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Introduction: Mobile and Wireless Network. 2 1. Paradigm Shift to Mobile Comm. S-curve S-curve...
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Transcript of Introduction: Mobile and Wireless Network. 2 1. Paradigm Shift to Mobile Comm. S-curve S-curve...
Introduction:Mobile and Wireless Network
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1. Paradigm Shift to Mobile Comm. S-curve
S-curve viewed from research and development The tool that helps to decide on whether an enterprise should continue
the use of a technology or replace the technology with something else Shift to the new paradigm at the emerging era
At a certain point the advantage of new paradigm suddenly increases, and the new paradigm suddenly settles down
Product company’s view Disadvantage: new technology costs more to adopt Advantage
To have good and renowned reputation More time era to sell a certain model Patents
User(businessmen)’s view Disadvantage: high product cost and high monthly fee Advantage: advantages coming from prompt or distinguished action
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1. Paradigm Shift to Mobile Comm. S-curve in communication (telephone) networks
1st generation Analog and mechanic and electric Advantage: talking on the phone instead of running to him(her)
2nd generation Digital and electronic Various flexible control Advantage: high speed data transfer, reliable and personalized
services 3rd generation
Mobile handset fixed networks Advantage: Quick access, Low cost in setup and maintaining (one
mobile phone acts for many phones, No cabling cost) 4th generation
Mobile handset, mobile network
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1. Paradigm Shift to Mobile Comm.
Pros and cons of mobile systems Advantages
Convenience: going to the phone the phone comes to us Quick access regardless one’s position Low cost in setup and maintaining
Disadvantages Limited frequency spectrum Complex technologies Quality of signals Power supply for the small portable units
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1. Paradigm Shift to Mobile Comm.
Wireless mobile communication system Mobile station: end-users can walk, or move in a car Land station(base station): the communication nodes
are built distributed throughout the service area Communication between two mobile nodes is done via
mobile station, and there is no direct communication between them
The first commercial mobile system AMPS(Advanced Mobile Phone System) is implemented in 1983
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1. Paradigm Shift to Mobile Comm.
(frequency) spectrum allocation problem Limited frequency (cost for use it) Frequency usage ratio is very important topic in
wireless communication Cell reuse or spread-spectrum is known to be much efficient
way
withoutcell reuse
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2. Cell Concepts Regular cells
Regular cells: dominant regions covered by each cells are all same There are only three regular cells – triangular, square, hexagonal
In N-gonal shapes, angle θ = (N-2) π / N For regular shaping, θ should be in the form of 2π/k where k is an
integer The satisfying N = 3, 4 and 6 only
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2. Cell Concepts
Why we only use hexagonal units? Hexagonal positioning of base station is the most
efficient Area of unit
Proportional to number of base stations = proportional to setup cost of base stations
Number of neighbors to a single unit Way of hand-off = proportional to base station networking and
control complexity
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3. Transmission using Frequency Spectrum
p. 23 ILF: voice frequency MF: AM radio VHF: FM radio UHF: TV broadcasting, PCS
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3. Transmission using Frequency Spectrum
Wireless transmission Electro-magnetic radiation is created, if enough current is loaded
into an antenna Antenna
Antenna length: approximately the same as the wavelength of the generated signal
Directed antenna: most radiation is focused to a certain direction Non-directed antenna: radiation is generated uniformly to all direction
As frequency(v) increases, Smaller wavelength λ = C/v (C=light speed) Energy increases, E = hv, h=Planck constant Tends to proceed in straight More data can be inserted
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3. Transmission using Frequency Spectrum
Type of radio waves (p. 24) Depending on the nature of the frequency and type of transmission Grounded or surface wave (LF, MF: 30k-3MHz)
Follows the curvature of the earth The long wavelength in this category is relatively immune to
terrestrial condition (tree, mountain, buildings,…), while the short wavelength is sensitive to them
Space wave (VHF, UHF, SHF or upper: 30MHz-) Covering more area than ground wave
Sky wave (3-30MHz) Transmitted upward to ionosphere, and reflected back to the ground For radio-broadcasting and long-distance telephone line
Satellite-based wave (2-40GHz) Upward transmission Downward transmission
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Speech transmission (p. 35) Voice waveform spoken into a phone creates an electrical
alternating current Sound wave consists of a band of frequencies
Spoken vowels: occupy mostly the lower portion of frequency band Consonants: use less power and generally occupy high frequency
band Due to the difficulties in transferring speech signal, spectrum is
cutoff in 200-3500Hz Low frequency FL: It is hard to reproduce low frequency exactly
using a normal speaker High frequency FH: high frequency usually is cut off during
transmission on electrical line Bandwidth B(=FH-FL) is proportional to transmission cost Guard band: transient part to guarantee that no signal generates out of
given bandwidth
3. Transmission using Frequency Spectrum
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4. Wireless Transmission System (Geo-synchronous) orbit satellite
Satellite whose position remains fixed according to the equator 22,300 miles high from the ground (1 mile = about 1.609 km) Moves with the speed of 6,900 mile/h A geo-synchronous satellite covers 30% of the surface of the earth
Microwave system Direct line of sight transmission 30-50km apart, 2-40GHz For wide-band transmission and radar
Infra-red transmission system Using directed infra-red signal 1 mile distance at maximum High data transfer rate with relatively low cost
Cellular radio system Wireless LAN
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Fundamentals of Cellular Systems
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Introduction Target: mobile system be efficient in the use of
limited spectrum bandwidth Generals
Mobile cellular components in early cellular systems Mobile station, base station, switch station
Wireless signal characteristics Multipath and its prevention
Cell design issues Wireless signal distortion Shapes: cell reuse Evolution of cells: cell splitting, cell sectoring
Roaming and handoff
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1. Early Cell System
Non-trunk radio system Does not use multiplexing scheme
Each radio channel is fixed to a specific user or a group of users
Trunk radio system (synchronous or asynchronous) multiplexing scheme Channels are shared and available to all users Advantage: increased efficiency of spectrum usage Disadvantage: more complex architecture required
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1. Early Cell System
Trunk radio system (AMPS) (p. 66) BTS (base station): controls the air interface between
the mobile station and MTSO Mobile station: having frequency-agile machine that
allows to change to a particular frequency designated for its use by the MTSO
MTSO: responsible for switching the calls to the cells providing
Interfacing with telephone network and backup Monitoring traffic Performing testing and diagnostics, network management
functions
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2. Wireless Signal Characteristics
Key issues of cellular system design Let the receivers accommodate to a wide variety of
signal characteristics Use frequency efficiently
Path loss (p. 68) Measured in dB(decibel) = 10 log (Pr/Pt) In wide range: decreasing as distance becomes apart In short range: very fluctuating (because of multipath)
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2. Wireless Signal Characteristics Multi-path propagation
There is little direct line-of-sight path between base and mobile station
Most paths are indirect path and their total distance are all different
multi-path signal Mixed with reflected, diffracted, and direct signal Direct signal is the strongest As signal gets reflected or diffracted, it loses considerable
portion of energy Different distance of multipath
Causes phase shift of signals think vector addition Causes multipath signal loss
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2. Wireless Signal Characteristics
Multipath delay variation (p. 69) Multipath delay varies as a mobile station moves Time dispersion (time delay spread) get worse as
Distance between base-station and mobile station increases Frequency becomes high
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2. Wireless Signal Characteristics Multi-path fading (Rayleigh fade) (p. 70)
Definition Suppose signal is sent in long distance It creates multi-path signals When all kinds of multi-path are combined in vector addition,
the signal tends to have special curve called Rayleigh fade Curve characteristics
Good signals are interspersed with narrow, but very poor signals, called fade
Signal peaks are relatively smooth Signal fades are very narrow, deep and totally unpredictable There is no way to predict at which position fade occurs in real
system
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2. Wireless Signal Characteristics Prevention from (reduction of) Rayleigh fade
effect use a pair of antennas At base station
Install a pair of antennas vertically in a few inches apart At mobile station
Install a pair of antennas one of them vertically and the other horizontally
As receiving device is randomly positioned, vertical or horizontal line positions differently
But these co-located antennas have very different reception characteristics (the fades received in from these two antenna are differently located)
If two signals from two co-located antennas are combined, most of fade shrinks or disappears
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3. Cell Design Issues
Frequency reuse = D/R D: the shortest distance between two cells that use the
same frequency R: radius of cells N: reuse pattern = number of different frequencies in a
cluster
For 7-cell group that has 3-mile radius cells, D 13.74 miles For 7-cell group that has 2-mile radius cells, D 9.16 miles
NRD 3/
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3. Cell Design Issues
Transmission interference Adjacent channel interference
Several frequency bands are effective in a cell Interference between two adjacent frequency bands
Co-channel interference Interference between signals of the same frequencies generated
from or to the different base station Co-channel interference reduction factor (q)
q = D / R
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3. Cell Design Issues Density of mobile node/cells
Mobile terminal is not equally distributed Increase of subscribers Required cell splitting
Cell splitting (p. 83) To increase cell capacity Install cells in half (a little bit larger than half) the length of
current ones New cell area = ¼ * old cell area New cell capacity = old cell capacity Maximum density of subscribers in the new system = 4 * maximum
density of subscribers in the old system With reduced power of transmission signal in both mobile and
base station
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3. Cell Design Issues
Cell sectoring (p. 86) Use 3 directional antennas instead of a non-directional
antenna
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4. Basic Operation of Cellular Call Initialization of mobile system
Power on: Power is turned off and then turned on in a mobile station Scanning: It begins to scan the paging channels
The unit monitors for signals broadcast to mobile stations Tuning: It chooses the strongest(best) signal and locks on it Registering: It registers its whereabouts to the mobile network Listening: It keeps on listening to the ongoing control messages from
base station Making a call
A user completes to keys in telephone number The unit finds and selects an available frequency The unit sends a call request message containing the phone number The MTSO receives the message and tries to establish call connection
to reach to another MTSO, normal public switched telephone network, or another mobile network
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4. Basic Operation of Cellular Call Receiving a call (in AMPS)
While the mobile unit listening to the page channel, it receives a page which informs that a call is tried to itself
MTSO chooses an available channel and orders the mobile unit to use the indicated channel
Mobile station tunes to the directed channel and accepts the call Roaming and handoff
A mobile station moves through a geographical region while talking on the phone
MSTO notices that the current signal is not good enough to maintain, and decides to initiated handoff procedure
MTSO seeks which cell(base station) has the strongest signal among cells that afford to give channels
MTSO provides the roaming mobile station with a new channel through the chosen cells
MTSO releases the old channel for other uses