I continued reading the Starlink patent US12003350 and summarized the information related to modulation and channel coding. The patent was filed in early 2021, so readers should consider its timeliness accordingly.
Starlink supports the following modulation schemes:
- BPSK
- QPSK
- 8-QAM
- 16-QAM
- 32-QAM
- 64-QAM
Unlike Wi-Fi and LTE, Starlink includes 8-QAM and 32-QAM. A possible reason is that satellite links are highly power-limited due to large propagation losses. Finer modulation granularity allows Adaptive Modulation and Coding (AMC) to utilize the available link margin more precisely and avoid wasting transmit power.
The patent explicitly mentions the Adaptive Modulation and Coding (AMC) mechanism.
User data is encoded using LDPC. Data scrambling uses a 15-stage Linear Feedback Shift Register (LFSR).
The physical-layer packet header, analogous to the Wi-Fi SIGNAL field, is encoded using a 64-state (constraint length 7), rate-1/3 convolutional code with BPSK modulation. This is similar to the coding used for LTE control/broadcast channels such as the PDCCH and PBCH.
The packet header contains:
- Sequence number
- Modulation and Coding Scheme (MCS)
- Packet length
Within a radio frame, the physical-layer header of the first PDU contains 27 bits of information, while the headers of subsequent PDUs contain 23 bits.
PDUs are divided into data PDUs and signaling PDUs. Signaling PDUs are used for communication between MAC entities, such as the random access procedure (RACH) between the terminal MAC and the satellite MAC.
Starlink also employs a PN-based symbol scrambling method for modulated symbols. A Linear Feedback Shift Register generates a random 2-bit value that rotates each transmitted symbol by 0°, 90°, 180°, or 270°. This scrambling can transform a BPSK symbol stream into a QPSK constellation.
All frequency-domain subcarriers, including both data and pilot subcarriers, are scrambled using PN phase rotations generated by a 15-stage LFSR. As a result, BPSK pilot symbols become QPSK after scrambling.
The 128-sample base sequence used for the time-domain Unique Word (UW) is also originally BPSK and is converted to QPSK through PN phase rotation scrambling. UW scrambling uses a 7-stage LFSR.