Microwave path availability at 19 and 23 GHz.
CCIR Rain Model
The GTE Lenkurt  model has been successfully used to establish the maximum available path (MAP) for links operating at frequencies below about 10 GHz. At higher frequencies this model yields overly optimistic results which can lead to system designs that will fail under heavy rain conditions. Several models for predicting path availability at the higher frequencies have been developed. The CCIR  model appears to be more useful. This model has developed the statistical rate rain rates around the world and presents a useful mathematical approach for the calculation of the effects of rain over a wide frequency range.
All of the models require the following inputs to determine the maximum available pathlength (MAP):
* antenna gains at the operating frequency
* microwave link system gain
* required system up-time or the allowable down-time
* predicted rain rates for the applicable local area.
The antenna gain for a parabolic reflective antenna is equal to:
G = 20 log 2.2 x d/l
d = reflector diameter in cm,
l = wavelength in cm.
The system gain is specified by the link manufacturer for the proposed system capable of providing the end-user's performance requirements. This is the absolute sum of the power output of the transmitter and the threshold level at the receiver expressed in dBm. The threshold level for digital systems, whether it be for voice or data, is usually determined by the bit-error-rate (BER). Most manufacturers specify [10.sup.-6]. Analog systems specify acceptable signal-to-noise ratios for the intended application.
The predicted rain rates must be obtained from historical data for the applicable area of the link installation. The percentage of time that the rain rates will not exceed the given rain rates determines the link up-time. Table I shows the CCIR rates for the rain zones assigned to the U.S.A.
Figure 1 shows the location of these zones. Figure 2 shows the zones for western Europe.
Maximum Available Path
The maximum available path for the required availability, selected link hardware and the location of the installed link is determined by the condition in which the sum of the system gain and the two antenna gains are equal to the sum of the free space loss (FSL), the path attenuation due to rain and the path losses due to water vapor and atmospheric gases.
The free space loss is equal to:
FSL = 20 log 4 x [pi] x D/I
D = path length in cm, and I = wavelength in cm.
The path loss due to rain is equal to
A = k x [rr.sup.e x P x L]
k = constant rr = rain rate in mm/hr. e = exponent of rr P = storm profile factor L = path length in km P = 90/(90 + C x L)
where C = constant (see Table 1).
The total path attenuation is the sum of FSL, rain attenuation and the vapor losses. The MAP length is then calculated from, Ant. gain + system gain = FSL + rain loss + vapor loss.
Note that the maximum available path length L is a variable in all three of the path parameters. The solution of the MAP formula is best solved by a computer or by the repeated calculation of the losses until they equal the sum of the system and antenna gains.
Table II shows the values of k and e for various microwave frequencies and Figure 3 shows the path losses due to water vapor and atmospheric oxygen.
To obtain the values of k and e at precise frequencies, a plot of Table II can be constructed.
A sample calculation will demonstrate the use of the above data. The assumptions are: system gain = 100 dB with a BER = [10.sup.-6]; P.O. = + 17 dBm and a sensitivity threshold of -83 dBm; data rate = T1; antenna gains = 2 x 40 dB at 23 GHz; rain zone K and a required availability of 99.99 percent up-time value of k = 0.108, e = 1.075, C = 4 and 0.29 dB/mi vapor loss.
The calculated MAP is 5.7 miles. This would be the maximum recommended path length for a microwave link whose path gain is 180 dB, with a required up-time of 99.99 percent of the time and located in rain zone K. This represents a down-time of 53 minutes per year. If the required path length is less than 5.7 miles, the proposed system will have superior performance as long as the rain rate is less than 42 mm/hr. The MAP for a clear day without rain would be over 50 miles.
The fade margin for the CCIR model would be the dB value of the rain loss which was 180 dB - FSL - vapor losses. The FSL for 5.7 miles is 143 dB and the vapor loss is 2 dB for a fade margin of 35 dB. If this fade margin is used for the GTE Lenkurt model, the MAP calculates to 28 miles. This demonstrates the optimistic performance results using this older model which is clearly not useful at 23 GHz.
Figure 4 shows the calculated maximum available path length for the other rain zones in the USA for the assumed link parameters.
Older rain rate/availability models should not be used for the 19 and 23 GHz microwave links. The maximum available path must be calculated to determine if the proposed microwave installation will meet the end-user's requirements. The MTBF's available for today's hardware should not be used to predict the system down-time. The downtime due to heavy rain may be greater.
This CCIR model has been used as a guide for several years and field results confirm its usefulness. The calculated confirmation that the hardware will perform as specified must be followed by proper link installation and path design. Only if path clearances are obtained will the conclusions drawn from the calculation of the MAP apply.
[1.] Robert F. White, Engineering Considerations for Microwave Communication Systems, GTE Lenkurt Inc., San Carlos, CA.
[2.] Recommendations and Reports of the International Radio Consultative Committee (CCIR), 1982, Volume V, Propagation in Non-ionized Media, Geneva, 1982.
|Printer friendly Cite/link Email Feedback|
|Date:||Aug 1, 1989|
|Previous Article:||MMIC Oscillator design techniques.|
|Next Article:||Leadless monolithic devices are drop-in replacements for beam lead devices.|