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<浼婅惤涓?gt; illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren
Of the Net, by the Net, for the Net
Free space path-loss is a major factor when considering the attenuation of EM signal strength. Basically it's defined by the following equation:
Pt/Pr = (4*Pi*d*f/c)^2 |
Pt: transmitted power
Pr: received power
d: the distance between transmitter and receiver
f: the signal frequency
c: light speed
Recast the equation in dB form, we get
FSPL (dB) = -147.56 + 20*log10(d) + 20*log10(f) |
Take the FSPL of a IEEE 802.11 WLAN signal for example. The relation between FSPL and distance is depicted below:
where the transmission power is about 10 mW, without considering antenna gain and other attenuation.
In ns2, path loss, which is an effect of the physical layer, is handled by class WirelessPhy (mac/wirelss-phy.{h,cc}). WirelessPhy calculates the path loss in its method sendUp(), by calling FreeSpace::Pr(PacketStamp *t, PacketStamp *r, WirelessPhy *ifp). FreeSpace and its parent Propagation are defined in mobile/propagation.{h, cc}.
The code snipit carrying out the call to FreeSpace's method is given below (wireless-phy.cc):
if(propagation_) {
|
The real meat of the path loss calculation is Propagation's method Friis(), which not only deals with free-space path loss but also considers antenna gains. (Note: By my obervation, antenna gain is actually 0 dB by default, revealed in Antenna::getTxGain().)
For a description of path loss and link budget analysis, turn to sub-section 5.2 of [Sklar 2001].
References
1. [Book] [Sklar 2001]
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<浼婅惤涓?gt; illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren
Of the Net, by the Net, for the Net
What
Fading is the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media.
Propagation models that characterize the rapid fluctions of the received signal strength over very short travel distances (a few wavelengths) or short time duration (on the order of seconds) are called small-scale or fading models. (Page 106 of [3])
Why
In wireless systems, fading is due to multipath propagation and is sometimes referred to as multipath induced fading. [1]
How
The effects of fading can be combated by using diversity to transmit the signal over multiple channels that experience independent fading and coherently combining them at the receiver.
Miscellaneous
The coherence time is a measure of the minimum time required for the magnitude change of the channel to become decorrelated from its previous value.
There's a good summary of large-scale and small-scall fadings on Page 40 and 41 of [5].
References
- Web http://en.wikipedia.org/wiki/Fading
- Web http://www.ylesstech.com/terminology.php?letter=all&id=15
- [Rappaport 2001]
- [Goldsmith 2005]
- [Tse 2005]
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| Name | Time domain | Frequency domain | ||
|---|---|---|---|---|
| Domain property | Function property | Domain property | Function property | |
| (Continuous) Fourier transform | Continuous | Aperiodic | Continuous | Aperiodic |
| Discrete-time Fourier transform | Discrete | Aperiodic | Continuous | Periodic (ƒs) |
| Fourier series | Continuous | Periodic (τ) | Discrete | Aperiodic |
| Discrete Fourier transform | Discrete | Periodic (N)[1] | Discrete | Periodic (N) |
http://en.wikipedia.org/wiki/Fourier_analysis
As on 09-03-30]]>