Digital Modulation Methods

In digital modulation, an analog carrier signal is modulated by a digital bit stream of either equal length signals or varying length signals. This can be described as a form of analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of alternative symbols (the modulation alphabet). In practical systems, these symbols are always chosen to be "orthogonal" to each other, resulting in a M-ary orthogonal modulation.

These are the most fundamental digital modulation techniques

• In the case of CW, groupings of on-off keying of varying length signals are used.
• In the case of PSK, a finite number of phases are used.
• In the case of FSK, a finite number of frequencies are used.
• In the case of ASK, a finite number of amplitudes are used.
• In the case of QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes. It can be seen as a two channel system, each channel using ASK. The resulting signal is a combination of PSK and ASK, with a finite number of at least two phases, and a finite number of at least two amplitudes.

Each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase.

If the alphabet consists of M = 2^N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is f_S symbols/second (or baud), the data rate is Nf_S bit/second.

For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate.

In the case of PSK, ASK and QAM, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol.

PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex valued signal called the equivalent lowpass signal or equivalent baseband signal. This is a representation of the valued modulated physical signal (the so called passband signal or RF signal).

The QAM principle can generate any possible band-limited signal. And any possible band-limited signal can be decomposed using QAM into the baseband I and Q signals necessary to generate it. The only reason we don't always use QAM modulator and demodulator hardware for every kind of signal modulation is that other kinds of modulators are simpler, cheaper, or require less battery power than QAM modulators or demodulators.

These are the general steps used by the modulator to transmit data:

1. Group the incoming data into codewords;
2. Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values.
3. Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum, typically using digital signal processing
4. Digital-to-analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using digital signal processing, DSP). Sometimes the next step is also achieved using DSP, and then the DAC should be done after that.
5. Modulate the high-frequency carrier waveform, resulting in that the equivalent low pas signal is frequency shifted into a modulated passband signal or RF signal
6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum

At the receiver, the demodulator typically performs:

1. Bandpass filtering
2. Automatic gain control, AGC (to compensate for attenuation)
3. Frequency shifting of the RF signal baseband I and Q signals, or to an intermediate frequency (IF) signal, or
4. Sampling and analog-to-digital conversion (ADC) (Sometimes before the above point)
5. Equalization filtering
6. Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal;
7. Quantization of the amplitudes, frequencies or phases to the nearest allowed values, using mapping.
8. Map the quantized amplitudes, frequencies or phases to codewords (bit groups);
9. Parallel-to-serial conversion of the codewords into a bit stream
10. Pass the resultant bit stream on for further processing such as removal of any error-correcting codes.

As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.

The most common digital modulation techniques are:

• Phase-shift keying (PSK)
• Frequency-shift keying (FSK) (see also audio frequency-shift keying (AFSK))
• Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)
• Quadrature amplitude modulation (QAM) a combination of PSK and ASK
• Polar modulation like QAM a combination of PSK and ASK.
• Continuous phase modulation (CPM)
  • Minimum-shift keying (MSK)
  • Gaussian minimum-shift keying (GMSK)
• Orthogonal frequency division multiplexing (OFDM) modulation, also known as discrete multitone (DMT).
• Wavelet modulation
• Trellis coded modulation (TCM) also known as trellis modulation

MSK and GMSK are particular cases of continuous phase modulation (CPM). Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signaling).

OFDM is based on the idea of Frequency Division Multiplex (FDM), but is utilized as a digital modulation scheme. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The sub-carriers are summarized into a OFDM symbol. OFDM is considered as a modulation technique rather than a multiplex technique, since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the Orthogonal Frequency Division Multiple Access (OFDMA) and MC-OFDM schemes, allowing several users to share the same physical medium by giving different sub-carriers to different users.

Of the two kinds of RF power amplifier, switching amplifiers cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as FM and some types of PM and polar modulation. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often QAM modulators are used to drive switching amplifiers with these FM and other waveforms, and sometimes sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers.