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Lte advanced

  1. 1. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® LTE-ADVANCED AND 4G WIRELESS COMMUNICATIONS LTE-Advanced: An Operator Perspective Prakash Bhat, Vodafone Satoshi Nagata, NTT DoCoMo Luis Campoy and Ignacio Berberana, Telefonica Thomas Derham, Orange Guangyi Liu and Xiaodong Shen, China Mobile Pingping Zong and Jin Yang, Verizon ABSTRACT edge topics, has a multitude of aspects we would like to have covered; however, the following key LTE-Advanced extends the capabilities origi- topics are dealt with herein: nally developed in LTE within the 3GPP. Carrier • Carrier aggregation — Pingping Zong and aggregationis the most significant, albeit complex, Jin Yang, Verizon (United States) improvement provided by LTE-Advanced. Band- • Advanced DL MIMO techniques — Satoshi widths from various portions of the spectrum are Nagata, NTT DOCOMO (Japan) logically concatenated resulting in a virtual block • Advanced UL MIMO techniques — of a much larger band, enabling increased data Thomas Derham, Orange (France) throughput. Additionally, enhancements to • Relaying — Prakash Bhat, Vodafone (Unit- MIMO antenna techniques in the uplink and ed Kingdom) downlink further increase the data throughput. • Coordinated multipoint TX and RX — Luis Cell coverage is improved by means of relay Campoy and Ignacio Berberana, Telefonica nodes, which connect to donor eNode-Bs. To (Spain) cope with the many varieties of cell types and • Enhanced ICIC/HetNets — Xiaodong Shen sizes (macro, pico, femto), intercell interference and Guangyi Liu, China Mobile (China) control is enhanced to handle these heteroge- Aspects touched on but not limited to neous networks. Operators hope to leverage include: motivations for LTE-Advanced, primary LTE-Advanced to offer their mobile wireless cus- use cases, scenarios, expectations, competitive tomers a vastly superior user experience. pressures, device and infra-structure challenges, and deployment and operational costs. INTRODUCTION CARRIER AGGREGATION The thirst for greater data rates exhibited by users of mobile wireless services has been on an MOTIVATION exponential trajectory. Long Term Evolution Wireless customers are increasingly using mobile (LTE)-Advanced seeks to improve voice quality devices as their main tool to surf the Internet, and expand broadband data services, to deliver play games, stay connected with friends and fam- high-definition video and audio and other on- ily, and watch real-time news, favorite TV pro- demand and real-time contentin an “anything- grams, or the latest blockbuster movie. Offering anywhere-anytime” manner. high-speed data over wireless networks to meet In addition, LTE-Advanced continues to and encourage such ever-increasing service advance means to lower latency and round-trip demands is of significant interest to the wireless delays, reduce intercell interference, and support operators around the world. The wireless indus- coexistence between the various flavors of cells try has evolved from using second-generation — macrocells, micocells, femtocells, and so on. (2G) technologies to today’s Third Generation Relays are being designed to provide greater Partnership Project (3GPP) LTE Release 8, with coverage, while using in-band backhaul via the increasing spectrum efficiency more than 100 existing radio interface. times. With the limitation of available contigu- In this article, operators representing a cross- ous spectrum being allocated and licensed to the section of the globe have come together to pro- wireless operators, carrier aggregation is needed vide a perspective on expectations for to meet the International Telecommunication LTE-Advanced from the operator point of view. Union — Radiocommunication Sector’s (ITU- Obviously, LTE-Advanced, as with all leading R’s) 1 Gb/s peak rate requirement for IMT- 104 0163-6804/12/$25.00 © 2012 IEEE IEEE Communications Magazine • February 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  2. 2. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Advanced. Carrier aggregation has been intro- The radio frequency (RF) performance aspect duced to 3GPP LTE-Advanced work since 2009 of Release 10 carrier aggregation is in progress. Based on the real as one of the key components for 3GPP LTE The main task yet to be completed is the RF Release 10. requirements specifications of each specific car- world spectrum rier aggregation band combination in which the allocation and the AN OVERVIEW OF RELEASE 10 operators are interested. Based on the real world CARRIER AGGREGATION operators’ need, spectrum allocation and the operators’ need, The 3GPP LTE Release 10 signaling specifica- 3GPP RAN4 has been working on 14 different 3GPP RAN4 has tion, which was completed in June 2011, sup- band combinations, most of which aggregate two been working on ports carrier aggregation of up to five component carriers to achieve up to 20–40 MHz total band- carriers (i.e., 100 MHz bandwidth) to achieve 1 width in both downlink and uplink. Based on the 14 different band Gb/s downlink peak rate and 500 Mb/s uplink demand to support the asymmetric downlink combinations, most peak rate. It supports two types of aggregation: heavy user traffic pattern, the current RF speci- of which aggregate intra-band contiguous spectrum aggregation and fication work has been prioritized to focus on inter-band spectrum aggregation. The support of downlink carrier aggregation. The work on two carriers to intra-band non-contiguous spectrum aggregation uplink carrier aggregation is expected to be achieve up to 20–40 has been deferred to Release 11, which is to be resumed as soon as the downlink carrier aggre- completed by December 2012. Furthermore, the gation requirements are finalized. MHz total Release 10 carrier aggregation signaling specifi- bandwidth in both cation supports the following downlink and POTENTIAL USE CASES OF downlink and uplink. uplink symmetry configurations: symmetric (i.e., RELEASE 10 CARRIER AGGREGATION same number of downlink and uplink compo- nent carriers) and asymmetric (i.e., more down- Carrier aggregation supports a higher peak rate link component carriers than uplink component of up to 1 Gb/s with up to 100 MHz bandwidth. carriers). The downlink and uplink component In the higher frequency bands, such as 3.5 GHz, carrier linkage is configurable via radio resource an operator may be allocated and licensed more control (RRC) signaling. It is also worth noting than 20 MHz of contiguous spectrum. Release that Release 10 carrier aggregation only sup- 10 carrier aggregation enables such operators to ports backward-compatible component carriers. provide higher peak rate and capacity without This ensures that non-carrier-aggregation-capa- being restricted by the 20 MHz bandwidth upper ble user equipment (UE, e.g., Release 8/9 UE) limit set in LTE Release 8/9. In the lower fre- can transmit and receive on one of the carriers quency bands, such as 700 MHz, spectrum allo- in a network supporting carrier aggregation. cation and licensing is normally done in much For an RRC_CONNECTED UE operating narrower blocks (e.g., 6 MHz/block in the 700 in carrier aggregation mode, there is always one MHz bands in the United States). For those pair of uplink and downlink primary component bands, Release 10 carrier aggregation enables an carriers (PCC) that corresponds to the primary operator to pool its spectrum resources together serving cell (PCell) and maybe one or multiple within the same band or across different bands pairs of uplink and downlink secondary compo- to achieve higher peak rate and capacity. nent carriers (SCCs) from the same eNB that Carrier aggregation also allows operators to correspond to the secondary serving cells provide multiple services simultaneously over (SCells). The UE monitors the PCell in the multiple carriers. For example, during a large same manner as in Release 8 for system infor- convention or sports event, LTE users could be mation, and performs RACH and downlink interested in accessing a high data rate broadcast radio link failure monitoring over PCell. The program (e.g., at 10 Mb/s), while still expecting security key input and non-access-stratum (NAS) the usual experience for unicast data services, information is sent only over the PCell as well. such as voice over IP (VoIP), email, and web The eNB can trigger a PCell change for UE due browsing. With Release 10 carrier aggregation, to mobility, load balancing, and so on. The PCell by configuring the multiple services of a particu- change is performed in the same manner as a lar user over multiple carriers, the throttling of Release 8/9 handover procedure. During the any of the abovementioned services for that user PCell change, the SCell will be deactivated by can be avoided. Hence, when operators are chal- the serving eNB, and then selected and activated lenged to provide a better user experience while by the target eNB. The downlink of a configured facing a lack of wider contiguous spectrum, the SCell can be activated and deactivated dynami- flexibility in simultaneous data delivery over cally to save power. The UE does not monitor multiple carriers becomes increasingly impor- the deactivated SCell for PDCCH. Only mobili- tant. ty-based measurement is performed on the deac- Furthermore, carrier aggregation can improve tivated SCell. There is no explicit network efficiency and user performance by activation/deactivation procedure specified for dynamically allocating traffic across the entire the uplink of a configured SCell: UE is required available spectrum. Carrier-aggregation-capable to be able to perform PUSCH transmission on devices will report channel quality for carriers any configured SCell uplink corresponding to across a wide spectrum. This will improve the PDCCH carrying the uplink grant. resource allocation and handover in a heteroge- The data aggregation of the multiple compo- neous network (HetNet), where both the num- nent carriers is performed at the medium access ber of carriers and the carrier transmit powers control (MAC) layer. Each component carrier can vary significantly across the sites, with high- has its independent hybrid automatic repeat er-power macrocells and lower-power pico- or request (HARQ) process, and modulation and femtocells. Both the macrocells and the coding schemes. pico/femto cells can transmit aggregated multi- IEEE Communications Magazine • February 2012 105C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  3. 3. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Capability LTE Release 8 LTE-Advanced IMT-Advanced (ITU-R) LTE-Advanced (3GPP) Further enhance- requirement capability capability requirement requirement ment of DL MIMO Downlink 16.3 (4 layers) 30.6 (8 layers) 15 30 technologies are required to improve Table 1. Peak spectrum efficiency requirements and capability of LTE-A DL (bits per second per Hertz). user experience throughput and ple carriers and dynamically allocate resources MIMO transmission would be a standard config- across the aggregated carriers. uration of the LTE deployment. For LTE- network capacity. Release 10 carrier aggregation supports Advanced, further advancement of LTE has Since the aims of “cross-carrier scheduling” that can mitigate the been envisioned in accordance with 3GPP oper- introducing MIMO control channel intercell interference resulting ators’ requirements and the need to exceed the from spectrum sharing between macrocells and IMT-Advanced requirements provided by ITU- technologies are pico-/femtocells. With cross-carrier scheduling, R [1]. Table 1 shows the downlink peak spec- different according the PDSCH resources on both PCC and SCC trum efficiency requirements and the capability can be scheduled by PCC’s PDCCH. Hence, in a of LTE and LTE-A systems. The ITU-R require- to the use case, synchronous deployment, as long as the PCCs of ment can be satisfied by the LTE Release 8 various MIMO macrocell and pico-/femtocells are configured to capability supporting a maximum of four spatial technology features different carriers, intercell interference to the layers. The LTE-Advanced requirements set by PDCCH can be prevented in a carrier-aggrega- operators in 3GPP are more challenging and can need to be tion-based HetNet deployment. be met by the enhanced MIMO capability sup- supported. Carrier aggregation can further enhance the porting a maximum of eight spatial layers. user experience at the cell edge of a frequency in some deployment cases. For example, if the FEATURES OF LTE-A DL MIMO UE aggregates the carrier X as its SCC, when LTE-Advanced adopted eight-Tx-antenna the UE moves from the coverage of carrier X to MIMO transmission with maximum spatial eight the coverage of carrier Y, the data transmission layers in DL to support high-end user terminals. and reception on the UE’s PCC will continue Eight-Tx-antenna MIMO transmissions would be without any interruption, while carrier Y can be beneficial to boost peak data rates for isolated added as the UE’s new SCC via configuration cell environments such as indoor deployments quickly without performing interfrequency han- through spatial multiplexing gain, and enhance dover between carrier X and carrier Y. network coverage though beamforming or MIMO precoding gain. Combined with carrier SUMMARY aggregation with five component carriers, eight- In this section, we provide an overview of the layer DL MIMO achieves peak data rates of 3.0 carrier aggregation feature supported in 3GPP Gb/s [2]. LTE Release 10. With carrier aggregation, the LTE-Advanced supports advanced multi-user bandwidth of the contiguous spectrum is less of (MU-) MIMO in the DL to improve the capacity a limiting factor in achieving a higher user peak of the network by spatially multiplexing multiple data rate. With the ability of pooling multicarri- user terminals, where each user terminal would er spectrum resources together, carrier aggrega- typically receive up to two spatial layers. The tion enables operators to utilize their spectrum advanced MU-MIMO techniques would be ben- holdings in a more flexible and efficient way. eficial for the capacity enhancement of highly populated urban areas. Advanced MU-MIMO is ADVANCED DL MIMO TECHNIQUES an essential feature to satisfy the ITU-R require- ment of cell spectrum efficiency and cell edge MOTIVATION spectrum efficiency for urban micro- and macro- To accommodate increasing traffic due to widely cells [3]. spreading smart phones, new mobile devices For LTE-Advanced, transmission mode (TM) such as tablets, and various cellular applications, 9 has been newly specified to jointly support the further enhancement of the LTE cellular net- maximum eight-layer single-suer (SU-) MIMO works is crucial for operators. Further enhance- and advanced MU-MIMO. TM9 is characterized ment of downlink (DL) MIMO technologies are by data demodulation based on demodulation required to improve user experience in terms of reference signal (DM RS) and channel state throughput and network capacity. Since the aims information (CSI) measurement based on CSI of introducing MIMO technologies are different RS. DM RS enables advanced beamforming and according to the use case, various MIMO tech- MIMO precoding to spatially multiplex multiple nology features need to be supported. For user terminals. For LTE-A, the DM RS struc- instance, user peak data rate enhancement might ture has been extended to support maximum 8- be important in indoor areas, and average user layer data transmission. CSI RS enables efficient data rate (or capacity) enhancement might be CSI — rank indicator (RI), precoding matrix more important in dense urban areas. Further- indicator (PMI), and channel quality indicator more, it is important that these enhancements (CQI) — measurements because CSI RS inser- be supported in a backward-compatible manner tion density is sparse in time and frequency. In for smooth migration from LTE to LTE- addition, a new codebook utilizing double code- Advanced. book structure has been specified to support 3GPP LTE Release 8 adopted MIMO tech- eight-Tx-antenna MIMO transmissions, where nologies for DL transmission, supporting up to the new codebook particularly targets a closely four spatial layers. Maximum two-layer DL spaced cross-polarized antenna arrangement that 106 IEEE Communications Magazine • February 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  4. 4. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Capability LTE Release 8 LTE-Advanced IMT-Advanced (ITU-R) LTE-Advanced (3GPP) requirement capability capability requirement requirement While mobile devices have been used pri- Uplink 4.3 16.8 (4 layers) 6.75 15 marily for consump- Table 2. Peak spectrum efficiency requirements and capability of LTE-A UL (bits/sec/Hz). tion of multimedia content, the evolu- has been identified as the most preferred anten- services are consumed. While conventionally tion of hardware and na arrangement by operators. mobile devices have been used primarily for con- sumption of multimedia content, the evolution user interface capa- LTE-A DL NETWORK DEPLOYMENT of hardware (CPU, touch display, camera, sen- bilities is spawning a Backward compatibility is an essential require- sors, storage, etc) and user interface capabilities new generation of ment for the LTE-Advanced operators, such that is spawning a new generation of mobile applica- LTE Rel-8 terminal can work in LTE-Advanced tions from which substantial volumes of multi- mobile applications network and vice versa. In order to fully support media content are generated by the user and from which substan- this requirement, LTE-Advanced enables mixed uploaded to the network — examples include operation between the LTE Rel-8 terminals and sharing of photos and videos on social networks, tial volumes of multi- the LTE-A terminals in the same subframe, the and continuous upload of sensor data for loca- media content are LTE terminals are configured in a Rel-8 trans- tion-based and wellness services. Furthermore, generated by the mission mode based on the common reference the maturity of cloud-based mobile services is signal (CRS) and the LTE-Advanced terminals increasing whereby certain functions such as user and uploaded are configured a LTE-A transmission mode, i.e., storage, data processing, or even complete appli- to the network. TM9. it is also possible to configure a different cations may be hosted on the network side. At number of configured antenna ports for DL data the same time, enterprise users increasingly transmission between LTE and LTE-A termi- expect to be able to use cellular networks to nals. The LTE-Advanced network can also uti- continue use of their full range of enterprise lize subframes where only LTE-Advanced UE applications (e.g., office, email, intranet, and can be scheduled by configuring these subframes database services) when they are on the road. as MBSFN subframes to LTE Release 8 termi- All of these trends are placing increased demand nals. As such, further overhead reduction is pos- on the cellular uplink, and it has become clear sible in these subframes by removing CRS on that further enhancement of the LTE uplink data regions without impacting the channel mea- (UL) spectral efficiency using MIMO technology surement behavior of LTE Release 8 terminals. would be crucial for operators to meet demand The LTE-Advanced operators could apply each for these services, while providing backward of the functionalities above for smooth network compatibility with existing LTE devices. migration from LTE to LTE-Advanced by con- 3GPP LTE Release 8 adopted a minimal sidering the number or ratios of LTE and LTE- MIMO implementation for UL transmission, A terminals, and their supporting features at supporting just a single spatial layer per UE, each phase of LTE-Advanced deployments. transmitted on a single antenna. If UE imple- While there are attractive benefits provided ments two transmit antennas, switched diversity by the advanced DL-MIMO technologies, there may be employed whereby the active antenna is are also some challenges to deploy higher order dynamically selected according to channel condi- MIMO base stations with four and eight Tx tions. This approach avoids the need for addi- antennas in practical cellular networks. Although tional RF transmitssignal chains (PAs, filters, antenna setting at both base stations and user etc.) in cost-sensitive UE. In addition, it ensures terminals would be one of the most difficult chal- that the low cubic metric (peak-to-average-power lenges, some technologies’ evolution would over- ratio, PAPR) properties of the discrete Fourier come these challenges. For base stations, space transform spread orthogonal frequency-division reduction of antenna elements could be achieved multiplexing (DFT-S-OFDM) UL signals are by utilizing multi-band antennas. Furthermore, maintained, which is important for power-con- high data rate support for indoor is becoming strained transmissions from mobile devices. How- more important and enhanced MIMO technolo- ever, as shown in Table 2, LTE Release 8 UL gies could be deployed in such an environment, spectral efficiency does not meet the require- where antenna size reduction could be realized ments for IMT-Advanced, nor the much more due to low transmit power and distributed anten- ambitious target set by 3GPP taking into account na setting could be potentially utilized for higher the increased importance of the uplink as order MIMO transmission. For user terminals, described above. Therefore, LTE-Advanced higher order MIMO will be supported by note- enhances the UL MIMO capability to support a book PCs and tablet phones for which high data maximum of four spatial layers. In addition to rate communication is particularly important. increased peak spectral efficiency, the enhanced MIMO capability is also designed to improve ADVANCED UL MIMO TECHNIQUES overall capacity, coverage, and user experience at the cell edge, which are key concerns for delivery MOTIVATIONS of reliable and robust connectivity. Development of the LTE-Advanced standard has coincided with an explosion in market FEATURES OF LTE-A UL MIMO demand for broadband mobile services and mul- LTE-Advanced adopts four-transmit-antenna timedia-oriented mobile devices (e.g., smart- SU-MIMO transmission with a maximum of four phones, tablets, and notebooks) on which these spatial layers in the UL for high-end user termi- IEEE Communications Magazine • February 2012 107C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  5. 5. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® nals. Very high UL data rates can be achieved provides significant gains with two transmit LTE-A provides signif- by spatial multiplexing of all layers in high sig- antennas, so these smaller devices will still bene- nal-to-interference-plus-noise ratio (SINR) sce- fit. There are also challenges to deploy the cor- icant gains with two narios (e.g. indoor deployments), while multiple responding four receive antennas in base transmit antennas, antennas can also be used for increased through- stations, which are largely common to the imple- so these smaller put and coverage extension at low/medium SINR mentation of enhanced downlink MIMO. through beamforming of a smaller number of devices will still ben- spatial layers. RELAYING efit. There are also LTE-Advanced also supports MU-MIMO in the uplink, where two UE units may transmit up MOTIVATION challenges to deploy to two spatial layers each — this can be consid- Operators have seen continued growth in data the corresponding ered an extension of virtual MIMO (V-MIMO) usage with increased numbers of smart devices four receive supported in LTE Release 8. The key point of (smart phones, USB devices, laptops/tablets, MU-MIMO is that even if the UE units have etc.). Furthermore, operators will be required to antennas in base less than four transmit antennas (e.g., lower-end support diverse sets of application data rate stations, which are or smaller devices), they may be paired in order requirements from low-data-rate machine-to- to obtain, on aggregate, the same peak spectral machine applications to high-data-rate applica- largely common to efficiency as for SU-MIMO. tions such as IPTV. This requires operators to the implementation Furthermore, LTE-Advanced introduces continually improve system capacity and cover- of enhanced DL transmit diversity for the uplink control channel age while meeting the challenges presented by (PUCCH) whereby the multiple antennas are the cost saving requirement. Fiber coverage as a MIMO. used to enhance robustness at the cell edge, backhaul solution will improve but will not reach and/or to reduce intercell interference by allow- everywhere. The operator is then presented with ing reduction of control channel transmit power, a challenge of improving capacity/coverage using which fundamentally improves network coverage either additional sites, more spectrum, or invest- and reliability. ing in a more spectrum-efficient technology. Not A new UL transmission mode, TM2, has been all the options are economical or feasible due to specified that allows the eNodeB to instruct the constraints such as cost and deployment (site UE to dynamically switch between multiple- acquisition, regulatory constraints, etc.). antenna (including spatial multiplexing) and sin- Whilst many solutions are envisaged to meet gle-antenna modes (e.g., according to channel the challenges posed, small cell deployments quality). TM1 specifies single-antenna mode look promising as a way to improve coverage only, corresponding to the LTE Rel-8 uplink and capacity demand. From an operator per- scheme. Data demodulation is based on an evo- spective, macro networks should be designed lution of the uplink demodulation reference sig- and deployed from the outset to support cluster- nals (DM RS) defined in LTE Rel-8 to support ing of low-powered cells. LTE will be extensively multiple spatial layers. Sounding reference sig- deployed on high carrier frequencies (i.e., 2.6 nals (SRS) are transmitted from each antenna GHz). Furthermore, the majority of mobile traf- separately, under control of the eNodeB to man- fic is generated indoors. Coverage improvement age over-head, and enable the eNodeB to effi- would then be key to meeting customer expecta- ciently determine CSI and hence control the UE tions. Heterogeneous networks with many differ- transmissions through rank indicator (RI), pre- ent types of small cell solutions will increasingly coding matrix indicator (PMI) and channel qual- be deployed to address the need for coverage ity indicator (CQI) feedback. The PMI codebook and capacity improvements. From a user’s per- is specially designed so the cubic metric of trans- spective, being closer to the cell should result in mitted signals from each antenna is not degrad- larger battery savings (due to reduced transmit ed even for multiple spatial layers, and also power) and improved throughput. emulates Rel-8 antenna switching, which may There are several solutions for coverage allow one PA to be switched off to reduce power improvements. These include solutions such as consumption if not needed. low-powered eNodeBs with traditional backhaul (e.g., microwave), repeaters with improved LTE-A UL NETWORK DEPLOYMENT antenna gains and echo cancellation techniques, The key challenge to implement UL MIMO and open femtocells. For many deployments, the enhancements will be how to integrate multiple backhaul could present a considerable challenge. transmit antennas and RF signal chains within Microwave backhaul, although successfully used the UE device packaging, while achieving the in macro deployments, needs to address many low mutual correlation required for maximum issues before it can be widely applied to small gains from spatial multiplexing and minimizing cell deployment. The availability of smart prod- power consumption (especially in battery pow- ucts in license exempt bands makes WiFi a ered devices). UE units comprising four transmit strong candidate; however, propagation charac- antennas are more likely to be physically large teristics of 5.8 GHz for specific deployments devices (e.g., tablets, notebook PCs), where for- need further consideration. The digital sub- tunately the throughput requirements are typi- scriber line access multiplexer (DSLAM) for cally also the highest. For devices such as backhaul is a cheaper option; however, availabil- smartphones and dongles, overcoming the con- ity and limited UL data rate represent significant straints of reduced antenna efficiency and issues. Fiber is an attractive solution, although increased mutual coupling within the small form very expensive and requiring a longer time to factor is challenging, although various R&D deploy. These challenges with backhaul can be activities on highly compact wideband antenna addressed by self-backhauling relay nodes. arrays are continuing. In any case, LTE-A also Relaying for small cells is an area where ven- 108 IEEE Communications Magazine • February 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  6. 6. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® dors can differentiate (with respect to cost and performance), and is one of the key enablers for eNB-to-relay transmission small cells. Relaying provides better fairness to users with homogeneous user experience; this One subframe comes at the cost of additional donor eNodeB access link resource utilized for the relay back- Transmission gap haul and also the associated latency. While Ctrl Data Ctrl (”MBSFN subframe”) inband relaying improves coverage, outband re- laying will also improve capacity. Outband relay, however, would require economical sub-4-GHz No relay-to-UE transmission spectrum dedicated for backhaul. Relays increas- ingly appear to be a possibility for rural broad- Figure 1. Type 1 relay. band coverage improvement where fiber is not available. While no additional spectrum is required for backhauling for inband relays, relay still transmit the control region (including the deployments have their own challenges. High reference signals). backhaul link quality is essential for the deploy- Further enhancements to relay backhaul, ment of relays. Lower penetration of wired back- such as advanced quality of service (QoS) man- haul to customer premises in emerging markets agement, carrier aggregation for backhaul, makes relays the most attractive solution for advanced MIMO schemes with support of 8 Tx indoor coverage in comparison to home eNBs antennas and up to 4 layers, improvements to (femtocells). However, no-touch (plug-and-play) relay control channel, header compression, and deployment is essential if relays are to be consid- enhancements to support mobile relays such as ered for indoor deployment. One of the key increased handover robustness for group mobili- metrics for deployment from an operator’s per- ty and enhancements to combat Doppler for the spective is the cost per bit. For widespread high-speed scenario, are being considered for deployment of relays for outdoor coverage future releases (Release 11 and later) of 3GPP. improvements, key would be to justify the rela- While use of mobile relays is envisaged primarily tive cost of relay deployment in comparison to for mass transit scenarios such as in buses and small cell deployment with traditional wireless trains, the self-backhauling nature could be use- backhaul solutions such as microwave. ful to provide emergency communication during Relays can provide throughput enhancement disasters when infrastructure may have been and coverage extension. Support for LTE relays impacted. Multihop relays is one other area that was standardized in Release 10 of 3GPP. With will require further study. no impact on UE implementation, it is expected Relay nodes, due to the nature of their all LTE UE should be able to benefit from relay deployment (indoors, rooftops, lampposts, etc.) node deployment. Relay nodes (RNs) connect may be particularly vulnerable to vandalism and wirelessly to the donor cell served by donor other malicious activities when security of the eNodeB and can be classified as transparent or entire mobile network may be compromised. non-transparent. Non-transparent relay controls This requires additional security measures (phys- cells of its own (similar to eNodeB), and has a ical as well as over the communication links). unique physical-layer cell identity; the same While ensuring security is an important aspect of radio resource management (RRM) mechanisms any small cell deployment, relays with wireless as normal eNodeB are used and shall appear as backhaul require additional considerations for a Release 8 eNB to Release 8 UE. Non-trans- backhaul security to prevent any eavesdropping parent relays can be further classified as type 1, over the backhaul. type 1a, and type 1b relays. A transparent RN is 3GPP has specified security procedures for part of the donor cell, does not have a separate RN deployments (Fig. 2). Mutual authentication physical cell identity, and is a type 2 relay. between RN and network is performed using Type 1 relays are half duplex relays by defini- Authentication and Key Agreement Protocol tion and are unable to transmit to the UE and (AKA) during the RN attach procedure with receive from the donor eNB simultaneously, and credentials stored on a universal integrated cir- require resource partitioning between the wire- cuit card (UICC; Fig. 2). Binding of RN and less backhaul link and the eNB, and the access USIM is based on either symmetric preshared link and the UE. keys or certificates. In either case, operators will Full duplex relays, on the other hand, can need to provision special RN-aware UICCs for operate as either outband relays (Type 1a) or RNs. Control plane traffic, and optionally user inband relays (Type 1b) with enough spatial sep- plane traffic, is integrity protected. aration, filtering, or enhanced interference can- In summary, from an operator perspective, cellation, thus requiring no specific resource relays could be a viable coverage (indoor and partitioning. outdoor) and capacity improvement tool. Relays Type 1 half duplex inband relay does not could be an important enabler for dense small transmit any signal to UE when it is supposed to cell deployment. Future evolution of relays such receive data from the donor eNB (Fig. 1). The as mobile relays and multihop relays could fur- relay then configures these subframes as multi- ther improve the user experience. Small low- cast/broadcast single-frequency network power relays could encourage and provide (MBSFN) subframes (fake MBSFN) when UE opportunities for further innovation and vendor units (including Release 8 UE) are not supposed differentiation from cost and performance per- to expect any DL transmission, avoiding any spectives. Specific consideration of robustness of legacy UE measurement issues. The relay should the backhaul link is required as several site con- IEEE Communications Magazine • February 2012 109C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  7. 7. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® UE NAS security UE user plane + RRC security Traditional backhaul link (fibre, microwave, etc.) Relay node IPsec (optional) UE (contains Core (contains UICC) network UICC) DeNB RN user plane + RRC security RN NAS security Figure 2. Relay node security. Deployment opportunities (1) (2) (3) Figure 3. Deployment scenarios. straints for deployment of relays may result in radio resources (RRs) among neighboring BSs less than desirable performance for backhaul. in order to serve users at the cell edge. A new approach, currently discussed for 3GPP Release 11, is CoMP, which provides a COORDINATED MULTIPOINT further step in the coordination of RRs among TX AND RX BSs. CoMP is not only based on RR sharing, but also takes advantage of the transmission and MOTIVATION reception capabilities of neighboring BSs to Coordinated multipoint (CoMP) TX/RX aims to increase SINR in both the UL and DL. increase the throughput available to UE at the cell edge, enhancing network throughput by COMP OVERVIEW means of combining a cluster of base stations CoMP is one of the important features of 3GPP (BSs) to simultaneously serve selected UE. LTE-A Release 11, with several techniques com- Convergence trends for fixed and mobile peting to prove enhanced performance for a set solutions have been widely advertised for more of scenarios agreed on by a large number of the than a decade, and are now a reality. Users are operators participating in the 3GPP. largely technology agnostic, and increasingly CoMP technologies could be classified as: expect seamless experience with a range of DL CoMP: In this type, more than one BS trans- devices, applications, and services being offered. mits signals in a coordinated manner to UE as if Therefore, the challenge is to offer users the it was a single transmitter with multiple antennas same quality of experience (QoE) perceived geographically distributed. BSs are clustered from their fixed broadband service but ubiqui- (either flexible or, more easily, fixed clusters) tously. However, the value of SINR, which limits coordinating DL transmission among all of them. available BS-UE throughput, degrades from the The main DL CoMP technologies are: UE near the base station to the UE approaching •Coordinated scheduling, in which one entity the cell edge due to the attenuation of the (usually one of the BSs in the cluster) assumes received signal strength and the increase of the scheduling functionalities of all the BSs in interference from other BSs or UE. As a conse- the cluster. For this technology to work, all DL quence, the QoS experienced by users at the cell channel state information (CSI) from all BSs to edge could be severely degraded if no coordina- the targeted UE are needed. Therefore, mecha- tion is present among different BSs. nisms should be implemented for the UE mea- 3GPP has developed different procedures to sured CSI to be sent to master BS. Any UE is overcome this degradation: fractional frequency served only by one BS, and therefore user data reuse (FFR) in 2G systems, code division in 3G only needs to be present at its serving BS. systems, and enhanced intercell interference •Coordinated beamforming, which uses CSI coordination (eICIC) in 4G systems. However, to precode transmitted signals in order to avoid all of them are based on arrangements (in a interference from different cells in a UE DL more or less dynamic manner) for sharing the channel. Beamforming precoding can be done 110 IEEE Communications Magazine • February 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  8. 8. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® locally at each cluster BS, in a distributed joint and modulation and coding schemes (MCS). In processing way. As in the previous case, any UE this scenario the master BS is in charge of the One of the more is served by only one BS; therefore, user data HARQ management of all BSs. only needs to be present at its serving BS. •Specific control and signaling among the significant changes is •Joint transmission, in which data is available master BS and the rest of the cluster BSs will be the one related to at several BSs of the cluster. The simplest sce- required, to exchange CSI information and com- the PDCCH, for nario for CoMP with data available at several mands with very low latency, since CSI may be BSs is dynamic point selection, in which the referred to radio channels with coherence time which an enhanced serving BS may be dynamically changed depend- below 1 ms. version, E-PDCCH, ing on the CSIs received. In order to improve •Time synchronization requirement, in order system performance of joint transmission, data is to align radio frames transmitted from different with an increase of transmitted simultaneously from different BSs of BSs to the UE. the number assigned the cluster. There are two options for the coor- •Phase synchronization, in case of coherent resources is currently dination of these transmissions: techniques previously discussed. — Non-coherent transmission, in which the The technology that could provide the highest being discussed for gain is obtained by pure signal power increase at system gains is joint processing with coherent 3GPP Release 11, the receiving UEs. Basic CSI is needed in order transmission, applied to cells geographically dis- to support scheduling decisions, and some tributed at different sites, but this is also the in order to be used degree of time synchronization which will be technology that presents more demanding for multiplexing related with the capacity of the radio interface to requirements from the backhaul. It should also control information deal with multipath delay. The main advantage be noted that hybrid technologies could be of this procedure is that phase synchronization is implemented, mixing the proposed technologies, of multiple UE units. not needed. each leading to a different set of requirements — Coherent transmission, which takes advan- for the backhaul. tage of a good knowledge of CSI, weighting CoMP technology is currently in the process physical resource block (PRB) allocations (PRBs of migrating from laboratory and proof of con- are the minimum value of RR allocable to UE, cepts tests to real deployments, but analysis and embracing several subcarriers along several simulations of its performance in different sce- OFDM symbols) in order to maximize the UE’s narios have identified some characteristics that received signal from several BSs. The UE, being may jeopardize its gain in some situations. able to combine coherently all received signals at The real performance of some CoMP tech- symbol level. For this technique to work, a very niques depends on traffic load and SINR distri- high definition of real CSI is needed, as well as a butions that are not easy to predict/model. Even tight time and phase synchronization among the more, the high volume of CSI information over cluster of BSs. This technique can be considered real X2 interfaces is subject to quantification an advanced distributed MIMO technology. error, delays, and increased acknowledgment UL CoMP. UEs UL signals are received at mul- (ACK)/negative ACK (NACK) round-trip time tiple BS that are geographically distributed in (RTT), which may have an impact on the tech- the neighborhood of the BSs in control of the nology’s performance. UE access to radio resources. This technology The time delay in information sharing leads to implements the mechanisms for coordinating CSI mismatch with actual values. According to [4], schedulers of all implied BSs and the received in order to take advantage of joint non-coherent signals analysis. One of the main advantages of CoMP, X2 delay should be in a range of 1 ms or UL CoMP is that it can be designed with no lower, meaning that care should be taken in design- impact on current UE specifications. One possi- ing the backhaul architecture. There is also a ble implementation consists of scheduling the dependence on the gain obtained by CoMP tech- same PRBs to several UE units; this case is simi- nology and the traffic load in the cells involved; the lar to implementing an MU-MIMO in UL but gain reduces with increase in traffic load. with several BSs receiving from the UE. The two For practical deployment of some CoMP basic approaches for analysis of signals received techniques changes will be needed not only in in different BS are: BS performance, but also in the UE’s, in order •Coherent reception: The received signals at to measure and send the CSI CQI/PMI/RI val- the BSs are combined at a central receptor, tak- ues related with all the BSs involved in the clus- ing advantage of coherent reception ter. Time-division duplex (TDD) may benefit •Non-coherent reception: There is a central- from channel reciprocity. ized scheduler for UL channels and multiple Another factor to be taken into account, receptions of UE signals. depending on the CoMP technique selected, is the possible degradation of the gains depending COMP DEPLOYMENT CONSIDERATIONS on the accuracy of time and phase synchroniza- One major drawback of CoMP is the impact this tion. feature has on the backhaul. It is not only the backhaul that will require Depending on the selected technique, if every some changes for the introduction of CoMP, but cell is not at the same site (intrasite deploy- also over-the-air control channels. One of the ment), CoMP technology will demand new more significant changes is the one related to requirements on backhaul links: the physical downlink control channel (PDCCH), •High data rate between the master BS and for which an enhanced version, E-PDCCH, with other BSs (or radiating points) in the cluster. In an increase of the number assigned resources is the more demanding scenario, the MAC packet currently being discussed for 3GPP Release 11, data unit (PDU) will be generated at the master in order to be used for multiplexing control BS along with the decision on PRB allocation, information of multiple UE units. IEEE Communications Magazine • February 2012 111C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  9. 9. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® cated with different carrier frequencies, and thus Environment Deployment scenario Non-traditional node the interference between macro and micro/pico can be ignored. On the other hand, as the spectrum available Macro + femtocell Example: CSG HeNB for a cellular system is rare and expensive, it is preferred that the macro eNB and low-power Macro + indoor Macro + indoor relay Indoor relay nodes share the same carrier as cell splitting and thus higher network capacity are expected from Macro + indoor RRH/hotzone Example: indoor pico the operator perspective. Such a deployment scenario is named heterogeneous network (Het- Macro + outdoor relay Outdoor relay Net), where severe interference between macro eNBs and low-power nodes may happen. Conse- Macro + outdoor quently, the enhancement of 3GPP Release 8/9 Example: outdoor ICIC mechanisms is important to efficiently sup- Macro + outdoor RRH/hotzone pico port HetNet deployment. Table 3. HetNet baseline deployment scenario. WHAT HETNET CAN OFFER Heterogeneous deployments are deployments where low-power nodes are placed throughout a SUMMARY macrocell layout. A baseline deployment sce- An intense research effort on CoMP technolo- nario for HetNet is described in Table 3. gies is currently taking place, and a Work Item has been opened on the subject in 3GPP for CHARACTERISTICS OF HETNET Release 11. This Work Item is receiving a great Co-channel deployment is one efficient way to amount of results from different vendors with utilize spectrum resources as much as possible. different approaches, from the simplest coordi- For an LTE system, the interference characteris- nated scheduler and beamforming to the most tics in a HetNet deployment can be significantly sophisticated coherent joint transmission tech- different from a homogeneous deployment in niques. co-channel deployment. Previous results on the selected scenarios •For a CSG cell, such as a macro and HeNB tend to support that these technologies are not (CSG) co-channel deployment, CSG cell trans- worthy to increase the spectral efficiency average mission causes interference in macro UE. If the over the full cell coverage. Results indicate gains macro and HeNB are co-channel deployed, a of 2 to 3 percent, or even decreases of 2 percent, macro user with no access to the CSG cell will depending on the used technology. be interfered with by the HeNB; the DL SINR These results point out that the real value of of some macro UE near the HeNB is around CoMP technology is the enhancement of cell –60 ~ –20 dB, which causes radio link failure edge UE performance. The gain in spectral effi- (RLF) and deteriorates the UE experience. ciency on these cell edge UE units reported •For an OSG cell, such as a macro and reach up to 80 percent with more sophisticated pico/relay cell co-channel deployment, path-loss- (and more backhaul demanding) mechanisms, based cell association (e.g., by using biased but even simpler coordinated scheduler results RSRP reports) may improve the UL but at the show gains of 15 to 20 percent. cost of increasing the DL interference of non- Therefore, CoMP technologies will represent macro users at the cell edge. For macro with in the future a way to guarantee the expected pico cells within coverage, a biased RSRP QoE of cellular users, avoiding the current bot- scheme leads to heavy DL interference to pico- tleneck of coverage in cell edges. For these tech- cell edge users. Figure 2 shows DL wideband nologies to become really deployed, geometry distribution with different values of modifications and tight requirements may be bias. With 6 dB bias or larger, the outage of needed on the backhaul interfaces. Even more, wrong or missed decode PDCCH joint with since its applications will be focused only on cell PCFICH is greater than 1 percent if required edge users, a dynamic UE selection mechanism demodulation SINR of PDCCH is assumed to of UE in which CoMP technologies are going to be –5 dB. Then it is natural to introduce the be applied should be developed. eICIC solution to mitigate dominant interfer- ence. ENHANCED ICIC/HETNETS FEATURES OF EICIC TECHNIQUES MOTIVATION To deal with the severe interference between a To provide flexible capacity expansion or macro eNB and a low-power node, carrier aggre- offloading, it is important to ensure that LTE- gation (CA)-based solutions are attractive for Advanced provides efficient support for a mix- situations with large availability of spectrum and ture of macrocells and low-power eNBs. Macro UE with CA capability. For non-CA (i.e., co- eNBs with high transmit power and high antenna channel) scenarios, HetNets afford interference height are deployed to provide wide coverage, suppression, where a dominant interferer may whereas some low-power nodes, such as micro- prevent some UE from establishing/maintaining cells, picocells, HeNBs, and RNs, are deployed reliable communications with corresponding for coverage extension or offloading. In order to serving cells. Non-CA-based solutions are impor- alleviate the complexity of network planning and tant to enable efficient HetNet deployments with optimization in hierarchical cellular deployment, small bandwidth availability and UE without CA the macro eNB and low-power nodes are allo- capability. 112 IEEE Communications Magazine • February 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®