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CELLULAR AND MOBILE COMMUNICATIONS
UNIT-I INTRODUCTION
HISTORY OF COMMUNICATION AND EVOLUTION
Evolution of Telecommunications WRITING SIGNALS
SIGNALLING
VISUAL TELEGRAPHY
ELECTRICAL TELEGRAPHY
MORSE CODE IN TELEGRAPHY
Telephone Technology
Wall mounted Telephone
TELEPHONES
TIMELINE OF TELECOMMUNICATIONS
TIMELINE OF TELECOMMUNICATIONS..
TELECOMMUNICATIONS TREE The trunk of the tree represents the technological prerequisites for successive unfolding of the various telecommunication domains into the branches of the tree The leaves of the branches represent evolution within the separate telecommunication domains
EVOLUTION OF TELECOMMUNICATION FROM TELEPHONY The basic theory of sound developed by Helmholtz supported the evolution from telegraphy to telephony. The early automation of industrial processes enabled the replacement of manual switchboards by automatic switching devices. The discovery of electromagnetic radiation and the subsequent development of devices for generating and detecting such waves led to the development of radio-telegraphy. The creation of electronic tubes (diodes and triodes) started the electronic era, which enabled the evolution from radio-telegraphy to radio-telephony and mobile radio.
The feedback principle applied in electronic circuitry facilitated the generation of high frequencies and thus the development of medium- and shortwave radio transmission and a new technology of circuit combination: carrier frequency, or multiplexing. The development of very high frequency generators in 1920 and velocity- modulated electronic tubes in the early 1930s made radio-relay transmission possible, Rockets, transistors, and solar cells were the ingredients for the satellite branch. The laser and extremely pure glass enabled the fiber optics branch to grow. ICs (integrated circuits) and microprocessors were the nourishment for the cellular radio branch. EVOLUTION OF TELECOMMUNICATION FROM TELEPHONY
Within two centuries telecommunications experienced tremendous progress. This development is best demonstrated by the example of transatlantic submarine cable transmission: 1866. The first transatlantic telegraph cable installed. Morse-coded telegraph channel with a speed of about 5 words per minute. 1956. The first transatlantic telephone cable installed. operated 36 telephone channels on two separate cables. 2000. The state-of-the art transatlantic fiber optic cable installed. 12 fibers each with a capacity of 40 WDM 10-Gbps channels, thus a total of 4.8 Tbps, which is equivalent to 58,060,800 telephone channels.
SIMPLE TELEPHONE COMMUNICATION
Simple telephone communication (simplex communication) The circuit consisting of a MIC, an Inductor, a battery and a loudspeaker. Microphone: Converts speech to electrical signal. Earphones: Converts electrical to audio signal.
Simple telephone communication.. Microphone: used carbon microphone. carbon granules placed in a box conduct electricity. when sound waves impinge on the diaphragm it vibrates causing the carbon granules to compress or expand thus changing resistivity offered by granules. Inductor: It acts as high impedance element for voice freq signals but permits dc from the battery to flow to the microphone and earphone. Voice freq signals generated by microphone reach the ear phone without being shunted by the battery arm and are converted to audio signals there. Earphone: It is usually an electromagnetic diaphragm which is positioned such that there is an air gap between it and the poles of electromagnet. when elctro magnet is energized by passing a current a force is exerted on the diaphragm thus vibrating the diaphragm and producing sound waves.
Simple telephone communication.. (HALF DUPLEX)
Basic terms of telecommunication Telecommunications represent the transfer of information, from an entity at one place to an entity at another place whereas the information can be in the form of data, voice or symbol. The entities can be human beings, computers, facsimile machines, telegraphy machines, phones or so on.
Basic terms of telecommunication.. In telephone conversation Calling Subscriber: The one who initiates the call. Called Subscriber: The one for whom the call is destined. Source and Destination: The communicating entities respectively.
Unit 1 CELLULAR MOBILE RADIO SYSTEMS ELEMENTS OF CELLULAR RADIO SYSTEM DESIGN
Contents CELLULAR MOBILE RADIO SYSTEMS: Introduction to Cellular Mobile System Performance Criteria Uniqueness of Mobile Radio Environment Operation of Cellular Systems Hexagonal Shaped Cells Analog and Digital Cellular systems. ELEMENTS OF CELLULAR RADIO SYSTEM DESIGN: General description of the problem Concept of frequency channels Co-channel Interference Reduction Factor Desired C/I from a normal case in a Omni directional Antenna system Cell splitting Consideration of the components of Cellular system.
Introduction to Cellular Mobile System Limitations of conventional mobile telephone systems limited service capability Poor service performance Inefficient frequency spectrum utilization
Limited Service Capability Lager coverage area of a Cell Transmitted Power is High The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the call will be dropped. (Problem of Handoff Process) The conventional system is that the number of active users is limited to the number of channels assigned to a particular frequency zone.
Poor Service Performance In the past, a total of 33 channels were allocated to three mobile telephone s ystems: Mobile Telephone Service (MTS) at 40MHz, provides 11 channels. Improved Mobile Telephone Service (IMTS) MJ systems at 150MHz , provides 11 channels. Improved Mobile Telephone Service (IMTS) MK systems at 450MHz , provides 11 channels. In 1976, New York City had 6 channels of MJ serving 320 customers, with another 2400 customers on a waiting list. New York City also had 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list. The large number of subscribers created a high blocking probability during b usy hours (demand will great). A high capacity system for mobile telephones was needed.
Inefficient Frequency Spectrum Utilization Frequency utilization measurement Mo, is defined as the maximum number of customers that could be served by one channel at the busy hour. Assume an average calling time of 1.76 min and apply the Erlang B model (lost- calls-cleared conditions). Calculate the blocking probability as follows: Use 6 channels, with each channel serving the two different numbers of customers
Inefficient Frequency Spectrum Utilization (Contd.) B1 = 50% (MJ system) B2 =30% (MK system) As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently since each channel can only serve one customer at a time in a whole area. This is overcome by the new cellular system.
Performance Criteria There are three categories for specifying performance criteria. 1. Voice quality 2. Service quality 3. Special features
VOICE QUALITY For any given commercial communications system, the voice quality will be based upon the following criterion: a set value x at which y% of customers rate the system voice quality (from transmitter to receiver) as good or excellent, the top two CIRCUIT MERITS (CM) of the five listed below. CM5 : Excellent (Speech Perfectly Understandable) CM4 : Good (Speech Easily Understandable, Some Noise) CM3 : Fair (Speech Understandable with a slight effort, occasional repetitions needed) CM2 : Poor (Speech Understandable only with considerable effort, frequent repetitions needed) CM1 : Unusable (Speech not Understandable) As the percentage of customers choosing CM4 and CM5 increases, the cost of building the system rises.
SERVICE QUALITY Three items are required for service quality. 1. Coverage: The system should serve an area as large as possible. With radio coverage, however, because of irregular terrain configurations, it is usually not practical to cover 100 percent of the area for two reasons: a. The transmitted power would have to be very high to illuminate weak spots with sufficient reception, a significant added cost factor. b. The higher the transmitted power, the harder it becomes to control interference. Therefore, systems usually try to cover 90 percent of an area in flat terrain and 75 percent of an area in hilly terrain. The combined voice quality and coverage criteria in AMPS
SERVICE QUALITY (Contd.) 2. Required grade of service: For a normal start-up system, the grade of service is specified for a blocking probability of .02 for initiating calls at the busy hour. This is an average value. However, the blocking probability at each cell site will be different. At the busy hour, near freeways, automobile traffic is usually heavy, so the blocking probability at certain cell sites may be higher than 2 percent, especially when car accidents occur. To decrease the blocking probability requires a good system plan and a sufficient number of radio channels. 3. Number of dropped calls: During Q calls in an hour, if a call is dropped and Q1 calls are completed, then the call drop rate is 1/Q. This drop rate must be kept low. A high drop rate could be caused by either coverage problems or handoff problems related to inadequate channel availability or weak reception.
Special Features Call Forwarding Call Waiting Voice Stored (VSR) Box Automatic Roaming Short Message Service (SMS) Multimedia Service (MMS) Push-to- talk (PTT) Navigation Services. However sometimes the customers have to pay extra charges for these special services.
A Basic Cellular System A basic analog cellular system consists of three subsystems: 1. Mobile Unit 2. Cell Site 3. Mobile Telephone Switching Office (MTSO) 4. CONNECTIONS to link the three subsystems
A Basic Cellular System (Contd.)
A Basic Cellular System (Contd.) Mobile Units: A mobile telephone unit contains a control unit, a transceiver, and an antenna system. Cell site: The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals. MTSO: The switching office, the central coordinating element for all cell sites, contains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, provides operation and maintenance, and handles billing activities. Connections: The radio and high-speed data links connect the three subsystems. Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units.
A Basic Cellular System (Contd.) The MTSO is the heart of the analog cellular mobile system. Its processor provides central coordination and cellular administration. The cellular switch, which can be either analog or digital, switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network. It uses voice trunks, It also contains data links providing supervision links between the processor and the switch and between the cell sites and the processor. The radio link carries the voice and signaling between the mobile unit and the cell site. The high-speed data links cannot be transmitted over the standard telephone trunks and therefore must use either microwave links or T-carriers (wire lines). Microwave radio links or T-carriers carry both voice and data between cell site and the MTSO.
Uniqueness of Mobile Radio Environment Description of Mobile Radio Transmission Medium The Propagation Attenuation The propagation path loss increases not only with frequency but also with distance. The incident (or Elevation) angle of the Direct Wave is 慮1. The incident angle of the Reflected Wave is 慮2. The propagation path loss would be 40 dB/dec at a signal receiver will be observed by the mobile unit. Therefore C is inversely proportional to W.
Operation of cellular systems 1. Mobile unit initialization The user activates the mobile unit. The receiver scan 21 set-up channels from designed 333 channels. It then selects the strongest cell sites. This is called self-location scheme.
Operation of Cellular Systems (Contd.) 2. Mobile Originated Call The user places the called number into an originated register in the mobile unit, checks to see that the number is correct, and pushes the "send" button. A request for service is sent on a selected set-up channel obtained from a self-location scheme. The cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use. At the same time the cell site sends a request to the mobile telephone switching office (MTSO) via a high-speed data link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it through the best directive antenna to link the mobile unit. The MTSO also con足 nects the wire-line party through the telephone company zone office.
Operation of Cellular Systems (Contd.) 3. Network Originated Call A land-line party dials a mobile unit number. the telephone company zone office recognizes that the number is mobile and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and the search algorithm. Each cell site transmits the page on its own set-up channel. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it. and responds to the cell site. The mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert. company zone office.
Operation of Cellular Systems (Contd.) 4. Call termination When the mobile user turns off the transmitter, a particular signal (signaling tone) transmits to the cell site, and both sides free the voice channel. The mobile unit resumes monitoring pages through the strongest set- up cannel.
Operation of Cellular Systems (Contd.) 5. Handoff Procedure During the call, two parties are on a voice channel. When the mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak. The present cell site requests a handoff. The system switches the call to a new frequency channel in a new cell site without either interrupting the call or alerting the user. The call continues as long as the user is talking. The user does not notice the handoff occurrences.
Hexagonal Shaped Cells The Hexagonal shaped cells properly fit into the planned area with no gap and no overlap between the cells. The cellular system which depend on the hexagonal cells is made implementable either by a simple mechanism or by a statistical approach.
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital
UNIT-I PPT (2).pptx Cell shape analog & Digital

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UNIT-I PPT (2).pptx Cell shape analog & Digital

  • 3. HISTORY OF COMMUNICATION AND EVOLUTION
  • 8. MORSE CODE IN TELEGRAPHY
  • 14. TELECOMMUNICATIONS TREE The trunk of the tree represents the technological prerequisites for successive unfolding of the various telecommunication domains into the branches of the tree The leaves of the branches represent evolution within the separate telecommunication domains
  • 15. EVOLUTION OF TELECOMMUNICATION FROM TELEPHONY The basic theory of sound developed by Helmholtz supported the evolution from telegraphy to telephony. The early automation of industrial processes enabled the replacement of manual switchboards by automatic switching devices. The discovery of electromagnetic radiation and the subsequent development of devices for generating and detecting such waves led to the development of radio-telegraphy. The creation of electronic tubes (diodes and triodes) started the electronic era, which enabled the evolution from radio-telegraphy to radio-telephony and mobile radio.
  • 16. The feedback principle applied in electronic circuitry facilitated the generation of high frequencies and thus the development of medium- and shortwave radio transmission and a new technology of circuit combination: carrier frequency, or multiplexing. The development of very high frequency generators in 1920 and velocity- modulated electronic tubes in the early 1930s made radio-relay transmission possible, Rockets, transistors, and solar cells were the ingredients for the satellite branch. The laser and extremely pure glass enabled the fiber optics branch to grow. ICs (integrated circuits) and microprocessors were the nourishment for the cellular radio branch. EVOLUTION OF TELECOMMUNICATION FROM TELEPHONY
  • 17. Within two centuries telecommunications experienced tremendous progress. This development is best demonstrated by the example of transatlantic submarine cable transmission: 1866. The first transatlantic telegraph cable installed. Morse-coded telegraph channel with a speed of about 5 words per minute. 1956. The first transatlantic telephone cable installed. operated 36 telephone channels on two separate cables. 2000. The state-of-the art transatlantic fiber optic cable installed. 12 fibers each with a capacity of 40 WDM 10-Gbps channels, thus a total of 4.8 Tbps, which is equivalent to 58,060,800 telephone channels.
  • 19. Simple telephone communication (simplex communication) The circuit consisting of a MIC, an Inductor, a battery and a loudspeaker. Microphone: Converts speech to electrical signal. Earphones: Converts electrical to audio signal.
  • 20. Simple telephone communication.. Microphone: used carbon microphone. carbon granules placed in a box conduct electricity. when sound waves impinge on the diaphragm it vibrates causing the carbon granules to compress or expand thus changing resistivity offered by granules. Inductor: It acts as high impedance element for voice freq signals but permits dc from the battery to flow to the microphone and earphone. Voice freq signals generated by microphone reach the ear phone without being shunted by the battery arm and are converted to audio signals there. Earphone: It is usually an electromagnetic diaphragm which is positioned such that there is an air gap between it and the poles of electromagnet. when elctro magnet is energized by passing a current a force is exerted on the diaphragm thus vibrating the diaphragm and producing sound waves.
  • 22. Basic terms of telecommunication Telecommunications represent the transfer of information, from an entity at one place to an entity at another place whereas the information can be in the form of data, voice or symbol. The entities can be human beings, computers, facsimile machines, telegraphy machines, phones or so on.
  • 23. Basic terms of telecommunication.. In telephone conversation Calling Subscriber: The one who initiates the call. Called Subscriber: The one for whom the call is destined. Source and Destination: The communicating entities respectively.
  • 24. Unit 1 CELLULAR MOBILE RADIO SYSTEMS ELEMENTS OF CELLULAR RADIO SYSTEM DESIGN
  • 25. Contents CELLULAR MOBILE RADIO SYSTEMS: Introduction to Cellular Mobile System Performance Criteria Uniqueness of Mobile Radio Environment Operation of Cellular Systems Hexagonal Shaped Cells Analog and Digital Cellular systems. ELEMENTS OF CELLULAR RADIO SYSTEM DESIGN: General description of the problem Concept of frequency channels Co-channel Interference Reduction Factor Desired C/I from a normal case in a Omni directional Antenna system Cell splitting Consideration of the components of Cellular system.
  • 26. Introduction to Cellular Mobile System Limitations of conventional mobile telephone systems limited service capability Poor service performance Inefficient frequency spectrum utilization
  • 27. Limited Service Capability Lager coverage area of a Cell Transmitted Power is High The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the call will be dropped. (Problem of Handoff Process) The conventional system is that the number of active users is limited to the number of channels assigned to a particular frequency zone.
  • 28. Poor Service Performance In the past, a total of 33 channels were allocated to three mobile telephone s ystems: Mobile Telephone Service (MTS) at 40MHz, provides 11 channels. Improved Mobile Telephone Service (IMTS) MJ systems at 150MHz , provides 11 channels. Improved Mobile Telephone Service (IMTS) MK systems at 450MHz , provides 11 channels. In 1976, New York City had 6 channels of MJ serving 320 customers, with another 2400 customers on a waiting list. New York City also had 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list. The large number of subscribers created a high blocking probability during b usy hours (demand will great). A high capacity system for mobile telephones was needed.
  • 29. Inefficient Frequency Spectrum Utilization Frequency utilization measurement Mo, is defined as the maximum number of customers that could be served by one channel at the busy hour. Assume an average calling time of 1.76 min and apply the Erlang B model (lost- calls-cleared conditions). Calculate the blocking probability as follows: Use 6 channels, with each channel serving the two different numbers of customers
  • 30. Inefficient Frequency Spectrum Utilization (Contd.) B1 = 50% (MJ system) B2 =30% (MK system) As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently since each channel can only serve one customer at a time in a whole area. This is overcome by the new cellular system.
  • 31. Performance Criteria There are three categories for specifying performance criteria. 1. Voice quality 2. Service quality 3. Special features
  • 32. VOICE QUALITY For any given commercial communications system, the voice quality will be based upon the following criterion: a set value x at which y% of customers rate the system voice quality (from transmitter to receiver) as good or excellent, the top two CIRCUIT MERITS (CM) of the five listed below. CM5 : Excellent (Speech Perfectly Understandable) CM4 : Good (Speech Easily Understandable, Some Noise) CM3 : Fair (Speech Understandable with a slight effort, occasional repetitions needed) CM2 : Poor (Speech Understandable only with considerable effort, frequent repetitions needed) CM1 : Unusable (Speech not Understandable) As the percentage of customers choosing CM4 and CM5 increases, the cost of building the system rises.
  • 33. SERVICE QUALITY Three items are required for service quality. 1. Coverage: The system should serve an area as large as possible. With radio coverage, however, because of irregular terrain configurations, it is usually not practical to cover 100 percent of the area for two reasons: a. The transmitted power would have to be very high to illuminate weak spots with sufficient reception, a significant added cost factor. b. The higher the transmitted power, the harder it becomes to control interference. Therefore, systems usually try to cover 90 percent of an area in flat terrain and 75 percent of an area in hilly terrain. The combined voice quality and coverage criteria in AMPS
  • 34. SERVICE QUALITY (Contd.) 2. Required grade of service: For a normal start-up system, the grade of service is specified for a blocking probability of .02 for initiating calls at the busy hour. This is an average value. However, the blocking probability at each cell site will be different. At the busy hour, near freeways, automobile traffic is usually heavy, so the blocking probability at certain cell sites may be higher than 2 percent, especially when car accidents occur. To decrease the blocking probability requires a good system plan and a sufficient number of radio channels. 3. Number of dropped calls: During Q calls in an hour, if a call is dropped and Q1 calls are completed, then the call drop rate is 1/Q. This drop rate must be kept low. A high drop rate could be caused by either coverage problems or handoff problems related to inadequate channel availability or weak reception.
  • 35. Special Features Call Forwarding Call Waiting Voice Stored (VSR) Box Automatic Roaming Short Message Service (SMS) Multimedia Service (MMS) Push-to- talk (PTT) Navigation Services. However sometimes the customers have to pay extra charges for these special services.
  • 36. A Basic Cellular System A basic analog cellular system consists of three subsystems: 1. Mobile Unit 2. Cell Site 3. Mobile Telephone Switching Office (MTSO) 4. CONNECTIONS to link the three subsystems
  • 37. A Basic Cellular System (Contd.)
  • 38. A Basic Cellular System (Contd.) Mobile Units: A mobile telephone unit contains a control unit, a transceiver, and an antenna system. Cell site: The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals. MTSO: The switching office, the central coordinating element for all cell sites, contains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, provides operation and maintenance, and handles billing activities. Connections: The radio and high-speed data links connect the three subsystems. Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units.
  • 39. A Basic Cellular System (Contd.) The MTSO is the heart of the analog cellular mobile system. Its processor provides central coordination and cellular administration. The cellular switch, which can be either analog or digital, switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network. It uses voice trunks, It also contains data links providing supervision links between the processor and the switch and between the cell sites and the processor. The radio link carries the voice and signaling between the mobile unit and the cell site. The high-speed data links cannot be transmitted over the standard telephone trunks and therefore must use either microwave links or T-carriers (wire lines). Microwave radio links or T-carriers carry both voice and data between cell site and the MTSO.
  • 40. Uniqueness of Mobile Radio Environment Description of Mobile Radio Transmission Medium The Propagation Attenuation The propagation path loss increases not only with frequency but also with distance. The incident (or Elevation) angle of the Direct Wave is 慮1. The incident angle of the Reflected Wave is 慮2. The propagation path loss would be 40 dB/dec at a signal receiver will be observed by the mobile unit. Therefore C is inversely proportional to W.
  • 41. Operation of cellular systems 1. Mobile unit initialization The user activates the mobile unit. The receiver scan 21 set-up channels from designed 333 channels. It then selects the strongest cell sites. This is called self-location scheme.
  • 42. Operation of Cellular Systems (Contd.) 2. Mobile Originated Call The user places the called number into an originated register in the mobile unit, checks to see that the number is correct, and pushes the "send" button. A request for service is sent on a selected set-up channel obtained from a self-location scheme. The cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use. At the same time the cell site sends a request to the mobile telephone switching office (MTSO) via a high-speed data link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it through the best directive antenna to link the mobile unit. The MTSO also con足 nects the wire-line party through the telephone company zone office.
  • 43. Operation of Cellular Systems (Contd.) 3. Network Originated Call A land-line party dials a mobile unit number. the telephone company zone office recognizes that the number is mobile and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and the search algorithm. Each cell site transmits the page on its own set-up channel. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it. and responds to the cell site. The mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert. company zone office.
  • 44. Operation of Cellular Systems (Contd.) 4. Call termination When the mobile user turns off the transmitter, a particular signal (signaling tone) transmits to the cell site, and both sides free the voice channel. The mobile unit resumes monitoring pages through the strongest set- up cannel.
  • 45. Operation of Cellular Systems (Contd.) 5. Handoff Procedure During the call, two parties are on a voice channel. When the mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak. The present cell site requests a handoff. The system switches the call to a new frequency channel in a new cell site without either interrupting the call or alerting the user. The call continues as long as the user is talking. The user does not notice the handoff occurrences.
  • 46. Hexagonal Shaped Cells The Hexagonal shaped cells properly fit into the planned area with no gap and no overlap between the cells. The cellular system which depend on the hexagonal cells is made implementable either by a simple mechanism or by a statistical approach.