Optimization of EGPRS Link Adaptation
Thesis work presentation
2007-01-16
Author:
Supervisor:
Instructor:
1
© 2007 Nokia
2007-01-16
Jussi Nervola
Professor Riku Jäntti
M.Sc. Petri Grönberg
jussi.nervola@nokia.com
Contents
• Objectives of the thesis
• Introduction
• Basics of EDGE and EGPRS
• EGPRS link adaptation
• Starting point for the thesis - performance with original link adaptation
• Methods
• Link adaptation optimization work
• Results of the optimization
• Laboratory measurement results
• Live network measurement results
• Conclusions
2
© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
Objectives of the Thesis
• Problem:
• Current EGPRS link adaptation (LA) algorithm is not behaving ideally in all radio
conditions. LA is not using the most suitable Modulation and Coding Scheme (MCS)
in every radio condition. That reduces the performance of the system.
• Objective:
• To adjust the operation of the EGPRS link adaptation so that its performance it at the
ideal level (i.e. LA chooses the best MCS in every radio condition)
• If ideal performance level can be achieved the quality of the service will be improved:
• Increased throughput
• Smaller latency
• Improved coverage for EGPRS service
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
EDGE and EGPRS
AuC – Authentication Centre
BSC – Base Station Controller
A-bis
Air
MS
BTS
A-ter
BSC+PCU
BTS – Base Transceiver Station
A
TC
PSTN
MSC/VLR
Gs
Gb
Gr
EIR – Equipment Identity Register
HLR
GGSN – Gateway GPRS Support Node
AuC
HLR – Home Location Register
EIR
MS – Mobile Station
MSC – Mobile Switching Center
Internet
Gn
BTS
SGSN
GGSN
PCU – Packet Control Unit
SGSN – Serving GPRS Support Node
TC – Transcoder
Base Station Subsystem
Network Switching Subsystem
VLR – Visitor Location Register
• EGDE (Enhanced Data Rates for GSM Evolution) offers both packet switched and
circuit switched data services for the users of GSM network:
• Packet switched EGPRS (Enhanced General Packet Radio Service)
• Circuit switched ECSD (Enhanced Circuit Switched Data)
• EGPRS has been more popular among the manufacturers than ECSD
• Link adaptation is working on the Air-interface between MS and BTS and is controlled
by the PCU in BSC
4
© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
EGPRS Link Adaptation
• In wireless networks the quality of the connection can change
from very good to very poor in short time
• EGPRS has nine different Modulation and Coding Schemes
(MCS-1 – MCS-9) in order to optimize the performance in different
radio conditions. All coding schemes have different amount of
robustness and error correction.
8.8 kbps
11.2 kbps
14.8 kbps
• When user bit rate is low, robustness is high (e.g. MCS-1)
17.6 kbps
• When user bit rate is high, robustness is low (e.g. MCS-9)
22.4 kbps
• Two modulations are used:
• GMSK (Gaussian Minimum Shift Keying) for MCS-1 – MCS-4
• 8-PSK (8-Phase Shift Keying) for MCS-5 – MCS-9
29.6 kbps
44.8 kbps
54.4 kbps
• MCS that offers the best performance in current radio
environment should be selected
• Automatic selection of most suitable MCS is called link
adaptation. The selection is based on link quality measurements
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
59.6 kbps
MCS-1
RLC/MAC block
(radio block)
22
MCS-2
28
MCS-3
redundancy from
channel coding
37
MCS-4
RLC data block,
number of bytes
44
MCS-5
56
MCS-6
74
MCS-7
56
56
MCS-8
68
68
MCS-9
74
74
Starting Point for the Thesis
Performance with Original Link Adaptation
LA is unable to change
to GMSK modulation
• Throughput performance of link adaptation is
compared to the performance of all MCSs
• Ideal LA would follow the envelope of the
throughput of different MCSs
DL FTP Throughput, variable RX-Level, 4 TSLs
1.00
MCS=1
0.90
MCS=2
MCS=3
FTP Th ro u g h p u t ( relat ive)
• Throughput is measured with FTP application in
variable RX-level environment
LA uses MCS-9 although MCS-8
w ould offer higher throughput
0.80
MCS=4
0.70
MCS=5
0.60
MCS=6
MCS=7
0.50
MCS=8
MCS=9
0.40
LA on
0.30
0.20
0.10
-110
-105
-100
-95
-90
-85
-80
-75
0.00
-70
RX- Level ( d Bm)
• Problem:
LA is unable to change
to GMSK modulation
• Original link adaptation was not always choosing
most suitable MCS
UL FTP Throughput, variable RX-Level, 2 TSL
1.20
MCS=1
MCS=2
1.00
• Solution:
• Change to GMSK modulation has to be done
earlier (when link quality decreases)
• MCS-8 should be used more
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
FTP Throughput (relative)
MCS=3
MCS=4
0.80
MCS=5
MCS=6
MCS=7
0.60
MCS=8
MCS=9
0.40
LA on
0.20
-110
-105
-100
-95
-90
RX-Level (dBm )
-85
-80
-75
0.00
-70
Operation of EGPRS Link Adaptation
•
•
In EGPRS link quality is measured in terms of Bit Error Probability (BEP).
BEP values are received in measurement reports from MS and BTS
•
Mean 8-PSK BEP and coefficient of variation of 8-PSK BEP
•
Mean GMSK BEP and coefficient of variation of GMSK BEP
MCS is selected according to BEP values by using look-up tables
1. Selection of modulation
2. Selection of MCS
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© 2007 Nokia
•
from GMSK MCS-selection table or
•
from 8-PSK MCS-selection table (below, from 3GPP specification)
2007-01-16
jussi.nervola@nokia.com
Optimization Work
• Methods:
• Measure the performance of the current link adaptation (LA)
• Specify new parameter / table values for LA
• Measure the performance of the optimized LA and adjust values further
• The measurements were executed in laboratory using real network elements:
MS
BTS
BSC
SGSN
GGSN
Server
• Three different laboratory scenarios:
• Variable RX-level, Variable C/I and Fading with variable RX-level
• Measured quantities:
• Downlink and Uplink throughput (with FTP and HTTP)
• RTT (with ping)
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
Results of the Optimization – Summary
Laboratory
measurements
Throughput
improvement
Coverage
improvement
Throughput
improvement
Coverage
improvement
RX-level
DL
+7%
+4 dB
UL
+17%
+5 dB
C/I
DL
+15%
+1 dB
UL
+3%
+6 dB
Throughput
improvement
Coverage
improvement
Throughput
improvement
Coverage
improvement
+11%
<not
measured>
+31%
<not
measured>
Live
Measurements
drive
route
DL
UL
• After the best version of optimized link adaptation was chosen, the performance
improvement was verified with several MSs in laboratory and live network
• Good results achieved through the optimization work:
• System performance was improved in all cases
• Throughput performance was improved in whole cell area in average by +11%
• Coverage of EGPRS service was improved by 1 – 6 dB depending on the conditions
• On the following slides some of the measurement results are explained briefly
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
Results of the Optimization – Laboratory Measurements
Variable RX-level Scenario
UL Throughput, variable RX-Level, 2 TSL
100%
• Throughput performance was compared:
Original LA
Optimized LA11
Throughput (relative)
• Original LA
• Optimized LA
• Optimal LA
80%
Optimal LA
60%
40%
20%
• Uplink throughput performance of
optimized LA is at the ideal level with all
MSs
0%
-110
-105
-100
-95
-90
-85
-80
-75
-70
RX-Level (dBm )
DL Throughput, variable RX-Level, 4 TSLs
100%
Original LA
Optimized LA11 6280
• With some older MSs there is 1 – 2 dB
wide area where throughput is not
optimal. This is not a problem in fading
environment.
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
80%
Optimized LA11 7270
Throughput (relative)
• Downlink throughput performance of
optimized LA is at the ideal level with new
MSs
Optimal LA
60%
40%
20%
0%
-110
-105
-100
-95
-90
RX-Level (dBm )
-85
-80
-75
-70
Results of the Optimization – Live Network Measurements
• Drive tests executed in
Ruoholahti with Nokia UltraSite
• FTP throughput was measured
on uplink and downlink
separately
• Improvement in average
Downlink FTP throughput
+11%
• Improvement in average Uplink
FTP throughput +31%
11
© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
Uplink Throughput Performance
• RLC uplink throughput figures along
different parts of the route:
RX-level during this part of the route: -85 – -105 dBm
Original link adaptation
RX-level during this part of the route: -70 – -88 dBm
Original link adaptation
RX-level
RX-level
Optimized link adaptation
Optimized link adaptation
RX-level
RX-level
-> Reason for improvement:
•Earlier change to GMSK modulation
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
-> Reason for improvement:
•Using more MCS-8 instead of MCS-9
Conclusions
• The results achieved in the thesis are well in accordance with the original
objectives
• Performance improvement of optimized link adaptation was clearly seen in
laboratory and live network
• The optimized link adaptation version created in this thesis is available for the
customers in newest BSC software update in 1/2007
• Further Study Items
• Measurements with various MSs from different vendors
• Effect of certain GSM features (such as frequency hopping) on Link Adaptation
• Analysis in commercial operators’ networks by using BSC KPI (Key Performance
Indicator) data
• Link adaptation algorithm enhancement by introducing new algorithm inputs
• This would optimize modulation change with all MSs also on static conditions
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com
Thank You!
Questions / Comments ?
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© 2007 Nokia
2007-01-16
jussi.nervola@nokia.com