ECE 4730: Lecture #6 1 Trunking & Grade of Service Trunked radio system radio system where a large # of users share a limited pool of channels Channel

ECE 4730: Lecture #6 1

Trunking & Grade of Service

Trunked radio system radio system where a large # of users share a limited pool of channels Channel allocated on demand & returned to channel pool upon call

termination Exploit statistical (random) behavior of users so that fixed # of

channels can accommodate large # of users Trunking theory used by telephone companies to allocate limited # of

voice circuits for large # of telephone lines Efficient use of equipment resources $$ savings Disadvantage is that some probability exists that mobile user will be

denied access to a channel» Blocked call : access denied “blocked call cleared”

» Delayed call : access delayed & call put into holding queue for specified amount of time (5-15 seconds)



Grade of Service (GOS) Measure of user access to a trunked radio system during

busiest hour of the week Specified as probability (Pr) that call is blocked or delayed Busiest hour typically 4-6 pm on Thu or Fri (cellular) Erlang (erl) : unitless measure of traffic intensity

» Example: 0.5 erl = 1 channel occupied 30 minutes during 1 hour

Table 3.3, pg. 76 Trunking theory definitions





Traffic Intensity (A)

Also called “Offered” Traffic Intensity Offered? not necessarily carried by system

(blocked/delayed)

Each user Au = H (erl)

System with U users A = U Au = U H (erl)

Intensity/channel AC = U Au / C (erl/channel)

Maximum carried traffic load = total # available channels = C (erl)



Erlang B formula Blocked Call Cleared (denied) BCC

GOS = Pr [blocked call] =

A = total offered traffic C = # channels in trunking pool (e.g. a cell or sector) Program in calculator or use computer to calculate

C

k

C

k

kA

CA

0 !

!



Erlang C formulas Blocked Call Delayed BCD put into holding queue Probability that call is delayed greater than t seconds

Probability that call is initially denied access and put into holding queue

Program in calculator or use computer to calculate

)/)(exp(]0[Pr][Pr HtACdelaytdelay

1

0 !1!

]0[Pr C

k

kC

C

kA

CA

CA

Adelay



Graphical form of Erlang Curves

Fig. 3.6 & 3.7 - pgs. 81, 82 Family of curves Pr vs. A for many values of C Provide approximate solutions

» Errors due to “eyeball” interpolation» Numerical formulas provide exact values


Erlang B Graph Number of Trunked Channels ( C )



Example: How many users can be supported in a cell containing 50 channels for a 5% GOS (BCC) if the average user calls twice/hr with an average call duration of 5 minutes?

Fig. 3.6 For GOS = 5% and C = 50 then A = 45 erl Twice/hr = 2 Call duration H = 5 minutes

Au = H = 2 (5 min / 60 min) = 1/6 erl (unitless!)

A = U Au U = A / Au = 45 / (1/6) = 270 users

See additional examples 3.4-3.7 in book


Trunking Efficiency

Trunking Efficiency measure of # users supported by specific configuration of fixed channels

Table 3.4, pg. 79 Lets assume 1% GOS 1 group of 20 channels 12 erl or (12/20) · 100 = 60%

efficiency 2 groups of 10 channels 2 · 4.46 = 8.92 erl or (8.92/20)

· 100 = 44.6% efficiency ***Larger channel group supports [(12/8.92) 1] · 100 =

35% more traffic!!*** Allocation of channel groups can substantially

change # users supported by trunked system As trunking pool size then trunking efficiency


Trunking Efficiency


Improving Cellular System Capacity

Cell Sectoring Method that capacity by reducing CCI Replace omni-directional antennas at base station with

several directional antennas» 3 sectors 3 @ 120° antennas» 6 sectors 6 @ 60° antennas

Cell channels broken down into sectored groups CCI reduced b/c only some of neighboring co-channel

cells radiate energy in direction of main cell Figs. 3.10 & 3.11, pgs. 90 & 91 Cell splitting (discussed next) keeps D / R unchanged

(same CCI) but increases frequency reuse/area


Cell Sectoring

3 sectors 3 @ 120° antennas 6 sectors 6 @ 60° antennas


Cell Sectoring

N = 7 cell cluster

6 CCI cells in first tier

120° Sectoring

io = 2 interfering cells


Cell Sectoring

How is capacity increased? By reducing CCI the cell system designer can choose smaller cluster

size (N )

Smaller N greater frequency reuse larger system capacity Much less costly than cell splitting

Only requires more antennas @ base station vs. multiple new base stations for cell splitting

Primary disadvantage is available channels in a cell subdivided into sectored groups Trunked channel pool trunking efficiency

*** Overall system capacity increased at the expense of reducing capacity of individual cells (w/sectors) *** Why? Cluster size N is reduced frequency reuse increases!!

W

W

i

N

iR

D

I

S

o

n

o

n31


Cell Sectoring

Other Advantages : More antenna gain sector antenna focuses signal

energy » Forward/reverse link budgets improved» More Tx power delivered to coverage area» Better building penetration

Flexibility in controlling CCI and dropped calls from poor handoff execution» Downtilt antennas in certain sectors to reduce CCI in specific cells» Uptilt antennas in certain sectors to increase coverage at cell

boundary to improve chance of successful handoff


Cell Sectoring

Other Disadvantages :

Must design network coverage with sectoring decided in advance

Can’t effectively (easily) use sectoring to increase capacity after setting cluster size N

Can’t be used to gradually expand capacity as traffic like cell splitting


Improving Cellular System Capacity

Cell Splitting

Subdivide congested cell into several smaller cells Must decrease antenna height & Tx power so smaller

coverage results and CCI level is held constant Each smaller cell keeps same # of channels as the

larger cell!! Capacity b/c channel re-use per unit area Smaller cells “micro-cells” Fig. 3.8, pg. 87


Cell Splitting

Base stationsplaced

at cell cornerfor illustration

purposes


Cell Splitting

Other Graphical Examples


Cell Splitting

Advantages : Only needed for cells that reach max. capacity not all

cells Implement when Pr [blocked call] > acceptable GOS System capacity can gradually expand as demand

Disadvantages : # handoffs/unit area Umbrella cell for high velocity traffic may be needed **More base stations $$ for real estate, towers, etc.**

Documents

ECE 4730: Lecture #6 1 Trunking & Grade of Service Trunked radio system radio system where a large # of users share a limited pool of channels Channel