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Cellular Concept-System Design FundamentalsPart 01
Udhay Prakash
Lecturer, ECE Dept., [email protected]
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Concepts under consideration Introduction to cellular systems
Frequency Reuse Scheme
Channel Assignment Strategies
Handoff Strategies and associated practicalconsiderations
Interference and system capacity
Channel planning Trunking and grade of service
Techniques to improve coverage and capacity
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Wireless networks Vs. fixed networksHigher loss-rates due to interference
emissions of, e.g., engines, lightning
Restrictive regulations of frequencies frequencies have to be coordinated, useful frequencies are almost all occupied
Low transmission rates local some Mbit/s, regional currently, e.g., 9.6kbit/s with GSMHigher delays, higher jitter
connection setup time with GSM in the second range, several hundred milliseconds for otherwireless systems
Lower security, simpler active attacking radio interface accessible for everyone, base station can be simulated, thus attracting calls
from mobile phones
Always shared medium secure access mechanisms important
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Wireless & Mobility
Wireless: Freedom of movement
No loss of connectivity
Increase in productivity
Mobility:
user mobility: users communicate (wireless) anytime, anywhere, with anyone
device portability: devices can be connected anytime, anywhere to the network.
Wireless vs. Mobile Examples stationary computer
notebook in a hotel room with Ethernet cable wireless LANs in historic buildings Personal Digital Assistant (PDA)
The demand for mobile communication creates the need for integration of wirelessnetworks into existing fixed networks. Eg: Mobile IP extension of IP, internetworking of GSM and ISDN
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Effects of device portabilityPower consumption
limited computing power, low quality displays, small disks due to limited batterycapacity
CPU: power consumption
Loss of data
higher probability, has to be included in advance into the design (e.g., defects,theft)
Limited user interfaces compromise between size of fingers and portability
integration of character/voice recognition, abstract symbols
Limited memory limited value of mass memories with moving parts
flash-memory or ? as alternative
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Wireless systems: overview of the
developmentcellular phones satellites
wireless LANcordless
phones
1992:GSM
1994:
DCS 1800
2001:IMT-2000
1987:
CT1+
1982:Inmarsat-A
1992:Inmarsat-B
Inmarsat-M
1998:
Iridium
1989:
CT 2
1991:
DECT 199x:proprietary
1997:
IEEE 802.11
1999:
802.11b, Bluetooth
1988:
Inmarsat-C
analogue
digital
1991:
D-AMPS
1991:
CDMA
1981:
NMT 450
1986:
NMT 900
1980:
CT0
1984:
CT1
1983:
AMPS
1993:
PDC
4G fourth generation: when and how?
2000:
GPRS2000:
IEEE 802.11a
200?:
Fourth Generation
(Internet based)
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Areas of researchWireless Communication
transmission quality (bandwidth, error rate, delay) modulation, coding, interference media access, regulations ...
Mobility location dependent services location transparency quality of service support (delay, jitter, security) ...
Portability power consumption limited computing power, sizes of display, ... usability ...
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Wireless Communication Evolution Demonstration of long distance wireless communication using
electromagnetic waves by Marconi, started an era of wireless
communications. Invention ofsolid state devices (vacuum tubes, transistors,..) led to
the manufacturing of compact handheld devices suitable for
mobility.
Thereby, the era ofmobile communication started.
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Wireless Communication EvolutionMilestone chronicle
First version of a mobile radio telephone being used in 1924. Source: www.bell-labs.com/technology/wireless/earlyservice.html
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Evolution of Mobile radio system Guglielmo Marconi invented wireless telegraph in 1896.
Communication by encoding alphanumeric characters inanalog signal.
Sent telegraphic signals across the Atlantic Ocean.
Motorola, in association with Bell Systems, operated thefirst commercial mobile telephone service MobileTelephone System (MTS) in 1946.
Analog frequency modulation High power BS tower to cover 50 miles radius
Inefficient (12kHz spectrum for a voice connection)
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Evolution of Mobile radio system Improved mobile telephone system (IMTS) developed in
1960. Full duplex services and direct-dialling
23 FM channels with BW reduced to 25-30KHz
Cellular Concept Exploits the attenuation of radio signal with distance to
achieve frequency reuse.
Originally proposed by D.H. Ring in 1947.
Bell Labs began work on cellular telephone system in the late1960s.
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Evolution of Mobile radio system First cellular system:
1979, Nippon Telephone and Telegraph Company (NTT) in Japan 600 FM duplex channels, 25 KHz channel bandwidth, 800 MHz
frequency range
In Europe: 1981, The Normadic Telephone system (NMT 450) 200 FM duplex channels, 25 KHz channel bandwidth, 450 MHz
frequency range
In USA:
1983, The Advanced Mobile Phone System (AMPS) 666 FM duplex channels, 30 KHz channel bandwidth, 800 MHz
frequency range
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Limitations ofearly mobile radio systems
Uses a single, high powered transmitter with an antennamounted on a tall tower, to achieve a large coveragearea.
Inefficient spectrum utilization. Very few calls blankets the complete bandwidth
available for a city.
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Cellular concept Replacing a single, high power transmitter (large cell) with
many low power transmitters (small cells).
Implements space division multiplex: base station covers a
certain transmission area (cell)
Mobile stations communicate only via the base station
Cell sizes from some 100 m in cities to, e.g., 35 km on thecountry side (GSM) - even less for higher frequencies.
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Cellular Structure Pros:
higher capacity, higher number of users
less transmission power needed
more robust, decentralized
base station deals with interference, transmission area etc.locally
Cons: fixed network needed for the base stations handover (changing from one cell to another) necessary
interference with other cells
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Cellular system Two types: circuit switched & Packet switched
Circuit switched systems- each traffic channel is dedicated to a user until its cell is
terminated. There are two variants of circuit switched systems
Analog Systems
Digital Systems
Packet-switched cellular systems.
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Circuit switched cellular systemAnalog System Digital System
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Packet Switched Cellular System
Node B: The name for base station in GSM.
RNC (Radio Network Controller) analogous to BSC. SGSN (Service GPRS Support Node) analogous to MSC/VLR.
GGSN (Gateway GPRS Support Node): Point of interface with external packet data
networks.
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Cellular Spectrum*A band
10 MHz
333 channels30khz
B band
10 MHz
333 channels30khz
825 835 845
870 880 890
824 846.5 849
869
A band
10 MHz
333 channels30khz
B band
10 MHz
333 channels30khz
Phone Transmit
Base Transmit
891.5 894
Aband
Aband
Aband
Aband
Bband
Bband
1 MHz33 chan
1.5 MHz50 chan
2.5 MHz83 chan
1 MHz
33 chan
1.5 MHz
50 chan
2.5 MHz
83 chan
20 MHz Guard
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Cellular System Overview
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Cellular Systems Terms Base Station (BS) includes an antenna, a controller, and
a number of receivers
Mobile telecommunications switching office (MTSO)
connects calls between mobile units Two types of channels available between mobile unit and
BS
Control channels used to exchange information having to dowith setting up and maintaining calls.
Traffic channels carry voice or data connection between users.
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Frequency Reuse It is a frequencyplanning strategy.
Base stations in adjacent cells are assigned channel groups which
contain completely different channels than neighbouring cells. Cell shape for theoretical planning: Square, equilateral triangle,
hexagon.
Among them, hexagon is more suitable, due to its maximal radial
coverage to its edges.
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Cell structure
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Frequency Reuse
Frequency reuse only with a certain distance between the base stations. Cluster No. of Cells sharing the complete assigned bandwidth, using
frequency reuse concept.
More users can be served
Drawback:
Generation of interference
Reduction of users which can be served
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Frequency Reuse Typical Cluster sizes are 4, 7 and 12; with 7 cell reuse
scheme being more popular.
Based on location of base station (BS), cell are
classified as center-excited cells with mainlyomni-directional
antennas, or set ofdirectional antennas, for BS.
corner-excited cells with directional antennas for BS.
f1f2
f3
f2 f1
f1
f2
f3f2
f3f1
f2f1f3f3
f3f3
f3
3 cell cluster
f5f3
f4f1
f3 f2
f6
f7
f2
f4f5
f1f3f5f6
f7f2
f2
7 cell cluster
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Frequency Reuse While tessellating the cells in hexagon geometry, in
frequency planning, the cluster size (N) must satisfythe condition: N=i2+ij+j2.
where i,j are any non-negative integers. To find nearest co-channel neighbours of a
particular cell:1. move i cells along any chain of hexagons, and then
2. turn 60 degrees counter-clockwise and move j cells.
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Frequency Reuse This figure illustrates the method of locating co-
channel cells, in a cellular system with cluster size,N=19, and for i=3 & j=2.
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Channel Assignment Strategies
For efficient utilization of bandwidth. Frequency reuse scheme with increasing capacity and minimizing
interference. Fixed channel assignment:
certain frequencies are assigned to a certain cell problem: different traffic load in different cells. To avoid blocking during overload conditions, follows channel borrowing
strategy.
Dynamic channel assignment: base station chooses frequencies depending on the frequencies already used
in neighbor cells
more capacity in cells with more traffic assignment can also be based on interference measurements. But, requires real-time data on channel occupancy, traffic distribution, and
radiosignal strength indications(RSSI) of all channels on a continuous basis.
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Handoff Also called handover (in Europe).
Handoff - When a mobile moves into a different cell while a
conversation is in progress, the MSC automatically transfers the call
to a new channel belonging to the new base station.
Before the advent of handoff concept, when a mobile user moves
into a different cell, then the mobile shouldgetregistered manually
in the new cell.
Handoff is automatic process, without getting noticed by the user.
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When to handoff? Minimum Usage Signal should be
normally taken as between90 dBm and100 dBm.
Margin, = Pr handoff Pr minimum_usable,cannot be too large or too small.
If is too small insufficient time tocomplete a handoff.
If is too large Unnecessary handoffsburden the MSC.
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Handoff Performance Metrics Cell blocking probability probability of a new call being blocked
Call dropping probability probability that a call is terminated due
to a handoff.
Call completion probability probability that an admitted call is
not dropped before it terminates.
Probability of unsuccessful handoff probability that a handoff is
executed while the reception conditions are inadequate.
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Handoff Performance Metrics Handoff blocking probability probability that a handoff cannot be
successfully completed.
Handoff probability probability that a handoff occurs before call
termination.
Rate of handoffnumber of handoffs per unit time.
Interruption duration duration of time during a handoff in which a
mobile is not connected to either base station.
Handoff delay distance the mobile moves from the point at which the
handoff should occur to the point at which it does occur.
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Handoff Strategies 1G analog cellular systems
signal strength measurements are made by BSs & supervised by MSC.
Each BS constantly monitors signal strengths of all of its reverse voice
channels to determine relative location of each mobile user w.r.t BS.
Locator receiver ( a spare receiver) in each BS, is used to scan and determine
signal strengths of mobile users which are in neighbouring cells.
This receiver is controlled by the MSC and is used to monitor the signal strength
of users in neighbouring cells which appear to be in need of handoff and reports
all RSSI values to the MSC.
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Handoff Strategies
2G cellular systems Mobile Assisted Handoff (MAHO) takes place.
every MS measures the received power from surrounding BSs and
continually reports the results of these measurements to the serving BS.
Handoff is initiated when the power received from adjacent BS is
greater than that of serving BS.
Mainly suited for microcell environment, where handoffs are initiated
more frequently.
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Handoff Strategies-to Determine Instant of Handoff Relative signal strength
Relative signal strength with threshold
Relative signal strength with hysteresis
Relative signal strength with hysteresis and threshold
Prediction techniques
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FOUR Types of Handoff
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MSC MSC
BSC BSCBSC
BTS BTS BTSBTS
MS MS MS MS
1
2 3 4
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Prioritizing Handoffs Strategies
Schemes to prioritize handoff requests over call initiation requestswhen allocating voice channels.
1) Guard Channel Concept-
a fractionof the totalavailable channels in a cell is reserved
exclusively for handoff requests. Reduced carried traffic
Efficient spectrum utilization, under dynamic spectrumassignment strategy.
2) Queuing of Handoffs- To reduce the probability of blocking.
Handoff delay time and sizeof the queue are determined from the trafficpattern of the particular service area.
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Practical Handoff ConsiderationsDesigning handoff strategy ofhigh speed vehicular users and thepedestrian users!
Ability to obtain new cell sites, particularly in urban areas, due tovarious geo-techno-political aspects!
Solution::: Umbrella Cell Approach.
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Practical Handoff ConsiderationsCell dragging results when the user has travelled wellbeyond the designed Cell range, and still possessesreceived signal at BS above handoff threshold.
Thus, handoff is not made.
This creates a potential interference and trafficmanagement problem
Particularly occurs in urban areas, where line of sight
(LoS) path exists between MS & BS. Solution: handoff thresholds and radio coverage
parameters must be adjusted carefully.
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Practical Handoff Considerations
1G analog cellular systems consumes 10s to make a handoff. So,should be in the order of 6 to 12dB.
Digital cellular systems takes 1-2s to make a handoff. So, should be in the order of 0 to 6dB.
Newer cellular systems are possessing the ability to make handoffdecisions based on a wide range of metrics other than signalstrength.
Handoff Hard handoff(eg: GSM)
Soft handoff (eg: CDMA)-Due to cluster size, N=1
soft handoff-The ability to select between the instantaneousreceived signals from a variety of BSs.
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Power Control Design issues making it desirable to include dynamic power
control in a cellular system
Received power must be sufficiently above the background noise for
effective communication
Desirable to minimize power in the transmitted signal from the mobile
Reduce co-channel interference, alleviate health concerns, save battery
power
In SS systems using CDMA, its desirable to equalize the received power
level from all mobile units at the BS
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Types of Power Control Open-loop power control
Depends solely on mobile unit.
No feedback from BS.
Not as accurate as closed-loop, but can react quicker to fluctuations in signal
strength.
Closed-loop power control
Adjusts signal strength in reverse channel based on metric of performance. BS makes power adjustment decision and communicates to mobile on control
channel.
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Gain, Loss & Decibels Ratio between two signal power levels is often
measured in decibels (dB):
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mW and dBm
Decibels refers to relative change in magnitude, not absolute values. So, we define the dBW (decibel-Watt) as a reference.
1 Watt of transmission power= 0dBw.
Example: Radio Mirchi 98.3 FM in hyderabad transmits at 100,000 Watts.
Power in dBW=10*log (100,000W/1W)=10*5=50 dBW.
For wireless networks, the dBm (decibel-milliwatt) is more useful.
1mW transmission power==0dBm
10 mW==10 dBm
0.1 mW==-10dBm
802.11b networks have a maximum transmit power of 100 mW== 20 dBm
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Decibel calculations
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References Theodore S. Rappaport, Wirelesss Communications-
Principles and Practice, 2e, PHI, 2002.
William C.Y. Lee, Wireless & Cellular
Telecommunications, 3e, ISBN:0-07-143686-3, TataMcGraw Hills.
T.K. Sarkar, R.J. Mailloux, A.A. Oliner, M. Salazar-palma and D.L. Sengupta, History of wireless, John
Wiley & Sons, Inc.., 2006.
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