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+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 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.