Digital Communications

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

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

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

Digital Radio Modulation Carrier Signal: V c (t) = V c sin(2πf c t + θ) Modulate V c to get Amplitude Shift Keying Modulate f c to get Frequency Shift Keying Modulate θ to get Phase Shift Keying Modulate V c and θ to get Quadrature Amplitude Modulation

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

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, , video, radio Claude Shannon: can pass digital data error free through any arbitrarily noisy system if you decrease the data rate

Digital Radio Applications 1. Computer Modems 2. DSL 3. Microwave and satellite communications 4. PCS 5. Digital TV 6. IEEE …

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 = log 2 M or M = 2 N

Symbols and Bits Example: if you want 3 bits per symbol, you need M = 2 3 = 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 Symbol # Bit Representation

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*log 2 M = 2*BW*N bps –Typical Maximum Data Rate: Baud rate = BW Bit Rate = BW*log 2 M = 2*BW*N bps

Amplitude Shift Keying (ASK) Simplest digital modulation technique Carrier Signal: v c (t) = V c cos(2πf c t) Modulating Signal is data sequence of 1s and 0s…m(t) = 1,0,0,1,1,1,0,1,1,1… Modulated Signal: v ask (t) = m(t)*v c (t)

ASK Example v ask (t) = m(t)*v c (t) m(t) = 1,0,1,1,0,1,1,1,0,1,0…

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.

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