School of Electrical and Information Engineering
University of the Witwatersrand, Johannesburg
Impact of Frequency and Radio Channel
Variability on a Converged Wireless
Access Network and Telecommunications
Service Provision in Sub-Saharan Africa
Peter J. Chitamu
Centre for Telecommunications Access and Services
p.chitamu@ee.wits.ac.za
www.ee.wits.ac.za/~comms/
• Introduction
Outline
– Background on CeTAS Research thrusts
– Convergence viewed from Access Network
parameters
• Presentation of Typical Wireless channel
parameters from measurements results
– Pathloss model parameters
– Multipath model parameters
• Examples of applications of the results
– Reflections on Coverage (Range)
– Reflections on throughput limitation (Rate)
• Conclusions
1
CeTAS Research Thrusts
How to Create
Innovative ICT
Applications
Using Telco
networks?
How to Drive the
Cost of telecom
Access down?
How to Create
Points of Integration
For Connections
And OSS?
CeTAS
How to provide
Application-related
Terminal-to-terminal
Quality of Service
Across networks?
CeTAS Research Thrusts
Services and
Applications
In an Open Service
Access Environment
Quality of Service
and Networks
Performance
The Next
Generation
Network
Access Networks
And Planning
2
Convergence and Networks
LAN
LAN
VoIP
Gateway
PSTN
Converged Network Model
New
Billing Server
OSS Server
Application
Connection
Control
Old
Gateways
IP Network
with QoS
SDP
Signalling System No 7
SG
Web
Server
SCP
TE
TG
IP
PSTN
LE
MG
LE
NAS
Access
DSLAM
BB-WLL
Campus
LAN
ADSL
Copper Loop
WLL
3
A view of Integrated Telecomms. Network
Call Collection
Area (Access)
Local
Transport
National
Transport
Global
Comms.
Voice
VoIP
Data
PSTN
Voice
VoIP
Data
Options:
1. cdma2000
fixed - mobile
2. WLL
3. WLAN
4. Broadband
Wirele ss
Gateway
Gateway
Local
Intranet
Internet
PSDN
Reach (Loss model) & Rate (Bps)
Reach / Rate
Traffic Source & Sink Models
From another node,
CN bps, N calls/sec
To another Node
1
1
2
2
Node
A
n
Node
B
Full duplex lines
CT=(NCL) bps
To another Node
m
Local loops,
CL bps, calls/sec
4
Reach problems in Sub-Saharan Africa
(Traffic density in proportion to Population density)
Typical Rural and Urban demand
1200
1000
800
600
400
200
0
1
Urban Europe
2
3
4
Cell Radius [km]
Urban Africa
5
6
Rural Africa
Network Convergence and Services
• Access Network to support a wide range of
services
– Rate Problems (what is average kBps?)
– Voice, data and multimedia (10 kbps – 500 kbps)
• Access Network to support users in different
operating environments
– Reach problems (How far subscriber can be?)
– Rural Vs Urban
– Low density Vs High density
• For wireless Access Technologies
– Convergence tied to radio channel characteristics
and hence Radio Access Network technology
5
Service Type vs Technology Capabilities
As Dec 2002
M El-Sayed and J Jaffe,”A View of Telecomms. Network
Evolution”, IEEE Comms. Mag., Dec 2002, pp. 74-81
Propagation measurements in Tanzania
• Purpose
– Obtain channel parameters for Reach estimation
– Obtain channel parameters for Rate estimation
– Use the models to specify a suitable RAN
• Propagation measurement data used
– Evaluation of performance of selected network
technologies
– Recommend features of a suitable Radio Access
– In the design of a Rural Radio Systems (DaRT)
6
Propagation Measurements
• Experimental Setup
– Transmitter
– Receiver
• Results
– Pathloss
– Multipath
• Model results and its implications
Example of Propagation Measurement
Equipment: Transmitter
1 3 0 .2 M Hz
x
x
DC
P RBS
3 0 0 M Hz
4 3 0 .2 M H z
TRAN S CEIVER
5 M Hz
7
Example of Propagation Measurement
Equipment: Receiver
A T TN .
TRANS C E IVE R
X
LT O P
3 00 M Hz
X
1 3 0 .2 M H z
DC
P RBS
5 M Hz -
fC
Technical Specs
Operating frequency
300 MHz
Antennae
Half wave dipoles, 11m vertically
polarised
Transmit power
10 watts (40dBm)
PRBS length
511 (29 - 1)
Chip rate
5 Mbits / sec
Time delay resolution
0.2 usec
Time delay range
102.2 usec
Time compression ratio
5000
Receiver noise figure
Overall around 2dB
8
Propagation Measurement sample sites
USA River
Propagation Measurement sample sites
Kilombero
9
Propagation Measurement sample sites
Mwanza
Propagation Measurement sample sites
Serengeti
10
Propagation Measurement sample sites
Moshi, Kilimanjaro
Receiver & Transmit test bed
11
Hilly areas in Lushoto
Different receiver sites
12
Soni, Lushoto (Families separated by nature)
Lushoto, Tanga
13
Different receiver sites
Mombo, Tanga
14
Njombe, Iringa
Transmit sites and some attractions
15
Multipath selected Results
2.0
1.0
0.0
-1.0
-2.0
-2.5
10220.0
10180.0
10300.0
10260.0
Time samples, PRBS correlator output
LU1162.LIN
1.2
Multipath selected Results
1.1
1.0
0.9
-6 dB
0.8
0.7
V
o
l
t
s
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-2.0
1.0
4.0
7.0
10.0
13.0
16.0
MICROSECONDS
19.0
22.0
25.0
28.0
16
AR1082.LIN
Multipath selected Results
1.2
1.1
1.0
0.9
0.8
0.7
V
o
l
t
s
-6 dB
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-3.0
2.0
7.0
12.0
17.0
22.0
27.0
MICROSECONDS
32.0
37.0
42.0
47.0
52.0
Multipath selected Results
ar1082.imp
1.2
USA River
Arusha, Tanzania
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
- 0 . 00 0 0 1 0
0. 0 0 0 0 1 0
0.000030
0 . 00 0 0 5 0
0. 0 0 0 0 7 0
17
AR1092.IMP
Multipath selected Results
1.2
1.0
Pre-Cursors and Post-cursors
0.8
Nearby echos and a group from far
0.6
0.4
0.2
0.0
-0.2
- 0 . 0 00 0 1 0
0 .0 0 0 0 10
Time
0 .0 0 0 0 30
in
0 .0 0 0 0 50
0 .0 0 0 0 70
seconds
Multipath selected Results
TA1 153.IM P
1.2
A real strange profile
1.0
0.8
0.6
Pre-Cursors
Post-Cursors
0.4
0.2
0.0
-0.2
-0.000015
-0.000005
Time
0.000005
in
0.000015
seconds
18
Scatter Plots
Pathloss models
PL(d) [dB] = PL(d0) + 10 n log (d/d0)
where n is path loss exponent &
constant PL(d0) from linear
regression of scatter plots
19
Path loss exponents
from propagation measurements
! Hilly and heavily forested terrain ( Tanga, Lushoto,
USA River, Arusha, Kilimanjaro )
P = 72 log d + 88
L
10
dB
! Undulating landscape with light vegetation
( Iringa, Morogoro, Njombe )
P = 53 log d + 93
L
10
dB
! Flat terrain with light vegetation ( Mwanza area )
P = 42 log d + 67
L
10
dB
Performance Evaluation
of selected RAN
• Reach example using derived models
– DECT Vs GSM
– Frequency Variability
• Fading (NLOS)
– Flat Vs Frequency Selected
– Rate and Reach Implications
• Techno-Economic Assessment
– TDMA Vs CDMA
– Sensitivity Analysis
20
Example 1: DECT and GSM
Band
Modulation Rate
Uplink power
(Ave)
Equalization
DECT
GSM (900)
1880 - 1900
Uplink
890 – 915
270.8333 kbps
(3.6923 s)
1W / 125 mW
30 dBm / 21 dBm
Rural Area, Hilly
Terrain, Typical
Urban, Test
Pattern
1152 kbps
(0.868 s)
10 mW
(10 dBm)
None: Assumed
Microcell, office
environment
Rural Africa ?
DECT Power budget
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
-150
-160
-170
-180
2
Area 1 ( 10 dBm)
Area 1 ( 24 dBm)
Area 2 ( 10 dBm)
Area 2 ( 24 dBm)
Area 3 ( 10 dBm)
Area 3 ( 24 dBm)
LogNormal Fading Margin
Receiver Threshold
4
6
Distance d [ km ]
8
10
21
GSM Power budget
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
-150
-160
-170
-180
2
Area 1 (13 dBm)
Area 1 (30 dBm)
Area 2 (13 dBm)
Area 2 (30 dBm)
Area 3 (13 dBm)
Area 3 (30 dBm)
LogNor mal Fading Mar gin
Receiver Threshold
4
6
8
10 12 14
Distance d [ km ]
16
18
20
Example 2: Impact of terrain and operating frequency
Free Space (n=2) Pathloss
80
60
40
20
0
0
2
4
6
8 10 12 14
Distance d [ km ]
Free Space (300 MHz)
16
18
20
Free Space ( 900 MHz)
Free Space (1800 MHz)
22
Pathloss from Measurements (300 MHz)
Flat, Undul &
Hilly at 300 MHz
200
180
160
140
120
100
80
60
40
20
0
0
2
4
6
300, 900 &
1800 MHz
8 10 12 14
Distance d [ km ]
16
Free Space ( 300 MHz)
Free Space ( 900 MHz)
Free Space ( 1800 MHz)
Prac Flat 300 MHz
Prac Undulating 300 MHz
Prac Hilly 300 MHz
18
20
Pathloss from Measurements
Impact of
terrain
200
180
160
140
120
100
80
60
40
20
0
We found that TDMA and CDMA
systems are affected differently
by channel variability
0
2
4
6
8 10 12 14
Distance d [ km ]
16
Prac Flat 300 MHz
Prac Undulating 300 MHz
Prac Hilly 300 MHz
Prac Flat 900 MHz
Prac Undulating 900 MHz
Prac Hilly 900 MHz
18
20
23
Implications on Large coverage areas
• High Density Traffic Areas e.g. Cities
– Close to the design parameters
• Low Density Traffic Areas e.g. Rural
Areas
– There could be large variations compared
to design values
– Coverage Limited possibility of outtage
PL (d ) = PL ( d 0 ) + 10n log
LogNormal
Shadowing
d
+ χσ
d0
P(out ) = P( PR ≤ Rth )
(
= P ( PT − PL (d ) − L M ) ≤ Rth
)
Example 3: Multipath Implications
• General Observations
– Usually more extensive that typical European cities
– Rural areas can have very large delay spread due to
NLOS large coverage areas (measured up to 50
microseconds delay spreads)
• Implications
– Throughput can be severely curtailed due to
Multipath induced Inter-symbol Interference
– Need a robust Radio Access Network (inbuilt
counter-measures against multipath delay spread)
– 3G is designed to take advantage of multipath, so
one possible option
24
GSM Rural Channel Impulse Response
Source: Raymond Steele,
Mobile Radio Comms, Pentech Press
GSM Urban Channel Impulse Response
Source: Raymond Steele, Mobile
Radio Comms, Pentech Press
25
GSM Hilly Channel Impulse Response
Source: Raymond Steele, Mobile
Radio Comms, Pentech Press
Delay resolution
Imaginary
2
1
Real
0
3
Time (1 sec intervals)
Delay resolution ( s)
DECT = 0.868
GSM = 3.69
CDMA = 0.814
UMTS = 0.26
Coherent Vector sum
3
3
2
1
0
Real
2
I
Delay
Resolution
II
III
Delay ( s)
26
Overall Coverage Uncertainty
BS
MS
PT
D IS T A N C E
L o g n o rm a l
S lo w fa d in g
PDF
S lo w
fa d in g
M a rg in
1 -2 %
R a y le i g h
F a s t fa d i n g
PD F
PR
F a st
fa d in g
M a rg in
1 -2 %
Southampton, England
27
Singapore
London, England
28
London, England
Some village in Lushoto
29
Some villages in Ifakara, Kilombero, Tanzania
Urban areas have many options
• Urban Reach
–
–
–
–
Cable
Fiber
Broadband WLL
3G (UMTS & cdma2000)
• Urban Rate
– Copper (ADSL)
– All of above
• Non-Urban Reach
–
–
–
–
Wireless (GSM)
3G (e.g. cdma2000)
Satellite (But expensive)
?
• Non-Urban Rate ( >
144kbps)
– 3G (e.g. cdma2000)
Most service areas in Africa can be
classified as Non-Urban
30
Summary
• Access Network Capability is a key area in
Converged Networks and services
– Should provide for both Reach and Rate (variable)
• Sub-Saharan Africa has unique conditions
– Requirements for large coverage areas/low traffic
– Propagation results suggest potential degradation
and coverage limitations for systems with low delay
resolution
– Requires careful selection of Access Network
Technology to meet both Reach and Rate
requirements of Multi-service platforms
concurrently
References
•
•
•
P. J. Chitamu and D. E. Vannucci, Networking Africa: A Case for
VoIP, The Transactions of the SA Institute of Electrical and
Engineers, Vol. 94 No: 2, June 2004, pp. 83-90
P. J. Chitamu, I. G. Kennedy, and R. Van Olst, “Technology
options to support telecommunications in countries with low teledensities”, 2nd International Symposium on Communications
and Info Tech (ISCIT2002), 23-25th October 2002, Pattaya,
Thailand.On CD-ROM ISBN 974-537-149-1, pg. 289-292.
P. J. Chitamu and P C Chen, “Economic Viability Study of
Cellular Network Technologies for Low Density Traffic Areas”,
Proceedings of 3rd International Conference on Information
(ICICS2001), Communications, and Signal Processing, 15-18
October 2001, Singapore, On CD-ROM ISBN 981-04-5149-0
31