Joona Vehanen Supervisor: Prof. Jyri Hämäläinen Instructor: M.Sc Markku Pellava

1. Handover between LTE to 3G Radio Access Technologies: Test measurement challenges and field environment test planning Joona Vehanen Supervisor: Prof. Jyri Hämäläinen Instructor: M.Sc Markku Pellava

2. Contents • Background • Goals and limitations of the thesis • Long Term Evolution of 3GPP • Mobility • System Verification and performance testing • Plan for Inter Radio Access Technology handover field performance testing • KPI measurements • Q & A

3. Background • LTE was developed to satisfy the need for increased mobile data usage • LTE provides higher data rates, reduced latencies and cost efficient operations • Initial rollout based on regioinal service hot spot networks in major cities • To provide an anywhere anytime type of mobile service, seamless and uninterrupted mobility across radio access technologies is required • Handover from LTE to 3G is seen as a high priority item • This feature needs to be developed by thorough functionality-, performance- and fault correction testing, preferably in realistic radio conditions

4. Goals and limitations of the thesis • The main goal of this thesis is to provide a test plan for Inter Radio Access Technology (I-RAT) handover performance measurements in a field environment • The field test network layout and configurations are presented • The necessary tools and methods for performing the measurements are introduced • The Key Performance Indicators (KPIs) of the measurement figures are described in detail • The original goal was to perform KPI measurements but due to e.g. limitations in the terminal equipment this was not possible at the time of writing • The secondary goal is to provide research documentation of the interworking procedures between LTE and legacy cellular networks • Little research work has been published in the field of I-RAT handovers so far • This thesis can be used for training purposes as an introductory material to new test engineers that are not familiar with I-RAT handovers or perhaps even with LTE • Theory part based on 3GPP specifications and white papers, practical part on interviews and discussions

5. Long Term Evolution of 3GPP (1/2) • Evolved Packet Core (EPC) • Flat ‘All-IP’ architecture • Packet data optimized • No CS core -> VoIP • Cost efficient operation • Seamless interoperability with legacy 3GPP cellular systems • Evolved Universal Terrestrial Radio Access Network (E-UTRAN) • OFDMA as downlink multiple access method provides spectral efficiency, while SC-FDMA in uplink minimizes terminal power consumption • Enhanced air interface techniques (MIMO, HARQ, etc.) • High data rates and low latencies

6. Long Term Evolution of 3GPP (2/2) • Spectrally efficient air interface physical layer • Evolved multiple antenna methods (Up to 2x2 MIMO in release 8) • Bandwidth scalable OFDMA as multiple access method in downlink • Adaptive modulation and Coding (up to 64QAM) • Enhanced medium access control layer procedures • Scheduling in both frequency and time domains per 1ms TTI • HARQ • Radio link Control and Packet Data Convergence Protocol • IP headers are compressed, and segmented packets are transported efficiently and securely over the air interface

7. Mobility (1/3) • Mobility is really the distinctive feature of cellular mobile networks • Anywhere anytime type of service provision • Seamless and uninterrupted service continuity during mobility • Various mobility scenarios according to user service requirements • Stringent delay requirements for active mode handovers • According to 3GPP, handover service interruption time should be <300ms for real time user traffic and <500ms for non-real time user data

8. Mobility (2/3) • Handovers in LTE are always network initiated • Intra-LTE, intra-frequency, inter-RAT, inter-technology handovers • Handover can be triggered by a measurement report sent by the UE or by a network decision due to e.g. service provision or network load coordination • Service provision or load control handovers are not possible in intra-frequency handovers • Handovers in LTE are always hard • Handovers are usually triggered by UE measurement reports • Neighbouring cell (Intra-LTE or inter-RAT) radio conditions (RSRP) are measured periodically • Triggering parameters (such as HO hysteresis, Time To Trigger) are set by the eNodeB • When the triggering conditions are satisfied the UE sends a measurement report. The eNodeB replies with a handover command, which initiates the handover procedure • I-RAT handover from LTE is usually an A2 coverage handover, which means that the triggering condition is defined merely by a lower limit of the measured LTE RSRP, for example -120dBm

9. Mobility (3/3) • I-RAT handover procedure is done in three distinctive phases • Handover preparation phase is started after the eNodeB receives a measurement report • Data flows towards the UE through P-GW, S-GW and eNodeB as usual • Resources are reserved in advance at the target RAT (Data forwarding tunnel setup, QoS bearer mapping etc.) • Handover execution phase is started when the eNodeB sends a handover command to the UE • Detach from old cell – complete radio connection switch to the new (3G) cell • UE data flow is interrupted. DL data is forwarded to the target RAN • Handover completion phase finalizes the handover procedure • Forwarded data packets are sent to the UE • Data path is updated. IP-mobility anchor remains at P-GW, and inter-RAT mobility anchor at S-GW, which tunnels the data to SGSN or in case of direct tunnelling, to the target RNC • Radio and mobility management resources are released in the source RAN (LTE)

10. System Verification and performance testing (1/2) • New software and hardware features go through a development process that consists of functionality, performance and fault correction testing • Knowledge of the technology standards and implementation features is essential for a test engineer so that the tests can be performed and analyzed correctly • Testing is done on several levels • Individual component functionality testing (System Component Testing) • Interworking with other network elements (Integration and Verification) • System level functional and performance testing in a laboratory environment (System Verification) • End-to-end testing in the field in real radio conditions (Field Verification) • Field Verification provides test results that are comparable to live operator networks • Some fault conditions may not be found in earlier test stages due to actual radio conditions

11. System Verification and performance testing (2/2) • I-RAT handover testing as well as LTE testing in general are challenging to the network equipment, test equipment as well as test engineers • Stringent performance and interworking requirements • High data rates, several RF bands, FDD and TDD modes of operation • Regardless of the simplified architecture the complexity of the devices continues to rise • Knowledge of both radio access technologies is required • Requirements set by 3GPP as well as individual vendor targets need to be satisfied • The test network is relatively small compared to live operator networks • Less interference – most of the base stations are at the ‘network edge’ • Geographically small area • Limited number of various fading profiles • Radio spectrum licences • Different technologies are generally tested by separate test teams • Coordination between test teams • Co-operation in test result analysis from each competence areas

12. Plan for Inter Radio Access Technology handover field performance testing (1/2)

13. Plan for Inter Radio Access Technology handover field performance testing (2/2) • The existing field test network can be utilized • No need to rollout an expensive dedicated I-RAT test network • Test coordination and competence sharing between test teams is required • The configuration is permanent from the network side • No time consuming base station configuration changes and resets • The UE can be configured to multimode or single mode operation according to testing needs • Handover locations and directions are well known • No unwanted ping-pong handovers • Measurements can be easily coordinated • Only a couple of configurable parameters on the network side • Neighbouring cell relations, A2 triggering thresholds, RAT prioritization • Core element interworking configurations (details are outside the scope)

14. KPI measurements (1/5) – Handover success rate • If a handover command is received but no handover completion -> handover failed • Failure can be caused by e.g. protocol errors, radio link layer failures etc. • Depending on the reason the service may be commenced after a short interruption • Straightforward indicator of overall handover performance • May have to be calculated manually in the first testing phase • Successful signalling flows / unsuccessful signalling flows with the XCAL tool • Base station KPI counters can be implemented as the level of automation is increase • Different traffic models need to be tested individually • TCP uplink and downlink, UDP uplink and downlink, HTTP, VoIP, traffic mixes HO failure Fault condition Successful HO

15. KPI measurements (2/5) – Call drop rate • Call drops can occur anywhere within the radio access network • Usually at the network edge before or right after a handover due to poor radio conditions • An interesting metric related to handover performance • Can be measured manually from e.g. XCAL signalling figures or automated BTS counters • From the user perspective call drop is usually similar to a handover failure • Call can be re-established after a short interruption as in the case of handover failure • For R&D purposes it is interesting to make a distinction between call drops and handover failures • Measurement report is not heard by the BTS or e.g. handover command not heard by the UE • Poor network dimensioning is the main cause to call drops • The optimal parameter set is not necessarily the one to minimize call drop rate • Short I-RAT measurement interval and short Time-To-Trigger may degrade performance and cause ping-pong handovers • It may not be meaningful to optimize a test network to a full extent

16. KPI measurements (3/5) – Cell throghput • Throughput is not exactly a handover related KPI but still interesting to measure • Verification of correct QCI-mapping after the I-RAT handover • Gives a quick overview of the network performance before and after the handover • Throughput degradation after a handover –scenarios have been found in earlier test cases • The impact of I-RAT neighbour cell measurement interval to throughput • Can be measured in real time or post processed after the testing • e.g. XCAL-tool or NetPerSec for real time monitoring • Post processing with MAC TTI-traces or BTS counters for average throughput • Further analysis and fault finding from base station internal logs and TTI-level traffic monitoring traces • Used traffic type and QCI-class need to be considered

17. KPI measurements (4/5) - Handover delay on control plane • Handover delay on UE control plane • Delay between handover command and handover confirm • The used traffic type should not be relevant • Can be calculated from the UE side from the received signalling message timestamps (XCAL) • Handover delay on network plane • Handover preparation phase delay included to the calculation • Can be longer than data interruption time since data is transferred normally during the preparation phase • Can be calculated from serving eNodeB timestamps

18. KPI measurements (5/5) - Handover delay on user plane • Handover delay on user plane • Time difference between the first received packet from target RAN and last packet from source RAN • Measured from the UE side with e.g. Wireshark • Used traffic model is significant (TCP acknowledged mode vs. UDP transparent mode) • Measurement process is manual and time consuming In this example IP-layer break can be calculated from the timestamps as follows: 240.13ms - 171.754 ms = 68.376 ms (only the millisecond part of the timestamp is considered). Wireshark packet capture from UE side highlighting measured intra-LTE X2 based handover interruption time

19. Conclusions • The introduced test plan provides the necessary tools and method for test execution • Simple to perform, yet complex enough for network testing and fault finding • Clearly defined test bed that utilizes the existing field test networks • Permanent from the network side • Coordination and competence sharing among test teams is needed • The plan can be developed according to any new testing needs that may arise • Automation and test complexity should be increased gradually • Testing is expected to be manual at the first stage • Automated test- and performance monitoring tools should be developed • The number of UEs in the test car and stationary UEs scattered around in the field network coverage area should be increased to add more complexity to the testing

20. Future work • Test development • Clear, simple and efficient test processes • Test automation • Other mobility features • Circuit Switched Fall Back (CSFB), Single Radio Voice Call Continuity (SRVCC) • Inter-RAT mobility towards 2G, Inter-technology mobility to WLAN, WiMaX, CDMA, etc. • Future 3GPP releases • Release 9 (Enhanced SON features) • Release 10 (LTE-Advanced: Bandwidths of up to 100MHz, enhanced MIMO operations etc.) • Backwards compatibility and mobility support to legacy systems

21. Q & A Any Questions?

22. Thank You