OE-7.4 Diversity Techniques & OFDM
You can compare selection, feedback, maximal ratio and equal gain combining, and explain OFDM's principle, cyclic prefix and fading mitigation.
Diversity fights fading with redundancy: send or receive the same information over paths unlikely to fade together, and combine them. Maximal ratio combining is optimal because it weights each branch by its own quality, which is worth being able to justify rather than just state. OFDM's elegance is that it converts one hard wideband problem into many easy narrowband ones, and the cyclic prefix is the trick that makes the channel look circular so equalization becomes one division per subcarrier.
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Practical space diversity considerations
The general answer to fading: send or receive over paths unlikely to fade together. The practical considerations are about how far apart the antennas must actually be.
Selection diversity; feedback or scanning diversity
The two simplest combining rules: take the best, or take the first acceptable one. Cheap, and they leave performance on the table, which the next item recovers.
Maximal ratio combining; equal gain combining
Weighting the copies before adding them, optimally or equally. The optimal rule is the best you can do and the equal one is nearly as good for much less effort.
Principle of OFDM
Splitting a wide channel into many narrow ones so each sees a simpler channel. It is the central idea of modern wireless and worth understanding properly rather than as a block diagram.
OFDM transceivers and the cyclic prefix
How the scheme is actually built, and the guard interval that makes it work. The cyclic prefix looks wasteful and is what turns a hard equalisation problem into a trivial one.
Spectrum of OFDM
What the transmitted signal looks like in frequency, and why the subcarriers can overlap without interfering. The orthogonality condition is the whole reason the scheme is efficient.
Fading mitigation in OFDM
How the scheme copes with the fading from the previous topic. Coding across subcarriers is what actually delivers the robustness, not the modulation alone.
Intercarrier interference
What goes wrong when the orthogonality above is broken, usually by frequency offset or motion. It is the scheme's main weakness and the reason synchronisation matters so much.
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