Radio Enabling Techniques for IMT-Advanced (4G) and beyond: WINNER+ Project Afif Osseiran#1, Mauro Boldi*, Jose F. Monserrat^, Jaakko Vihriälä&, Antti Tölli%, Alexandre Gouraud$ #

Ericsson AB, Stockholm, Sweden

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Telecom Italia Lab, Italy ^Universidad Politécnica de Valencia, Spain & Nokia Siemens Networks, Finland % University of Oulu, Finland $ Orange Labs, France Spectrum innovations techniques that may be applied in the future IMT-Advanced standards within the scope of the LTEAdvanced development process. The main goal is to harmonize the innovations within a WINNER+ and select the most promising techniques in order to allow: • System Integration and evaluation of innovations in areas with high potential of exploitation in IMT-Advanced; • Harmonization of innovations in the prestandardization phase; • Participation in the evaluation of selected technology proposals and strong contribution to ITU-R WP5D. The evaluation of IMT-Advanced toward ITU-R WP5D group was successfully completed [11]. Further some of the selected techniques for IMT-Advanced techniques were evaluated in depth, for more details see [7]. This article provides an overview of the WINNER+ most promising techniques within the above stated innovation areas of ARRM, CoMP, Network Coding, and Spectrum innovations. The remainder of this paper outlines the main techniques associated with each area.

Abstract— Enabling radio innovation techniques for IMTAdvanced and beyond investigated in the WINNER+ project are summarized in this paper. These techniques survey five innovation areas: Advanced Antenna Systems (AAS), Advanced Radio Resource Management (ARRM), Network Coding, Coordinated Multipoint (CoMP) transmissions and Spectrum Technologies. In AAS, under the framework of cellular multiuser MIMO systems, the proposals focus on how to make the channel state information (CSI) available at the transmitter, to reduce the pilot overhead and to utilize the acquired CSI for efficient downlink precoding. Within CoMP transmissions, coordinated scheduling and/or beamforming and joint processing are briefly presented and analysed. In ARRM carrier aggregation is described. In network coding the application of non-binary codes is presented. Concerning spectrum technologies special attention is given to the co-existence of macro-layer and femto-layer and consequently how to coordinate the interference.

I. INTRODUCTION The race towards International Mobile Telecommunications (IMT)-Advanced is progressing rapidly. At the 3GPP, the Long Term Evolution (LTE)-Advanced Study Item was launched in May 2008 and is being finalized early 2010. In IEEE 802.16 standard, IEEE 802.16m is the IMT-Advanced candidate. It is described in the System Description Document (SDD) and was already finalized in September 2009. Both LTE-A and IEEE 802.16 were submitted to ITU (International Telecommunication Union) as IMT-A technology candidates. The final evaluation results was submitted to ITU for Radio (ITU-R) in June 2010. Consequently ITU-R is expected to announce the accepted terrestrial radio interface technologies through a Circular Letter in October 2010. The CELTIC WINNER+ (Wireless World Initiative New Radio) [1] project has contributed to the definition of IMTAdvanced technology proposals and beyond, especially the 3GPP LTE-Advanced. WINNER+ maintains the technological development path by targeting IMT-Advanced technologies and their evolutions. The first phase of the project saw the development of innovative techniques integrated with the overall system functionalities [2]. In the second and final phase of WINNER+, we pursued the development of promising AAS, ARRM, CoMP, Network Coding and

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II. ADVANCED ANTENNA SYSTEMS The spectral efficiency of multiantenna transmission can be significantly increased if channel state information (CSI) is available at the transmitter, allowing the system to effectively adapt to the radio channel and take full advantage of the available spectrum. Providing full CSI feedback may cause an excessive overhead, and hence quantized instantaneous and/or statistical CSI are preferable in practice. A simple alternative in using time division duplex (TDD) system where the same carrier frequency is alternately used for transmission and reception, and thus the CSI can be tracked at the transmitter during receive periods, provided that fading is sufficiently slow and the radio chains are well calibrated. The innovations introduced in WINNER+ project [7] focus mostly on seeking for system performance improvements from advances in the acquisition of channel state information at transmitter (CSIT) – short term or long-term – via new

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signaling and estimation solutions. The framework of cellular multiuser MIMO systems is considered, where a base station employing an antenna array communicates with user terminals, each equipped with one or more antenna elements. The framework of the presented solutions consists of spatial user multiplexing or scheduling, and beamforming by means of linear transmit precoding. The problem of acquiring the CSIT consists of multiple tasks, such as pilot signal design, channel state and quality estimation, as well as feedback signal design. All these aspects were addressed in order to enhance the system performance. The innovations can be grouped into four main categories presented. Each subgroup includes a multitude of proposals of which a more detailed description can be found in [7]. The performance gains and the applicable scenario of the proposed innovations are summarized in TABLE I.

of the major drawbacks related to joint processing is its high complexity, in particular regarding the backhaul and signaling overhead. In order to reduce these complex requirements, clustering solutions that restrict joint processing techniques to a limited number of BSs (statically or dynamically) have been proposed, and constitute an important achievement of the project.

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TABLE I ADVANCED ANTENNAS CATEGORIES

Enhancements for codebook based multiantenna transmission Feedback methods for multi-user MIMO zeroforcing Resource allocation schemes for TDD systems Coding and decoding

Applicable to FDD/TDD Applicable to UL/DL FDD (and TDD) downlink FDD (and TDD) downlink TDD downlink and uplink Any duplexing mode and link

Expected performance

Improved spectral efficiency for cell-edge users

Fig. 1 Schematic representation of a CoMP scenario.

Improved spectral efficiency in environments with high antenna correlation Highly increased overall performance if accurate CSI available Provides diversity, coding, and decoding gain for pointto-point links

In a first approach, a single and static cluster of BSs is considered. In this case, a user-centric partial joint processing (PJP) scheme is proposed to reduce both the inter-base information exchange and the feedback from the users compared to a fully centralized approach. The average sumrate per cell for cell border users can be increased by a factor ranging from 2 to 4 depending on the different configurations of PJP with respect to the uncoordinated case. In a second step, focus is cast on a multi-cluster level and a dynamic and network-centric clustering approach including issues of user scheduling. A star topology is requested with a master central unit. Based on the CSI and on the scheduling requirements, the central unit jointly creates the clusters of collaborating BSs, schedules the users in these clusters and calculates the beamforming coefficients and the power allocation. In this approach, substantial gains are obtained with respect to a static clustering scenario. As an example, for cell edge users the throughput could be doubled. A dynamic clustering technique is combined with multiantenna receivers in another proposed solution, with, in addition a concept for a scalable CSI feedback. The basic idea is to enable each user to generate and provide CSI feedback by selecting a preferred receive strategy. Each user can choose its desired receive strategy according to its own computational capabilities and knowledge of the CSIs at the receiver including interference, independently from other users. This allows to benefit from two major advantages: first, the multiple receive antennas are efficiently used for suppression of external interference at the user side. Second, by reducing the number of data streams per user, the system can serve instantaneously a larger set of active users. With this approach the sector spectral efficiency could be increased by a factor of 1.5-2 also with very small clusters.

III. COORDINATED MULTI-POINT TRANSMISSIONS CoMP transmission and reception is one of the most promising techniques currently proposed in order to increase the data rates especially of the cell edge users. CoMP refers to a system where the transmission and/or reception at multiple, geographically separated antenna sites is dynamically coordinated in order to improve system performance (see Fig. 1 for a schematic representation of a CoMP scenario). At a high level, downlink coordination schemes can be divided into two categories: coordinated scheduling/beamforming (CB CoMP) and joint processing transmission (JP CoMP). In the first category the data to a single user equipment (UE) is instantaneously transmitted from a single transmission point, while in the joint processing/transmission category the data to a single UE is simultaneously transmitted from multiple transmission points so that the received signal quality and/or cancel actively interference for other UEs. It can be noted that in the case of coordinated beamforming the requirements on the link coordination and on the backhaul are significantly reduced. On the other hand, within the framework of CoMP the best performances are from JP CoMP schemes (in case of prefect CSI), especially in conjunction with a proper selection of the cells to be conveniently gathered in a cluster where the scheme is applied. Indeed, from a practical point of view, one

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IV. NETWORK CODING Network coding (NC) is currently emerging in multi-hop or multi-user wireless networks as a new class of information processing and transmission techniques. Compared to traditional routing techniques, network coding allows information processing in the intermediate nodes. Performance gains in e.g., energy-efficiency, fairness, robustness, or coverage are obtained. Though network coding was originally proposed for error-free computer networks, the principles of network coding can be applied also to implement wireless communications. An illustration of network coding in a bidirectional scenario is shown in Fig.2. In WINNER+ we proposed and investigated the performance of non binary network coding in cooperative and multiple-relay scenarios. In addition we investigated the relay selection and user grouping in a relay multiple access scenario.

Carrier aggregation is one of the main factors of the success of the next 4G technologies. This concept implies transmitting data on multiple contiguous or non-contiguous sub-bands, called component carriers. Each component carrier occupies up to 20 MHz of bandwidth in which it can be transmitted information towards LTE or LTE-Advanced mobiles. In WINNER+ the concept of carrier aggregation has been deeply assessed from different points of view; specifically from physical layer, MAC layer and signaling. Three are the main conclusions drawn. Regarding channel coding, transport block segmentation should be avoided as much as possible, since it naturally entails some degradation in the system performance. The improvement achieved with low density parity check (LDPC) code is, in most cases, limited to 0.5 dB. Provided that LDPC are not backward compatible, its inclusion in next generation networks seems not justified. Second, from the scheduling perspective, a significant advantage of noncontiguous carrier aggregation over contiguous aggregation has been observed, mostly due to the higher spectral diversity of the former strategy. The disadvantage regards hardware redundancy, i.e. the employment of more than one physical (and possibly MAC) layer processing chains. Third, with regards to CQI signaling in CA, from the point of view of the CQI reporting procedure in a bandwidth aggregation scenario, the research proposes a method to define the CQI report granularity in the time domain and in the frequency domain depending on the carrier the UE is using aiming to save uplink bandwidth without degrading the system performance.

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VI. SPECTRUM TECHNOLOGIES Flexible spectrum use (FSU) and spectrum sharing are Fig. 2 An example of three phases’ bidirectional network coding. promising candidates for increasing spectral efficiency of a The multiple-user multiple-relay (MUMR) using non- cellular network on the system level. An important technique binary network coding consists of several users have in the area of FSU is that of femtocells which are low-cost, independent information to be transmitted to a common base low-power, short-range, plug-and-play base stations, which station (BS), with the help of multiple relays. In fact the non can be used for offloading traffic from the macro cells, thus binary codes are used on the top of channel codes to rebuild resulting in throughput gain. source information from the minimum possible set of coded Due to their uncoordinated nature, minimal changes are blocks. Further the used linear non binary network codes are needed in the macro network. On the other hand, if some asymptotically optimal in terms of diversity (diversity order 3), amount of coordination is allowed, the performance can be see [7]. improved by more advanced interference control and channel In a multi-cell network coding wireless relaying system the allocation methods. This would affect both macro- and introduction of adequate joint or disjoint user grouping and femtocells; decreased femto-to-macro interference would relay selection allows exploit fully the system performance [9]. increase macro cell throughput, and more optimal channel In fact the usage of user grouping and relay selection for allocation in the femtocell would improve its performance. Network Coding improves the cell throughput by 70%. Since femtocells are deployed by the end users, their placement can be modeled as a random drop, example of V. ADVANCED RADIO RESOURCE MANAGEMENT which is given in Fig. 3. Femtocells are represented by circles, Due to the huge complexity of the WINNER+ system, the macro eNB’s by triangles and UE’s by dots. innovative algorithms related to resource allocation are Interference caused by the femtocell is illustrated in Fig. 4. referred to as Advanced Radio Resource Management The most detrimental interference is the femto-to-macro (ARRM) techniques. Four main ARRM concepts were interference, caused by femto base station (HeNB in 3GPP identified [3]: Coordinated Multipoint (CoMP) Systems, terminology) DL transmission to the macro UE (MUE) DL Cross-layering for QoS, Carrier Aggregation (CA) and reception. The MUE in the cell edge may receive weak signal Enhanced Single Frequency Network (SFN) Multicasting. from the eNB, and a nearby HeNB may cause unacceptable This section focuses only on CA given its special relevance. level of interference. On the other hand, also macro-to-femto

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In the uncoordinated case, there is no signalling between macro and femto networks. Instead, femto-to-macro interference is avoided by using TDD on UL band in the femtocell [4], [7]-[8]; this way, when the distance to the nearest macro BS is large enough, the femtocell does not cause any interference if the transmission powers in the femtocells are kept low enough. The distance is estimated by measuring the path loss from the macro BS.

interference is a problem from the femtocell point of view; a nearby MUE causes interference to the femtocell. An obvious method to decrease interference is to limit transmission power of FUE and HeNB, but even this does not decrease the interference to and acceptable level. The femtocell innovations in WINNER+ can be divided into two categories: coordinated and uncoordinated. 1400

VII. CONCLUSIONS This paper described briefly the most important selected techniques for IMT-Advanced technologies in the following innovation areas within WINNER+: advanced antenna schemes (AAS), coordinated multipoint (CoMP) systems, advanced RRM (ARRM), network coding and spectrum technologies. These techniques improve substantially the system performance. For instance CoMP allows lifting the cell edge spectral efficiency while being sensitive to channels impairments. Uncoordinated femtocell deployment is another technique which can be used to lift the system throughput.

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ACKNOWLEDGMENT This article has been written in the framework of the CELTIC project CP5-026 WINNER+. The authors would like to acknowledge the contributions of their colleagues.

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Fig. 3 An example of a femtocell deployment.

In the coordinated approach, the interference from the macro network to the femtocell is avoided by extending the intercell interference coordination (ICIC) mechanism to the femtocells [4]. This method is used to deny the HeNB’s use of physical resource block (PRB) which would cause high interference to the macro network. Game theoretical approach [7] is then used to further optimize the resource usage for those PRB’s where interference is on an intermediate level. Even though this method is called “coordinated”, the nature of the femtocells is still uncoordinated; their deployment is done by the end user, in a plug-and-play fashion, and the network has no control over the femto BS locations.

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Fig. 4 Femtocell interference.

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WINNER Project IST 2004-507581, WINNER II Project IST-4-027756 and WINNER+ Project CELTIC CP5-026, http://projects.celticinitiative.org/winner+/ A. Osseiran et al., Radio Innovation Areas for IMT-Advanced & Beyond: WINNER+ System Concept, Proc. of ICT Mobile Summit 2009, Santander, Spain, June 2009 Celtic CP5-026 WINNER+, Intermediate Report on System Aspect of Advanced RRM, Deliverable D1.5, November 2009, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html. Celtic CP5-026 WINNER+, Intermediate Report on Flexible Spectrum Use, Deliverable D1.6, November 2009, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html Celtic CP5-026 WINNER+, Intermediate Report on Advanced Multiple Antenna Systems, Deliverable D1.7, November 2009, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html Celtic CP5-026 WINNER+, Intermediate Report on Coordinated Multipoint and Relaying in the Framework of CoMP, Deliverable D1.8, November 2009, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html. Celtic CP5-026 WINNER+, Final Innovation Report, Deliverable D1.9, April 2010, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html. Celtic CP5-026 WINNER+, Initial Report on Flexible Spectrum Use, Deliverable D1.2, January 2009, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html Celtic CP5-026 WINNER+, concepts in Peer-to-Peer and Network Coding, Deliverable D1.3, January 2009, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html Celtic CP5-026 WINNER+, Enabling Techniques for LTE-A and beyond, Deliverable D2.2, July 2010, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html Celtic CP5-026 WINNER+, Final conclusions on end-to-end performance and sensitivity analysis, Deliverable D4.2, July 2010, http://projects.celticinitiative.org/winner+/deliverables_winnerplus.html