Wireless Networks Lecture 14 Fundamentals of Cellular Networks

Wireless Networks Ü Lecture 14 Ü Fundamentals of Cellular Networks (Part IV) Ü Dr. Ghalib A. Shah 1

Outlines Ü Trunking and Grade of Service ► Measuring Traffic Intensity ► Trunked Systems • • Blocked Calls Cleared Blocked Calls Delayed ► Erlang Charts Ü Improving Coverage and Capacity ► ► Cell Splitting Sectoring Repeaters for Range Extension Microcell Zone Concept 2

Last lecture review Ü Interference and system capacity ► Co-channel interference and capacity ► Adjacent channel interference and capacity Ü Channel Planning for Wireless System 3

Ü Trunking ► Allows a large number of users to share a small number of channels ► Channel allocated per call basis from a pool of available channels ► Relies on statistical behavior of users so that a fixed number of channels (circuits) may accommodate a large random user community ► Trunking theory is used to determine number of channels for particular area (users) ► Tradeoff between the number of available channels and likelihood of call blocking during peak calling hours 4

Ü Trunking Theory ► Developed by Erlang, Danish Mathematician, how a large population can be accommodated by a limited number of servers, in late 19 th century ► Today, used to measure traffic intensity ► 1 Erlang represents the amount of traffic intensity carried by a completely occupied channel • • i. e. one call-hour per hour or one call-minute per minute 0. 5 Erlang: Radio channel occupied 30 minutes during 1 hour 5

Grade of Service Ü GOS is a benchmark used to define performance of a particular trunked system ► Measure of the ability of a user to access trunked system during the busiest hour. • • Busy hour is based on the demands in an hour during a week, month or year. Typically occur during rush hours between 4 pm to 6 pm. Ü GOS is typically given as likelihood of call blocking or delay experienced greater than certain queue time 6

Traffic intensity Ü Traffic intensity is measured as call request rate multiplied by call holding time User traffic intensity of Au Erlang is (1) Au= λH Where H is average call duration or holding time and λ is average number of call requests. For system of U users and unspecified channels, the total offered traffic intensity A is (2) A = UAu In a C channel trunked system, traffic equally distributed, traffic intensity per channel Ac is (3) Ac= UAu/C 7

Ü Note that traffic is not necessarily the carried traffic but offered to the trunked system Ü If offered load increases the system capacity, the carried traffic becomes limited Ü In Erlang, max possible carried traffic is the number of channels C Ü AMPS is designed for a GOS of 2% blocking ► i. e. 2 out of 100 calls will be blocked due to channel occupancy Ü There are two types of commonly used trunked systems ► Blocked Calls Cleared ► Blocked Calls Delayed 8

Block Calls Cleared Ü User is given immediate request if a channel is available. Ü If no channel available, the requesting user is blocked and free to try later Ü Assume call arrivals as Poisson Distribution Ü the Erlang B formula determines the probability that call is blocked with no queuing, is a measure of GOS for trunked system 9

Erlang B Trunking GOS Capacity of an Erlang B System Number of Chan nels C Capacity (Erlangs) For GOS = 0. 01 = 0. 005 = 0. 002 = 0. 001 2 0. 153 0. 105 0. 065 0. 046 4 0. 869 0. 701 0. 535 0. 439 5 1. 36 1. 13 0. 900 0. 762 10 4. 46 3. 96 3. 43 3. 09 20 12. 0 11. 1 10. 1 9. 41 24 15. 3 14. 2 13. 0 12. 2 40 29. 0 27. 3 25. 7 24. 5 70 56. 1 53. 7 51. 0 49. 2 100 84. 1 80. 9 77. 4 75. 2 10

Erlang B 11

Block Calls Delayed Ü Queue is provided to hold blocked calls. Ü Call request may be delayed until a channel becomes available Ü Its measure of GOS is defined as the probability that a call is blocked after waiting specific length of time in the queue Ü The likelihood of a call not having immediate access is determined by Erlang C formula 12

Erlang C 13

Ü if no channels are available immediately, the call is delayed, probability that call is forced to wait more than t seconds is Ü Average delay D in all calls in queued system is 14

Trunking Efficiency Ü A measure of the number of users which can be offered a particular GOS with particular configuration of channels Ü The way channels are grouped can alter the number of users handled Ü For example, From table ► 10 trunked channels at GOS of 0. 01 can support 4. 46 Erlang of traffic ► Whereas 2 groups of 5 channels can support 2 x 1. 36=2. 72 Erlangs of traffic, 60% lesser 15

Improving Coverage and Capacity Ü As demand increases, number of channels per cell become insufficient Ü Cellular design techniques needed to provide more channels per unit coverage area Ü Various techniques developed to expand the capacity of system ► Cell splitting ► Sectoring ► Micro cell zone concept 16

Cell Splitting Ü Achieve capacity improvement by decreasing R and keeping D/R (cell reuse ratio) unchanged Ü Divide the congested cells into smaller cells ► Smaller cells are called micro cells Ü If radius of cell is cut to half, approximately four cells would be required ► Increased number of cells would increase the number of clusters, which in turn increase the capacity Ü Allows a system to grow by replacing larger cells with smaller cells without upsetting the allocation scheme 17

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Ü For new cells to be smaller in size, tx power must be reduced. By which factor? Ü If n = 4 then the received powers equal to each other becomes Ü Power must be reduced by 12 d. B in order to maintain S/I requirements 19

Ü Thus low speed and high speed users can simultaneously handled Ü Channels in old cell must be broken down into two groups corresponding to smaller and larger cells Ü At beginning of cell splitting, fewer channels to smaller power groups. Ü As demand grows, more channels will be required and thus more micro cells Ü In the end, the whole system will be replaced with micro cells 20

Sectoring Ü Keep cell radius unchanged and decrease D/R Ü Increases SIR so that cluster size may be reduced ► SIR is improved using directional antennas ► Hence increasing frequency reuse without changing transmission power Ü Cell is partitioned into 3 120 o sectors or 6 60 o sectors as shown in Fig 21

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Ü Instead of interference from 6 cells, only 2 sectors interfere Ü thus S/I can be found to be 24. 2 d. B, where it is 17 d. B in worst case presented before Ü This S/I improvements allow designers to decrease cluster size N and hence enhances capacity Ü Drawbacks ► Increased number of handoffs 24

Microcell Zone Concept Ü A cell is divided into zones with a single BS sharing the same radio equipment Ü Zones are connected through coaxial cable, fiber optics or microwave links to the BS Ü Superior to sectoring since antennas are placed at outer edges of the cells and any channel may be assigned to any zone by BS Ü As mobile travels from one zone to other, it retains same channel, BS simply switches the channel to a different zone. Ü Co-channel interference is minimized becuase ► Large BS is replaced by several low powered tx ► Improves S/I 25

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