无线通信系统第2章

2011-6-15
17
Choosing Handover Margins
Handover margin ∆ ∆ = P r handover - Pr minimum usable – If ∆ is too large unnecessary handover will occur, burdening the MSC – If ∆ is too small, there maybe insufficient time to complete the handover before a call is lost due to weak signals – Therefore ∆ is chosen carefully
2011-6-15
24
7-cell reuse pattern
2011-6-15
25
12-cell reuse pattern
2011-6-15
26
19-cell reuse pattern
2011-6-15
27
Co-channel reuse ratio vs. frequency reuse pattern
2011-6-15
9
Example 2
If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses four-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and traffic channels in each cell for each of the system.
A t = 1.3r 2
A s = 2.0r 2
A h = 2 .6 r 2
2011-6-15
3
Cellular coverage representation
2011-6-15
4
The size of a cell
Macrocell large, covering a wide are range of several hundred kilometres (km) to ten km mostly deployed in rural and sparsely populated areas Microcell medium cell, coverage are a smaller than in macro cells range of several hundred metres to a couple of metres deployed mostly in crowded areas, stadiums, shopping malls Picocell small, covering a very small area range of several tens of metres low power antennas, indoor
2011-6-15
20
Interference and System Capacity
2011-6-15
21
Six Effective Interfering Cells of Cell 1
2011-6-15
22
Co-channel reuse ratio Q
2011-6-15
23
Co-channel reuse ratio vs. frequency reuse pattern
The Cellular Concept： System Design Fundamentals
2011-6-15
1
Cellular Networks
2011-6-15
2
Cell shape
Circle: Equilateral triangle: Square: Hexagon:
A c = π r 2 = 3.14r 2
2011-6-15
33
Trunking and Grade of Sere of Service (GOS)
2011-6-15
35
About Erlang
2011-6-15
36
Erlang B formula
where C is the number of trunked channels offered by a trunked radio system and A is the total offered traffic.
2011-6-15
10
Method of locating co-channel cells
i = 3, j = 2
2011-6-15
11
Channel Assignment Strategies
Three assignment approaches Fixed and static (most common) Dynamic Hybrid Fixed channel assignment all channels in a cell could be in use all the time –new calls are then blocked (no channels left) –may be solved by borrowing spare channels from nearby cells
2011-6-15
14
Handover Process
Parties in communication share the same channels – Received signal weakens as mobile moves out of cell – Cell site at some point requests handover to cell with stronger signal strength – MSC switches call to new cell after allocating channels.
2011-6-15
28
Propagation model
α Di− n
2011-6-15
29
Co-channel interference ratio
2011-6-15
30
Example 3
2011-6-15
31
More exact geometry layout
2011-6-15
32
Closely approximately as
2011-6-15
37
Example 4
Trunked mobile network provide cellular service in this area. System has 394 cells with 19 channels each. Find the number of users that can be supported at 2% blocking if each user averages two calls per hour at an average call duration of three minutes. Compute the total number of subscribers that can be supported by this system. Probability of blocking = 2% =0.02 Number of channels per cell used in the system, C=19 Traffic intensity per user, Au = λH= 2 x (3/60) = 0.1 Erlangs from the Erlang B chart, the totalcarried traffic, A, is obtained as 12 Erlangs. Therefore, the number of users that can be supported per cell is U=AIAu= 12/0.1 = 120 Since there are 394 cells, the total number of subscribers that can be supported by System A is equal to 120 x 394 = 47280
2011-6-15
15
Handover Process
BS2
BS1
BS3
PST N
MSC Trunks
2011-6-15
16
Handover Process
Handover must not be too frequent or system is kept busy servicing handover requests – handover threshold is set slightly stronger – Minimum usable signal level is normally set to be between -90 dBm and -100 dBm
2011-6-15
13
Handover (Handoff)