The Cellular Concept - System Design Fundamental

7/30/2019 The Cellular Concept - System Design Fundamental

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Handoff Decisionsignal level decreases due to

signal fading dont handoff mobile moving away from base station handoff

must monitor received signal strength over a periodof time moving averagetime allowed to complete handoff depends onmobile speed

large negative received signal strength (RSS) slope high speed quick handoff

statistics of the fading signal are important tomaking appropriate handoff decisions Chapters4 and 5


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1st Generation Cellular (Analog FM AMPS)Received signal strength (RSS) of RVC measuredat base station & monitored by MSCA spare Rx in base station (locator Rx) monitors

RSS of RVC's in neighboring cellsTells Mobile Switching Center about these mobiles andtheir channels

Locator Rx can see if signal to this base station is

significantly better than to the host base stationMSC monitors RSS from all base stations &decides on handoff


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2nd Generation Cellular w/ digital TDMA (GSM,IS-136)

Mobile Assisted HandOffs (MAHO)important advancement

The mobile measures the RSS of the FCCs fromadjacent base stations & reports back to serving basestationif Rx power from new base station > Rx power fromserving (current) base station by pre-determinedmargin for a long enough time period handoff initiated by MSC


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MSC no longer monitors RSS of all channelsreduces computational load considerablyenables much more rapid and efficient handoffsimperceptible to user


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A mobile may move into a different systemcontrolled by a different MSC

Called an intersystem handoff What issues would be involved here?

Prioritizing HandoffsIssue: Perceived Grade of Service (GOS) servicequality as viewed by users

quality in terms of dropped or blocked calls (notvoice quality)assign higher priority to handoff vs. new call requesta dropped call is more aggravating than an occasional

blocked call


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Guard Channels% of total available cell channels exclusively setaside for handoff requestsmakes fewer channels available for new call

requestsa good strategy is dynamic channel allocation (notfixed)

adjust number of guard channels as needed by demand

so channels are not wasted in cells with low traffic


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Queuing Handoff Requestsuse time delay between handoff threshold andminimum useable signal level to place a blockedhandoff request in queue

a handoff request can "keep trying" during that time period, instead of having a single block/no block decision

prioritize requests (based on mobile speed) and

handoff as neededcalls will still be dropped if time period expires


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VI. Practical Handoff Considerations

Problems occur because of a large range of mobile velocities

pedestrian vs. vehicle user

Small cell sizes and/or micro-cells larger #handoffsMSC load is heavy when high speed users are

passed between very small cells


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Umbrella CellsFig. 3.4, pg. 67use different antenna heights and Tx power levels to

provide large and small cell coverage

multiple antennas & Tx can be co-located at singlelocation if necessary (saves on obtaining new tower licenses)

large cell high speed traffic fewer handoffs

small cell low speed trafficexample areas: interstate highway passing thruurban center, office park, or nearby shopping mall


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Cell Dragginglow speed user w/ line of sight to base station (very strongsignal)strong signal changing slowlyuser moves into the area of an adjacent cell without handoff

causes interference with adjacent cells and other cellsRemember: handoffs help all users, not just the one which ishanded off.

If this mobile is closer to a reused channel interferencefor the other user using the same frequencySo this mobile needs to hand off anyway, so other users

benefit because that mobile stays far away from them.


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Typical handoff parametersAnalog cellular (1st generation)threshold margin 6 to 12 dBtotal time to complete handoff 8 to 10 sec

Digital cellular (2nd generation)total time to complete handoff 1 to 2 seclower necessary threshold margin 0 to 6 dBenabled by mobile assisted handoff


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benefits of small handoff time

greater flexibility in handling high/low speedusersqueuing handoffs & prioritizing

more time to rescue calls needing urgenthandoff fewer dropped calls GOS increased

can make decisions based on a wide range of

metrics other than signal strengthsuch as also measure interference levelscan have a multidimensional algorithm for making decisions


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Soft vs. Hard HandoffsHard handoff: different radio channels assignedwhen moving from cell to cell

all analog (AMPS) & digital TDMA systems (IS-136,GSM, etc.)

Many spread spectrum users share the samefrequency in every cell

CDMA IS-95Since a mobile uses the same frequency in every cell, it

can also be assigned the same code for multiple cellswhen it is near the boundary of multiple cells.The MSC simultaneously monitors reverse link signalat several base stations


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MSC dynamically decides which signal is bestand then listens to that one

Soft Handoff passes data from that base station on to the PSTN

This choice of best signal can keep changing.Mobile user does nothing for handoffs except

just transmit, MSC does all the work

Advantage unique to CDMA systemsAs long as there are enough codes available.


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VII. Co-Channel Interference

Interference is the limiting factor in performance of all cellular radio systemsWhat are the sources of interference for amobile receiver?

Interference is in bothvoice channelscontrol channels

Two major types of system-generatedinterference:1) Co-Channel Interference (CCI)2) Adjacent Channel Interference (ACI)


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First we look at CCIFrequency Reuse

Many cells in a given coverage area use the sameset of channel frequencies to increase system

capacity ( C )Co-channel cells cells that share the same set of frequenciesVC & CC traffic in co-channel cells is aninterfering source to mobiles in Several differentcells


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Possible Solutions?1) Increase base station Tx power to improve radio

signal reception? __ this will also increase interference from co-channelcells by the same amountno net improvement

2) Separate co-channel cells by some minimumdistance to provide sufficient isolation from

propagation of radio signals?if all cell sizes, transmit powers, and coverage patterns same co-channel interference is independent of Tx power


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co-channel interference depends on: R : cell radius D : distance to base station of nearest co-channel cell

if D / R then spatial separation relative to cellcoverage area improved isolation from co-channel RF energy

Q = D / R : co-channel reuse ratio

hexagonal cells Q = D/R = 3 N


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Fundamental tradeoff in cellular system design:

small Q small cluster size more frequencyreuse larger system capacity greatBut also: small Q small cell separation increased co-channel interference (CCI) reducedvoice quality not so greatTradeoff: Capacity vs. Voice Quality


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Signal to Interference ratio S / I, ____________

S : desired signal power I i : interference power from ith co-channel cellio : # of co-channel interfering cells


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Approximation with some assumptions

D i : distance from ith interferer to mobileRx power @ mobile ( ) ni D


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n : path loss exponent

free space or line of sight (LOS) (no obstruction) n = 2urban cellular n = 2 to 4, signal decays faster with distance away from the base stationhaving the same n throughout the coverage areameans radio propagation properties are roughly thesame everywhere

if base stations have equal Tx power and n is thesame throughout coverage area (not always true)then the above equation (Eq. 3.8) can be used.


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Now if we consider only the first layer (or tier)of co-channel cells

assume only these provide significant interference

And assume interfering base stations are

equidistant from the desired base station (all atdistance D) then


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What determines acceptable S / I ?voice quality subjective testingAMPS S / I 18 dB (assumes path loss exponentn = 4)

Solving (3.9) for N

Most reasonable assumption is io : # of co-channelinterfering cells = 6 N = 7 (very common choice for AMPS)


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Many assumptions involved in (3.9) :same Tx power hexagonal geometryn same throughout area

D i D (all interfering cells are equidistant from the base station receiver)optimistic result in many cases

propagation tools are used to calculate S / I whenassumptions arent valid


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S / I is usually the worst case when a mobile is at the

cell edgelow signal power from its own base station & highinterference power from other cellsmore accurate approximations are necessary in those cases

4

4 4 42( ) 2( ) 2

S R

I D R D R D


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N =7 and S / I 17 dB


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Eq. (3.5), (3.8), and (3.9) are ( S / I ) for forward link

only, i.e. the cochannel base Tx interfering withdesired base station transmission to mobile unit

so this considers interference @ the mobile unit

What about reverse link co-channel interference?less important because signals from mobile antennas (near the ground) dont propagate as well as those from tall basestation antennasobstructions near ground level significantly attenuate mobile

energy in direction of base station Rxalso weaker because mobile Tx power is variable basestations regulate transmit power of mobiles to be no larger than necessary


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HW1:1-9, 1-11, 1-18, 3-5, 3-7

The Cellular Concept - System Design Fundamental

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