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RF MICROELECTRONICS BEHZAD RAZAVI

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

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  1. RF MICROELECTRONICSBEHZAD RAZAVI 지능형 마이크로웨이브 시스템 연구실 박 종 훈

  2. 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

  3. 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. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 4.1 Mobile RF Communications • 7. Delay Spread • Exhibit equal loss and unequal delays • Second cosine factor relates the fading to the delay spread

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 4.2.4 Code-Division Multiple Access • Increases the bandwidth of the data spectrum by the number of pulses in the code • Spectral Efficiency

  23. 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

  24. 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

  25. 4.2.4 Code-Division Multiple Access • 2. Frequency-Hopping CDMA

  26. 4.3 Wireless Standards • All the details and constraints that govern the design of transceivers used in a wireless system

  27. 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

  28. 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

  29. 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

  30. 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

  31. 4.3.4 Qualcomm CDMA • Based on direct-sequence CDMA • Proposed by Qualcomm, Inc., adopted for the North America as IS-95

  32. 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

  33. 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

  34. 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

  35. 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

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