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Chapter 3
The Cellular Concept - System DesignFundamentals
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I. Introduction
Goals of a Cellular System High capacity Large coverage area Efficient use of limited spectrum
Large coverage area - Bell system in New York City had early mobile radio Single Tx, high power, and tall tower Low cost Large coverage area - Bell system in New York City had 12
simultaneous channels for 1000 square miles Small # users Poor spectrum utilization
What are possible ways we could increase the number of channels available in a cellular system?
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Cellular concept Frequency reuse pattern
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Cells labeled with the same letter use the same group of channels.
Cell Cluster: group of N cells using complete set of available channels
Many base stations, lower power, and shorter towers Small coverage areas called “cells” Each cell allocated a % of the total number of
available channels Nearby (adjacent) cells assigned different channel
groups to prevent interference between neighboring base
stations and mobile users
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Same frequency channels may be reused by cells a “reasonable” distance away reused many times as long as interference between same
channel (co-channel) cells is < acceptable level As frequency reuse↑ → # possible simultaneous
users↑→ # subscribers ↑→ but system cost ↑ (more towers)
To increase number of users without increasing radio frequency allocation, reduce cell sizes (more base stations) ↑→ # possible simultaneous users ↑
The cellular concept allows all mobiles to be manufactured to use the same set of freqencies
*** A fixed # of channels serves a large # of users by reusing channels in a coverage area ***
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II. Frequency Reuse/Planning
Design process of selecting & allocating channel groups of cellular base stations
Two competing/conflicting objectives:1) maximize frequency reuse in specified area
2) minimize interference between cells
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Cells base station antennas designed to cover specific cell
area hexagonal cell shape assumed for planning
simple model for easy analysis → circles leave gaps actual cell “footprint” is amorphous (no specific shape)
where Tx successfully serves mobile unit
base station location cell center → omni-directional antenna (360°
coverage) not necessarily in the exact center (can be up to R/4
from the ideal location)
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cell corners → sectored or directional antennas on 3 corners with 120° coverage. very commom Note that what is defined as a “corner” is
somewhat flexible → a sectored antenna covers 120° of a hexagonal cell.
So one can define a cell as having three antennas in the center or antennas at 3 corners.
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III. System Capacity
S : total # of duplex channels available for use in a given area; determined by: amount of allocated spectrum channel BW → modulation format and/or standard
specs. (e.g. AMPS)
k : number of channels for each cell (k < S) N : cluster size → # of cells forming cluster S = k N
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M : # of times a cluster is replicated over a geographic coverage area
System Capacity = Total # Duplex Channels = C
C = M S = M k N (assuming exactly MN cells will cover the area)
If cluster size (N) is reduced and the geographic area for each cell is kept constant: The geographic area covered by each cluster is smaller, so
M must ↑ to cover the entire coverage area (more clusters needed).
S remains constant. So C ↑ The smallest possible value of N is desirable to maximize
system capacity.
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Cluster size N determines: distance between co-channel cells (D) level of co-channel interference A mobile or base station can only tolerate so much
interference from other cells using the same frequency and maintain sufficient quality.
large N → large D → low interference → but small M and low C !
Tradeoff in quality and cluster size. The larger the capacity for a given geographic area,
the poorer the quality.
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Frequency reuse factor = 1 / N each frequency is reused every N cells each cell assigned k ≒ S / N
N cells/cluster connect without gaps specific values are required for hexagonal geometry
N = i2 + i j + j2 where i, j 1≧ Typical N values → 3, 4, 7, 12; (i, j) = (1,1), (2,0),
(2,1), (2,2)
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To find the nearest co-channel neighbors of a particular cell (1) Move i cells along any chain of hexagons, then (2)
turn 60 degrees and move j cells.
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IV. Channel Assignment Strategies
Goal is to minimize interference & maximize use of capacity lower interference allows smaller N to be used → greater
frequency reuse → larger C
Two main strategies: Fixed or Dynamic Fixed
each cell allocated a pre-determined set of voice channels calls within cell only served by unused cell channels all channels used → blocked call → no service
several variations MSC allows cell to borrow a VC (that is to say, a FVC/RVC
pair) from an adjacent cell donor cell must have an available VC to give
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Dynamic channels NOT allocated permanently call request → goes to serving base station → goes
to MSC MSC allocates channel “on the fly”
allocation strategy considers: likelihood of future call blocking in the cell reuse distance (interference potential with other cells
that are using the same frequency) channel frequency
All frequencies in a market are available to be used
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Advantage: reduces call blocking (that is to say, it increases the trunking capacity), and increases voice quality
Disadvantage: increases storage & computational load @ MSC requires real-time data from entire network related
to: channel occupancy traffic distribution Radio Signal Strength Indications (RSSI's) from all
channels
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V. Handoff Strategies
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 a new base station new voice and control channel frequencies very important task → often given higher priority
than new call It is worse to drop an in-progress call than to deny a
new one
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Minimum useable signal level lowest acceptable voice quality call is dropped if below this level specified by system designers typical values → -90 to -100 dBm
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Quick review: Decibels
S = Signal power in WattsPower of a signal in decibels (dBW) is Psignal = 10 log10(S)
Remember dB is used for ratios (like S/N)dBW is used for Watts
dBm = dB for power in milliwatts = 10 log10(S x 103)dBm = 10 log10(S) + 10 log10(103) = dBW + 30
-90 dBm = 10 log10(S x 103)10-9 = S x 103
S = 10-12 Watts = 10-9 milliwatts-90 dBm = -120 dBW
Signal-to-noise ratio:N = Noise power in Watts
S/N = 10 log10(S/N) dB (unitless raio)
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choose a (handoff threshold) > (minimum useable signal level) so there is time to switch channels before level
becomes too low as mobile moves away from base station and
toward another base station
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Handoff Margin △ △ = Phandoff threshold - Pminimum usable signal dB
carefully selected △ too large → unnecessary handoff → MSC loaded down △ too small → not enough time to transfer → call dropped!
A dropped handoff can be caused by two factors not enough time to perform handoff
delay by MSC in assigning handoff high traffic conditions and high computational load on MSC
can cause excessive delay by the MSC no channels available in new cell
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Handoff Decision signal level decreases due to
signal fading → don’t handoff mobile moving away from base station → handoff
must monitor received signal strength over a period of time → moving average
time allowed to complete handoff depends on mobile speed large negative received signal strength (RSS) slope →
high speed → quick handoff statistics of the fading signal are important to
making appropriate handoff decisions → Chapters 4 and 5
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1st Generation Cellular (Analog FM → AMPS) Received signal strength (RSS) of RVC measured
at base station & monitored by MSC A spare Rx in base station (locator Rx) monitors
RSS of RVC's in neighboring cells Tells Mobile Switching Center about these mobiles and
their channels
Locator Rx can see if signal to this base station is significantly better than to the host base station
MSC 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 FCC’s from
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 a long enough time period → handoff initiated by MSC
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MSC no longer monitors RSS of all channels reduces computational load considerably enables much more rapid and efficient handoffs imperceptible to user
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A mobile may move into a different system controlled by a different MSC Called an intersystem handoff What issues would be involved here?
Prioritizing Handoffs Issue: Perceived Grade of Service (GOS) – service
quality as viewed by users “quality” in terms of dropped or blocked calls (not
voice quality) assign higher priority to handoff vs. new call request a dropped call is more aggravating than an occasional
blocked call
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Guard Channels % of total available cell channels exclusively set
aside for handoff requests makes fewer channels available for new call
requests a good strategy is dynamic channel allocation (not
fixed) 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 Requests use time delay between handoff threshold and
minimum useable signal level to place a blocked handoff 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 needed
calls 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 # handoffs
MSC load is heavy when high speed users are passed between very small cells
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Umbrella Cells Fig. 3.4, pg. 67 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 (saves on obtaining new tower licenses)
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
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Cell Dragging low speed user w/ line of sight to base station (very strong
signal) strong signal changing slowly user moves into the area of an adjacent cell without handoff causes interference with adjacent cells and other cells
Remember: handoffs help all users, not just the one which is handed off.
If this mobile is closer to a reused channel → interference for the other user using the same frequency
So 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 parameters Analog cellular (1st generation)
threshold margin △ ≈ 6 to 12 dB total time to complete handoff ≈ 8 to 10 sec
Digital cellular (2nd generation) total time to complete handoff ≈ 1 to 2 sec lower necessary threshold margin △ ≈ 0 to 6 dB enabled by mobile assisted handoff
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benefits of 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 → GOS increased
can make decisions based on a wide range of metrics other than signal strength such as also measure interference levels can have a multidimensional algorithm for
making decisions
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Soft vs. Hard Handoffs Hard handoff: different radio channels assigned
when moving from cell to cell all analog (AMPS) & digital TDMA systems (IS-136,
GSM, etc.) Many spread spectrum users share the same
frequency in every cell CDMA → IS-95 Since a mobile uses the same frequency in every cell, it
can also be assigned the same code for multiple cells when it is near the boundary of multiple cells.
The MSC simultaneously monitors reverse link signal at several base stations
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MSC dynamically decides which signal is best and 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 systems
As 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 systems
What are the sources of interference for a mobile receiver?
Interference is in both voice channels control channels
Two major types of system-generated interference:1) Co-Channel Interference (CCI)2) Adjacent Channel Interference (ACI)
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First we look at CCI Frequency Reuse
Many cells in a given coverage area use the same set of channel frequencies to increase system capacity (C)
Co-channel cells → cells that share the same set of frequencies
VC & CC traffic in co-channel cells is an interfering source to mobiles in Several different cells
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Possible Solutions?1) Increase base station Tx power to improve radio
signal reception? __ this will also increase interference from co-channel
cells by the same amount no net improvement
2) Separate co-channel cells by some minimum distance 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 cell coverage area ↑ improved isolation from co-channel RF energy
Q = D / R : co-channel reuse ratio hexagonal cells → Q = D/R = 3N
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Fundamental tradeoff in cellular system design: small Q → small cluster size → more frequency
reuse → larger system capacity great But also: small Q → small cell separation →
increased co-channel interference (CCI) → reduced voice quality → not so great
Tradeoff: Capacity vs. Voice Quality
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Signal to Interference ratio → S / I, ____________
S : desired signal power Ii : interference power from ith co-channel cell
io : # of co-channel interfering cells
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Approximation with some assumptions
Di : distance from ith interferer to mobile
Rx power @ mobile ( ) niD
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n : path loss exponent free space or line of sight (LOS) (no obstruction) →
n = 2 urban cellular → n = 2 to 4, signal decays faster
with distance away from the base station having the same n throughout the coverage area
means radio propagation properties are roughly the same everywhere
if base stations have equal Tx power and n is the same 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 at distance ≈ D) then
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What determines acceptable S / I ? voice quality → subjective testing AMPS → S / I 18 dB (assumes path loss exponent ≧
n = 4) Solving (3.9) for N
Most reasonable assumption is io : # of co-channel interfering cells = 6
N = 7 (very common choice for AMPS)
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Many assumptions involved in (3.9) : same Tx power hexagonal geometry n same throughout area Di ≈ D (all interfering cells are equidistant from the
base station receiver) optimistic result in many cases propagation tools are used to calculate S / I when
assumptions aren’t valid
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S / I is usually the worst case when a mobile is at the cell edge low signal power from its own base station & high
interference power from other cells more accurate approximations are necessary in those cases
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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 with desired 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) don’t propagate as well as those from tall base station antennas
obstructions near ground level significantly attenuate mobile energy in direction of base station Rx
also weaker because mobile Tx power is variable → base stations 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