ECE & TCOM 590 Microwave Transmission for Telecommunications

1. ECE & TCOM 590Microwave Transmission for Telecommunications April 15, 2004 Microwave Systems: Digital Modulation Receiver Design

2. Binary Digital Modulation • Advantages of digital over analog modulation techniques: • improved performance in the presence of noise and fading • lower transmit power requirements • better suitability for transmission of digital data with error correction and encryption • Carrier: A cos (t + ) • Three degrees of freedom: A, , 

3. Pulse Code Modulation (PCM) • Binary digital modulation • PCM codes a binary “1” as a signal voltage s1(t) and a binary “0” as s2(t) each of bit duration, T. • PCM signals: using shift (S) keying (K) techniques: ASK (amplitude), FSK (frequency), and PSK (phase)

4. Binary Signals • On-off, return to zero (RZ) coding • return to zero of a “one” before the end of the period • Non-returning to zero (NRZ) coding • less bandwidth required • Polar NRZ coding (“1” represented by a high voltage, “0” represented by a negative voltage such that the average is zero)

5. Data Rate • Source produces data that the transmitter converts into signal or waveforms to be sent over communications channel • Twisted-pair (telephone), coaxial (TV), air (acoustical) or E&M wave through space • Binary transmission: two distinguishable signals (by amplitude, frequency, phase) • M-ary transmission – more than two signals to represent data; resulting in faster data transmission

6. Data Rate Measurment • Let R = signal transmission rate (signals produced every second) • 1/R is the time duration of each signal • Data Rate: D = R log2 M

7. Decoding M-ary Signals(figure 8.2, Kuc)

8. Frequency Shift Keying • Switching a sinusoidal carrier wave between two frequencies 1,2 where • 1 = 0 - ; and 2 = 0 +  • v(t) = cos t, • where, = 1 for m(t) = 1 and = 2 for m(t) = 0 • use of tunable oscillator to switch between 1 & 2

9. Frequency Shift Keying (figure 8.8, Kuc)

10. Frequency Shift Keying Frequency-shift keying uses different frequencies • 300 to 3300 Hz bandwidth of the telephone network • example, two different frequencies might represent 1s & 0s • Or, more practically, four frequencies, each one assigned to a two-bit value – Baud rate the same, but the data rate doubles with the two bits per sample period.

11. Phase-Shift Keying • Changes the phase at a constant frequency and amplitude • Can make M-ary transmission by having each value have a different phase shift relative to the immediately preceding sinusoidal signal • M=4: dibits with dibit varying by 360/4 = 90o • M=8: tribits, with tribits varying by 360/8=45o • Phase shift occurs every Tbaud seconds

12. Quadrature Phase-Shift Keying (figure 8.11, Kuc)

13. Phase-Shift Keying • Phase shift occurs every Tbaud seconds and if M=4, every shift encodes 2 bits, so the data rate is twice the baud rate. • Modem factor: 1 bit/cycle = 1 bps/Hz • If M=8, we transmit 3 bits every 2 cycles of the waveform for a modem factor of 1.5 bps/Hz

14. AM and Phase-Shift Keying (figure 8.14, Kuc)

15. Channel Noise • Noise – commonly from thermal energy • Atomic (charged) particles vibrating randomly • Disturbs the data signal • Higher temperatures cause greater thermal motion •  Sensitive receivers are placed in low-temp environments • Noise power level: n2 • Maximum signal power level produced by transmitter: s2

16. Receiving: Demodulation • Synchronous demodulation • Requires the frequency and phase of the local oscillator to be the same as the incoming signal • transmission of low level pilot carrier to phase lock the local oscillator (LO) • or use carrier recovery circuit that uses phase or frequency information from the received signal to synchronize the local oscillator • Envelope detection • demodulate noncoherently with an envelop detector (avoids the requirement for coherent LO) • not possible for polar NRZ with negative signals

17. Amplitude Shift Keying (ASK) • ASK- carrier turned on and off according to binary baseband data sequence. • Local oscillator with a mixer driven by NRZ binary signal • ASK modulated signal: v(t) = m(t) cos 0t, where m(t) =0 or 1 • Output of the mixer is v1(t) = v(t) cos 0t = 0.5 m(t) (1+ cos 20t) • After filtering, v0(t) = ½ m(t) • this requires LO to have the same phase and frequency as the incoming signal

18. Coherent Detection of FSK • Use of demodulator with two coherent local oscillators operating at 1and2. Let the incoming signal be = 1, then the outputs of the two mixers will be • v1(t) = v(t) cos 1t = cos21t =½ (1+cos 21t) • v2(t) = v(t) cos 2t = cos 1t cos 2t = ½ [cos(1- 2)t + cos(1+ 2)t ] • After low-pass filtering only the DC term remains, resulting in a positive pulse at the output of the summer, hence a “1” is received • Similarly if = 2, then the output of the summer will by a negative pulse so a “0”; hence polar NRZ

19. Phase Shift Keying • Phase of the carrier wave is switched between two states, usually 0 and 180o. Hence v(t) = m(t) cos 0t, where m(t) =1 or -1 • Generated by mixing a polar NRZ of the data with a LO. Spectrum relatively wide due to sharp transitions caused by the phase reversals.

20. Phase Shift Keying • After mixing v(t) = m(t) cos 0t with LO, the output becomes v1(t) = v(t) cos 0t = ½ m(t) [(1+cos 20t)] • After low-pass filtering the output voltage is v0(t) = ½ m(t), which is proportional to the original polar NRZ data.

21. PCM Signals

22. PCM Signals with Noise

23. Probability of Error with Synchronous PSK

24. Bit Rate and Bandwidth Efficiency

26. Channel Capacity • Developed by Claude Shannon: • C=B log2 (1 +S/N) • Where N = n0B, n0/2=two sided power spectral density of the gaussian white noise • C = maximum data rate capacity of the channel (bps) • B = Bandwidth • S = signal power • Gives the upper bound on the maximum data rate that can be achieved for a given channel in the presence of additive gaussian white noise.

27. Receiver Design - Introduction • Recovery of desired signal from a wide spectrum of transmitting sources, interference, and noise • Requirements: • High Gain (approx. 100 dB) • Selectivity (receive the desired signal while rejecting adjacent channels and interference) • Down-conversion from received RF to IF for processing to the actual desired signal • Isolation from transmitter to avoid saturation

28. Superheterodyne Receiver • Most popular type of receiver • IF between RF and baseband frequencies • allows the use of sharper cutoff frequencies for improved selectivity • For microwave and millimeter wave frequencies - use two stages of IF of down conversion to avoid problems due to LO stability

29. Duplexing • Most wireless systems use full-duplex where transmission and reception can occur simultaneously • Normally use separate frequency bands for transmit and receive • If a single antenna is used then a duplexer must be used to separate transmitter from receiver (isolation of 100 dB needed)

30. Modem – Two Way Communication (figure 8.9, Kuc)

31. Minimum Detectable Signal (MDS) • Reliable communication requires a receive signal power at of above a certain minimum level, called the MDS • For a given system noise power, the MDS determines the minimum SNR at the demodulator of the receiver. • Alternative is the SINAD signal + noise to noise ratio = 1 + SNR

32. Minimum Detectable Signal (MDS)

33. Minimum Detectable Signal (MDS)

34. Dynamic Range • Receiver dynamic range, DRr, is • DRr maximum allowable signal power / minimum detectable signal power • DRr depends upon the noise characteristics of the receiver as well as the type of modulation being used, and the required minimum SNR. • Recall that the maximum allowable signal power could alternately be defined by the third-order intercept point, P3.

35. Automatic Gain Control • Typically, a gain on the order of 80 to 100 dB is required to bring the minimum detectable signal up to a usable level. • This large gain should be distributed throughout the RF, IF, and baseband stages. • Avoid high levels of RF gain such that the 1 dB compression point and third-order intercept point are not exceeded. On the other hand, a moderate RF gain is good to establish a good noise figure for the receiver system