Block 8 — modulation and synchronization workflow

Block 8 — modulation and synchronization workflow

This block turns the TX/RX chain from Block 7 into a digital data link: modulation, frequency/phase/timing mismatch, error estimation, correction and BER/EVM validation.

Main engineering chain

flowchart LR
    BITS[Bits / frame] --> MOD[Modulator: BPSK/QPSK]
    MOD --> SHAPE[Pulse shaping]
    SHAPE --> CH[Channel: noise, CFO, phase, timing]
    CH --> CFO[CFO estimation/correction]
    CFO --> TIMING[Timing recovery]
    TIMING --> PHASE[Phase recovery]
    PHASE --> DEC[Decision device]
    DEC --> METRICS[EVM, BER, sync error]
    METRICS --> REPORT[Sync report]

What changes after Block 7

Block 7	Block 8
signal is known and synchronization is almost ideal	real synchronization errors are introduced
DDC places the signal near DC	residual frequency error must be estimated
constellation is plotted after simple correction	constellation becomes a diagnostic tool
BER is computed in a simplified model	BER is linked to synchronization and decisions

Main synchronization errors

Error	What it looks like	Consequence
Carrier frequency offset	rotating constellation	BER increases rapidly
Phase offset	constellation is rotated	wrong symbol decisions
Timing offset	points are smeared	ISI and EVM growth
Sample-rate offset	phase/time slowly drift	tracking loop is needed
IQ imbalance	constellation is stretched/tilted	image and EVM penalty

Minimal debug order

1. Check spectrum and channel centering.
2. Estimate CFO.
3. Correct CFO.
4. Estimate residual phase.
5. Correct phase offset.
6. Select symbol samples.
7. Plot constellation before/after.
8. Compute EVM and BER.

Block metrics

Metric	Meaning
CFO estimate	estimated carrier frequency mismatch
residual CFO	error after correction
EVM	constellation quality after correction
BER	final decision quality
phase error	residual constellation rotation
decision margin	distance margin to decision boundaries

Expected result

After this block, the student should be able to:

• explain why a constellation rotates;
• estimate CFO using a preamble or an M-th power method;
• correct CFO;
• separate frequency offset from phase offset;
• interpret EVM/BER after synchronization;
• prepare a report with before/after correction plots.