1. Digital Communications

2. What is Digital Communications • Communication using digital data • Digital Data = bits, nibbles, bytes…1’s and 0’s • Two Broad Categories of Digital Communications: • Digital Transmission • Digital Radio

3. Digital Transmission • Communicating digital information in digital form • Pulses representing 1’s and 0’s sent back and forth to each other • Baseband communication • Not wirelessly transmittable • Requires physical connection via conductor or fibre

4. Digital Radio • Communicating digital data using analog signals • Similar to analog communications – a carrier is modulated by the information containing signal • Difference is modulating signal is a digital one • Amplitude, frequency and phase can all be modulated

5. Digital Radio Modulation • Carrier Signal: Vc(t) = Vcsin(2πfct + θ) • Modulate Vc to get Amplitude Shift Keying • Modulate fc to get Frequency Shift Keying • Modulate θ to get Phase Shift Keying • Modulate Vc and θ to get Quadrature Amplitude Modulation

6. Digital Radio Advantages • Noise Reduction • Data Repeaters remove noise easily • Easy Multiplexing in Time (TDMA), Frequency (FDMA), Code (CDMA) • Analog can multiplex, but only FDMA is easy • Signal Processing capabilities – filtering, amplification and other effects • All done in software. Limited only by imagination and technical skills

7. More Digital Radio Advantages • Becoming cheaper and easier to build • Integrated circuits and digital systems • System on a Chip (SOC) • Many forms of data/info already in digital form • Internet, databases, email, video, radio • Claude Shannon: can pass digital data error free through any arbitrarily noisy system if you decrease the data rate

8. Digital Radio Applications • Computer Modems • DSL • Microwave and satellite communications • PCS • Digital TV • IEEE 802.11 • …

9. Bit Rate and Baud Rate • Bit Rate is the number of bits that can be sent per second over a channel • Baud Rate is the number of symbols that can be sent per second over a channel • If we designate N to be number of bits per symbol, and M to be number of symbols necessary to represent N bits per symbol: N = log2M or M = 2N

10. Symbols and Bits • Example: if you want 3 bits per symbol, you need M = 23 = 8 symbols • For ASK, different symbols represented by different amplitudes • For FSK – different frequencies • For PSK – different phases • For QAM – combo different amplitudes and phases

11. Binary vs M-ary Communications • Binary Communication: • Sending only symbols representing a 1 or a 0 • Absolute Maximum Data Rate:bps = 2*BW • Typical Maximum Data Rate:bps = BW • M-ary Communications: • Sending symbols representing more than 1 bit each (i.e., there are M symbols representing N bits each • Absolute Maximum Data Rate: • Baud rate = 2*BW • Bit rate = 2*BW*log2M = 2*BW*N bps • Typical Maximum Data Rate: • Baud rate = BW • Bit Rate = BW*log2M = 2*BW*N bps

12. Amplitude Shift Keying (ASK) • Simplest digital modulation technique • Carrier Signal: vc(t) = Vccos(2πfct) • Modulating Signal is data sequence of 1s and 0s…m(t) = 1,0,0,1,1,1,0,1,1,1… • Modulated Signal: vask(t) = m(t)*vc(t)

13. ASK Example • vask(t) = m(t)*vc(t) • m(t) = 1,0,1,1,0,1,1,1,0,1,0…

14. ASK Example System • Switch turned on when input bit is a 1, and turned off when input bit is a 0 • Rate of input bits must be constant so that each bit has a fixed, known length of time.

15. ASK Demodulated Signal • Demodulation is fairly simple • Incoming signal will have an envelope like analog AM • Detect the envelope, and this will give first approximation of incoming data • Add threshold detector to determine whether data is a 1 or 0 to get regenerated data