difference between sat and cellular communication

[bvishwanath]

1what is the difference between satellite and cellular communication?
2Why non sinusoidal signals are not used as carrier in modulation?
3Why the area of coverage in FM radio is less when compared with AM radio?

 

[enjunear]

1what is the difference between satellite and cellular communication?

Many things are different between satcom and cellular.
1. The power level required (you need more energy to go many miles into space, compared with a few miles between you and the nearest tower)
2. The modulation used (you need a more robust modulation and error correction scheme to counteract the distortion seen by passing through the upper layers of the atmosphere).
3. Polarization of the signals, most cellular antennas are linearly polarized whereas satcom signals are circularly polarized. As signals propagate through the ionosphere, they receive an extra "twist", which would take a vertically polarized signal and rotate it to some unknown angle, which would immediately result in lower received signal at the receiver, by up to 3 dB (1/2 the power).

Those are some of the bigger points.

2Why non sinusoidal signals are not used as carrier in modulation?

Because a non-sinusoid carries harmonic content. These harmonics can easily end up on the same channel as a weak signal you are tying to receive. By having this very large interfering signal coming out of your transmitter (an undesired harmonic), the incoming received signal in that same channel will be drowned out and you'll never hear it. This issue becomes paramount in a co-site installation with multiple antennas. Think of a fighter jet... they have one or two radios for voice communication, one for SATCOM, one for a high speed datalink, and maybe a couple for radio navigation (TACAN, VOR/ILS, etc). They all have to play nice with each other and not splatter more noise into the adjacent frequencies than absolutely necessary. Reducing your transmitted harmonic levels is a big requirement in a system like that. Now think about a military ship like an aircraft carrier... think about all of those antennas on the mast, and some down on the deck.... your "playing nicely together" problem just increased about ten-fold. That's why you don't use anything other than a sinusoid for a carrier.

3Why the area of coverage in FM radio is less when compared with AM radio?

Broadcast AM operates at a much lower frequency than broadcast FM stations. AM runs around 500 to 1400 kHz, whereas FM operates from around 88-108 MHz. The higher the frequency, the worse the attenuation vs. distance becomes in air.

The first-order path loss equation is:
\[L=20*{log}_{10}(\frac{4*\pi*d}{\lambda})\]
where L = attenuation in dB, d = distance in meters, and lambda = wavelength in meters.

As frequency increases, lambda decreases. As the denominator decreases, the value inside the parenthesis increases, so loss increases. So, in the end, loss increases as frequency increases.