It is important to note that (with a few exceptions) bands are generally allocated to more than one radio service

• Sharing
• Interference
• -> CONSTRAINTS

Bands have traditionally been divided into "commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilise "government" bands.

Antenna Gain: 

g = (Power density on-axis at given distance) / (power density from isotropic source at same distance)

g = 4π.a/λ²

Antenna Aperture:

a = η.π.r²

Power density from transmit antenna at receive aperture is:

p_den = (p_t.g_t)/(4π.d²)

Power collected by receive antenna:

p_r = ((p_t.g_t)/(4π.d²)).a_r.α = p_t.g_t.g_r(λ/(4π.d))².α

Path transmission loss ("free space loss"):

l_p = ((4π.d)/λ)²

Thermal noise at output of receive antenna:

n = k.T.B

Carrier-to-noise ratio:

p_r/n_r = (p_t.g_t.g_r.α)/(l_p.k.T.B)

Hence "Link Budget Equation" expressed in dB:

C/N = P_t+G_t-L_p-L_a+(G/T)|_r-k-10.log(B)

Note: |_r means evaluated at r.

When calculating the link budget, we must take account of additional factors:

• Interferences from other radio systems
• Intermodulation noise
• Additional loss, interference and noise due to propagation events (e.g. rain, rain-scatter, noise degradation, ducting, etc.)

Figure of merit (G/T) is a useful way of categorising the performance of a receiving system. 

In a constant RF field, different receiving systems (with the same receive bandwidth) will yield a C/N proportional to the system G/T.

EIRP is the usual way of combining transmit power and gain:

EIRP = P_t+G_t

So, the link budget equation is normally expressed as:

C/N = EIRP - L_p - L_a - L_m + G/T - 10log(B) + 228.6

The satellite link:

Uplink and downlink are analysed seperately as individual radio links:

Overall Link Quality:

C/N = C/(N_up+N_down+I_up+I_down+N_imod)

(c/n_tot)^-1=(c/n_up)^-1+(c/n_down)^-1+(c/i_up)^-1+...