Introduction to the analog mobile phone system

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  • Needed method for better spectrum use, break coverage area into small cells
  • Requires timing synchronization More advanced versions use channelization where time/frequency domains are split into smaller channels and time slots, then allocated to users, packetized
  • At each time slot the signal is assigned a frequency band, signal moves around band, immune from narrowband noise interference. Users are defined pseudorandom codes. Time synchronization among users not required. Direct Sequence Spread Spectrum – spreads the signal over a wide bandwidth to lower power.
  • Base stations communicate with the Mobile Switching Center which controls subscriber verification, what tower subscriber is near, handoffs, links to Public Switched Telephone Network
  • Cells with the same letter reuse the same frequencies, cell clusters are outlined in bold. The typical cluster size is N=7, with a frequency reuse factor of 1/7. Many low power cells, increases the number of possible users by reusing frequencies, for example: we go from 12 conversations with one transmitter to 1200 conversations with 100 smaller transmitters. Note that user power control is critical here. – to prevent interference with adjoining cells.
  • Frequency reuse: S channels =k N; k – group of k channels divided among N cells in a cluster (typically 4,7,12 C= total number of duplex channels = M S = M k N; M is the number of clusters in a system Small cluster size means that co-channel cells are closer, but higher capacity in system Large cluster size, less interference N=I^2 + I*J + j^2 for no gaps in coverage Example of frequency reuse calculation see notes
  • Dynamic is more complex, needs channel occupancy, traffic distribution, signal strength indicators Depends on MSC capabilities, coordination between MSCs
  • Passing call between cells, when, where, what channel. When a user passes between cells, needs voice and control signals to be reallocated. Based on power margins = P_handoff –P_minimum =delta, if delta too large, system is handing off frequently, if too low, calls can drop below minimum and be lost due to insufficient time to handoff. Need to prioritize handoffs –don’t want to drop calls, better not to let a call be placed. Newer systems (second generation): Mobile assisted handoffs- mobiles measure signal strength from nearby base stations and reports results to the active base stations, when power from adjacent exceeds that of active, handoff initiated. Good for microcells. Can get complicated for microcells and high-speed users, when users roam into another company’s network Need to make decision at right time, don’t want to switch due to momentary fading Depends on power level, loading in adjacent cells, channel availability Try to predict user dwell time in cells – works for highways which have known speed ranges and length; not good for microcells, complex environments.
  • High speed users use larger cells, too many handoffs for microcells and slow users- urban environments MSC tries to estimate user speed Use same tower for different power transmitters Cell Dragging – a slow moving user can have a line-of sight signal be quite strong and signal strength does not decay quickly, this can “stretch” the cell Newer systems-spread spectrum users share channels in different cells, received by multiple base stations,
  • -Can’t overcome by increasing power, just causes more interference, need to separate (propogation isolation), Q=D/R, R- radius of cell, D istance between centers of co-channel cells Q – co-channel reuse ratio (N=3,Q=3;N=7,Q=4.6;N=12, Q=6) SIR – signal to interference ratio = S/I=signal power/(sum of all interferers)
  • Can reduce co-channel interference due to directionality Improves SIR
  • More precise power control needed, can add small cells in dense areas, leave others be. Requires new basestations,more handoffs
  • BCH=block coding method for error detection and correction, non-binary, part of Reed-Solomon Codes
  • SAT –supervisory tones during voice transmission to confirm that the base station/mobile are properly connected, handshake repeated at least every 250 ms. Mobile decodes and transmits back to base. Wideband blank-and-burst encoding- wideband 10 kbps data streams, uses NRZ, Manchester (bi-phase coding), concentrated at 10kHz, sends a wide array of commands to users, uses BCH block codes less than 100 ms in duration
  • Introduction to the analog mobile phone system

    1. 1. Introduction to the AdvancedMobile Phone System (AMPS) EE421 Fall 2001 Dr. Julie Dickerson
    2. 2. Information Sources• Theodore Rappaport, Wireless Communications: Principles & Practice, Prentice-Hall, 1996• Gallagher & Snyder, Mobile Telecommunications Networking with IS-41, McGraw-Hill• Tutorial on Mobile Communications from the International Engineering Consortium - on course web site03/22/12 EE421 Fall 2001 2
    3. 3. History• 1946 First mobile phones deployed, FM 120KHz, half-duplex• 1950 FM 60KHz, better RF filters available• Mid 1960’s: 30KHz, full-duplex, IMTS (Improved Mobile Telephone Service), limited capacity: 12 channels, 543 paying customers in New York City (1976)03/22/12 EE421 Fall 2001 3
    4. 4. Early Mobile Systems03/22/12 EE421 Fall 2001 4
    5. 5. Cellular Communications• 1968 – Cell concept proposed by AT&T• 1983 – FCC allocates 40 MHz of spectrum in 800 MHz band, 30 kHz for each one-way channel (for full-duplex) analog• Channels split between two carriers• More frequencies allocated, 1.8-1.9 GHz, etc. over time• Bandwidth limited03/22/12 EE421 Fall 2001 5
    6. 6. Cellular Concept03/22/12 EE421 Fall 2001 6
    7. 7. Digital Communications• Early 1990’s digital systems begin with the goal of increasing the number of users, D- AMPS• TDMA• FDMA• CDMA03/22/12 EE421 Fall 2001 7
    8. 8. FDMA-Frequency Division F Multiple Access r e q u e n c y Time -Requires no synchronization or central timing, channels independent.03/22/12 EE421 Fall 2001 8
    9. 9. TDMA-Time Division Multiple Access: Fixed Slots F r e q u e n c y Time03/22/12 EE421 Fall 2001 9
    10. 10. CDMA-Code Division Multiple Access: Frequency Hopping F r e q u e n c y Time03/22/12 EE421 Fall 2001 10
    11. 11. 03/22/12 EE421 Fall 2001 11
    12. 12. Terminology• Base Station – Fixed station used for radio communication with mobiles. Located at the center or edge of coverage region. Consists of radio channels, transmit/receive antennas.• Control Channels – Radio channels used for transmission of call setup, request, initiation and other control purposes• Full Duplex – Communication system that allows simultaneous two-way communication, transmission reception usually on two different frequencies (FDD)03/22/12 EE421 Fall 2001 12
    13. 13. • Forward Channel – Radio channel for transmission from base station to mobile• Reverse channel – Radio channel for transmission from mobile to base station• Handoff – Process of transferring a mobile from one channel or base station to another• Mobile Switching Center – Switching center that coordinates call routing in a large service area. MSC connects cellular base stations and the mobiles to the PSTN (also called Mobile Telephone switching office (MTSO)03/22/12 EE421 Fall 2001 13
    14. 14. Wireless System Basics Reverse Link Forward Link Control or Setup Channels Mobile UnitBase Station 03/22/12 EE421 Fall 2001 14
    15. 15. Cellular System03/22/12 EE421 Fall 2001 15
    16. 16. Making calls from a cellphone1. Mobile sends call initiation request, its ID number to base station on reverse control channel2. Base station receives and relays to the MSC3. MSC validates user, instructs base station to move mobile to voice channels4. Base station pages mobile with instructions5. Mobile moves to voice channel6. MSC connects mobile to PSTN7. Voice transmission/reception between mobile and base station begins03/22/12 EE421 Fall 2001 16
    17. 17. Making calls to a cellphone 1. MSC receives call from PSTN , sends page to base stations 2. Base stations send page on forward control channel to mobiles 3. Mobile receives page and acknowledges on reverse control channel; sends validation information 4. MSC validates mobile, asks base station to put user on voice channel pair 5. Base station sends voice channel information on FCC, mobile moves to voice channel 6. Voice transmission/reception initiated; MSC connects with PSTN.03/22/12 EE421 Fall 2001 17
    18. 18. Forward/Reverse Channels 849-851 Air Mobile, air cellular 851-869 Private land mobile, public safety frequencies, trunk lines03/22/12 EE421 Fall 2001 18
    19. 19. Cell Frequency Reuse03/22/12 EE421 Fall 2001 19
    20. 20. Locating Cells N=19 (i= 3, j=2)03/22/12 EE421 Fall 2001 20
    21. 21. Channel Assignment• Need to assign frequencies to users/cell• Fixed – Each cell has predetermined number of channels – If all channels used, cell is “blocked”, no service – Cells can borrow channels from neighboring cells• Dynamic – No permanent allocation – Frequency band requested from MSC, assigned using reuse distance, blocking probabilities, use of channel, signal strength03/22/12 EE421 Fall 2001 21
    22. 22. HandoffScenarios03/22/12 EE421 Fall 2001 22
    23. 23. Umbrella Cells03/22/12 EE421 Fall 2001 23
    24. 24. Interference and Capacity• Interference causes – cross-talk, poor quality,(voice channel) – blocking and missed calls (control channel)• Co-channel interference – Frequency reuse in nearby cells• Adjacent channel interference – Signal in adjacent frequency band – Signals from other cell companies03/22/12 EE421 Fall 2001 24
    25. 25. Co-Channel Interference03/22/12 EE421 Fall 2001 25
    26. 26. Adjacent Channel Interference• Comes from imperfect filters that allow frequency leakage into the band• Serious problem if interferer is nearby, near-far effect – Nearby mobile transmits on a frequency near to that of a weak mobile• Base station receivers need high-Q filters to reject adjacent channel interference.03/22/12 EE421 Fall 2001 26
    27. 27. Power Control• Each mobile should use the minimum amount of power to have good quality.• Base station controls power – CDMA power toggles up and down continuously, like delta modulation – TDMA, AMPS adjusted by base station03/22/12 EE421 Fall 2001 27
    28. 28. Increasing Capacity• Cell Sectoring – Divide cell up into angular sections, typically 3 or 6 – Increases reuse• Cell Splitting – Subdivide cell into smaller cells, increases the number of channels, keep same structure – Needs power control03/22/12 EE421 Fall 2001 28
    29. 29. Cell Sectoring03/22/12 EE421 Fall 2001 29
    30. 30. Cell Splitting03/22/12 EE421 Fall 2001 30
    31. 31. AMPS Specs03/22/12 EE421 Fall 2001 31
    32. 32. AMPS Voice Processing Compander – compresses signal in amplitude, roughly 2:1 ratio Deviation Limiter –assures that the max. deviation is the +/- 12 kHz Postdeviation limiter filter – LPF, attenuated signal to keep in band and avoid interfering with SAT tones03/22/12 EE421 Fall 2001 32
    33. 33. Other Cell Specs• IS-54 – Digital cellular in US, DQPSK• IS136 PCS• IS 95 A – CDMA spread spectrum• Global system for mobiles (GSM) – TDMA with channelization03/22/12 EE421 Fall 2001 33

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