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
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


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
 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
 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 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 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 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 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 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 Communications
 2) Multipath Fading
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
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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 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 Communications
 7. Delay Spread
 Exhibit equal loss and unequal delays


Second cosine factor relates the fading to the delay spread
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
 4.2.1 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
 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 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 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
 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
 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 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
 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 Access
 Increases the bandwidth
of the data spectrum by
the number of pulses in
the code
 Spectral Efficiency
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 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 Access
 2. Frequency-Hopping CDMA
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
 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
 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 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
 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
 Based on direct-sequence CDMA
 Proposed by Qualcomm, Inc., adopted for the North
America as IS-95
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 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 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
 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