Duplex digital communication system vs analog

In duplex (full-duplex) mode both terminals send and receive at the same time, e.g. a mobile phone call. Each terminal therefore has a complete digital transmitter and receiver, and the two directions share the channel by separate frequencies (FDD) or time slots (TDD) through a duplexer.

Functional digital communication system (duplex)

  TERMINAL A                         TERMINAL B
 +-----------+                      +-----------+
 | Source    |                      | Sink      |
 | Formatting|                      | Formatting|
 | Src encode|                      | Src decode|
 | Encrypt   |                      | Decrypt   |
 | Chan code |                      | Chan dec. |
 | Modulator |                      | Demod.    |
 +-----+-----+                      +-----^-----+
       |   +-------+  CHANNEL  +-------+  |
       +-->|       |==========>|       |--+
           |Duplex-|  (noise,  |Duplex-|
       +<--|  er   |<==========|  er   |<-+
       |   +-------+  fading)  +-------+  |
 +-----v-----+                      +-----+-----+
 | Demod.    |                      | Modulator |
 | Chan dec. |                      | Chan code |
 | Decrypt   |                      | Encrypt   |
 | Src decode|                      | Src encode|
 | Sink      |                      | Source    |
 +-----------+                      +-----------+

Main blocks of each direction:

1. Formatting: converts the source output to digital form (sampling, quantization, PCM).
2. Source encoding: removes redundancy to reduce bit rate (e.g. Huffman, speech codecs).
3. Encryption: secures the data.
4. Channel encoding: adds controlled redundancy for error detection/correction (parity, Hamming, CRC, convolutional codes).
5. Multiplexing / multiple access: combines several streams or users (TDM, FDMA, CDMA).
6. Modulation: maps bits to waveforms (ASK, FSK, PSK, QAM); pulse shaping limits bandwidth.
7. Duplexer and channel: separates the transmit and receive paths over the shared medium.
8. At the receiver, the blocks work in reverse: demodulation and detection, channel decoding, decryption, source decoding, and formatting back to the user's form.

Contrast with analog communication

Impairments

• Additive noise (thermal, shot, atmospheric) causes bit errors.
• Attenuation reduces signal strength with distance.
• Bandwidth limitation and dispersion cause inter-symbol interference (ISI).
• Interference: co-channel, adjacent channel, crosstalk, echo in duplex links.
• Fading and multipath in wireless channels.
• Timing jitter and carrier/clock synchronization errors.

Performance parameters

• Probability of bit error (BER) at a given $E_b/N_0$.
• Data rate (bit/s) and bandwidth efficiency (bit/s/Hz).
• Power efficiency: $E_b/N_0$ needed for a target BER.
• Channel capacity limit $C = B\log_2(1 + S/N)$.
• Latency/delay and system complexity and cost.