Relaying Operation in 3GPP LTE: Challenges and Solutions

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Wanshi Chen and Juan Montojo, Qualcomm Inc. Alexander Golitschek, Panasonic R&D Center. Chrysostomos Koutsimanis, Ericsson Research. Xiaodong Shen ...
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LTE-ADVANCED AND 4G WIRELESS COMMUNICATIONS

Relaying Operation in 3GPP LTE: Challenges and Solutions Christian Hoymann, Ericsson Research Wanshi Chen and Juan Montojo, Qualcomm Inc. Alexander Golitschek, Panasonic R&D Center Chrysostomos Koutsimanis, Ericsson Research Xiaodong Shen, China Mobile Research Institute

ABSTRACT With the ever growing demand of data applications, traditional cellular networks face the challenges of providing enhanced system capacity, extended cell coverage, and improved minimum throughput in a cost-effective manner. Wireless relay stations, especially when operating in a halfduplex operation, make it possible without incurring high site acquisition and backhaul costs. Design of wireless relay stations faces the challenges of providing backward compatibility, minimizing complexity, and maximizing efficiency. This article provides an overview of the challenges and solutions in the design of relay stations as one of the salient features for 3GPP LTE advanced.

INTRODUCTION Recent advancements in wide area cellular networks provide high data rates for a variety of scenarios not relying on the availability of wireless local area access networks or wired connectivity, and providing seamless mobility support. Mobile broadband networks deployments span scenarios from very dense urban areas to remote rural areas, operating in a large range of carrier frequencies with different propagation characteristics and different coverage levels. The cost of the backhaul to the various network nodes has been identified as an essential cost component in the deployment of cellular networks, especially for future deployment of micro- and picocells for which the site acquisition cost is expected to be lower than that for macrocells. Therefore, the ability to deploy network nodes not relying on a wired backhaul is an appealing option to reduce total network deployment cost and operating costs. Relaying operation provides the means to extend the coverage of broadband cellular networks coping with diverse radio propagation conditions without a wired backhaul.

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With the help of relay nodes (RNs), the radio link between the base station (eNB) and user equipment (UE) is divided into two hops. The link between the eNB (also called the donor eNB or DeNB) and the RN is referred to as the backhaul link, while the link between the RN and the UE is referred to as the access link, as shown Fig. 1. Both links are expected to have better propagation conditions than the direct link from the DeNB to the UE. An RN has a dual personality. On one hand, it communicates like UE with the DeNB. On the other hand, it communicates like an eNB with UE. Figure 1 illustrates how the RN and its respective physical links are integrated into the cellular concept. Relaying has been a hot research topic for years [1]. The International Telecommunication Union — Radiocommunication Sector (ITU-R) has conferred IMT-Advanced status (fourth generation, 4G) to two technologies: 3GPP Long Term Evolution (LTE) and IEEE 802.16m, both including relaying as one of their key features. Both technologies support fixed and two-hop relays [2]. This article describes relaying in 3GPP LTE and the challenges faced to define this new type of network node within the existing LTE structure. One key requirement set forth at the beginning of the LTE relay study was the support of LTE UE of earlier and current releases (LTE Rel-8/9 and Rel-10, respectively). For these UE terminals, the RN is required to appear as a regular eNB. The article is organized as follows. We present various types of relays that were discussed as part of the LTE-Advanced study. Challenges and requirements for LTE relaying are provided. Detailed solutions for LTE relaying are given, focusing on architecture and protocols, and physical and medium access control (MAC) layer aspects, respectively. Simulation results are shown demonstrating the benefits of relaying

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Outband relaying

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RN

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operation does not

f1 Ba

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ck

at the lower layers

ha

ul

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:t 3, Ac f3 ce ss l

No air interface ink

changes are required UL

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Macrocell

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f4 Relay cell

TYPE OF RELAYS Amplify-and-forward relays, or repeaters, amplify and forward the received analog signals. Repeaters are transparent to both the UE and the eNB. Since a repeater amplifies whatever it receives, including noise and interference, it is mainly useful in high signal-to-noise ratio (SNR) environments. Decode-and-forward relays decode and reencode the received signal prior to forwarding it to the receiver. Since this class of relays does not amplify noise and interference, they are also useful in low-SNR environments. Separate rate adaptation and scheduling for the backhaul and access links is possible. If two or more RNs per DeNB are spatially separated, radio resources on the access link can be reused, which paves the way to increased system capacity. However, the decode-and-forward operation implies a larger delay than for a simple repeater. Two families of decode-and-forward relays were studied as part of the LTE-Advanced study [4]. Type 1 relays are non-transparent to the UE, have their own physical identity, and transmit all the necessary physical channels to appear as a regular eNB to all the UE. Type 2 relays do not have their own physical identity and are transparent to the UE (i.e., the UE is unaware of their existence). Type 2 relays attempt to exploit early decoding on the backhaul link and boosting DeNB retransmissions by participating in the retransmissions. Since the relay transmitter causes interference to its own receiver, simultaneous backhaul link and access link transmissions are not feasible unless sufficient isolation of the outgoing and incoming signals is provided. The means for the isolation include: • Frequency multiplexing the used frequency bands (a.k.a. outband relaying) • Spatial separation of the relay antennas (a.k.a. full-duplex inband relaying) [4] • Time multiplexing the access and backhaul subframes (a.k.a. half-duplex inband relaying) Outband relaying operation does not pose

IEEE Communications Magazine • February 2012

of relay. However, an additional LTE carrier is required.

Figure 1. Relay deployment example. operation. Future trends are briefly discussed. Finally, conclusions are drawn.

to support this kind

any challenges at the lower layers of the LTE protocol stack. No air interface changes are required to support this kind of relay. However, an additional LTE carrier is required. Inband relaying operation requires the separation of backhaul and access links either in time or in space. Spatial separation between access and backhaul links can be attained by sophisticated implementation and deployment (e.g., by sufficient antenna separation or isolation). Under sufficient spatial separation, concurrent operation of backhaul and access links is possible, resulting in full-duplex relaying. Temporal separation implies that in the downlink (DL) carrier frequency, at a given point in time, the RN either transmits on the access link or receives on the backhaul link. Also, in the uplink (UL) carrier frequency, at a given point in time, the RN either receives on the access link or transmits on the backhaul link. This duplexing of transmission/reception yields an effective half-duplex operation on the backhaul and access links. Looking at the notation introduced in Fig. 1, time multiplexing implies f1 = f3 and f2 = f4, as well as t1 ≠ t3 and t2 ≠ t4. The 3GPP specifications support relays of Type 1 according to time and frequency multiplexing [4]. Since outband relaying has no physical layer impact, we will mainly focus on the solution applicable for half-duplex inband relaying.

CHALLENGES AND REQUIREMENTS LTE BASICS In LTE, two duplexing schemes are defined: frequency-division duplexing (FDD) and time-division duplexing (TDD). The basic scheduling unit is a subframe, which is 1 ms long, and10 subframes constitute a radio-frame [5]. One of the characteristics of LTE in the DL is the time-division multiplexing (TDM) of control channels and data channels within each subframe. The control channel consists of grants allocating physical time/frequency resources for the same subframe for DL data assignments and a subsequent subframe for UL data assignments. The control region carrying DL control channels is concentrated in the earlier part of the subframe, while the rest of the subframe constitutes the data region of the subframe.

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MME/S-GW

S11 S1

S1

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S1

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E-UTRAN DeNB

S1 X2 Un

eNB

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Figure 2. E-UTRAN architecture including relay nodes.

The LTE frame structure supports regular and so-called multicast/broadcast over single-frequency network (MBSFN) subframes. While regular subframes are meant to have valid content in the entire duration of a subframe, MBSFN subframes only require transmission of the control region for legacy UE support. Indeed, MBSFN subframes were originally introduced to provide a subframe structure different from regular subframes in support of multicast services using single-frequency network operation with their own reference signals. The concept of MBSFN subframes makes it possible to introduce new features in a backward-compatible manner, as detailed later.

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traffic and a demultiplexer for DL traffic. As a consequence, the backhaul link is most probably the bottleneck, and should be targeted to operate at high spectral efficiency. • A half-duplex relay continuously switches its radio frequency (RF) circuitry between backhaul and access link operations. Switching should be as fast as possible, since a longer switching period automatically means a loss of physical resources (time) that could be available for backhaul and access link operations. A major challenge in the backward compatibility requirement of RNs arises from the UE’s assumption that the DL cell-specific reference signals (CRS) are present in each subframe. CRS are used not only for demodulation and channel state information feedback, but also for mobility measurements and radio link monitoring. Therefore, an RN has to transmit CRS in each (DL) subframe while still having opportunities to communicate with the DeNB. It was desirable that the protocol operations on the backhaul link have no adverse effects on the protocol operations on the access link. hybrid automatic repeat request (HARQ) is an important means to recover from transmission errors. While a retransmission on the access link affects only the traffic between a single UE terminal and the RN, a retransmission on the backhaul link affects the traffic between the UE attached to the RN and the DeNB. It is therefore required that the HARQ on the backhaul link can be operated as efficiently as possible, particularly with respect to the delay caused by HARQ retransmissions. On the access link, the backward compatibility requires that the existing HARQ methods from Rel-8 are used without modifications.

SOLUTIONS: ARCHITECTURE AND PROTOCOLS

CHALLENGES AND REQUIREMENTS

In this section, we discuss the architecture and protocols of the relay operation in 3GPP LTE.

A major challenge for the manufacturing and deployment of RNs is cost efficiency. RNs should have an advantage in operational and/or capital expenditure over the installation of a full-fledged eNB. Consequently, it is necessary to evaluate the RN not only from a technical perspective, but also keeping its attractiveness for operators in mind. The development and manufacturing cost of an RN should reuse functionality for earlier LTE releases as much as possible, and consider new enhancements only when necessary. Costly upgrades of network nodes upon introducing RNs in existing LTE networks should be avoided. In addition, cost effectiveness is also achieved from the requirement that RNs be backward compatible, such that legacy UE can fully benefit from the deployment of RNs. Half-duplex inband relaying puts several restrictions and challenges particularly on the physical layer transmission and relay behavior: • Since the RN is basically a forwarding node, the additional delay caused by the relay operation should be as small as possible. • The RN is acting as a multiplexer for UL

As in the physical layer, the RN also has a dual personality from an architecture and protocols perspective. On one hand, it supports normal eNB functionality, such as terminating the radio protocols and the base-station-to-core network (S1) and inter-base station (X2) interfaces. The Evolved Universal Terrestrial Radio Access (E-UTRAN) architecture, including relay nodes, is shown in Fig. 2. On the other hand, an RN supports a subset of UE functionality (e.g., the procedure to attach to the network). In order to meet the above mentioned design guidelines, a proxy concept has been developed that minimizes the impact on network nodes: from a core network perspective, the RN is seen as a normal eNB and from an RN perspective, the DeNB mimics a regular core network. Hence, existing interfaces and functionality can be reused. Each RN sets up S1 and X2 connections with its DeNB. S1 connects the RN to the mobility management entity (MME) via the DeNB, and

PROXY CONCEPT

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X2 connects the RN to other eNBs via the DeNB. The DeNB, acting as a proxy, will report the RN as one of its controlled cells to neighbor eNBs, and it will also forward UE-dedicated signaling messages (e.g., during handover). The DeNB also provides gateway functionality to its connected RNs [6].

One subframe First slot

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SIGNALING PROCEDURES

Bearer Management — Bearers for the RN can be activated and modified by the DeNB. This procedure is the same as the normal network-initiated bearer activation/modification procedure for UE [8], with the exception that in this case gateway functionality is embedded within the DeNB. Handover — As it is built on base stations’ functionality, LTE relaying supports legacy mobility in RRC connected state. The DeNB, aware of a certain set of UE being attached to the RN, proxies the relevant S1 messages between the RN and the MME (S1 handover) and, correspondingly, the X2 messages between the RN and the target base station (X2 handover).

RADIO PROTOCOLS According to the RN’s dual personality, it appears as a regular base station to its own UE, fully reusing the LTE radio interface with its protocols and procedures. The same radio protocols are reused on the backhaul, with certain control plane protocol additions. The most significant addition is the possibility to configure backhaul subframes and a newly defined control channel, as discussed later.

SOLUTIONS: PHYSICAL AND MAC LAYER ASPECTS Here, we focus on the physical [9] and MAC layer solutions of the relay operation in 3GPP LTE.

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R-PDCCH (DL grants)

Frequency

Relay Startup and Attach — Before initial attach, the RN can be preconfigured with information about which cells (or DeNBs) it is allowed to access. Such a list can be either downloaded over the air or preprogrammed. At power up, the RN can first attach to the network using the normal attach procedure for UE [8] and retrieve its initial configuration parameters (e.g., the list of available DeNBs). During this phase, the RN is allowed to connect only to its management system, thereby reducing possible security threats from a “rogue” RN. The RN then detaches from the network. The RN now connects to a DeNB selected from the previous list. The startup procedure is the same, except that this time the RN signals its identity to the chosen DeNB using a specific RN indicator at radio connection setup. Based on this information, the DeNB now also acts as a gateway, and it selects an MME that supports relay functionality. After the MME successfully sets up the context and the DeNB sets up bearers for S1 and X2, the RN will set up S1 and X2 connections with its DeNB.

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Figure 3. Relay DL backhaul design.

SUBFRAME CONFIGURATION In order to create a transmission gap on the access link that even LTE Rel-8 UE can handle, the RN can declare an MBSFN subframe in the DL. When a UE terminal is informed of an MBSFN subframe, it processes the first few symbols carrying control information, but can ignore the remainder of the subframe (unless the UE is configured to process the multicast/broadcast service). An RN can thus use the remainder of such subframes for switching its RF circuitry and receiving channels from the DeNB on the backhaul link. This operation is illustrated in Fig. 3, where the DL data channel is denoted physical DL shared channel (PDSCH), and R-PDCCH denotes the new relay physical downlink control channel. Due to the transmission of synchronization and broadcast signals, only 6 out of 10 subframes can be declared MBSFN subframes. The DeNB has the flexibility of configuring different backhaul subframes for an RN and across different RNs, excluding subframes that cannot be declared MBSFN subframes by the RN. In particular, a UL backhaul subframe implies that the RN cannot receive UL access link transmissions from UE in that subframe; however, no special provisions are necessary since the RN can avoid UE UL transmissions by not scheduling any UL transmission resources in such a subframe. Reconfiguration of the subframes used for backhaul communication is supported in order to accommodate long-term changing link qualities on the access and backhaul links, and longterm changing traffic to/from the UE connected to the RN. However, reconfiguration is not expected to be frequent.

BACKHAUL CHANNEL DESIGN The design of backhaul channels (R-PDCCH and PDSCH, Fig. 3) should have minimal standardization and implementation impact. In other

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User distribution

Non-uniform user distribution: 4 clusters per macrocell, hotspot probability = 4/15 according to configuration 4a of [4]

Traffic model

Equal UL buffer size across UE

Network environment and deployment

3GPP case 1 (intersite distance: 500 m) [4] 2 or 4 RNs deployed randomly or planned based on the user clusters 25% indoor users (hotspot users are indoors) Exterior wall loss 20 dB

Bandwidth

10 MHz FDD at 2 GHz

Transmission scheme

2 × 2 MIMO in DL 1 × 2 SIMO in UL

Antenna gains

14 dB at the DeNB 2 dB at the RN

Output power

DeNB: 43 dBm RN: 30 dBm UE: 21 dBm

Cell selection

Based on DL reference signal received power (RSRP)

Table 1. Simulation assumptions.

for several subframes in the future. Therefore, it is sufficient to have TDM-like R-PDCCH for DL grants. Since a subframe consists of two slots, and many features in LTE Rel-8/9 are slotbased, it is natural to enforce a boundary for RPDCCH for DL grants at the end of the first slot of a subframe. Now, in order to utilize the second slot of a subframe and for simplicity, R-PDCCH for UL grants or PDSCH transmission can utilize the second slot. With this, the entire subframe can be efficiently utilized. At the same time, a PDSCH for direct link UE is thus still multiplexed with R-PDCCH in an FDM manner, resulting in minimal standardization and implementation impact. If there is no DL grant in the first slot, and there is a UL grant in the second slot in the same resource pair, the first slot can not be used by a PDSCH and is left empty. The demodulation and decoding of RPDCCH can be based on either CRS or UE-specific reference signals (UE-RS). CRS-based R-PDCCH can be more frequency distributed, resulting in improved frequency diversity and interference diversity gain. UE-RS-based RPDCCH facilitates the usage of beamforming, especially when reliable channel information feedback is available.

HYBRID ARQ OPERATION words, the backhaul channel design should reuse the existing design for direct link UE as much as possible. At the same time, the design of backhaul channels should aim for maximal backhaul efficiency. To that end, the design should consider the follow needs: • Multiplexing R-PDCCH with PDSCH in the same set of resources • Multiplexing relay backhaul channels with the data channels of direct link UE in the same subframe • Early decoding of R-PDCCH for DL traffic • Supporting various backhaul operations One approach for multiplexing is based on frequency-division multiplexing (FDM), where R-PDCCH utilizes resources covering the entire duration of the Rel-8/9 data region. R-PDCCH is thus clearly separated from other channels, which greatly facilitates multiplexing and minimizes scheduling complexity. One disadvantage is the difficulty in early R-PDCCH decoding. An RN may have to wait until the end of a subframe before R-PDCCH decoding, only after which can PDSCH decoding start. Another possibility is to have a TDM-based R-PDCCH design. A few OFDM symbols can be set aside for R-PDCCH across all or a fraction of the system bandwidth, similar to the way control and data channels are multiplexed in Rel8/9. While it offers the early decoding benefit, it is not desirable either, because it prevents multiplexing with legacy data transmissions and complicates the reuse of existing reference signals. A hybrid approach of FDM and TDM was adopted. Note that early decoding of R-PDCCH is more relevant to DL assignments as these typically have a more immediate impact on UE operation than UL assignments, which are meant

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Access Link HARQ Operation — No changes are made to the HARQ operation to ensure backward compatible. HARQ acknowledgments to UL data transmissions can still be transmitted by the RN in the control region of MBSFN subframes. If necessary, ongoing PUSCH transmissions can be suspended by HARQ acknowledgments. In DL, HARQ acknowledgments to DL PDSCH transmissions for access link UE are guaranteed by design. For instance, in FDD, the 4 ms HARQ acknowledgment delay is ensured by implicit UL backhaul subframe configuration, which always occurs four subframes after DL backhaul subframes. Backhaul-Link HARQ Operation — For FDD, DL backhaul subframes are configured with a periodicity of eight subframes, excluding subframes that cannot be declared as MBSFN subframes by the RN. The MBSFN subframes follow a 10 ms structure, while the HARQ timing follows an 8 ms periodicity. Due to this inherent mismatch, the DeNB would not always be able to transmit HARQ acknowledgments in response to a UL transmission, unless revised HARQ timing is designed. Hence, for simplicity, negative HARQ acknowledgments are not used on the backhaul link for UL transmissions. Instead, the DeNB can issue a new scheduling grant, indicating an unsuccessful transmission and thus triggering the corresponding retransmission. In TDD, for each supported DL/UL configuration, one or several backhaul configurations are supported, providing necessary flexibility for relaying operation. Note that for most TDD downlink/uplink configurations, UL HARQ timing follows a 10 ms round-trip time, which matches the 10 ms MBSFN structure.

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PERFORMANCE

Uplink 1 Mb/s per macrocell

This section discusses performance results of an urban cellular LTE network with non-uniform user distribution. Two different outdoor relay deployments are considered: random deployment and planned deployment. The number of RNs per DeNB is 0, 2, and 4. More detailed parameters are given in Table 1.

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SYSTEM CAPACITY Figure 5 shows the cell edge and mean UL user throughput as a function of the traffic load. For a given traffic load, the results show that randomly deployed relays improve the cell edge throughput only at low loads. However, planned relays can greatly improve cell edge throughput even at high loads.

FUTURE TRENDS LTE relaying is expected to evolve along with future application scenarios. High-speed public transportation is being deployed worldwide at an increasing pace. Hence, providing good quality data service to users on high-speed vehicles is important, yet more challenging than typical mobile wireless environments due to a typical reduced handover success rate and a higher Doppler effect. Mobile relay (a relay mounted on a vehicle and wirelessly connected to the

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80 70

CDF (%)

With a certain resource partitioning 0 ≤ α ≤ 1, between backhaul and access links, the end-toend user throughput can be approximated by min[αR backhaul , (1 – α)R access ], where R backhaul and Raccess are the individual throughput values on the backhaul and access links, respectively. An optimal value of α is RN-specific, but it can only be configured per DeNB. In the simulations, α is fixed at 0.5. Conventional cell selection, which is modeled here, is based on the DL reference symbol receive power of the access link only. This may be improved by considering the quality of the wireless backhaul. In the following, we focus on the UL performance since this link is limited by the restricted UE transmit power. Figure 4 shows the cumulative distribution functions (CDFs) of the UL user throughput for the five different cases (0, 2, 4 relays per cell and planned/random deployment). Cell edge user throughput (5th percentile) can be significantly enhanced, especially by planned relays (e.g., an increase up to 650 percent when deploying four well planned RNs). UE terminals that are not connected to a relay might also benefit like cell edge users due to cell offloading. The discontinuity around half of the peak rate is a result of the resource partitioning. With a backhaul allocation α = 0.5, the end-to-end peak rate is limited to half of the single-hop peak rate. It can be observed that not all users benefit from being connected via relay. As shown in Fig. 4, with four planned relays per cell, 51 percent of users are connected via relays, although the targeted hotspots only attract 25 percent of all users. This indicates the potential of advanced cell selection.

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Figure 4. UL user throughput.

macro networks) is a potential technique to solve the above problems. The backhaul link has been identified as the performance bottleneck, and improvements to the relay backhaul link may be considered in future enhancements of LTE. This could, for instance, be achieved by carrier aggregation or improved DL and UL backhaul efficiency (e.g., with a larger number of transmit antennas, more efficient resource management). Relaying in LTE Rel-10 supports two-hop communications. Supporting more than two hops is worth further investigation in the future to assess the trade-off between performance benefits, and standardization and implementation complexity.

CONCLUSIONS Wireless relay stations in 3GPP LTE, especially when operating in half-duplex operation, make it possible to achieve enhanced system capacity, extended cell coverage, and improved minimum throughput. Different types of relays have been discussed as part of the LTE-Advanced study. Non-transparent half-duplex inband relays are fully backward compatible, and are deemed an efficient, effective, and practical means to complement the existing cellular networks. Non-transparent half-duplex relaying operation in 3GPP LTE imposes great design challenges stemming from the support of legacy LTE UEs, and maximizing the reuse of the existing LTE physical layer, MAC layer, and upper layer standards. Detailed solutions for LTE relaying operation are provided in this article. Simulations show promising gains in certain relay scenarios, most notably when the RNs are placed close to the UE in a hotspot fashion. The key to relay gains is a significant increase of the quality of the access and backhaul links compared to the direct link, especially when proper

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from RWTH Aachen University in 2002. At RWTH’s Chair of Communication Networks he worked toward his doctoral degree, which he received in 2008. Since 2007 he has worked at Ericsson Research, where he focuses on advancing 3GPP LTE, especially in the areas of relaying, CoMP, and heterogeneous networks. He served as rapporteur of the Relaying work item in 3GPP. Currently, he is technical coordinator of Ericsson’s RAN1 delegation.

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Macro mean Macro 5th perc 2Rel/M, random mean 2Rel/M, random 5th perc 2Rel/M, planned mean 2Rel/M, planned 5th perc 4Rel/M, random mean 4Rel/M, random 5th perc 4Rel/M, planned mean 4Rel/M, planned 5th perc

10

W ANSHI C HEN ([email protected]) received a B.S. degree (with highest honors) from Southwest Jiaotong University, Chengdu, China, an M.S. degree from the Ohio State University, Columbus, and a Ph.D. degree from the University of Southern California, Los Angeles, respectively. From 2000 to 2006 he was with Ericsson working on CDMA2000 related research, implementation, and standardization. Since May 2006 he has been with Qualcomm Inc., where he is involved in research and development efforts related to 3GPP LTE standards.

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Figure 5. UL cell-edge user throughput vs. traffic demand.

deployment of relays is possible and sophisticated cell selection is used. Finally, we have touched upon possible areas of future research and standardization for enhanced relaying operation in LTE.

REFERENCES [1] B. Walke et al., 2009, Layer-2 Relays for IMT-Advanced Cellular Networks, in Radio Technologies and Concepts for IMT-Advanced, M. Döttling, W. Mohr, and A. Osseiran, Eds., Wiley, Sept. 2009. [2] K. Loa et al., “IMT-Advanced Relay Standards,” IEEE Commun. Mag., Aug. 2010. [3] 3GPP TS 36.300 v10.1.0, E-UTRAN Overall Description; Stage 2 (Release 10), Apr. 2010. [4] 3GPP TR 36.814 v9.0.0, Further Advancements for EUTRA (Physical Layer Aspects), Mar. 2010. [5] E. Dahlman, S. Parkvall, and J. Sköld, 4G: LTE/LTEAdvanced for Mobile Broadband: LTE/LTE-Advanced for Mobile Broadband, Academic Press, Apr. 2011. [6] M. Olsson et al., SAE and the Evolved Packet Core — Driving the Mobile Broadband Revolution, Academic Press, 2009. [7] H. Ekström, “QoS Control in the 3GPP Evolved Packet System,” IEEE Commun. Mag., Feb. 2009. [8] 3GPP TS 23.401, GPRS Enhancements for E-UTRAN Access (Release 10). [9] 3GPP TS 36.216 v10.3.0, Physical Layer for Relaying Operation (Release 10), June 2011.

BIOGRAPHIES C HRISTIAN H OYMANN ([email protected]) received his Diploma degree in electrical engineering

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ALEXANDER GOLITSCHEK ([email protected].ºcom) holds the position of principal engineer at the Panasonic R&D Center Germany GmbH, where he is mainly responsible for physical layer mobile communication research related to 3GPP. His research interests currently include physical layer control procedures and corresponding signaling for carrier aggregation, multipleantenna techniques, and relaying. He has regularly contributed to and participated in 3GPP meetings since 2006, and is the rapporteur for the technical specification “Physical Layer for Relaying Operation.” He received his Dipl.-Ing. degree from Universität Karlsruhe in 1997. J UAN M ONTOJO ([email protected]) is a principal engineer and manager in the Systems Engineering department of Qualcomm’s Corporate R&D group where he has been since 1997. He holds telecommunications engineering degrees from the Universitat Politècnica de Catalunya, Barcelona, Spain, and Institut Eurecom, Sophia Antipolis, France. He also holds an M.S. from the University of Southern California and a Ph. D. from the University of California San Diego, both in electrical engineering. He has been involved in the design, specification, and implementation of multiple wireless systems: Globalstar, cdma2000 1xEV-DO, TD-SCDMA, and WCDMA/HSPA. Since 2005 he has been heavily involved in the design and specification of the physical layer of 3GPP’s LTE and is the editor of one of the physical layer specifications (3GPP TS 36.212, “Multiplexing and Channel Coding”). XIAODONG SHEN ([email protected]) received his M.S. in electrical engineering from Beijing University of Posts and Telecommunications in 2007, and his B.S. (Phy) from Peking University in 2004. He has been a project manager at China Mobile Research Institute since 2007. His current interests include LTE, LTE-Advanced, and IMTAdvanced research and standardization for 3GPP, ITU, and NGMN. CHRYSOSTOMOS KOUTSIMANIS ([email protected]) is a senior research engineer at Wireless Access Networks of Ericsson Research in Sweden. He obtained his Diploma in electrical and computer engineering from the National Technical University of Athens, Greece, in 2004, and his M.Sc. degree in wireless systems from Royal Institute of Technology, Stockholm, Sweden, in 2007. His main research interests are in the area of radio network algorithms, covering optimization, performance evaluation, and concept development.

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