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Note for Mobile Communication - MC By Indhumathi

  • Mobile Communication - MC
  • Note
  • Pondicherry University - PU
  • Electronics and Communication Engineering
  • 10 Topics
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Ajeena A Al- Ameen Engg. College Wireless Mobile Communication Question Bank with Solutions 2 MARKS QUESTIONS previous question paper questions 1. What is Microcell? State it Drawbacks Microcell network is served by a low power cellular base station and is connected to base station by fiber/microwave link, Drawbacks : Dropped Calls,Cell Dragging Cell Dragging : Slow moving mobiles (pedestrians) may have very slow decay in RSSI and may stay with one Base Station until it has moved deep within another cell. This results in Channel Interference. 2.What are the three basic propagation mechanisms which impact propagation in a mobile communication system ? Reflection ,Diffraction and Scattering are the basic propagation mechanism which impact propagation in mobile communication system. Reflection : occurs when propagating electromagnetic wave impinges upon on object which has very large dimension compared to the wavelength of propagating wave. It occurs from surface of earth and from buildings and walls. Diffraction : Radio path between transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). Scattering : Occurs when medium through which wave travels consists of objects with dimensions that are small compared to the wavelength and where the number of obstrucles per unit volume is large 3.If a cellular operator is allocated 12.5 Mhz for each simplex band and if Bt is 12.5 Mhz Bguard is 10khz and Bc is 30khz find the number of channels available in a FDMA system The number of channels available in FDMA system is N= Bt - 2Bguard Bc 6 3 = 12.5 *10 -2(10*10 ) = 3*103 416 4 Define Jamming Margin The level of interference (jamming) that a system is able to accept and still maintain a specified bit error ratio even though the signal to noise ratio is decreasing . Or It is the maximum jamming power to signal power ratio that a spread spectrum receiver can tolerate while still maintaining the specified bit error rate www.edutalks.org Page 1

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Ajeena A Al- Ameen Engg. College of average powers of interference j and data signal ps 5. What is the cut off frequency of baseband ,Gaussian ,pulse shaping filter used in GSM System? Baseband filter cut off frequency from few kilohertz to 20 Mhz & Gaussian filter 5 Mhz 6.Why Hexagon Geometry are always proffered ?Explain Hexagon Compared to circle has largest area and therefore has large number of users. A Cell must be designed to serve the weakest mobile within footprint Hexagon cell is universally adopted and manageable in handling performance analysis & System modeling Base Station transmitter placed either in centre of cell or in edge of cell, 7. What is trunking in cellular Radio system? Trunking allows a large number of users to share small number of channels in a cell by providing access to each user on demand from set of available channels. In a trunked system each user will be allocated a channel on a per call basis and when terminated the pervious occupied channel is returned to pool of available channels. 8. Find the far field distance for an antenna with maximum dimension of 1m and operating frequency of 900 mhz Df =2D2 /lambda = lambda = C /V = 3*108 /900 * 106 = .33 Df = 2*(1)2 / .33 = 6m ESSAY Questions 1. Explain adjacent and co channel interference Interference and System Capacity Interference is the major limiting factor in the performance of cellular radio systems. Sources of interference another mobile in the same cell a cell in progress in a neighboring cell other base stations operating in the same frequency band... www.edutalks.org Page 2

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Ajeena A Al- Ameen Engg. College Interference on voice channels causes cross talk, where the subscriber hears interference in the background due to an undesired transmission. On control channels, interference leads to missed and blocked calls due to errors in the digital signaling. Interference is more severe in urban areas, due to the greater RF noise floor and the large number of base stations and mobiles. The two major types of system-generated cellular interference are co-channel interference and adjacent channel interference. Co-channel Interference and System Capacity Frequency reuse implies that in a given coverage area there are several cells that use the same set of frequencies. These cells are called co-channel cells, and the interference between signals from these cells is called co-channel interference. Unlike thermal noise which can be overcome by increasing the signal-to-noise ratio (SNR), co-channel interference cannot be combated by simply increasing the carrier power of a transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells. To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation. When the size of each cell is approximately the same and the base stations transmit the same power, the co-channel interference ratio is independent of the transmitted power and becomes a function of the radius of the cell (R) and the distance between centers of the nearest co-channel cells (D). By increasing the ratio of D/R, the spatial separation between co-channel cells relative to the coverage distance of a cell is increased. The parameter Q, called the co-channel reuse ratio, is related to the cluster size For a hexagonal Geometry D = 3N R • N small, Q small, larger capacity N large, Q large, better transmission quality due to a small level of co-channel interference. Q= • signal-to-interference ratio for a mobile receiver which monitors a forward channel: S = I S i0 ∑ i =1 www.edutalks.org Ii Page 3

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Ajeena A • Al- Ameen Engg. College the average received power at a distance d from the transmitting antenna is approx. by Pr = P0 ( d −n ) d0 OR d ) d0 where P0 is the power received at a close-in reference point in the far field region of the antenna at a small distance d0 from the transmitting antenna, and n is the path loss exponent (is the reduction of power density of electromagnetic wave propogates through space) Pr (dBm) = P0 (dBm) − 10n log( Adjacent Channel Interference • Results from imperfect receiver filters which allow nearby frequencies to leak into the pass band. • Near-far effect (the adjacent channel interference is particularly serious.) * • • • • • • • • An adjacent channel user is transmitting in very close range to s subscriber’s receiver, while the receiver attempts to receive a base station on the desired channel. It also occurs when a mobile close to a base station transmits on a channel close to one being used by a weak mobile. The base station may use by a weak mobile. The base station may have difficulty in discriminating the desired mobile user from the “bleed over” caused by the close adjacent channel mobile Adjacent channel interference can be minimized through careful filtering and channel assignments. Since each cell is given only a fraction of the available channels, a cell need not be Assigned channels which are all adjacent in frequency. By keeping the frequency separation between each channel in a given cell as large as possible, the adjacent channel interference may be reduced considerably. Channel allocation schemes also prevent a secondary source of adjacent channel interference by avoiding the use of adjacent channels in neighboring cell sites.. If the subscriber is at the distance d1 and interferer is at d2 then Signal To Interference Ration is o S/I = (d1/d2)n where n is path loss exponent Power Control for Reducing Interferences www.edutalks.org Page 4

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