[OpNet] 默认无线设置中的自由空间路损

frost.wrath

   
   
A Test on Free Space Pathloss in OpNet Default Transceiver SettingsIllidan.modeler@gmail.comNorthern Capital, Republic of PandariaAbstract:
In some occasions we need to test a protocol in a radio environment. At
this point we don't want to bother the radio settings. But our protocol
module has to cope with it, for example, placing radio nodes in
different geographical places to test protocol performance. In this
document, I will make an analysis on transmission power andTx-Rx distance for default radio settings. In
this project, the test scenario is made up of a sending node and a
receiving node. The sending node consists of a traffic source module
(simple_source) and a radio transmitter. While the receiving node
consists of a sink module and a radio receiver. The two nodes are in
approximate latitude and longitude (x, y). Major position difference
lies in altitude, shown in the parameter list below. The altitude of
the sending node is set as that of a medium earth orbit satellite.

At
first, I try to figure out how small the transmission power can be, as
long as the packet can be successfully received. So let's make a short
analysis based on the following conditions.Parameters:Transmitter's altitude(m): 10,000,000Receiver's altitude(m): 2Min frequency(MHz): 30Radio pipeline models: default (dra_*)Modulation method: bpskPacket length (bit): 400Simulation duration(sec): 60Traffic inter-arrival interval (s): 5Denotation: Pt  (transmission power)Pr  (received power)Pn (noise power)Gp (processing gain, 9.67 dB by default)c (light speed)f (transmission frequency)d (Tx-Rx distance)By default, the ECC threshold is 0. So any bit of error causes drop of a packet. When bit error rate(ber)
is approaching 0, the packet tends to have no bit error (packet length
also being a factor). For a 400-bit packet to be error free, theber is no higher than 2.5E-3. According to bpsk modulation curse in OpNet library, a ber lower than 2.5E-2 for bpsk requires an S/N (or SNR) higher than 6.2 dB. And note that for longer packet to be error free, it requires higher S/N. (Pr/N)dB + Gp > SNR = 6.2 dB(Pr/N) > 10^((SNR-Gp)/10).The noise, by default, is only the background noise (i.e. thermal and ambient noise). So Pr > N*10^((SNR-Gp)/10)For a free space path loss, Pt/Pr = (4*pi*f*d/c) ^ 2Pt = Pr*(4*pi*f*d/c)^2 > N*10^((SNR-Gp)/10)*(4*pi*f*d/c)^2 = 2.84E-3(W)                                                     (*)Finally I've got the minimum transmission power, 0.00284W, or 2.84 mW. Let's try this in the simulation, by setting the transmitter's 'power' attribute.For the parameters listed above, I ran 10 simulations, with transmission power 1e-3, 2e-3, 3e-3, ..., 10e-3, respectively.Results show that, scenarios with transmission power 3, 4, 5 and 6mW have successful packet receptions, as well as several failure receptions. Receptions are all successful with transmission power higher than 7mW. (According to simulation duration and traffic inter-arrival interval, there'll be 10 transmissions).In case of 2E-2 transmission power, ber is 0.007028, resulting in 1 bit error. For 3e-3 transmission power, ber is 0.001719. This is lower than the  threshold ber
for error free reception, 2.5E-3, mentioned above.  But also note that
default error pipeline model uses an algorithm with random number to
determine bit error probability. That's why a closeber to the threshold has chances to cause bit errors, resulting in drop of packets. Recast equation (*) to dB form and use a unit of dBm for the power.Pt(dB) = 20*log10(f)+10*log10(N)+SNR-Gp-147.56+20*log10(d) Pt(dB) = -135.47 + 20*log10(d)                                 (**)d is power of 10. Thus equation (**) is easy to calculate. Pt(dB) = -135.47+140 = 4.53 (dBm).Pt = 2.84 mW
   
   


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