Receiver Chain

AD9361 ADC -> rx_ingress -> ddc_core -> rx_switch -> agc -> matched_filter
           -> timing_recovery -> carrier_recovery -> slicer
           -> demod (BPSK | QPSK | GMSK) -> deframer -> fec_rx -> DMA (bytes)

In PL loopback the input comes from the channel emulator instead of the ADC (the loopback mux sits in front of the RX chain). The DDC-side blocks live in the window at 0x4001_0000, the demodulator-side blocks at 0x4002_0000.

rx_ingress#

Packs the AD9361's 12-bit I/Q into the 16-bit datapath with a digital gain.

ddc_core#

A complex NCO mixer, a decimator by DECIM (a single-stage running-sum CIC), and a compensation FIR (identity by default).

rx_ddc.rx_switch then routes the DDC output to the demodulator and to the monitor taps.

agc#

Estimates |x|², compares it with TARGET, and integrates the gain up (DECAY step) or down (ATTACK step).

The target sits 12 dB under full scale because the AGC acts before the matched filter, whose DC gain is about 2.8: at full scale the filter clipped most samples and blinded the carrier loop. A gain pinned at its Q4.12 ceiling (just under 16) means the input is too weak: raise the RX gain.

matched_filter#

A 33-tap complex FIR, root-raised-cosine α = 0.35 at 4 samples per symbol, built at synthesis time.

timing_recovery#

Gardner timing recovery with a fractional (Farrow, piecewise-parabolic) interpolator: two interpolants per symbol, on-time and mid-symbol, driven by a PI loop.

The lock detector also requires a minimum input magnitude, so silence releases the lock.

carrier_recovery#

A second-order Costas loop, one update per symbol, with acquisition aids.

What a profile arms depends on the path:

The blind sweep (bit 3) is left off: it false-locked on frequency aliases in bench tests.

slicer and demodulators#

The slicer takes a hard, Gray-coded decision on the sign of I and Q and counts symbols (rx_demod.slicer.sym_cnt → telemetry rx_symbols). The demodulator matching the profile is enabled; the others are switched off.

deframer#

Searches the sync marker, delimits frames and descrambles.

The marker is correlated against every rotation of the constellation the modulation allows (four for QPSK, two for BPSK and GMSK) with at most one bit of error. The matched rotation is undone on the payload before byte assembly and descrambling, so the bytes come out as sent whichever point the carrier loop settled on.

The deframer applies back-pressure: if nothing consumes its output (no RX block submitted), it parks with a byte pending and the jam propagates back to the transmitter. The reader must be armed before the feed; the API and CLI measurement commands do this for you.

fec_rx#

Viterbi decoder (64 states, hard decision) followed by the Reed-Solomon (255,223) decoder (syndromes, Berlekamp-Massey, Chien search, Forney).

RS corrects up to 16 byte errors per codeword; beyond that it declares the failure rather than returning a wrong word silently, and the codeword is still passed on, errors included. The RS decoder is double-buffered, so a codeword is accepted while the previous one is corrected and the decoder closes its input for only about 20 cycles per codeword.

Reading the chain from the outside#