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Sat & mob commn lizy

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Based on Kerala University 8th semester Satellite & Mobile Communication subject of ECE branch.

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Sat & mob commn lizy

  1. 1. 08.804 Satellite & MobileCommunication (T) KERALA UNIVERSITY B-TECH 8th SEMESTER B-lizytvm@yahoo.com Lizy Abraham+919495123331 Assistant Professor ` Department of ECE LBS Institute of Technology for Women (A Govt. of Kerala Undertaking) Poojappura Trivandrum -695012 Kerala, India
  2. 2. SYLLABUS• 08.804 SATELLITE & MOBILE COMMUNICATION (T) L-T-P : 3-1-0 Credits: 4• Module I Communication Satellite- Orbits & launching methods-Kepler‘s law-Inclined Orbits- Geostationary orbits, Effect of Orbital Inclination, Azimuth and Elevation, Coverage Angle and Slant Range, Eclipse, Satellite Placement. Space segment subsystems & description, Earth Station- Antenna, High Power Amplifiers, Up converter, Down converters, Monitoring and Control. Satellite link- Basic Link and Interference analysis, Rain Induced Attenuation and Cross Polarization Interference-Link Design.Mobile Satellite Networks.• Module II Cellular concept:-hand off strategies, Interference and system capacity-: Cell splitting, Sectoring, Repeaters, Microcells. Link budget based on path loss models. Propagation models(outdoor):- Longely-Rice Model, Okumura Model. Mobile Propagation:- Fading and doppler shift, impulse response model of multipath channel, parameters of multipath channel. Fading effect due to multipath time delay spread and doppler shift. Statistical models for multipath flat fading:- Clarks model, Two-ray Rayleigh Model. Multiple Access- TDMA overlaid on FDMA,SDMA, FHMA. GSM:- Architecture, Radio subsystem, Channel types, Frame Structure. Introduction to Ultra Wideband Communication System.• Module III Direct sequence modulation, spreading codes, the advantage of CDMA for wireless, code synchronization, channel estimation, power control- the near-far problem, FEC coding and CDMA, multiuser detection, CDMA in cellular environment. Space diversity on receiver techniques, multiple input multiple output antenna systems, MIMO capacity for channel known at the receiver -ergodic capacity, space division multiple access and smart antennas.28-12-2012 2
  3. 3. SYLLABUS - TEXTBOOKS• Text books:• 1. Dennis Roody, Satellite communication,2/e, McGraw Hill.• 2. Theodore S. Rappaport: Wireless communication principles and practice,2/e, Pearson Education• 3. Simon Haykin, Michael Mohar, Modern wireless communication, Pearson Education,2008• References:• 1. Tri. T. Ha, Digital satellite communication,2/e, Mcgraw Hill.• 2. M. Ghavami, L. D. michael, k Rohino, Ultra-wide band signals in communication engineering, Wiley Inc.• 3. William stallings: Wireless communication and networks, Pearson Education, 2006• 4. William C Y Lee: Mobile cellular Telecommunications,2/e, McGraw Hill.• 5. MadhavendarRichharia: Mobile satellite communications: principles and trends, Pearson Education,2004.• Question Paper• The question paper shall consist of two parts. Part I is to cover the entire syllabus, and carries 40 marks. This shall contain 10 compulsory questions of 4 marks each. Part II is to cover 3 modules, and carries 60 marks. There shall be 3 questions from each module (10 marks each) out of which 2 are to be answered.• (Minimum 40% Problem, derivation and Proof)28-12-2012 3
  4. 4. `ÉwâÄx D ftàxÄÄ|àxVÉÅÅâÇ|vtà|ÉÇ Liz28-12-2012 4
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  46. 46. Kepler’s 3rd Law: Law of Harmonics• The squares of the periods of two planets’ orbits are proportional to each other as the cubes of their semi-major axes: T12/T22 = a13/a23• Orbits with the same semi-major axis will have the same period.28-12-2012 46
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  59. 59. Equinox• Earth’s axis of rotation is not perpendicular to that of sun’s equatorial plane and instead is tilted at an angle of about 23 degrees.• The day that the Earths North Pole is tilted closest to the sun is called the summer solstice. This is the longest day (most daylight hours) of the year• The winter solstice, or the shortest day of the year, happens when the Earths North Pole is tilted farthest from the Sun.• In between, there are two times when the tilt of the Earth is zero, meaning that the tilt is neither away from the Sun nor toward the Sun. These are the vernal equinox — the first day of spring — and the autumnal equinox – the first day of fall.• Equinox means "equal." During these times, the hours of daylight and night are equal. Both are 12 hours long.28-12-2012 59
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  62. 62. • The right ascension of the ascending node is the angle measured eastward from the Vernal Equinox to the ascending node.• The Vernal Equinox is the Suns apparent ascending node (marking the beginning of the Northern hemispheres spring.28-12-2012 62
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  64. 64. Solar Eclipses for Geo-stationary Satellites• Between 28 February and 11 April, and between 2 September and 14 October, roughly 21 days either side equinoxes, satellites in geostationary orbits will pass through the shadow of the earth once every day.• While in the earth’s shadow the satellite gains no power from its all important solar cells. So, either a satellite is forced to shut down, or if 24-hour operation is necessary, to switch over to batteries.• Earth caused eclipses can continue around equinox with the satellite being in the shadow for up to 70 minutes each day.28-12-2012 64
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  72. 72. • A solar day is the length of time between two successive passes of the sun across the same spot in the sky. That time period is, on average, 24:00:00, hours, or one mean solar day.• A sidereal day is the length of time between two successive passes of the fixed stars across the sky. That time period is 23:56:04, or one sidereal day.28-12-2012 72
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  76. 76. azimuth and elevation• azimuth and elevation - an angular coordinate system for locating positions in the sky.• Azimuth is measured clockwise from true north to the point on the horizon directly below the object.• Elevation is measured vertically from that point on the horizon up to the object.28-12-2012 76
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  101. 101. Wideband Receiver• A duplicate receiver is provided so that if one fails, the other is automatically switched in.• The combination is referred to as a redundant receiver, meaning that although two are provided, only one is in use at a given time. Refer fig.7.14 and 7.16(Dennis Roody, Satellite communication,2/e, McGraw Hill)28-12-2012 101
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  104. 104. • Directional beams are usually produced by means of reflector-type antennas. Eg:-Paraboloidal reflector• Gain of a paraboloidal reflector relative to an isotropic radiator, G=ηI(πD/λ)2• λ -wavelength of the signal •D-reflector diameter ηI-aperture efficiency •3dB beamwidth, Ɵ3dB=70 λ/D •Gain can be increased and the beamwidth made narrower by increasing the reflector size or decreasing the wavelength.28-12-2012 104
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  106. 106. AOCS• Attitude- orientation of satellite in space• Attitude control-ensure the directional antennas point in the proper directions.• Disturbance torques-forces which alter the attitude. Eg:-gravitational fields of earth & moon, solar radiation• Station keeping:- maintaining a satellite in its correct position using thrusters.28-12-2012 106
  107. 107. • Sensors- measures satellite’s orientation in space and of any tendency for this to shift. Eg:-Infrared sensors(horizon detectors)• With 4 such sensors, one for each quadrant-any shift in orientation is detected by one or other of the sensors, and a corresponding control signal is generated, which activates a restoring torque.• Attitude maneuver-a shift in attitude is required, this is executed. The control signals needed to achieve this maneuver is transmitted from earth station.28-12-2012 107
  108. 108. • Controlling torques may be generated by passive or active attitude control.• 3 axes which define satellite’s attitude are roll, pitch and yaw.• In spin stabilization (cylindrical satellites), mechanically balanced about one of the axes and is set spinning around this axis.• Also achieved by a spinning fly wheel (noncylindrical satellites), rather than by spinning the satellite itself.• If the average momentum referred as momentum bias is zero, this is termed as momentum wheel or reaction wheel.28-12-2012 108
  109. 109. • If each axis is stabilized by a reaction wheel, called as 3 axis stabilization.• The wheel is attached to the rotor, which consists of a permanent magnet providing the magnetic field for motor action.• The stator of the motor is attached to the body of the satellite. Thus the motor provides the coupling between the flywheel and the satellite structure.28-12-2012 109
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  111. 111. • The demands on the attitude and orbit control system (AOCS) differ during the two main phases of the mission- the orbit-raising phase and the operational phase.• Two types of attitude control systems are in common use-1. spin stabilization and2. Three-axis stabilization (Momentum wheel stabilization)• The specifications of the attitude-control system depend on the desired spacecraft pointing accuracy which is a function of the satellite antenna beam width.• The attitude control may be either active or passive.• A passive attitude-control system maintains the attitude by obtaining an equilibrium at the desired orientation without the use of active attitude devices. Eg:- Spin stabilization• An active control system maintains the attitude by the use of active devices in the control loop. Eg:- Momentum wheel stabilization28-12-2012 111
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  161. 161. Mobile Satellite Systems• Like cellular systems, except that the base stations (i.e., satellites) move as will as mobile devices• Satellite coverage attractive for areas of world not well served by existing terrestial infrastructure: ocean areas, developing countries.28-12-2012 161
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  163. 163. • Mobile Satellite Systems • Geostationary Systems– INMARSAT– MSAT • Big “LEO” Systems– ARIES– ELLIPSO– IRIDIUM– ODYSSEY • Little “LEO” Systems– Orbcomm– LEOSAT– STARNET– VITASAT28-12-2012 163
  164. 164. Inmarsat• is a British satellite telecommunications company, offering global, mobile services.• It provides telephony and data services to users worldwide, via portable or mobile terminals which communicate to ground stations through eleven geostationary telecommunications satellites.28-12-2012 164
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  167. 167. • Carrier to Noise Ratio (C/N) The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. This figure is directly related to G/T and S/N; and in a video signal the higher the C/N, the better the received picture.• G/T A figure of merit of an antenna and low noise amplifier combination expressed in dB. "G" is the net gain of the system and "T" is the noise temperature of the system. The higher the number, the better the system.• dBW:• decibels with respect to one Watt. A Logarithmic representation of a power level reference to 1W of power.• Figure of Merit:• A Figure of merit is a quantity used to characterize the performance of a device relative to other devices of the same type. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application.• The overall Earth station figure of merit is defined as the ratio of receive gain to system noise temperature expressed in decibels per Kelvin• e.g. G/T is a measure of the performance of a downlink station expressed in units of dB/K, depending on the receive antenna and low noise amplifier28-12-2012 167
  168. 168. • An isotropic radiator is an antenna which radiates in all directions equally.• Effective Isotropic Radiated Power (EIRP) is the amount of power the transmitter would have to produce if it was radiating to all directions equally.• A measure of the strength of the signal radiated by an antenna.• The calculation of received signal based on transmitted power and all losses and gains involved until the receiver is called “Link Power Budget”, or “Link Budget”.• The received power Pr is commonly referred to as “Carrier Power”, C.28-12-2012 168
  169. 169. • The satellite link is probably the most basic in microwave communications since a line-of- sight path typically exists between the Earth and space.• This means that an imaginary line extending between the transmitting or receiving Earth station28-12-2012 169
  170. 170. Design of the Satellite Link LNB (LOW NOISE BLOCK DOWN CONVERTER) A device mounted in the dish, designed to amplify the satellite signals and convert them from a high frequency to a lower frequency. LNB can be controlled to receive signals with different polarization. The television signals can then be carried by a double-shielded aerial cable to the satellite receiver while retaining their high quality. A universal LNB is the present standard version, which can handle the entire frequency range from 10.7 to 12.75 GHz and receive signals with both vertical and horizontal polarization. Critical Elements of the Satellite Link28-12-2012 170
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  225. 225. Interference Analysis28-12-2012 225
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  227. 227. Refer 5.4,5.5,5.6 (Dennis Roody, Satellitecommunication,2/e, McGraw Hill)28-12-2012 227
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  234. 234. Refer 12.9.2 (Dennis Roody, Satellitecommunication,2/e, McGraw Hill)28-12-2012 234
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  243. 243. Interference Analysis (contd..)• Refer 5.4, 5.5, 5.6, 12.9.2, 13.1, 13.2, 13.2.1, 13.2.2, 13.2.3. (Dennis Roody, Satellite communication,2/e, McGraw Hill)28-12-2012 243
  244. 244. • Eb/No (Energy per bit per Noise Power Density)• Is the performance criterion for any desired BER• It is the measure at the input to the receiver• Is used as the basic measure of how strong the signal is• Directly related to the amount of power transmitted from the uplink station• Eb/No = (C/N)T + Noise BW – Information Rate28-12-2012 244
  245. 245. `ÉwâÄx E`Éu|Äx VÉÅÅâÇ|vtà|ÉÇ Liz
  246. 246. The Cellular Concept28-Dec-12 246
  247. 247. Basic Concept• Cellular system developed to provide mobile telephony: telephone access “anytime, anywhere.”• First mobile telephone system was developed and inaugurated in the U.S. in 1945 in St. Louis, MO.• This was a simplified version of the system used today.28-Dec-12 247
  248. 248. System Architecture• A base station provides coverage (communication capabilities) to users on mobile phones within its coverage area.• Users outside the coverage area receive/transmit signals with too low amplitude for reliable communications.• Users within the coverage area transmit and receive signals from the base station.• The base station itself is connected to the wired telephone network.28-Dec-12 248
  249. 249. First Mobile Telephone System One and only one high power base station with which all users communicate. NormalTelephone Entire Coverage System Area Wired connection 28-Dec-12 249
  250. 250. Problem with Original Design• Original mobile telephone system could only support a handful of users at a time…over an entire city!• With only one high power base station, users phones also needed to be able to transmit at high powers (to reliably transmit signals to the distant base station).28-Dec-12 250
  251. 251. Improved Design• Over the next few decades, researchers at AT&T Bell Labs developed the core ideas for today’s cellular systems.• Although these core ideas existed since the 60’s, it was not until the 80’s that electronic equipment became available to realize a cellular system.• In the mid 80’s the first generation of cellular systems was developed and deployed.28-Dec-12 251
  252. 252. The Core Idea: Cellular Concept• The core idea that led to today’s system was the cellular concept.• The cellular concept: multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations tessellate the system coverage area.28-Dec-12 252
  253. 253. Cellular Concept• Thus, instead of one base station covering an entire city, the city was broken up into cells, or smaller coverage areas.• Each of these smaller coverage areas had its own lower- power base station.• User phones in one cell communicate with the base station in that cell.28-Dec-12 253
  254. 254. 3 Core Principles• Small cells tessellate overall coverage area.• Users handoff as they move from one cell to another.• Frequency reuse.28-Dec-12 254
  255. 255. Basic Cellular System PSTN/ISDN Switch28-Dec-12 255
  256. 256. Wireless communication definitions28-Dec-12 256
  257. 257. Wide area Paging systemPaging is usually one way. It can be Numeric, alphanumeric or a voicemessage. They are used to notify a subscriber that they need to call back orget in touch with somebody. Some applicatios are:New headlinesStock quoattionsFaxNetwork managementDistance and coverage:Inside a buildingSimple : 2 to 5 KmsWide area paging : worldwide coverage.Concept is simple but the transmission systems are quite complicated 28-Dec-12 257
  258. 258. Cordless handsets28-Dec-12 258
  259. 259. Cellular system Concept MSC is also called Mobile telephone switching office (MTSO)28-Dec-12 259
  260. 260. • A basic system comprises :• Cellular subscriber phones• Base station• Mobile switching center• The cellular network is connected to public telephone network.• High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area is called cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.• Each cell uses different freq channels.• Cellaular systems use standard freq plan. The voice and control channels are defined. Normally 95% of channels are used for information communication while only 5% are used for signaling purposes.• Switching system, called handoff, enables call to proceed uninterrupted when the user moves from one cell to another.• Typical MSC handles 100,000 cellular users and 5,000 simultaneous conversations at a time.28-Dec-12 260
  261. 261. Tessellation• Some group of small regions tessellate a large region if they cover the large region without any gaps or overlaps.• There are only three regular polygons that tessellate any given region.28-Dec-12 261
  262. 262. Tessellation (Cont’d)• Three regular polygons that always tessellate: – Equilateral triangle – Square – Regular Hexagon Triangles Squares Hexagons28-Dec-12 262
  263. 263. 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 circular coverage area.28-Dec-12 263
  264. 264. Circles Don’t 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.• For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three.• Thus, early researchers started using hexagons to represent the coverage area of a base station, i.e., a cell.28-Dec-12 264
  265. 265. Thus the Name Cellular• With hexagonal coverage area, a cellular network is drawn as: Base Station• Since the network resembles cells from a honeycomb, the name cellular was used to describe the resulting mobile telephone network.28-Dec-12 265
  266. 266. Handoffs• A crucial component of the cellular concept is the notion of handoffs.• Mobile phone users are by definition mobile, i.e., they move around while using the phone.• Thus, the network should be able to give them continuous access as they move.• This is not a problem when users move within the same cell.• When they move from one cell to another, a handoff is needed.28-Dec-12 266
  267. 267. A Handoff• A user is transmitting and receiving signals from a given base station, say B1.• Assume the user moves from the coverage area of one base station into the coverage area of a second base station, B2.• B1 notices that the signal from this user is degrading.• B2 notices that the signal from this user is improving.28-Dec-12 267
  268. 268. A Handoff (Cont’d)• At some point, the user’s signal is weak enough at B1 and strong enough at B2 for a handoff to occur.• Specifically, messages are exchanged between the user, B1, and B2 so that communication to/from the user is transferred from B1 to B2.28-Dec-12 268
  269. 269. 2.4 Handoff Strategies • 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. • Handoff operation – identifying a new base station – re-allocating the voice and control channels with the new base station. • Handoff Threshold – Minimum usable signal for acceptable voice quality (-90dBm to -100dBm) – Handoff margin cannot be too large or too small. – If ∆ = Pr ,handoff − Pr ,minimum burden the MSC is too large, unnecessary handoffs usable – If is too small, ∆ there may be insufficient time to complete handoff ∆ before a call is lost. ∆28-Dec-12 269
  270. 270. 28-Dec-12 270
  271. 271. • Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station. • Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided. – Depends on the speed at which the vehicle is moving. – Steep short term average -> the hand off should be made quickly – The speed can be estimated from the statistics of the received short-term fading signal at the base station • Dwell time: the time over which a call may be maintained within a cell without handoff. (Avg. time having a smooth conversation before going for a handoff.) • Mean Dwell time- fixed, well-defined path of constant speed. Eg:- Highway users • Dwell time depends on – propagation – interference – distance28-Dec-12 – speed 271
  272. 272. • RSSI of reverse voice channels • Locator Receiver in each BS controlled by MSC • Monitor the signal strength of MUs in neighboring cells and report all RSSI values to the MSC • Handoff measurement – In first generation analog cellular systems, signal strength measurements are made by the base station and supervised by the MSC. – In second generation systems (TDMA), handoff decisions are mobile assisted, called mobile assisted handoff (MAHO) – Every MU measures the received power from BSs and continually reports the results to the serving BS. – A handoff is initiated when the power received from the neighboring BS begins to exceed that of current BS by a certain level for a certain period of time. • Intersystem handoff: If a mobile moves from one cellular system to a different cellular system controlled by a different MSC.28-Dec-12 272
  273. 273. Prioritizing Handoffs• Guard Channel for handoff requests• Queuing of handoff requests28-Dec-12 273
  274. 274. Practical Handoff Consideration • Different type of users – High speed users need frequent handoff during a call. – Low speed users may never need a handoff during a call. • Microcells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells. • Minimize handoff intervention – handle the simultaneous traffic of high speed and low speed users. • Large and small cells can be located at a single location (umbrella cell) – different antenna height – different power level • Cell dragging problem: pedestrian users provide a very strong signal to the base station – The user may travel deep within a neighboring cell28-Dec-12 274
  275. 275. 28-Dec-12 275
  276. 276. • Handoff for first generation analog cellular systems – 10 secs handoff time – ∆ is in the order of 6 dB to 12 dB • Handoff for second generation cellular systems, e.g., GSM – 1 to 2 seconds handoff time – mobile assists handoff – ∆ is in the order of 0 dB to 6 dB – Handoff decisions based on signal strength, co-channel interference, and adjacent channel interference. • IS-95 CDMA spread spectrum cellular system – Mobiles share the channel in every cell. – No physical change of channel during handoff – MSC decides the base station with the best receiving signal as the service station •28-Dec-12 276
  277. 277. Frequency Reuse• Extensive frequency reuse allows for many users to be supported at the same time.• Total spectrum allocated to the service provider is broken up into smaller bands.• A cell is assigned one of these bands. This means all communications (transmissions to and from users) in this cell occur over these frequencies only.28-Dec-12 277
  278. 278. Frequency Reuse (Cont’d)• Neighboring cells are assigned a different frequency band.• This ensures that nearby transmissions do not interfere with each other.• The same frequency band is reused in another cell that is far away. This large distance limits the interference caused by this co-frequency cell.28-Dec-12 278
  279. 279. Example of Frequency Reuse Cells using the same frequencies28-Dec-12 279
  280. 280. 2.2 Frequency Reuse • Each cellular base station is allocated a group of radio channels within a small geographic area called a cell. • Neighboring cells are assigned different channel groups. • By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells. • Keep interference levels within tolerable limits. • Frequency reuse or frequency planning •seven groups of channel from A to G •footprint of a cell - actual radio coverage •omni-directional antenna v.s. directional antenna28-Dec-12 280
  281. 281. • Consider a cellular system which has a total of S duplex channels. • Each cell is allocated a group of k channels, k < S. • The S channels are divided among N cells. • The total number of available radio channels S = kN • The N cells which use the complete set of channels is called cluster. • The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity C = MkN = MS • The capacity is directly proportional to the number of replication M. • The cluster size, N, is typically equal to 4, 7, or 12. • Small N is desirable to maximize capacity. • The frequency reuse factor is given by 1/ N28-Dec-12 281
  282. 282. • Only certain cluster sizes and cell layout are possible. • The geometry of hexagon is such that the number of cells per cluster, N, can only have values which satisfy N = i 2 + ij + j 2 • Co-channel neighbors of a particular cell, eg, i=3 and j=2 and N=19. • To find the co-channel neighbours of a particular cell, (a) move i cells along any chain of hexagons (b) turn 600 conuter clockwise and move j cells.28-Dec-12 282
  283. 283. 2.5 Interference and System Capacity • Sources of interference – another mobile in the same cell – a call in progress in the neighboring cell – other base stations operating in the same frequency band – noncellular system leaks energy into the cellular frequency band • Two major cellular interference – co-channel interference – adjacent channel interference28-Dec-12 283
  284. 284. 2.5.1 Co-channel Interference and System Capacity • Frequency reuse - there are several cells that use the same set of frequencies – co-channel cells – co-channel interference • To reduce co-channel interference, co-channel cell must be separated by a minimum distance. • When the size of the cell is approximately the same and the BSs transmit the same power, – co-channel interference is independent of the transmitted power – co-channel interference is a function of • R: Radius of the cell • D: distance between the centers of the nearest co-channel cells • Increasing the ratio Q=D/R, the interference is reduced. • Q is called the co-channel reuse ratio28-Dec-12 284
  285. 285. • For a hexagonal geometry D Q= = 3N R • A small value of Q provides large capacity • A large value of Q improves the transmission quality - smaller level of co-channel interference • A tradeoff must be made between these two objectives28-Dec-12 285
  286. 286. • Let i0 be the number of co-channel interfering cells. The signal-to- interference ratio (SIR) for a mobile receiver can be expressed as S S = i0 I ∑I i =1 i S: the desired signal power I i : interference power caused by the ith interfering co-channel cell base station • The average received power at a distance d from the transmitting antenna is approximated by −n d  Pr = P0   d  d0 or  0 d  P0 :measued power Pr (dBm ) = P0 (dBm ) − 10n log  d  XT  0 n is the path loss exponent which ranges between 2 and 4.28-Dec-12 286
  287. 287. • When the transmission power of each base station is equal, SIR for a mobile can be approximated as S R−n = i0 I ∑ (Di )−n i =1 • Consider only the first layer of interfering cells S ( D / R )n = = ( 3N )n i0 = 6 I i0 i028-Dec-12 287
  288. 288. • For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interference ratio for the worst case can be approximated as S R −4 = I 2 ( D − R ) − 4 + 2( D + R ) − 4 + 2 D − 428-Dec-12 288
  289. 289. 2.5.2 Adjacent Channel Interference • Adjacent channel interference: interference from signals which are adjacent in frequency to the desired signal. – Imperfect receiver filters allow nearby frequencies to leak into the passband – Performance degrade seriously due to near-far effect. retlif gniviecer esnopser lennahc tnecajda no langis lennahc tnecajda no langis langis derised RETLIF ecnerefretni ecnerefretni langis derised28-Dec-12 289
  290. 290. • The near-far problem is a condition in which a receiver captures a strong signal and thereby making it impossible for the receiver to detect a weaker signal. • Consider a receiver(BS) and two transmitters(MUs), one close to the BS, the other far away. If both transmitters transmit simultaneously and at equal powers, the SNR for the farther transmitter is much lower. • This makes the farther transmitter more difficult to detect. • Adjacent channel interference can be minimized through careful filtering and channel assignment. • Keep the frequency separation between each channel in a given cell as large as possible • If the frequency reuse factor is large (ie, small N), a channel separation greater than six channel bandwidth separations is needed to bring the adjacent channel interference to an acceptable level.28-Dec-12 290
  291. 291. 2.5.3 Power Control for Reducing Interference • Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel – long battery life – increase SIR – solve the near-far problem28-Dec-12 291
  292. 292. 2.7 Improving Capacity in Cellular Systems • Methods for improving capacity in cellular systems – Cell Splitting: subdividing a congested cell into smaller cells. – Sectoring: directional antennas to control the interference and frequency reuse. – Coverage zone : Distributing the coverage of a cell and extends the cell boundary to hard-to-reach places.28-Dec-12 292
  293. 293. 2.7.1 Cell Splitting • Split congested cell into smaller cells. – Preserve frequency reuse plan. – Reduce transmission power. – Increase the capacity of the Reduce R to R/2 cellular system microcell28-Dec-12 293
  294. 294. •The microcell BS labeled G is placed half way between 2 larger stations using the same channel G Illustration of cell splitting within a 3 km by 3 km square28-Dec-12 294
  295. 295. • Transmission power reduction from Pt1 to Pt 2 • Examining the receiving power at the new and old cell boundary Pr [at old cell boundary ] ∝ Pt1R − n Pr [at new cell boundary ] ∝ Pt 2 ( R / 2) − n • If we take n = 4 and set the received power equal to each other Pt1 Pt 2 = 16 • The transmit power must be reduced by 12 dB in order to fill in the original coverage area. • Problem: if only part of the cells are splited – Different cell sizes will exist simultaneously • Handoff issues - high speed and low speed traffic can be simultaneously accommodated28-Dec-12 295
  296. 296. 2.7.2 Sectoring • Decrease the co-channel interference and keep the cell radius R unchanged – Replacing single omni-directional antenna by several directional antennas – Radiating within a specified sector28-Dec-12 296
  297. 297. • Interference Reduction position of the mobile interference cells28-Dec-12 297
  298. 298. Directional Antenna• One way to get more capacity (number of users) while maintaining cell size is to use directional antenna.• Assume antenna which radiates not in alldirections (360 degrees) but rather in 120 degrees only.28-Dec-12 298
  299. 299. Directional Antenna (Cont’d)• Because these directional antenna only receive signals in particular direction, the amount of interference power they receive assuming a clustersize of 7 is reduced by 1/3.• With less interference power, the speech quality is much better than it needs to be.• So we can reduce the clustersize (increase interference power) and still have good speech quality.28-Dec-12 299
  300. 300. Directional Antenna• Trials show that in systems with 120 degree antenna, the clustersize can be as small as 3.• This allows more users to be supported, while keeping cell size fixed.• Because of the benefits offered by 120 degree antenna, these are most readily used by base station towers.28-Dec-12 300
  301. 301. 2.7.3 Microcell Zone Concept • Antennas are placed at the outer edges of the cell • 3 antennas at 3 corners and all are connected to the BS • Any channel may be assigned to any zone by the base station • Mobile is served by the zone with the strongest signal. • Handoff within a cell – No channel re- assignment – Switch the channel to a different zone site • Reduce interference – Low power transmitters are employed28-Dec-12 301
  302. 302. Complete Cellular NetworkA group of local base stations are connected (by wires) toa mobile switching center (MSC). MSC is connected tothe rest of the world (normal telephone system). MSC Public (Wired) Telephone MSC Network MSC MSC28-Dec-12 302
  303. 303. Terminologies involved in Cellular Phone Systems1) Mobile Identification Number (MIN): Subscriber’s Telephone No.2) Electronic Serial Number (ESN): Serial No. of the Mobile3) Station Class Mark(SCM): It indicates the maximum Transmitter power level for a particular user.International Mobile Subscriber identity number ( IMSI) Ex: GSM First 3 digit ( Mobile country code : MCC); next 2 mobile Network code ( MNC); Next 10 Mobile subscriber Identity no.( MSIC)262 02 454 275 1010 ( Germany; Optus commun; MSIC ) (India-404,405) (Airtel 02-Punjab,03 Himachal Prade, 10- Delhi NCR 900 MHz) 28-Dec-12 303
  304. 304. Cell Phone Codes Electronic Serial Number (ESN) - a unique 32- bit number programmed into the phone when it is manufactured Mobile Identification Number (MIN) - a 10- digit number derived from your phones number System Identification Code (SID) - a unique 5- digit number that is assigned to each carrier by the FCC28-Dec-12 304
  305. 305. All cell phones have special codes associated with them. These codes are used to identify the phone, the phones owner and the service provider. When you first power up the phone, it listens for an SID on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message. When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system. Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of your phones location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.28-Dec-12 305
  306. 306. The MTSO gets the call, and it tries to find you. It looks in its database to see which cell you are in. The MTSO picks a frequency pair that your phone will use in that cell to take the call. The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. Now, you are talking by two-way radio to a friend. As you move toward the edge of your cell, your cells base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phones signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell.28-Dec-12 306
  307. 307. As you travel, the signal is passed from cell to cell. Lets say youre on the phone and you move from one cell to another -- but the cell you move into is covered by another service provider, not yours. Instead of dropping the call, itll actually be handed off to the other service provider. If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds.28-Dec-12 307
  308. 308. Timing Diagram when a call is made by a Landline User to a Mobile 28-Dec-12 308
  309. 309. Timing Diagram when a call is made by a Mobile user to a Landline User 28-Dec-12 309
  310. 310. Roaming • All cellular systems provide a service called roaming. – This allows subscribers to operate in service areas other than the one from which service is subscribed. – When a mobile enters a city or geographic area that is different from its home service area, it is registered as a roamer in the new service area. – Periodically, the MSC issues a global command over each FCC in the system, asking for all mobiles which are previously unregistered to report their MIN and ESN over the RCC for billing purposes. – If a particular mobile user has roaming authorization for billing purposes, MSC registers the subscriber as a valid roamer.28-Dec-12 310
  311. 311. Outdoor Propagation Model• Radio transmission in a mobile communication system often takes place over irregular terrain.• The terrain profile may vary from a simple curved earth profile to a highly mountainous profile.• Presence of trees, buildings and other obstacles may also taken into account.• A number of propagation models are available to predict the path loss over irregular terrain.28-Dec-12 311
  312. 312. Longely-Rice Model• Applicable to point-to-point communication systems in the frequency range from 40MHz to 100GHz, over different kinds of terrain.• Point-to-point mode prediction-When a detailed terrain profile is available, the path specific parameters can be easily determined.• Area mode prediction- If the terrain profile is not known, the method provides techniques to estimate the path-specific parameters.28-Dec-12 312
  313. 313. • Certain modications over the rudimentary model like an extra urban factor (UF) due to urban clutter near the reciever is also included in this model.28-Dec-12 313
  314. 314. Disadvantages• Does not provide a way of determining corrections due to environmental factors.• Multipath is also not considered.28-Dec-12 314
  315. 315. 28-Dec-12 315
  316. 316. 28-Dec-12 316
  317. 317. 28-Dec-12 317
  318. 318. Fig. 4.23 & 4.24 (Theodore S. Rappaport: Wireless communication principles and practice,2/e, Pearson Education)28-Dec-12 318
  319. 319. Multipath Propagation• In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths.• Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.• The effects of multipath include constructive and destructive interference, and phase shifting of the signal.28-Dec-12 319
  320. 320. Multipath Fading• Multipath signals are received in a terrestrial environment, i.e., where different forms of propagation are present and the signals arrive at the receiver from transmitter via a variety of paths.• Therefore there would be multipath interference, causing multipath fading.• Adding the effect of movement of either Tx or Rx or the surrounding clutter to it, the received overall signal amplitude or phase changes over a small amount of time. Mainly this causes the fading.28-Dec-12 320
  321. 321. Fading• The term fading, or, small-scale fading, means rapid fluctuations of the amplitudes, phases, or multipath delays of a radio signal over a short period or short travel distance.28-Dec-12 321
  322. 322. Multipath Fading Effects28-Dec-12 322
  323. 323. Factors Influencing Fading28-Dec-12 323
  324. 324. Doppler Shift Geomerty28-Dec-12 324
  325. 325. • a mobile moving at a constant velocity v, along a path segment length d between points X and Y, while it receives signals from a remote BS source S.• The difference in path lengths travelled by the wave from source S to the mobile at points X and Y is ∆l = d cosƟ = v ∆ t cosƟ , where ∆ t is the time required for the mobile to travel from X to Y.28-Dec-12 325
  326. 326. • source is assumed to be very far away.• The phase change in the received signal due to the difference in path lengths is therefore28-Dec-12 326
  327. 327. • The small scale variations of a mobile radio signal can be considered as impulse response of the mobile radio channel.• Mobile radio channel may be modelled as a linear filter with time varying impulse response in continuous time.• consider the channel impulse response (time varying impulse response) h(d,t) and x(t), the transmitted signal.• The received signal y(d,t) at any position d28-Dec-12 327
  328. 328. Channel issues28-Dec-12 328
  329. 329. 28-Dec-12 329
  330. 330. 28-Dec-12 330
  331. 331. Time-varying impulse response28-Dec-12 331
  332. 332. 28-Dec-12 332
  333. 333. k is the gain of transmitter.28-Dec-12 333
  334. 334. Parameters of Mobile Multipath Channel (Ref: 5.4)• Multipath delay spread- due to the different multipath waves which have propagation delays which vary over different spatial locations of the receiver.• Coherence BW- the range of frequencies over which we get a flat response of the channel.• Doppler Spread- Spectral broadening of the signal at the receiver due to doppler shift.• Coherence time- time duration over which 2 signals arriving at the receiver have a strong correlation.28-Dec-12 334
  335. 335. Types of Small-Scale Fading Bs<Bc Bs>Bc στ <Ts στ >Ts Bs<BD Bs>BD Tc<Ts Tc>Ts28-Dec-12 335
  336. 336. Flat Fading• Such types of fading occurs when the bandwidth of the transmitted signal is less than the coherence bandwidth of the channel.• Equivalently if the symbol period of the signal is more than the delay spread of the channel, then the fading is flat fading.• Bs-Signal BW• Bc-Coherence BW• Ts-Symbol (signal) period• Tc- Coherence time• στ- rms delay spread• Bd- Doppler spread28-Dec-12 336
  337. 337. 28-Dec-12 337
  338. 338. • Over time, the received signal r(t) varies in gain, but the spectrum of transmission is preserved.• But in freq. selective fading, the received signal includes multiple versions of the transmitted waveform, which are attenuated (faded) & delayed in time, and hence the received signal is distorted.• Refer fig. 5.12 & 5.13(Theodore S. Rappaport: Wireless communication principles and practice,2/e, Pearson Education)28-Dec-12 338
  339. 339. Flat-fading (non-freq. Selective)28-Dec-12 339
  340. 340. 28-Dec-12 340
  341. 341. 28-Dec-12 341
  342. 342. Frequency selective fading28-Dec-12 342
  343. 343. 28-Dec-12 343
  344. 344. 28-Dec-12 344
  345. 345. Two independent fading issues28-Dec-12 345
  346. 346. Statistical models for multipath propagation• Many multipath models have been proposed to explain the observed statistical nature of a practical mobile channel.• The most popular of these models are Rayleigh model, which describes the NLoS propagation.• The Rayleigh model is used to model the statistical time varying nature of the received signal.28-Dec-12 346
  347. 347. Two ray NLoS multipath, resulting in Rayleigh fading28-Dec-12 347
  348. 348. Rayleigh Fading Model• Let there be two multipath signals S1 and S2 received at two different time instants due to the presence of obstacles as shown in Figure.• there can either be constructive or destructive interference between the two signals.28-Dec-12 348
  349. 349. Above distribution is known as Rayleigh Distribution and is shown in the figure for different σ values. It has been derived for slow fading.28-Dec-12 349
  350. 350. 28-Dec-12 350
  351. 351. 2nd order parameters of fading• Level Crossing Rate(LCR):- expected rate at which the Rayleigh fading envelope, normalized to the local rms signal level, crosses a specified level ‘R’ in a positive going directon.• The number of level crossings /second,28-Dec-12 351
  352. 352. • Average Fade Duration(AFD):- average period of time for which the received signal is below a specified level R.• Average fade duration,28-Dec-12 352
  353. 353. 28-Dec-12 353
  354. 354. 28-Dec-12 354
  355. 355. • Clarke’s model consider only flat fading conditions.• Do not consider multipath time delay or frequency selective fading conditions.• Impulse response of the model is,28-Dec-12 355
  356. 356. 28-Dec-12 356

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