Page 395 - Complete Wireless Design

Communications System Design

394 Chapter Nine

Because of the vagaries of wireless communications caused by atmospheric
and multipath fading, a certain amount of fade margin will be necessary to
make sure that the link will remain up—even under infrequent but severe
weather conditions—for a certain percentage of the year. This fade margin is
a safety allowance of excess receiver NF and gain, transmitter power, or
antenna gain placed in our link budget to assure a dependable wireless con-
nection, with a certain amount of permitted downtime (in seconds) per year.
For instance, a 20-dB fade margin for digital communications systems can be
added to the link budget, which will not only cover atmospheric anomalies and
multipath, but also equipment aging and repairs. A common figure for depend-
able operability that could be expected for certain digital systems, and with
this 20 dB of fade margin, might be 99.99 percent with a BER of 10 8 through-
out a year-long period.
It is important when calculating two-way links that each direction of the
link has the required link budget to function at a desired BER, considering
that all duplex links may not be power-, bandwidth-, SNR-, frequency-, or even
modulation-symmetrical.

9.3.2 Link budget design
After we find the range (in kilometers or miles) over which the communica-
tions link must reliably transmit information, we must then calculate the
wireless link’s free-space path loss. This is the loss, in dB, that occurs to an RF
signal at a specific frequency over a specific range—but it does not account for
any impairments.
Another figure we must obtain is the fade margin, as mentioned above. This
will be required to assure the link of reception reliability during any unex-
pected, but persistent, atmospheric anomalies and multipath effects.
We would also like to find the minimum power that must be available at the
receiver’s output for proper demodulation within the modem or demodulator,
and what SNR is required for the particular modulation and error correction
in use.
In the following example case, it is being assumed that the RF designer will be
told what the maximum transmitter equivalent isotropically radiated power
(EIRP) will be. But in order to obtain the necessary BER at the receiver’s output,
a compromise will normally have to be made between transmitter output power
(which must generally be minimized to the lowest level that assures reliable com-
munications), and the receiver’s NF and gain versus IMD generation. However,
higher-gain antennas and lower-loss coax cable on either the transmit and
receive side, or both, can sometimes be an easy way to increase the fade margin.
To perform a link budget analysis:
1. Calculate the free-space path loss over the desired link distance. As men-
tioned, free-space path loss does not include any losses caused by the atmo-
sphere or by multipath, but merely accounts for the inverse square law
signal spreading of the RF wavefront as it leaves the transmitting antenna

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