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單元 2 HetNet 以小基站為基礎之異質性行動網路

教育部行動寬頻尖端技術人才培育計畫 - 小細胞基站聯盟 中心 「小基站與 WiFi 之異質性網路存取」課程 模組 使用 ITRI LWA-Small Cell ( 含 WiFi) 實驗平台. 單元 2 HetNet 以小基站為基礎之異質性行動網路. 助理教授：吳俊興 助教：王瑞元、許力中 國立 高雄大學 資訊工程學 系. Outline. Introduction to Heterogeneous Networks (HetNet) - Heterogeneous Network Deployments

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單元 2 HetNet 以小基站為基礎之異質性行動網路

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  1. 教育部行動寬頻尖端技術人才培育計畫-小細胞基站聯盟中心教育部行動寬頻尖端技術人才培育計畫-小細胞基站聯盟中心 「小基站與WiFi之異質性網路存取」課程模組 使用ITRI LWA-Small Cell (含WiFi) 實驗平台 單元2HetNet以小基站為基礎之異質性行動網路 助理教授：吳俊興 助教：王瑞元、許力中 國立高雄大學 資訊工程學系

  2. Outline • Introduction to Heterogeneous Networks (HetNet) - Heterogeneous Network Deployments - Features of Heterogeneous Networks - Evolution of Cellular Technology and Standards • Dense Small Cell Deployments - Introduction - Evolution of Small Cells - Efficient Operation of Small Cells - Control Signaling Enhancement - Reference Signal Overhead Reduction • TD-LTE Enhancements for Small Cells - Enhancements for Dynamic TDD - FDD-TDD Joint Operation • Future Trends in Heterogeneous Networks and Summary Reference: Joydeep Acharya, Long Gao, and Sudhanshu Gaur, Heterogeneous Networks in LTE-Advanced, John Wiley & Sons, Ltd, 2014

  3. Motivation • Challenge: increasing number of • Mobile broadband data subscribers, and • Bandwidth-intensive services Competing for limited radio resources • Operatorshave met this challenge by • Increasing capacity with new radio spectrum • Adding multi-antenna techniques • Implementing more efficient modulation and coding schemes

  4. Expand a Homogeneous Network • These measures alone are insufficient in the most crowded environments and at cell edges • Adding small cells and tightly-integrating these with their macro networks to spread traffic loads • Widely maintain performance and service quality while reusing spectrum most efficiently • Add more sectors per eNB or deploying more macro-eNBs • Maintaining it as a homogeneous network

  5. Toward a Heterogeneous Network Finding new macro-sites becomes increasingly difficult and can be expensive • Introduce small cells through the addition of low-power base stations (eNBs, HeNBs or Relay Nodes (RNs)) or Remote Radio Heads (RRH) to existing macro-eNBs • Added to increase capacityin hot spots with high user demand and to fill in areas not covered by the macro network – both outdoors and indoors • They also improve network performance and service quality by offloading from the large macro-cells • The result is a heterogeneous network with large macro-cells in combination with small cells providing increased bitrates per unit area

  6. A Heterogeneous Networkwith Large and Small Cells Large cell • High-power eNB • Macro-eNB site can be difficult to find • Small cell • Low-power base station or RRH • Off load for large cell • Small site size • Indoor coverage • Hot-spot coverage • Coverage at cell edge of large cell • Coverage in area not covered by the macro-network In heterogeneous networks the cells of different sizes are referred to as macro-, micro-, pico- and femto-cells; listed in order of decreasing base station power

  7. History of Heterogeneous Network Planning • Already used in GSM • Separated through the use of different frequencies • LTE networks mainly use a frequency reuse of one to maximize utilization of the licensed bandwidth • The actual cell size depends not only on the eNB power but also on antenna position, as well as the location environment; e.g. rural or city, indoor or outdoor • LTE Standardization • HeNB (Home eNB) introduced in LTE Release 9 (R9) (March 2010) • Introduces the complete integration of the Femtocell concept (Home eNodeB) • eICIC and Relay Node (RN) in LTE R10 (LTE-Advanced, June 2011) • feICIC, LTE-CA, CoMP in LTE R11 (March 2013) • LTE-CA/Small cell enhancements in LTE R12 (March 2015) • Small cell dual-connectivity and architecture in LTE R13 (March 2016)

  8. HeNB (Home eNB) in LTE Release 9 • The HeNB (Home eNB) was introduced in LTE R9 • It is a low power eNB which is mainly used to provide indoor coverage, femto-cells, for Closed Subscriber Groups (CSG), for example, in office premises • They are privately owned and deployed without coordination with the macro-network • There is a risk of interference between the femto-cell and the surrounding network if • The frequency used in the femto-cell is the same as the frequency used in the macro-cells • The femto-cell is only used for CSG

  9. Relay Node (RN) in LTE Release 10 • Roles or a Relay Node (RN) • From the UE perspective the RN will act as an eNB, and • from the DeNB’s view the RN will be seen as a UE. • The RN is connected to a Donor eNB (DeNB) via the Un radio interface, which is based on the LTE Uu interface • RRHs connected to an eNB via fibre can be used to provide small cell coverage • When the frequencies used on Uu and Un for the RN are the same, there is a risk of self interference in the RN

  10. HeNB (R9) and RN (R10)

  11. Cell Selection Under Mixed Cells • In a network with a frequency reuse of one, the UE normally camps on the cell with the strongest received DL signal (SSDL) • Hence the border between two cells is located at the point where SSDL is the same in both cells (SSDLsmall < SSDLmacro in the grey area) • In a heterogeneous network, with high-power nodes in the large cells and low-power nodes in the small cells, the point of equal SSDL will not necessarily be the same as that of equal path loss for the UL (PLUL)

  12. Cell Range Extension (CRE) • Cell Range Extension (CRE) • To increase the area served by the small cell, through the use of a positive cell selection offset to the SSDL of the small cell • To ensure that the small cells actually serve enough users • Negative effect: increased interference on the DL experienced by the UE located in the CRE region and served by the base station in the small cell • Especially the reception of the DL control channels in particular

  13. Trend • Rapid proliferation in mobile broadband data • Strategy Analytics* estimates that • Mobile data traffic grew by 100% in 2012 • The data traffic is expected to increase by about 400% by 2017 • The major contributors to the traffic are bandwidth-intensive real-time applications such as mobile gaming and video Growth forecast in annual mobile data traffic *Reference: Strategy Analytics (2013) Handset data traffic (2001–2017), June 2013. Strategy Analytics

  14. Challenges to Operators • Challenges • Increasing data traffic: network capability using traditional macrocell-based deployments is growing at about 30% less than the demand for data • Decreasing profit margins: the profit margins of most operators have also been decreasing globally • The flat rate pricing policies prevent the mobile data revenues of an operator to scale proportionately with the increased usage of mobile broadband data • The cost incurred as a result of setting up more base stations to provide increased capacity and coverage • Rethink methods of operating their networks • Key principle: deliver higher capacity at a reduced cost

  15. Ways to Increase Capacity • A 1000× increase in capacity is required to support rising demand in 2020* • High capacity can be achieved by • Improving spectral efficiency • Employing more spectrum • Increasing network density • The major gains are expected throughincreasing network density by deployingan overlay network of small cellsover the macro coverage area Related to link level enhancements(but already at near optimal) *Reference: Mallinson, K. (2012) The 2020 vision for LTE. Available at http://www.3gpp.org/2020-vision-for-LTE (accessed November 2013)

  16. Towards Heterogeneous Networks • A small cell could be • An indoor femtocell or an outdoor picocell • A compact base station ora distributed antenna system (DAS)controlled by a central controller • The different types of small cells • have low transmit power and coverage • and together with the macro cells are referred to asHeterogeneous Networksor simply HetNets small cell Macro cell HetNet: a wireless network comprised of different types of base stations and wireless technologies, including macro base stations, small cells, distributed antenna systems (DAS), and even Wi-Fi access points

  17. Licensed Small Cells Source: http://electronicdesign.com/engineering-essentials/understanding-small-cell-and-hetnet-movement

  18. Benefits of Heterogeneous Networks • Improve capacity • Mobile broadband data is highly localized as the majority of current traffic is generated indoors and in hotspots such as malls and convention centers • Add capacity where it is needed by deploying an overlay of small cells in those regions of the macro coverage area which generates heavy data demand • Small cells offload data from the macro coverage area and improve frequency reuse • They can offer higher capacity than the macro as they can better adapt to the spatio-temporal variations in traffic by dynamic interference management techniques • Reduce cost • A small cell-based heterogeneous network is much more energy efficient than a macrocell network • A macrocell needs high transmit power which requires a cooling unit • A low transmit power of the small cell reduces power consumption (by 25–30%*) • Incorporating small cells into the network can save service providers 12–53% in CAPEX and 5–10% in OPEX, depending on traffic loading (Bell Labs study)

  19. Heterogeneous Network Deployments There are many different kinds of small cells which results in different kinds of heterogeneous networks: each has unique deployment, coverage, and capacity characteristics • Distributed Antenna Systems (DAS) • Consisting of a network of DAS nodesthat are connected via fiber toa central processing unit • Public Access Picocells/Metrocells • Open to all members of the public • Covering a smaller area and are specificto a particular wireless access technology • Consumer-Grade Femtocells • Small stand-alone low-power nodes thatare typically installed indoors • WiFi Systems • Operated in the unlicensed band • Integrated with an existing cellular network by offloading some of its load Single Antenna Distributed Antenna Systems

  20. Features of Heterogeneous Networks • Association and Load Balancing • One of the main functions is to offload UE traffic from the macro • Downlink reference signal received power (RSRP) is the most basic criterion but this does not lead to much offloading • Since the transmit power of a macrocell is much greater than that of the small cell • The macrocell and the picocell can operate at different carrier frequencies • Reference signal received quality (RSRQ) leads to better load balancing • Load balancing will distribute UE load across all base stations uniformly • Interference Management • The dense deployment of small cells increases interference • System performance will degrade if intercell interference is not managed properly • Various techniques proposed • Frequency-domain (R8): two neighboring cells can coordinate their data transmission and interference in frequency domain • Time-domain (R10, R11): a cell can mute some subframes to reduce its interference to its neighboring cell • R12 • Dynamic activation/deactivation • Full-dimension (FD) MIMO • CoMP: a macrocell and a small cell can cooperate to simultaneously serve a UE

  21. Features of Heterogeneous Networks (cont.) • Self-Organizing Networks • Base stations (notably the small cells) can sense their environment, coordinate with other base stations and automatically configure their parameters such as cell ID, automatic power control gains, and so on • SONs are therefore critical to small cell deployments • A SON optimizes network parameters for controlling interference • Manages the traffic load among different cells and different radio access networks • Provides the user with the best possible service • Mobility Management • Using the same set of handover parameters for all cells/UEs may degrade the mobility performance in a heterogeneous network • Desirable to have a cell-specific handover offset for different classes of small cells • For high-mobility UEs passing through a dense heterogeneous network, the normal handover process between small cells will lead to very frequent changes in the serving cell • solved by associating this UE to the macrocell at all times, leading to UE-specific handover parameter optimization

  22. Evolution of Cellular Technology and Standards • 1G: Analog systems • 2G: Hybrid FDMA and TDMA • 3G: CDMA (IMT-2000 / 3GPP LTE R8) • 4G: OFDMA and flat all-IP • IMT-Advanced • 3GPP2 LTE-Advanced (LTE-A) R10 • IEEE 802.16m • 5G • IMT-2020 • 3GPP LTE-Advanced Pro LTE Release 11 introduced coordination among different base stations of a HetNet

  23. Performance Requirements ofVarious 3GPP Releases • Influenced by guidelines of International Telecommunications Union (ITU) • International Mobile Telecommunication (IMT) requirements

  24. UE Categories • http://www.3gpp.org/keywords-acronyms/1612-ue-category • 3GPP TS 36.306, E-UTRA; UE radio access capabilitieshttp://www.3gpp.org/DynaReport/36306.htm

  25. 3GPP Standardization Process

  26. 3GPP UMTS/LTE Specification Releases

  27. 3GPP UMTS/LTE Specification Series The most useful TSs:TS 23.401 (EPC) andTS 36.300 (Air Interface) http://www.3gpp.org/specifications/specification-numbering

  28. Outline • Introduction to Heterogeneous Networks (HetNet) - Heterogeneous Network Deployments - Features of Heterogeneous Networks - Evolution of Cellular Technology and Standards • Dense Small Cell Deployments - Introduction - Evolution of Small Cells - Efficient Operation of Small Cells - Control Signaling Enhancement - Reference Signal Overhead Reduction • TD-LTE Enhancements for Small Cells - Enhancements for Dynamic TDD - FDD-TDD Joint Operation • Future Trends in Heterogeneous Networks and Summary

  29. Dense Small Cell Deployments • Mobile data traffic is expected to grow tremendously in the future • In indoor and outdoor hotspot areas • Network densification via overlaid • Pico- and femto-cells • Advanced techniques • CoMP and FeICIC • Enhanced small cells to meet the future capacity requirements • For diverse applications and traffic types • The initial developments in LTE Release 12 • State-of-the-art technologies for small cell enhancements

  30. Evolution of Small Cells • Prior Release 12, LTE Releases 10 and 11 had considered • The co-channel heterogeneous deployments of small cells with the macro • Limited to isolated small cells deployed • A macro coverage area supports bursty traffic • A larger number of overlaid small cells • Operating on different carriers than macrocells • Realistic backhaul constraints, and • Emphasis on real-time traffic for evaluations • Differences between prior LTE releases and Release 12

  31. A Conceptual Small Cell Deployment Scenario Being Considered in Release 12

  32. Key Aspects of Small Cell Deployments • Co-channel and separate frequency deployment of small cells • Both co-channel and separate carrier deployment of small cells under the macro coverage area have been considered • One of the most important uses concerns the utilization of higher-frequency bands • Macro coverage • Deployment scenarios are being considered where small cells can be deployed without concurrent coverage of a microcell • Small cell density • Given their potential to offload more user traffic intelligently • Network densification via the deployment of multiple small cell clusters covering a hotspot area is being considered

  33. Key Aspects of Small Cell Deployments (cont.) • Outdoor and indoor • Prior LTE releases have considered • LTE Release 12 continues to focus on • Backhaul connectivity • Practical constraints are unlikely to allow ideal backhaul connectivity between macrocells and small cells as well as between small cells • Impact the choice of potential solutions • Efficient operation of small cells • Traffic characteristics • Have a small number of associated UEs per small cell due to small coverage • Highly asymmetrical in the uplink and downlink directions

  34. Deployment Scenarios • Scenario 1 Small cells deployed in the same carrier (F1) as the macrocell • Clustered outdoor deployment of small cells with 4–10 small cells per cluster • Coordination between macro and small cells via both the ideal and non-ideal backhaul • Realistic buffer traffic models are prioritized • Scenario 2 Small cells deployed in carrier (F2) different from the macrocell carrier (F1) • Clustered deployment of small cells similar to Scenario 1. Both outdoor (Scenario 2a)and indoor (Scenario 2b) deployments have been considered • Non-ideal backhaul-based coordination is prioritized • Legacy UEs can access the small cell as a legacy cell • Realistic buffer traffic models are prioritized • Scenario 3 Standalone deployment of small cells • Small cells deployed as a stand-alone cell regardless of macrocell coverage • Coordination among small cells via both the ideal and non-ideal backhaul • Realistic buffer traffic models are prioritized

  35. Co-channel Deployment of Macrocell with Overlaid Outdoor Small Cell Clusters

  36. Deployment of Macrocell with Overlaid Small Cell Clusters on Separate Frequencies

  37. Deployment of Standalone Indoor Small Cell Clusters with no Coordination with Macrocell

  38. Envisioned Small Cell Deployment Scenarios • Correspondence between small cell scenarios for evaluation in LTE Release 12 and real-lifesmall cell deployments

  39. Macro Association Ratios • Evaluated for different association methods • Traditional RSRP-based association methods • Clustered around locations of high UEdensity • Thus leading to higher small cell RSRPs • The trend as the number of clusters is varied • RSRQ+bias yield intermediate association ratios compared to • The extreme examples of RSRPand RSRQ

  40. Distribution of Small Cells • The distribution of small cells as per the number of UEs associated with them based on the RSRQ criterion • Fewer UEs associatedper small cell than Traditional systems • Have a lower number of small cells • The majority of small cells donot have any associated UEs • Important • consequences on the performance of small cells and also • the associated control signal design

  41. Efficient Operation of Small Cells • Network densification via small cell deployments offers opportunities as well as challenges • That must be overcome to enhance network performance • Dense and clustered deployment of small cells • Realistic traffic characteristics leads to a smaller number of active UEs offering opportunities • Throughput improvement • Reduction in control signaling overheads or • Use of higher order modulation

  42. Dense Small Cell Deployment Issues • Interference among small cells • eICIC and FeICIC, protect small cells • From the dominant interference arising from the downlink transmission of a microcell • The fluctuating interference arising • From the macrocell • From strongly coupled small cells • The use of dynamic UL/DL configurations • TDD leads to several new interference conditions • Small cell interference • Adverse impact on the discovery of small cells • Mobility management • Impact on operation of legacy CoMP and CA mechanisms • Limited coordination via non-ideal backhaul and network synchronization issues • Likely to render tight coordination among cells infeasible • Adversely impact CoMP and CA operations

  43. Dense Small Cell Deployment Issues (cont.) • Impact on core network: • With dense small cell deployments • Frequent and unnecessary handovers (between small cells) • Expected to increase even for low UE mobility • The smaller coverage area of small cells • Directly proportional to the number of small cells in a cluster • Co-channel deployments of macro and small cells • Result in severe interference conditions resulting in increased handover failure (HOF) rate • A homogeneous macrocell deployment

  44. Potential Technologies for Small Cell Enhancements in Release 12 The relationship between some of the candidate technologies and their applicability to particular small cell scenarios

  45. Dual Connectivity • A UE has the ability to maintain simultaneous connections to both the macrocell and the small cell • A dual-connectivity-capable UE • Perform RRM/RLM measurements • At both the macro and small cell layers • Receive downlink transmissions and possibly • Perform uplink transmissions • At macro and small cell layers, either simultaneously or in a TDM manner

  46. Key Benefits of Dual Connectivity • Mobility enhancement • Dual connectivity via control-plane/user-plane split between the macro and small cell layers • Expected to help achieve efficient mobility management • CoMP • The UE receives the control-plane transmission and user-plane information from different nodes • An example usage of dual connectivity that allows a UE to maintain its RRC connection with a macrocell while it receives uplink/downlink data from the small cells (below) • As the UE moves between coverage areas of the small cells, • It can receive data from the nearest small cell at any given instant

  47. Key Benefits of Dual Connectivity (cont.) • Throughput enhancements • Dual connectivity can allow an advanced UE • Receive PDSCH transmissions from macro and small cell layers simultaneously • Improving throughput performance • UL/DL power imbalance • Small cell deployments with overlapping macro coverage • Characterized by UL/DL power imbalance for UEs connected to a small cell • Such offloading is beneficial for Scenarios 1 and 2 and can • Be realized via dual connectivity

  48. Different Levels of Specification • Whether theinter-eNodeB connectivity • Co-channel deployment • Deployments in differentfrequency bands • Scenario1: co-channel deployments • Be limited to enabling UL/DL decoupled operation • Scenario 2: inter-band deployments • The UE capability of simultaneous receptionand/or transmission to both the macro and the small cell • Dual connectivity operation will have • Different requirements for the physical layerenhancements

  49. Three Modes of Dual Connectivity

  50. Impact of Non-Ideal Backhaul on Dual Connectivity • The carrier aggregation mechanism be categorized under dual connectivity • CA Scenario 4 is an intra-eNodeBinter-frequency dual connectivity scheme • CoMP Scenario 4 is an intra-eNodeB intra-frequencydual connectivity scheme • Difference : the required backhaul support • Release 12 will specify support for Uplink Control Information (UCI) transmission over PUCCH in each SCell

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