RF MicroelectronicMultiple access standards, TDM, CDM, OFDM
TRx architecture
IS-19 cellular telephone RF section block diagram
A 45 MHz offset frequency oscillator generates the required receiver and transmitter local oscillator frequency.
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IS-55 block diagram
A narrowband IF filter is required for digital operation, as well as an ADC in the baseband.
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I&Q transmitter
Disciplines required in RF design
As the industry moves toward higher integration and lower cost, RF and wireless design demands increasingly more “concurrent engineering”.
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RF design hexagon
The trade-offs involved in the design of such circuits can be
summarized in the “RF design hexagon”.
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Technology
Three critical factors influencing the choice of technologies in the competitive RF industry:
Performance
Cost
Time to market
Issues play an important role in the decisions made by the designers:
Level of integration
The technologies constitute the major section of the RF market:
GaAs
BiCMOS
SiGe
CMOS
CMOS technology must resolve a number of practical issues: Substrate coupling of signals that differ in amplitude by 100dB, parameter variation with temperature and process, and devices modeling for RF operation.
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Multiple access standards, TDM, CDM, OFDM
TRx architecture
Effects of nonlinearity
Model nonlinearity as a Taylor series expansion up to its third order term:
If a sinusoid is applied to a nonlinear system:
The term with input frequency is called the fundamental and the higher-order terms the harmonics.
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1 dB Compression Point
The small-signal gain of a circuit is usually obtained with the
assumption that harmonics are negligible.
In RF circuit,
1-dB Compression point defined as:
The input signal level that causes the small-signal gain to drop by 1 dB.
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To calculate the 1-dB compression point, we can write from
gain equivalent:
That is,
In typical front-end RF amplifiers:
The 1-dB compression point occurs around -20 to -25 dBm (63.2 to 35.6 mVpp in a 50 Ohm system).
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Desensitization and Blocking
If a small signal and a large interferer are applied to a compressive system, the “average” gain for the small signal is reduced:
Assume,
The gain for the desired signal is equal to
Many RF receivers must be able to with stand blocking signals 60 to 70 dB greater than the wanted signal.
A decreasing function of A2 if a3<0 .
For sufficiently large A2, the gain drops to zero, and we say the signal is “blocked”.
The interferer is called a blocking signal.
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Cross Modulation
When a weak signal and a strong interferer pass through a nonlinear system,
Weak signal:
Strong interferer:
Then,
Cross modulation is the transfer of modulation on the amplitude of the interferer to the amplitude of the weak signal.
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Cross Modulation
If two signals experience nonlinearity, amplitude modulation in one appears in the other.
Most important in “multi-carrier” systems. Example include cable TV transmitters and base station transmitters.
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Intermodulation
If the input sinusoid frequency is chosen such that its harmonics fall out of the passband,
The output distortion appears quite small even if the input stage of the filter introduces substantial nonlinearity.
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Intermodulation
Assume;
Thus,
Expanding the left side and discarding dc terms and harmonics, obtain the following Intermodulation products:
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Corruption of a signal due to Intermodulation between two interferer:
Intermodulation in a nonlinear system:
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IP3 is measured by two-tone test
A is chosen to be sufficiently small so that higher-order nonlinear terms are negligible and the gain is relatively constant and equal to a1
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IIP3|dBm=Poutput|dBm -GaindB+DPdB/2 .
OIP3|dBm=Poutput|dBm +DPdB/2 .
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Then,
Thus,
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Cascade nonlinear stages
This equation readily gives a general expression for three or more stages:
Typical receiver IP3 is -15 dBm.
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AIP3,tot= AIP3,2 /a11
Stage 1
Stage 2
distort the signal.
a periodic square wave
output with exponential tail
With a random sequence of ONES and ZEROS as the
input :
Low-pass
filter
Vin
Vout
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Intersymbol Interference
Each bit level is corrupted by decaying tails created by previous bits. Called “Intersymbol Interference” (ISI).
Leads to higher error rate in the detection of random waveforms transmitted through band-limited channels
Particularly troublesome in wireless communications
because of narrow bandwidth allocated to each channel
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The correlation between two sources must be taken into
account
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in terms of their “noise figure” rather the input-referred
noise
Noise figure is a measure of how much the SNR
degrades as the signal passes through a system
Signal-to-noise ratio at the input
Signal-to-noise ratio at the output
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Calculation of Noise Figure
SNR in is the ratio of the input signal power to the noise generated by the source resistance, R s, modeled by
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Calculation of Noise Figure
Voltage gain from Vin to the input port of the circuit (node P)
Voltage gain from P to V out
V n and I n R s are added before squaring to account for their correlation
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Calculation of Noise Figure
(for the spot noise figure to emphasize the very small bandwidth )
=Total noise at the output
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we divide the total output noise power by the square
of the voltage gain from V in to V out and normalize it
to the noise of R s.
As an example:
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Calculation of Noise Figure
What is the noise figure of this circuit with respect to a
source resistance R s ?
with acceptable signal-to-noise ratio
The overall power is distributed across the channel bandwidth, B. Thus the two sides of the equation must be integrated over the bandwidth to obtain the total mean square power
for a flat channel
The noise power that R s delivers to the receiver
Assuming conjugate matching at the input
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Noise Floor
Since P in,min is a function of the bandwidth, a receiver may appear very sensitive because it employs a narrowband channel
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Dynamic Range (DR)
Generally is the ratio of the maximum input level that the circuit can tolerate to the minimum input level at which the circuit provides a reasonable signal quality
In RF design, this definition is based on the Intermodulation behavior and the sensitivity called :
Spurious-free dynamic range
The upper end of the dynamic range is:
The maximum input level in a two-tone test for which the
Third-order IM products do not exceed the noise floor
And hence,
Dynamic Range (DR)
The input level for which the IM products become equal to the noise floor is:
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Dynamic Range (DR)
SFDR is the difference (in dB) between P in, max and
P in, min
SFDR represents the maximum relative level of interferers that a receiver can tolerate while producing an acceptable signal quality from a small input level
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Multiple access standards, TDM, CDM, OFDM
TRx architecture
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(transmitter)
(transceiver)
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Quadrature Modulation
A binary data stream could be subdivided into pairs of two bits and each pair represented with one of four levels
before performing modulation.
Bits bm and bm+1 are impressed upon a single carrier
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orthogonal functions
This operation is illustrated:
To obtain constellation, we assume bits bm and bm+1
are rectangular pulses with a height ±1 and write the
modulated signal as :
Where 1 and 2 can each take on a value of +Ac and -Ac
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QPSK
If the bit waveform is a rectangular pulse, a QPSK signal is obtained.
One of four phases of a sinusoid is selected according
to the symbol
at the end of each symbol
[-1 -1]
[1 1]
QPSK
180º phase step or, equivalently, a transition between two diagonally opposite points in the constellation
Such transitions are undesirable if the waveform is to be filtered
and subsequently processed by a nonlinear power amplifier
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Modulation with half-the-symbol-period-offset in time
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noncoherent receivers, is not executable in OQPSK
So /4-QPSK is another variant of QPSK
The /4-QPSK signal consists of two QPSK schemes,
one rotated by 45º with respect to the other:
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output from each QPSK generator
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abrupt phase changes which;
2.present difficulties in the design of power amplifiers
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A modulated waveform
is said to have a constant envelope if A(t) does not vary
with time.
Constant-and Variable-Envelope-Signals behave differently
in a nonlinear system
passes through a nonlinear system
QAM and OFDM are examples of variable-envelope signals
which are less power efficient
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Multiple access standards, TDM, CDM, OFDM
TRx architecture
Answer : Duplexing
The same frequency band is utilized for both transmit
(TX) and receive (RX) paths, but the system transmits
for half of the time and receives for the other half.
Fast enough to be transparent to the user
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TDM
Merits:
The transmitter is disabled during reception, so the two TX and RX paths do not interfere.
Direct (peer-to-peer) communication between two
transceivers, an especially useful feature in short-range, local area network applications, is allowed.
Drawback:
The strong signals generated by all of the nearby mobile
transmitters fall in the receive band thus desensitizing the receiver
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FDD
Employs two different frequency bands for the transmit and receive paths.
Merit:
The “Duplexer Filter” (the two combined front-end band-pass filters), makes the receiver immune to the strong signals transmitted by other mobile units
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FDD
Drawbacks:
Components of the transmitted signal that leak into the receive band are attenuated by typically only 50dB
Owing to the trade-off between the loss and the quality factor of the filters, the loss of the duplexer is typically higher than that of a TDD switch.
Spectral leakage to adjacent channels in the transmitter output which occurs:
When the power amplifier turns on and off to save energy
When the local oscillator driving the modulator undergoes a transient
By contrast, in TDD such transients can be timed
to end before the antenna is switched to the power
amplifier output.
among multiple transceivers
FDMA
The available frequency band partitioned into many channels each assigned to one user
The channel assignment remains fixed until the end of the call
Principal access method in early cellular networks because of its relative simplicity
Insufficient capacity in crowded areas
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TDMA
The same band is available to each user but at different times (time-Division multiple access)
User 3
User 2
User 1
Time assigned to all users Frame : Tf
Every Tf seconds each user finds access to the channel for Tsl seconds
Speech data
Synchronization data
Control data
TDMA
Problem:
What about the data of all other users when only one user is allowed to transmit?
The data stored for (Tf-Tsl) seconds and
transmitted as a “Burst”
Data requires to be in Digital form to be buffered
Speech compression and Coding would be allowed
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TDMA vs. FDMA
TDMA is a power saving system since the Power Amplifier of the transmitter is turned on for only one time slot in every frame
Even with FDD, proper timing of TDMA bursts prevents simultaneous enabling of the transmit and receive paths in each transceiver
TDMA more complex than FDMA because of:
A/D conversion,
Digital modulation,
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Assigns a certain code to each transceiver
Each bit of Data translated to that code before modulation
User 1
User 2
User 3
The baseband data spread over the entire
available bandwidth
The code is called “spreading sequence”
or
CDMA
In the receiver, the demodulated signal is decoded by multiplying it by the same code
Upon multiplication the desired signal is “dispread” with its bandwidth returning to its original value
The unwanted signal remains spread
Correlation
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increases the Noise Floor
among others
CDMA
The receiver monitors the signal strength of each transmitter and periodically sends the power adjustment requests to each one
Received signal levels are typically within 1 dB of each other
Reduction in the average power dissipation
of the mobile unit
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AMPS
NADC
CDMA
UMTS
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Communication method: analog
Duplex-Method: FDD
Multiple-Access: FDMA Band 824-849/869-894 MHz, 832 „Channels“ with 30 kHz width
Modulation: FM
Mobile Phone System USA (“2. Generation”)
Communication Method: digital
gives 4992 „traffic channels“
(“2. Generation”)
Mobile Phone System of Germany from 1991
(2nd Generation) worldwide adoption
GSM
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Communication Method: digital
Band 1900-2025/2110-2200 MHz,
5 MHz Bandwidth
Communication: digital
medium ranges )
gives 120 “traffic channels”