Cellular Concept-System Design Fundamentals

Lecture 5

Adjacent Channel Interference

Imperfect Tx filters allow energy from adjacent channels to leak into the passband of other channels

⇒ desired filter response

Controlling ACI Use better filters maximize channel separation

separation of as many as N channel bandwidths

not assigning adjacent channels in a cell

Example In US AMPS system there are 832

channels. These 832 are divided equally between

two operators (416 + 416 channels) Out of 416 channels 395 are voice

channels and 21 are control channels For N=7 each cell in a cluster will have

one control channel, neighbouring clusters will have remaining 14 control channels

Control channels are reused according to N=21 pattern

AMPS Duopoly Channels

Trunking and Grade of Service Cellular radio systems rely on trunking to accommodate a

large number of users in a limited radio spectrum.

Trunking system A mechanism to allow many users to share fewer

number of channels. Not every user calls at the same time Penalty: Blocking Effect.

If traffic is too heavy, call is blocked!! Small blocking probability is desired. There is a trade-off between the number of available

circuits and blocking probability.

Trunking Trunking exploits the statistical behavior of users so that a

fixed number of channels may accommodate a large random user community

There is a trade-off between the number of available telephone circuits and the likelihood of a particular user finding that no circuits are available during the peak calling times

As the number of phone lines decreases, it becomes more likely that all circuits will be busy for a particular user

In a trunked mobile radio system, when a particular user requests service and all of the radio channels are already in use, the users is blocked or denied access to the system. In some systems a queue may be used to hold requesting users until a channel becomes available

Erlang The fundamentals of truncking theory were developed

by Erlang, a Danish mathematician who, in the late 19th century embarked on the study of how a large population could be accommodated by a limited number of servers.

Erlang: a “dimensionless unit,” The basic unit of telecom traffic intensity carried a channel

that is completely occupied (one call-hour/hour or one call-min/min)

Since a single circuit used continuously carries 60 minutes of calling in one hour, one Erlang is usually defined as 60 minutes of traffic

A radio channel that is occupied for 30 minutes during an hour carries 0.5 Erlangs of traffic

Grade of Service GOS is a benchmark used to define the desired

performance of a particular trunked system

Grade of Service (GOS): probability that a call is blocked (or delayed).

The probability that all servers will be busy when a call attempt is made. For example, on a trunk group: P.02 means that there is a 2% probability of getting a busy signal (being “blocked”) when you have a given amount of traffic and a given number of trunks.

Grade of Service

It is a wireless designer’s job to estimate the maximum required capacity and to allocate the proper number of channels in order to meet GOS

AMPS is designed for GoS of 2% blocking. This means that channel allocations for cell sites are designed so that 2 out of 100 calls will be blocked due to channel occupancy during the busiest hour

Common Terms of Trunking Theory Set-up Time: The time required to allocate a

trunked radio channel to a requesting user

Blocked Call: Call which cannot be completed at time of request, due to congestion. (lost call)

Holding Time: Average duration of a typical call. Denoted by H (in seconds)

Traffic Intensity: Measure of channel time utilization, which is the channel occupancy measured in Erlangs. Denoted by A

The traffic intensity offered by each user is equal to the call request rate multiplied by the holding time. Each user generates a traffic intensity of

Au Erlangs given by

H is the average duration of a call λ =avg No. of call requests per unit time

for each user

For a system containing “U” users and an unspecified No. of channels,

Total offered traffic Intensity=A=UAu

In a “C” channel trunked system, if the traffic is equally distributed among the channels, traffic intensity per channel, Ac = UAu / C

Important! Offered traffic is not necessarily the traffic

which is carried by the trunked system It is only that which is offered to the trunk

system When the offered traffic exceeds the

maximum capacity of the system, the carried traffic becomes limited due to the limited availability of the No. of channels

Block Calls cleared/Erlang B System In Erlang B System, a call request is simply

denied if all channels in the pool are in use

Assumptions Any user can require a channel any time Probability of user occupying a channel is exponential Finite trunk channels available No queuing, for call requests if no channels are

available, requesting user is blocked it is assumed that there is no setup time and user is

given immediate access to a channel if one is available

Probability of Blocking The GOS measure for Block Calls

Cleared System is the probability that a user’s call request is blocked

The Erlang B formula determines the blocking probability, p, given a certain total offered traffic intensity, A, and a certain number of channels C in the pool

A is the total offered traffic Since some calls are blocked, A is not

the traffic carried by the system

Erlang-B Formula

Capacity of Erlang B System

Erlang B Chart

Blocked Calls delayed/Erlang C A queue is provided to hold calls which are blocked If a channel is not available immediately, the 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 a specific length of time in the queue

To find the GOS, it is first necessary to find the likelihood that a call is initially denied access to the system

This likelihood of a call not having immediate access to a channel is determined by Erlang C formula

Erlang-C Formula

Erlang C Chart

Example

Example

Example

Example

Example

ConclusionTrunking efficiency is a measure of No of users which can be offered a particular GOS with a

particular configuration of fixed channels Way in which channels are grouped substantially alters

the number of users handled by a trunked system 10 trunked channels at a GOS of 0.01 can support 4.46

Erlangs of traffic But two groups of 5 trunked channels can support 2 ×

1.36 Erlangs, or 2.72 Erlangs of traffic 10 channels trunked together support 60% more traffic

at a specific GOS than do two five channel trunks! So allocation of channels in a trunked radio system has

a major impact on overall system capacityDo look at remaining examples!