EC-23.6 Synchronisation, and Simulating the Whole Link
You can describe how a receiver recovers symbol timing, carrier frequency and phase from the signal itself, and build a simulation of a complete link that produces a bit error rate curve you would be willing to defend.
Before:S5-3. Digital Signal ProcessingEC-11. Data Communications and Computer Networks
A receiver is handed a voltage and has to decide where each symbol begins, what the carrier frequency really is and where its phase sits, all before any of the previous topics apply. Getting this wrong makes a perfectly designed system produce noise. The second half of the topic assembles everything in the module into one simulation, because a curve of error rate against signal-to-noise ratio is the standard evidence that a design works.
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Symbol timing recovery, and what happens when you sample at the wrong instant
Sampling away from the centre of a symbol collects energy from its neighbours and shrinks the eye. Timing recovery finds the right instant from the signal itself, without a separate clock being sent.
NPTEL: Principles of Signal Estimation for MIMO/OFDM Wireless Communication (IIT Kanpur) · CourseCarrier frequency offset, and what it does to a constellation
A small difference between the two oscillators makes the whole constellation rotate steadily, which destroys any decision after a few symbols. It has to be estimated and corrected before phase correction can begin.
NPTEL: Principles of Modern CDMA, MIMO and OFDM Wireless Communications (IIT Kanpur) · CoursePhase-locked loops and decision-directed tracking
Once the offset is roughly removed, the receiver keeps up with the remaining drift by comparing its decisions against what it received. The loop works well while decisions are mostly right and fails badly when they are not.
NPTEL: Principles of Modern CDMA, MIMO and OFDM Wireless Communications (IIT Kanpur) · CourseFrame synchronisation, and finding the start of anything
Knowing where symbols begin is not the same as knowing where a message begins, so a recognisable pattern is inserted for the receiver to search for. Its correlation properties are what make it findable in noise.
Building the simulation: source, channel, receiver and the error count
A link simulation is a loop over signal-to-noise values, each running enough symbols to count a statistically meaningful number of errors. How many is enough is a question the simulation itself has to answer.
NPTEL: Principles of Signal Estimation for MIMO/OFDM Wireless Communication (IIT Kanpur) · CourseReading an error rate curve, and the mistakes that make one look too good
A curve that beats the theoretical limit is a bug, usually a receiver given information it would not really have. Comparing against the known uncoded curve is the first check on any simulation.
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