Flashcards
Wireless Channel Basics
Flashcards do conjunto (34)
- 1Impulse Response The fundamental result in LTI system theory is......that any LTI system can be characterized entirely by a single function called the system's impulse response. The output is the convolution of the input with the impulse response.
- 2Impulse ResponseImpulse response is a property of a system that depends only on the system itself. It is the system's output when presented with a brief input signal, called an impulse.
- 3Give an example of impulse responseIn acoustic and audio applications, impulse response captures the acoustic characteristics of a location, such as a concert hall.
- 4What is the generic sinusoidal input signal φ(t)?φ(t)=cos 2πft
- 5Suppose there is a fixed receive antenna at the location u, which is r distance in angular direction from the transmit antenna. What is the formula received waveform at u?
- 6Antenna PatternsPlacing a receive antenna in position u changes the electric field in the vicinity of u. The coefficient alpha() accounts for the product of the antenna patterns of transmit and receive
antennas
- 7What is meant by saying that this electric field equation is 'linear in the input'?The received field (waveform) at u in response to a weighted sum of transmitted waveforms is simply the weighted sum of responses to those individual waveforms.
- 8What conditions are required for an electric field equation to be 'linear in the input'?The transmit and receive antennae must be stationary relative to each other.
- 9Large scale fadingDue to path loss of signal as a function of distance and shadowing by large obstacles
Occurs as the mobile moves through a distance of the order of the cell size.
Frequency independent. - 10Small scale fadingDue to constructive and destructive interference of multiple signal paths.
Occurs at the spatial scale of order of the carrier wavelength.
Frequency dependent. - 11Cellular communication in the USA is limited by the Federal Communication Commission (FCC), and by similar authorities in other countries, to one of which three frequency bands?Approx. 0.9 GHz, 1.9 GHz,
and 5.8 GHz - 12What approximate wavelengths (λ) are associated with the main three frequency bands used for cellular communication?Fractions of a metre, ie. λ <1m
- 13Describe the relationship of the electric field and magnetic field in the far field.The electric and magnetic fields are perpendicular both to each other and to the direction of propagation from the antenna.
- 14In response to a transmitted sinusoid cos(2ft), we can express the electric far field at time t as...?
- 15What do the variables in this electric field equation represent?(r,θ,ψ) : the point u in space at which the electric field is being measured
r: distance from the transmit antenna to u
θ,ψ: vertical and horizontal angles from antenna to u.
c: speed of light
α_s (f,θ,ψ): radiation pattern of antenna - 16Why is there a factor of r in the denominator of the equation:To account for antenna losses over distance.
- 17Describe H(f), the system function for an LTI (linear time-invariant) channel
- 18How can the impulse response be found from the system function H(f)?The impulse response is the inverse Fourier transform of H(f).
- 19How does H(f) relate to the electric field E?E is the real part of H(f)e^{j2πft}.
- 20Describe the electric field E in terms of distance, frequency etc. when the transmit and receive antenna are moving relative to one another at speed v
- 21Describe the change made to the waveform by the motion of the observation pointWrite f(t−r0/c−vt/c) as ft(1−v/c)−fr0/c. Then,
(1) the sinusoid at frequency f is converted to a sinusoid of frequency f(1−v/c) and
(2) there has been a Doppler shift of −fv/c due to the motion of the observation point. - 22Using the method of ray tracing, describe in words the waveform at the receive antenna.Approximated by sum of direct wave from transmitter (intensity E at distance r) plus the reflected wave (!minus! intensity E at distance 2d-r).
- 23Describe the electric field of a wave reflected off a wall, assuming the wall is very large relative to it's distance from the receiver?It is the same (except for a sign change) as the wave that would exist on the wall's other side if the wall didn;t exist.
- 24What happens to r1 and r2 as the distance r becomes very large relative to the vertical displacements h1 and h2?The difference between the distances r1 and r2 tends to zero (vanishes) relative to the wavelength c/f.
- 25What problem is faced as r increases?The difference between distances r1 and r2 tends to zero, and the two waves start to cancel out due to the sign reversal upon reflection.
- 26How bad is the effect of the cancelation in terms of received wave form and power?The electric wave at the receiver is attenuated as r−2, and the received power decreases as r−4.
- 27Where is this effect seen?This is particularly
important in rural areas where base-stations tend to be placed on roads. - 28Evidence from experimental field studies suggests that while power decay near the transmitter is like r−2, at large distances......the power can decay exponentially with distance.
- 29power decays exponentially in distance at a rate that is proportional to......the density of the obstacles.
- 30Cellular Capacity and its LimitationsThe number of mobiles that can be supported simultaneously within a
cell so that reliable communication can still take place. Limited due to interference between users. - 31ShadowingThe density of obstacles between antennas depends on the physical environment.
This randomness is captured by modeling obstacle density and absorption behavior as random numbers; the overall phenomenon is called shadowing. - 32In what important way does fading due to shadowing differ from that due to multipath?The duration of a shadow fade lasts for
multiple seconds or minutes, and hence occurs at a much slower time-scale
compared to multipath fading. - 33Why is shadowing called shadowing?Because it is similar to the effect of clouds partly blocking sunlight
- 34How do we model scattering?The received waveform is still a linear combination but of many more paths.
Thus, we model it as as an integral over paths with infinitesimally small differences in their lengths, rather than as a sum.
Informações
- Descrição
- PhD Comms (The Wireless Channel) FlashCards sobre Wireless Channel Basics, criado por L H em 03-01-2018.
- Criado por
- L H
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