Florida Institute of technologies ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 21: Congestion control Spring 2011

Florida Institute of technologies Page 2  Traffic congestion  Methods for control of traffic ongestion  Examples Outline Important note: Slides present summary of the results. Detailed derivations are given in notes.

Florida Institute of technologies Congestion control  Initial deployment – coverage driven  As network traffic grows – capacity becomes main issue  Non uniform distribution of traffic – congestion problems occurs at different places at different times  Level of cell congestion monitored and reported by the “switch”  Two approaches for congestion control oNew resource provisioning oTraffic balancing Page 3 Some methods for congestion control

Florida Institute of technologies Page 4 Addition of traffic resources  Provides additional capacity at the site  Practical concerns oAbility of the base station hardware oFrequency planning constraints Example. Consider iDEN site serving 4.65 E of traffic using 3 radios (8 trunks for voice and 1 for control). Current GOS is 7%. Using Erlang B formula determine the number of radios that will bring GOS below 2% Offered traffic Erlang B formula The site needs 10 trunks. Addition of a radio provides 11 trunks for traffic data PropertyValue Channel bandwidth25KHz Access technologyFDMA/TDMA with TDD # of TS/channel3 Properties of iDEN

Florida Institute of technologies Page 5 Sectorization (trunking efficiency)  Omni vs. Sector Antenna Configurations oSectorization provides less capacity for a given number of channels oDecrease in trunking efficiency oSectorization provides an increase in system capacity through channel reuse efficiency Decrease in capacity of 37% ( E / 14E) N=21 GOS=2% 14 E N=7 GOS=2% 2.94(3)=8.82 E 7 Ch. 21 Ch.  Trunking efficiency - example

Florida Institute of technologies Page 6 Cell splitting  Cell Splitting oProvides additional traffic resources to the same geographical area oMaintains a sound reuse plan oAdd a cell between 1  and 2  to relieve traffic between the two sectors Example: Cell Splitting oSectors 1  and 2  in the diagram have excessive blocking  Sector 1  has 100 users  S ector 2  has 80 users oAssume uniform traffic distribution over faces 1  and 2  oRelieve the traffic by performing a cell split side split Geometrical solution

Florida Institute of technologies Page 7 Microcell deployment  Special form of cell splitting  Microcells olow power oantenna below rooftops osmall cell radii  Implemented in places of traffic “hot-spots”  Allow for frequency super- reuse Deployment of microcells

Florida Institute of technologies Page 8 Optimization of system parameters  RF Changes  Decrease cell footprint  Downtilt / Uptilt  Azimuth Change  Antenna Change  Power modifications  By pulling in the coverage from a high capacity cell, other cells can pick up the traffic  Implementation of smart antennas  System parameter changes oHandoff thresholds oNeighbor lists oCell preference oVarious timing parameters Original handoff threshold New handoff threshold High capacity area (another site will pick up this area)

Florida Institute of technologies Page 9 Underlay and overlay cell configurations  Channels are separated in two groups  Channels for overlay are re-used as higher rate  Need for inter-cell handoff  Offers significant capacity improvements on account of reuse  Can be implemented in FDMA/TDMA based systems  Coverage between overlay and underlay needs to be carefully balanced Underlay/overlay implementation

Florida Institute of technologies Page 10 Underlay/overlay - example Assume that the total number of channels in an imaginary cellular network equals 126. Determine the capacity increase resulting from introduction of underlay/overlay if  Overlay is planned using 4/12 reuse  The number of channels for underlay is 42 and the number of channels for overlay is 84 Before - number of channels per site After - number of channels per site Capacity - (at 2% GOS) before after

Florida Institute of technologies Page 11 Hierarchical cell structure  Divides cells into hierarchical layers  Layers have different preference  Similar to overlay/underlay  Different hierarchical layers do no have to co-locate  Supported in IS-136, GSM and W-CDMA  Typical layers oumbrella cells omacro-cells omicro-cells

Florida Institute of technologies Page 12 Use of network planning tools  Traffic planning - essential part of network planning process  Planning tools us GIS data to determine ogeographical distribution of traffic otype of traffic demand ooffered traffic per site othe number of communication resources required to meet QoS objectives  Typical predictions otraffic served otraffic offloaded oGOS othroughput odelay  In cellular systems traffic and RF performance are interconnected. For example oin CDMA/WCDMA traffic load determines cell breathing oin GSM/iDEN traffic load determines system interference and limits throughput