Intermezo

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.