无线通信 Lecture 4 Cellular Cont.

Lecture 4: Cellular Fundamentals
Chapter 3 - Continued
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I. Adjacent Channel Interference
Two major types of system-generated interference:
1) Co-Channel Interference (CCI) – discussed in last lecture 2) Adjacent Channel Interference (ACI)
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maximize channel separation
separation of as many as N channel bandwidths some schemes also seek to minimize ACI from neighboring cells by not assigning adjacent channels in neighboring cells
Control of ACI
don’t allocate channels within a given cell from a contiguous band of frequencies
for example, use channels 1, 4, 7, and 10 for a cell. no channels next to each other
Provider A is a company that has not traditionally provided telephone service Provider B is a traditional wireline operator 21 VC groups with ≈ 19 channels/group
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III. Trunking & Grade of Service (GOS)
Trunked radio system: radio system where a large # of users share a pool of channels
channel allocated on demand & returned to channel pool upon call termination exploit statistical (random) behavior of users so that fixed # of channels can accommodate large # of users
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dramatically improves adjacent channel S / I ratio, since mobiles in other cells only transmit at high enough power as transmitter controls (not at full power) most beneficial for ACI on reverse link will see later that this is especially important for CDMA systems
system with U users → A = UAu = UλH Erlangs capacity = maximum carried traffic = C Erlangs = (equal to total # of available channels that are busy all the time)
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trunking theory is used by telephone companies to allocate limited # of voice circuits for large # of telephone lines efficient use of equipment resources → savings disadvantage is that some probability exists that mobile user will be denied accol
technique to minimize ACI base station & MSC constantly monitor mobile received signal strength mobile Tx power varied (controlled) so that smallest Tx power necessary for a quality reverse link signal is used (lower power for the closer the mobile is to the base station) also helps battery life on mobile
at least 21 channel separation for each group
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for N = 7 → 3 VC groups/cell
For example, choose groups 1A, 1B, and 1C for a cell – so channels 1, 8, 15, 22, 29, 36, etc. are used. ∴ ≈ 57 channels/cell at least 7 channel separation for each cell group
actual filter response
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This affects both forward & reverse links Forward Link → base-to-mobile
interference @ mobile Rx from a ______ Tx (another mobile or another base station that is not the one the mobile is listening to) when mobile Rx is ___ away from base station. signal from base station is weak and others are somewhat strong.
λ = traffic intensity (average arrival rate of new calls, in new requests per time unit, say calls/min). H = average duration of a call (also called 1/ µ in queueing theory)
to have high quality on control channels, 21 cell reuse is used for CC’s
instead of reusing a CC every 7 cells, as for VC’s, reuse every 21 cells (after every three clusters) greater distance between control channels, so less CCI
Trade-off between the number of available channels that are provided and the likelihood of a particular user finding no channels available during the busy hour of the day.
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GOS : measure of the ability of user access to a trunked system during the _______ hour
specified as probability (Pr) that call is blocked or delayed designed to handle the busiest hour → typically ______ Erlang : unitless measure of traffic intensity
Adjacent Channel Interference (ACI)
Imperfect Rx filters allow energy from adjacent channels to leak into the passband of other channels
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desired filter response
Reverse Link → mobile-to-base
interference @ base station Rx from nearby mobile Tx when desired mobile Tx is far away from base station
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Near/Far Effect
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Originally 666 channels, then 10 MHz of spectrum was added 666+166 = 832 channels 395 VC plus 21 CC per service provider (providers A & B)
395*2 = 790, plus 42 control channels
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use high quality filters in base stations
better filters are possible in base stations since they are not constrained by physical size and power as much as in the mobile Rx makes reverse link ACI less of a concern than forward link ACI