无线通信原理与应用第二章

2020/3/5
26
Tradeoff
2020/3/5
27
Propagation model
2020/3/5
Din
28
Co-channel interference ratio
2020/3/5
29
Example 2
2020/3/5
S 12dB(15.9), n 4 Q (615.9)0.25 3.12 I
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
Chapter2: The Cellular Concept -System Design Fundamentals
2020/3/5
1
2.1 The Concept :Cellular Networks
2020/3/5
2
geometric shapes
Circle:
Ac r 2 3.14r 2
i0
（3.10），
Using
N 4, D R Q 4.6
，
S 54.3 17.3dB
I
32
2.5 Trunking and Grade of Service (GOS)
• Trunking: large number of users share a relatively small number of channels
2020/3/5
11
Dynamic channel assignment – MSC allocates frequencies when a call is made – Provides high channel utilization
To do this it needs real-time information on – channel occupancy – traffic distribution – radio signal strength indication (RSSI) – high computational load and increased storage
2020/3/5
centre-excited
edge-excited
7
2.2 Frequency Reuse and Capacity
S kN
C MkN MS
C can be used as a measure of capacity
2020/3/5
8
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
Picocell small, covering a very small area range of several tens of metres low power antennas, indoor
2020/3/5
5
Three different kinds of cells
2020/3/5
6
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
very quickly
2020/3/5
13
Handover Process
– 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.
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
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
2020/3/5
17
Setting Handover Thresholds
2020/3/5
18
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
2020/3/5
14
Illustration of a handoff scenario at cell boundary
BS2 BS1
PST N
Trunks
MSC
2020/3/5
BS3
15
The basic concept of handover strategies
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
Q2 (3.12)2
N
3.24 4
3
3
30
More exact geometry for the co-channel interferences
2020/3/5
31
2020/3/5
S/I is approximated as
S
R－n
R n
I i0 Di n 2 D Rn 2 D Rn 2 Dn
22
Co-channel reuse ratio Q
2020/3/5
23
12-cell reuse pattern
2020/3/5
24
19-cell reuse pattern
2020/3/5
25
Co-channel reuse ratio vs. frequency reuse pattern
2020/3/5
19
2.4 Interference
Interference and System Capacity:
2020/3/5
20
7-cell reuse pattern
2020/3/5
21
Six Effective Interfering Cells of Cell 1
2020/3/5
2020/3/5
9
Method of locating co-channel cells
2020/3/5
i 3, j 2
10
2.3 Channel Assignment and handoff
Channel Assignment Strategies:
Two assignment approaches Fixed and static (most common) Dynamic
2020/3/5
33
Grade of Service (GOS)
• Grade of Service (GOS): measure of the ability of a user to access a trunked system during the busiest hour.
• Prob. [call is blocked] or Prob.[delay>T] • Measure of traffic intensity: Erlang • 1 Erlang=amount of traffic intensity carried by a
2020/3/5
16
Choosing Handover Margins
Handover margin
= P r handover - Pr minimum usable – If is too large unnecessary handover will occur, burdening
Equilateral triangle:
At 1.3r 2
Square: Hexagon:
As Biblioteka Baidu2.0r 2
Ah 2.6r 2
2020/3/5
3
Cellular coverage representation
2020/3/5
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
channel that is • Completely occupied
2020/3/5
34
About Erlang
• On average, during the busy hour, a company makes 120 outgoing calls of average duration 2 minutes. It receives 200 incoming calls of average duration 3 minutes. Find (1)the outgoing traffic,(2)the incoming traffic,(3)the total traffic.
2020/3/5
12
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
• Each user is allocated a channel on PER Call basis • All channel in use: new user is blocked or has to • To wait in a queue • Trunking theory: A.K. Erlang(1917)