无线通信系统第2章

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Handover (Handoff)
Provides continuity of communication across cells • Difficulty – dropping a call before reconnecting is unacceptable – different cells use different frequencies – mobile phone users usually move from place to place and very quickly too
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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
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Cellular coverage representation
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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
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7-cell reuse pattern
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12-cell reuse pattern
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19-cell reuse pattern
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Co-channel reuse ratio vs. frequency reuse pattern
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Setting Handover Thresholds
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Handoff in different system
•1G signal strength measurements are made by the base stations and supervised by the MSC •GSM MAHO handoff decisions are mobile assisted •CDMA pilot channel, soft handoff
The Cellular Concept： System Design Fundamentals
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Cellular Networks
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Cell shape
Circle: Equilateral triangle: Square: Hexagon:
A c = π r 2 = 3.14r 2
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Handover Process
BS2
BS1
BS3
PST N
MSC Trunks
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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
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Dynamic channel assignment – MSC allocates frequencies when a call is made – Provides high channel utilisation To do this it needs real-time information on – channel occupancy – traffic distribution and – radio signal strength indication (RSSI) – high computational load and increased storage
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Trunking and Grade of Service (GOS)
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Grade of Service (GOS)
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About Erlang
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Erlang B formula
where C is the number of trunked channels offered by a trunked radio system and A is the total offered traffic.
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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
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Base station transmitters
In hexagonal cells, base stations transmitters are either: –centre-excited, base station is at the centre of the cell –edge-excited, base station at 3 of the 6 cell vertices
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Interference and System Capacity
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Six Effective Interfering Cells of Cell 1
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Co-channel reuse ratio Q
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Co-channel reuse ratio vs. frequency reuse pattern
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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
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Method of locating co-channel cells
i = 3, j = 2
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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
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Propagation model
α Di− n
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Co-channel interference ratio
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Example 3
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More exact geometry layout
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Closely approximately as
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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.
centre-excited
edge-excited
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Cellular coverage representation
S = kN
C = MkN = MS
C can be used as a measure of capacity
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Example 1
A mobile communication system uses a frequency reuse factor of 1/7 and 416 channels available. If 21 channels are allocated as control channels, compute its system capacity. Assume a channel supports 8 users Channels available for allocation = 416 - 21 = 395 Channel Number of a cell = 395 / 7 = 57 Number of simultaneous users per cell = 8 x 57 = 456 Number of simultaneous users in system = 7 x 456 = 3192