1. ECE 4730: Lecture #4 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

2. ECE 4730: Lecture #4 2 Illustration of Handoff at Cell Boundary P HT P MUS  P HT P MUS

3. ECE 4730: Lecture #4 3 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    = P HT  P MUS  Carefully selected   too large  unnecessary handoff  MSC loaded down   too small  not enough time to transfer  call dropped!  Fig. 3.3, pg. 63 Note

4. 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

5. ECE 4730: Lecture #4 5 Mobile Unit Handoff  1 st 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

6. ECE 4730: Lecture #4 6 Mobile Unit Handoff  2 nd 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

7. 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 > P HT  handoff not initiated but adjacent cell has better signal!!

8. 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

9. 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 a 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

10. 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

11. 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

12. ECE 4730: Lecture #4 12 Umbrella Cell

13. 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

14. 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

15. 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

16. 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 ***

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

18. 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)