Wireless networks - Lecture 14: Fundamentals of cellular networks (Part 4)

Wireless networks - Lecture 14: Fundamentals of cellular networks (Part 4). The main topics covered in this chapter include: trunking and grade of service; improving coverage and capacity; block calls cleared; trunking efficiency; microcell zone concept;...

Wireless Networks

Lecture 14 Fundamentals of Cellular Networks (Part IV)

Dr Ghalib A Shah

 Trunking and Grade of Service

► Measuring Traffic Intensity

► Trunked Systems

• Blocked Calls Cleared

• Blocked Calls Delayed

► Erlang Charts

 Improving Coverage and Capacity

► Cell Splitting

► Sectoring

► Repeaters for Range Extension

► Microcell Zone Concept

Last lecture review

 Interference and system capacity

► Co-channel interference and capacity

► Adjacent channel interference and capacity

 Channel Planning for Wireless System

► 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

 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

Grade of Service

 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.

 GOS is typically given as likelihood of call

blocking or delay experienced greater than certain queue time

 Note that traffic is not necessarily the carried traffic but

offered to the trunked system

 If offered load increases the system capacity, the

carried traffic becomes limited

 In Erlang, max possible carried traffic is the number of

channels C

 AMPS is designed for a GOS of 2% blocking

► i.e 2 out of 100 calls will be blocked due to channel occupancy

 There are two types of commonly used trunked

systems

► Blocked Calls Cleared

► Blocked Calls Delayed

Block Calls Cleared

 User is given immediate request if a channel is

available

 If no channel available, the requesting user is

blocked and free to try later

 Assume call arrivals as Poisson Distribution

 the Erlang B formula determines the probability

that call is blocked with no queuing, is a

measure of GOS for trunked system

Erlang B Trunking GOS

Capacity of an Erlang B System

Erlang B

Block Calls Delayed

 Queue is provided to hold blocked calls

 Call request may be delayed until a channel

becomes available

 Its measure of GOS is defined as the

probability that a call is blocked after waiting specific length of time in the queue

 The likelihood of a call not having immediate

access is determined by Erlang C formula

Erlang C

 if no channels are available immediately, the

call is delayed, probability that call is forced to wait more than t seconds is

 Average delay D in all calls in queued system

is

Trunking Efficiency

 A measure of the number of users which can

be offered a particular GOS with particular configuration of channels

 The way channels are grouped can alter the

number of users handled

 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

Improving Coverage and Capacity

 As demand increases, number of channels per

cell become insufficient

 Cellular design techniques needed to provide

more channels per unit coverage area

 Various techniques developed to expand the

Cell Splitting

 Achieve capacity improvement by decreasing R and

keeping D/R (cell reuse ratio) unchanged

 Divide the congested cells into smaller cells

► Smaller cells are called micro cells

 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

 Allows a system to grow by replacing larger cells with

smaller cells without upsetting the allocation scheme

 For new cells to be smaller in size, tx power must be reduced By

which factor?

 If n = 4 then the received powers equal to each other becomes

 Thus low speed and high speed users can

simultaneously handled

 Channels in old cell must be broken down into

two groups corresponding to smaller and larger cells

 At beginning of cell splitting, fewer channels to

smaller power groups

 As demand grows, more channels will be

required and thus more micro cells

 In the end, the whole system will be replaced

with micro cells

 Keep cell radius unchanged and decrease D/R

 Increases SIR so that cluster size may be

reduced

► SIR is improved using directional antennas

► Hence increasing frequency reuse without changing

transmission power

 Cell is partitioned into 3 120o sectors or 6 60o

sectors as shown in Fig

 Instead of interference from 6 cells, only 2

sectors interfere

 thus S/I can be found to be 24.2 dB, where it is

17 dB in worst case presented before

 This S/I improvements allow designers to

decrease cluster size N and hence enhances capacity

 Drawbacks

► Increased number of handoffs

Microcell Zone Concept

 A cell is divided into zones with a single BS sharing the

same radio equipment

 Zones are connected through coaxial cable, fiber optics

or microwave links to the BS

 Superior to sectoring since antennas are placed at

outer edges of the cells and any channel may be

assigned to any zone by BS

 As mobile travels from one zone to other, it retains

same channel, BS simply switches the channel to a

different zone.

 Co-channel interference is minimized becuase