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Technical Note

Microwave Radio System Gain and Fade Margin

Overview

Fade margin calculations are necessary to establish the expected performance of aproperly designed microwave link. A path with higher fade margin will accommodate pathfading and interference better than a path with lower fade margin. Higher system gain, thedifference between the maximum transmit power and the receiver threshold at 10

-6BER,

can better accommodate higher fade margins.

This technical note looks at what constitutes the fade margin of a digital microwave radiolink and introduces the primary fade margin path planning variables, and their designconstraints. When considering fade margin, system gain is the primary performanceindicator of a microwave radio, and not Tx power or Rx gain in isolation.

Fade Margin Definit ion

The fade margin of a digital microwave radio link is the amount by which a received signallevel may be reduced without causing system performance to fall below a thresholdvalue, which is typically specified at a BER of 10

-6.

From a path planning perspective it represents the design allowance that provides forsufficient system gain to accommodate expected fading, for the purposes of ensuring thatthe required quality of service is maintained.

Fading events are most commonly caused by multipath fading and precipitation.

Fade Margin Calculation

Figure 1 shows how path gains and losses, the link budget, relate in the calculation of alink fade margin, which begins with the transmitter and moves across to the receiver. It

shows a split-mount installation with the ODU direct-mounted to its antenna.The transmitter has been set for a Tx power of 20 dBm and assumes:

The antennas have a gain of 40 dB

Total path loss is 160 dB

The Rx 10-6

threshold is -80 dBm

The resulting fade margin is 20 dB, that is, the link can accommodate a reduction (fade)of 20 dB on the received signal level (RSL) before the onset of a 10

-6BER.

Figure 1. Link Gains and Losses

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Fade Margin Variables

For a given path the total path loss is fixed, meaning the variables available to an

operator to change the fade margin are:Tx power

Typically Tx power can be software-set over a range of 15 to 20 dB, to a maximum thatdepends on the frequency band and the capacity/modulation selected.

On the lower 6 to 8 GHz bands, typical industry maximums are 28 to 30 dBm forQPSK and 24 to 25 dBm for 128 QAM. On the higher bands Tx power reduces. At23 GHz typical industry maximums are 20 dBm for QPSK and 16 dBm for 128QAM.

It is worth noting that power consumption and therefore the heat generated by anODU does not change significantly when Tx power is adjusted because the poweramplifier (PA) stage operates in what is termed Class A, to achieve optimum

linearity (minimum distortion) over power and modulation selection.

- As the modulation rate increases, so do the demands for distortion-freeoperation to maintain optimum amplitude and phase relationships within theQAM constellation.

- Hence the upper TX power limit is generally determined by the ability of thePA device to meet the constellation design objectives and ultimately thetransmit mask.

- On higher frequency bands this requirement for linear PA operation is furtherconstrained by the ability of the RF devices available to the industry. A higherTx power is more difficult to achieve cost-effectively on the higher frequencybands.

- Ultimately, Tx power is a key consideration in the design of an ODU. A higherTx power means more heat must be dissipated, requiring compromises in thequest for more compact and lighter ODUs. Bear in mind that heat stress is aprimary cause of premature component failure; any reduction in operatingtemperatures will assist long term reliability of the ODU. Design for heatdissipation means ODU temperature limits must not be exceeded, even underhigh ambient conditions and solar gain, such as found in hot equatorialclimates.

Antennas

The shielded parabolic antennas typically used on licensed-band point-to-point links

range in size from 0.3 m to 1.8 m. Larger and smaller sizes are available, but their use isthe exception.

Key electrical specifications include frequency, gain, beamwidth, cross polarizationdiscrimination, front-to-back ratio and VSWR.

Key mechanical specifications include size, weight, wind loading, and the environment.

But from a link budget viewpoint it is all about gain.

Antenna gain is a measure of directivity and efficiency, and for parabolic antennasis primarily a function of antenna size. As the diameter of an antenna increases itsgain increases and its beamwidth decreases.

- Directivity is the ability of an antenna to focus energy in a particular direction.

- Efficiency is how much of the energy fed to an antenna is actually transmitted(that which is not transmitted is lost as heat). Conversely, it is how much of the

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White PaperTechnical Note

incident received energy is converted to a receive signal at the antenna port.The antennas used have efficiencies ranging from 50 to 70 percent.

The gain figure for an antenna is measured in dBi, as gain relative to an isotropicradiator. Three gain figures are usually given; the bottom, top and mid-point of the

specified frequency band. Generally, the gain of an antenna increases by between 3 to 5 dB for one increase

in antenna size ( 0.3 m, to 0.6 m, to 1.2 m, to 1.8 m).

As frequency increases the gain of an antenna increases for the same antennasize. For example, at 7 GHz the gain of a 0.6 m antenna is typically 30 dBi. At 23GHz the same size antenna has a gain of about 40 dBi.

Ultimately, the selection of an antenna size / gain is determined by the link budget,though can often be constrained by the size of antenna that can be supported dueto weight and wind-loading considerations on its support structure. Also by localcouncil environmental planning limitations.

Receiver Threshold

While strictly not a variable, it is in the sense that it does change with capacity/modulationand by frequency band, so does provide an input to appropriate selection of a link bandand channel bandwidth for a required link capacity.

Receiver threshold is about the minimum signal-to-noise (S/N) required at the inputto the receiver to achieve the threshold BER, where the noise constraints are thenoise figure of the receiver, and background thermal noise. As bandwidthincreases, so does the background thermal noise.

Receiver thresholds are usually specified for a 10-6

BER. For a 10-3

BER thethreshold is typically 1 to 1.5 dB lower (more sensitive).

On the lower 6 to 8 GHz bands, typical industry 10-6

thresholds range from -92

dBm for a 7MHz QPSK channel, to -70 dBm for a 28 MHz 128 QAM channel. On the higher bands thresholds are higher (less sensitive).

Summary

Datasheets for digital microwave radios specify Tx Power and Receiver Thresholds, andalso System Gains. System gain is the difference between the maximum Tx power andRx 10

-6threshold, and is specified for all capacity/modulation options on each frequency

band. In essence, system gain is the primary indicator of a radios ability to support a hopwhere fade margin is a critical factor.

On their own, Tx power and Rx threshold do not provide a complete indication ofthe RF performance of a link. Together, as a system gain, they provide a complete

picture. The better the system gain, the better the performance under faded pathconditions.

In many situations, particularly on short hops, adequacy of fade margin will not bea primary consideration. The available system gain coupled with the smallestpractical antenna (typically 0.3 m), will provide a fade margin in excess of what isneeded to support the link availability objectives. In these situations Tx Power isbacked-off in the interests of interference-reduction / frequency-reuse, andultimately to ensure that receiver inputs are not overloaded.

Generally, in the path planning stages the availability (reliability) figure for a path isdecided first and the path variables adjusted to meet this figure. This involves thevariables listed under Fade Margin Variables above, where typically the smallest antenna(lowest cost) is chosen for the available system gain.

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