Transcript Chapter 3

Chapter 3
The Cellular Concept - System Design
Fundamentals
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?
2
 Cellular concept
 Frequency reuse pattern
3
 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
4
 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 ***
5
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
6
 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)
7
 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.
8
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=kN
9
 M : # of times a cluster is replicated over a
geographic coverage area
 System Capacity = Total # Duplex Channels = C
C=MS=MkN
(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.
10
 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.
11
 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)
12
 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.
13
14
15
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
16
 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
17
 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
18
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
19
 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
20
Quick review: Decibels
S = Signal power in Watts
Power 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)
21
 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
22
23
 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
24
 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
25
 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
26
 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
27
 MSC no longer monitors RSS of all channels
 reduces computational load considerably
 enables much more rapid and efficient handoffs
 imperceptible to user
28
 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
29
 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
30
 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
31
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
32
 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
33
34
 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.
35
 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
36
 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
37
 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
38
 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.
39
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)
40
 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
41
 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
42
 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
43
 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
44
 Signal to Interference ratio → S / I, ____________
 S : desired signal power
 Ii : interference power from ith co-channel cell
 io : # of co-channel interfering cells
45
 Approximation with some assumptions
 Di : distance from ith interferer to mobile
 Rx power @ mobile  ( Di )n
46
 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.
47
 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
48
 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)
49
 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
50
 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
S
R4

I 2( D  R)4  2( D  R)4  2D4
51
N =7 and S / I ≈ 17 dB
52
 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
53
 HW1:
1-9, 1-11, 1-18, 3-5, 3-7
54