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Chapter 33

Radar and communications

Two lessons in Part XI, Applications. Read them in order, or start anywhere: a prerequisite is a link, never a gate.

Start with 33.1
0 of 2 readabout 42 minutes

Lessons in this chapter

A long chirp, a short peak: echoes of two targets at 4500 and 4530 m, amplitude 0.5 each, no noise, from a 10 µs, 10 MHz chirp. Raw echo, range 4300 to 6200 m: the two echoes, each 1500 m long, overlap from 4530 to 6000 m and reach 1 where they add. Compressed: two narrow peaks near 4500 and 4530 m, about 0.5 high, with a deep dip between them. Diamonds mark the peaks at 4498.0 m and 4532.0 m, each about 13.3 m wide at −3 dB; the highest side lobes reach −10.5 dB.A long chirp, a short peak: echoes of two targets at 4500 and 4530 m, amplitude 0.5 each, no noise, from a 10 µs, 10 MHz chirp. Raw echo, range 4300 to 6200 m: the two echoes, each 1500 m long, overlap from 4530 to 6000 m and reach 1 where they add. Compressed: two narrow peaks near 4500 and 4530 m, about 0.5 high, with a deep dip between them. Diamonds mark the peaks at 4498.0 m and 4532.0 m, each about 13.3 m wide at −3 dB; the highest side lobes reach −10.5 dB.

Lesson 122 minYou are hereRead

Radar signal processing

Range from an echo's delay, speed from its Doppler turn: compress a chirp with a matched filter, then build a range–Doppler map across pulses.

One panel over time from −1 to 1 symbol around the decision instant at 0, heights from −2.5 to 2.5, with a dotted threshold at 0. The 200 received traces, within ±2.28, cross the threshold at the decision instant: the echoes add to 1.1, and the eye opening is −0.10. The 5800 noisy decision samples are dots at time 0, within ±2.37: 1307 of 5800 on the wrong side.One panel over time from −1 to 1 symbol around the decision instant at 0, heights from −2.5 to 2.5, with a dotted threshold at 0. The 200 received traces, within ±2.28, cross the threshold at the decision instant: the echoes add to 1.1, and the eye opening is −0.10. The 5800 noisy decision samples are dots at time 0, within ±2.37: 1307 of 5800 on the wrong side.

Lesson 220 minYou are hereRead

Channels and equalisation

Echoes in a channel close the eye; a 15-tap equaliser opens it again, by zero-forcing, by the Wiener filter's MMSE taps, or learned with LMS.

After this chapter

Where to go next.

The chapters either side, and the rest of Part XI in the library.

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