Extending Base Station Active Radio Link
Set for Improved Uplink Scheduling
Esa-Pekka Pullola
Supervisor: Professor Raimo Kantola
1
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Contents of the presentation
• Background
• UMTS basics
• The problem
• Test environment and simulation tool application
• RAKE receiver modification
• Test cases and results
• Summary
2
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Background
• The thesis has been carried out at Nokia Networks, WCDMA Radio Access
Network business unit in Espoo, Finland
• The research concentrated on the performance of the RAKE receiver of the Base
Transceiver Station (BTS)
• The main objective of this work was to design and implement a modified radio
link control procedure for the RAKE receiver
• The new procedure was implemented in RAKE DSP software
• In addition, the creation of an appropriate test environment and an application for
controlling the environment was required
• This research handled the behavior of only one base station of the third
generation mobile network
3
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Universal Mobile Telecommunications System (UMTS)
basics
User Equipment
UMTS Terrestrial Radio Access Network
Core Network
Circuit switched domain
Node B
RNC
Node B
Packet switched domain
Node B
RNC
Node B
Node B
4
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
UMTS basics
• The radio access technology used in
UMTS is Wideband Code Division
Multiple Access (WCDMA)
• Users are separated by individual
codes
• Even though users transmit at the
same frequency channel, other
transmissions appear only as
background noise
• As the number of transmissions
and/or bit rates increase, also the
interference in the air interface
increases
• Efficient power control is essential for
avoiding excessive interference in the
transmission medium
5
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
UMTS basics
• An essential feature
of the UMTS
network is the
capability to perform
soft and softer
handovers in
addition to hard
handovers
• Soft and softer
handover procedure
Measurement
• user equipment
measures the
pilot channel and
sends reports to
RNC
• RNC makes
handover
decisions and
commands the
base station to
perform radio link
updates
6
© 2006 Nokia
∆t
Quantity
pullola_101006.ppt / 2006-10-10
∆t
∆t
CPICH 1
Handover
Handover
margin
margin
CPICH 3
CPICH 2
Time
Add Cell 2
Cell 1
Cell 1
Cell 3
Add Cell 3
Cell 2
Cell 2
Cell 2
Cell 1
Remove Cell 1
Cell 3
Cell 2
Cell 1
Cell 3
Cell 3
The problem
• A single, high bit rate user, interferes other users in the adjacent cell
• Handover is not yet performed
• Radio Network Controller (RNC) not able to see how the granted transmission
capacity for one user effects the users in another cell
• As a result, the transmission rates of the users in the other cell might have to be
decreased
7
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Test environment
• The purpose of the test environment was to simulate user movement in the base
station coverage area
• Creation of movement was based on performing softer handovers for the moving
user and controlling the transmitted power to the different cells of the base station
according to the location of the moving user
• The moving and the stationary users were simulated by individual signal
generators
• The amount of power of the moving user received at the base station antennas
was handled with remote controlled attenuators
• The radio link setups and deletions and the control of the attenuators were
operated by a simulation tool created specifically for this work
• Results used for the performance analysis were collected by another application
normally used in base station research and development
• The performance analysis is based on the bit error ratio calculated by the RAKE
receiver
8
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Test environment
Attenuators
Signal generator
BTS
Mobile user
Filter Filter Filter Filter
4
Power
splitter
3
2
TR
TR
TR
1
TR
Signal generator
Application
Manager
Signal
processor
9
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Static user
Hub
Test environment
• Four sector configuration
• Radio channel phenomena not considered in this environment
4
1
Mobile user
Sector 4
Sector 1
Sector 3
Sector 2
3
10
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
2
Static user
Simulation tool
• The simulation tool used for controlling the simulations was implemented with
Visual Basic
• Simulation tool supports three and four sector configurations
• Works as a replacement for RNC in this test environment
• Purpose of the testing tool
• provide visual information on the progress of the simulation
• update the location of the moving user according to movement model
• perform radio link setups and deletions
• control the operation of the attenuators
• receive acknowledgements and measurement results from the base station
• perform simple power control for the signal generator of the moving user
• operate a transmission rate granting procedure for the moving user
11
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Simulation tool
• User interface of the
simulation tool
application
• Connect to simulation
environment equipment
• Set simulation
parameters and select
features
• Displays information
• sent attenuation
commands
• power control
commands
• user location
• measurement results
• active radio links
12
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Simulation tool
90o
• Softer handover procedure
• Predetermined locations for radio
link updates
Radio link 1
Set up radio link 2
180o
0o
Delete radio link 1
Set up radio link 3
• Power control procedure
• Based on signal to interference
(SIR) measurements performed by
the base station
• The transmission power of the
moving user’s signal generator is
decreased or increased in order to
reach the target SIR level which has
been set to it
13
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Delete radio link 2
270o
Horizontal
angle
Simulation tool
0
1
2
3
4
5
6
7
8
9
10
• Attenuator control procedure
• Individual attenuation values
are calculated for each
attenuator using an antenna
pattern equation
• According to the user location,
the testing tool reads the
appropriate values and sends
commands to the attenuators
Attenuator 1 Attenuator 2 Attenuator 3 Attenuator 4
-5.9
-5.8
-5.6
-5.5
-5.4
-5.3
-5.2
-5.0
-4.9
-4.8
-4.7
-3.0
-3.1
-3.3
-3.4
-3.6
-3.7
-3.9
-4.0
-4.2
-4.3
-4.5
-23.9
-23.9
-23.9
-23.9
-23.9
-23.9
-23.9
-23.9
-23.9
-23.9
-23.9
-23.0
-23.0
-23.4
-23.4
-23.4
-23.4
-23.4
-23.4
-23.4
-23.4
-23.4
4 sectors
-3
• Bit rate control procedure
• SIR target is lower when the
user enters cell two
-8
Gain (dB)
• The bit rate changes for the
moving user are simulated by
increasing the SIR target
-13
-18
-23
0
45
90
135
180
225
270
315
Horizontal angle (degrees)
Sector 1 antenna
14
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Sector 2 antenna
Sector 3 antenna
Sector 4 antenna
RAKE receiver modification
• In this work the radio link control procedure of the RAKE receiver is modified
• The basic idea behind the modification is to leave old radio links on even though
according to the softer handover procedure they should be released
• As a result, more power of the user in question is received at the base station and the
transmission is received through several paths which is advantageous for diversity
reception
• Only the RAKE receiver is aware of the undeleted radio links and the radio link
controller is normally acknowledged of performed radio link deletions
• Due to the limited maximum number of three radio links the radio link control
procedure has to be able to decide which old radio link to delete if a cell to which
a connection is not currently maintained is entered
• The deletion decision is based on the strength of the impulse response (IR)
measurement performed for each of the active radio links
• The radio link to be deleted is chosen from the radio links that the RNC has deleted,
i.e. the radio links that only the RAKE receiver is aware of
15
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
RAKE receiver modification
Handle RNC
radio link
command
Check radio link
message
True
Radio link setup
Number of active radio links =
True
max number of radio links
False
False
Set up radio link
according to setup
message
True
Radio link deletion
False
RNC radio link
command
handled
16
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Send
acknowledgement
to RNC
Delete weakest
radio link
Simulation test cases
90o
90o
• Circular movement
• Passing by BTS
180o
0o
180o
270o
270o
90o
• Passing through loaded cell
180o
0o
270o
17
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
0o
90o
The circular motion test case
180o
0o
Circular motion - stationary user
17%
270o
16%
BER
15%
14%
13%
12%
11%
0
360
720
1080
Passed degrees
Original SW
18
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Modified SW
1440
90o
The passing by BTS test case
180o
0o
Passing by BTS - stationary user
14.0%
270o
13.5%
Border between
sectors 4 and 1
BER
13.0%
12.5%
12.0%
Softer
11.5%
handover
11.0%
0
50
100
150
200
250
300
Passed distance (m)
Original SW
19
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Modified SW
350
400
450
90o
The passing through loaded cell test case
180o
0o
Passing through loaded cell - stationary user
20%
19%
18%
BER
17%
Softer
Softer
handover
handover
Border between
Border between
sectors 3 and 2
sectors 2 and 1
16%
15%
14%
13%
12%
0
50
100
150
200
250
300
Passed distance (m)
Original SW
20
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
Modified SW
350
400
450
270o
Summary
• The correct operation of the modified radio link control procedure could be
verified
• Test cases showed a general decrease of 0 - 0.5 % in bit error ratio
• the amount of obtained performance enhancement is highly dependent on the test
environment, i.e. the precalculated attenuation values
• Clear improvement was noticed when the moving user left the cell where the
stationary user was located
• For future development, the radio link control procedure could be allowed to set
up radio links experimentally in advance and thereby avoid interference problems
beforehand
• Radio propagation phenomena could be taken into account more carefully
because in the current test environment they have not been considered in great
detail
21
© 2006 Nokia
pullola_101006.ppt / 2006-10-10
RAKE receiver modification
• Advantages of the RAKE receiver modification
• More of the power transmitted by the moving user is received at the base station
(depends on environment, received at every antenna)
• In some case, the interfering, moving user stays in the sector-wise scheduling area
and it is not allowed to increase its transmission rate
• Additional assistance in fighting the pingpong effect during which the mobile
continuously switches between cells due to varying conditions in the air interface
• Implementable with current base station equipment
• Disadvantages
• Increases complexity in resource management (only one BTS considered now)
• Consumes base station resources
22
© 2006 Nokia
pullola_101006.ppt / 2006-10-10