Need to determine/analyse link performance (e.g Bit Error Rate (BER)
or Packet Loss Rate) in terms of:

Transmit power

Antenna parameters (e.g. gain)

Received system noise levels (usually specified as noise temperature)

Other factors (e.g. propagation losses, interference, intermodulation)
Consider an isotropic antenna  radiates in all directions. Imagine antenna at
centre of hollow sphere.
Power flux density at radius d is:
PFD = P_{T} / (4πd^{2}) W/m^{2}
'Inverse Square Law'
Antennas have gain to concentrate power in a wanted direction.
Gain is relative to Isotropic with units of dBi
Trivial Example:
Transmitter and receiver spaced distance d.

PFD in wanted direction is increased by a factor of G_{T
}

Then at receiver it looks as though there is an isotropic transmitter with
power P_{T}G_{T
}

P_{T}G_{T} = Effective Isotropic Radiated Power (EIRP).

In dB: P_{T} + G_{T} = EIRP (dBW)
Examples:

A transmit antenna has a gain of 50 dB and is fed with a power of 20
watts. What is its EIRP?
P_{T} = 13 dBW, G_{T} = 50 dB, so EIRP = 13 + 50 = 63 dBW

A transmit antenna has a gain of 27 dB and is fed with a power of
2 mW. What is its EIRP?
P_{T} = 27 dBW, G_{T} = 27 dB, so EIRP = 0 dBW.
