Transcript Receiver

Transmitter and Receiver System Parameters
• Nonlinearity
• Intersymbol interference
• Noise
• Sensitivity and dynamic range
• Receiver architecture (heterodyne and homodyne)
• IF and Image
• Transmitter architecture
Code-Division Multiple Access (CDMA)
• Direct-sequence CDMA (DS-CDMA)
pseudonoise (PN) code
pseudonoise (PN) code
Overlapping spectra
Despreading operation
Code-Division Multiple Access (CDMA)
• Frequency-hopping CDMA (FH-CDMA)
Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly
switching a carrier among many frequency channels, using a pseudorandom sequence known to
both transmitter and receiver. It is utilized as a multiple access method in the frequency-hopping
code division multiple access (FH-CDMA) scheme.
Nonlinearity (1)
• Harmonics
Nonlinearity (2)
• Gain compression
• Cross modulation
x(t)=A2(1+mcosmt)cos2t
Nonlinearity (3)
• Intermodulation
Nonlinearity (4)
IP3 (third intercept point)
IIP3 (input IP3)
OIP3 (output IP3)
Nonlinearity (5)
Nonlinearity (6)
• Cascaded nonlinear stages
Nonlinearity (7)
Nonlinearity (8)
Intersymbol Interference (1)
Intersymbol Interference (2)
• Raised-cosine pulse and filter
Noise (1)
• Thermal noise
Power spectral density
k: Boltzmann constant
Thermal noise is generated by resistors, base
and emitter resistance of bipolar devices, and
channel resistance of MOSFETs.
Noise (2)
• Shot noise
Shot noise is a Gaussian white process
associated with the thransfer of charge across
an energy barrier (e.g. pn junction)
• Flicker noise
Noise (3)
• Input-referred noise
The noise of a two-port system can be
Modeled by two input noise generators:
A series voltage source and a parallel
current source.
g m V n  I nD
2
2
I nD  4 kT (
2
2
2
2
g m I n Z in
2 gm
)
3
V n  8 kT /( 3 g m )
2
2
I n  8 kT /( 3 g m Z in )
2
2
 I nD
2
Noise Figure (1)
Noise Figure (2)
If
R in  R S
Output noise voltage of M2
Noise Figure (3)
• NF in cascaded stages
Noise Figure (4)
• NF of Lossy circuit
L
Pin
Pout
Noise Figure (5)
• Cascade of filter and amplifier
Sensitivity and Dynamic Range (1)
Sensitivity and Dynamic Range (2)
The sensitivity of an RF receiver is defined as the
Minimum signal level that the system can detect with
acceptable signal-to-noise ratio.
Psig: input signal power
PRS: source resistance noise
MDS (minimum detectable signal)= Pin,min + 3 dB
Sensitivity and Dynamic Range (3)
Dynamic range is generally defined as 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.
• Dynamic range (DR)
DR=Pout - G + 1 dB - MDS
• Spurious-free dynamic range (SFDR)
Consideration of Transceiver (1)
transmitter
GSM mobile communication system
receiver
Desensitization of LNA by PA output leakage
Rejection required of a hypothetical
front-end bandpass filter
Consideration of Transceiver (2)
Effect of nonlinearity in the front-end circuit
Band selection at the front end of a receiver
Transmitter (1)
• Baseband/RF interface
Pulse shaping based on digital signal process
Transmitter (2)
• RF signal leakage
Injection pulling
• PA/Antenna interface
Signal loss either from the duplexer
or switch circuit will dissipate 30 % to
50 % of PA output power.
Transmitter (3)
• Direct-Conversion Transmitter
I and Q mismatch will become worse
due to injection pulling.
To alleviate the phenomenon of LO pulling,
offsetting LO architecture can be used.
Transmitter (4)
• Two-step Transmitter
Advantages:
• Low I and Q mismatch
• Suppress transmitted noise and spurs in adjacent channels
Disadvantages:
• narrowband BPF is hard to achieve
Receiver (1)
• Heterodyne receiver
10.7MHz
87 MHz—108.8 MHz
•
•
•
FM radio receiver
即：f本振-f信号=f中频
如接收信号频率是：
100.1 MHz，则本振频率是
110.8 MHz；
Receiver (2)
• Problem of image
high IF
low IF
Receiver (3)
• Problem of half IF
( in   LO )
 IF
2
2
If in the downconversion path, the interferer experiences
second-order distortion and the LO contains a significant
second harmonics as well, then the IF output exhibits a
component at | ( in   LO )  2 LO |  IF
( in   LO )
 IF
2
2
In order to suppress the half-IF phenomenon,
second-order distortion in the RF and IF paths must
be minimized, and a 50 % LO duty cycle must be
maintained.
Receiver (4)
• Dual IF technology
The trade-off between sensitivity
and selectivity in the simple
heterodyne architecture often
proves quite severe:
If the IF is high, the image can
be suppressed but complete
channel selection is difficult, and
vice versa.
Receiver (5)
• Homodyne receiver
• Direct-conversion
• Zero-IF
Advantages:
Unwanted by-product beat signals from the
mixing stage do not need any further processing,
as they are completely rejected by use of a lowpass filter at the audio output stage. The receiver
design has the additional advantage of high
selectivity, and is therefore a precision
demodulator. The design also improves the
detection of pulse-modulated transmission mode
signals.
Disadvantages:
Signal leakage paths can occur in the receiver.
Local-oscillator energy can leak through the mixer
stage back and feed back to the antenna input and
then re-enter the mixer stage. The overall effect is
that the local oscillator energy would self-mix and
create a DC offset signal. The offset could be large
enough to overload the baseband amplifiers and
overcome the wanted signal reception. There were
subsequent modifications to deal with this issue but
added to the complexity of the receiver.
Receiver (6)
• Channel selection
• DC offset
LO self-mixing
A strong interferer self-mixing
Offset cancellation
Receiver (7)
• I/Q mismatch
In practice, it is desirable to maintain the amplitude
mismatch below 1 dB and phase error below 5,
but these bounds depend on the type of modulation.
Receiver (8)
• Image-reject receiver (Hartley)
Problem:
Incomplete image rejection due to gain and phase
mismatch.
Receiver (9)
• Image-reject receiver (Weaver)
No gain
imbalance !
Problem:
Secondary image if the second downconversion
translates the spectrum to a nonzero frequency.
Receiver (10)
• Digital-IF receiver
Digital processing avoids the problem of I and Q mismatch.
The principal issue in this approach is the performance
required of high-speed and wide dynamic-range A/D
converter.