Interference
Co-channel Interference (CCI)
Co-channel interference is the factor that causes reduction in the
capacity and performance of the cellular system. The distance between
two cells having same frequency is limited by the co-channel
interference and so the capacity of the cellular system is driven by the
co-channel interference. Co-channel interference or CCI is crosstalk
from two different radio transmitters using the same frequency.
When considering the size of cell almost constant and all the base
station is transmitting same power, in such a case the co-channel
interference is independent of the transmitted power and is function of
'D' and 'R'. The relationship between co channel interference distance
(D) and radius (R) is given by
Adjacent channel Interference
Adjacent-channel interference or ACI is interference caused by
extraneous power from a signal in an adjacent channel. ACI may be caused
by inadequate filtering, such as incomplete filtering of unwanted
modulation products in frequency modulation (FM) systems, improper
tuning, or poor frequency control, in either the reference channel or
the interfering channel, or both.
Traffic Engineering
The task of teletraffic theory is to design systems as cost
effectively as possible with a predefined grade of service when we know
the future traffic demand and the capacity of system elements.
In teletraffic engineering the word traffic is used to denote the
traffic intensity i.e traffic per unit time. Traffic intensity is the
average number of calls simultaneously in progress during a particular
period of time. It is measured in Erlang or CCS.
Traffic engineering is a method adapted to optimize the performance
of a telecommunications network by dynamically analyzing, predicting and
regulating the behavior of data transmitted over that network. Traffic
engineering is also known as tele traffic engineering and traffic
management.
Traffic Intensity
The instantaneous traffic intensity in a pool of resources is the
number of busy resources at a given instant of time. Most commonly used
unit of traffic is Erlang. A traffic intensity of one Erlang means
continuous occupancy of a facility during the time period under
consideration.
Carried traffic
It is the volume of traffic carried by the switch. It refers to the maximum capacity of the network.
Offered traffic
Offered traffic refers to the average generated total traffic
including the traffic that is blocked in the system. So the capacity
should be higher than offered traffic; otherwise, many users would not
be able to get service because all lines would be occupied all the time
on average.
Relation between offered traffic and carried traffic
Carried traffic (Ct) = offered traffic (1- Blocking probability)
Lost or Rejected traffic or overflow
The difference between offered traffic and carried traffic is equal
to the rejected traffic. The value of this parameter can be reduced by
increasing the capacity of the system.
Overview of Taffic Flow
Inter arrival time
Interarrival time is a value used in queuing theory. Queuing theory
uses models to analyze systems that involve waiting in lines for a
service. The interarrival time is the amount of time between the arrival
of one customer and the arrival of the next customer. It is calculated
for each customer after the first and is often averaged to get the mean
inter arrival time, represented by ?.
Holding Time
It is average duration of a typical call and denoted by 'H'.
The traffic intensity offered by each user is equal to the call
request rate multiplied by the holding time. That is, each user
generates a traffic intensity of Au Erlang given by
Where ? is average number of call request per unit time for each user and H is holding time.
For a system containing u users and an unspecified number of channels the total offered traffic intensity A, is given by
Moreover, in a C channel trunk system, if the traffic is equally
distributed among the channel, then the traffic intensity per channel,
Ac, is given by
When the offered traffic exceeds the maximum capacity of the system,
the carried traffic becomes limited due to the limited capacity. The
maximum possible carried traffic is the total number of channels, C, in
Erlang.
Grade of Service (GOS)
It is the measured of proportion of calls that is lost due to
congestion (Erlang B). In AMPS cellular system GOS of 2 % is specified
which means 2 out of 100 called were rejected. Mathematically we defined
GOS as
So the GOS is specified for the traffic at the busy hour. At other
times it is much better. If it is too large, subscribers are unable to
complete the call and are unsatisfied with the operator and also this
means congestion in the system may occurred at the busy hour. So if the
offered traffic increases the number of trunks must obviously be
increased to prove a given grade of service.
There are two type of trunking system available. First one is if the
channel is available, the requested user will get that channel
immediately upon request. If the channel is not available, the requested
user is blocked without access and is free to try again later. This is
called Loss Call Cleared (LCD). Erlang B formula is used to measure the
GOS or for measurement of blocking probability which is given by the
following relation
Where C is the number of trunked channels offered by a trunked radio system and A is the total offered traffic.
Different Channel Assignment Strategies
Fixed Channel Assignment Strategy
In this strategy the number of channels is permanently assigned to a
base station. Any call attempt within the cell can only be served by the
unused channel in that particular cell. If channel is occupied, call is
blocked and subscriber does not get the service. The same set of
channels is used in other cells at some distance away. The distance
between any two cells using the same set of channels must be equal to or
greater than the co-channel reuse distance, which is the minimum
distance at which radio frequency can be reused with acceptable
signal/interference ratio.
Borrowing Channel Assignment Strategy
In this strategy, if the assign channel is fully occupied, a channel
is borrowed from a neighboring cell. While borrowing it should ensure
that the borrowing channel does not disrupt any of the calls in progress
in the donor cell. In cellular system borrowing of the channel from one
cell to another cell is monitor by the MSC.
Hybrid Channel Assignment Strategy
In hybrid channel allocation, all the available channels are divided
into two groups, fixed and dynamic. Channels in the fixed group are
assigned to cells as nominal channels, as in fixed channel allocation
methods. Channels in the dynamic group are shared among all the cells.
Nominal channels are preferred for use in their respective cells. When a
call request comes in a cell, it is assigned a nominal channel if it is
available. When no nominal channel is available, a channel from the
dynamic group is assigned.
Dynamic Channel Assignment Strategy
Dynamic channel allocation methods do not assign permanent channels
to cells. The Mobile Switching center, instead of the base station, is
responsible for channel allocation. When there is a call request, the
base station notifies the mobile switching center, which searches for
the most suitable channel. While assigning the channel by the MSC,
these factors are considered such as reuse distance, future call
blocking probability, and channel use frequency.
The main advantages of dynamic channel allocation are flexibility and
traffic adaptability. It is because channels are not assigned into the
cells as nominal channel, and each channel assignment decision is made
dynamically based on the network condition.
The underlying strategy in fix channel assignment is the permanent
assignment of a set of channels to both new calls and handoff calls. If
the call is requested from the cells then the call will be served from
the available channel that has been allocated as the nominal channel. If
resources are not available then that call will be blocked. Same thing
happens for handoff calls also. GOS for newly generated call and the
handoff call are independent. In fixed channel assignment algorithm
there is no provision for sharing resources although the resources are
free.
