Category: Wireless & Mobile Communications Objective Questions

Wireless & Mobile Communications Multiple Choice Questions on “Types of Small- Scale Fading”.

1. Which of the following is not a channel parameter?
a) Bandwidth
b) Coherence time
c) Rms delay spread
d) Doppler spread
Answer: a
Clarification: Channel parameters are coherence bandwidth, rms delay spread and Doppler spread. Signal parameters are bandwidth and symbol period. Different types of fading occur depending on the relation between channel and signal parameters.

2. ______ leads to time dispersion and frequency selective fading.
a) Doppler spread
b) Multipath delay spread
c) Time dispersive parameters
d) Frequency delay spread
Answer: b
Clarification: Multipath delay spread leads to time dispersion and frequency selective fading. Doppler spread leads to frequency dispersion and time selective fading. These two propagation mechanism are independent of each other.

3. Which of the following s not a characteristic of flat fading?
a) Mobile radio channel has constant gain
b) Linear phase response
c) Non linear phase response
d) Bandwidth is greater than the bandwidth of transmitted signal
Answer: c
Clarification: The received signal will undergo flat fading if mobile radio channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of transmitted signal. It is the most common type of fading.

4. Spectral characteristics of the channel changes with time in flat fading.
a) True
b) False
Answer: b
Clarification: In flat fading, the multipath structure of the channel is such that spectral characteristics of the transmitted signal are preserved at the receiver. But, the strength of the received signal changes with time due to fluctuations in the gain of channel caused by multipath.

5. Flat fading channel is also known as _______________
a) Amplitude varying channel
b) Wideband channel
c) Phase varying channel
d) Frequency varying channel
Answer: a
Clarification: Flat fading channel is also known as amplitude varying channel. They are also sometimes referred to as narrowband channel. In flat fading channel, the bandwidth of the applied signal is narrow as compared to the channel flat fading bandwidth.

6. In a frequency selective fading, mobile radio channel possess a constant gain and a linear phase over bandwidth smaller than bandwidth of transmitted signal.
a) True
b) False
Answer: a
Clarification: If the channel possesses a constant gain and linear phase response over a bandwidth that is smaller than the bandwidth of transmitted signal, then the channel creates frequency selective fading on the received signal. Frequency selective fading channel are much more difficult to model than flat fading channel.

7. Frequency selective fading channels are also known as ________
a) Narrowband channel
b) Wideband channel
c) Amplitude varying channel
d) Phase varying channel
Answer: b
Clarification: Frequency selective fading channels are also known as wideband channels. The bandwidth of the signal is wider than the bandwidth of channel impulse response.

8. Frequency selective fading does not induce intersymbol interference.
a) True
b) False
Answer: b
Clarification: Frequency selective fading is due to time dispersion of the transmitted symbols within the channel. Thus, the channel induces intersymbol interference (ISI). The channel have certain frequency components in the received signal spectrum that have greater gain than others.

9. For fast fading channel, the coherence time of the channel is smaller than _______ of transmitted signal.
a) Doppler spread
b) Bandwidth
c) Symbol period
d) Coherence bandwidth
Answer: c
Clarification: In fast fading channel, the channel impulse response changes rapidly within the symbol duration. Thus, the coherence time of the channel is much smaller than the symbol period of the transmitted signal.

10. In slow fading channel, Doppler spread of the channel is much less than the ________ of baseband signal.
a) Symbol period
b) Phase
c) Coherence time
d) Bandwidth
Answer: d
Clarification: Slow fading channel may be assumed to be static over one or several reciprocal bandwidth intervals. In the frequency domain, this implies that the Doppler spread of the channel is much less than the bandwidth of the baseband signals.

.

Wireless & Mobile Communications Questions and Answers for Campus interviews on “Algorithms for Adaptive Equalization”.

1. Which of the following factor could not determine the performance of algorithm?
a) Structural properties
b) Rate of convergence
c) Computational complexity
d) Numerical properties
Answer: a
Clarification: The performance of an algorithm is determined by various factors. These factors are rate of convergence, computational complexity and numerical properties. The performance of algorithm does not depend on structural properties.

2. Rate of convergence is defined by __________ of algorithm.
a) Time span
b) Number of iterations
c) Accuracy
d) Complexity
Answer: b
Clarification: Rate of convergence is required as number of iterations required for the algorithm to converge close enough to the optimum solution. It enables the algorithm to track statistical variations when operating in non stationary environment.

3. Computational complexity is a measure of ________
a) Time
b) Number of iterations
c) Number of operations
d) Accuracy
Answer: c
Clarification: Computational complexity is the number of operations required to make one complete iteration of the algorithm. It helps in comparing the performance with other algorithms.

4. Choice of equalizer structure and its algorithm is not dependent on ________
a) Cost of computing platform
b) Power budget
c) Radio propagation characteristics
d) Statistical distribution of transmitted power
Answer: d
Clarification: The cost of the computing platform, the power budget and the radio propagation characteristics dominate the choice of an equalizer structure and its algorithm. Battery drain at the subscriber unit is also a paramount consideration.

5. Coherence time is dependent on the choice of the algorithm and corresponding rate of convergence.
a) True
b) False
Answer: a
Clarification: The choice of algorithm and its corresponding rate of convergence depends on the channel data rate and coherence time. The speed of the mobile unit determines the channel fading rate and the Doppler spread, which is directly related to coherence time of the channel.

6. Which of the following is not an algorithm for equalizer?
a) Zero forcing algorithm
b) Least mean square algorithm
c) Recursive least square algorithm
d) Mean square error algorithm
Answer: d
Clarification: Three classic equalizer algorithm are zero forcing (ZF) algorithm, least mean squares (LMS) algorithm and recursive least squares (RLS) algorithm. They offer fundamental insight into algorithm design and operation.

7. Which of the following is a drawback of zero forcing algorithm?
a) Long training sequence
b) Amplification of noise
c) Not suitable for static channels
d) Non zero ISI
Answer: b
Clarification: The zero forcing algorithm has the disadvantage that the inverse filter may excessively amplify noise at frequencies where the folded channel spectrum has high attenuation.

8. Zero forcing algorithm performs well for wireless links.
a) True
b) False
Answer: b
Clarification: ZF is not often used in wireless links as it neglects the effect of noise altogether. However, it performs well for static channels with high SNR, such as local wired telephone links.

9. LMS equalizer minimizes __________
a) Computational complexity
b) Cost
c) Mean square error
d) Power density of output signal
Answer: c
Clarification: LMS equalizer is a robust equalizer. It is used to minimize mean square error (MSE) between the desired equalizer output and the actual equalizer output.

10. For N symbol inputs, LMS algorithm requires ______ operations per iterations.
a) 2N
b) N+1
c) 2N+1
d) N²
Answer: c
Clarification: The LMS algorithm is the simplest algorithm. For N symbol inputs, it requires only 2N+1 operations per iteration.

11. Stochastic gradient algorithm is also called ________
a) Zero forcing algorithm
b) Least mean square algorithm
c) Recursive least square algorithm
d) Mean square error algorithm
Answer: b
Clarification: The minimization of the MSE is carried out recursively, and it can be performed by the use of stochastic gradient algorithm. This more commonly called the least mean square (LMS) algorithm.

12. Convergence rate of LMS is fast.
a) True
b) False
Answer: b
Clarification: The convergence rate of the LMS algorithm is slow. It is slow due to the fact that it uses only one parameter i.e. step size that control the adaptation rate.

13. Which of the following does not hold true for RLS algorithms?
a) Complex
b) Adaptive signal processing
c) Slow convergence rate
d) Powerful
Answer: c
Clarification: Recursive least square (RLS) algorithm uses fast convergence rate as opposed to LMS algorithms. They are powerful, albeit complex, adaptive signal processing techniques which significantly improves the convergence of adaptive equalizer.

14. Which of the following algorithm uses simple programming?
a) LMS Gradient DFE
b) FTF algorithm
c) Fast Kalman DFE
d) Gradient Lattice DFE
Answer: a
Clarification: Advantages of LMS gradient DFE algorithm are low computational complexity and simple programming. While fast tranversal filter (FTF) algorithm, Fast Kalman DFE and gradient lattice DFE uses complex programming.

of Wireless & Mobile Communications for Campus Interviews, .

Wireless & Mobile Communications Multiple Choice Questions on “Capacity of Cellular Systems”.

1. ____________ is the maximum number of channels that can be provided in a fixed frequency band.
a) Channel capacity
b) Radio capacity
c) Spectral capacity
d) Carrier capacity
Answer: a
Clarification: Channel capacity for a radio system can be defined as the maximum number of channels or users that can be provided in a fixed frequency band. It is useful in computer science, in electrical engineering, and in other disciplines evaluating the capacity of a channel or conduit.

2. Which of the following measures spectrum efficiency of a wireless system?
a) Channel capacity
b) Radio capacity
c) Spectral capacity
d) Carrier capacity
Answer: b
Clarification: Radio capacity is a parameter which measures spectrum efficiency of a wireless system. This parameter is determined by the required carrier to interference ratio and the channel bandwidth.

3. If D is the distance between co-channel cells and R be the cell radius, co-channel reuse ratio is given by __________
a) D*R
b) D²/R
c) D/R
d) D/R²
Answer: c
Clarification: Let, D is the distance between co-channel cells and R be the cell radius. Then the minimum ratio of D/R that is required to provide a tolerable level of co-channel interference is called the co-channel reuse ratio.

4. __________ is the interference at a base station receiver that comes from the subscriber units in the surrounding cells.
a) Forward channel interference
b) Carrier interference
c) Receiver interference
d) Reverse channel interference
Answer: d
Clarification: In a cellular system, the interference at a base station receiver will come from the subscriber units in the surrounding cells. This is called reverse channel interference.

5. In practice, TDMA systems improve capacity by ____________ compared to analog cellular systems.
a) Three to six times
b) Equal capacity
c) Two time
d) Ten to twenty times
Answer: a
Clarification: In practice, TDMA systems improve capacity by a factor of three to six times as compared to analog cellular radio systems. Powerful error control and speech coding enable better link performance in high interference environment.

6. __________ allows subscribers to monitor neighbouring base stations.
a) TDMA
b) MAHO
c) FDMA
d) ACA
Answer: b
Clarification: Mobile assisted handoff (MAHO) allows subscribers to monitor the neighbouring base stations, and the best base station choice may be made by each subscriber. MAHO allows deployment of densely packed microcells, thus giving substantial capacity gains in a system.

7. Time division multiple access introduces ACA, which stands for ________
a) Acute carrier allocation
b) Adaptive carrier allocation
c) Adaptive channel allocation
d) Acute channel allocation
Answer: c
Clarification: TDMA makes it possible to introduce adaptive channel allocation (ACA). ACA eliminates system planning since it is not required to plan frequencies for cells.

8. Which of the following standard had not adopted digital TDMA for high capacity?
a) USDC
b) GSM
c) PDC
d) IS-95
Answer: d
Clarification: Various proposed standards such as the GSM, US digital cellular (USDC), and Pacific Digital Cellular (PDC) have adopted digital TDMA for high capacity. IS-95 does not use TDMA.

9. Capacity of CDMA is bandwidth limited.
a) True
b) False
Answer: b
Clarification: The capacity of CDMA systems is interference limited. It is bandwidth limited in FDMA and TDMA. Therefore, any reduction in the interference will cause a linear increase in the capacity of CDMA.

10. In a CDMA system, link performance for each user __________ as the number of users _______
a) Increase, decrease
b) Increase, increase
c) Decrease, increase
d) Decrease, decrease
Answer: a
Clarification: In a CDMA system, the link performance for each user increases as the number of users decreases. A straightforward way to reduce interference is to use multisectorized antennas, which results in spatial isolation of users.

11. Capacity of CDMA can be increased by operating in DTX, which stands for _______
a) Discrete transmission mode
b) Discrete transmission modulation
c) Discontinuous transmission mode
d) Digital transmission mode
Answer: c
Clarification: Capacity of CDMA can be increased by operating in DTX, which stands for Discontinuous transmission mode. It takes advantage of the intermittent nature of speech. In DTX, the transmiiter is turned off during the periods of silence in speech.

12. Frequency reuse factor for CDMA system is ___________
a) One
b) Two
c) Zero
d) Ten
Answer: a
Clarification: The amount of out-of-cell interference determines the frequency reuse factor, f, of a CDMA cellular system. Ideally, each cell shares the same frequency and the maximum possible value of f (f=1) is achieved.

.

Wireless & Mobile Communications Multiple Choice Questions on “TCP over Wireless”.

1. A packet in Transmission Control Protocol (TCP) is called a ____________
a) Transmittable slots
b) Packet
c) Segment
d) Source Slots
Answer: c
Clarification: Applications working at the Application Layer transfers a contiguous stream of bytes to the bottom layers. It is the duty of TCP to pack this byte stream to packets, known as TCP segments, which are passed to the IP layer for transmission to the destination device.

2. Cable TV and DSL are examples of ____________
a) Interconnection of network
b) LAN
c) MAN
d) WAN
Answer: c
Clarification: A MAN often acts as a high speed network to allow sharing of regional resources. It typically covers an area of between 5 and 50 km diameter. Examples of MAN are telephone company network that provides a high speed DSL to customers and cable TV network.

3. Station on a wireless ALOHA network is maximum of ________
a) 400 Km
b) 500 Km
c) 600 Km
d) 700 Km
Answer: c
Clarification: The stations on wireless ALOHA networks are a maximum of 600 km apart. It was designed for a radio (wireless) LAN, but it can be used on any shared medium. It is obvious that there are potential collisions in this arrangement. The medium is shared between the stations.

4. IEEE 802.11 defines basic service set as building block of a wireless ___________
a) LAN
b) WAN protocol
c) MAN
d) ALOHA
Answer: a
Clarification: The IEEE 802.11 topology consists of components interacting to provide a wireless LAN. It enables station mobility transparent to higher protocol layers, such as the LLC.

5. In wireless LAN, there are many hidden stations so that __________ cannot be detected.
a) Frames
b) Collision
c) Signal
d) Data
Answer: b
Clarification: In wireless networking, the hidden node problem or hidden terminal problem occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with said AP. This leads to difficulties in media access control and collisions could not be detected.

6. A set that makes stationary or mobile wireless station and also have optional central base station is known as ___________
a) Basic service set
b) Extended service set
c) Network point set
d) Access point
Answer: a
Clarification: A set that makes stationary or mobile wireless station and also have optional central base station is known as a basic service set. BSS is made of stationary or mobile wireless stations and an optional central base station, known as the access point (AP).

7. Wireless communication started in _________
a) 1869
b) 1895
c) 1879
d) 1885
Answer: b
Clarification: In England, Guglielmo Marconi began his wireless experiments in 1895. On 2 June 1896, he filed his provisional specification of a patent for wireless telegraphy. He demonstrated the system to the British Post Office in July.

8. Wireless transmission is divided into ___________
a) 3 broad groups
b) 6 broad groups
c) 9 broad groups
d) 8 broad groups
Answer: a
Clarification: We can divide wireless transmission into three broad groups: radio waves, microwaves, and infrared waves. Radio waves are used for multicast communications, such as AM and FM radio, television, maritime radio, cordless phones and paging systems. Microwave propagation is line-of-sight.

9. Transmission Control Protocol/Internet Networking Protocol have ___________
a) Four Layers
b) Five Layers
c) Six Layers
d) Seven Layers
Answer: a
Clarification: TCP/IP functionality is divided into four layers, each with its own set of agreed-upon protocols: The datalink layer consists of methods and protocols that operate only on a link. The Internet layer connects independent networks to transport the packets. The Transport layer handles communications between. The Application layer standardizes data exchange for applications.

10. Packets of data that is transported by IP is called __________
a) Datagrams
b) Frames
c) Segments
d) Encapsulate message
Answer: a
Clarification: The format of data that can be recognized by IP is called an IP datagram. It consists of two components, the header and data, which need to be transmitted. The fields in the datagram, except the data, have specific roles to perform in the transmission of data.

11. Parameter that is normally achieved through a trailer added to end of frame is ___________
a) Access Control
b) Flow Control
c) Error Control
d) Physical addressing
Answer: c
Clarification: The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

.

Wireless & Mobile Communications Multiple Choice Questions on “Handoff Strategies”.

1. What is the condition for handoff?
a) A mobile moves into a different cell while in conversation
b) A mobile remains in the same cell while in conversation
c) A mobile moves to different cell when idle
d) A mobile remains in the same cell and is idle
Answer: a
Clarification: Handoff occurs when a mobile moves into a different cell while a conversation is in progress. The MSC automatically transfers the call to a new channel belonging to the new base station.

2. Handoff does not require voice and control channel to be allocated to channels associated with the new base station.
a) True
b) False
Answer: b
Clarification: Handoff operation involves identifying a new base station. It also requires that the voice and control signal be allocated to channels associated with the new base station.

3. The time over which a call can be maintained within a cell without handoff is called _________
a) Run time
b) Peak time
c) Dwell time
d) Cell time
Answer: c
Clarification: The time over which a call is maintained within a cell without handoff is called as dwell time. Dwell time vary depending on speed of user and type of radio coverage.

4. Dwell time does not depend on which of the following factor?
a) Propagation
b) Interference
c) Distance between subscriber and base station
d) Mobile station
Answer: d
Clarification: Dwell time of a particular user is governed by a number of factors. They include propagation, interference, distance between the subscriber and the base station, and other time varying effects.

5. Which of the following is associated with the handoff in first generation analog cellular systems?
a) Locator receiver
b) MAHO
c) Cell dragging
d) Breathing cell
Answer: a
Clarification: Locator receiver is a spare receiver in each base station. It is used to scan and determine signal strengths of mobile users which are in neighbouring cells.

6. MAHO stands for ______
a) MSC assisted handoff
b) Mobile assisted handoff
c) Machine assisted handoff
d) Man assisted handoff
Answer: b
Clarification: MAHO stands for mobile assisted handoff. In 2G systems, handoff decisions are mobile assisted. In MAHO, every mobile station measure the received power from surrounding base station and continuously reports the results to serving base station.

7. A handoff is initiated when the power received from the base station of a neighbouring cell falls behind the power received from the current base station by certain level.
a) True
b) False
Answer: b
Clarification: MAHO measures the power received from the surrounding base station. And a handoff is initiated when the power received from the base station of a neighbouring cell begins to exceed the power received from current base station.

8. What is the condition for intersystem interference?
a) Mobile moves from one cell to another cell
b) Mobile remains in the same cell
c) Mobile moves from one cellular system to another cellular system
d) Mobile remains in the same cluster
Answer: c
Clarification: An intersystem handoff is initiated when a mobile moves from one cellular system to another during a course of a call. An MSC engages in an intersystem interference when a mobile becomes weak in a given cell and MSC cannot find another cell to which call can be transferred.

9. What is the disadvantage of guard channel?
a) Efficient utilization of spectrum
b) Cross talk
c) Near far effect
d) Reduce total carried traffic
Answer: d
Clarification: Guard channel is a concept for handling priority in handoff. Here, a fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls. This method has the disadvantage of reducing the total carried traffic, as fewer channels are allocated to originating calls.

10. Which of the following priority handoff method decrease the probability of forced termination of a call due to lack of available channels?
a) Queuing
b) Guard channel
c) Cell dragging
d) Near far effect
Answer: a
Clarification: Queuing of handoff requests is a method to decrease the probability of forced termination of a call due to lack of available channels. Queuing of handoff is possible due to the fact that there is a finite time interval between the time the received signal level drops below the handoff threshold and the time the call is terminated.

11. Umbrella cell approach is possible by using _________
a) Antenna of same heights
b) Antenna of different heights
c) Different voice channels
d) Different control channels
Answer: b
Clarification: Umbrella cell approach is possible by using different antenna heights and different power levels. By using this approach, it is possible to provide large and small cells which are co-located at a single location.

12. Cell dragging is a problem occur due to __________
a) Pedestrian users
b) Stationary users
c) High speed mobile systems
d) Base stations having same frequency
Answer: a
Clarification: Cell dragging is a practical handoff problem in microcell system. It results from pedestrian users that provide a very strong signal to the base station.

13. What was the typical handoff time in first generation analog cellular systems?
a) 1 second
b) 10 seconds
c) 1 minute
d) 10 milliseconds
Answer: b
Clarification: In first generation analog cellular system, the typical time to make a handoff once the signal level is below the threshold, is about 10 seconds. This requires the value for threshold to be 6 dB to 12 dB.

14. How much time it takes for handoff in digital cellular systems like GSM?
a) 1 second
b) 10 seconds
c) 1 minute
d) 10 milliseconds
Answer: a
Clarification: In digital cellular systems, the mobile assista with the handoff procedure by determining the best candidate. Once the decision is made, it typically requires 1 to 2 seconds for handoff.

15. Soft handoff is also known as _________
a) MAHO
b) Hand over
c) Break before make
d) Make before break
Answer: d
Clarification: Soft handoff is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case, the connection with the receiver target is established before the connection to the source is broken, hence this handover is called make-before-break.

.

Wireless & Mobile Communications Multiple Choice Questions on “Rayleigh and Ricean Distribution”.

1. Which of the following distribution is used for describing statistical time varying nature of received envelope of multipath component?
a) Log normal distribution
b) Levy distribution
c) Rayleigh distribution
d) Gaussian distribution
Answer: c
Clarification: Rayleigh distribution is the most common distribution for statistical modelling. It is used to describe the statistical time varying nature of the received envelope of a flat fading signal. It also describes the envelope of an individual multipath component.

2. Envelope of the sum of two quadrature Gaussian noise signal obeys _________ distribution.
a) Rayleigh
b) Inverse Gaussian
c) Nakagami
d) Gamma
Answer: a
Clarification: It is well known that the envelope of the sum of two quadrature Gaussian noise signal obeys Rayleigh distribution. This fading distribution could be applied to any scenario where there is no line of sight path between transmitter and receiver antennas.

3. For a Rayleigh fading signal, mean and median differ by _______
a) 2 dB
b) 10 dB
c) 0.55 dB
d) 100 dB
Answer: c
Clarification: The mean and median differ by only 0.55 dB in a Rayleigh fading signal. The differences between the rms values and the other two values are higher.

4. It is easy to compare different fading distributions using mean values instead of median values.
a) True
b) False
Answer: b
Clarification: By using median values instead of mean values, it is easy to compare different fading distributions which may have widely varying means. Median is used in practice since fading data is measured in the field and a particular distribution cannot be assumed.

5. For a nonfading signal component present, the small scale fading envelope distribution is ____________
a) Rayleigh
b) Gaussian
c) Log normal
d) Ricean
Answer: d
Clarification: The small scale fading envelope is Ricean when there is a dominant stationary (nonfading) signal component, such as line of sight propagation path. In such a situation, random multipath components arriving at different angles are superimposed on a stationary dominant signal.

6. Ricean distribution degenerates to ________ distribution when the dominant component fades away.
a) Log normal
b) Gamma
c) Rayleigh
d) Gaussian
Answer: c
Clarification: Ricean distribution degenerates to Rayleigh distribution when the dominant component fades away. As the dominant signal becomes weaker, the composite signal resembles a noise signal which has an envelope that is Rayleigh.

7. The envelope of a bandpass noise is __________
a) Uniformly distributed
b) Rayleigh
c) Ricean
d) Gaussian
Answer: b
Clarification: The envelope of only bandpass noise is Rayleigh distribution. Rayleigh distribution is a continuous probability density function for positive random variables.

8. The envelope of a sinusoid plus bandpass noise has __________
a) Uniformly distributed
b) Rayleigh
c) Ricean
d) Gaussian
Answer: c
Clarification: The envelope of a sinusoid plus bandpass noise has Ricean distribution. In probability theory, Ricean distribution is the probability distribution which has magnitude of a circular bivariate normal random variable with potentially non-zero mean.

9. What do you call an attenuation that occurs over many different wavelengths of the carrier?
a) Rayleigh fading
b) Ricean fading
c) Wavelength fading
d) Slow fading
Answer: d
Clarification: Slow fading does not vary quickly with the frequency. It originates due to effect of mobility. Slow fading is the result of signal path change due to shadowing and obstructions such as tree or buildings etc.

10. Which of the reception problems below that is not due to multipath?
a) Delayed spreading
b) Rayleigh fading
c) Random Doppler shift
d) Slow fading
Answer: d
Clarification: Slow fading arises when the coherence time of the channel is large relative to the delay requirement of the application. Slow fading is caused by events such as shadowing, where a large obstruction such as a hill or large building obscures the main signal path between the transmitter and the receiver.

.