Modulation Schemes for Satellite Communications
2020-08-24 | 7 min read
With strong demand for faster data throughput, satellite communications use high-order modulation schemes to improve their spectral efficiency. However, satellite channel impairments such as large path losses, delays, and Doppler shifts pose severe challenges to the realization of a satellite network. The modulation techniques for satellite communications require not only faster data rates but also minimizing the impacts of the channel impairments. This post discusses the modulation techniques for modern satellite communications.
The Requirements of Satellite Communications
In digital modulation systems, a vector signal can be in changing the carrier signal’s magnitude, phase, or some combination of those. The most fundamental digital modulation schemes are amplitude-shift keying (ASK), phase-shift keying (PSK), frequency-shift keying (FSK), and quadrature amplitude modulation (QAM).
In satellite transmission, RF power amplifiers often operate at their compression levels to maximize conversion efficiency. Operating at compression levels causes AM/AM and AM/PM distortion, as shown in Figure 1. For example, the I/Q constellation outer points have higher output power levels, and the compression is because of the saturated output power in the RF power amplifier. Thus, nonlinear amplifiers require a modulation scheme tolerant to distortion. Also, the higher output power creates more noise to the signal.
Constant Envelope Digital Modulation Schemes
The constant envelope modulation schemes such as FSK and PSK are the most suitable for satellite communications because they minimize the effect of nonlinear amplification in the high-power amplifier. Figure 2 illustrates the constellation diagrams of binary PSK (BPSK), quadrature PSK (QPSK), and 8PSK. They transmit 1, 2, and 3 bits per symbol, correspondingly. For higher-order PSK, the constellation points are closer to each other, and the system is more sensitive to channel impairments. For FSK, 4FSK (2 bits per symbol) has higher spectral efficiency than 2FSK’s but the smaller frequency deviation will cause a bad sensitivity in the receiver.
Nonconstant Envelope Digital Modulation Schemes
Quadrature amplitude modulation (QAM) is a nonconstant modulation that changes both phase and amplitude to increase spectral efficiency. Figure 3 illustrates the constellation diagram of 16PSK and 16QAM. 16QAM increases the distance between the constellation points and has better resistance to signal impairments. However, 16QAM also increases the amplitude levels to three (rings) compared with 16PSK. RF power amplifiers require a wider linear range for nonconstant modulation schemes.
Satellite equipment must be capable of transmitting at a high power level while maintaining high output linearity. Also, the higher modulation schemes enable higher data throughput but are sensitive to signal impairments.
Resist Nonlinear Distortion with APSK
Satellite communications employ amplitude phase-shift keying (APSK) to resist nonlinear distortion. Figure 4 illustrates a constellation diagram for APSK and QAM modulation schemes. The APSK’s states are in rings such that the amplitude compression is the same in a specific ring. The 16APSK constellation has only two amplitudes (rings), whereas 16QAM has three amplitudes. The 32APSK constellation has three amplitudes versus five in 32QAM. More amplitude levels make the rings closer together and more difficult to compensate for nonlinearities.
There are several variable parameters for APSK modulation such as the number of rings, number of symbols on a ring, and spacing between rings. A designer can also reach a balance between lower peak-to-average power ratio (PAPR) and better resistance to distortion.
Enhance Data Rate Using OFDM
The orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier technique that possesses many unique advantages over single-carrier approaches. The technique has been adopted for many broadband wireless communication standards, such as 4G/5G, Wi-Fi, digital video broadcast for terrestrial and satellite communication systems.
OFDM uses many closely spaced orthogonal subcarrier signals to transmit data in parallel. That process provides better spectral efficiency than traditional digital modulation schemes, such as QAM and PSK, and robustness against channel linear distortion. Figure 5 shows a single OFDM carrier (the left plot) and multiple subcarriers (the right plot). The peak of each subcarrier occurs at zero crossings of the others. The signal is orthogonal in the frequency domain, and each subcarrier does not interfere with the others. The subcarriers can apply different modulation formats and channel coding, depending on the noise and interference level of individual sub-bands that provide a robust communication link.
However, the OFDM signal has a higher PAPR than traditional modulation schemes, requiring a large back off to avoid the compression at a high output power level. Nonlinear effects generated by the high-power amplifier may introduce more distortions to a satellite system that causes a system failure. Therefore, characterizing the distortion performance of satellite RF components is essential for making a good system design.
Most communications systems optimize efficiencies in system designs, including spectral, power, and cost. The selection of modulation schemes for satellite communications depends on the communication channels, hardware limitations, and data throughput requirements.
Also, both custom APSK and OFDM modulation schemes bring in test challenges – generating and analyzing custom, proprietary modulation schemes. Next post, we will discuss how to simplify the custom signal generation and analysis.