Interference Coordination in LTE/LTE-A (2): eICIC


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Interference Coordination in LTE/LTE-A (2): eICIC

  1. 1. 1 NETMANIAS TECH-BLOG Please visit to view more posts Interference Coordination in LTE/LTE-A (2): eICIC August 6, 2014 | By Dr. Michelle M. Do and Dr. Harrison J. Son ( Enhanced Inter-Cell Interference Coordination (eICIC) As noted in the previous post about ICIC, we will find out about enhanced Inter-Cell Interference Coordination (eICIC), an interference control technology in LTE-A in this post. In LTE/LTE-A, one key challenge for operators is that they have to increase network capacity to keep up with fast-growing traffic. Especially, crowded areas in metropolitan cities have hotspots with extremely high traffic. For these hotspots, just reducing the size of macro cells is not quite enough to handle the high traffic. So, network operators want to increase the network capacity in a more economical way - by installing small cells. Networks consisting of the same type of cells (e.g. existing macro networks), as presented in the previous post, are called homogeneous networks while ones with different types of cells are called heterogeneous networks (HetNet). So, HetNet is a network where small cells are deployed within a macro cell coverage. From Release 10 on, HetNet environments are also considered when discussing LTE-A standards. Figure 1. Homogeneous network and heterogeneous network (HetNet) What is eICIC? eICIC is an interference control technology defined in 3GPP release 10. It is an advanced version of ICIC, previously defined in 3GPP release 8, evolved to support HetNet environments. To prevent inter-cell interference, ICIC allows cell-edge UEs in neighbor cells to use different frequency ranges (RBs or sub-carriers). On the other hand, eICIC allows them to use different time ranges (subframes) for the same purpose. That is, with eICIC, a macro cell and small cells that share a co-channel can use radio resources in different time ranges (i.e. subframes). Two main features of eICIC are: Almost Blank Subframe (ABS) technology defined in Release 10 and Cell Range Expansion (CRE) technology defined in Release 11. ABS can prevent cell-edge UEs in small cells from being interfered with by the neighboring macro cell by having both cells still use the same radio resources, but in different time ranges (subframes). CRE expands the coverage of a small cell so that more UEs near cell edge can access the small cell. In this post, we will discuss ABS only. Cell A Cell B Homogeneous Network Cell A Cell C Heterogeneous Network (HetNet) Macro Cell Cell B Macro Cell Macro Cell Small Cell Small Cell
  2. 2. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (2): eICIC 2 Figure 2. eICIC technology: ABS Problems with ICIC First, you may wonder what issues ICIC had that made HetNet choose eICIC over ICIC. ICIC enables cell-edge UEs to use different frequency resources (RBs) in communicating, by having neighboring base stations exchange interference information with each other over X2 interface. This is effective in reducing inter-cell interference in an existing macro cell-based homogeneous network, but causes interference between control channels in a HetNet. When a base station communicates with a UE, each DL subframe of 1 msec consists of two periods - one for delivering control channel and the other for delivering data channel. ICIC can allocates different frequency resources to cell-edge UEs only when delivering data channels (Physical Downlink Shared Channel; PDSCH). Resource information allocated to UEs is delivered through control channels (Physical Downlink Control Channel; PDCCH). Here the thing is, unlike data channels, control channels are not delivered through different frequency ranges, but distributed across the entire channel bandwidth first and then delivered. This may cause UEs in neighbor cells to share the same frequency resources. Figure 3. Control channel (PDCCH) and data channel (PDSCH) In a homogeneous network, this is not a big problem because there isn't much difference in Tx power from neighbor cells' antenna, and hence no significant inter-channel interference by control channels is caused between neighbor cells at cell edge. On the other hand, in HetNet where a macro cell has much higher Tx power than a small cell 1 , the small cell's control channel is inevitably interfered with by the macro cell's, making ICIC applied to the data channel ineffective. t t f f … … Almost Blank Subframe (ABS) Radio frame (10 subframes) Small Cell Macro Cell subframe Macro Cell Small Cell Edge t f Channelbandwidth DL Subframe (1 ms) time slot time slot 0 1 2 6 0 1 23 4 5 3 4 5 6 Symbols RBs PDCCH period PDSCH period … PDCCH: Physical Downlink Control Channel PDSCH: Physical Downlink Shared Channel RB: Radio Block Control Data
  3. 3. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (2): eICIC 3 Figure 4. Issues with ICIC in HetNet: Interference by macro cell's control channel eICIC Concept: Problems with ICIC solved by having cells use radio resources in different time As seen above, in a HetNet, even ICIC cannot completely prevent control information of UEs in a small cell from being significantly interfered with by its neighbor macro cell. To avoid this inter-cell interference, ICIC effect through inter-cell cooperation should be obtained through taking advantage of time domain, instead of frequency domain. That's why eICIC was introduced. The basic idea of eICIC is that it allows a macro cell and its neighbor small cells to deliver data, subframe by subframe, by using different time ranges. Figure 5. eICIC Concept Macro Cell Small Cell f f DL Subframe (1 ms) : interference t t Control CH Data CH no interference Î Î Î Î Î Î Î Data CH: No Interference Control CH: Interference A B C Macro Cell Small Cell f1-9 f1-9 f1-9 High Power Low Power A B C Macro Cell Small Cell f7 - f9 f3 - f6 f1, f2 f7 - f9: UE A in macro cell f3 - f6: UE B in small cell Edge Edge f1, f2: UE C in small cell symbols Subframe (#1) ABS Macro cell Small cell ABS Subframe (#2) Data CH (no interference) Data CHControl CH symbols symbols f f Control CH ( : interference) ICIC eICIC Macro cell UE Î Î Î Î Î Î Î Î Î Î Î Î Î Small cell UE Macro cell UE Small cell UE Subframe (#1) Subframe (#2) t t symbols t t
  4. 4. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (2): eICIC 4 So, when communicating with cell-edge UEs, small cells use the subframes that are not used by their neighbor macro cell, avoiding interference by the macro cell. Of course, when communicating with UEs at cell centers, the small cells can use any subframe available whether the macro cell is delivering data or not at the time. A subframe carrying no data is called Almost Blank Subframe (ABS) because obviously there is barely any information being carried in the subframe. ABS subframes carry the minimum control information (reference signal, radio frame sync info., paging and cell access info.), and hence interference to be caused by control signals can be mitigated. eICIC Operation: Delivering ABS pattern information over X2 interface Figure 6. Basic idea of ABS: Macro cell UEs and small cell UEs at cell-edge use different subframes for communication A base station and UE exchange data in radio frames, and one radio frame consists of 10 subframes. The decision on how many ABS subframes are to be included in a radio frame is made based on traffic load and network operators' policy. In the figure below, a macro base station decides which subframes will carry data ("0") and which ones will not ("1"), and then saves it as ABS pattern information (e.g. “0011000110”). Then, it prepares a Load Information message, 2 and send it to a small cell base station over X2 interface. Upon receiving this message, the small cell learn from the pattern which subframes are to be used by the macro cell, and deliver data to cell-edge UEs through ABS subframes only. Below is the result of LTE-A eICIC field test conducted by Softbank in March 2014. The figure shows a case where ABS ratio of macro cell to small cell is 5:5. The graph in the upper right displays radio resource allocation (macro cell in blue and small cell in purple), and the one in the bottom shows cell throughputs of macro cell (in blue), small cell with eICIC (in yellow), and small cell without eICIC (in purple). From this, we can tell the small cell throughput has been improved when eICIC was employed. Increased number of small cells or higher ratio of ABS in a macro cell (e.g. 7:3) will result in even higher throughput in small cells. #0 #1 #2 #3 #4 #5 #6 #7 #8 #9 ABS: Almost Blank Subframe Small Cell Macro Cell ABS pattern: 0011000110 X2 Load Information Cell Edge 0 0 1 1 0 0 0 1 1 0 Macro cell ABS pattern (Macro cell) Tx Power (Macro cell) Small cell (Edge) Control signal ABS Macro cell data Small cell data (Edge) Small cell data (Center) ABS Small cell (Center) t f t t f f Subframe Radio frame (#N)
  5. 5. Netmanias Tech-Blog: Interference Coordination in LTE/LTE-A (2): eICIC 5 Figure 7. Result of an eICIC field test: Resource allocation and throughput in case ABS ratio of macro cell to small cell is 5:5 (source: Softbank) Footnotes 1. In general, Tx power is a few Ws to tens of Ws for a macro cell, and a few hundreds of mWs for a small cell. 2. In Release 8, Load Information message provides interference information for ICIC only. However, in Release 10, the message provides ABS pattern information as well, supporting eICIC. ABS Ratio of Macro Cell and Small Cell – 5 : 5 Macro Cell Small Cell Macro Cell Small Cell (eICIC) Small Cell (no eICIC) Resource allocation Throughput
  6. 6. About NMC Consulting Group ( 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 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