CBOC (composite binary offset carrier)

Composite Binary Offset Carrier (CBOC) is a radio frequency (RF) modulation technique used in Global Navigation Satellite Systems (GNSS), such as GPS (Global Positioning System) and Galileo. CBOC is an enhancement to the existing binary offset carrier (BOC) modulation scheme, which is used in GPS L1 C/A (Coarse/Acquisition) and L2C signals.

CBOC is a combination of two modulated carriers: a binary modulated carrier (BOC) and a subcarrier modulated by a secondary binary signal. The binary modulated carrier is a square wave that is modulated with the navigation data bits. The subcarrier is a sine wave that is modulated with a secondary binary signal, which is generated by multiplying the navigation data bits with a spreading code.

The spreading code is a pseudo-random noise (PRN) code that is unique to each satellite and is used to distinguish the signal from other signals in the same frequency band. The PRN code is a sequence of binary digits that is generated using a mathematical algorithm. The sequence is designed to have good autocorrelation and cross-correlation properties, which allows for the signal to be easily distinguished from other signals.

The CBOC modulation scheme has several advantages over the BOC modulation scheme. One of the advantages is that it reduces the peak-to-average power ratio (PAPR) of the signal, which reduces the distortion and interference caused by the signal. The reduction in PAPR is achieved by using a subcarrier that is modulated by a secondary binary signal, which spreads the energy of the signal over a wider frequency range.

Another advantage of CBOC is that it provides a better signal-to-noise ratio (SNR) compared to BOC. This is because the subcarrier provides additional energy to the signal, which increases the power spectral density (PSD) of the signal. The increased PSD improves the signal's immunity to noise and interference.

CBOC also provides better multipath performance compared to BOC. Multipath is the phenomenon where the signal bounces off objects such as buildings and terrain before reaching the receiver. The bounced signal can cause interference and errors in the received signal. CBOC reduces the effects of multipath by spreading the energy of the signal over a wider frequency range, which reduces the likelihood of the signal being completely cancelled out by the multipath signal.

In addition to the above advantages, CBOC also provides better interoperability with other GNSS systems. This is because CBOC is a modulation scheme that is used in both GPS and Galileo. The use of a common modulation scheme makes it easier for receivers to track signals from multiple satellite systems.

The CBOC modulation scheme has two variants: CBOC(6,1) and CBOC(10,5). The numbers in the brackets represent the chip rate of the BOC signal and the subcarrier, respectively. The CBOC(6,1) variant is used in GPS L1C signals, while the CBOC(10,5) variant is used in Galileo E1 signals.

The CBOC(6,1) variant has a BOC chip rate of 1.023 MHz and a subcarrier chip rate of 6.138 MHz. The CBOC(10,5) variant has a BOC chip rate of 1.023 MHz and a subcarrier chip rate of 10.23 MHz. Both variants use a spreading code that is 1023 chips long.

To summarize, CBOC is a modulation scheme used in GNSS that provides several advantages over the existing BOC modulation scheme. CBOC uses a combination of a binary modulated carrier and a subcarrier modulated by a secondary binary signal. This combination reduces the peak-to-average power ratio (PAPR) of the signal, increases the signal-to-noise ratio (SNR), and provides better multipath performance compared to BOC. CBOC also provides better interoperability with other GNSS systems, as it is used in both GPS and Galileo.

The CBOC modulation scheme is implemented using digital signal processing (DSP) techniques. The DSP algorithms are implemented in the satellite payload and the receiver to generate and decode the CBOC signal. The receiver extracts the navigation data and the PRN code from the received signal using a process called correlation. The correlation process involves multiplying the received signal with a locally generated replica of the PRN code and integrating the result over a period of time. The correlation result is used to detect the presence of the signal and extract the navigation data.

The CBOC modulation scheme has been standardized by the International Telecommunication Union (ITU) and is included in the Radio Regulations as a recognized GNSS modulation scheme. The standardization of CBOC ensures that the modulation scheme is compatible with existing regulations and can be used in GNSS applications worldwide.

CBOC has been adopted by various GNSS systems around the world, including GPS and Galileo. The use of CBOC has improved the performance of GNSS systems and has enabled new applications such as high-precision positioning and timing, autonomous vehicles, and unmanned aerial vehicles (UAVs).

In conclusion, CBOC is a modulation scheme used in GNSS that combines a binary modulated carrier and a subcarrier modulated by a secondary binary signal. CBOC provides several advantages over the existing BOC modulation scheme, including reduced PAPR, increased SNR, and better multipath performance. The use of CBOC has been standardized and adopted by various GNSS systems, enabling new applications and improving the performance of GNSS systems worldwide.