To achieve stable and reliable satellite communications, ground terminal antennas must be precisely aligned with distant satellites. This process involves continuous searching and approaching, with the goal of precisely pointing the antenna beam toward the satellite to maximize signal reception and transmission efficiency.
The alignment technology of satellite communication terminal antennas is the key to ensuring the stability of satellite communication links. Currently, the commonly used alignment modes include the following five:
- Beacon tracking
- Carrier tracking
- Reference satellitetracking
- Program tracking
- Manual tracking
We introduced beacon tracking in “One of the Five Satellite Tracking Modes of Earth Station Antenna: Beacon Tracking .” The satellite continuously transmits a dedicated beacon signal during the downlink. This signal is typically a narrowband, high-power continuous wave signal with a fixed frequency and polarization. Ground-based satellite terminals perform alignment operations by detecting the beacon signal’s power. Today, we will focus on another common and efficient automatic alignment method: DVB tracking.
- What is a DVB signal?
DVB stands for Digital Video Broadcasting. DVB signals are a series of open international standards for digital television broadcasting, not a single modulation signal.
The core of the DVB standard is the use of digital modulation technology to compress and encode video streams (such as MPEG) and then modulate them onto a carrier for transmission. The modulated signal can be transmitted via satellite links, terrestrial cable networks, or terrestrial broadcasting.
What are the common DVB standards? What are their corresponding modulation methods?
The DVB standard is a family of standards that includes multiple sub-standards. Each standard uses a different modulation method to adapt to different transmission environments and requirements.
The main categories are as follows:
- DVB-S
This is the most basic and common DVB standard for satellite communications. It uses QPSK (Quadrature Phase Shift Keying) modulation, transmitting two bits of information per symbol. DVB-S, introduced around 1995, was one of the earliest standards used in satellite television broadcasting and remains widely used today.
QPSK has good noise resistance and can maintain good communication quality even when the signal-to-noise ratio is low, so it has been widely used.
- DVB-S2
DVB-S2 is the second generation of DVB-S, also known as enhanced DVB-S. In addition to supporting QPSK, it also introduces a variety of higher-order modulation methods, improving spectrum efficiency and transmission capacity, and adapting to different link conditions and service requirements.
The modulation modes supported by DVB-S2 include:
o• QPSK: 2 bits are transmitted per symbol;
o• 8PSK: Each symbol transmits 3 bits, suitable for high-definition television broadcasting;
o• 16APSK: 4 bits are transmitted per symbol;
o• 32APSK: Each symbol transmits 5 bits, but requires a higher signal-to-noise ratio.
From QPSK to 8PSK, to 16APSK, and finally to 32APSK, the modulation order is getting higher and higher, and the transmission efficiency is getting higher and higher, but the required signal-to-noise ratio is also getting higher and higher. When the signal quality is good and the signal-to-noise ratio is high, high-order modulation can be used.
In conventional satellite communication methods, once the modulation and coding methods are set, they will always be used, which is also called constant coding modulation (CCM).
DVB-S2 introduces Adaptive Coding and Modulation (ACM) technology. Unlike traditional Constant Coding Modulation (CCM), ACM allows the transmitter to dynamically adjust the modulation scheme and coding rate based on channel conditions fed back by the receiver.
DVB-S2 Adaptive Coding and Modulation
The satellite receiver measures the quality of the satellite communication link (primarily signal-to-noise ratio and bit error rate) in real time. It then transmits this information back to the transmitter via the satellite backhaul link. Based on this information, the transmitter dynamically adjusts the modulation method and coding rate for the next frame.
When the signal-to-noise ratio (SNR) at the receiver is high, the transmitter can use higher-order modulation (such as 16APSK or 32APSK) and a higher code rate (such as LDPC 8/9). Conversely, if the channel deteriorates, the transmitter will reduce the modulation order (such as from 16APSK to 8PSK or QPSK) and lower the code rate (such as LDPC 1/4) to maintain communication quality.
- DVB-S2X
DVB-S2X is an extended version of DVB-S2, adding higher-order modulation methods, including:
o• 64APSK: 6 bits are transmitted per symbol;
o• 128APSK: 7 bits are transmitted per symbol;
o• 256APSK: 8 bits are transmitted per symbol;
DVB-S2X is suitable for large-bandwidth data transmission in high signal-to-noise ratio scenarios, and supports a minimum roll-off factor of 0.05, effectively reducing adjacent frequency interference and improving bandwidth utilization.
- DVB-T and DVB-C
DVB-T is suitable for terrestrial broadcasting, while DVB-C is suitable for cable television networks. They respectively use COFDM, 16QAM, 32QAM, and 64QAM modulation schemes. Since these are terrestrial transmission standards and unrelated to satellite communications, they will not be discussed in detail in this article.
- Principles of DVB signal tracking
As can be seen from the above description, DVB signals are essentially modulated carrier signals. In theory, the power detection method described in Power Detection Carrier Tracking can also be used .
As we’ve mentioned, telemetry, image transmission, and data link are all wireless communications that can transmit video images and various data information, so they share commonalities. However, telemetry signals have their own standards, including encoding, frame format, and modulation methods, and therefore require their own unique signal capture, tracking, and demodulation methods.
Similarly, DVB signals have their own unique structure and standards, including frame format, encoding method, modulation method, etc., so its signal capture, tracking and demodulation process is also unique.
Why use carrier-to-noise ratio instead of signal power? The beacon tracking and power-detection carrier tracking methods described earlier both use signal power as the basis for antenna alignment. However, the signal reception performance of satellite communication terminals, such as the bit error rate, is directly related to the received signal-to-noise ratio.
However, signal power doesn’t directly reflect communication quality, while the carrier-to-noise ratio (C/N0) is closely related to the bit error rate (BER). If we use the C/N ratio as the basis for satellite tracking, antenna alignment is directly linked to signal quality (not just power) and directly correlated with the bit error rate (BER) of data demodulation.
In DVB tracking, the system doesn’t rely directly on signal power. Instead, it calculates the carrier-to-noise ratio (C/N0) by demodulating the received signal, performing channel estimation, and evaluating signal quality. This value serves as the basis for the antenna servo system’s satellite alignment. The antenna controller (ACU) monitors C/N0 changes in real time to determine whether the antenna is aligned with the satellite.
- The role of pilot signal and training sequence
DVB signals usually contain pilot signals or training sequences. These known reference signals are periodically inserted into the data stream for synchronization, channel estimation, and carrier-to-noise ratio calculation.
Pilots/Training Sequences are pre-known, specific symbol sequences periodically inserted into the data stream. They do not carry any user data but serve as a reference for receiver synchronization, channel estimation, and performance monitoring.
During DVB tracking, the receiver only needs to demodulate these pilot signals to estimate the current channel state and calculate the carrier-to-noise ratio (C/N0). This process does not require demodulating all data, significantly reducing computational complexity.
- Composition and workflow of DVB tracking system
4.1 System connection structure
A DVB tracking system usually consists of the following parts:
- Antenna
- LNB (Low Noise Converter)
- DVB Tracking Receiver/DVB Demodulator
- Antenna Controller ACU
The specific structure is as follows:
Composition and workflow of DVB tracking system
- Basic process of DVB tracking
The main steps of DVB tracking are as follows:
5.1 Service Carrier Signal Reception
Ground station A transmits DVB signals to the satellite, which are transparently forwarded to ground station B. Ground station B receives the signals and sends them to a DVB tracking receiver.
5.2 DVB Signal Tracking
The DVB tracking receiver demodulates and processes the pilot or training sequence in the signal, estimates the current channel state, and calculates the carrier-to-noise ratio C/N0.
The tracking receiver/DVB demodulator outputs the signal quality parameter (C/N ratio) to the antenna controller in real time. The C/N ratio is usually provided to the antenna controller via the RS-232/485 serial port or Ethernet interface.
5.3 Antenna Scanning and Signal Detection
The ACU controls the antenna to scan a small range in azimuth and elevation, and records the carrier-to-noise ratio values at different angles.
5.4 Peak Tracking and Alignment
The ACU records the carrier-to-noise ratio at each angle, finds the maximum value as the optimal alignment position, and continuously makes fine adjustments to maintain the alignment.
DVB tracking is a highly efficient automatic satellite tracking method based on the carrier-to-noise ratio (C/N0), particularly suitable for satellite communications scenarios where DVB signals are predominant. Compared to traditional power detection methods, it more accurately reflects signal quality, thereby improving satellite tracking accuracy and stability.
In the next chapter, we will continue to introduce other types of satellite communication antenna alignment methods.
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