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RF MICROELECTRONICS BEHZAD RAZAVI. 지능형 마이크로웨이브 시스템 연구실 박 종 훈. Contents. Ch.4 Multiple Access Techniques and Wireless Standards 4.1 Mobile RF Communications 4.2 Multiple Access Techniques 4.2.1 Time-and Frequency Division Duplexing 4.2.2 Frequency-Division Multiple Access

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rf microelectronics behzad razavi

RF MICROELECTRONICSBEHZAD RAZAVI

지능형 마이크로웨이브 시스템 연구실

박 종 훈

contents
Contents
  • Ch.4 Multiple Access Techniques and Wireless Standards
    • 4.1 Mobile RF Communications
    • 4.2 Multiple Access Techniques
      • 4.2.1 Time-and Frequency Division Duplexing
      • 4.2.2 Frequency-Division Multiple Access
      • 4.2.3 Time-Division Multiple Access
      • 4.2.4 Code-Division Multiple Access
    • 4.3 Wireless Standards
      • 4.3.1 Advanced Mobile Phone Service
      • 4.3.2 North American Digital Standard
      • 4.3.3 Global System for Mobile Communication
      • 4.3.4 Qualcomm CDMA
      • 4.3.5 Digital European Cordless Telephone
ch 4 multiple access techniques wireless standards
Ch.4 Multiple Access Techniques & Wireless Standards
  • 1. Multiple Access Techniques
    • For a large number of transceivers in a network, additional methods are required to ensure proper communication among multiple users
  • 2. Wireless Standard
    • frequency bands
    • timing
    • data coding
4 1 mobile rf communications
4.1 Mobile RF Communications
  • 1. Mobile System
    • One in which users can physically move while communicating with one another
      • e.g. pager, cellular phones, cordless phones
    • Mobile unit
      • Transceiver carried by the user
      • Mobile, Terminal, Hand-held unit
    • Base station
      • Mobiles communicate only through a fixed, relatively expensive unit
    • Channel
      • Forward channel or downlink
      • Reverse channel or uplink
4 1 mobile rf communications1
4.1 Mobile RF Communications
  • 2. Cellular System
    • Frequency Reuse
      • Physically far enough from each other can use the same carrier frequency
      • MTSO(Mobile Telephone Switching Office)
        • All of the base stations are controlled by a MTSO
4 1 mobile rf communications2
4.1 Mobile RF Communications
  • 3. Co-Channel Interference(CCI)
    • How much two cells that use the same frequency interfere with each other
    • This effect depends on the ratio of the distance between two co-channel cells to the cell radius and is independent of the transmitted power
    • 7-cell pattern : approximately 4.6
      • Signal to co-channel interference ratio : 18dB
4 1 mobile rf communications3
4.1 Mobile RF Communications
  • 4. Handoff
    • Since the power level received from the base station is insufficient to maintain communication, the mobile must change its base station and channel
    • Performed by MTSO
4 1 mobile rf communications4
4.1 Mobile RF Communications
  • Handoff process
    • Received level Drops below a threshold -> hands off
    • Problem
      • Fails with high Probability -> Dropped calls
    • Solution
      • Measure the received signal level from different base station
      • Handoff when the path to the second base station has sufficiently low loss
4 1 mobile rf communications5
4.1 Mobile RF Communications
  • 5. Path Loss and Multipath Fading
    • 1) Path Loss
      • Signal power loss proportional to the square of the distance
      • Direct Path / Reflective Path
        • Loss increases with the fourth power of the distance
4 1 mobile rf communications6
4.1 Mobile RF Communications
  • 2) Multipath Fading
    • Since the two signal generally experience different phase shifts, possibly arriving at the receiver with opposite phase and roughly equal amplitudes, the net received signal may be very small
    • Moving objects tend to soften the fading -> P(Amplitude=0) ≈ 0
    • Reflected by Building, moving cars
    • Fluctuations are quite irregular
4 1 mobile rf communications7
4.1 Mobile RF Communications
  • 6. Diversity
    • Higher probability of receiving a nonfaded
    • 1) Frequency diversity
      • Multiple carrier frequency are used
      • Two frequencies sufficiently far from each other
    • 2) Time diversity
      • Transmitted or received more than once to overcome short-term fading
4 1 mobile rf communications8
4.1 Mobile RF Communications
  • 7. Delay Spread
    • Exhibit equal loss and unequal delays
      • Second cosine factor relates the fading to the delay spread
4 1 mobile rf communications9
4.1 Mobile RF Communications
    • Difficulty
      • Fading bandwidths of several hundreds of kilohertz
        • Entire communication channel may be suppressed
      • Delay spread ≈ bit period(digital modulating waveform)
        • Rise to considerable intersymbol interference
  • 8. Interleaving
    • For lower the effect of errors, the baseband bit stream in the transmitter undergoes interleaving before modulation
    • Interleaver scrambles the time order of the bits according to an algorithm known by the receiver
4 2 multiple access techniques
4.2 Multiple Access Techniques
  • 4.2.1 Time-and Frequency Division Duplexing
  • 4.2.2 Frequency-Division Multiple Access
  • 4.2.3 Time-Division Multiple Access
  • 4.2.4 Code-Division Multiple Access
4 2 1 time and frequency division duplexing
4.2.1 Time-and Frequency Division Duplexing
  • 1. TDD(Time Division Duplexing)
    • Same frequency band is utilized for both transmit and receive paths
    • Transmits for half of the time and receives for the other half
    • E.g. walkie-talkies
4 2 1 time and frequency division duplexing1
4.2.1 Time-and Frequency Division Duplexing
  • 2. FDD(Frequency Division Duplexing)
    • Incorporates bandpass filters to isolate the two paths
    • Transceivers cannot communicate directly
4 2 1 time and frequency division duplexing2
4.2.1 Time-and Frequency Division Duplexing
  • 3. Merit and Drawback
    • 1) TDD
      • Merit
        • RF switch with a lossless than 1dB, output power may be 100dB
        • Two paths do not interfere
        • Direct communication
      • Drawback
        • Weak nearby strong signals
    • 2) FDD
      • Drawback
        • Leak into the receive band
        • Loss of the duplexer is higher than TDD switch
        • Leakage to adjacent channels in the transmitter output
          • Occures when the power amplifier is turned on and off
      • Despite the above drawbacks, FDD is employed in many RF systems
4 2 2 frequency division multiple access
4.2.2 Frequency-Division Multiple Access
  • FDMA
    • Available frequency band can be partitioned into many channels
    • Minimum number of simultaneous users is given by the ratio of the total available frequency band and the width of each channel
4 2 3 time division multiple access
4.2.3 Time-Division Multiple Access
  • TDMA
    • Data stored(Buffered) for TF-Tsl seconds
    • Advantage over FDMA
      • PA can be turned off during the rest of the frame
      • Digitized speech can be compressed in time
      • Receive and transmit paths are never enabled simultaneously with FDD
4 2 3 time division multiple access1
4.2.3 Time-Division Multiple Access
  • TDMA more complex than FDMA
    • With the advent of VLSI DSPs, drawback is no longer a determining factor
    • Combination of TDMA and FDMA is utilized
4 2 4 code division multiple access
4.2.4 Code-Division Multiple Access
  • 1. Direct-Sequence CDMA
    • Certain code is assigned to each transmitter/receiver pair
    • Walsh’s code
    • Each codes are orthogonal
4 2 4 code division multiple access1
4.2.4 Code-Division Multiple Access
  • Increases the bandwidth of the data spectrum by the number of pulses in the code
  • Spectral Efficiency
4 2 4 code division multiple access2
4.2.4 Code-Division Multiple Access
  • Upon multiplication
    • Desired signal : returning to the original bandwidth value
    • Unwanted signal : remains spread because of its low correlation
4 2 4 code division multiple access3
4.2.4 Code-Division Multiple Access
  • CDMA is its soft capacity limit
    • FDMA and TDMA the maximum number of users is fixed
  • Power control
4 2 4 code division multiple access4
4.2.4 Code-Division Multiple Access
  • 2. Frequency-Hopping CDMA
4 3 wireless standards
4.3 Wireless Standards
  • All the details and constraints that govern the design of transceivers used in a wireless system
4 3 1 advanced mobile phone service
4.3.1 Advanced Mobile Phone Service
  • AMPS
    • Employs FDMA with analog FM and FDD
    • Support approximately 830 users simultaneously
    • Requires control and supervisory signals to initiate, maintain, and terminate a call
4 3 2 north american digital standard
4.3.2 North American Digital Standard
  • NADC
    • Employs TDMA with π/4-DQPSK and FDD
    • Each frame is 1944bits(40ms)
    • Each time slot carries approximately 260bits of data along with 64 bits of control and synchronization information
4 3 2 north american digital standard1
4.3.2 North American Digital Standard
  • First digital cellular system in the United States
  • Most first-generation digital phones in the United States actually operate with both AMPS and NADC to provide a wider coverage for users
    • IS-54(Interim Standard 54)
      • Developed by the Electronic Association and the Telecommunication Industry Association
4 3 3 global system for mobile communication
4.3.3 Global System for Mobile Communication
  • GSM
    • Unified wireless standard for Europe
    • Supports many other services such as facsimile and ISDN
    • Employs TDMA/FDD system using GMSK
4 3 4 qualcomm cdma
4.3.4 Qualcomm CDMA
  • Based on direct-sequence CDMA
  • Proposed by Qualcomm, Inc., adopted for the North America as IS-95
4 2 4 code division multiple access5
4.2.4 Code-Division Multiple Access
  • 1. Power Control
    • Open loop procedure
      • Perform a rough, but fast adjustment
      • Mobile measures the signal power it receives from the base station and adjusts its transmitted power so that the sum of the two(in dB) is approximately -73dBm
        • Pbs – k + Pm = -73dBm
          • Pbs : power transmitted by the base station
          • k : Receive and transmit paths entail roughly attenuation
          • Pm : mobile output power
4 2 4 code division multiple access6
4.2.4 Code-Division Multiple Access
  • Closed-loop procedure
    • Base station measures the power level received from the mobile unit and sends a feedback signal requesting power adjustment
    • This command is transmitted once every 1.25ms
4 2 4 code division multiple access7
4.2.4 Code-Division Multiple Access
  • 2. Frequency and Time Diversity
  • 3. Variable Coding Rate
    • 9600, 4800, 2400, 1200 b/s
  • 4. Soft handoff
    • Make-before-break operation
4 3 5 digital european cordless telephone
4.3.5 Digital European Cordless Telephone
  • DECT
    • Allow connection to other systems such as GSM
    • Provides mobility to local area network users
    • Employs GFSK
    • 24 time slots(12 for transmit and 12 for receive)
    • Total duration of 10ms
    • Each time slot contains 32 preamble bits
    • 388 data bits
    • 60 guard bits