Weeks 1-4 Flashcards
Whats the difference between a linear and time invariant system?
Linear: The response to 𝑎 × input1(𝑡) is 𝑎 × output1(𝑡) for any complex number and The response to input1 𝑡 + input2(𝑡) is output1 𝑡 + output2(𝑡)
Time Invariant: A system is time-invariant if the response to a time-shifted input is the response to the original signal shifted in time with the same amount
Whats the bandwidth and baseband bandwidth?
Bandwidth: Range of frequencies present in a signal
Baseband Bandwidth: Range of frequencies from zero to the highest frequency present in a signal
What happens in and ideal communication channel and what adverse effects do you get in reality?
The “ideal” communication channel outputs the transmitted signal unchanged as the received signal. The Communication channel is frequency selective.
In reality, the signal can be open to attenuation, noise and distortion.
Amplifier is used to increase transmission power across transmission medium. Signal power is lost to the atmosphere between antennas but has enough information from the message signal to be recovered. The signal is amplified once again before it is received.
Whats the purpose of the transmitter and receiver?
Transmitter: Converts message signal into form suitable for transmission.
Receiver: Converts receiver signal from the channel into a an estimate of the message signal Does this by demodulating and reducing frequency of received signal)
They are both designed to combat the deleterious effects from the transmission channel.
What is amplitude modulation, what happens in the suppressed carrier variant of AM and what does an AM-SC demodulator do?
Modulates the amplitude of a high-frequency carrier to enable electromagnetic propagation and simultaneously, information transfer.
In the “suppressed carrier” (SC) variant, the modulator simply multiples the message signal with a sinusoidal carrier
The AM-SC demodulator multiplies the received signal with a sinusoidal carrier of the same frequency and low-pass filters the product.
What do coherent receivers do?
Estimate synchronisation errors and compensate for them
What does amplitude modulation with a large carrier (AM-LC) do?
Transmits the carrier alongside the modulated message to simplify the demodulation process in maintaining coherency
What is the signal envelope?
Smooth curve outlining the extremes of the signal
function. It can be understood as the shape of the signal.
What are the conditions for envelope detection for AM-LC?
- Ka is small enough such that Ka*M(t)<1
- Carrier frequency is much larger than than message frequency (fc»fm)
Whats the differences between AM-LC vs AM-SC in terms of power efficiency?
AM-LC is highly inefficient due to spending power budget on transmitting carrier
AM-SC is more power efficient but transmitted signal has twice the baseband bandwidth requiring a commensurate channel bandwidth
What does a single side band (SSB) modulation do and when do you get air gaps?
Single sideband (SSB) modulation only transmits either the upper or the lower pair of sidebands.
Energy gap forms for a baseband signal that has little or no energy at low frequencies.This gap enables to use a filter with a reasonable transition band to obtain an SSB signal
What is vestigial side band AM and how does it differ to SSB?
VSB transmits a single sideband, plus a small amount (or vestige) of the other sideband. (requires coherent demodulation)
What is angle modulation and what are its advantages?
Angle modulation aims to maintain a constant envelop modulated signal and carry information in the angle (phase or frequency) of the modulated signal
Advantages: Constant transmit power, better noise immunity
What does a zero cross detector do?
Only uses the points of the modulated signal
where it crosses zero
