Protocol Data Unit Header Decoding and Payload Coding Recovery in the Starlink Ku-Band Downlink

Abstract

Existing Starlink downlink studies describe synchronization, pilots, and predictable waveform elements, but these descriptions do not establish how received header bits specify payload allocations and channel coding. Using 18 frames from one continuous complex-baseband capture spanning approximately 24 ms, we identify two protocol data unit (PDU) header regions in each frame and decode their contents after receiver correction and reference-phase removal. We recover the headers' terminated rate-1/3 convolutional code with constraint length seven and an eight-bit cyclic redundancy check over the concatenated information words. The decoded bits support a conditional 20-bit layout of modulation and coding scheme (MCS) index--length pairs, linking format labels and quantities to subsequent payload regions. Predicted allocation endpoints agree with signal-based estimates in 16 of 18 frames and differ by at most two symbols in the remaining frames. We further recover modulation mappings and equivalent quasi-cyclic low-density parity-check coding relations for several payload formats, including rate-1/2 and rate-5/8 families. Fixed generators predict 78,848, 9,984, and 811,008 parity bits for formats 5, 9, and 21 without disagreement on codewords excluded from their generator fitting. Cross-modulation tests support shared encoders. Formats 28 and 37 have verified shortened cores, with 256 and 384 additional bits unexplained, while format 68 remains unresolved.