Reasons for modulation; digital communication block diagram

Reasons for modulation

Modulation shifts the message onto a high-frequency carrier. It is needed because:

1. Antenna size: efficient radiation needs an antenna of about $\lambda/4$. A 3 kHz voice signal has $\lambda = 100$ km, i.e. a 25 km antenna; on a 100 MHz carrier the antenna is only 0.75 m.
2. Multiplexing: different messages are put on different carriers so many users share one medium (FDM in radio, TV, telephone trunks).
3. Noise reduction: wideband schemes (FM, spread spectrum) improve SNR at the cost of bandwidth.
4. Narrowbanding: reduces the ratio of highest to lowest frequency, making antenna and amplifier design easier.
5. Channel matching and long-range propagation: places the signal in a band the channel passes well and in the band allotted by regulation.

Advantages of digital over analog communication

1. Noise immunity: only a decision between a few levels is needed; regenerative repeaters remove accumulated noise, so quality does not fall with distance.
2. Error detection and correction coding is possible.
3. Security: encryption is easy.
4. Integration: voice, data, image and video all become bits and share the same network (TDM, packet switching).
5. Storage and processing: easy to store, compress and process with DSP and computers.
6. Hardware: cheap, reliable, flexible VLSI circuits; easy to reconfigure (software radio).
7. Bandwidth/power trade-off and better performance in low SNR via coding.

Generic digital communication system (full-duplex)

In full-duplex both ends transmit and receive simultaneously. Each end has a transmitter chain and a receiver chain, joined to the channel by a duplexer (FDD or TDD).

   END A (transmit path)          END B (receive path)
 Source                              Sink
   |                                   ^
 Formatting                        Formatting
   |                                   ^
 Source enc.                       Source dec.
   |                                   ^
 Channel enc.                      Channel dec.
   |                                   ^
 Modulator                         Demodulator
   |                                   ^
 [Duplexer]====== CHANNEL + noise ==[Duplexer]
   |                                   ^
 Demodulator ... (same chain in reverse) ...
   v                                   |
 Sink   <---- B transmits to A ----  Source

• Formatting: sampling, quantization, PCM to obtain bits.
• Source encoder/decoder: compression.
• Channel encoder/decoder: adds/uses redundancy to correct errors.
• Modulator/demodulator: maps bits to waveforms (ASK, FSK, PSK, QAM) and back; the receiver also performs synchronization and detection.
• Duplexer: lets one antenna or line carry both directions (different frequency bands in FDD, alternate time slots in TDD).