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Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era
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Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era Document Transcript

  • 1 NETMANIAS TECH-BLOG Please visit www.netmanias.com to view more posts Evolution of Mobile RAN Architecture in LTE/LTE- Advanced Era April 16, 2014 | By Dr. Michelle M. Do and Dr. Harrison J. Son (tech@netmanias.com) 1. Traditional RAN Architecture and Issues 1.1 Issue No. 1: Degraded service quality and network performance due to inter-cell interference 1.2 Issue No. 2: Increased costs of building and operating cell sites 2. Degradation of service quality and network performance due to inter-cell interference: Prevented by reducing X2 distance 3. Costs of building and operating cell sites: Reduced by BBU Centralization (C-RAN) The past few years have seen smartphones rapidly gain popularity and become one of the most loved daily essentials, especially with all of their ever-advancing multimedia processing features. Due to these advanced technologies behind mobile devices, the size of contents (video, music, picture, etc.) that users can enjoy on the devices are growing bigger and bigger every day (e.g. for videos, resolution SD (480p) → HD (720p) → now Full HD (1080p), and encoding rates, 500Kbps → 1Mbps → 2Mbps → now 4~8Mbps). Because of this growth, data traffic in mobile operators' network is soaring, and will do even more so from now on (This is not the same with voice traffic, which has already declined drastically compared to data traffic that is big-sized and steadily increasing). Then what about the network? In the previous voice traffic-centric networks, securing coverage for uninterrupted voice services (so that phones can pick up signals) was the most important issue. However, in today's LTE/LTE-A networks that put more focus on data traffic, increasing network capacity to ensure reliable and high data throughputs (rate, Mbps) has become the No. 1 priority. So, now it's clear. It's not coverage, but capacity. To cover soaring data traffic, network capacity needs to grow as well.1 To this end, LTE/LTE-A macro cells are getting smaller (In some metropolitan cities of Korea, the radius of an LTE/LTE-A cell is merely several hundred meters long). And this results in: ❶ more inter-cell interference, and ❷ higher cost (Capex/Opex) of building and operating smaller and more cell sites. With these changes, mobile network architectures have evolved to deliver reliable and high quality services to users, and at the same time to reduce the costs of investment in networks. So, it would be worthwhile to look into these trends here in this post. 2Km2Km Legacy Macro Cell in Seoul LTE/LTE-A Macro Cell in Seoul
  • Netmanias Tech-Blog: Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era 2 1. Traditional RAN Architecture and Issues Figure 1. Traditional RAN Architecture and Issues 1.1 Issue No. 1: Degraded service quality and network performance due to inter-cell interference In LTE/LTE-A networks that use OFDM for wireless access, a mobile device in the border areas of cells, experiences inter-cell interference caused among the neighbor cells that use the same frequency. This of course results in quality degradation. In order to prevent such quality degradation, CoMP (Coordinated MultiPoint) technology is employed in LTE-A networks. This technology allows each base station to schedule its radio resources in a way that minimizes interference, by exchanging current status information about its devices with its neighbor stations in real time over X2 interface. As a result, the service quality of the device as well as the network performance can be upgraded. CO BBU RRH AC Power (UPS) RRH CSG IP Edge Issue No. 1: Degraded service quality and network performance due to inter-cell interference - Very low latency between base stations, as low as 1~2 msec, is required in order for CoMP to improve performance at cell edges. - Hairpin in IP edge of backbone networks: Long X2 distance (several ~ several tens of msec) - CoMP not working properly (degraded service quality and network performance) - Then, why this architecture was used? Before LTE, no communication Between base stations was needed. So, a line (circuit/virtual circuit, e.g. T1/VLAN) was given for every base station in IP edge. CoMP IP/MPLS Physical X2 connection Issue No. 2: Increased cell site costs (Capex/Opex) - BBU, RRH, A/C, UPS (power), transport, etc. are all installed at cell sites (located in leased spaces) - High costs of lease, installation, utilities, and maintenance RRH AC Power (UPS) CSG AC Power (UPS) CSG t2=t1+tBH 4CSI = t1 Base Station 1 Base Station 2 sends UE1's CSI at t1 Base Station 3 UE1 SONET, MSPP, PTN, Carrier Ethernet, etc. SAE-GW 4 ≠ 10 t UE1 10CSI = UE1 Base Station 3 receives UE1's CSI at t2 1 t1 CSI=10 2 Base Station 2 UE1: CSI=10 UE1: CSI=10 Cell Site t2 t2=t1+tBH CSI = 10 UE1 3 At t2, the actual CSI of UE1 is much different from the CSI that Base Station 3 assumed t1 t2 Tens of Km or sometimes over 100 Kms Base Station Edge Router Backhaul SAE-GW Point-to-point circuit/virtual circuit IP/MPLS
  • Netmanias Tech-Blog: Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era 3 In order for the CoMP to work as designed, a base station must deliver the Channel State Information (CSI) of all the devices in each cell within the station's coverage (for example, information on what radio resources are currently used by each device, how strong or weak the signal that the base station sent is received by each device, and how severely each device is interfered by its neighbor cells, etc.) to the neighbor base stations in real time. If it is not delivered in time, the station's scheduling would be based on outdated and thus meaningless information, failing to achieve significant quality improvement (In the example in Figure 1, 4≠10. If so, it would rather be better not to use CoMP). Especially while a mobile device is moving from place to place, this CSI value changes very fast. So, the latency caused when two base stations exchange the information over X2 interface (X2 delay) should be minimized. As radio resource scheduling is performed at an interval of 1 ms (which corresponds to the LTE subframe duration), X2 delay of 1 ms would work the best, theoretically. In the traditional RAN architecture as seen in Figure 1, base stations are located at cell sites, and layer 1 or layer 2 backhaul network, such as TDM, MSPP, PTN, carrier Ethernet etc., aggregates base station traffic to IP edge (e.g., router) In a network with this type of architecture, X2 traffic generated at one base station must be sent through this long backhaul network, and on to IP edge. Then there, the traffic is routed to a base station in the neighbor cell site through the same backhaul network. In general, a base station is tens of Kms or sometimes over 100 Kms away from IP edge, and there are many hops between them for the traffic to pass. Thus, X2 delay in this architecture can easily be as high as several ~ tens of msecs, making it very unlikely for CoMP to improve performance at cell edges. Being based on a long X2 delay is like assuming today's weather will be nice because it was so a week ago, and being based on a short X2 delay is like assuming the same because it was so yesterday. Therefore, in LTE-A networks, it is inevitable to re-design the backhaul network architecture to ensure minimized the X2 delay. 1.2 Issue No. 2: Increased costs of building and operating cell sites The traditional RAN has a standalone base station where both Digital Unit (DU) and Radio Unit (RU) are installed at a cell site. Also it is generally installed inside of the medium or large scaled building for stable power supply and air conditioning. Because of these natures, each cell site must have a base station installed, AND each station must have its own power supply system, A/C, etc. Of course, the more devices and facilities there are, the more spaces and thus the higher costs are required. In addition to that, installation costs (civil works, labor charges) are increased, network installation takes longer, and monthly electric bill climbs as well. It will cost N (No. of cell sites) times more.
  • Netmanias Tech-Blog: Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era 4 And that means soaring Capex/Opex costs for building and operating a nation-wide network are unavoidable. Besides, as the network evolves into 4G, 5G and so on, the size of each cell is becoming smaller. Smaller cells mean, to mobile operators, more cell sites to build and operate. And more cell sites means more money. Let's take a look at the RAN architecture that is designed to reduce X2 delay for improved LTE-A radio performance. 2. Degradation of service quality and network performance due to inter-cell interference: Prevented by reducing X2 distance The CoMP technology is designed to improve service quality on mobile devices and network performance by controlling inter-cell interference. In order for the technology to do its job, X2 distance should be minimized. And this can be achieved by placing routers as close as possible to their associated base station as seen in Figure 2. Figure 2. Switching backhaul network to L3 routing network for reduced X2 delay CO BBU RRH AC Power (UPS) RRH IP Edge CoMP IP/MPLS Physical X2 connection RRH CSI UE1 10 t2=t1+tBH 9 CSI UE1 t1 Base Station 1 UE1 IP/MPLS Backhaul (IP/MPLS) AC Power (UPS) Base Station 2 AC Power (UPS) Base Station 3 CSI UE1 10 9 » 10 hundreds of meters to several Kms Placing routers closer to base station SAE-GW SAE-GW t1 t2 CSI=10 Base Station 2 sends UE1's CSI at t1 1 UE1: CSI=10 UE1:CSI=10 Base Station 3 receives UE1's CSI at t2 2 t2=t1+tBH t 3 At t2, the actual CSI of UE1 is pretty close to the CSI that Base Station 3 assumed Base Station Edge Router In LTE/LTE-A, the distance between a base station and CO is ranging from hundreds of meters to several kilometers. If delivered via fiber, it takes 5 μsec for data to travel 1 km. So, X2 delay (the period of time needed to send data from one base station to another, e.g. from Base Station 2 to Base Station 3 in the figure) will be significantly reduced as low as several μsecs, or tens of μsecs at most. ❶ X2 delay reduced by placing routers closer to base station
  • Netmanias Tech-Blog: Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era 5 In LTE/LTE-A, the distance between a base station and CO is ranging from hundreds of meters to several kilometers. If delivered via fiber, it takes 5 μsec for data to travel 1 km. So, X2 delay (the period of time needed to send data from one base station to another, e.g. from Base Station 2 to Base Station 3 in the figure) will be significantly reduced as low as several μsec, or tens of μsec at most. So, mobile operators in many countries tend to choose to place L3 hop as close as possible to base stations. 3. Costs of building and operating cell sites: Reduced by BBU Centralization (C-RAN) As macro cells have become even smaller to provide increased network capacity, more cell sites were required. To build and operate more sites, high costs were inevitable. To address this cost issue, a new RAN architecture, C-RAN (Centralized/Cloud RAN), was introduced. C-RAN allows operators to separate BBU and RRH in each cell site, and move all BBUs to a centralized location. Operators can then leave only RRHs and antennas unmoved at each cell site where actual radio signal reception is taken place. BBUs and RRHs, now located in different places away from each other, are connected using fiber cables (Dedicated Fiber per RRH or Dedicated λ per RRH). RRHs, designed for outdoor use, are simple but very hardened devices that run well without A/C facilities, which means no indoor space to rent. So, operators can minimize their rental costs as they only need rooftop spaces for RRHs and antennas. This also means reduced electricity bills as they only need to supply power for RRHs.
  • Netmanias Tech-Blog: Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era 6 Figure 3. Reducing costs of building and operating cell sites: C-RAN (Centralized/Cloud-RAN) In summary, for efficient accommodation of soaring data traffic, mobile network architectures have recently evolved as follows: i) IP layer has been placed as close as possible to base stations so that shorter X2 delay is ensured and by that degradation of service quality and network performance caused by inter-cell interference is prevented. ii) To minimize costs of building and operating cell sites, BBUs are removed from each cell site and are moved upward (to mobile operator's CO or master cell site). Footnotes 1. The advent of small cells intended to increase network capacity has made the two issues even worse. This HetNet environment will be discussed later in another post, and only homogeneous networks will be concerned here. CO IP Edge IP/MPLS UE1 IP/MPLS RRHs (Outdoor) RRHs (Outdoor) RRHs (Outdoor) BBU SAE-GW Backhaul (IP/MPLS)Fronthaul (CPRI)RRH BBU SAE-GW CPRI Physical X2 connection X2 BBU centralized at CO ❷ BBU Centralization (C-RAN) · C-RAN allows operators to separate BBU and RRH in each cell site, and move all BBUs to a centralized location. Operators can then leave only RRHs and antennas unmoved at each cell site where actual radio signal reception is taken place. · RRHs, designed for outdoor use, are simple but very hardened devices that run well without A/C facilities, which means no indoor space to rent. So, operators can minimize their rental costs as they only need rooftop spaces for RRHs and antennas. This also means reduced electricity bills as they only need to supply power for RRHs. BBU BBU BBU
  • About NMC Consulting Group (www.netmanias.com) NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2014 NMC Consulting Group. All rights reserved. 7 Carrier WiFi Data Center Migration Wireline Network LTE Mobile Network Mobile WiMAX Carrier Ethernet FTTH Data Center Policy Control/PCRF IPTV/TPS Metro Ethernet MPLS IP Routing 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 eMBMS/Mobile IPTV Services CDN/Mobile CDN Transparent Caching BSS/OSS Cable TPS Voice/Video Quality IMS LTE Backaul Netmanias Research and Consulting Scope Visit http://www.netmanias.com to view and download more technical documents. Future LTE IP/MPLS CarrierEthernet Networks Consulting POC Training Wi-Fi Infrastructure Services CDN Transparent Caching IMS Concept Design DRM eMBMS protocols Analyze trends, technologies and market Analysis Report Technical documents Blog One-Shot gallery We design the future We design the future We design the future