Interference ,Trunking and GOS

1. Interference ,Trunking and GOS

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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.

9. 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.

10. 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

11. 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.

12. 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.

13. 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

14. 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

15. 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

16. Trunking and GOS

17. 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.

18. 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)

19. 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

20. 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.

21. 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

22. 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