APD	Avalanche Photodiode (or ~Detector)
BERT	Bit Error Ratio Tester
BGL	Background Light
FoV	Field-of-View
InGaAs	Indium Gallium Arsenide semiconductor
IM/DD	Intensity Modulation/Direct Detection
IRT	Index-of-Refraction Turbulence
LEO	Low Earth Orbit
LP	Low Pass, or reception filter (between TIA and limiter)
OOK	On/Off-Keying modulation
PRBS	Pseudo-random bit sequence
Ppb	Photons per symbol-bit
RFE	Receiver front-end
RSSI	Received signal strength indicator
SNL	Shot Noise Limited
SNR	Signal-to-noise ratio
TIA	Trans-impedance amplifier
TNL	Thermal Noise Limited
B	Bandwidth, e.g., of the RFEs reception filter
c	Speed of light in vacuum (2.998 × 108 m/s)
FA	Excess noise factor of an APD
Fn	Amplifier noise figure
h	Planck constant (6.626 × 10−34 Ws2)
it	Thermal noise current density from amplifier
Id	Dark current of a photodiode
Idm	Part of dark current that will get multiplied with M
Idu	Part of dark current that will not get multiplied
kA	Ionization coefficient ratio of electrons vs. holes
kB	Boltzmann constant
Le,Ω,λ	Spectral irradiance (typically per nm wavelength)
M	Multiplication factor
Mopt	optimum multiplication factor
$\langle N\rangle$	Mean number of photons per bit
$\langle P_{Rx}\rangle$	Mean received optical power
$P_{bgl}$	Background light power seen by the APD area
pBE	Probability of bit error
Q	Receiver quality factor
q	Elementary charge (1.6022 × 10−19 As)
R	Unmultiplied detector responsivity
RTI	Transimpedance resistor
r	Data rate = ½ B
T	Temperature
UBD	Breakdown voltage of APD
UR	Reverse voltage applied to APD
$\widehat{..}$	Peak signal value (e.g., pulse amplitude)
$\langle ..\rangle$	Mean value of a binary symbol sequence