Transcript of The Cellular Concept Cellular radio systems accommodate a large number of users (subscribers) over a...
- Slide 1
- The Cellular Concept Cellular radio systems accommodate a large
number of users (subscribers) over a large geographic area, within
a limited frequency spectrum. High capacity is achieve by limiting
the coverage area of each base station transmitter to a small
geographic area called a cell. The cellular structure allow the re-
use of frequency across the network. 1 Top of Cellular Radio
tower
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- Basics: Structure Cells Different Frequencies or Codes Base
Station (BS) Fixed transceiver Mobile Station (MS) Distributed
transceivers Downlink, forward direction Uplink, reverse direction
Handoff Handover Multiple Access To differentiate between different
transmissions in the same direction (share the same broadcast
channel) Multiple access technique is used, e.g., FDMA, TDMA, CDMA,
OFDMA. To differentiate between the downlink and the uplink
(provide full-duplex system), use Frequency division duplexing
(FDD) or Time division Duplexing (TDD). Division Duplexing Separate
Tx and Rx antennas Single antenna for Tx and Rx, a device called
duplexer is used to separate. Cell 2
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- Duplexer principle 3 Tx Rx
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- FDD: 4 X HzY Hz Allocated Frequency Spectrum for a cell
UplinkDownlink Allocated Frequency Spectrum for the whole cellular
system 12 uplinkdownlink 1k121k1212
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- TDD: 5 Uplink X Hz Y Hz Allocated Frequency Spectrum for a cell
Downlink
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- Basic Multiple Access Methods 6 Time Frequency Codes TDMA: Time
Division Multiple Access FDMA: Frequency Division Multiple Access
CMDA: Code Division Multiple Access
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- Channels allocation 7 Allocated channels for one cell 12
uplinkdownlink FCCFCC 1 RCCRCC 34 K-1 234 RVCFVC
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- Terminology Subscriber: A user who pays subscription charges
for using a mobile communication system. Mobile Station (MS): A
station in the cellular radio service intended for use while in
motion at unspecified locations. Base Station: a fixed station in a
mobile system used for radio communication with mobile station.
Base stations are located at the center or on the edge of a
coverage region. Mobile Switching Center (MSC) or Mobile Telephone
Switching Office (MTSO): Switching center which coordinates the
routing of calls in a large service area. It connects the cellular
base stations and the mobiles to the PSTN. Handoff: the process of
transferring a mobile station from one channel or base station to
another. Hard handoff: break before make. Soft handoff: make before
break. Page: A brief message which is broadcast over the entire
service area, by many BS at the same time. Roamer: A subscriber
which operates on a service area (market) over than that from which
service has been subscribed. 8
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- Make a cellular Call Communication between MS and BS is defined
by: Standard Common Air Interface (CAI): FVC: forward voice
channel. RVC: reverse voice channel. FCC: forward control channel.
RCC: reverse control channel. 9 Called Setup channels
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- MSCPSTN Mobile Terminated call On MIN#1 downlink FCC Power
level Page MIN#1 FCC Ack RCC Ack Move MS to U VC FCC 12
uplinkdownlink FCCFCC 1 RCCRCC 34 K-1 234 RVCFVC Alert FVC3 10
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- MSCPSTN Mobile Originated call RCC request FCC request Move MS
to U VC 12 uplinkdownlink FCCFCC 1 RCCRCC 34 K-1 234 RVCFVC FVC4
tone 11
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- idealized radio coverage of the cell hexagon shape of the cell
cell segmentation of the area into cells Cell planning use of
several carrier frequencies not the same frequency in adjoining
cells cell sizes vary from some 100 m up to 35 km depending on user
density, geography, transceiver power etc. hexagonal shape of cells
is used (cells overlap, shapes depend on geography) Actual radio
coverage of a cell is known as the footprint, is determined: From
field measurement or, Propagation prediction models 12 Actual radio
coverage of the cell cell Ameba shape
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- Circular Coverage Areas Original cellular system was developed
assuming base station antennas are omnidirectional, i.e., they
transmit in all directions equally. Users located outside some
distance to the base station receive weak signals. Result: base
station has Ideally circular coverage area. Weak signal Strong
signal 13
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- Tessellation Some group of small regions tessellate a large
region if they overlay the large region without any gaps or
overlaps. There are only three regular polygons that tessellate any
given region. 14
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- Tessellation (Contd) Three regular polygons that always
tessellate: Equilateral triangle Square Regular Hexagon Triangles
Squares Hexagons 15
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- Circles Dont Tessellate Thus, ideally base stations have
identical, circular coverage areas. Problem: Circles do not
tessellate. The most circular of the regular polygons that
tessellate is the hexagon. Thus, early researchers started using
hexagons to represent the coverage area of a base station, i.e., a
cell. 16
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- 17 Circles to Hexagons
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- Thus the Name Cellular With hexagonal coverage area, a cellular
network is drawn as: Since the network resembles cells from a
honeycomb, the name cellular was used to describe the resulting
mobile telephone network. Base Station 18
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- Regular Hexagon 19 R R R R h triangle
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- Frequency Reuse (Frequency Planning) Cellular structure allows
carrier frequencies to be re-used High frequency re-use will lead
to: Short distance between same carriers High traffic capacity Low
C/I ratio (i.e. worse interference) Frequency planning involves a
compromise between requirements for capacity and interference
Digital systems like GSM can cope with lower values of C/I than
analog systems Simple frequency plans assume a homogeneous
distribution of carriers and equal sized cells. A cell-cluster is a
group of adjacent cells, which are allocated all the frequency
channels without duplication 20
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- Cell Cluster 21 tier 1 tier2
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- 22 tier1 7 Cell Cluster
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- 23 tier1 12 Cell Cluster
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- 24 D R Frequency Re-use Distance
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- Interference and System Capacity Interference is the main
limiting factor in the performance of cellular radio system.
Interference has been recognized as a major bottleneck in
increasing capacity and is often responsible for dropping calls.
Sources of interference: Another mobile in the same cell. A call in
progress in a neighboring cell. Other base stations operating in
the same frequency band The two major types of system-generated
cellular interference are: Co-channel interference (C/I). Adjacent
channel interference (C/A). 25
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- Adjacent Channel Interference 26
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- Co-channel interference 27 1 2 3 4 5 6 7 1 2 3 4 5 6 7 2 1 2 3
4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 2 2 Serving Cell
6 nearest interfering (co-channel) cells D
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- Interference Calculations 28 1 2 3 4 5 6 7 1 2 3 4 5 6 7 2 1 2
3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 2 2 Serving
Cell 6 nearest interfering (co-channel) cells D
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- Estimating C/I for Re-use Patterns 29
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- Estimating C/I for first tier 30
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- C/I for Typical Cluster Sizes 233.54 Cluster Size (N)
31.766.538.9211.3 43.018.4111.113.8 75.4412.0515.3618.66
96.5313.6817.2720.85 127.7815.5619.4523.34 2110.2119.2123.7128.21
31 Analog systems require a minimum C/I of about 20 dB Digital
systems can cope with C/I as low as 9 dB
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- Cell Splitting 32
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- Cell Splitting Techniques To increase capacity, split each cell
into 3 using sectored antennas 33 Center-excited cell:
omnidirectional antennas Edge(corner)-excited cell: sectored
directional antennas tri-sectored site antenna systems with space
diversity Top view
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- Further Splitting As the network grows, capacity can be further
increased by another 3 way split as shown 34
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- 1:4 Cell Split Alternative way of further splitting the cells
No re-alignment of antennas needed Increases traffic capacity,
frequency re-use and number of sites by a factor of 4 35
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- Effect of Cell Splitting on Interference Directional pattern of
sectored antennas reduces response to interference Increases C/I
significantly Allows greater frequency re-use, i.e. smaller cells
If cells A and B use the same carrier: B will cause co-channel
interference in A A will cause very little co-channel interference
in B Interference is no longer mutual 36
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- Transition Zones Problems may occur at the boundaries between
high and low traffic areas Large cells in rural areas will use
higher power - can cause interference with smaller urban cells
nearby Requires careful frequency planning - possibly reserve
carriers for use in such transition zones Alternatively, hierarchy
of cells (e.g. overlay / underlay) may be used 37
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- GSM Frequency Patterns Two common re-use patterns in GSM are
3/9 and 4/12 3/9 consists of 3 sites, each of which has been tri-
sectored giving a cluster of 9 cells 38
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- Interference in the 3/9 Pattern 3/9 pattern allows frequencies
to be allocated so no physically adjacent cells use the same
frequency C/I is about 9 dB, which is the minimum specified for GSM
with frequency hopping Cells A1 and C3 are physically adjacent and
are allocated adjacent carriers On the boundary of A1 and C3: C/A =
0 dB GSM specifies a minimum C/A of -9 dB 39
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- 4/12 Re-use Pattern 4 sites, each tri-sectored to give a 12
cell cluster Numbering of D cells allows carriers to be allocated
so that no adjacent carriers are used in physically adjacent cells
40
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- Interference in the 4/12 Pattern 4/12 pattern has no physically
adjacent cells with co- channel or adjacent channel carriers C/I is
about 12 dB This is adequate in GSM without frequency hopping C/A
is higher than in 3/9 pattern Traffic capacity is lower than 3/9 as
there are fewer carriers per cell 41
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- Adjustments for Capacity Simple re-use patterns assign same
number of carriers to each cell Practical traffic may not be evenly
distributed Moving carriers to other cells to handle traffic will
introduce new interference problems This can be avoided by reducing
base station power - e.g. introduce an overlay cell 42
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- Multiple Reuse Patterns (MRP) MRP : A technique to vary the
reuse pattern for different channels and different levels of
quality of service (QoS) Combines conservative control channel
reuse with aggressive traffic channel reuse to achieve a tighter
average reuse Frequency Hopping, Power Control and DTX are
necessary: These techniques reduce the impact of interference on
calls and allow close reuse distances to work more reliably
Frequencies can be reserved for microcells and picocells Best used
with lots of spectrum Performance results with 15 MHz (75 GSM
carriers) are better than for 5 MHz (25 GSM carriers) because there
are more frequencies to hop across 43
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- 44 MRP BCCH1 BCCH: N=12, i=2, j=2 TCH1:N=7, i=1, j=2 TCH2: N=3,
i=1, j=1 BCCH1 TCH1 TCH2
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- Frequency Hopping in GSM When using frequency hopping, the
actual carrier frequency used by a TRX changes on each frame (8
timeslots) The frequency follows either a sequential or pseudo-
random pattern One frame is 4.6 ms long Rate of hopping = 1/ (4.6 x
10 -3 ) = 217 hops / second This is also known as Slow Frequency
Hopping (SFH) to distinguish it from Fast Frequency Hopping used in
CDMA systems 45
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- Frequency Hopping at the BTS If the BTS has implemented SFH:
TRXs used only for traffic channels will hop through set sequences
TRX used for the BCCH carrier will not hop - mobiles must be able
to access this for periodic signal level measurements 64 hopping
sequences are available in GSM: 1 sequence is cyclic - 1,2,3 , 1,2
63 others are pseudo random patterns Hop Sequence Number (HSN)
defines the sequence in use HSN = 0 indicates the cyclic sequence
The set of carrier frequencies assigned to the sequence (Mobile
Allocation) may be the same for each TRX provided the sequence
starts at a different point (Mobile Allocation Index Offset, MAIO)
46
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- Frequency Hopping at the Mobile Base stations need not
implement frequency hopping Mobile must be capable of SFH in case
it enters a cell in which it is implemented In addition to hopping
in step with the BTS, the mobile must also make measurements on
adjacent cells This is why the rate of hopping is limited to SFH in
GSM The mobile needs to know: Frequencies used for hopping (Mobile
Allocation) - coded as a subset of the Cell Allocation frequencies
Hop Sequence Number (HSN) Start frequency (Mobile Allocation Index
Offset, MAIO) 47
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- Traffic Theory and Channel Dimensioning 48
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- Traffic Capacity Capacity is the ability of the network to
handle traffic, i.e. calls made by subscribers Traffic theory is
based on the concept of trunking, where the links between potential
callers are routed through a limited number of channels or trunks
Cellular radio systems rely on trunking to accommodate a large
number of users in a limited radio spectrum This leads to the
concepts of blocking and grade of service which must be considered
when dimensioning the channels 49
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- Trunking Without trunking: With trunking: 50 In wireless
networks trunks correspond to traffic channels
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- Terminology used in Trunking Theory 51
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- Blocking Since there are fewer trunks (channels) than potential
calls, some calls will be blocked Grade of Service (GoS) is a
measure of the ability of a user to access a trunked system during
the busiest hour. % of calls blocked % of calls experiencing a
delay greater than a certain queuing time So a low figure for Grade
of Service is good for the subscriber Low Grade of Service may not
be good for the network, as channels will be under-used at times
Trunking efficiency describes the percentage usage that is made of
the channels 52 Offered Traffic = Carried Traffic + Blocked Traffic
Offered Traffic : Total traffic offered to channel by all users
Carried Traffic : Traffic successfully carried by the channel
Blocked Traffic: Traffic which is blocked at call setup
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- Grade of Service (GoS) Grade of Service is the fraction of
incoming calls (offered traffic) allowed to be blocked due to
congestion in the channel Typical Grade of Service is 0.02 (2%)
Grade of Service is also called blocking probability or loss
probability 53
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- Traffic Channel Dimensioning To dimension the network for
traffic capacity: Find the total traffic generated by the
subscribers in the network area Find the traffic that can be
handled by one TRX at a base station Divide to find the number of
base station TRXs needed 54
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- Traffic Measurement The fundamentals of trunking theory were
developed by Erlang, a Danish mathematician. The measure of traffic
intensity bears his name. Unit of traffic measurement: Erlang (E)
One Erlang represents the amount of traffic intensity carried by a
channel that is completely occupied. Traffic in Erlangs is the
number of call-hours per hour: e.g. A radio channel that is
occupied for thirty minutes during an hour carries 0.5 Erlang of
traffic 55
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- Cont. 56
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- Cont. Typical traffic per subscriber during the busy hour is 25
mE which corresponds to a mean call holding time of 90 sec (How?).
Another traffic unit, used mostly in the USA, is the Call Centum
Second (CCS): 1 CCS = 100 call seconds per hour 1 Erlang = 3600
call seconds per hour 1 Erlang = 36 CCS 57
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- Erlang Models of Traffic Two commonly used models of trunked
systems are: Erlang B and Erlang C Erlang B (Blocked calls
cleared)- blocked calls are lost or cleared Erlang C (Blocked calls
delayed)- calls that cannot be handled are put in a queue until a
channel becomes available 58
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- Erlang B Formula 59
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- Erlang B Calculations Tables based on the Erlang B model allow
calculations to be made relating: Offered traffic Grade of Service
Number of channels Structure of Erlang B table: Example: at 2%
blocking (0.02 GoS), 2 traffic channels can carry 0.22347 Erlangs
of traffic 60 C
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- Erlang C Formula 61
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- Channel Dimensioning - Example In GSM channel dimensioning, the
number of channels must be related to the number of carriers
(frequencies) available: 8 channels (timeslots) per carrier Some
channels will be required for signalling Example - in a particular
cell: Mean call holding time = 90 seconds Grade of Service = 1 %
Total number of available carriers = 4 3 timeslots allocated for
signaling How many subscribers can this cell support ? 62
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- Channel Dimensioning - Solution 63
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- Trunking Efficiency Trunking efficiency or channel utilisation
is given by: carried traffic / number of channels In the Erlang B
model: Trunking Efficiency = A (1- GoS) / C Using the previous
example: A = 19.487 E, GoS = 0.01, C = 29 Trunking Efficiency =
19.487 (1 - 0.01) / 29 =0.665 = 66.5 % 64
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- Example2: A certain city has an area of 1300 square miles and
is covered by a cellular system using a seven-cell reuse pattern.
Each cell has a radius of four miles and the city is allocated 40
MHz of spectrum with a full duplex channel bandwidth of 60 kHz
(each channel serves only one subscriber). Assume a GoS of 2% for
an Erlang B system is specified. If the offered traffic per user is
30 mE, compute: a) The number of cells in the service area. b) The
number of channels per cell. c) Traffic intensity of each cell. d)
The maximum carried traffic. e) The total number of users that can
be served for 2% GoS. f) The number of mobiles per unique channel.
g) The theoretical maximum number of users that could be served at
one time by the system. 65
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- Solution: 66
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- Example3: 67
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- Solution: 68
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- UL/DL capacity limitation Scenario 1: Capacity limitation due
to UL interference The cell cant serve UE1 because the increase in
UL interference by adding the new user would be too high, resulting
in a high risk of drops Scenario 2: Capacity limitation due to DL
power The cell cant serve UE2 because its using all its available
power to maintain the connections to the other UEs UE 1 UE 2
Scenario 1 Scenario 2 69
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- RBS 1RBS 2 Fully loaded system Unloaded system Cell breathing
The more traffic, the more interference and the shorter the
distance must be between the RBS and the UE The traffic load
changes in the system causes the cells to grow and shrink with time
70