B1P1 - Channels for communication Flashcards
absorption
where EMR is attenuated from passing through a medium;
amplitude
the maximum absolute value of a periodic signal;
analogue signal
a signal that can take any value within a continuous range;
attenuation
reduction in the power of a signal, e.g. through absorption;
attenuation coefficient
how much a signal is attenuated when passing through a specific medium; units are dB m^-1;
bandwidth (B)
the amount of spectrum occupied by the signal spectrum: B = f_max - f_min;
beamwidth
the cone angle containing the antenna’s radiation above maxPower - 3dB;
antenna
EMR <==> current
bipolar signal
a signal that uses positive and negative voltages to rep data 1s and 0s;
Carson’s rule (formula)
gives bandwidth of a freq-modulated signal:
B_fm = 2 * (delta_f + f_mod) = 2 * (1 + beta) * f_mod
…where
delta_f = f_max - f_min
f_mod = is the max(mod_freq)
beta = mod_index;
cladding
the layer (with a lower refractive index) surrounding the central core of an optical fibre;
combiner
a device taking input signals from 2+ optical fibres and merging them into a single output;
core
the innermost part (with a higher refractive index) of an optical fibre;
crosstalk
where neighbouring copper cables carrying different signals interfere with each other;
current
transfers energy in a cable through the movement of electrons;
decade
a range of values covering a ten-fold increase, e.g. 20 to 200;
decibel
an alternative rep of = power1/power2 = G_dB:
G_dB = 10 * log_10(power1 / power2);
power1/power2 = 10^(G_dB/10);
demultiplexing
converting a multiplexed signal back to the original set of signals;
dielectric (insulating) loss
where the signal energy is dissipated through the material separating the copper cable’s conductors;
diffraction
where EMR spreads / bends / wraps round when passing through a gap or by a sharp corner;
digital signal
can take one of a set of discrete values;
dipole
an antenna consisting of two straight conductors in a line:
———-<>———-;
directional coupler
combines or splits optical signals;
dispersion
where the signal is distorted due to there being light of varying frequencies and speeds in the optical fibre;
distributed amplifier
a device which laser-zaps an optical fibre’s full transmission path to amplify the signal;
envelope
the shape of an amplitude modulated signal;
erbium-doped fibre amplifier (EDFA)
where erbium atoms are added to a section of the optical fibre and then lazer-zapped to amplify the signal;
far field
all space outwith a few wavelengths from the antenna and where 1/d^2 law applies;
filtering
where only a subset of frequencies are allowed to pass through;
frequency deviation
for a freq-modulated signal, the max. difference between mod-frequency and carrier-frequency;
frequency modulation
where the mod-wave inflects the carrier-wave’s frequency to rep symbols;
graded-index fibre
where the refractive index changes smoothly from high in the core to low in the cladding;
inverse fourth-power law
1 / (d^4);
inverse square law
1 / (d^2);
ionosphere
a layer of the atmosphere containing molecules with a net charge [i.e. #protons != #eletrons];
isotropic
an antenna that has a 3D spherical radiation envelope;
light-emitting diode (LED)
current ==> EMR;
logarithmic scale
where each tick on the scale is the previous tick multiplied by n, where n is the base of the log (typically 2, 10, or e);
modulating signal
the signal inflecting the carrier-signal to rep symbols;
modulation
where a carrier-signal is inflected by a mod-signal to rep symbols;
modulation index
the ratio of frequency deviation to max frequency in the mod-signal:
beta = delta_freq / freq_max;
multimode fibre
where the core diameter is large compared to wavelength, allowing light to take many paths;
multipath reception
where EMR signals are received via 2+ paths;
multiplexing
where 2+ signals are transmitted through a single medium;
near field
all space within a few wavelengths of the antenna and where 1/d^2 law does not apply;
omnidirectional
an antenna that has a flat 2D disc-like radiation envelope;
passband
the frequencies that a filter allows to pass through;
period
T = 1 / freq.
periodic signal
a signal that repeats at regular time intervals;
phase
from the interval [0, 2pi), the amount a periodic signal has been shifted along the horizontal axis;
phase modulation (PM)
where the phases of the mod-wave and carrier-wave differ by for example pi, and each are switched on and off to rep symbols;
photodiode
EMR ==> current
polarisation mode distortion
where some light zig-zags and some zooms in narrow fibres thereby distrorting the signal;
pulse spreading
where the sharp states of a digital signal diffuse in transmission;
[ |’’‘| ==> _/’’‘_ ]
quantising
where, at regular time intervals, an analogue signal is rounded to the nearest value from an value-alphabet;
refractive index
refractive_index = c / v_medium;
regenerator
a midway device that relays a corrected version of the received signal;
repeater
a midway device that relays just an amped up version of the received signal;
resistance
how difficult it is for an electric current to pass through a conductor;
resolution
for quantising, the number of discrete levels chosen to rep an analogue signal: bits_to_rep_level = log_2(numLevels);
(frequency) response
how sensitive an antenna is to frequencies at / around its tuned frequency;
sampling
measuring an analogue signal at regular time intervals;
selectivity
how well an antenna can reject signals outside its passband;
single-mode fibre
where light can travel only one way through due to the fibre’s narrow core;
sinusoid
a periodic signal with the general form v(t) = A * sin((2pif*t) + phi);
sky wave
where the ionosphere reflects / refracts EMR back to Earth;
step-index fibre
where there is an abrupt change of refractive index between core and cladding;
threshold detection
where a received binary signal value is tested against the predefined value separating a data 1 from a data 0;
total internal reflection
where light reflects at the boundary between two media with different refractive indices;
voltage
volt = power / current;
voltage-controlled oscillator (VCO)
voltage (signal) ==> frequency;
waveform
a periodic signal’s shape in time domain;
wavelength division multiplexing (WDM)
where multiple signals of different wavelengths share a single optical fibre;
