Joined: 14 Mar 2009
|Posted: Mon Mar 28, 2011 11:03 am Post subject: Friis Transmission Formula in the Media
|Here is an article, by The Financial Times. It discusses the "US spectrum crunch", or lack of bandwidth available for mobile communications (ipads, smartphones, etc).
Interestingly, there is a statement in the article:
"Mr Ahuja says the airwaves he plans to use are more attractive because they are at a lower frequency where wireless signals travel further. This means it should cost LightSquared less than Clearwire to build a network."
Now, recall the Friis Transmission Formula, which does state that pathloss increases with higher frequency (that is, the transmitted power dies off quicker at higher frequencies):
Now, in regards to Mr Ahuja's statement, it is true that the pathloss is less at LTE (700 MHz band) as opposed to WiMax (2 GHz and up). However,
the difference in path loss between 700 MHz and 2100 MHz (3 times higher in frequency) is 3^2 = 9 = 9.5 dB. This does sound like a large gain.
The statement is true, but note that cell phones transmit less than a watt of power (1/60th of what comes out of a small crappy light bulb). Hence, to achieve the same "expected receive power at a mobile phone", the power from a cell station simply be increased from 1 W to 9W, which is insignificant.
However, the mobile phone power is regulated (primarily by SAR), so the uplink (data from mobile device to base station) will be improved for a given distance at lower frequency.
One extra advantage of lower frequencies is that low frequency energy "spreads out" more than high frequency energy. That is, light beams (very high frequency) tend to travel in a straight lines, whereas the lower the frequency the more uniform or spread out in space the signal will be.