Unit
- Spectrum range: Microwave links commonly use frequencies from 6 GHz to 38 GHz, with licensed bands allocated for fixed point-to-point services. The choice of band affects antenna size, rain attenuation, and available channel widths, so operators align band selection with required capacity and hop length.
- Wavelength and frequency: The relationship between wavelength and frequency is inverse, since wavelength in meters equals 299,792,458 divided by frequency in hertz. Higher frequencies produce shorter wavelengths, which require smaller antennas but are more sensitive to physical obstructions and atmospheric absorption.
- Licensed versus unlicensed: Licensed microwave requires coordination with a national regulator to prevent harmful interference, while unlicensed bands such as 24 GHz ISM allow faster deployment but offer no protection from other users. Operators must weigh interference risk before choosing unlicensed frequencies.
- Capacity scaling: Higher bands generally support wider channel bandwidths and higher modulation orders, yielding greater throughput. For example, a 28 MHz channel in the 18 GHz band can carry several hundred megabits per second with high modulation, making it suitable for urban backhaul.
- Rain fade correlation: At frequencies above 10 GHz, raindrops absorb and scatter the signal, creating attenuation that grows rapidly with frequency. Link designers compensate by shortening hop lengths, adding fade margin, or using adaptive modulation to maintain availability.
- Equipment standardization: Radio vendors offer indoor and outdoor unit configurations that follow industry-standard interfaces such as Gigabit Ethernet. This allows operators to mix equipment from different suppliers when the radio frequency unit and modem comply with open standards.
- Operational considerations: Band selection also affects spares inventory, training needs, and test equipment availability. A network with multiple frequency bands requires more calibration tools and more careful documentation than a single-band network.
- Regulatory framework: National spectrum authorities publish frequency allocation tables and license conditions that define permitted emissions, antenna mask limits, and coordination obligations. Operations staff must read these documents before turning up a new link.
- Fresnel zone clearance: Path planning must account for the first Fresnel zone, an elliptical region around the line of sight that carries most of the radio energy. Obstructions that penetrate this zone cause additional attenuation, so operators calculate required clearance for the specific frequency and verify it during site surveys.
- Adaptive modulation: Many modern microwave radios automatically adjust modulation order between QPSK and 4096 QAM based on real-time link conditions. During heavy rain, the link drops to lower modulation to maintain connectivity, and when clear weather returns, it raises throughput again. Operators configure thresholds to balance availability and capacity.
- Path availability targets: Fixed microwave links are designed against annual availability objectives, commonly 99.99 percent for carrier-class service. This objective determines how much fade margin is required and how hop distances are constrained. Operators record these targets in link budgets and use them when evaluating proposed routes.
System Architecture
Pattern: Point-to-Point Microwave Link
Components:
- Transmitter: upconverts baseband data to RF and feeds the antenna
- Receiver: downconverts RF to baseband with low noise figure
- Antenna: focuses the RF beam for directional transmission
- Modem: applies modulation and coding for the digital payload
- Protection switch: selects active path in hot standby configurations
Code Example
Python
frequency_ghz = 18.0
speed_of_light = 299792458
wavelength_m = speed_of_light / (frequency_ghz * 1e9)
print('Wavelength for {} GHz is {:.2f} m'.format(frequency_ghz, wavelength_m))This script helps operations staff quickly convert between frequency and wavelength, which is useful when choosing antennas and measuring cable lengths.
Quick Revision
- The microwave spectrum spans 6 GHz to 38 GHz for fixed links, with higher bands offering more capacity but higher rain fade.
- Licensed bands provide interference protection, while unlicensed bands enable rapid deployment.
- Wavelength and frequency are inversely related, and equipment choices must match the band plan.
- Operators must understand national spectrum rules before link activation.