MOBILE COMMUNICATIONS (650520) Part 3

Transcription

1 Philadelphia University Faculty of Engineering Communication and Electronics Engineering MOBILE COMMUNICATIONS (650520) Part 3 Dr. Omar R Daoud 1

2 Trunking and Grade Services Trunking: A means for providing access to users on demand from available pool of channels (large number of users share a pool of available channels (relatively small number of channels)). With trunking, a small number of channels can accommodate large number of random users. Telephone companies use trunking theory to determine number of circuits required (Trade off between the number of available telephone circuits and the likelihood of a particular users finding that no circuits on the peak time) Trunking theory is about how a population can be handled by a limited number of servers. Dr. Omar R Daoud 2

3 Trunking and Grade Services Trunking: What will happen to the user if all channels are in use?? (in some systems a queue may be used to solve the blocking or denying process) In the 19th century, Erlang (Danish mathematician) study how a large number of population could be accommodated by a limited number of services (today its known by the measure of traffic intensity) Dr. Omar R Daoud 3

4 Trunking and Grade Services Trunking- Terminology: Traffic intensity is measured in Erlangs (One Erlang: traffic in a channel completely occupied. 0.5 Erlang: channel occupied 30 minutes in an hour). Grade of Service (GOS): It is the probability that a call is blocked (or delayed). It is a measure of the ability of a user to access a trunked system during the busiest hour It s the designer s job to estimate the max required capacity to allocate the proper number of channels to meet the GOS Dr. Omar R Daoud 4

5 Trunking and Grade Services Trunking- Terminology: Set-Up Time: time to allocate a channel. Blocked Call: Call that cannot be completed at time of request due to congestion. Also referred to as Lost Call. Holding Time: (H) average duration of typical call. Load: Traffic intensity across the whole system. Request Rate: (λ) average number of call requests per unit time. Dr. Omar R Daoud 5

6 Trunking and Grade Services GOS is typically given as the likelihood that a call is blocked or experiencing a delay greater than a certain queuing delay time. If blocked call are cleared (i.e. queued), then under some model assumptions, the probability of a blocked call is given by Erlang B model as: C is the number of trunked channels offered by a radio system A is the total offered traffic Dr. Omar R Daoud 6

7 Trunking and Grade Services The second type is one in which a queue is provided to hold calls which were blocked (queues blocked calls Blocked Calls Delayed). This is known as an Erlang C model. The likelihood of a call not having immediate access to a channel is determined by Erlang C formula: Pr( delay 0) A C A C! 1 C A C C 1 k 0 k A k! Dr. Omar R Daoud 7

8 Trunking and Grade Services Erlang C Procedure. Determine Pr[delay> 0] = probability of a delay from the chart. Pr[delay > t delay > 0 ] = probability that the delay is longer than t, given that there is a delay ( ( CA) t ) H Pr( delay t) Pr( delay 0) e Pr( delay Unconditional Probability of delay > t : t) Pr( delay 0) Pr( delay t delay 0) Average delay time D D Pr( delay 0) ( H ( C ) A) Dr. Omar R Daoud 8

9 Erlang B Chart Erlang B chart Dr. Omar R Daoud 9

10 Capacity of Erlang B System Dr. Omar R Daoud 10

11 Erlang C Chart Dr. Omar R Daoud 11

12 Trunking and Grade Services A u is defined as a traffic intensity and is given by A u = λh where λ is the average number of call requests per unit time for each user, H is the average duration of a call. The total offered traffic intensity for U users is A= A u U In a C channel trucked system, the traffic intensity per channel, A c, is given by A c = A/C The max possible carried traffic is the total number of channels, C, in Erlang Dr. Omar R Daoud 12

13 Trunking and Grade Services Dr. Omar R Daoud 13

14 Trunking and Grade Services Ex.1 Dr. Omar R Daoud 14

15 Trunking and Grade Services Dr. Omar R Daoud 15

16 Trunking and Grade Services Dr. Omar R Daoud 16

17 Trunking and Grade Services Dr. Omar R Daoud 17

18 Trunking and Grade Services Dr. Omar R Daoud 18

19 Trunking and Grade Services Dr. Omar R Daoud 19

20 Trunking and Grade Services Dr. Omar R Daoud 20

21 Trunking and Grade Services Dr. Omar R Daoud 21

22 Trunking and Grade Services Dr. Omar R Daoud 22

23 Trunking and Grade Services Dr. Omar R Daoud 23

24 Trunking and Grade Services Dr. Omar R Daoud 24

25 Improving Capacity in Cellular Systems Cost of a cellular network is proportional to the number of Base Stations. The income is proportional to the number of users. Its depends on the demand of users. Because the assigned number of channels becomes in sufficient, the cellular design needed to expand the capacity Ways to increase capacity: New spectrum expensive. Architectural approaches: cell splitting, cell sectoring, reuse partitioning, microcell zones. Dynamic allocation of channels according to load in the cell (non-uniform distribution of channels). Improve access technologies. Dr. Omar R Daoud 25

26 Improving Capacity in Cellular Systems Methods for improving capacity in cellular systems Cell Splitting: subdividing a congested cell into smaller cells (orderly growth of the cellular system; increases the number of BS) Sectoring: directional antennas to control the interference and frequency reuse (relay on BS elements) Coverage zone : Distributing the coverage of a cell and extends the cell boundary to hard-to-reach place. Repeaters for Range Extension A Microcell Zone Concept (distribute the coverage of a cell and extends the cell boundary to hard-to-reach places, relay on BS elements; antennas) Dr. Omar R Daoud 26

27 Improving Coverage and Capacity Cell Splitting It s the process of subdividing a congested cell into smaller cells (each with its own BSs and a reduction in antenna height and transmitter power) Dr. Omar R Daoud 27

28 Improving Coverage and Capacity Cell Splitting The number of times that channels are reused increases (the capacity increases), that s means that there are additional number of channels per unit area The system grow by replacing large cells with smaller ones Dr. Omar R Daoud 28

29 Improving Coverage and Capacity Cell Splitting Transmission power reduction from Pt1 to P t 2 Examining the receiving power at the new and old cell boundary P Pr [at old cell boundary] Pt 1 r[ at new cell boundary] Pt 2 If we take n = 4 and set the received power equal to each other Pt 1 Pt 2 16 R n ( R / 2) n Dr. Omar R Daoud 29

30 Improving Coverage and Capacity Cell Splitting The transmit power must be reduced by 12 db in order to fill in the original coverage area. Problem: if only part of the cells are splitted Different cell sizes will exist simultaneously Handoff issues - high speed and low speed traffic can be simultaneously accommodated Dr. Omar R Daoud 30

31 Improving Coverage and Capacity Sectoring By rescaling the system the capacity can be achieved The cell radius is kept R by sectoring try to reduce the cochannel reuse ratio D/R, thus will increase the frequency reuse Dr. Omar R Daoud 31

32 Improving Coverage and Capacity Sectoring Decrease the co-channel interference and keep the cell radius R unchanged Replacing single Omnidirectional antenna by several directional antennas Radiating within a specified sector Dr. Omar R Daoud 32

33 Improving Coverage and Capacity Sectoring The co-channel interference increases (it can be reduced by replacing the omnidirectional antennas by directional antennas, each radiated within a specific area) Dr. Omar R Daoud 33

34 Improving Coverage and Capacity Sectoring Dr. Omar R Daoud 34

35 Improving Coverage and Capacity Sectoring The factor by which the cochannel interference is reduced by using directional antennas depends on the amount of sectoring used The channels used are broken down into sectored group and are used only within a particular sector Dr. Omar R Daoud 35

36 Improving Coverage and Capacity Sectoring Consider the interference experienced by a mobile located in the right-most sector in the center cell labeled by 5 Out of the three in right, two have sectors radiate to interfere the sector labeled 5 in the center cell Find S/I. its 24.4 db which has a significant improvement over the omindirectional antenna (worst case 17 db) If N decreased, the frequency reuse is improved and thus the system capacity Dr. Omar R Daoud 36

37 Improving Coverage and Capacity Repeaters Radio re-transmitters are needed to provide dedicated coverage for hard-to-reach areas They simultaneously send signals and received signal from a serving BS (bidirectional) Don't add capacity to the system, but serves to radiate signals to required areas Dr. Omar R Daoud 37

38 Improving Coverage and Capacity Microcell Concept Zone Antennas are placed at the outer edges of the cell Any channel may be assigned to any zone by the base station Mobile is served by the zone with the strongest signal. Dr. Omar R Daoud 38

39 Improving Coverage and Capacity Microcell Concept Zone Sectoring increases the number of handoffs (results in an increased load on switching and control link elements ) As an example; shown below; each of the three zone sites are connected to a single BS and share the same radio equipment Dr. Omar R Daoud 39

40 Improving Coverage and Capacity Microcell Concept Zone The antennas are placed at the outer edge of the cell As a mobile travels in this zone, it will be served by the strongest signal Dr. Omar R Daoud 40

41 Improving Coverage and Capacity Microcell Concept Zone The handoff is not required???? (MS retains the same frequency channel, BS simply switches the channel into a different zone site) No channel re-assignment Switch the channel to a different zone site Dr. Omar R Daoud 41

42 Improving Coverage and Capacity Microcell Concept Zone Co-channel interference reduced (large central BS replaced by several low powered transmitters), and thus improves the signal quality Dr. Omar R Daoud 42

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