Popular Interview Wireless Question with Answer

Popular Interview Question in Wireless Communication
Dr. Varun Kumar
**************************************************************************************************
1. What do you mean favourable propagation?
Ans It refers the mutual orthogonality among vector value wireless channel that can maximize the
total system throughput. Ex- Let H is the channel matrix and P is the transmitted power
and σ2
is noise variance. Hence, channel capacity is
C = log2

Let H =

h11 h12
h21 h22
#
⇒ HH
=

h∗
11 h∗
21
h∗
12 h∗
22
#
C = log2
P
σ2
n
(1 + |h2
11| + |h12|2
)(1 + |h2
21| + |h22|2
) − (h11h∗
21 + h12h∗
22)(h21h∗
11 + h22h∗
12)
o
Favourable propagation refers: (h11h∗
21 + h12h∗
22)(h21h∗
11 + h22h∗
12) → 0
C = Cmax = log2
P
σ2
n
(1 + |h2
11| + |h12|2
)(1 + |h2
21| + |h22|2
)
o
2. What are the challenges of TDD ?
Ans The challenges of TDD are as follow
• It is relatively more complex compare to FDD. How??
• It works on larger power compare to FDD. Why??
• The UL and DL channel matrix doesn’t remain same for UL/DL same carrier frequency.
The major reason is hardware mismatch across BS and UE side both.
3. Can we achieve the channel reciprocity using frequency division duplexing ?
Ans For reciprocal channel, the UL and DL carrier frequency should be same. For practical
consideration, if the UL/DL carrier frequencies are nearly same the associated hardware
circuitry will respond same. In this way, we can say reciprocity can be achieved through
FDD. But if UL and DL signals are not nearly equal then, channel reciprocity can’t be
achieved using FDD.
4. What is coherent and non-coherent detection ?
Ans Message signal is detected, when received signal is passed through product modulator then
after through LPF. Ex- Let received signal is m(t) cos 2πfct. Hence, there will be two case
arise.
1

Case 1: When local oscillator (LO) produces the same carrier, ie cos 2πfct then the resultant
signal can be expressed as
m(t) cos 2πfct × cos 2πfct =
m(t)
2
+
m(t) cos 4πfct
2
LPF
−−−−−−→
m(t)
2
This detection is called as coherent detection.
Case 2: When local oscillator (LO) produces the same carrier, ie cos 2π(fc + ∆fc)t then
the resultant signal can be expressed as
m(t) cos 2πfct × cos 2π(fc + ∆fc)t =
m(t) cos 2π∆fct
2
+
m(t) cos 2π(2fc + ∆fc)t
2
LPF
−−−−−−
→
m(t) cos 2π∆fct
2
This detection is called as non-coherent detection.
5. Why FDD is not suitable for massive MIMO communication ?
Ans For coherent detection, the coherence time Tc can be expressed as follow
Tc = Tch + TUL(Data) + TDL(Data) + Toth
Tch → Time elapsed in channel estimation
TUL(Data) → Time allocation for UL data transmission
TDL → Time allocation for DL data transmission
Toth → Guard period etc
Let a base station (BS) has M antenna and K UEs connected to the respective BS. The
channel estimation time for TDD and FDD scheme can be expressed as
Tch = (2M + K)τ → FDD
= Kτ → TDD
If channel estimation time is more, then it will not support the highly mobile user or when
operating carrier frequency is very high.
6. What is TDD and FDD ?
Ans TDD refers to time division duplexing, whereas FDD refers to frequency division multiplexing.
The basic difference has been described through below table.
7. What are practical difficulty with reciprocal channel ?
Ans Channel reciprocity: When Tx − Rx position are interchanged then the transfer charac-
teristics of channel should remain same. This process or act referred as channel reciprocity.
Practical difficulties with BS-UE based wireless network.
• Man-made noise levels: The antenna of the BS is at more height, so that it does not affected by
obstacles in its vicinity. On the other hand, UE antenna is at most mounted a few meters above ground
level. The man-made noise level is likely to be substantially higher at the mobile antenna than at the
BS antenna.
2

TDD FDD
MAC layer complexity is less, because
it operates on a single frequency.
MAC layer complexity is relatively high.
Latency is more, hence it is not suitable
for large distance communication.
No major issues of latency in FDD.
Spectrum efficiency is high, because in bi-
directional communication most of the DL
channel utilized.
Spectrum efficiency is less in unbalanced
traffic scenario.
Equipment cost is less, because no diplexer
is used due to single carrier frequency.
Equipment cost is high.
Signal processing complexity is more. Signal processing complexity is less.
• Effect of antenna diversity: Multipath scatters mostly occur in the immediate vicinity of the mobile
antenna. The base station receives more or less a transversal electromagnetic wave, whereas the mobile
station receives a superposition of a set of reflected waves from random angles. Two antennas at the
mobile terminal are likely to receive uncorrelated signal powers if their separation is more than a wave
length. At the base station site, however, all reflections arrive from almost identical directions. Therefore,
diversity at the base station requires much larger separation of the antennae to ensure uncorrelated
received signal powers at the two antennas. For the same reason, antenna directivity has different effects
at the mobile and the base station.
• Correlation of shadow fading of desired signal and interfering signals: In a cellular network,
shadow fading of the wanted signal received by the mobile station is likely to be correlated with the
shadow fading of the interference caused by other base stations, or, in a spread-spectrum network, with
the shadowing of simultaneously transmitted signals from the same base station. In contrast to this, at
the base station, shadow fading of the wanted signal presumably is mostly statistically independent from
shadow fading of the interference. However, experimental results for correlation of shadow attenuation
are scarce.
• Full-duplex channels: In full-duplex operation, multipath fading of inbound and outbound channel,
which operate at widely different frequencies, may be uncorrelated. This will particularly be the case if
the delay spread is large.
• Multiplexing and multiple access In a practical multi-user system with intermittent transmissions,
inbound messages are sent via a multiple-access channel, whereas in outbound channel, signals destined
for different users can be multiplexed. In the latter case, the receiver in a mobile station can maintain
carrier and bit synchronisation to the continuous incoming bit stream from the base station, whereas the
receiver in the base station has to acquire synchronisation for each user slot. Moreover, in packet-switched
data networks, the inbound channel has to accept randomly occurring transmissions by the terminals in
the service area. Random-access protocols are required to organise the data traffic flow in the inbound
channel, and access conflicts (’contention’) may occur. In cellular networks with large traffic loads per
base station, spread-spectrum modulation can be exploited in the downlink to combat multipath fading,
whereas in the uplink, the signal powers from the various mobile subscribers may differ too much to
effectively apply spread- spectrum multiple access unless sophisticated adaptive power control techniques
are employed.
• industrial design From a practical point of view, the downlink and the uplink will be designed under
entirely different (cost) constraints, such as power consumption, size, weight and other ergonomic aspects,
energy radiated into the human body, and consumer cost aspects.
• Data traffic patterns In packet data networks applied for traffic and transportation, the characteristics
3

of the data traffic flows are known to differ for the uplink and the downlink. For instance, (outbound)
messages from a fleet management centre to the vehicles are likely to be of a more routine type, of a more
uniform length and occur in a more regular pattern than messages in the opposite (inbound) direction.
8. Can massive MIMO nullify the effect of inter-user interference ?
Ans Beamforming is the essential feature of massive MIMO. In same time-frequency slot all UEs
observed the vector valued channel. Due to large number of links, it is supposed that all
wireless link are mutually orthogonal. This orthogonality implies the nullification of inter-
user interference.
9. Difference between Rayleigh, Racian and Nakagami fading.
Ans Fading refers the fluctuation of signal, when it travel into a wireless media.
(a) Rayleigh fading: When transmitter and receiver are not in the LOS, then observed
fluctuated channel can be modeled as a Rayleigh distributed random variable.
f(r) =
r
σ2
e− r2
2σ2
(b) Racian fading: When transmitter and receiver are in the LOS, then observed fluctuated
channel can be modeled as a Racian distributed random variable.
(c) Nakagami fading: When transmitter and receiver are in the LOS, then observed fluc-
tuated channel can be modeled as a Racian distributed random variable.
10. Difference between time flat and time selective, frequency-flat and frequency-
selective, and doubly selective channel.
Ans The received signal has two components, ie channel coefficient and transmitted data symbol.
Mathematically, it can be expressed as
y(t) =
N
X
i=1
h(t − τi)x(t − ti) ∀ i = 0, 1, 2, ...N
Case 1: When y(t) = hx(t − t0)
(a) h(t) = h → Time flat
(b) x(t − ti) = x(t − t0) → Single delay element → Frequency flat
Case 2: When y(t) = h0x(t − t0) + h1x(t − t0) + ... + hN x(t − t0)
(a) h(t − τ0) = h0, ...., h(t − τN ) = hN → Time selective
Case 3: When y(t) = h0x(t − t0) + h1x(t − t1) + ... + hN x(t − tN )
(a) h(t − τ0) = h0, ...., h(t − τN ) = hN → Time selective
4

11. What is coherence time and coherence bandwidth ?
Ans Coherence time and coherence bandwidth are the statistical measures for a communication
system. We can understand the mathematical relation of the received wireless signal
y = hx + n (1)
h → Channel coefficient, that is also a time varying quantity.
x → Transmitted symbol/data/information
n → Additive noise
Coherence time refers a time, where channel is supposed to highly correlated. Let h(t)
and h(t + τ) are channel coefficient for two time instant. For 70% correlation, the value of
coherence time can be expressed as
Tc =
9
16πfd
, fd =
vfc
c
→ Doppler frequency
v → Velocity of UE, fc → Carrier frequency, c → Speed of light
Coherence bandwidth: From (1), coherence bandwidth is associated with x. Supportable
bandwidth, so that channel became flat.
12. What is equalizer and how it improves the signal quality ?
Ans Equalization is the reversal of distortion incurred by a signal transmitted through a channel.
X(f) =⇒ H(f)
Channel
| {z }
=⇒ H(f)X(f) =⇒
1
H(f)
Equalizer
| {z }
=⇒ X(f)
Equalizer is a method for combating with multi-path fading. Ex- let a current received symbol
is y(k) that depends on current and one unit delayed input signal, i.e, x(k), and x(k − 1).
Mathematically, it can be expressed as
y(k) = h0x(k) + h1x(k − 1) + n(k)
y(k + 1) = h0x(k + 1) + h1x(k) + n(k + 1)
y(k + 2) = h0x(k + 2) + h1x(k + 1) + n(k + 2)
Let current input is a three tapped output symbol, i.e
x̂(k) = f y(k), y(k + 1), y(k + 2)

Above expression can also be written in the matrix form



y(k + 2)
y(k + 1)
y(k)


 =



h0 h1 0 0
0 h0 h1 0
0 0 h0 h1








x(k + 2)
x(k + 1)
x(k)
x(k − 1)





+



n(k + 2)
n(k + 1)
n(k)



5

Let x̂(k) = c0y(k+2)+c1y(k+1)+c2y(k), C̄ = [c2 c1 c0]T
, X̄ = [x(k+2), x(k+1), x(k), x(k−
1)]T
and Ȳ = [y(k + 2), y(k + 1), y(k)]T
. The aim of equalizer is to find out the vector C̄,
such that C̄T
H → [0 0 1 0]. Let 12 = [0 0 1 0] Here,
x̂(k) = C̄T
Ȳ = C̄T
HX̄ + N̄

Applying the least square based equalization, the value of C̄ is
C̄ = HT
H
−1
HT
12 (2)
13. What is the significance of auto-correlation ?
Ans It refers the degree of similarity between signal itself. It is widely applicable in wireless
communication, when we work on a random process. The best example of auto-correlation
that gives some practical sense is coherence time, finding the energy and power of the
signal.
Rxx(τ) =
Z ∞
−∞
x(t)x(t + τ)dt ⇒ Rxx(0) =
Z ∞
−∞
|x(t)|2
dt = Eg
P = lim
T→∞
1
T
Z T/2
−T/2
|x(t)|2
dt =
14. What is the difference between large and small-scale effect?
Ans When signal propagate into a wireless media then two types of effect is observed.
(a) Large scale effect: When separation is of the order of 100m or more. Ex-
i. Propagative loss
• Free space
• Indoor
• Outdoor
ii. Propagative mode
• Reflection
• Refraction
• Diffraction
iii. Shadowing
(b) Small scale effect: When separation is of the order of 10 to 20λ. Ex- Multi-path
fading
• Rayleigh fading
• Ricean fading
• Flat fading
• Frequency selective fading
• Fast fading
6

• Slow fading
• Time dispersion
• Random FM
15. Difference between FDM, OFDM, and OFDMA.
Ans Multiplexing is process when resources are shared over a common channel.
(a) FDM:(Frequency division multiplexing) It refers a process where frequency re-
sources are shared over a common channel. In FDM, Guard band is required and the
spectral efficiency of FDM is low.
(b) OFDM (Orthogonal frequency division multiplexing): It is a special kind of FDM,
where two sub-carrier are orthogonal to each other and no need of guard band. Hence,
it maximize the overall spectrum efficiency.
(c) OFDMA(Orthogonal frequency division multiplexing access): It is also a mul-
tiple access technique. It can be understood as the combination of TDMA+OFDM.
16. What do you mean by interleaver in communication system?
Ans Interleaving in communication is a process by which the ordering sequence of the encoded
bits is changed so that the burst of errors gets distributed across various codewords present
in the data rather than being bound to a single codeword.
17. What is the difference between multiplexing gain and diversity gain.
Ans ⇒ Multiplexing gain refers the number of bits/symbol transmission from the transmitter
node at a time through a common channel.
⇒ Diversity gain refers the link reliability, when signal is coming from multiple link.
Higher the number of link greater be the diversity gain. Capacity can also be increased
by diversity combining. In MIMO system, let Nt and Nr be the total number of transmit
and receive antenna. Mathematically capacity expression can be written as as
R = min{Nt, Nr}
| {z }
Multiplexing
log2

1 +
P
σ2
(|h1|2
+ |h2|2
+ ... + |hN |2
)
| {z }
Diversity

18. What is water-filling algorithm ?
7

Ans It is a method for power allocation across each spatial direction. This algorithm is used in the
MIMO communication network. Let a MIMO network of size N × N having spatial gain α1,
α2,....., αN and allocated power across each spatial directions are P1, P2,....,PN , respectively.
During communication, let the noise variance is σ2
. Hence, the observed data rate under
such a scenario can be expressed as
R =
N
X
i=1
log2

1 +
Piαi
σ2

In communication system the total transmit power (PT ) is a fixed quantity. Hence,
P1 + P2 + ...... + PN ≤ PT
It is a optimization problem and solving these problem (minimize the power and maximize
the data rate), we get
Pi = max
n1
λ
−
σ2
αi
+
, 0
o
∀ α1 ≥ α2...... ≥ αN
Here, 1
λ
is the reference allocated power. From above relation, it is clear that as per water
filling algorithm maximum power is allocated for the highest spatial gain.
19. What do you mean by transmit diversity and receive diversity ?
Ans Diversity is a technique that provides the robustness in communication system. Multiple
diversity techniques are as follow
(a) Time diversity
(b) Frequency diversity
(c) Antenna diversity or Space diversity
(d) Macro diversity
20. What are the functions of precoder and decoder ?
Ans (a) Precoder: It is a device that send or transmit the signal for fulfilling the following
objective.
⇒ Mitigate the interuser interference
⇒ Cancellation of inter base station interference
⇒ Nullify the effect of pilot contamination
(b) Decoder It is a device that send or transmit the signal for fulfilling the following objec-
tive.
21. What is the usage of circular convolution and correlation in communication sys-
tem ?
8

Ans Circular convolution is the convolution of two periodic functions that have the same period.
For example, the DTFT of the product of two discrete sequences is the periodic convolution
of the DTFTs of the individual sequences. And each DTFT is a periodic summation of a
continuous Fourier transform function. Although DTFTs are usually continuous functions
of frequency, the concepts of periodic and circular convolution are also directly applicable to
discrete sequences of data.
22. What is the usage of I-FFT block in OFDM transmitter ?
Ans There are two major advantage of I-FFT block in OFDM transmitter.
(1) It reduces the hardware requirement like filter bank across the receiver unit.
(2) Sample based sub-carrier information can be coherent decoded by only the IFFT opera-
tion. Because, each adjacent samples are orthogonal to each other.
In OFDM communication system, let total bandwidth and number of allocated sub-carriers
are B and N respectively. Bandwidth allocation across each sub-carrier is
4f =
B
N
Total time scale integration = 1
4f
= N
B
. Let us see the mathematical operation of IFFT
xn =
1
N
N−1
X
k=0
X(k)ej2π kn
N
From the mathematics of the IFFT, nth
sub-carrier for integer value of k = 0, 1, ...N − 1, all
are orthogonal among each other.
23. What do you mean by channel correlation and its drawback on communication
system ?
Ans Channel correlation: Correlation is a statistical measure between the similarity between
two random variable. Let h(t) is the time-varying channel and h(t − τ) is the delayed version
of channel. The auto-correlation can be expressed as
Rh(τ) = lim
T→∞
1
T
Z T/2
−T/2
h(t)h(t + τ)dt
If a channel is highly correlated then it is supposed to less time varying. In another words,
if auto-correlation has Gaussian PDF then larger variance shows the high correlation. It can
be easily observed from above figure.
Impact on communication system:
(a) A mobile user experience correlated wireless channel and if channel gain is poor then the
mobile user experience poor SNR for a very time.
9

(b) In a MIMO wireless network, if all links are highly correlated and at any instant the
channel gain is very poor or in deep fade then the diversity gain can’t be reach to its
theoretical expectation.
(c) Channel correlation also helps for estimating the coherence time, that provides a time
resource to the wireless network for doing the channel estimation, uplink/downlink data
transmission and other signal processing activity.
24. Difference between random variable and random process.
Ans (a) Random variable: A fundamental quantity, whose outcome is not certain called as
random variable. Ex- temperature, pressure, attenuation, velocity, etc.
Note: For converting a random quantity into its deterministic equivalent, we need prob-
ability density function (PDF), mean, variance and higher order moment.
(b) Random process: Time dependent random variable is called as random process. Ex-
i Variation of temperature in day, week or year is an example of random process.
ii Wireless channel coefficient, etc
As per the types of random process, it has been classified into several categories.
– Stationary process (strictly stationary)
– Wide-sense stationary (WSS)
– Cyclo-stationary
– Non-stationary
25. What are the differences among stationary, non-stationary, wide sense stationary
and cyclo-stationary processes ?
Ans Fundamental differences are as follow
(a) Strictly stationary process: A random process, whose outcome remain same for all
time or a time invariant. In other words, variance of this time dependent random variable
is equal to zero. In real world, strictly stationary process is not found.
(b) Wide-sense stationary process: It is also a stationary process but not in strict sense.
Time dependent random variable is classified in terms of mean and auto-correlation.
10

Higher order moment is not required for analyzing this process. Ex- Wireless channel
coefficient.
(c) Cyclo-stationary process: It is also a weak stationary process. A cyclostationary
process is a signal having statistical properties vary cyclically with time. Ex- 21 July is
considered to be hot and 20 December to be cold day of the year.
(d) Non-stationary process: A random process whose outcome is un-certain is called
as non-stationary process. For hardware making, no hardware can be made for non-
stationary process.
26. What is the significance of linear phase ?
Ans Linear phase assure the distortionless wireless communication. Let y and x are output and
input respectively. If
y = kx(t − τ)
Now taking the Fourier transform
Y (jΩ) = kX(jΩ)e−jΩτ
= kX(jΩ)e−θ(f)
Here, θ(f) = 2πfτ → Linear phase.
Note: It means constant gain/attenuation and fixed delay in input signal has linear phase
and it is main condition for distortion free communication.
27. What do you mean by linear and non-linear distortion ? What are the cause for
arising these type of distortion ?
Ans For general consideration, the channel is supposed to be linear time invariant. Signal dis-
tortion can happen either due to magnitude distortion or phase distortion. This type of
distortion is called as linear distortion. Linear distortion causes time domain spreading of
a signal
Let a memoryless non linear channel where input g and output y are related by some
non-linear equation
y = f(g)
and
y(t) = a0 + a1g(t) + a2g2
(t) + a3g3
(t) + ....+
If we take the spectrum of the above signal then output signal has larger bandwidth compare
to input one. Non-linear distortion causes spectral broadening.
28. What do you mean by vector space and signal space ?
Ans Signals that are defined for only a finite number of time instant (say N) can be written as
vectors (of dimension N). Consider g(t) is defined over a time interval [a, b]. Let we pick N
point uniformly on the time interval [a, b] such that
t1 = a, t2 = a + , ....., tN = a + (N − 1) = b =
b − a
N − 1
11

Then we can write a signal vector g as an N− dimensional vector.
g = [g(t1), g(t2), ...., g(tN )]
29. What is the significance of negative frequency ?
Ans Negative frequency does not describes the rate of variation of sine or cosine wave. It describes
the direction of rotation of a unit length exponential sinusoid and its rate of revolution.
30. What do you mean by group delay and phase delay ?
Ans The output envelope is the same as the input envelope delayed by
tg = −
1
2π
dθh(f)
df
called as group delay. If output carrier is the same as the input carrier delayed by
tp = −
θh(f)
2πf
is called as phase delay.
31. What is uniform and non-uniform quantization ?
Ans Quantization is process, where amplitude of signal is divided into several chunks. If this
division is done in a uniform fashion then it is called as uniform quantization otherwise it is
a non-uniform quantization.
32. What do you mean by source coding, channel coding, and line coding ?
Ans Fundamental differences between all coding schemes are as follows
(a) Source coding: It is process to reduce the redundancy so that bandwidth requirement
became less in communication system.
(b) Channel coding: It is process by which we add some redundancy with the information
bits so that the error detection and correction could be easily possible. Due to redundant
bit addition the BER reduces with greater extent in the communication system.
(c) Line coding: It is process by which information bits are converted into electrical pulses
that is used for following purposes.
– Bandwidth efficiency
– Power efficiency
– Reducing the BER
– Maintaining the transparency for long bit stream.
Most popular line coding are like, RZ, NRZ, polar, manchester, etc.
33. What is the difference between regenerative repeaters and amplifier ?
Ans The basic difference between regenerative repeaters and amplifier are as follow.
12

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