1 / 18

Mobile Unit Handoff

Mobile Unit Handoff. Handoff : when a mobile unit moves from one cell to another while a call is in progress, the MSC must transfer (handoff) the call to a new channel belonging to new base station New voice and control channel frequencies

malana
Download Presentation

Mobile Unit Handoff

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Mobile Unit Handoff • Handoff : when a mobile unit moves from one cell to another while a call is in progress, the MSC must transfer (handoff) the call to a new channel belonging to new base station • New voice and control channel frequencies • Very important task  often given higher priority than new call!! • Which is more annoying to customers? • Dropping call during middle of conversation • Blocking a new call request ECE 4730: Lecture #4

  2. Mobile Unit Handoff • Minimum Useable Signal (MUS) • Lowest acceptable voice quality • Specified by system designers • Typical values 100 dBm = 0.1 pW! • Handoff Threshold (HT) > MUS Level • Handoff Margin  •  = PHTPMUS • Carefully selected •  too large  unnecessary handoff  MSC loaded down •  too small  not enough time to transfer  call dropped! • Fig. 3.3, pg. 63 Note ECE 4730: Lecture #4

  3. PHT  PMUS PHT PMUS Illustration of Handoff at Cell Boundary ECE 4730: Lecture #4

  4. Mobile Unit Handoff • Dropped Call Occurrence – 3 typical causes • Excessive delay by MSC in assigning handoff • High traffic times and  computational load on MSC • No channels available in new cell • Mobile unit moves into dead zone • Handoff Decision • Signal level decreasing due to • Momentary signal fading  do NOT handoff • Mobile moving away from base station • Must monitor RSS over a period of time  moving average • Time allowed to complete handoff depends on mobile speed • Large negative RSS slope  high speed  quick handoff • Statistics of small-scale (short time) fading signal important to making appropriate handoff decision  Chpt. 5 ECE 4730: Lecture #4

  5. Mobile Unit Handoff • 1st Generation (1G) Cellular (Analog FM  AMPS) • RSS of RVC measured at base station & monitored by MSC • Another Rx in base station (locator Rx) monitors RSS of RVC in adjacent cells (controlled by MSC) • MSC monitors RSS from all base stations & decides on handoff • Call handoff took 12 sec  very noticeable to user • Break in conversation and “white noise” before new channel established in new cell ECE 4730: Lecture #4

  6. Mobile Unit Handoff • 2nd Generation (2G) Cellular w/ digital TDMA (USDC, GSM, IS136) • Mobile Assisted HandOffs (MAHO) • Important advancement • Mobile measures RSS of FCC from multiple adjacent base stations & reports back to serving base station • If Rx power from new base station > Rx power from serving (current) base station by pre-determined margin for certain time period  handoff initiated by MSC • MSC makes handoff decision but is assisted by mobile which reports RSS values from serving and neighboring cells ECE 4730: Lecture #4

  7. Mobile Unit Handoff • 2G MAHO • MSC no longer monitors RSS of all channels like in 1G • MSC computational load considerably reduced • Enables rapid (~ 20 msec) and efficient handoffs • Imperceptible to user • Improves call quality as well • RSS from adjacent cell may be much better than serving cell and RSS > PHT  handoff not initiated but adjacent cell has better signal!! ECE 4730: Lecture #4

  8. Mobile Unit Handoff • Prioritizing Handoffs • Perceived Quality of Service (QoS) • Assign higher priority to handoff vs. new call request • Dropped call more aggravating than occasional blocked call • Guard Channels • % of total available cell channels exclusively set aside for handoff requests • Fewer channels available for new call requests!! • Good strategy for dynamic channel allocation (not fixed) • Guard channels only allocated/used in needed cells (high traffic) • Not “wasted” on cells with low traffic ECE 4730: Lecture #4

  9. Mobile Unit Handoff • Prioritizing Handoffs (continued) • Queuing Handoff Requests • Use time delay between handoff threshold and MUS level to place handoff request in queue • Prioritize requests (mobile speed) and handoff as needed • High speed mobile user  rapid variation of RSS  strong negative slope of RSS when leaving cell  short time duration before MUS level crossed  high priority handoff • Low speed mobile user can be placed lower in queue priority even though handoff threshold crossed before another high speed user ECE 4730: Lecture #4

  10. Mobile Unit Handoff • Practical Handoff Considerations • Problems occur b/c of large range of mobile velocities • Pedestrian vs. vehicle user • Small cell sizes and/or micro-cells  larger # handoffs • MSC load is heavy when high speed users are passing between very small cells • # handoffs/time increases ECE 4730: Lecture #4

  11. Mobile Unit Handoff • Practical Handoff Considerations • Umbrella Cells • Use different antenna heights and Tx power levels to provide large and small cell coverage • Multiple antennas & Tx can be co-located at single location if necessary • Large cell  high speed traffic  fewer handoffs • Small cell  low speed traffic • Example areas: interstate highway passing thru urban center, office park, or nearby shopping mall • Fig. 3.4, pg. 67 ECE 4730: Lecture #4

  12. Umbrella Cell ECE 4730: Lecture #4

  13. Handoff Parameters • Typical 1G Analog Cellular parameters • Threshold margin  6 to 12 dB • Total time to complete handoff  8 to 10 sec • Transfer time  1 to 2 sec ECE 4730: Lecture #4

  14. Handoff Parameters • Typical 2G Digital Cellular parameters • Threshold margin  2 to 6 dB • Total time to complete handoff  1 to 2 sec • Transfer time  20 msec • Small handoff time • Greater flexibility in handling high/low speed users • Queuing handoffs & prioritizing • More time to “rescue” calls needing urgent handoff • Fewer dropped calls  QoS increased ECE 4730: Lecture #4

  15. Mobile Unit Handoff • Soft vs. Hard Handoffs • Hard handoff different radio channels assigned when moving from cell to cell • All 1G analog (AMPS) & 2G digital TDMA systems (USDC, GSM, IS-136, etc.) • Spread spectrum users share the same channel in every cell • CDMA  IS95, JSTD8 (Verizon Wireless, Sprint PCS) • MSC simultaneously monitors reverse link signal at several base stations • Soft Handoff  MSC chooses best signal and passes on to PSTN • Advantage unique to CDMA systems ECE 4730: Lecture #4

  16. Cellular Interference • Interfering Sources: • Another mobile in same cell (adjacent frequency channel) • Base stations operating at same frequencies (co-channel) • Other cellular carriers transmitting out-of-band • Non-linear Tx products  intermodulation • More severe in urban areas with many service providers • Other microwave, electronic, and wireless devices *** Interference is the limiting factor in performance of all cellular radio systems *** ECE 4730: Lecture #4

  17. Cellular Interference • Voice Channel Interference • Cross “talk”  data interference • Lower voice quality • Dropped calls • Control Channel Interference • Missed, blocked, and dropped calls • Reduced system capacity ECE 4730: Lecture #4

  18. Cellular Interference • Most interference generated by same system • Cannot be completely controlled due to random propagation effects of radio signals • Two major types of system-generated interference : 1) Co-Channel Interference (CCI) 2) Adjacent Channel Interference (ACI) ECE 4730: Lecture #4

More Related