Download - Overview of CMC Slides
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 1/60
Cellular Wireless Networks
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 2/60
Cellular Network OrganizationUse multiple low-power transmitters (100 W or less)Areas divided into cells
Each served by its own antennaServed by base station consisting of transmitter,receiver, and control unitBand of frequencies allocatedCells set up such that antennas of all neighbors areequidistant (hexagonal pattern)
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 3/60
Cells
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 4/60
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 5/60
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 6/60
Cell size and capacityCell size determines number of cellsavailable to cover geographic area and(with frequency reuse) the totalcapacity available to all usersCapacity within cell limited by availablebandwidth and operationalrequirementsEach network operator has to size cellsto handle expected traffic demand
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 7/60
Cell structureImplements space division multiplex: base station covers acertain transmission area (cell)Mobile stations communicate only via the base station
Advantages of cell structures:
higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc.locally
Problems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on thecountry side (GSM) - even less for higher frequencies
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 8/60
Capacity of a Cellular SystemFrequency R e-Use DistanceThe K factor or the cluster size
Cellular coverage or Signal tointerference ratioSectoring
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 9/60
i
j
1
2
34
5
6
7
Frequency re-use distance is based on the cluster size K
The cluster size is specified in terms of the offset of the center of a cluster fromthe center of the adjacent cluster
K = i2 + ij + j2
K = 2 2 + 2*1 + 1 2
K = 4 + 2 + 1
K = 7
D = D 3K * R
D = 4.58R
12
35
6
7
D
R
The K factor and Frequency Re-Use Distance
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 10/60
K = i2 + ij + j2
K = 2 2 + 2*0 + 0 2
K = 4 + 0 + 0
K = 4D = D 3K * R
D = 3 .46R i
D
R
The Frequency Re-Use for K = 4
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 11/60
12
34
5
6
7
12
34
5
6
7
21
12
34
5
6
7
12
34
5
6
7
12
34
5
6
7
The Cell Structure for K = 7
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 12/60
1
2
3
4
1
1
1
1
1
12
2
2
2
2
3
3
3
3
3
4
4
4
4
4
4
3
2
Cell Structure for K = 4
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 13/60
1
11
1
2 2
223
3
3
3
4
4 4
45
5 5
5
6
6 6
6
7
7
7
7
8 8
889
99
9
10
1010
10
1111
1111
1212
12 12
Cell Structure for K = 12
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 14/60
Increasing cellular systemcapacity
Cell sectoringDirectional antennas subdivide cell into 3or 6 sectorsMight also increase cell capacity by factorof 3 or 6
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 15/60
Increasing cellular systemcapacity
Cell splittingDecrease transmission power in base andmobileR esults in more and smaller cellsR euse frequencies in non-contiguous cellgroups
Example: ½ cell radius leads 4 foldcapacity increase
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 16/60
Tri-Sector antenna for a cell
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 17/60
Highway
TownSuburb
R ural
Cell Distribution in a Network
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 18/60
Optimum use of frequencyspectrum
Operator bandwidth of 7.2MHz (36 freq of 200 kHz)TDMA 8 traffic channels per carrier
K factor = 12What are the number of traffic channelsavailable within its area for these three cases
Without cell splitting
With 72 cellsWith 246 cells
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 19/60
One Cell = 288 traffic channels
72 Cell = 1728 traffic channels
246 Cell = 5904 traffic channels
R e-use of the frequency
8 X 36 = 288
8 X (72/12 X 36) = 1728
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 20/60
F requency ReuseAdjacent cells assigned different frequencies toavoid interference or crosstalk Objective is to reuse frequency in nearby cells
10 to 50 frequencies assigned to each cellTransmission power controlled to limit power at thatfrequency escaping to adjacent cellsThe issue is to determine how many cells must
intervene between two cells using the same frequency
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 21/60
Approaches to Cope withIncreasing Capacity
Adding new channelsF requency borrowing ± frequencies are taken fromadjacent cells by congested cells
Cell splitting ± cells in areas of high usage can besplit into smaller cellsCell sectoring ± cells are divided into a number of wedge-shaped sectors, each with their own set of
channelsMicrocells ± antennas move to buildings, hills,and lamp posts
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 22/60
Cellular System Overview
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 23/60
Cellular Systems TermsBase Station (BS) ± includes an antenna, acontroller, and a number of receiversMobile telecommunications switching office
(MTSO) ± connects calls between mobile unitsTwo types of channels available between mobileunit and BS
Control channels ± used to exchange informationhaving to do with setting up and maintaining callsTraffic channels ± carry voice or data connection
between users
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 24/60
Steps in an MTSO ControlledCall between Mobile Users
Mobile unit initializationMobile-originated call
PagingCall acceptedOngoing call
Handoff
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 25/60
Additional F unctions in anMTSO Controlled Call
Call blockingCall termination
Call dropCalls to/from fixed and remote mobilesubscriber
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 26/60
Mobile Radio PropagationEffects
Signal strengthMust be strong enough between base station and mobileunit to maintain signal quality at the receiver
Must not be so strong as to create too much cochannelinterference with channels in another cell using thesame frequency band
F ading
Signal propagation effects may disrupt the signal andcause errors
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 27/60
Handoff Performance MetricsCell blocking probability ± probability of a newcall being blockedCall dropping probability ± probability that a call
is terminated due to a handoff Call completion probability ± probability that anadmitted call is not dropped before it terminatesProbability of unsuccessful handoff ± probability
that a handoff is executed while the receptionconditions are inadequate
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 28/60
Handoff Performance MetricsHandoff blocking probability ± probability that ahandoff cannot be successfully completedHandoff probability ± probability that a handoff occurs before call terminationRate of handoff ± number of handoffs per unittimeInterruption duration ± duration of time during ahandoff in which a mobile is not connected toeither base stationHandoff delay ± distance the mobile moves fromthe point at which the handoff should occur to the
point at which it does occur
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 29/60
Handoff Strategies Used toDetermine Instant of Handoff
Relative signal strengthRelative signal strength with threshold
Relative signal strength with hysteresisRelative signal strength with hysteresis andthreshold
Prediction techniques
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 30/60
Power ControlDesign issues making it desirable to includedynamic power control in a cellular system
Received power must be sufficiently above the
background noise for effective communicationDesirable to minimize power in the transmitted signalfrom the mobile
Reduce cochannel interference, alleviate health concerns, save battery power
In SS systems using CDMA, it¶s desirable to equalizethe received power level from all mobile units at the BS
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 31/60
Types of Power ControlOpen-loop power control
Depends solely on mobile unit No feedback from BS
Not as accurate as closed-loop, but can react quicker tofluctuations in signal strength
Closed-loop power controlAdjusts signal strength in reverse channel based onmetric of performanceBS makes power adjustment decision andcommunicates to mobile on control channel
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 32/60
Traffic EngineeringIdeally, available channels would equalnumber of subscribers active at one timeIn practice, not feasible to have capacityhandle all possible loadF or N simultaneous user capacity and L
subscribers L < N
± nonblocking system L > N ± blocking system
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 33/60
Blocking System PerformanceQuestions
Probability that call request is blocked?What capacity is needed to achieve a certainupper bound on probability of blocking?What is the average delay?What capacity is needed to achieve a certainaverage delay?
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 34/60
Traffic Intensity
Load presented to a system:
P = mean rate of calls attempted per unit timeh = mean holding time per successful callA = average number of calls arriving during averageholding period, for normalized P
h A P!
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 35/60
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 36/60
F irst-Generation Analog
Advanced Mobile Phone Service (AMPS)In North America, two 25-MHz bands allocatedto AMPS
One for transmission from base to mobile unitOne for transmission from mobile unit to base
Each band split in two to encouragecompetitionF requency reuse exploited
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 37/60
AMPS OperationSubscriber initiates call by keying in phonenumber and presses send keyMTSO verifies number and authorizes user
MTSO issues message to user¶s cell phoneindicating send and receive traffic channelsMTSO sends ringing signal to called partyParty answers; MTSO establishes circuit and
initiates billing informationEither party hangs up; MTSO releases circuit,frees channels, completes billing
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 38/60
Differences Between F irst andSecond Generation Systems
Digital traffic channels ± first-generation systemsare almost purely analog; second-generationsystems are digital
Encryption ± all second generation systems provide encryption to prevent eavesdroppingError detection and correction ± second-generationdigital traffic allows for detection and correction,giving clear voice receptionChannel access ± second-generation systems allowchannels to be dynamically shared by a number of users
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 39/60
Mobile Wireless TDMA DesignConsiderations
Number of logical channels (number of time slotsin TDMA frame): 8Maximum cell radius (R): 35 km
F requency: region around 900 MHzMaximum vehicle speed ( V m ):250 km/hr Maximum coding delay: approx. 20 msMaximum delay spread ( ( m ): 10 QsBandwidth: Not to exceed 200 kHz (25 kHz per channel)
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 40/60
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 41/60
GSM Network Architecture
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 42/60
Mobile StationMobile station communicates across Um interface(air interface) with base station transceiver insame cell as mobile unit
Mobile equipment (ME) ± physical terminal, suchas a telephone or PCSME includes radio transceiver, digital signal processorsand subscriber identity module (SIM)
GSM subscriber units are generic until SIM isinserted
SIMs roam, not necessarily the subscriber devices
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 43/60
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 44/60
Network Subsystem (NS) NS provides link between cellular network and public switched telecommunications networks
Controls handoffs between cells in different BSSs
Authenticates users and validates accountsEnables worldwide roaming of mobile users
Central element of NS is the mobile switchingcenter (MSC)
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 45/60
Mobile Switching Center (MSC)Databases
Home location register (HLR) database ± storesinformation about each subscriber that belongs toitVisitor location register (VLR) database ± maintains information about subscribers currently
physically in the regionAuthentication center database (AuC) ± used for authentication activities, holds encryption keysEquipment identity register database (EIR) ± keeps track of the type of equipment that exists atthe mobile station
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 46/60
TDMA F ormat ± Time SlotF ields
Trail bits ± allow synchronization of transmissionsfrom mobile unitsEncrypted bits ± encrypted data
Stealing bit - indicates whether block containsdata or is "stolen"Training sequence ± used to adapt parameters of receiver to the current path propagationcharacteristics
Strongest signal selected in case of multipath propagation
Guard bits ± used to avoid overlapping with other bursts
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 47/60
GSM Speech Signal Processing
l l
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 48/60
GSM Signaling ProtocolArchitecture
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 49/60
F unctions Provided by ProtocolsProtocols above the link layer of the GSMsignaling protocol architecture providespecific functions:
Radio resource managementMobility managementConnection managementMobile application part (MAP)BTS management
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 50/60
Advantages of CDMA Cellular F requency diversity ± frequency-dependenttransmission impairments have less effect onsignal
Multipath resistance ± chipping codes used for CDMA exhibit low cross correlation and lowautocorrelationPrivacy ± privacy is inherent since spreadspectrum is obtained by use of noise-like signalsGraceful degradation ± system only graduallydegrades as more users access the system
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 51/60
Drawbacks of CDMA Cellular Self-jamming ± arriving transmissions frommultiple users not aligned on chip boundariesunless users are perfectly synchronized
Near-far problem ± signals closer to the receiver are received with less attenuation than signalsfarther awaySoft handoff ± requires that the mobile acquiresthe new cell before it relinquishes the old; this ismore complex than hard handoff used in F DMAand TDMA schemes
M bil Wi l CDMA D i
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 52/60
Mobile Wireless CDMA DesignConsiderations
RAKE receiver ± when multiple versions of asignal arrive more than one chip interval apart,RAKE receiver attempts to recover signals frommultiple paths and combine them
This method achieves better performance than simplyrecovering dominant signal and treating remainingsignals as noise
Soft Handoff ± mobile station temporarily
connected to more than one base stationsimultaneously
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 53/60
Principle of RAKE Receiver
T f Ch l S t d b
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 54/60
Types of Channels Supported byF orward Link
Pilot (channel 0) - allows the mobile unit toacquire timing information, provides phasereference and provides means for signal strengthcomparisonSynchronization (channel 32) - used by mobilestation to obtain identification information aboutcellular systemPaging (channels 1 to 7) - contain messages for one or more mobile stationsTraffic (channels 8 to 31 and 33 to 63) ± theforward channel supports 55 traffic channels
F d T ffi Ch l
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 55/60
F orward Traffic ChannelProcessing Steps
Speech is encoded at a rate of 8550 bpsAdditional bits added for error detectionData transmitted in 2-ms blocks with forwarderror correction provided by a convolutionalencoder Data interleaved in blocks to reduce effects of errorsData bits are scrambled, serving as a privacy mask
F d T ffi Ch l
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 56/60
F orward Traffic ChannelProcessing Steps (cont.)
Power control information inserted into trafficchannelDS-SS function spreads the 19.2 kbps to a rate of
1.2288 Mbps using one row of 64 x 64 WalshmatrixDigital bit stream modulated onto the carrier usingQPSK modulation scheme
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 57/60
Third-Generation CapabilitiesVoice quality comparable to the public switchedtelephone network 144 kbps data rate available to users in high-speedmotor vehicles over large areas384 kbps available to pedestrians standing or moving slowly over small areasSupport for 2.048 Mbps for office use
Symmetrical / asymmetrical data transmissionratesSupport for both packet switched and circuitswitched data services
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 58/60
Third-Generation CapabilitiesAn adaptive interface to the Internet to reflectefficiently the common asymmetry betweeninbound and outbound traffic
More efficient use of the available spectrum ingeneralSupport for a wide variety of mobile equipmentF lexibility to allow the introduction of newservices and technologies
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 59/60
Alternative Interfaces
8/3/2019 Overview of CMC Slides
http://slidepdf.com/reader/full/overview-of-cmc-slides 60/60
CDMA Design ConsiderationsBandwidth ± limit channel usage to 5 MHzChip rate ± depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps
or more is reasonableMultirate ± advantage is that the system canflexibly support multiple simultaneousapplications from a given user and can efficientlyuse available capacity by only providing thecapacity required for each service