AltBOC (alternate binary offset carrier)
Alternate Binary Offset Carrier (AltBOC) is a modulation scheme used in Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS). It is a form of binary phase-shift keying (BPSK) modulation, which is a type of digital modulation technique that encodes digital data onto a sinusoidal carrier signal by varying its phase.
In traditional BPSK, the carrier signal is a pure sine wave that oscillates between two phase states, 0 degrees and 180 degrees. AltBOC, on the other hand, uses two separate carrier signals that are offset in frequency and phase from each other. The two carriers are combined in such a way that their phases are alternated in a binary pattern, creating a composite signal with a unique spectral shape that provides several advantages over traditional BPSK.
The AltBOC signal consists of two sub-carriers: a main carrier and a sub-carrier. The main carrier is a continuous wave that oscillates at the center frequency of the AltBOC signal, while the sub-carrier is an intermittent wave that alternates between two frequencies that are offset from the main carrier by a specific amount. The sub-carrier is also phase-shifted by 180 degrees each time it switches frequency, so that its phase is always opposite to that of the main carrier.
The alternating phase relationship between the two sub-carriers in the AltBOC signal produces a unique spectral shape that has a much lower peak-to-average power ratio (PAPR) than traditional BPSK. PAPR is a measure of how much the peak power of a signal exceeds its average power, and it is a major concern in many wireless communication systems because it can cause distortion and interference. AltBOC's low PAPR makes it more robust to interference and noise, and it also allows for more efficient use of the available transmit power.
Another advantage of AltBOC is its ability to provide increased positioning accuracy. The two sub-carriers in the AltBOC signal have different wavelengths, which means they experience different levels of ionospheric delay as they travel through the Earth's atmosphere. By measuring the difference in phase between the two sub-carriers, GNSS receivers can estimate the amount of ionospheric delay and correct for it, which improves positioning accuracy.
AltBOC also has improved multipath performance compared to traditional BPSK. Multipath occurs when a signal is reflected off of nearby surfaces and arrives at the receiver at slightly different times and angles, causing interference and distortion. AltBOC's unique spectral shape makes it less susceptible to multipath, which improves receiver performance in urban environments and other areas with high levels of multipath.
In summary, AltBOC is a modulation scheme that uses two sub-carriers with different frequencies and phases to create a composite signal with a unique spectral shape that has several advantages over traditional BPSK. These advantages include lower PAPR, increased positioning accuracy, and improved multipath performance. AltBOC is currently used in some GNSS systems, including the European Galileo system and the Chinese BeiDou system.
AltBOC is also being considered as a potential modulation scheme for future GNSS systems, including the next generation GPS system known as GPS III. This is because AltBOC offers several advantages over other modulation schemes, including its low PAPR, improved multipath performance, and ability to provide increased positioning accuracy.
One of the challenges of using AltBOC is that it requires more complex receiver hardware than traditional BPSK. Because the AltBOC signal consists of two sub-carriers, the receiver must be able to detect and process both sub-carriers separately in order to decode the signal. This requires specialized hardware and algorithms, which can increase the cost and complexity of GNSS receivers.
Despite these challenges, AltBOC has shown promise as a modulation scheme for GNSS systems, particularly in environments with high levels of interference and multipath. Its ability to provide increased positioning accuracy and lower PAPR also make it an attractive option for future GNSS systems.
In addition to its use in GNSS systems, AltBOC has also been studied for other applications, such as in communication systems and radar systems. In communication systems, AltBOC has been shown to provide better performance in multipath environments than traditional BPSK, and it has also been proposed as a potential modulation scheme for 5G cellular networks. In radar systems, AltBOC has been studied as a possible alternative to traditional pulse compression techniques, which are used to improve radar resolution and range.
In conclusion, AltBOC is a modulation scheme that uses two sub-carriers with different frequencies and phases to create a composite signal with a unique spectral shape that offers several advantages over traditional BPSK. Its low PAPR, improved multipath performance, and ability to provide increased positioning accuracy make it an attractive option for GNSS systems and other applications. While AltBOC requires more complex receiver hardware than traditional BPSK, it has shown promise in a variety of applications and is likely to continue to be studied and used in the future.