Fichas
Input/output model
Fichas del conjunto (39)
- 1What is the generic sinusoidal input signal φ(t)?φ(t)=cos 2πft
- 2Overall AttenuationThe product of attenuation factors due to 1) antenna patterns
2) the nature of reflectors
3) the distances from the transmitting antenna to the reflector and from the reflector to the receive antenna. - 3What are... ?Respectively, the overall attenuation and propagation delay at time t from the transmitter to the receiver on path i
- 4What is the received signal to the sinusoidal input φ(t)=cos 2πft?
- 5What assumption must we make in order to generalize the relation to an arbitrary input x(t) with non-zero bandwidth?That a(f,t) and τ(f,t) do not depend on the frequency f.
- 6If we use the principle of superposition to generalize the relation to an arbitrary input x(t) with non-zero bandwidth, we get the received signal...
- 7In practise what causes the overall attenuation and propagation delay to vary with frequency?Time-varying path lengths and frequency-dependent antenna gains.
- 8Why is it acceptable to ignore the frequency dependence of attenuation and delay?Because we are interested in transmitting over bands that are narrow relative to the carrier frequency.
- 9Although individual attenuations and delays are assumed to be independent of frequency...... the overall channel response can still vary with frequency because different paths have different delays.
- 10What is represented by h(τ,t)?The response of the channel at time t to an impulse transmitted at time t-τ.
- 11Give the expression for h(τ,t) in terms of a_i(t) and τ_i(t)
- 12What is meant by this expression?The overall channel response 'h' to a transmitted signal is the sum of the impulse responses, of all the signal's paths (weighted by their attenuations) to a scaled and time-shifted Kronecker delta function.
- 13Give the input/output relationship between the transmit and receive antennas in terms of h(τ,t)
- 14Explain what is shown by this expressionComplexities of solving Maxwell’s equations reduce to an input/output relation represented as the impulse response of a linear time-varying channel filter.
- 15What expression do we use to represent a more general input than the generic φ(t)=cos 2πft?x(t)
- 16In the special case when the transmitter, receiver and environment are stationary, the attenuations a_i and propagation delays τ_i......do not depend on time t (since the distances do not vary with t either in this situation).
- 17For the time-varying impulse response h(τ,t) define the time-varying frequency response H(f;t)
- 18Any signal that can be represented as an amplitude that varies in time has a corresponding......frequency spectrum
- 19The time-scale at which the ___________ varies is typically much longer than the _______________ of the ___________________ at a fixed time.channel
delay spread
impulse response - 20At the transmitter, the last stage is to__________ the ____________ signal to the carrier frequency and transmit it via the antenna.up-convert
baseband - 21The first step at the receiver is to __________ the RF (radio-frequency) signal to the _________ before further processing.down-convert
baseband - 22In typical wireless applications, communication occurs in a passband _________ of bandwidth ___ around a center frequency ___.[fc-W/2, fc+W/2]
W
fc - 23Describe what the Fourier transform does to a signal (function of time)Decomposes it into the frequencies that make it up.
- 24For a real signal s(t) with Fourier transform S(f), band-limited in [fc−W/2, fc+W/2] with W<2fc. How might we define its complex baseband equivalent?
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- 25has baseband equivalent S_b(f)...
- 26This baseband representation is equivalent toShifting S(f) left by fc, omitting the sub-zero section branch of the graph and scaling by root 2.
- 27S(f)=S*(−f) since s(t) is real, therefore......s_b(t) contains exactly the same information as s(t).
- 28—
- 29What is this implementation of a passband communication system known as?quadrature amplitude modulation (QAM)
- 30What are the 'in phase' (I) and 'quadrature' (Q) components of the passband signal s_b(t)?Respectively the real Re[s_b(t)]and imaginary Im[s_b(t)] parts of the signal, where the imaginary part is rotated
by π/2. - 31What is the baseband equivalent channel?
- 32Define
- 33What is the baseband equivalent impulse response, h_b(τ,t)?
- 34The ________ ______ is the sum over each path, of the delayed replicas of the _________ ______. The magnitude of the ith such term is the magnitude of the __________ on the given path.baseband output
baseband input
response - 35The baseband output changes ________ over time, with significant changes occurring on the order of ___________ or ________.slowly
seconds
more - 36What is considered a significant change in phase for a signal of frequency f? when does this occur?A change in phase of π/2 is significant. This occurs when the delay on the path changes by 1/4f, ie. when the path length changes by a quarter wavelength c/4f.
- 37If a signal of frequency f has a path length changing at velocity v, how long does it take for a significant phase change to occur?c/4fv
where c is the speed of light - 38What is the Doppler shift D (per second) at frequency f for a path length altering at velocity v?D=fv/c
- 39the Fourier transform H_b(f;t) of h_b(t) for a fixed time t is simply...... H((f+f_c);t), the frequency response of the original system(at a fixed t) shifted by the carrier frequency.
Información
- Descripción
- PhD (The Wireless Channel) Comms Fichas sobre Input/output model, creado por L H el 10/01/2018.
- Creado por
- L H
- Visitas
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- Valoración