Demystify TwoRayGround propagation model

In this model, the shadowing fading factor is not considered. Therefore, for a unique distance, the Pr is a determinstic value. Usually, tworayground has two formulas:

• when d is less than the crossing distance: Free Space model is used.
• when d is larger than the crossing distance: the follownig formula is applied:

P_r =   P_tG_tG_rh_t²h_r² / (Ld⁴)
Based on the default settings in ns-2, we can derive a relationship between Pr and distance
Default parameter in ns-2:

• Pt = 0.28183815
• L = 1
• Antenna Gain : 1
• Antanna height 1.5m

Distance	Pr
15m	2.81838e-5
20m	8.91754e-6
22m	6.0908e-6
30m	1.7615e-6
60m	1.1009e-7
120m	6.8808e-9
200m	8.9175e-10
210m	7.3365e-10
220m	6.908e-10
230m	5.0986e-10
240m	4.3005e-10
250m	3.6526e-10
300m	1.7615e-10
350m	9.5081e-11
400m	5.5735e-11
450m	3.4795e-11
500m	2.2829e-11
550m	1.5592e-11
600m	1.1009e-11
650m	7.993e-12
700m	5.9425e-12
750m	4.5094e-12

Cross-distnace Effect

In reality, path loss index is only 2 in near distance. Therefore, for near-distance (<25m) , the above formula does not apply. When distance is less than ~25 meter, the Friss model will be used. Check tworayground.cc for detail