ECE 5221 Personal Communication Systems

1. ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 21: Congestion control Spring 2011

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

3. 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 • New resource provisioning • Traffic balancing Some methods for congestion control

4. Provides additional capacity at the site Practical concerns Ability of the base station hardware Frequency planning constraints Example. Consider iDENsite 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% Addition of traffic resources Offered traffic Properties of iDEN Erlang B formula The site needs 10 trunks. Addition of a radio provides 11 trunks for traffic data

5. Sectorization (trunking efficiency) Trunking efficiency - example • Omni vs. Sector Antenna Configurations • Sectorization provides less capacity for a given number of channels • Decrease in trunking efficiency • Sectorization provides an increase in system capacity through channel reuse efficiency N=21 GOS=2% 14 E 21 Ch. N=7 GOS=2% 2.94(3)=8.82 E 7 Ch. 7 Ch. 7 Ch. Decrease in capacity of 37% (1 - 8.82E / 14E)

6. Cell Splitting Provides additional traffic resources to the same geographical area Maintains a sound reuse plan Add a cell between 1b and 2g to relieve traffic between the two sectors Example: Cell Splitting Sectors 1b and 2g in the diagram have excessive blocking Sector 1b has 100 users Sector 2g has 80 users Assume uniform traffic distribution over faces 1b and 2g Relieve the traffic by performing a cell split Cell splitting Geometrical solution side split

7. Microcell deployment • Special form of cell splitting • Microcells • low power • antenna below rooftops • small cell radii • Implemented in places of traffic “hot-spots” • Allow for frequency super-reuse Deployment of microcells

8. 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 Handoff thresholds Neighbor lists Cell preference Various timing parameters Optimization of system parameters Original handoff threshold New handoff threshold High capacity area (another site will pick up this area)

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

10. Underlay/overlay - example Before - number of channels per site 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 After - number of channels per site Capacity - (at 2% GOS) before after

11. 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 umbrella cells macro-cells micro-cells Hierarchical cell structure

12. Traffic planning - essential part of network planning process Planning tools us GIS data to determine geographical distribution of traffic type of traffic demand offered traffic per site the number of communication resources required to meet QoS objectives Typical predictions traffic served traffic offloaded GOS throughput delay In cellular systems traffic and RF performance are interconnected. For example in CDMA/WCDMA traffic load determines cell breathing in GSM/iDENtraffic load determines system interference and limits throughput Use of network planning tools