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Hard Handoff Scheme Exploiting Uplink and Downlink Signals in IEEE 802.16e Systems

Hard Handoff Scheme Exploiting Uplink and Downlink Signals in IEEE 802.16e Systems. Sunghyun Cho, Jonghyung Kwun, Chihyun Park, Jung-Hoon Cheon, Ok-Seon Lee, Kiho Kim Vehicular Technology Conference, 2006. VTC 2006-Spring. Zhang Hao Ming. Outline. Introduction

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Hard Handoff Scheme Exploiting Uplink and Downlink Signals in IEEE 802.16e Systems

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  1. Hard Handoff Scheme Exploiting Uplink and Downlink Signals in IEEE 802.16e Systems Sunghyun Cho, Jonghyung Kwun, Chihyun Park, Jung-Hoon Cheon, Ok-Seon Lee, Kiho Kim Vehicular Technology Conference, 2006. VTC 2006-Spring. Zhang Hao Ming

  2. Outline • Introduction • Handoff Schemes in IEEE 802.16e • The Proposed Hard Handoff Scheme • Performance and Simulation • Conclusion

  3. Introduction • In the conventional handoff schemes, only downlink signal is used to determine handoff initiation and execution. • In the next generation, handoff schemes should consider not only the QoS (Quality of Service) of voice service but also QoS of data services.

  4. Introduction • Combine uplink and downlink signal levels to determine handoff direction. • While measure the downlink signal, the scanning period will interruption the data transmission. • Using the joint hysteresis to reduce the outage probability.

  5. Handoff Schemes in IEEE 802.16e • Break-Before-Make (BBM) handoff • Traditional Hard Handoff • Make-Before-Break (MBB) handoff • Marco Diversity Handoff (MDHO) • Fast Base Station Switching (FBSS)

  6. Hard Handoff • A default mode in IEEE 802.16e. • Messages are defined in IEEE802.16e standard to reduce handoff delay. • Mobile Station (MS) can reduce scanning time by using MOB_NBR-ADV (neighbor advertisement message) message. • MS acquires downlink timing synchronization and estimates downlink signal quality by scanning period.

  7. Hard Handoff (cont.) • After scanning, begin with handoff procedure. • Cell reselection • Handoff decision and initiation • May originate either MS or Serving BS • Synchronization to Target BS downlink • Ranging • Termination of MS Context

  8. MDHO and FBSS • Two optional handoff processes in IEEE 802.16e. • It is necessary to maintain diversity set and anchor BS. • Diversity set: a list of active base stations. • Anchor BS: a node where mobile stations are registered and synchronized.

  9. MDHO • MS communicates with all BSs in the diversity set. • BS supporting MDHO shall broadcast the DCD message with H_add and H_delete threshold. • Mean CINR < H_Delete threshold：delete the BS from the diversity set. • Mean CINR > H_Add threshold：add the BS to the diversity set. • CINR：Carrier-to-Interference-and-Noise Ratio

  10. FBSS • MS communicates only with anchor BS. • To support FBSS, BS in an active set： • Use same frequency • Correspond to the time and frame synchronization. • BS in an active set share the same user contexts. • Perform FBSS: • FBSS handoff decision and initiation • Active set selection and update • Anchor BS selection and update

  11. Four Conditions of Proposed Scheme • Only consider the MS which have uplink and downlink data simultaneously. • During handoff process, Serving BS can monitor uplink traffic signal at any time. • Serving BS determines handoff initiation time and direction based on both of the signals. • MS reports downlink signal measurement result to BS to use downlink signal in the BS.

  12. Determine Thresholds • The thresholds are used to determine handoff initiation and direction • TH1: handoff initiation using uplink signal level • TH2: handoff initiation for Adaptive Modulation & Coding (AMC) users • TH3: handoff direction using uplink signal level (UL Hysteresis ) • TH4: minimum downlink signal level to prevent call drop • TH5: handoff direction using downlink signal level (DL Hysteresis ) • TH6: minimum uplink signal level to prevent call drop

  13. Adaptive Modulation & Coding According to channel condition, modulate adaptive modulation and coding way in a short time, it can increase Throughput.

  14. Proposed Scheme • Mobile terminal must adjust its time and frequency same with neighbor BSs. • Handoff initiation when uplink signal strength or SINR (Signal-to-Interference & Noise Ratio) is less than TH1→Uplink signal level. (by serving BS) • It can prevent MSs monitoring and reporting downlink signal at non-handoff region.

  15. AMC User How to Trigger Handoff Process? • Because of selective fading, channel quality of the subcarriers in use can dramatically become worse. • Users should be allocated another frequency band and a handoff process is unnecessary. • The serving BS triggers handoff process only if |Paverage-Pused| < TH2 →handoff initiation for the AMC users • Paverage：average uplink received signal power of unusedfrequency bands. • Pused：average uplink received signal power of used frequency bands.

  16. How to Determine Handoff Direction? • Once the handoff process is triggered based on uplink signal: • The serving BS send NBR_MOR_REQ messages to the neighbor BSs. • The neighbor BSs monitor the uplink signals of the relevant MS. • BSs report measurement results to the serving BS. • It also can prevent ping-pong and outage probability.

  17. Use the Advantage of Downlink • Serving BS sends DL_MEASUREMENT_REQ message to MS after handoff initiation. • MS reporting downlink signal strength of all BSs. • Serving BS then determines handoff execution and direction.

  18. Determine Handoff Execution and Direction • PDL_target：downlink signal strengths in a target BS • PDL_serving：downlink signal strengths in a serving BS • PUL_target：uplink signal strengths in a target BS • PUL_serving：uplink signal strengths in a serving BS →handoff direction using uplink signal level →minimum downlink signal level to prevent call drop →handoff direction using downlink signal level →minimum uplink signal level to prevent call drop

  19. Handoff Process in a Serving Base Station

  20. Performance Evaluations • Evaluated average number of handoff and the outage probability during handoff. • Compare with mobile assisted handoff. • Consider multi-cell environment in TDD OFDMA system.

  21. Simulation parameters • Absolute threshold：critical value to initiate handoff process. • Hysteresis is used to reduce ping-pong. • The outage probability can be reduce because of symmetry between uplink and downlink channel.

  22. Outage Probability • DL/UL Outage Threshold = -10dB • Joint Hysteresis = -3dB

  23. Average Number of Handoff • DL/UL Outage Threshold = -10dB • Joint Hysteresis = -3dB

  24. The Correlation between the Average Number of Handoff and the Outage Probability • DL/UL Outage Threshold = -10dB • Joint Hysteresis = -3dB

  25. Conclusion • Proposed a handoff scheme which exploits not only downlink signal but also uplink signal. • Overcome the uplink channel quality which can dramatically grow worse during handoff process. • With joint hysteresis, the proposed scheme reduces outage probability approximately 10%.

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