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Interference Coordination in LTE/LTE-A (1): ICIC

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  • 1. 1 NETMANIAS TECH-BLOG Please visit www.netmanias.com to view more posts Interference Coordination in LTE/LTE-A (1): ICIC June 5, 2014 | By Dr. Michelle M. Do and Dr. Harrison J. Son (tech@netmanias.com) As mobile communication technology has evolved dramatically, from LTE (10 MHz) to LTE-A (10+10 MHz), and then to wideband LTE (20 MHz), South Korea's mobile market is hotter than ever with its big 3 operators competing fiercely in speed and quality (see Netmanias Report, LTE in Korea UPDATE - May 1, 2014). Operators can offer different maximum speeds depending on how wide frequency bandwidths they can actually use. All three, with pretty much same amount of LTE frequency bandwidths obtained, practically support the same maximum speeds. However, these theoretical maximum speeds are not available to users in real life. What users experience, i.e., Quality of Experience (QoE) is affected by various factors, and so the actual QoE is far from the maximum speeds. One of the biggest factors that causes such quality degradation is Inter-cell Interference. In 2G/3G networks, it was base station controllers, i.e., upper nodes of base stations, that control inter-cell interference. In 4G networks like LTE/LTE-A, however, inter-cell interference can be controlled through coordination among base stations. This was made possible because now LTE networks have X2 interfaces defined between base stations. By exchanging interference information over these X2 interfaces, base stations now can schedule radio resources in a way that avoids inter-cell interference.1 There are several Interference Coordination technologies in LTE and LTE-A: • LTE: Inter-Cell Interference Coordination (ICIC) • LTE-A: Enhanced ICIC (eICIC) which is an adjusted version of ICIC for HetNet, and Coordinated Multi-Point (CoMP) which uses Channel Status Information (CSI) reported by UE In this and next few posts, we will learn more about these Interference Coordination technologies. First, let's find out ICIC, the most basic interference coordination technology. Inter-Cell Interference Coordination (ICIC) What causes inter-cell interference? The biggest cause of lower mobile network capacity is interference. Interference is caused when users in different neighbor cells attempt to use the same resource at the same time. Suppose there are two cells that use the same frequency channel (F, e.g., 10MHz at 1.8GHz band), and each cell has a UE that uses the same frequency resource2 (fi, fi∈F). As seen in the figure below, if the two UEs are located in cell centers like A2 and B2, no interference is caused because they use low power to communicate. However, if they are at cell edges like A1 and B1, their signals cause interference for each other because the two use high power to communicate.
  • 2. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (1): ICIC 2 Interference is caused because cells only know what radio resources their own UEs are using, and not what other UEs in the neighbor cells are using. For example, in the figure above, Cell A knows what resources A1 is using, but not about what B1 is using, and vice versa. And the cells independently schedule radio resources for their own UEs. So, to the UEs at cell edges (A1 in Cell A and B1 in Cell B), same frequency resource can be allocated. ICIC Concept ICIC is defined in 3GPP release 8 as an interference coordination technology used in LTE systems. It reduces inter-cell interference by having UEs, at the same cell edge but belonging to different cells, use different frequency resources. Base stations that support this feature can generate interference information for each frequency resource (RB), and exchange the information with neighbor base stations through X2 messages. Then, from the messages, the neighbor stations can learn the interference status of their neighbors, and allocate radio resources (frequency, Tx power, etc.) to their UEs in a way that would avoid inter-cell interference. For instance, let's say a UE belonging to Cell A is using high Tx power on frequency resouce (f3) at the cell edge. With ICIC, Cell B then allocates a different frequency resource (f2) to its UE at the cell edge, and f3 to its other UE at the cell center, having the one at the center use low Tx power in communicating. f1 Interference in Cell Edge signalinterference signalinterference f3 No interference in central region f1 Cell A Cell B A1 B1 f3 Low power High power High power Low power A2 and B2 use the same frequency resource (f1), and use low Tx power in communicating with their serving cells because they are in cell centers. So, no interference is caused. A1 and B1 use the same frequency resource (f3), and use high Tx power in communicating with their serving cells because they are at cell edges. So, they both cause interference for each other. A2 B2 F F F = {f1, f2, …, fN}, fi : resource blocks (RBs) or sub-carriers Low power High power f1 No Interference in Cell Edge signal signal f3 No interference in central region f1 Cell A Cell B A1 B1 f2 High power Low power Cell A and Cell B coordinate to ensure: • different frequency resources (f3 and f2) are allocated to A1 and B1 at the cell edge, allowing for enhanced communication quality with no interference. • Cell B allocates f3 to a UE that uses low Tx power if Cell A is already using f3 for a UE at the cell edge that uses high Tx power. A2 B2 B3 f3 Low power F F F = {f1, f2, …, fN}, fi : RBs or sub-carriers
  • 3. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (1): ICIC 3 Interference Information used in ICIC eNBs exchange interference information of their cells with the neighbor eNBs by sending an X2 message (Load Information message3 ) after each ICIC period. At this time, the message includes information like Relative Narrowband Tx Power (RNTP), High-Interference Indicator (HII), and Overload Indicator (OI).  RNTP: Indicates frequency resources (RBs) that will be using high Tx power for DL during the next ICIC period. Power strength of each RB is measured over the current ICIC period and shown in bits (0: low, 1: high). For example, the strength can be averaged over the current ICIC period.  HII: Indicates frequency resources (RBs) that will be using high Tx power for UL during the next ICIC period, just like RNTP, but for UL this time. RBs with high allocated power are used by UEs at cell edges, and thus are very likely to cause interference for neighbor cells. The power strength of each RB is measured and shown in bits (0: low, 1: high).  OI: Indicates frequency resources (RBs) that have experienced most interference during the last ICIC period. Degree of interference caused to each RB is measured and marked as Low, Medium or High. RNTP and HII are information about interference to be caused by a cell to its neighbor cell. However, OI is information about interference that has already been caused by the neighbor cell to the cell during the last ICIC period. HII information is mandatory and serves as the most important information. Cell A Cell B Signal Signal f1 f2 3. UL: OI {f1, f2, f3, f4, f5} = {High, High, Low, Low, Low} Cell A Signal Signal f2 f1 1. DL: RNTP {f1, f2, f3, f4, f5} = {0,1,1,0,0} 2. UL: HII {f1, f2, f3, f4, f5} = {0,1,1,0,0} A2 A1 A3 Signal f3 t f 1 2 3 4 5 0 1 1 0 0 Interference caused by a neighbor cell to Cell A - by frequency (RB) Power strength of each UE in Cell A - by frequency (RB) Low power High power Ave. None B1 B2 OI high high low ICIC period 1msec high high low TTI TTI: Transmission Time Interval ICIC: Inter-Cell Interference Coordination Interference Interference
  • 4. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (1): ICIC 4 Basic ICIC Behavior eNBs send interference information to neighbor eNBs after each ICIC period. In general, an ICIC period (ranging tens ~ hundreds of ms) is longer than a scheduling period, TTI (1 ms). Below is the illustration of an example that shows how ICIC works. Here, the ICIC period of both Cell A and Cell B is 20 ms. Cell B Cell A X2 ICIC Period (e.g. 20 ms) X2 Load Information Load Information Measure signal/ interference and generate interference information Send interference info. through X2 message ICIC-based Scheduling (ICIC power control) RNTPHIIOI Tx (X2 Delay) ICIC Calc. Tx (X2 Delay) ICIC Calc. #12 #13#11 Local (Cell A) SchedulingSignal/Interference Measurement f 1 2 5 3 4 Local (Cell B) SchedulingSignal/Interference Measurement t t A3 ... Resource Coordination for ICIC period #13 f 21 4 RNTPHIIOI Input ICIC results into scheduler Maximum Tx power allowed for allocation f1 f2 f3 f5 f Cell A Cell B Low Tx power used, so no Interference for neighbor cells Maximum Tx power allowed for allocation f4 f1 f2 f3 f5f4 f 1 2 5 3 4 B1 B2 ... t t A2 High power Low power High power Low power ICIC Calculation (Coordina- tion) 3
  • 5. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (1): ICIC 5 Basic operations of ICIC are: ❶ Generate interference information (ICIC period #11) Cell A and B measure signal/interference strength during an ICIC period, and generate interference information (RNTP, HII, OI). ❷ Share interference information (ICIC period #12) Cell A and B share the interference information with neighbor cells through X2 message. X2 delay between neighbor cells must be shorter than the ICIC period. ❸ Resource Coordination: ICIC calculation (ICIC period #12) Both cells run an ICIC algorithm based on the neighbor cell's interference information received, and determine frequency resources (RBs or sub-carriers) that will be used at cell edges, and thus will be using high Tx power. ❹ Coordinated resource-based local scheduling (ICIC period #13) The results of ICIC calculation are applied to local schedulers. Based on coordinated resources, cells perform local scheduling (i.e. allocating radio resources to the UEs accessed to them) depending on the channel status of each UE. With ICIC, each cell can carry out local scheduling based on resources coordinated with neighbor cells, consequently reducing inter-cell interference. Next time, we will discuss eICIC, an adjusted version of ICIC for HetNet environment. Footnotes 1. Over X2 interfaces, not only interference information, but also information on handover, resource status, neighbor cells, etc., can be exchanged. However, only interference information is discussed here in this post. 2. Frequency resources are allocated in resource blocks (RBs). In this post, RBs (or sub-carriers) that are allocated to UEi are referred to as fi. 3. 3GPP TS 36.423
  • 6. 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. 6 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