Interference ,Trunking and GOS

Interference Interference in voice channels can cause Cross talk Interference in control channels leads to missed and blocked calls Interference is major bottleneck in increasing capacity and is often responsible for dropped calls Interference is of two types: Co-channel interference Adjacent channel interference

Interference Sources of interference : Another mobile in the same cell A call in progress in a neighboring cell Other base stations operating in the same frequency band Any non-cellular system which leaks energy into cellular frequency band

Interference(Cont) The interference created by out of band users is very difficult to control The transmitters from competing cellular carriers are often a significant source of out-of band interference because they often locate their base stations in close proximity to one another in order to provide comparable coverage

Co-Channel Interference Cells which use the same set of frequencies are called as co-channel cells The interference between signals from these cells is called as co-channel interference The co-channel interference cannot be combated by increasing the carrier power of transmitter (as we do in case of thermal noise) To reduce this, co-channel cells must be physically separated by minimum distance to provide isolation

Co-channel Reuse Ratio The parameter Q, called as co-channel re-use ratio is defined as : Q=D/R where D=distance between centers of the nearest co-channel cells R=radius of the cell By increasing the D/R, the spatial separation between co-channel cells relative to the coverage distance of a cell is increased

Co-channel Reuse Ratio (Q) A small value of Q ---- larger capacity Large value of Q----Improvement in transmission quality due to small co-channel interference

Adjacent Channel Interference Interference resulting from signals which are adjacent in frequency is called as adjacent channel interference. It results from imperfect receiver filters which allow nearby frequencies to leak into the pass band. This issue is very serious if an adjacent channel user is transmitting in very close range to a subscriber’s receiver while the receiver attempts to receive a base station on the desired channel This is referred to as near-far effect.

Cont Near-far effect also occurs in a case when a mobile close to a base station transmits on a channel close to one used by a weak mobile. The base station may have difficulty in discriminating the desired mobile user from the “bleedover” caused by the close adjacent channel mobile. This can be minimized through careful filtering and channel assignments. By keeping the frequency separation between each channel in a given cell as large as possible, this interference can be reduced considerably.

Cont.. By sequentially assigning successive channels in the frequency band to different cells , many channel allocation schemes also prevent a secondary source of adjacent channel interference by avoiding the use of adjacent channels in neighboring cell sites

Power Control for reducing Interference In practical cellular radio systems, the power level transmitted by every subscriber unit are under constant control by the serving base station. This is done to ensure that each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel. Power control helps prolong battery life for the subscriber unit. It also reduces the reverse channel S/I in the system.

Trunking Cellular radio systems rely on trunking to accommodate a large number of users in a limited radio spectrum Trunking allows a large number of users to share the relatively small number of channels in a cell by providing access to each user , on demand from a pool of available channels. A channel is allocated to user on per call basis and upon termination , the channel is returned to a pool of available channels.

Cont For designing trunked radio system that can handle a specific capacity and at specific GOS ,it is essential to understand trunking theory and queuing theory. Erlang(A Danish mathematician) developed the fundamentals of trunking theory One Erlang represents the amount of traffic intensity carried by the channel that is completely occupied A radio channel that is occupied for 30 min during an hour carries 0.5 erlangs of traffic

GOS GOS is the measure of the ability of user to access a trunked system during the busiest hour. The GOS is the benchmark used to define the desired performance of a particular trunked system by specifying a desired likelihood of a user obtaining channel access given a specific number of channels available in the system GOS is typically given as the likelihood of a call experiencing a delay greater than a certain queuing time

The traffic intensity offered by each user is equal to the call request rate multiplied by the holding time. i.e. each user generates a traffic intensity of Au Earlangs given by : Au=lamdaH Where H is the average duration of call and lamda is the average number of call requests per unit time for each user. For a system containing U users and an unspecified number of channels , the total offered traffic intensity A , is given as A=UAu In a C channel trunked system ,if the traffic is equally distributed among the channels ,then the traffic intensity per channel Ac is given by Ac=UAu/C

Trunking and GOS

The offered traffic is not necessarily the traffic which is carried by the trunked system only that which is offered to the trunked system When the total traffic exceeds the maximum capacity of the system , the carried traffic becomes limited due to limited capacity. The maximum possible carried traffic is the total number of channels C, in erlangs The AMPS cellular system is designed for GOS of 2% blocking.

Probability of Blockage Let C be the total number of trunks(channels),and A be the offered traffic in erlangs, then the probability of all the C trunked channels are busy or “Probability of blocking” is given by eq 3.16 (Please refer to text book)

Example 1 Consider a small system of 4 channels.There are altogether 20 subscribers and each subscriber is expected to generate a traffic of 0.1 erlang. Determine the probability of blockage that at any time all four channels get busy

Example2 GOS required is 0.02 and it is expected that 1000 calls are generated per hour with average call duration of 2 minutes. Calculate the total number of channels required by the system per cell.

System Capacity Improvement Techniques As the demand of wireless service increases , the number of channels assigned to a cell eventually becomes insufficient to support the required number of users. At this point , cellular design techniques are needed to provide more channels per unit area. System Capacity:C=MKN In terms of traffic intensity , the total traffic handled by the system is given by: A=UAu Where U=number of users in system Au=traffic generated by typical user

Cont.. Maximum value of A can assume C. Given an allocated spectrum S=KN which is fixed ,we have to use some cellular design techniques to improve system capacity C, or A is a function of C Following techniques are used in practice to expand the capacity of cellular systems: (1) Cell Splitting (2) Sectoring (3)Micro-cell zone Approach