Satellite Communication

Why it matters

Satellite links connect distant and remote terminals. A link budget separates transmitted power, antenna gain, propagation losses, receiver noise and bandwidth so received signal quality can be assessed consistently.

Key ideas

An uplink travels from an Earth station to a satellite; a downlink travels back to Earth. A transparent transponder filters, translates frequency and amplifies signals; a regenerative payload may demodulate and process them. GEO, MEO and LEO geometries differ in coverage, path loss, propagation delay and tracking needs. The footprint describes coverage subject to antenna pattern and link requirements.

Link-budget equations

EIRP(dBW) = P_t(dBW) + G_t(dBi) − transmit feeder loss(dB). Received carrier C(dBW) = EIRP + G_r − L_path − other losses. Noise spectral density N0 = kT_s in W/Hz, with k = 1.380649×10^−23 J/K. N0(dBW/Hz) = −228.6 + 10log10(T_s/1 K). C/N0(dB-Hz) = C(dBW) − N0(dBW/Hz). C/N(dB) = C/N0(dB-Hz) − 10log10(B/1 Hz), where B is equivalent noise bandwidth.

G/T = G_r(dBi) − 10log10(T_s/1 K) is commonly expressed in dB/K. Temperature must be converted logarithmically before entering a dB sum. C/N0 and C/N are different quantities.

Worked example

Let P_t = 20 dBW, G_t = 30 dBi, G_r = 40 dBi, path loss = 200 dB and T_s = 290 K. Neglect all other losses. Take B = 1 MHz.

EIRP = 20+30 = 50 dBW. C = 50+40−200 = −110 dBW. 10log10(290) = 24.624 dB relative to 1 K. N0 = −228.6+24.624 = −203.976 dBW/Hz. C/N0 = −110−(−203.976) = 93.976 dB-Hz. C/N = 93.976−10log10(10^6) = 33.976 dB.

Without B, one can calculate C/N0 but not the integrated C/N. If independent uplink and downlink noise contributions are referred consistently, their linear inverse carrier-to-noise ratios add; do not simply add their dB C/N values.

Common mistakes

Subtracting 290 K directly in a dB equation; reporting C/N0 as C/N; mixing dBm with dBW; or omitting atmospheric, pointing, feeder and polarization losses when they are specified.

Quick check

1. What is EIRP?
2. What extra quantity converts C/N0 to C/N?
3. Does doubling noise bandwidth change C/N for fixed C and N0?

Answers: 1. Equivalent isotropically radiated power. 2. Equivalent noise bandwidth. 3. Yes, it reduces C/N by about 3.01 dB.