5G NR Transmitter and Receiver Chain: Following the Bits From MAC to Antenna and Back

The mental model: an assembly line and its reverse

The transmit chain is an assembly line. It receives a bag of bits from the MAC (a transport block) and, station by station, wraps it in protection, spreads it over antennas, and turns it into a time-domain waveform. The receive chain is the same line run backwards: every station is undone in reverse order. If you remember the order on one side, you know it on the other.

Two questions to ask at every station: what problem does this stage solve, and why here and not earlier or later.

The transmit chain (downlink PDSCH as the example)

Transport block from MAC
 │
 1  CRC attachment           "Did the whole block arrive intact?"
 2  Base-graph selection      Pick LDPC BG1 (big/high-rate) or BG2 (small/low-rate)
 3  Code-block segmentation   Cut into ≤ 8448 / 3840-bit pieces, each with its own CRC
 4  LDPC encoding             Add parity bits
 5  Rate matching + RV        Trim/repeat to fit the allocated REs; choose redundancy version
 6  Bit interleaving          Spread bits so a burst of errors hits many code bits lightly
 7  Code-block concatenation  Reassemble into one codeword
 8  Scrambling                XOR with a UE/cell-specific sequence
 9  Modulation mapping        Bits → QPSK … 256QAM symbols
10  Layer mapping             Split the codeword(s) across 1–8 spatial layers
11  [Transform precoding]     Uplink DFT-s-OFDM only
12  Precoding / port mapping  Layers → antenna ports (implementation-specific in DL)
13  VRB → PRB → RE mapping    Place symbols on the grid, skipping DM-RS, CSI-RS, reserved REs
14  OFDM: IFFT + CP           Frequency-domain grid → time-domain samples
15  RF                        DAC, up-conversion, analog beamforming, PA, antenna

Stages 1–7 are channel coding (TS 38.212, the "upper PHY"). Stages 8–14 are modulation and mapping (TS 38.211, the "lower PHY"). That boundary is also where O-RAN's 7-2x split puts the fronthaul.

Why each stage sits where it does

• CRC before coding: the receiver must be able to check the whole block after decoding, so the CRC has to be inside the code.
• Segmentation before LDPC: LDPC decoders work on fixed-size blocks; giant transport blocks (up to ~1.2 Mbit) are cut into code blocks, each with a 24-bit CRC so the receiver can ask for retransmission of only the failed pieces (code-block groups).
• Rate matching after coding: the code produces a fixed mother-code output; rate matching adapts it to however many REs the scheduler allocated. The redundancy version chooses which part of the circular buffer to send, enabling incremental redundancy across HARQ retransmissions.
• Scrambling before modulation: scrambling is a bit operation. Done here it randomises the bit pattern so neighbouring cells' interference looks like noise rather than structured signal, and so that a UE cannot accidentally decode another UE's codeword.
• Layer mapping before precoding: layers are the logical spatial streams; precoding maps them onto antenna ports (and hence beams). Splitting first, then mixing, keeps the MIMO processing clean.
• Precoding is not standardised in the downlink: the gNB may use any precoder because the DM-RS goes through the same one — the UE estimates the effective channel. The spec only fixes the uplink codebooks (so the gNB can tell the UE which precoder to use).
• IFFT last: everything before it lives on the frequency-domain grid; the IFFT is the single point where the grid becomes a waveform. The CP is appended to each output symbol.

The receive chain (the same line backwards)

Antenna → LNA, down-conversion, ADC
 │
 1  Timing/frequency sync     Using SSB / TRS (and QCL info); align the FFT window
 2  CP removal + FFT          Time-domain samples → frequency-domain grid
 3  RE de-mapping             Pull out data REs, DM-RS REs, PT-RS REs of this allocation
 4  Channel estimation        From DM-RS (per port, per PRG); interpolate across REs
 5  Phase-noise correction    From PT-RS (FR2)
 6  Equalisation / MIMO detection   Undo the effective channel (MMSE, etc.), separate layers
 7  [Inverse transform precoding]   Uplink DFT-s-OFDM only
 8  Demodulation              Symbols → soft bits (LLRs)
 9  Descrambling              Flip LLR signs per the scrambling sequence
10  Rate de-matching          Place LLRs into the circular buffer; combine with earlier HARQ copies
11  LDPC decoding             Iterative belief propagation
12  Code-block CRC check      Report failed CBs / CBGs
13  Concatenation + TB CRC    Deliver to MAC; generate HARQ ACK/NACK

Two stages have no transmit-side twin — synchronisation and channel estimation — because they exist only to undo what the channel did, not what the transmitter did. They are also where most of the receiver's complexity and most of the performance differences between vendors live.

Where the reference signals plug in

Uplink differences

The PUSCH chain is the same with three changes: transform precoding may be inserted (stage 11), precoding uses a codebook the gNB signals in DCI 0_1 (or non-codebook based on SRS), and uplink control information (UCI) may be multiplexed into the PUSCH before modulation. The gNB receiver is the same list, generally with more antennas and more sophisticated MIMO detection.

A numbers example

A 30 kHz PDSCH, 50 PRBs, 12 symbols (one of them DM-RS), 2 layers, 64QAM, R ≈ 0.6. Data REs per layer = 50 × 12 × 11 = 6 600, so 13 200 REs over 2 layers, × 6 bits = 79 200 coded bits. At R ≈ 0.6 that carries a TBS of about 47.5 kbit, which is cut into 6 code blocks (BG1, ≤ 8448 bits each), LDPC-encoded to roughly 3× their size, rate-matched down to the 79 200 bits available, scrambled, mapped to 13 200 64QAM symbols on 2 layers, placed on 6 600 REs per layer, then IFFT'd with a 4096-point transform (of which 600 bins carry this UE's data) 12 times, each output getting a 2.34 µs CP. The UE reverses all of it in well under a slot.

Common confusions

Spec pointers

Recap

Interview questions

The questions below are drawn from this topic and phrased the way they tend to come up. Try each one out loud before revealing the answer.