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