Types of Digital Modulation
Modulation methods are divided into digital and analog formats.
Digital modulation applies discrete modulation levels to a carrier signal, producing a discrete set of signal characteristics that are used to encode digital information.
Wireless and fiber systems use digital modulation formats to transmit at high data rates.
Amplitude modulation and frequency modulation are probably the most familiar modulation formats, but they both fall within a larger set of modulation formats: analog and digital modulation. These modulation formats aren’t limited to AM and FM transmission; phase modulation, pulse modulation, and combinations of all of these modulation formats can be used to transmit digital and analog information over wireless, copper, or fiber channels.
Today, digital modulation formats are standard in wireless communications and they enable higher data rates in wireless channels. For systems designers and RF designers, it’s important to understand the various types of digital modulation, as each format can place performance constraints on a design.
The Types of Digital Modulation vs. Analog Modulation
Modulation refers to the technique of mixing two signals to transmit information. The two signals that are used in modulated communication are the carrier signal and the information signal. The only difference between analog and digital modulation is found in the information signals. In digital modulation, the information signal takes discrete levels, while in analog modulation, the information signal has no discretization and can take any level. The information signal is then used to modulate (vary) the phase, amplitude, and/or frequency of the carrier signal.
Amplitude, Phase, and Frequency Shift Keying (ASK, PSK, FSK)
These methods are among the simplest to implement, as they only involve variations in one signal quality: amplitude for ASK, phase for PSK, and frequency for FSK. Although these are easier to implement in terms of circuit design compared to other digital modulation methods, they can be limited in terms of bit rate.
FSK, ASK, and PSK (middle graph to bottom) shown in the time domain
It is sometimes assumed or implied that ASK, PSK, and FSK are “binary” methods (BASK, BPSK, and BFSK), meaning the signal characteristics are only varied between two values. In fact, ASK, FSK, and PSK can be multilevel methods, where the signal characteristic is varied between multiple discrete values. For example, if we want to transmit a 3-bit digital bitstream using multilevel FSK (MFSK), we could use 23 = 8 frequency levels; the same applies to multilevel ASK (MASK) and multilevel PSK (MPSK).
ASK, PSK, and FSK all have a variant that involves demodulating the received signal in quadrature. In quadrature modulation schemes, two portions of a digital bit stream are transmitted in parallel in a communications channel. The two bit streams are transmitted with a half-wave phase shift between them so that only one portion of the carrier wave is active at any time. The two bit streams are then combined with an adder and transmitted. At the receiver, the two signals are demodulated to recover the two modulating waveforms that correspond to the dual input bit streams.
A block diagram showing the transmission method used in a QAM technique is shown below. In this method, a phase shift is applied to the lower leg of the diagram to produce two out-of-phase signals.
Example block diagram for generating QAM signals
Quadrature modulation methods are some combination of single-level or multilevel shift keying methods. Quadrature amplitude modulation (QAM) is most common; this technique is really a combination of N-level MASK with BPSK. At the receive side, this gives N2 possible logic states that can be transmitted in a communications channel. The possible list of states can be visualized using a constellation diagram, which shows the list of possible states.
Example 16-QAM constellation diagram
QAM combined with frequency division multiplexing (FDM), we have a multichannel technique called orthogonal frequency division multiplexing (OFDM). This modulation technique is used in 5G, where 64-QAM or 256-QAM is used in different frequency channels to provide multiple bit streams with a high data rate. Carrier aggregation provides another doubling of the data rate, ultimately reaching Gbps levels over the air.
Combined Shift Keying Methods
Shift keying methods can be combined in tandem, similar to QAM. One method used in satellite communication and tactical wireless communication as an alternative to QAM is amplitude phase shift keying (APSK). In this method, the transmitted signal is shifted between multiple phase levels and multiple amplitude levels, rather than having two different ASK signals that are out-of-phase.
Example 32-APSK constellation diagram. The red bits correspond to 4 possible phase values and the green bits correspond to 8 possible amplitude values. [Source: DVB-S2 Spectrum Efficiency Improvement with Hierarchical Modulation]
Combined shift keying methods can be at a disadvantage compared to quadrature methods in terms of their bandwidth and noise sensitivity. Phase-modulated signals have broader bandwidth than amplitude-modulated signals, so the majority of the bandwidth in a transmitted signal’s power spectrum cannot be smaller than the limit imposed by the applied phase modulator. However, by combining MASK and MPSK, you can transmit data at the same rate as with an equivalent QAM signal. Although QAM has tighter noise margin allowances, APSK is a better option for applications like satellite and military communications, as there is less interference in the deployment environment.
Pulse code modulation methods use a stream of pulses rather than a sinusoidal carrier signal to transmit data. In these modulation schemes, the pulse stream is modulated by varying the pulse duration (pulse duration modulation, or PDM), phase between pulses (pulse phase modulation, or PPM), frequency (pulse frequency modulation, or PFM), or amplitude (pulse amplitude modulation, or PAM). Digital PAM is the method used today to transmit high data rates in Gbps channels over copper and fiber, and it may be the key to getting to 224 Gbps and higher data rates.
Each type of digital modulation has a corresponding analog modulation method. This correspondence between the basic analog and digital modulation methods is summarized below:
Cadence’s PCB design and analysis software can help you understand and visualize signal behavior for any type of digital modulation. Circuit and signal modeling applications can help you simulate systems with arbitrary waveforms, including digitally modulated signals. Cadence’s software suite can also be used to perform a range of signal integrity simulations and evaluate your system’s functionality before you start your physical layout.
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