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)

Overlapping spectra

Despreading operation

pseudonoise (PN) code

pseudonoise (PN) code

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+mcosmt)cos2t

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

Thermal noise is generated by resistors, base and emitter resistance of bipolar devices, and channel resistance of MOSFETs.

Power spectral density

k: Boltzmann constant

Noise (2) • Shot noise • Flicker noise

Shot noise is a Gaussian white process associated with the thransfer of charge across an energy barrier (e.g. pn junction)

Noise (3) • Input-referred noise

222nDnm IVg 2222

nDinnm IZIg

)3

2(42 m

nD

gkTI

)3/(82mn gkTV

)3/(822

inmn ZgkTI

The noise of a two-port system can beModeled by two input noise generators:A series voltage source and a parallel current source.

Noise Figure (1)

Noise Figure (2)

Sin RR If

Output noise voltage of M2

Noise Figure (3) • NF in cascaded stages

Noise Figure (4) • NF of Lossy circuit

out

in

P

PL

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 theMinimum signal level that the system can detect withacceptable signal-to-noise ratio.

Psig: input signal powerPRS: source resistance noise

MDS (minimum detectable signal)= Pin,min + 3 dB

Sensitivity and Dynamic Range (3)

Dynamic range is generally defined as the ratio ofthe maximum input level that the circuit can tolerate tothe minimum input level at which the circuit provides a reasonable signal quality.

• Dynamic range (DR)

• Spurious-free dynamic range (SFDR)

DR=Pout - G + 1 dB - MDS

Consideration of Transceiver (1)

GSM mobile communication system

receiver

transmitter

Rejection required of a hypothetical front-end bandpass filterDesensitization of LNA by PA output leakage

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

• PA/Antenna interface

Injection pulling

Signal loss either from the duplexeror switch circuit will dissipate 30 % to50 % of PA output power.

Transmitter (3)• Direct-Conversion Transmitter

To alleviate the phenomenon of LO pulling,offsetting LO architecture can be used.

I and Q mismatch will become worsedue to injection pulling.

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

• 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；

FM radio receiver

10.7MHz87 MHz—108.8 MHz

• Problem of image

Receiver (2)

high IF

low IF

Receiver (3)

• Problem of half IF

IFLOLOin |2)(|

In order to suppress the half-IF phenomenon,second-order distortion in the RF and IF paths mustbe minimized, and a 50 % LO duty cycle must bemaintained.

If in the downconversion path, the interferer experiencessecond-order distortion and the LO contains a significant second harmonics as well, then the IF output exhibits a component at

2

)( LOin

2

)( LOin

2IF

2IF

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 low-pass 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.

• Image-reject receiver (Hartley)

Receiver (8)

Problem:Incomplete image rejection due to gain and phase mismatch.

Receiver (9)

Problem:Secondary image if the second downconversion translates the spectrum to a nonzero frequency.

No gain imbalance !

• Image-reject receiver (Weaver)

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.