2013 Multi-Technology Synergies: Multi-RAT Small Cells Radio Resource Management M. Rafie, Ph.D. Argela 11/7/2013
Table of Contents 1. Introduction... 5 2. Multi Technology Synergies: Radio Resource Management... 5 2.1 Need for Multi-Technology RRM... 5 2.2 Multi-RAT RRM Benefits... 6 2.3 Multi-RAT RRM Functions and Procedures... 7 2.4 Multi-RAT RRM Challenges... 8 3. Multi-RAT RRM Interaction Model and Dependencies... 9 3.1 Multi-RAT RRM Interaction Model... 9 3.2 Multi-RAT RRM Dependencies... 10 4. Multi-Technology RRM Implementation Models and Topologies... 11 4.1 Integrated Multi-RAT RRM: Single Access Point... 12 4.2 Integrated Multi-RAT RRM: Multi-Access Point... 13 4.3 Server/Cloud-based Multi-RAT RRM: Multi-Access Point... 15 4.4 Integrated Multi-RAT RRM: UE-Based... 15 4.5 Hierarchical Multi-Tier Architecture... 16 5. Conclusion... 17 References... 18 Page 1
Index of Tables Table 1: Various multi-rat RRM models and associated use cases... 12 Page 2
Index of Figures Figure 1: Multi-RAT RRM in a multi-standard HetNet... 6 Figure 2: Multi-RAT RRM functionalities and procedures... 8 Figure 3: Multi-RAT RRM Architecture... 9 Figure 4: Interaction between multi-rat RRM and local RRM entities... 10 Figure 5: Single access point Integrated multi-rat RRM topology... 13 Figure 6: Multiple access point Integrated multi-rrm topologies, (a) integrated multi-rat RRM in each intra-rat AP, (b) integrated multi-rat RRM in some APs, and (c) integrated intra-rat in single AP with integrated multi-rat RRM in some APs.... 14 Figure 7: Multiple access point Server/Cloud-based multi-rat RRM topology... 15 Figure 8: Integrated multi-rat RRM UE based scenario... 15 Figure 9: Generic Multi-Radio Access Technology - RRM Topology... 16 Figure 10: Multi-tier hierarchical inter-operator RRM Topology... 17 Page 3
Abbreviations 2G 3G 3GPP ANDSF ANQP C-plane CAPWAP enodeb eicic enb EMS EPC EPS EUTRAN GERAN GPRS GSM HeNB HeNB-GW HeMS HNB HNB-GW HMS IFOM LTE MME MSC/VLR NMS OAM PGW RAN RAT RRM S1 S11 S1-MME S1-U SeGW SGSN UE UTRAN U-plane WLAN Second generation Third generation 3rd Generation Partnership Project Access Network Discovery and Selection Function Access Network Query Protocol Control Plane Control And Provisioning of Access Point Evolved NodeB Enhanced Inter-Cell Interference Coordination E-UTRAN NodeB Element Management System Evolved packet core Evolved Packet System Evolved Universal Terrestrial Radio Access Network GSM and EDGE radio access network General Packet Radio Service Global System for Mobile Communications Home enb HeNB Gateway HeNB Management System Home NB HNB Gateway HNB Management System IP Flow Mobility Long Term Evolution Mobility Management Entity Mobile Switching Center (MSC) / Visited Location Register (VLR) Node B Management System Operation and Maintenance Public Data Network (PDN) Gateway Radio Access Network Radio Access Technology Radio Resource Management Interface between enb, MME, and S-GW Interface between the MME and S-GW S1 for the control plane S1 for the user plane Security Gateway Serving General Packet Radio Serving Support Node User Equipment Universal Terrestrial Radio Access Network User plane Wireless LAN Page 4
1. Introduction As a response to the tsunami of mobile data growth, operators are forced to adopt more resilient, cost effective, high performance, high coverage and high capacity multi-standard network solutions through a multitude of technical, regulatory and commercial innovations for their ever-expanding ecosystem. Operators can best achieve substantial capital expenses (OPEX) and operational expenses (CAPEX) benefits and performance gains through an aggregated view of well-coordinated and managed heterogeneous networks (HetNet). The potential drawback of the multi-technology small cells deployment (residential, enterprise, public or rural) is the possibility of an increase in CAPEX/OPEX and added complexities involved in procuring multiple sites of their networks. Additionally, the lack of methods to jointly coordinate resources and manage interference among multi-layer small cells with different radio access technologies, frequency planning and multiple backhaul connections may severely degrade the system performance at the end-user. The next generation of small cells deployments must enable multi-technology small cell deployments combining 3G and 4G technology with integrated Wi-Fi connectivity ensuring an end-to-end ubiquitous and optimized traffic performance to users. This means that the design, operation and optimization of future networks present novel challenges and require new methodologies and capabilities for 3G, 4G, and Wi-Fi wireless devices. Mobile operators seek ways to provide additional performance and costs saving for both operators and end users. In fact, operators are required to use a combination of various technologies as appropriate to meet their requirements, delivering a high-quality user experience (QoE) with a limited set of resources in the form of spectrum, transmit power and radio technology. 2. Multi Technology Synergies: Radio Resource Management 2.1 Need for Multi-Technology RRM The next generation mobile wireless communication services are envisioned to be supported by heterogeneous networks, often labeled HetNets, accommodating a variety of radio access technologies (RATs), as shown in Figure 1. Most of the existing Radio Resource Management (RRM) strategies and algorithms are RAT-specific and they are implemented separately for given access networks. Heterogeneous networks require efficient use of multi-technology RRM strategies to optimally coordinate and manage radio resources among multiplicity of RATs. Multi-technology radio resource management (multi-rat RRM) functions constitute assignment, control and sharing of radio resources among the users across such multi-standard HetNet. The placement, design and the distribution of multi-technology RRM strategies are highly dependent on the integration, harmonization, and optimization of the following multi-level network architectures and services: Page 5
Multi-access technology: 4G, 3G (UMTS/HSPA), GPRS/EDGE, and WLAN Multi-layer topology: macro cell, metro cell, and small cell Multi-band spectrum: regional and global spectrum fragmentation Multi-vendor RAN: interface for seamless interworking Multi-services: diverse and often conflicting QoS requirements for services A preferred multi-rat RRM model should exploit the presence of the synergy between multilayer, multi-band, multi-vendor, multi-technology, and multi-service diversity in a HetNet environment. Figure 1 illustrates the importance and the challenges faced by multi-rat RRM in a multistandard HetNet. Figure 1: Multi-RAT RRM in a multi-standard HetNet 2.2 Multi-RAT RRM Benefits A key aspect of heterogeneous systems is the implementation of efficient joint radio resource management mechanisms through the coexistence of a variety of radio access technologies with different, but also complementary, technical characteristics and performance. Multi-RAT RRM can bring significant benefits in heterogeneous networks. Some of the key benefits are: Page 6
Multi-RAT RRM methods can outperform existing local-based RRM methods in terms of call blocking probability and capacity gains, in both real-time and non-real time services Multi-RAT RRM can provide flexible scalability for operators depending on the implementation or topology approaches Multi-RAT RRM can provide an integrated and adaptive radio to coordinate and manage the fragmented license spectrum over an entire region of interest through spectrum aggregation Multi-RAT RRM can enable a common framework for measurement and diagnostic purposes through a unified pool of KPI for multi-rat Aps Other multi-rat RRM benefits include a mechanism for active load balancing, interference distribution, reduction of unnecessary handovers and call dropping between layers of the HetNet 2.3 Multi-RAT RRM Functions and Procedures The ultimate multi-technology RRM objective is to satisfy the QoS requirements of the individual radio bearers and to optimize the pool of available radio resources among a variety of the multiaccess technologies at the smallest possible cost for the network. The trends in next-gen wireless network evolution indicate a desire to integrate a variety of wireless access technologies for an always best connected environment for mobile users. This can be accomplished through efficient use of resources (for new and existing users) by minimizing call blocking, dropped call rates and handover failures and maximizing overall network performance, capacity and coverage at the minimum cost to the operators. Multi-technology RRM functionalities can be split into the following three procedures: 1. Measurement, monitoring, and reporting 2. Multi-RAT RRM functionalities, strategies, and algorithms 3. Decision making and execution Examining the RRM functions shows that a high degree of common information is shared by each RRM function across multi-rat access points. In particular, a good portion of measurements, reporting, and monitoring pool of resource procedures is RAT agnostic. Note that, in multi-rat RRM the decision enforcement procedure may result in triggering other RRM functionalities. For example, handling the task of uneven distribution (load balancing) of the traffic load over multiple inter-frequency and inter-rat cells can lead to handover and/or cell reselection decisions. Figure 2 illustrates the main functions and procedures of a multi-rat RRM. Page 7
Figure 2: Multi-RAT RRM functionalities and procedures Although from the topology point of view, multi-rat RRM and multi-rat SON implementation models might have a seemingly similar topologies in a HetNet, SON functionality is to do more with the self- configuration, optimization and maintenance aspects of multi-cell/multi-rat networks. 2.4 Multi-RAT RRM Challenges Efficient implementation of multi-rat RRM strategies within multi-standard networks is vital for a proper operation of a HetNet. The utmost challenge is to provide the desired QoS level with the minimum resources, minimizing operators investment while meeting the network design requirements. Some of the key challenges for the management and coordination of radio resources in a multi-standard HetNet environment are: Every RAT is based on a specific multiple access mechanism (e.g., WCDMA, OFDMA, Wi-Fi) Without suitable load balancing or traffic steering techniques, multi-rat network traffic may exhibit significant spatial and temporal variations resulting in underutilization or overutilization of radio resources at certain cells Layer balancing, local loading, UE services requests, (e)icic configurations, link adaptation, dynamic scheduling and interference management across multi-rat networks need to be jointly optimized Local RAT-specific RRM mechanisms are needed for every RAT used in the network (e.g., RRM-eUTRAN, RRM-UTRAN, RRM-GERAN, RRM-WLAN) Multi-RAT RRM entities are required to control and manage a pool of radio resources that belong to multiplicity of RATs available at various layers in a multi-standard HetNet Intelligent management of service delays due to possible increase in signaling overhead in multi-rat small cell networks is required A way of speeding up the scheduling process (LTE - 1ms TTI) for the radio resources should be considered. This combined with other multi-rat RRM functionalities places a lot of processing load in enbs Page 8
A defined set of interfaces is needed to enable o multi-vendor IOT for wider accessibility and enhanced coverage (seamless service continuity) o multi-technology synergy between 3G, 4G and WiFi for efficient use of spectrum and enhanced coverage o measurements and information reports that are to be exchanged between intra- and inter-rat access-points A unified approach is required for multi-rat RRM performance, verification, testing, and optimization 3. Multi-RAT RRM Interaction Model and Dependencies 3.1 Multi-RAT RRM Interaction Model A possible architecture for an efficient implementation of multi-rat RRM is a two-tier RRM architecture [1] as shown in Figure 3. Local RRM: this RRM entity physically resides within each access point (AP) or User Equipment (UE). It manages, allocates, and de-allocates RAT-specific radio resources for the given radio technology network. Multi-RAT RRM: this common RAT or controlling RRM entity has the knowledge of the overall radio resource pool, and is responsible for efficient management of multiple local RRM entities as well as exchanging information reporting with other multi-rat RRMs Figure 3: Multi-RAT RRM Architecture Based on the degree of interaction between local RRM and multi-ram entities, the following functions can be performed by either local RRM or multi-rat RRM entities: a) Information reporting and exchanging function: a. Static cell information: QoS, cell relations, capabilities, priorities, capacities, buffer delay, buffer size, maximum bit rate for a given service, policies, etc. b. Dynamic cell information: TX power, RX power, layer load, and interference measurements, etc. b) Decision making function a. Multi-RRM centred decision-making: Multi-RRM entity makes decision and informs local RRM to execute those decisions b. Local RRM centred decision-making: Final decisions are made by the local RRM entities. Multi- RRMs only advise local RRMs In general, the multi-rat RRM and the local RRM will work in a hierarchical manner, where the resources managed by the local RRM will depend on the assignments performed by the multi- RAT RRM, which will depend on feedback information coming from the local RRM and external policies. Page 9
The level of granularity of local RRM (which typically contains the scheduler) will be usually smaller than the multi-rat RRM. For example, LTE and LTE-A can define scheduling decisions in the order of 1ms (TTI) that is more appropriate to be implemented in local RRMs. Multi-RAT RRM is typically operational in seconds or minutes granularity. The use of a multi-rat RRM as a central entity could only be required and used when the local RRM entities are not able to further fulfill the network and user requirements. 3.2 Multi-RAT RRM Dependencies Most of the interactions between local RRMs and multi-rat RRMs are on the low or intermediate time-scale levels. This allows the multi-rat RRM entity to perform functions such as RAT selection, vertical handover (Inter-RAT), admission control, and congestion control functions. A higher degree of interaction between local RRM and multi-rat RRM entities can achieve more efficient resource management. However, it requires more frequent interactions between these entities, which may lead to a higher amount of signaling overhead. The interaction between the local RRM and multi-rat RRM is not directly related to the coupling topologies between radio access networks. Nevertheless, for frequent local RRM and multi-rat RRM interactions, tight coupling topologies are required in order to reduce the delays in communications between levels of hierarchies. Various degrees of coupling between multi-rat RRM and local RRM entities in terms of short/long-term time scales ranging from less than 1ms (e.g., inner loop power control in WCDMA) to thousands of frames (e.g., admission control, Handover or outer loop power control) can occur. In general, the more frequent interactions between multi-rat RRM and local RRM, the tighter the coupling architectures are required as shown in Figure 4. The lowest degree of interaction occurs when the multi-rat RRM is only responsible for dictating operator s policy enforcement as various configurations for lower local RRM functions. Figure 4: Interaction between multi-rat RRM and local RRM entities Page 10
4. Multi-Technology RRM Implementation Models and Topologies From the implementation point of view and coupling architectures, the simplest solution for heterogeneous networks integration is the so-called loose coupling architecture. In this implementation, different networks are connected together through gateways, while still maintaining their independence. In general, a number of possible configurations can be considered for multi-rat RRM implementations: Centralized RRM o Resides in a small number of locations (network and/or domain management) such as OAM, NMS, EMS, GW, controller or servers o More control for the operators o Less scalable o Slower reaction, potentially less accurate Distributed RRM o Resides in many locations o Algorithms are executed at the network elements (e.g., enb, HeNB, or APC) o Offers less control for the operator o More scalable Localized RRM o Distributed but autonomous o No control for the operators o Decisions are made totally autonomously o Most scalable option Hybrid RRM o Some RRM resides in the Operation and Maintenance (OAM) servers and some on network management (e,g, enb) o Operator can move to the distributed option as they become more confident about multi-vendor Interoperability In general, distributed or hybrid approaches are favored since they enable lower delay, less signaling, and lower cost, even though they risk losing some performance gains compared to their centralized counterparts. Several multi-technology RRM use cases can be identified to cover multiple aspects of the network operations, including planning, deployment, optimization and maintenance [3]. A. Single AP a. Local RRM B. Enterprise Cluster APs a. Local RRM w/ peer-to-peer communications b. Controller hosted RRM C. Cloud RAN a. Single or clustered APs D. HetNet (Macrocell overlay, smallcell underlay) The Multi-RAT RRM topologies can be, possibly, split into the following models based on the implementation constraints and the use cases as shown in Table 1: 1. Integrated Single Access Point (AP): Localized, AP centric 2. Integrated Multi-Access Point: Single tire, distributed, AP centric 3. Server-based Multi-Access Point: Multi-tier, centralized, server / cloud / network centric 4. Integrated UE-based: UE centric, centralized 5. Hybrid Multi-RRM: Hierarchical / distributed Page 11
Table 1: Various multi-rat RRM models and associated use cases # 1 2 Multi-RAT RRM Implementation Models Integrated Single Access Point Cell or AP centric Integrated multi-access Point Single horizontal tier RRM Cell or AP centric Description Implications Use Cases Localized multi-rrm All multi-technology stacks are in the same access point Distributed multi-rrm Local multi-rrms with peer-topeer communication Multi-technology stacks are in multiple access points No Enterprise controller No new interface / standard is required within the APs Decision processes are made locally New interfaces may be required for information reposting across APs (inter-rat) Dynamic RRM handling, requiring frequent signal exchanges New interfaces may be required The existing I/F may not be adequate eutran: X2, S1 UTRAN: Iuh, Iub, Iur, Iuhr WLAN: IuWLAN, CAPWAP, ANDSF, ANQP, IFOM Standardized Itf-N/Itf-S (3GPP type 1/2 OAM) Dynamic RRM handling, requiring frequent signal exchanges Single cell Enterprise cluster Peer-to-peer communications 3 4 5 Server based multi- Access Point (multi- RRM controller) Multi-tier RRM Network/server/cloud centric Integrated multi-rrm UE Based RRM UE centric Hybrid (hierarchical) RRM Centralized multi-rrm Enterprise controller Cloud controller Server/cloud based management Centralized Multi-RRM Localized RRM in UE and AP Same as case 2 Signalling delays Suited for long-term RRM functions (overall network load balancing) Additional cost Intra/inter-AP Communication Same as case 1 UE-centric RRM assigns resources to users first Bottom-up Hybrid Multi-RRM Same as cases 2 and 3 Trade-off between integrated and server-based multi-rat RRM Top-down RAN global optimization System-centric RRM RRM hosted server Enterprise cluster Cloud RAN DAS RAN Single cell (Typically) Same as 2, 3 and 4 4.1 Integrated Multi-RAT RRM: Single Access Point In an integrated multi-rat RRM single access point, the execution of the multi-rrm functions can be performed locally between various RATs rather than through multiple AP s or CN s. In this topology, no additional delays will be incurred. Since the RRM decision processes between RATs are co-located within the same physical entities, the support functions do not need to be standardized and no new open interfaces need to be defined. The entire RRM process is a cell or a single Access Point (AP) centric (intra-ap communication). Figure 5 shows an integrated multi-rat RRM topology. Page 12
Figure 5: Single access point Integrated multi-rat RRM topology 4.2 Integrated Multi-RAT RRM: Multi-Access Point In this topology, an integrated multi-rat RRM functionality may be included in each AP or only in some of the APs. This suggests that the intra-ap RRM decision processes are performed locally in the same physical entities. However, in this case, the reporting information functions and interfaces between different multi-rrms (inter-rat) entities must be standardized. Figure 6 shows various possible scenarios for an integrated multi-rat RRM across inter-rat APs. In scenario (a), the reporting information between intra-rat can be communicated through standard interfaces such as X2 or Iur. In the case of inter-rat RRMs such as UTRAN and GERAN, the communication between BSC and Radio Network Controller (RNC) NC can be performed through the MSC of the core network. Alternatively, in case (b), where multi-rrm entities are not included in some of the access points, either new interfaces need to be defined or the communication between the multi-rrms should be performed through the core network, using the existing interfaces that are defined in the 3GPP standards (e.g., S1, Iu, Iuh, Iur-g, Gb). Finally, in case (c), a combination of an integrated multi-rat RRM in a single AP and another multi-rat RRM in another AP is shown. Note that the dashed-line interface connections may or may not exist. In cases where the dashed interfaces are missing the communications between APs are performed through the core network (or using a cloud RAN). Page 13
Figure 6: Multiple access point Integrated multi-rrm topologies, (a) integrated multi-rat RRM in each intra-rat AP, (b) integrated multi-rat RRM in some APs, and (c) integrated intra-rat in single AP with integrated multi-rat RRM in some APs Page 14
4.3 Server/Cloud-based Multi-RAT RRM: Multi-Access Point In a server-based multi-rat RRM topology, a server may reside in the core network. The serverbased multi-rat RRM is centralized and common to multiple technologies (eutran, UTRAN, GERAN, WLAN) as shown in Figure 7. Multi-RAT RRMs and local RRMs are located in different physical nodes and are inter-connected from the server towards the access points through a set of defined (open) interfaces. Since the multi-rrms can collect the information from all available RATs, layers and nodes, they can provide a more optimal decision in cases of call admission control or inter-system handover. Figure 7: Multiple access point Server/Cloud-based multi-rat RRM topology 4.4 Integrated Multi-RAT RRM: UE-Based This topology allows a greater part of the RRM decisions to be assigned to the UE including making the RAT selection decisions. User-centric RRM typically assigns resources to users first and then to radio ports. This approach requires more computation and power consumption from the UE, in addition to information from the RAN and core network. Figure 8 illustrates a UE-centric multi-rat RRM topology. Figure 8: Integrated multi-rat RRM UE based scenario Page 15
4.5 Hierarchical Multi-Tier Architecture A generic (canonical) topology for a multi-rat RRM can consist of a hierarchical-mesh topology. The multi-tier RRM can be distributed at UEs, access points, and/or core networks. In general, the upper layers control and coordinate a number of lower layers. This topology is in effect a trade-off between the centralized and distributed topologies. The advantage of having a hierarchical RRM is that it allows the lower-level entities in the hierarchy to perform and communicate the RRM decisions faster and with less overhead than in a scheme that only depends on a central RRM entity. However, in the case of low network load or a smaller number of local RRM entities, the use of multi-rat RRM can be avoided if the local RRMs are available and capable of managing the resources autonomously. The concept of a multi-technology RRM model based on a multi-tier RRM architecture is shown in Figure 9. Figure 9: Generic Multi-Radio Access Technology - RRM Topology Another scenario in a multi-tier hierarchical topology could be in the case where operators A and B are sharing the spectrum in a given region [2]. This allows operators to access underutilized spectrum on a shared basis, without interfering with incumbent spectrum holders. This proposed Authorized Shared Access (ASA) can potentially unlock hundreds of MHz of highquality spectrum suitable for (LTE) small cell deployment. In this case, an entity(s) will be in charge of coordinating and managing the spectrum sharing between the different operators, as shown in Figure 10. The speed at which the spectrum is dynamically shared between operators will be slower than for the multi-rat RRM and Local RRM. Page 16
Figure 10: Multi-tier hierarchical inter-operator RRM Topology 5. Conclusion Multi-RAT RRM can bring significant benefits in the heterogeneous network including load balancing, interference distribution, reduction of unnecessary handovers and reduction of dropping/blocking probability in both licensed and unlicensed spectrum. Depending on the topology and the coupling interaction between local RRM and multi-rat RRM, the amount of signaling overhead may increase. Trade-offs between multi-rat RRM architecture, performance, strategies and signaling overhead are required to find the optimum solution. Although the Multi- RAT RRM algorithms do not need to be standardized, the issue of the interoperability between multi-vendors and multi-rat RANs could be addressed through a set of defined (open) interfaces for a full deployment of always best connect future networks. Page 17
References [1] Jordi Pe rez-romero, Oriol Sallent, Ramon Agustı and Miguel Angel Dı az-guerra, Radio Resource Management Strategies in UMTS, John Wiley & Sons Ltd, 2005 [2] Ian F. Akyildiz, David M. Gutierrez-Estevez and Elias Chavarria Reyes, The evolution to 4G cellular systems: LTE-Advanced, Physical Communications 3. 2010 [3] Nick Johnson, Radio and Physical Layer (RPH) Working Group multi-technology RRM, Small Cell Forum, Manchester, UK, 2012 Page 18