Introduction: Mobile and Wireless Network

1. Introduction:Mobile and Wireless Network

2. 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

3. 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

4. 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

5. 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

6. 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 without cell reuse

7. 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

8. 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

9. 3. Transmission using Frequency Spectrum • p. 23 • ILF: voice frequency • MF: AM radio • VHF: FM radio • UHF: TV broadcasting, PCS

10. 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

11. 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

12. 3. Transmission using Frequency Spectrum • 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

13. 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

14. Fundamentals of Cellular Systems

15. 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

16. 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

17. 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

18. 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)

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 3. Cell Design Issues • Cell sectoring (p. 86) • Use 3 directional antennas instead of a non-directional antenna

27. 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

28. 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