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

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 conversationBlocking a new call request


Illustration ofHandoffat Cell

Boundary

PHT

PMUS

PHT

PMUS


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

mW1log10dBm

P

Note


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


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 12 sec very noticeable to user

» Break in conversation and “white noise” before new channel established in new cell


Mobile Unit Handoff

2nd Generation (2G) Cellular w/ digital TDMA (USDC, GSM, IS136) 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


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!!


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


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


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


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


Umbrella Cell


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


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


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 IS95, JSTD8 (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


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 ***



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



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)

Documents

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