I understand there is concern over the limited amount of wireless spectrum available for allocation but am somewhat confused as to why. Couldn't we just divide the electromagnetic spectrum into as many radio frequency bands as we need by making the communication frequency more precise, i.e. broadcasting over 101.713 MHz instead of 101.7 MHz in order to make more room. What are the limiting factors preventing us from doing this?
Electronic – Why can’t we divide the electromagnetic spectrum into arbitrarily many radio frequency bands
cellphonefmradioRFwireless
Related Solutions
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).
Think of the vibratory modes on a string clamped at both ends. When you clamp a length of string at both ends, the allowed vibration patterns all have one thing in common; they go to zero at the ends.
So, the length of the string determines the natural frequencies of vibration; the longer the string, the lower the fundamental frequency.
Very roughly speaking, an antenna acts similarly. For example, the current must be zero at the ends of a center-fed dipole antenna in analogy with the clamped ends of the string.
So, for the antenna to resonate at a lower frequency, it must be longer, just as the string must be.
Related Topic
- Electronic – What does the term “fixed” mean on the radio spectrum
- Electronic – Where on a cellphone does the electromagnetic radiation(radio frequencies) come from? the antennae or the transmitter
- Electronic – If an FM signal has different frequencies, how does the radio receive the signal in a fixed frequency
- Electronic – Why are radio signals clearer near the human body
- Electronic – Consistent Radio wave detection timing
- Electrical – How many watts does a 40 MHz radio frequency signal create
Best Answer
Signals don't exist at a single frequency. They have a bandwidth that spreads across spectrum. The radio station at 101.7MHz takes up the spectrum from 101.6-101.8MHz.