Block 7 — TX/RX chain workflow

Block 7 — TX/RX chain workflow

This block combines earlier course results into a complete transmit/receive chain: baseband generation, DUC, RF frontend, channel, DDC, filtering, decimation and metric estimation.

Main engineering chain

flowchart LR
    SRC[Symbols / tone / waveform] --> TXBB[TX baseband shaping]
    TXBB --> DUC[DUC: interpolation + mixer]
    DUC --> TXRF[AD9363 TX]
    TXRF --> CH[RF path / loopback / channel]
    CH --> RXRF[Receiver / AD9363 RX / RTL-SDR]
    RXRF --> DDC[DDC: mixer + FIR + decimation]
    DDC --> SYNC[Timing / frequency alignment]
    SYNC --> METRICS[FFT, SNR, EVM, BER]
    METRICS --> REPORT[Engineering report]

Why this block exists

Before Block 7, the student studied separate elements:

• FFT, FIR, mixer and decimation;
• fixed-point conversion;
• RTL/testbench;
• RF frequency/gain planning.

Block 7 shows how these elements become a system. The main output is not a single filter or mixer, but a coherent TX/RX chain with explicit frequencies, formats, latencies and validation checks.

Main design decisions

Decision	Options	What it affects
TX waveform	tone, QPSK, test frame	synchronization complexity
Pulse shaping	none, RRC, FIR	bandwidth and EVM
DUC	mixer only, interpolation + mixer	sample-rate plan
RF loop	cable, attenuator, over-the-air	reproducibility and safety
RX path	RTL-SDR, AD9363 RX, file replay	availability and accuracy
DDC	mixer + FIR + decimator	channel selection
Metrics	FFT/SNR, EVM, BER	signal type

Signal interface map

Every transition between blocks should have an explicit interface:

Stage	Data type	Sample rate	Format	Notes
TX source	complex		float / Q1.15	symbols or waveform
TX FIR	complex		float / Q1.15	pulse shaping or channel filter
DUC output	complex		Q1.15	shifted baseband
RF capture	complex		ci16 / cu8 / cf32	receiver-dependent
DDC output	complex		float / Q1.15	baseband channel
Metrics input	complex / symbols		float	aligned signal

Frequency plan through the chain

RF frequency = TX_LO + TX_baseband_offset
RX observed offset = RF frequency - RX_LO
DDC output offset = RX observed offset + DDC_shift

For a correct chain, the target signal should end near DC after DDC:

DDC_shift ≈ -RX_observed_offset

Loopback levels

Block 7 must reuse the Block 6 safety discipline:

• start with attenuation;
• use manual gain;
• avoid overload;
• record metadata;
• compare loopback and external observation.

Verification ladder

flowchart TB
    SIM[Pure simulation] --> FILE[File replay]
    FILE --> LOOP[Digital loopback]
    LOOP --> RFLOOP[RF cable loopback]
    RFLOOP --> OTA[Controlled over-the-air]

Do not start from RF if the pure simulation chain does not work.

Minimal Block 7 report

A complete report should contain:

1. TX/RX block diagram;
2. sample-rate table;
3. frequency-plan table;
4. data-format table;
5. loopback method;
6. FFT before/after DDC;
7. metric table;
8. limitations and next experiment.