RF MICROELECTRONICS BEHZAD RAZAVI

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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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


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


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 T
F
-
T
sl

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


P
bs



k + P
m

=
-
73dBm


P
bs

: power transmitted by the base station


k : Receive and transmit paths entail roughly attenuation


P
m

: 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