5.0 Advanced Digital Modulation and Encoding

Digital modulation techniques offer greater information capacity, data security, and communication quality compared to analog methods.

5.1 Digital Modulation Schemes

• Amplitude Shift Keying (ASK): Represents binary data by varying the amplitude of the carrier signal. For example, a high-level input corresponds to the carrier being present, and a low-level input corresponds to its absence (zero amplitude).
• Frequency Shift Keying (FSK): Represents binary data by varying the frequency of the carrier signal. Binary ‘1’s and ‘0’s are represented by two different frequencies, known as Mark and Space frequencies.
• Phase Shift Keying (PSK): Represents binary data by changing the phase of the carrier signal.
  • Binary PSK (BPSK): Uses two phase shifts (e.g., 0° and 180°).
  • Quadrature PSK (QPSK): Uses four phase shifts (0°, 90°, 180°, 270°), allowing two bits of information (bigits) to be sent per symbol, thus halving the data bit rate and improving bandwidth efficiency.
  • Differential PSK (DPSK): The phase is shifted relative to the previous signal element, eliminating the need for a reference oscillator at the receiver.

5.2 Delta and Differential Modulation

These techniques improve upon PCM by encoding the difference between samples rather than their absolute values, which is efficient for highly correlated signals.

• Differential PCM (DPCM): Transmits the quantized difference between the current sample and a predicted value, reducing redundancy.
• Delta Modulation (DM): A simplified, 1-bit version of DPCM. It uses a much higher sampling rate and represents the signal as a staircase approximation. It is simpler to implement but can suffer from slope overload distortion (if the signal changes too fast for the step-size) and granular noise (if the signal is flat and the step-size is too large).
• Adaptive Delta Modulation (ADM): Improves upon DM by allowing the step-size to vary according to the slope of the modulating signal, reducing both types of noise.

5.3 M-ary Encoding

M-ary encoding techniques transmit two or more bits at a time using a single signal, thereby reducing the required channel bandwidth. The number of possible conditions (M) is related to the number of bits (N) by the equation M = 2^N.

• M-ary ASK: The carrier signal takes on M different amplitude levels.
• M-ary FSK: The carrier signal takes on M different frequency levels.
• M-ary PSK: The carrier signal takes on M different phase levels.