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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 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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Ajeena A • • • • Al- Ameen Engg. College In practical cellular radio and personal communication systems, the power levels transmitted by every mobile unit are under constant control by the serving base stations. This is done to ensure that each mobile transmits the smallest power necessary on the reverse channel. Power control not only helps prolong battery life, also reduces the signal to interference ratio on the reverse channel. It is especially important for CDMA systems, because every user in every cell share the same radio channel. (to reduce the co-channel interference). 2. Derive the expression for blocking probability (Erlang B formula ) of a trunked system which provides no queuing for blocked calls Trunking and Grade of Service • • • • • • • Cellular radio systems rely on trunking to accomodate a large number of users in a limited radio spectrum In a trunked radio system, each user is allocated a channel on a per call basis and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels The fundamentals of trunking theory were developed by Erlang One Erlang represents the amount of traffic intensity carried by a channel that is completely occupied (i.e. one call-hour per hour or one call-minute per minute) The GOS is a measure of the ability of a user to access a trunked system during the busiesthour GOS is typically given as the likelihood that a call is blocked or the likelihood of a call experiencing a delay greater than a certain queuing time • The traffic intensity offered by each user is equal to the call request rate multiplied by the holding time Each user generates a traffic intensity of Au Erlangs given by • For a system containing U user and an unspecified number of channels, the total offered traffic intensity is • In a C channel trunked system, if the traffic is equally distributed among the channels, then the traffic intensity per channel is • • There are two types of trunked systems: blocked calls cleared and blocked calls delayed www.edutalks.org Page 5

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