Lecture 11 Outline: Digital Modulation Announcements: Jeremy will cover my 11:30-12:30 OHs today Homework 3 due today 5pm, HW 4 posted tonight Reading: “Communication Systems”, pg Posted lecture slides on Digital Modulation. Friday lecture: I may arrive a bit late, so TAs may start the lecture Review of Friday Lecture FM is an optional/extra credit topic Digital Communication System Block Diagram Baseband Digital Modulation Passband Digital Modulation ASK and PSK ASK and PSK Demodulation Quadrature Modulation: MQAM

Review of Friday Lecture DSBSC, SSB, Broadcast AM, FM (DSBSC) Modulated signal is s(t)=m(t)cos(  c t+  ) Single Sideband (SSB) only sends upper or lower sidebands Broadcast AM has s(t)=[1+k a m(t)]cos(  c t) with [1+k a m(t)]>0 DSBSC/SSB Demodulator: multiply by cos(  c t+  ) and LPF Recovering the transmitter phase  a big challenge for all demodulators Broadcast AM demodulator uses an envelope detector No need to recover phase  very simple receiver design Quadrature modulation sends independent AM signals modulated onto the sin and cosine carriers FM encodes information in frequency of carrier (optional topic) LSB USB W -W   cc  c W 2W 1 kaka

Digital Communication System Block Diagram (only in ppt) Channel is a physical entity (wire, cable, wireless channel, string) Cannot send a complex signal over a physical channel: s(t) must be real S(j  )=S * (-j  ): s(t) real/even  S(j  ) real/even; s(t) real/odd  S(j  ) imaginary/odd Often write s(t) in terms of in-phase/quadrature components: s(t)=s I (t)cos(  c t)-s Q (t)sin(  c t) ADC Analog Source Analog Sink DAC Compression Decompression Error-Correction Encoding Error-Correction Decoding Baseband Modulation Baseband Demodulation Passband Modulation Passband Demodulation Channel Modulated Waveform s(t) ŝ(t): Corrupted Copy of s(t) Compressed Source Bits Encoded Bits Baseband Waveform m(t) Decoded Compressed Source Bits Detected Encoded Bits converts continuous-time to bits Removes redundancyintroduces controlled redundancy binary or multi-level shifts waveform to carrier frequency propagates signal but adds distortion, noise & interference shifts waveform to baseband compares waveform to thresholds to detect bits corrects errors in detected bits restores source redundancy converts bits to continuous-time Analog System Digital Source Bits Digital Sink Bits

Baseband digital modulation converts bits into analog signals y(t) (bits encoded in amplitude) Pulse shaping (optional topic) Instead of the rect function, other pulse shapes used Improves bandwidth properties and timing recovery Explored in extra credit Matlab problem Baseband Digital Modulation On-Off Polar t t TbTb m(t) A A -A

Changes amplitude (ASK), phase (PSK), or frequency (FSK) of carrier relative to bits We use baseband digital modulation as information signal m(t) to encode bits, i.e. m(t) is on-off or polar Passband digital modulation for ASK/PSK is a special case of DSBSC For m(t) on/off (ASK) or polar (PSK), modulated signal is Passband Digital Modulation

ASK and PSK Amplitude Shift Keying (ASK) Phase Shift Keying (PSK) AM Modulation m(t) Assumes carrier phase  =0, otherwise need phase recovery of  in receiver A -A A

ASK/PSK Demodulation Similar to AM demodulation, but only need to choose between one of two values (need coherent detection) Decision device determines which of R 0 or R 1 that R(nT b ) is closest to For ASK, R 0 =0, R 1 =A, For PSK, R 0 =-A, R 1 =A Noise immunity  N is half the distance between R 0 and R 1 Bit errors occur when noise exceeds this immunity s(t)  cos(  c t+  ) nT b Decision Device “1” or “0” r(nT b ) R0R0 R1R1 aa r(nT b )+  Integrator (LPF) NN

Quadrature Digital Modulation: MQAM Sends different bit streams on the sine and cosine carriers Baseband modulated signals can have L>2 levels More levels for the same TX power leads to smaller noise immunity and hence higher error probability Sends 2log 2 (L)=M bits per symbol time T s, Data rate is M/T s bps Called MQAM modulation: 10 Gbps WiFi has 1024-QAM (10 bits/T s ) t m i (t) m 1 (t)cos(  c t)+ m 2 (t)sin(  c t)+ n(t) X -90 o cos(  c t) sin(  c t) X Decision Device “1” or “0” R0R0 R1R1 aa r I (nT b ) +N I Decision Device “1” or “0” R0R0 R1R1 aa r Q (nT b ) +N Q A -A A/3 -A/3 TsTs Data rate: log 2 L bits/T s T s is called the symbol time

Main Points Digital communication system block diagram includes compression, error-correction coding and modulation Digital baseband modulation encodes bits in analog signal Digital passband modulation encodes binary bits into the amplitude or phase of the carrier. ASK/PSK special case of AM Noise immunity in receiver dictates how much noise is required to make an error MQAM modulation sends independent bit streams on cosine and sine carriers where baseband signals have L levels Leads to data rates of M/T s bps for M=2log 2 (L) (can be very high)