1 © 2006 Nokia pullola_ ppt / Extending Base Station Active Radio Link Set for Improved Uplink Scheduling Esa-Pekka Pullola Supervisor: Professor Raimo Kantola

2 © 2006 Nokia pullola_ ppt / Contents of the presentation Background UMTS basics The problem Test environment and simulation tool application RAKE receiver modification Test cases and results Summary

3 © 2006 Nokia pullola_ ppt / 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

4 © 2006 Nokia pullola_ ppt / Universal Mobile Telecommunications System (UMTS) basics RNC UMTS Terrestrial Radio Access Network RNC Node B User Equipment Core Network Circuit switched domain Packet switched domain

5 © 2006 Nokia pullola_ ppt / 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

6 © 2006 Nokia pullola_ ppt / 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 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 CPICH 1 CPICH 2 CPICH 3 Handover margin Handover margin ∆t Add Cell 2Remove Cell 1Add Cell 3 Time Measurement Quantity Cell 1 Cell 2 Cell 3 Cell 1 Cell 2 Cell 3 Cell 1 Cell 2 Cell 3 Cell 1 Cell 2 Cell 3

7 © 2006 Nokia pullola_ ppt / 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

8 © 2006 Nokia pullola_ ppt / 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

9 © 2006 Nokia pullola_ ppt / Test environment Signal generator Power splitter Filter TR Hub Signal processor Application Manager Attenuators BTS Signal generator Static user Mobile user Filter

10 © 2006 Nokia pullola_ ppt / Test environment Four sector configuration Radio channel phenomena not considered in this environment Static user Mobile user Sector 1 Sector 2 Sector 3 Sector 4

11 © 2006 Nokia pullola_ ppt / 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

12 © 2006 Nokia pullola_ ppt / 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

13 © 2006 Nokia pullola_ ppt / Simulation tool Softer handover procedure Predetermined locations for radio link updates 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 Radio link 1 0o0o 90 o 180 o 270 o Set up radio link 2 Delete radio link 1 Set up radio link 3 Delete radio link 2

14 © 2006 Nokia pullola_ ppt / Simulation tool 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 Bit rate control procedure The bit rate changes for the moving user are simulated by increasing the SIR target SIR target is lower when the user enters cell two

15 © 2006 Nokia pullola_ ppt / 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

16 © 2006 Nokia pullola_ ppt / RAKE receiver modification Radio link setup True False Check radio link message Handle RNC radio link command Radio link deletion RNC radio link command handled Number of active radio links = max number of radio links Delete weakest radio link Set up radio link according to setup message Send acknowledgement to RNC False True

17 © 2006 Nokia pullola_ ppt / Simulation test cases Circular movement 0o0o 90 o 180 o 270 o 0o0o 90 o 180 o 270 o 0o0o 90 o 180 o 270 o Passing by BTS Passing through loaded cell

18 © 2006 Nokia pullola_ ppt / The circular motion test case 0o0o 90 o 180 o 270 o

19 © 2006 Nokia pullola_ ppt / The passing by BTS test case Border between sectors 4 and 1 Softer handover 0o0o 90 o 180 o 270 o

20 © 2006 Nokia pullola_ ppt / The passing through loaded cell test case Border between sectors 3 and 2 Softer handover Border between sectors 2 and 1 Softer handover 0o0o 90 o 180 o 270 o

21 © 2006 Nokia pullola_ ppt / Summary The correct operation of the modified radio link control procedure could be verified Test cases showed a general decrease of % 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

22 © 2006 Nokia pullola_ ppt / 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