Electronic – Why can’t the orthogonal PN codes help in the near-far effect

Freescale AN2253 "Channel Estimation for a WCDMA Rake Receiver" by Ahsan Aziz 2004 has a pretty good explanation -- see Figure 1.

The signal from from an antenna is split into several fingers.

Each finger takes that input signal and produces one output signal by:

• (a) delays the signal a certain amount of time (often measured in "chips"), different for every finger, compensating for the physical length of that particular path of the multipath between the transmitter and reciever
• (b) despreads the signal with a fixed despreading code (the same for every finger)
• (c) multiplies the signal by a "complex attenuation" value, different for every finger -- compensating for some kinds of reflections that turn the signal "upside down" relative to the direct path, and emphasizing paths with better signal/noise ratio over other paths with worse signal/noise ratio.

Then the receiver combines the output from all outputs from all the fingers from all the antennas, hopefully producing a combined signal with a better signal/noise ratio than it would get without all this rake complexity.

The part of the radio that independently tunes the delay for each path in part (a) is the Path Searcher. The part of the radio that independently tunes the complex attenuation for each path in part (c) is the Channel Estimator.

People familiar with AM and FM radio may think that part (c) is the only one needed -- in those kinds of radios, time delays have approximately the same effect as phase changes. But with spread spectrum technology, time delays act very different from phase changes. A phase change of +20 degrees can be compensated by either an additional phase change of -20 degrees or a phase change of +340 degrees or a phase change of +700 degrees -- all three methods of compensation are identical. A spread-spectrum signal with a relative time delay of -21.5 chips must be compensated by a time delay of close to +21.5 chips; anything below +20.5 chips and anything above +22.5 chips is useless.

The part of the radio that "can be used to find the multipaths" is the Path Searcher. I suppose you might have one integrated component that simultaneously tunes the time delays in (a) and the complex attenuations in (c); that component includes the Path Searcher and the channel estimator. (This happens often with highly-integrated electronics -- you have a bunch of conceptually distinct blocks on the overview block diagram, but often there is some single physical component that is shared between two blocks).

Tesla coils generally use near-field energy transfer. But ...

The page cited is sloppy in its wording and it would better to say "near field" where it says "RF". But, (again) ...

The answer is necessarily not black and white.
"Near field" has a precise meaning BUT obtaining ONLY near field coupling in a given case is not certain.

There is a gradual transition from NF to FF and a boundary region where both can occur. Near Field essentially occurs where certain geometry dependant terms form a major or significant part in describing interactions between transmit and receive structures - these relate to cyclical transfer of energy between the antenna I and V and adjacent electrostatic and magnetic fields. As distance increases these terms become less significant until they can be ignored.

You'll always get "some of both" with "some" varying as you move away from the antenna.

(1) At distances of well under a wavelength there is substantial interaction between the electric and magnetic fields produced by the aerial and current and voltage in the antenna. Energy is transferred to and fro between fields and aerial throughout with losses caused by non idealities but no energy loss due to energy "leaving" the aerial structure. This close in zone is termed the "reactive zone" where power may be absorbed by a tuned load which has voltage and current induced in it and which then dissipates energy (ie has a resistive component). Coupling involving power transfer is magnetic.

(3) [note number] "RF communications or energy transfer occur at distances beyond several wavelengths form the"antenna" structure. Here the ratio if electric and magnetic coupling have "settled down" and any energy present is not coupled to the structure I & V so is "lost", whether "received" or not. One way of viewing this one is that the two aerials are geometrically distance and secondary terms which account for the filed coupling and which have a strong distance dependent component have become insignificant - the field has become essentially homogeneous over lengths of the order of the receiving antenna.

(2) At distances past about half a wavelength the "second order" terms on which pure NFC coupling depends start to get small and the field starts to become homogeneous. This is termed the "Fresnel zone" (the guy has his name all over) and there is a degree of non ideality in field coupling to the antenna.

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This wikipedia page on near and far field does a better than usual job of commenting.

Their summary section says, in part:

• The near-field is remarkable for reproducing classical electromagnetic induction and electric charge effects on the EM field, which effects "die-out" with increasing distance from the antenna (with magnetic field strength proportional to the inverse-cube of the distance and electric field strength proportional to inverse-square of distance), far more rapidly than do the classical radiated EM far-field (E and B fields proportional simply to inverse-distance). Typically near-field effects are not important farther away than a few wavelengths of the antenna. Far near-field effects also involve energy transfer effects which couple directly to receivers near the antenna, affecting the power output of the transmitter if they do couple, but not otherwise. In a sense, the near-field offers energy which is available to a receiver only if the energy is tapped, and this is sensed by the transmitter by means of answering electromagnetic near-fields emanating from the receiver. Again, this is the same principle that applies in induction coupled devices, such as a transformer which draws more power at the primary circuit, if power is drawn from the secondary circuit. This is different with the far-field, which constantly draws the same energy from the transmitter, whether it is immediately received, or not.