of the LTE standardization process, the 3GPP conducted several deep evaluations of the developing technology to ensure the achievement of requirements.This assessment allows getting an insight into to which extent LTE implies a revolution in comparison with UMTS. As shown in next section, LTE results demonstrate that this technology is quite close to the requirements established for the Fourth-Generation mobile, although further improve- ments are expected in LTE-Advanced..

2.3.1. Peak Spectral Effciency.

The peaks pectral effciency is the highest theoretical data rate assignable to a single mobile user divided by the allocated bandwidth. The highest data rate is calculated as the received data bits assuming error-free conditions and excluding radio resources that are used for control issues and guard bands. At the end, the radio access technology is classiﬁed as more or less powerful according to the achievable eﬃciency what makes this measurement perfect for comparative purposes.

Assuming a transmission bandwidth of 20MHz the maximum achievable rates in downlink are: 91.2Mbps for SIMO 1 × 2, 172.8Mbps for MIMO 2 × 2 and 326.4Mbps for MIMO 4 × 4. The resulting peak spectral eﬃciencies are 4.56, 8.64 and 16.32 b/s/Hz for the considered multiantenna schemes. These values have been calculated taking into account realistic overhead due to the reference signals and assuming that control signals overhead is equal to one OFDM symbol in each subframe. In uplink with SIMO 1 × 2themaximumachievablerateis86.4Mbpswitha transmission bandwidth of 20MHz. Thus, the peak spectral eﬃciency is 4.32 b/s/Hz. These values have been calculated assuming that two OFDMsymbols are occupied by reference signals. Both in downlink and uplink calculations 64QAM is the considered modulation and code rate is assumed to be 1.

The calculated peak spectral eﬃciencies of LTE are depicted in Figure 2 for both downlink and uplink together with the eﬃciencies of UMTS Release 6, that is, including HSDPA and HSUPA. From this peak spectrum eﬃciency it can be seen that LTE with 20MHz meets and exceeds the 100Mbps downlink and 50Mbps uplink initial targets. Besides, the comparison with UMTS demonstrates that LTE is a major step forward in mobile radio communications. With these achievable data rates mobile systems will give a greater user experience with the capability of supporting more demanding applications.

Figure 2: LTE peak spectral eﬃciencies in downlink (a) and uplink (b).

2.3.2. LTE Link Level Performance.

Based on link level simulations it can be assessed the relation between eﬀective throughput (correctly received bits per time unit) and signal- to-noise plus interference ratio (SINR). Simulations assessed for this paper used 10MHz of bandwidth for both downlink and uplink. This bandwidth is equivalent to 50 LTE resource blocks. The evaluation was focused on the performance experienced by a pedestrian user and hence the user mobility model used was the extended pedestrian A model [14]with a Doppler frequency of 5Hz. The central frequency has been set to 2.5GHz, the most likely band for initial LTE deployment. The set of modulation and coding schemes has been selected from the CQI table included in LTE speciﬁcations [15]. This set was selected by 3GPP to cover the LTE SINR dynamicmargin with approximately 2 dB steps between consecutive curves. A distinction from other studies is that channel estimation was realistically calculated at the receivers. In order to exploit the multiantenna conﬁguration at the receiver side, minimum mean-square error (MMSE) equalization was considered. The remaining parameters considered in the simulations are summarized in Table 1.

Table 1: Simulation parameters.

Common parameters
Bandwidth		10 MHz (50 RB)
Channel		Tapped delay line: EPA with 5 Hz Doppler frequency at link level, ETU at system level
Central frequency		2.5 GHz
MCS		CQI 1-15
Multiantenna schemes	DL	SIMO 1x2, MIMO 2x2/4x4
	UL	SIMO 1x2/1x4
Control channels overhead	DL	2 OFDM symbols per subframe
	UL	Not considered
System level parameters
Inter site distance (ISD)		500 m
Cell deployment		3-sector cells, reuse 1
Pathloss		130.2+37.6log₁₀(d(km))dB
Shadowing		log_normal,СКО=8dB
eNB transmission power		46 dBm

Concerning LTE downlink, diﬀerent multiantenna con- ﬁgurations weremodeled including SIMO 1×2,MIMO 2×2 and MIMO 4 × 4. Simulated MIMO scheme followed the open loop spatial multiplexing scheme as speciﬁed by the 3GPP [16], the number of codewords was 2 and the number of layers was equal to the number of transmit antennas, that is, 2 and 4. Additionally, the multiple channels among anten- nas were supposed uncorrelated. Control channel and signals overhead were taken into account and hence the ﬁrst two OFDM symbols in each subframe were reserved for control channels. Besides, reference signals were emulated in detail, although neither broadcast information nor synchronization signals overhead was considered.

In the uplink, two diﬀerent multiantenna conﬁgurations were simulated: SIMO 1 × 2 and SIMO 1 × 4. The multiple channels among antennas were supposed uncorrelated too Nowadays, the LTE standard does not allowMIMO in uplink so that MIMO schemes were not simulated. Therefore as established in the 3GPP speciﬁcations [17], only one codeword was considered. Moreover, 12 of the 14 available SC-FDMA symbols in a subframe were occupied by codiﬁed data since the other 2 were reserved for reference signals needed for the channel estimation at the receiver.

Taking into account these assumptions and parameters, a set of simulations was performed whose results are shown in Figure 3 for LTE downlink and in Figure 4 for LTE uplink.

Figure 3: Link level evaluation of throughput versus SINR in LTE downlink.

Figure 4: Link level evaluation of throughput versus SINR in LTE uplink.

In both ﬁgures it can be observed that the maximum throughputs are not equal to the peak throughputs previ- ously calculated. The reason is threefold: the used bandwidth is not 20MHz but 10MHz, the highest coding rate used is 0.93 instead of 1 and downlink control signals overhead is assumed to be 2 OFDM symbols instead of 1.

In LTE downlink, according to the results shown in Figure 3,MIMO4 × 4schemeprovidesaclearlybetter performance than the other schemes for almost all the useful SINR margin. Nevertheless,MIMO 2×2schemedoes not provide an important performance improvement until SINR reaches a value of 15 dB. Also, it can be observed that improvement factor in peak throughput due to MIMO schemes is far frombeing equal to the number of antennas (2 or 4). Instead, peak throughput is multiplied by 1.7 and 3.6 inMIMO 2×2andMIMO4×4 respectively. This is basically due to the higher quantity of reference signals needed in the MIMO schemes.

In LTE uplink, there is not any peak throughput gain when using more receiver antennas. But a nonnegligible SINR gain can be achieved. This gain is about 5 dB for a throughput of 20Mbps. Note that in SIMO 1 × 4maximum rate is achieved 10 dB before than in SIMO 1× 2..

2.3.3. LTE System Level Performance.

LTE performance anal- ysis at system level requires the deﬁnition of system level statistics. The cell spectral eﬃciency and the cell edge user spectral eﬃciency are the more important ones. Given a multiuser/multicell scenario, the cell spectral eﬃciency is deﬁned as the aggregate throughput of all users (the number of correctly received bits over a certain period of time) normalized by the overall cell bandwidth and divided by the number of cells. In the same scenario, the cell edge user spectral eﬃciency is the 5% point of CDF of the user throughput normalized with the overall cell bandwidth.

In order to calculate these values in the downlink, a dynamic system level simulator has been used. The main parameters of the considered scenario are shown in Table 1. The scenario is similar to the “Case 1” scenario in [9]. The main diﬀerences in this assessment are that the channel has been implemented using a tapped delay line model and a low correlation among channels has been assumed. Speciﬁcally, anETUchannelhasbeenused[14]. The scheduler operation follows the proposal of [18] where scheduling algorithm is divided in two parts: one for the time domain and another for the frequency domain. For both domains a proportional fair approach has been used.

Following the proposed approach, average cell spectral eﬃciency in downlink was obtained yielding 1.52 bps/Hz/cell for SIMO 1 × 2, 1.70 bps/Hz/cell for MIMO 2 × 2and 2.50 bps/Hz/cell for MIMO 4 × 4.Thecelledgeuser spectral eﬃciencies are 0.02 bps/Hz/user, 0.03 bps/Hz/user and 0.05 bps/Hz/user, for the same antenna conﬁgurations. Note that the LTE values for the uplink have been extracted from the results presented by the 3GPP partners in [12], since the downlink values obtained in this assessment ﬁt with 3GPP results. Since LTE requirements were deﬁned as relative to HSPA performance, Table 2 includes HSPA ﬁgures extracted also from [12, 13]. After direct inspection, it can be concluded that most of the requirements speciﬁed by 3GPP are fulﬁlled by the current Release 8 version of LTE.

Table 2: LTE requirements related to technical performance.

		Requirements LTE TR 25.913	LTE simulation results
Peak data rate (Gbps)		0.1	0.172 (2 x2) 0.326 (4x4)
Latency		C-Plane < 100ms U-Plane<5 ms	—
Peak spectral eﬃciency (bps/Hz)	DL UL	5(1x2) 2.5(1x2)	4.56(1x2) 8.64(2x2) 16.32(4x4) 4.32(1x2)
Average spectral eﬃciency (bps/Hz/cell)	DL UL	EUTRA (2x2)3-4 times HSDPA R6 {0.53} EUTRA (1x2)2-3 times HSUPA R6(1x2){0.33}	1.52(1x2) 1.70(2x2) 2.50 (4x4) 0.73 (1x2)
Cell edge user spectral eﬃciency (bps/Hz/cell/user)	DL UL	EUTRA (2x2)2-3 times HSDPA R6 {0.02} EUTRA (1x2)2-3 times HSUPA R6(1x2){0.01}	0.02(1x2) 0.04 (2x2) 0.05 (4x4) 0.02(1x2)
Mobility		Up to 350 km/h	30 km/h
Bandwidth		Up to 20MHz	10MHz

3. LTE-Advanced and the Fourth-GenerationMobile

The process of deﬁning the future IMT-Advanced family was started with a Circular Letter issued by ITU-R calling for submission of candidate Radio Interface Technologies (RITs) and fvcandidate sets of Radio Interface Technologies (SRITs) for IMT-Advanced [1]. However, all documents available in that moment concerning IMT-Advanced did not specify any new technical details about the properties of future 4G systems. Instead, they just reference the Recommendation M.1645 [19], in which the objectives of the future development of IMT-Advanced family were barely deﬁned:toreach100Mb/sformobileaccessandupto1Gb/s. for nomadic wireless access. Unfortunately, it was not until November 2008 when the requirements related to technical performance for IMT-Advanced candidate radio interfaces were described [20].

		Requirement ITU-R M.2134	Requirements LTE-A TR 36.913
Peak data rate (Gbps)		1	1-(DL) 0.5-(UL)
Latency		C-Plane < 100ms U-Plane < 10ms	C-Plane < 50ms U-Plane < 5ms
Peak spectral eﬃciency (bps/Hz)	DL UL	15(4x4) 6.75(2x4)	30(8x8) 15(4x4)
Cell spectral eﬃciency (bps/Hz/cell)	DL UL	2.2(4x2) 1.4(2x4)	2.4(2x2) 2.6(4x2) 3.7(4x4) 1.2(1x2) 2.0(2x4)
Cell edge user spectral eﬃciency (bps/Hz/cell/user)	DL UL	0.06(4x2) 0.03(2x4)/	0.07(2x2) 0.09 (4x2) 0.12(4x4) 0.04(1x2) 0.07 (2x )
Mobility		Up to 350 km/h	Up to 350 km/h
Bandwidth		Up to 40 MHz	Up to 100 MHz

3.1. LTE-Advanced Requirements.

The requirement speciﬁ- cation list was also included in TR 36.913. Although it is expected a list extension, these are some of the current agreements on the requirements for LTE-Advanced [21].

3.2. LTE-Advanced Technical Proposals.

LTE Re l e a s e 8 c an already fulﬁll some of the requirements speciﬁed for IMT- Advanced systems. However, it is also clear that there are more challenging requirements under discussion in the 3GPP, which would need novel radio access techniques and system evolution. The 3GPP working groups, mainly RAN1 working on the physical layer, are currently evaluating some techniques to enhance LTE Release 8 performance. This section oﬀers an overview of some of these proposals.

Support of Wider Bandwidth.

A signiﬁcant underlying fea- ture of LTE-Advanced will be the ﬂexible spectrum usage..

The framework for the LTE-Advanced air-interface technol- ogy is mostly determined by the use of wider bandwidths, potentially even up to 100MHz, noncontiguous spectrum deployments, also referred to as spectrum aggregation, and a need for ﬂexible spectrum usage.

In general OFDM provides a simple means to increase bandwidth: adding additional subcarriers.Due to the discon- tinuous spectrum reserved for IMT-Advanced, the available bandwidth might also be fragmented. Therefore, the user equipments should be able to ﬁlter, process and decode such a large variable bandwidth. The increased decoding complexity is one of the major challenges of this wider bandwidth.

Concerning the resource allocation in the eNB and the backward compatibility, minimum changes in the speciﬁca- tions will be required if scheduling, MIMO, Link Adaptation and HARQ are performed over groups of carriers of 20MHz. For instance, a user receiving information in 100MHz bandwidth will need 5 receiver chains, one per each 20MHz block.

Coordinated Multiple Point Transmission and Reception. Coordinated multi point transmission and reception are considered for LTE-Advanced as one of the most promising techniques to improve data rates, hence increasing average cell throughput. It consists in coordinating the transmission and reception of signal from/to one UE in several geograph- ically distributed points. So far, the discussions have focused on classifying the diﬀerent alternatives and identifying their constraints. Potential impact on speciﬁcations comprises three areas: feedback and measurement mechanisms from the UE, preprocessing schemes and reference signal design.

Relaying Functionality. Relaying can be aﬀorded from three diﬀerent levels of complexity. The simplest one is the Layer 1 relaying, that is, the usage of repeaters. Repeaters receive the signal, amplify it and retransmit the information thus covering black holes inside cells. Terminals can make use of the repeated and direct signals.However, in order to combine constructively both signals there should be a small delay, less than the cyclic preﬁx, in their reception..

4. Conclusions

LTE has been designed as a future technology to cope with next user requirements. In this paper two complete LTE Release 8 link and system level simulators have been presented together with several performance results. Based on these results, this paper concludes that LTE will oﬀer peak rates ofmore than 150Mbps in the downlink and 40Mbps in the uplink with 10MHz bandwidth. Besides, in the downlink the minimum average throughput will be around 30Mbps, which represents a quite signiﬁcant improvement in the cellular systems performance. As compared with current cellular systems, LTE entails an enhancement of more than six times the performance of HSDPA/HSUPA.

This paper has also given an initial insight into the new technical proposals currently under discussion in the framework of 3GPP. .

References

[1] ITU-R Circular Letter 5/LCCE/2, 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, March 2008.

[2] J. J. Sanchez, D. Morales-Jimenez, G. Gomez, and J. T. Enbrambasaguas, Physical layer performance of long term evolution cellular technology, in Proceedings of the 16th IST Mobile and Wireless Communications Summit, pp. 1-5, Budapest, Hungary, July 2007.

[3] A. Osman and A. Mohammed, Performance evaluation of a low-complexity OFDM UMTS-LTE system, in Proceedings of the IEEE Vehicular Technology Conference (VTC '08), pp. 21422146, Singapore, May 2008.

[4] K. Manolakis, A. Ibing, and V. Jungnickel, Performance evaluation of a 3GPP LTE terminal receiver, in Proceedings of the 14th European Wireless Conference (EW '08), pp. 1-5, Prague, Czech Republic, June 2008.

[5] C. Spiegel, J. Berkmann, Z. Bai, T. Scholand, and C. Drewes, MIMO schemes in UTRA LTE, a comparison, in Proceedings of the IEEE Vehicular Technology Conference (VTC '08), pp. 2228-2232, Singapore, May 2008.

[6] N. Wei, A. Pokhariyal, C. Rom, et al., Baseline E-UTRA downlink spectral efficiency evaluation, in Proceedings of the IEEE Vehicular Technology Conference (VTC '06), pp. 21312135, Montreal, Canada, September 2006.

[7] P. Mogensen, W. Na, I. Z. Kovacs, et al., LTE capacity compared to the shannon bound, in Proceedings of the 65th IEEE Vehicular Technology Conference (VTC '07), pp. 12341238, Dublin, Ireland, April 2007.

[8] 3GPP TR 25.913, Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN), v.8.0.0, December 2008.

[9] 3GPP TR 25.814, Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA), September 2006.

[10] 3GPP R1-070674, LTE physical layer framework for performance evaluation, February 2007.

[11] 3GPP TR 25.912, Feasibility Study for evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN), v.8.0.0, December 2008.

[12] 3GPP R1-072261, LTE Performance Evaluation—Uplink Summary, May 2007.

[13] 3GPP R1-072578, Summary of Downlink Performance Evaluation, May 2007.

[14] 3GPP TS 36.101, User Equipment (UE) radio transmission and reception, v.8.5.1, March 2008.

[15] 3GPP TS 36.213, Physical layer procedures, v.8.6.0, March 2009.

[16] 3GPP TS 36.211, Long Term Evolution physical layer; General description, v.8.6.0, March 2009.

[17] 3GPP TS 36.212, Multiplexing and channel coding, v.8.6.0, March 2009.

[18] G. Monghal, K. I. Pedersen, I. Z. Kovacs, and P. E. Mogensen, QoS oriented time and frequency domain packet schedulers for the UTRAN long term evolution, in Proceedings of the 67th IEEE Vehicular Technology Conference (VTC '08), pp. 25322536, May 2008.

[19] ITU-R Recommendation M.1645, Framework and overall objectives ofthe future development of IMT-2000 and systems beyond IMT-2000, June 2003.

[20] ITU-R Report M.2134, Requirements related to technical performance for IMT-Advanced radio interface(s), November 2008.

[21] 3GPP TR 36.913, Requirements for Further Advancements for E-UTRA, v.8.0.1, March 2009.