EE 445 S RealTime Digital Signal Processing Lab

EE 445 S Real-Time Digital Signal Processing Lab Spring 2017 Quadrature Amplitude Modulation (QAM) Receiver Lab 6 – Part 2 QAM/QPSK Receiver Yeong Choo and Sam Kanawati Based on Lecture 16 QAM Receiver Slides by Prof essor Brian L. Evans

Outline • QAM transmitter demonstration • QAM demodulation 2

Baseband QAM Transmitter i[m] i[n] L Index Bits 1 Serial/ parallel converter J L samples/symbol m sample index n symbol index s[m] Pulse shapers (FIR filters) Map to 2 -D constellation cos( c m) sin( c m) + q[m] q[n] L s(t) D/A fs g. T[m] QAM Demodulation Receiver X r(t) A/D Carrier recovery, symbol recovery, equalization, and automatic gain control are not shown g. T[m] fs r[m] LPF L 2 cos( c m) X -2 sin( c m) 3

Baseband QAM Demodulation • Goal: Recovers baseband in-phase/quadrature signals • Assumptions: perfect auto. gain control, equalizer, sym. recovery Low Pass Filter Roles: 1. QAM demodulation filter fmax = 2 fc + B and fs > 2 fmax X LPF x[m] 2 cos( c m) 2. Anti-aliasing filter X 3. Matched filter LPF -2 sin( c m) Maximize SNR at downsampler output Minimize symbol error probability at downsampler output 4

LPF for QAM Demodulation = Modulate and lowpass filter to obtain baseband signals X LPF s[m] 2 cos( c m) X baseband high frequency component centered at 2 wc ½S 1(w + wc) -w c - w 1 -wc -w c + w 1 S 1(w) ½ 0 ½S 1(w - wc) wc w 1 wc wc + w 1 j ½S 2(w + wc) LPF -2 sin( c m) SQ(w) -j ½S 2(w - wc) j w -w – -w c + c w 2 - wc w 2 Upconverted signal ½ wc – wc w 2 -j ½ 5 wc + w 2 w

LPF for Anti-Aliasing Filtering Ref: Slide from Prof M. Mansour, American University of Beirut 6

LPF as Matched Filter g(t) x(t) Pulse signal w(t) h(t) Matched filter y(T) y(t) t=T a b g(t) is a low pass filter (raised cosine)