Frequency Planning and Frequency Assignment

9.2.6.1 Introduction. To derive optimum performance from an LOS microwave system, the design engineer must set out a frequency-usage plan that may or may not have to be approved by the national regulatory organization. The problem has many aspects. First, the useful RF spectrum is limited from above dc (0 Hz) to about 150 GHz. The upper limit is technology-restricted. To some extent it is also propagation-restricted. The frequency ranges of interest for this discussion cover the bands listed in Table 9.3. The frequencies above 10 GHz could also be called rainfall-restricted, because at about 10 GHz is where excess attenuation due to rainfall can become an important design factor.

Then there is the problem of frequency congestion. Around urban and built-up areas, frequency assignments below 10 GHz are difficult to obtain from national regulatory authorities. If we plan properly for excess rainfall attenuation, nearly equal performance is available at those higher frequencies.

9.2.6.2 Radio-Frequency Interference (RFI). There are three facets to RFI in this context: (1) own microwave can interfere with other LOS microwave and satellite communication earth stations nearby, (2) nearby LOS microwave and satellite communication facilities can interfere with own microwave, and (3) own microwave can interfere with itself. To avoid self-interference (3), it is advisable to use frequency plans of CCIR (ITU-R organization) as set forth in the RF Series (Fixed Service). Advantage is taken of proper frequency separation, transmit and receive, and polarization isolation. CCIR also provides methods for interference analysis (coordination contour), also in the RF series. Another alternative is specialist companies, which provide a service of electromagnetic compatibility analysis.

Table 9.3 LOS Microwave Frequency Bands

2110-2130 MHz 3700-4200 MHz 5925-6425 MHz 6525-6875 MHz 10,700-11,700 MHz 17,700-18,820 MHz

18,920-19,160 MHz 19,260-19,700 MHz 21,200-23,600 MHz 27,500-29,500 MHz 31,000-31,300 MHz 38,600-40,000 MHz

The IEEE defines electromagnetic compatibility (EMC) as "The requirements for electromagnetic emission and susceptibility dictated by the physical environment and regulatory governing bodies in whose jurisdiction a piece of equipment is operated."

We'll call electromagnetic emission (EMI) RFI. It just means the level of RF interference caused by a certain piece of equipment such as a microwave terminal. Susceptibility deals with how well a piece of equipment can operate in an RFI environment. EMC can be a real headache for a microwave engineer.

0 0

Post a comment