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0 0.5 1 1.5 2 2.5 3
12:00am 1:00am 2:00am 3:00am 4:00am 5:00am 6:00am 7:00am 8:00am 9:00am 10:00am 11:00am 12:00pm 1:00pm 2:00pm 3:00pm 4:00pm 5:00pm 6:00pm 7:00pm 8:00pm 9:00pm 10:00pm 11:00pm
Time
Clear-air attenuation (dB)
Fig. 4- 6: Clear-air attenuation over 24 hours, 15th March, 2004
A Semi-Empirical Formulation for Determination of Rain Attenuation on Terrestrial Radio Links
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-46 -45 -44 -43 -42 -41 -40
Febr uary
Marc h
April May
June Aug
ust Sept
embe r
October Nove
mber Dec
embe r
Calender Months
Non-rain faded signal level (dBm)
Mean recieved signal level Median received signal level (value exceeded 50% of the time
Fig. 4- 7a: Monthly mean and median received signal level in clear-air for 10 months in 2004
0.68 0.76
1.86
2.55
3.61
3.87
2.46 2.46
1.41
2.17
0 1 2 3 4 5
Februa ry
Marc h
April
May
June
August
Septembe r
Oct obe
r
Novembe r
Decem ber
Calender month
Average clear-air attenuation (dB)
Fig. 4-7b: Mean excess attenuation (dB) above free space loss on clear-air days for 10 months in 2004
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From Fig. 4-7b, the average clear-air attenuation fluctuates between 0.68 dB to 3.87 dB for the entire measurement period. With the month of August recording the highest excess attenuation (dB) above free space loss on clear air days and February with the lowest. With these values, the actual rain attenuation values in each of the month can be determined. From Fig. 4-7a, the non- rain faded average for February and March is – 41.68 dBm and – 41.76 dBm respectively while that of April is – 42.86 dBm which tends to be lower than that of the other two months and relatively lower than the expected received signal level. For the months of May, June, and August which is normally referred to as the winter months in South Africa [13]; [South Africa year book, 2006] a very low non-rain faded average values of – 43.55 dBm, – 44.61 dBm, and – 44.87 dBm are recorded respectively.
It should be noted at this point that the noise floor of the receiver has been calculated to be between –80.5 to –80.2 dBm, but in the measurements, this noise value varied from –79.5 dBm to –82 dBm (as stated in section 4.2). This has been adequately catered for during the sorting and the processing of the non-rain-faded received signal level in each of the month.
Several factors contributed to this clear air attenuation. Taking into considerations some factors discussed in the second chapter of this thesis, and Durban being a location that has a rugged and hilly terrain, the excess free space loss recorded especially in the winter months where there no significant rains might have arise due to k-factor fading. The value of the k used in the design of the terrestrial link is 1.33, while the median value of k-factor for Durban is 1.21, with a value of k
= 0.5, exceeded 99.9% of the time [Odedina and Afullo, 2005; 2006]. The “worst” month for k- factor fading is February (with value of k0.2 exceeded 99.9% of the month); while the month of August has k-factor value exceeded 99.9 % of the time of 0.9. Note also that the median value of k for August is 1.27 as opposed to the 1.21 for February [Odedina and Afullo, 2006]. Thus, this type of fading may contribute 1-1.5 dB over the path (see clearance of the first Fresnel zone, Fig.
4-4), and results in Odedina and Afullo, [2005], and Odedina and Afullo, [2006]).
Water vapour effect is another contributor to the measured non-rain faded signal levels. The highest contribution to the water vapour effects occur in the summer months with an average pressure of about 25.76 mb (see Table 4-2). This gave an average attenuation of about 0.34 dB/km, or 2.2 dB over the 6.7 km path. On the other hand in winter, the average water vapour pressure is about 12.37 mb (see Table 4-2), resulting in attenuation of about 0.13 dB/km, and 0.9 dB over the 6.7 km path [Ajayi et al, 1996]. Thus, water vapour attenuation contributes about 1
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dB in winter and 2.2 dB in summer. There are some intervening rivers along the propagation path (see Fig 4-2 and 4-3) which may cause some little reflections of the radio signals along the path, thus, the multipath fading may be limited to below 1 dB.
Also, due to the coastal nature of Durban, as well as the industries, fog attenuation is also a contributor. From the climatic characteristics of South Africa discussed in Chapter two of this thesis, the east coast of South Africa are said to have abundant low stratus cloud and fog [South Africa year book, 2006]. Durban, lying along the eastern coastal side of South Africa has the tendency of being influenced by this fog and low stratus cloud which in turn can affect any signal level even when there are no rains. At the operating frequency of the link of 19.5 GHz, an average attenuation of 0.1 dB/km is expected, resulting in a value of 0.7 dB along the propagation path (see Fig. 2-4) [Hall et al., 1996]. This thus accounts for the attenuation during non-rainy days.
Table 4- 2: Average water vapour recorded for Durban in 2004 at 19.5 GHz