New guy with WiMax question

[stevefigo]

Hi
Even though I'm an electrical engineer, I haven’t looked this stuff since I graduated in 1754... a long time ago.

1 - Is there an optimal frequency for WiMax? A private broadcaster in Italy is looking at using lower frequencies than established by the current WiMax Forum profiles. The range would be in the 600-800Mhz.

2 - Given a 700Mhz frequency, what is the difference in power requirements for the same footprint/distance from a 3.5Ghz standard? As I remember the lower the frequency the more the penetration and the higher the power requirements, but I cant find any references to this.

If anyone could help, and/or send a reference link, Id really appreciate it.
1000 thanks
Steve

 

[VEBill]

You can explore the relationship between frequency, path length and pathloss by starting with the following link:

http://www.google.com/search?q=pathloss+32.45

There are other factors.

High antenna gain is easier at higher frequencies. If the clients tend to be clustered on the surface of the Earth, then the antenna gain at the fixed station can even be omnidirectional (aimed along horizon).

External noise is often higher at lower frequencies (a huge simplification), but noise figure generally gets worse at higher frequencies.

Higher frequencies obviously offer higher bandwidth. Higher bandwidth can be used to provide coding gain.

Early cell-phone trials (AMPS trials in Chicago IIRC)indicated that 400 MHz was best for penetrating buildings, but those bands were already taken. They had to settle for 800 MHz.

In general, you take what you can get.

 

[Higgler]

If you have two antennas (say horn antennas whose gain increases naturally with increasing frequency), when you point them at each other, the coupling loss from one antenna to the other is exactly the same at all frequencies. Hence, only if the antennas used are both the same gain, say low gain zero dBi do you get advantage by lowering frequency. Sounds odd I know, but the math is there. Increasing antenna gain cancels out the "space loss" which is really a spreading of the antenna signal power reduction, called space loss, but no power is actually lost, it's just spread out over a different area.

One consideration; If have your wireless penetrate rebar strengthened concrete in buildings, the cutoff frequency via waveguide cutoff physics depends on your rebar spacing. I believe the 200-400 Mhz range is where rebar cutoff occurs mostly. Tighter rebar spacings for future buildings could up that 8.4 inches is half wave at 700 MHz, but concret is not dielectric of air, it's 5 or 6 at 2 ghz, Maybe higher, but not sure at 0.7 ghz. Most rebar is in the 4-6 inch squares as a guess. If you decide to drop your frequency even more, be aware of that. 700 Mhz may still be a problem, find out the construction rules in Italy on rebar spacing requirements.

UHF TV is still on that frequency range, but I think that 680 Mhz will be the new upper frequency of TV soon in the US. Frequencies 680-880 Mhz are being opened up for new wireless action.

For range difference or power difference, every time you cut your frequency in half, you only need 1/4 of the power to transmit the same distance with identical gain antennas( only applies from an airplane to airplane, i.e. nothing in the way but air), Hence 3.5 Ghz to 875 Mhz is 1/16th the power. 3500 Mhz to 700 Mhz is 1/25th power change if both systems use the same low gain omni antenna and have a clear path. Pretty big improvement in air, absolutely not true in real life though and it's a very tough calculation to find the answer in a point to point urban connection. Antennas on cars would end up being 4-5 inches tall at 700 Mhz (1/4 wave monopole).

Seems like a good idea going lower frequency.
Good luck,

KCH