Cellular system design fundamentals

Mobile Communication (ET4 153) 01/05/00

Mobile Communications (ET4 153)

2. Cellular System Design Fundamentals
Part 2

Jos Nijhof
Delft University of Technology

Cellular Systems mc_02 # 1

Cellular Systems – Overview Part 1

• Frequency reuse
• Cluster size (N)
• Frequency reuse factor (1/N)
• Co-channel interference
• Co-channel reuse ratio (Q)
• Signal-to-Interference ratio (S/I)
• Trunking
• Grade of Service (GOS)
• Erlang-B formula
• Cell splitting
• Sectoring


Cellular Systems 1


Cellular Systems – Overview Part 2
• Exercises on cellular planning
• Channel assignment strategies
• Handoff (handover) strategies
• Power control


Example 1.1 - Problem statement
(2)

(1) (5)
Compute:
66.7

30.8 (4) 38.2 (1) (a) The number of channels
(3) required in each cell
33.2 (7) (b) The number of subscribers
(6) 32.6
48.6 served by the system
(c) The average number of
37.8
subscribers per channel
Given: (d) The number of calls
Total available channels: 395 supported by the system
Each subscriber generates 0.03 erlang (e) The subscriber density per
Average holding time: 120 s square mile
System area: 1200 miles2
Grade of service: 2%
(f) The cell radius in miles


Cellular Systems 2


Example 1.1 - Solution
Cell Traffic No. of No. of No. of Channel
Number (erlang) Channels Subscribers Calls Utilisation
Required Per Cell Per Cell
(An) (a) (b) (d)

1 30.8 40 1026.7 924 0.77

2 66.7 78 2223.3 2001 0.86

3 48.6 59 1620.0 1458 0.82

4 33.2 43 1106.7 996 0.77

5 38.2 48 1273.3 1146 0.80

6 37.8 48 1260.0 1134 0.79

7 32.6 42 1086.7 978 0.78

Total 287.9 358 9596.7 8637

From An An
erlang (b) x 0.9
0.03 (a)
table/chart



(a), (b) : see table
(c) : avg. number of subscriber s per channel : 9597 358 = 26.8
(d) : number of calls supported by system : A = λh ⇒ 0.03 = λ × 120

= 0.00025 [calls/s] = 0.00025 × 3600 = 0.9 [calls/hr ]
0.03
λ=
120
Cell (1) : Number of calls supported : 1026.7 × 0.9 = 924
(e) : subscriber density per mile 2 : 9597/1200 = 8.0
(f) : cell radius in miles : area/cell = 1200/7 = 171.4 miles 2 ⇒ radius


Cellular Systems 3


Example 1.3 - Problem statement
Compare the spectral efficiency of the digital system with respect to the
analogue system using the following data:

(a) The total number of channels in the analogue cellular system = 416
(b) The number of control channels = 21
(c) The number of voice channels = 395
(d) The channel bandwidth = 30 kHz. The digital systems has 3 channels
per 30 kHz
(e) The reuse factor N = 7
(f) The total available bandwidth in each direction = 12.5 MHz
(g) The total coverage area = 10,000 km2
(h) The required S/I ratio for the analogue system = 18 dB (63.1)
(I) The required S/I ratio for the digital system = 14 dB (25.1)
(j) The call blocking (GOS) = 2%



Spectral efficiency - η m =
( Total traffic carried by the system )
( Bandwidth ) × ( Total coverage area)
ANALOGUE SYSTEM:
No. of voice channels / cell: 395 / 7 = 56.4 ⇒ 56
Offered traffic load: N = 56, B = 0.02 ⇒ A = 459 (from erlang table)
.
Carried traffic load: C = (1 − B) × A = (1 − 0.02) × 45.9 = 44.98 [erlang / cell]
10,000 10,000
Number of cells: =
Acell 2.6 R 2
10,000
44.98 ×
⇒ Spectral efficiency = 2.6 R 2 = 1384
.
12.5 × 10,000 R2


Cellular Systems 4


Total traffic carried by the system
Spectral efficiency = ηm =
(Bandwidth ) × (Total coverage area )
DIGITAL SYSTEM :
No. of channels per 30 kHz = 3
Number of voice channels per cell = 56 × 3 = 168
Offered traffic load : N = 168,B = 0.02 ⇒ A = 154.5 (from erlang table)
Carried traffic load : C = (1 − B ) × A = (1 − 0.02 ) × 154.5 [erlang/cel l]
1 1
 S 4  S 2 Qdigital 2 25.1
Q = 6  ⇒ Q = 6  ⇒
2
= = 0.6307
 I  I Qanaloge 2
63.1
151.4 7.386
⇒ Spectral efficiency = = erlang/MHz /km 2
12.5 × 2.6 R × 0.6307
2
R 2

ηm (digital ) 7.386
⇒ = = 5.34
ηm (analogue ) 1.384

GSM system architecture (1)
PLMN
& International
OMC
ISC
PSTN
ISDN
PDN
MS BTS
BSC
GMSC

BSC MSC
BTS
MS
EIR
MS AUC
BTS HLR
VLR


Cellular Systems 5


GSM system architecture (2)
BTS Base Transceiving Station
BSC Base Station Controller
MSC Mobile Switching Center
GMSC Gateway MSC
ISC International Switching Center
MS Mobile Station
HLR Home Location Register
VLR Visitor Location Register
EIR Equipment Identity Register
AUC Authentication Center
OMC Operation and Maintenance Center


A mobile radio environment

Multipath fading
Ra
dio
pa
th Medium

Pro
pa
g
los ation
s

Base
station
Mobile
station


Cellular Systems 6


The mobile radio channel: fading

10
Shadowing
0
Signal Level (dB)

-10

-20

-30
Rayleigh fading
-40 (multipath reception)
λ
-50
2
-60
0 1 2 3 4 5 6 7
time


GSM: Carrier frequencies, duplexing, and TDMA frames
960 MHz
959.8 MHz 124
123 Downlink
25 MHz
⋅⋅⋅
1 2 3 4 5 6 7 8
200 kHz ⋅⋅⋅
2
935.2 MHz 1
935 MHz
Data burst, 156.25 bit periods = 15/26 ms ≈ 576.9 µs
915 MHz
914.8 MHz 124
45 MHz 123 Delay
separation 1 2 3 4 5 6 7 8
⋅⋅⋅
200 kHz ⋅⋅⋅ Uplink
2
890.2 MHz 1
890 MHz


Cellular Systems 7


Channel Assignment Strategies (1)
• Fixed channel allocation (FCA)
– fixed assignment of frequencies to cell clusters and
cells.
– not very efficient if traffic load varies
– simple to use, but requires careful traffic analysis
before installation
– used in the GSM system
• Variation: Borrowing channel allocation (BCA)
– heavy loaded cell can “borrow” channels from a light
loaded neighboring cell
– problem: interference

Channel Assignment Strategies (2)
• Dynamic channel allocation (DCA)
– each time a call request is made, the base station
requests a channel from the MSC
– MSC takes into account:
• probability of future blocking within the cell
• frequency of use of the channel
• frequency reuse distance
– Advantages:
• lower probability of blocking, increases trunking capacity of
the system
– Disadvantages:
• increased storage and computational load

Cellular Systems 8


The handover process
Handover: Changing physical channels, radio channels of fixed
network connections involved in a call, while
maintaining the call
Two phases:

1. MONITORING PHASE
• measurement of the quality of the current and
possible candidate radio links
• initiation of a handover when necessary
2. HANOVER HANDLING PHASE
• determination of a new point of attachment (PoA)
• setting up of new links, release of old links
• initiation of a possible re-routing procedure


Two basic reasons for a handover
• MS moves out of the range of a BTS
– signal level becomes too low
– error rate becomes too high
• Load balancing
– traffic in one cell is too high ⇒ shift some MSs to other
cells with a lower load

The GSM standard identifies about 40 reasons for a handover!


Cellular Systems 9


Handover types
• Intra-cell handover
– narrow-band interference ⇒ change carrier frequency
– controlled by BSC
• Inter-cell, intra-BSC handover
– typical handover scenario
– BSC performs the handover, assigns new radio channel in the
new cell, releases the old one
• Inter-BSC, intra-MSC handover
– handover between cells controlled by different BSCs
– controlled by the MSC
• Inter-MSC handover
– handover between cells belonging to different MSCs
– controlled by both MSCs


Handover types

PLMN

MSC MSC MSC
MSC

BSC BSC BSC BSC BSC

old handover new handover
PoA PoA

handover

Intra-BSC handover Inter-BSC / intra-MSC Inter-MSC handover
handover

Cellular Systems 10


Intra-MSC handover (mobile assisted)
MS BTSold BSCold MSC BSCnew BTSnew
measurement measurement
report result HO required

HO decision ch. activation
HO request

resource allocation

HO request ack ch. activation ack
HO command HO command
HO command
HO access

Link establishment

HO complete HO complete
clear command clear command

clear complete clear complete


Handover scenario at cell boundary
Level at point A
Received signal level

Improper Handoff threshold
handover situation Minimum acceptable signal level
Level at point B
Received signal level

Level at point B
Proper
handover situation Level at which handover is made

BS1 A B BS2


Cellular Systems 11


Handover decision depending on receive level
receive level receive level
BTSold BTSnew

average level

HO_MARGIN

MS MS

BTSold BTSnew


Handover – 1st generation systems
• 1st generation systems (analog cellular):
– signal strength measurements made by the BSs and
supervised by the MSC
– the BS constantly monitors the signal strengths of all
the voice channels
– locator receiver measures signal strength of MSs in
neighboring cells
– MSC decides if a handover is necessary or not.


Cellular Systems 12


Handover – 2nd generation systems
• 2nd generation systems (digital TDMA):
– handover decisions are mobile assisted
– every MS measures the received power from
surrounding BSs and sends reports to its own BS
– handover is initiated when the power received from a
neighbor BS begins to exceed the power from the
current BS (by a certain level and/or for a certain
period)


Avoiding handovers: Umbrella cells

Small microcells for
low speed traffic
Large “umbrella” cell for
high speed traffic


Cellular Systems 13


Power control
• Power levels transmitted by every MS are under
constant control by the BSC.
• Assures that each MS within a BTS coverage area
provides the same signal level to the BTS
receiver.
• Goals:
– to reduce interference
– to prolong battery life
– to combat the near-far problem in CDMA systems


Cellular Systems 14

Cellular system design fundamentals

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