Wireless Interview Questions and Answers

This page provides a collection of frequently asked questions (FAQs) related to wireless communication and signal processing. These questions are designed to help individuals preparing for interviews and can also be useful as viva questions.

Here’s a list of wireless FAQs:

Basic Wireless Concepts

1. What is OFDM?

OFDM stands for Orthogonal Frequency Division Multiplexing. It’s a multi-carrier modulation technique that divides a high-data-rate stream into multiple lower-data-rate streams, which are then transmitted simultaneously over a number of orthogonal subcarriers. This helps to minimize inter-symbol interference (ISI) and improve spectral efficiency.

2. Explain advantages and disadvantages of OFDM over a single carrier system

Advantages:

• High Spectral Efficiency: OFDM packs more data into a given bandwidth.
• Robustness to Multipath Fading: The use of multiple subcarriers makes the system less susceptible to frequency-selective fading.
• Simplified Equalization: Equalization becomes simpler as the channel is divided into smaller sub-channels.
• Resistant to ISI: OFDM’s use of a cyclic prefix helps mitigate ISI.

Disadvantages:

• Sensitivity to Frequency Offset and Phase Noise: OFDM systems are more vulnerable to these impairments.
• High Peak-to-Average Power Ratio (PAPR): This can lead to distortion in power amplifiers.
• Complexity: OFDM systems are generally more complex to implement than single-carrier systems.

3. What is OFDMA?

OFDMA stands for Orthogonal Frequency Division Multiple Access. It’s a multi-user version of the OFDM modulation scheme. In OFDMA, multiple users are assigned different subcarriers, allowing them to transmit simultaneously.

4. What is the difference between OFDM and OFDMA?

OFDM is a modulation technique that divides a single channel into multiple subcarriers. OFDMA, on the other hand, is a multiple access technique that allows multiple users to share the same channel by assigning them different subcarriers within the OFDM framework. OFDM is a modulation scheme, while OFDMA is a multiple access scheme.

Antenna Fundamentals

5. What is the gain of an antenna?

Antenna gain is a measure of how well an antenna focuses radio frequency (RF) energy in a particular direction compared to an isotropic antenna (which radiates equally in all directions). It’s usually expressed in dBi (decibels relative to an isotropic antenna).

6. What is the beamwidth of an antenna?

The beamwidth of an antenna is the angle (in degrees) within which the radiated power is at least half of the maximum power. It’s a measure of the antenna’s directivity. A narrower beamwidth means the antenna is more focused.

7. What is G/T of an antenna?

G/T stands for Gain-to-Noise Temperature ratio. It is a figure of merit used to characterize the performance of an antenna system, especially in satellite communications. It is calculated by dividing the antenna gain (G) by the system noise temperature (T).

Wireless System Parameters

8. What is the Friis path loss formula?

The Friis transmission equation calculates the power received by a receiving antenna from a transmitting antenna over a free space path of distance d. The formula is:

Where:

• $P_r$ is the received power (in dBm or Watts)
• $P_t$ is the transmitted power (in dBm or Watts)
• $G_t$ is the gain of the transmitting antenna (in dBi)
• $G_r$ is the gain of the receiving antenna (in dBi)
• $\lambda$ is the wavelength of the signal (in meters)
• $d$ is the distance between the antennas (in meters)

9. What is the function of a satellite transponder?

A satellite transponder receives signals from an uplink station, amplifies them, changes their frequency, and retransmits them back to Earth on a downlink frequency. It essentially acts as a repeater in space.

10. How are the basic parameters do you test to validate a Wireless Transmitter?

To validate a wireless transmitter, you would typically test the following parameters:

• Output Power: Ensuring the transmitter is outputting the correct power level.
• Frequency Accuracy: Verifying that the carrier frequency is within the specified tolerance.
• Modulation Accuracy: Evaluating the quality of the modulated signal (e.g., EVM).
• Spurious Emissions: Measuring the levels of unwanted signals outside the intended bandwidth.
• Harmonics: Checking for harmonics of the carrier frequency.
• Occupied Bandwidth: Ensuring that the signal occupies the correct amount of bandwidth.

11. How are the basic parameters do you test to validate a Wireless Receiver?

To validate a wireless receiver, you would typically test the following parameters:

• Sensitivity: Determining the minimum signal level the receiver can reliably detect.
• Selectivity: Measuring the receiver’s ability to reject signals at adjacent frequencies.
• Blocking/Intermodulation: Assessing the receiver’s performance in the presence of strong interfering signals.
• Noise Figure: Measuring the amount of noise added by the receiver.
• Dynamic Range: Determining the range of signal levels the receiver can handle without distortion.

12. What is the difference between harmonics and spurious emissions?

• Harmonics are integer multiples of the fundamental frequency of the transmitted signal. They are related to the intended signal.
• Spurious emissions are unwanted signals generated by the transmitter at frequencies that are not harmonically related to the fundamental frequency. They can be caused by various nonlinearities and are generally undesirable.

13. What are the basic building blocks in a typical wireless system?

A typical wireless system includes the following basic building blocks:

• Transmitter: Includes components like the modulator, upconverter, power amplifier, and transmit antenna.
• Receiver: Includes components like the receive antenna, low noise amplifier (LNA), downconverter, demodulator.
• Channel: The medium (air, space, etc.) through which the signal propagates.
• Baseband Processing: Includes functions like source coding, channel coding, and data formatting.

14. What all impairments get incorporated from the transmitter section of data to be transmitted till the receiver section in a common wireless system?

Several impairments can affect a wireless signal as it travels from the transmitter to the receiver:

• Noise: Thermal noise, interference from other signals.
• Fading: Rayleigh fading, Rician fading due to multipath propagation.
• Path Loss: Signal attenuation due to distance.
• Interference: Co-channel interference, adjacent channel interference.
• Distortion: Nonlinearities in the transmitter and receiver.
• Frequency Offset and Phase Noise: Impairments in the local oscillators.
• I/Q Imbalance: Imperfections in the quadrature modulation and demodulation process.

15. What is Rayleigh Fading?

Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices. Rayleigh fading models assume that the magnitude of a signal that has passed through a transmission medium will vary randomly, or fade, according to a Rayleigh distribution. This type of fading is caused by multipath propagation, where the signal arrives at the receiver via multiple paths with different delays and amplitudes.

16. What is BER?

BER stands for Bit Error Rate. It’s a measure of the number of bit errors that occur in a given number of transmitted bits. It is usually expressed as a ratio (e.g., 1 error in 100,000 bits, or BER = 10^-5). A lower BER indicates better performance.

17. What is EVM?

EVM stands for Error Vector Magnitude. It is a measure of the quality of a digitally modulated signal. It quantifies the difference between the ideal transmitted signal and the actual received signal. A lower EVM indicates better modulation quality.

Diversity and MIMO Techniques

18. Explain various types of Antenna Diversity?

Antenna diversity is a technique used to improve the reliability of a wireless communication system by providing multiple independent signal paths. Common types of antenna diversity include:

• Spatial Diversity: Using multiple antennas spaced far enough apart so that they experience independent fading.
• Polarization Diversity: Using antennas with different polarizations to capture different signal components.
• Frequency Diversity: Transmitting the same data on multiple frequencies.
• Time Diversity: Transmitting the same data at different times.

19. Explain techniques of MIMO?

MIMO stands for Multiple-Input Multiple-Output. It’s a wireless communication technology that uses multiple antennas at both the transmitter and receiver to improve data rates and link reliability. Common MIMO techniques include:

• Spatial Multiplexing: Transmitting multiple independent data streams simultaneously on different antennas to increase data rates.
• Spatial Diversity (e.g., Alamouti coding): Transmitting redundant copies of the data on different antennas to improve link reliability.
• Beamforming: Focusing the transmitted signal towards the intended receiver to increase signal strength and reduce interference.

20. What is SIMO?

SIMO stands for Single-Input Multiple-Output. It’s a wireless communication technique that uses a single antenna at the transmitter and multiple antennas at the receiver. SIMO can improve link reliability through diversity techniques.

21. Why is Forward Error Correction required in a wireless system?

Forward Error Correction (FEC) is required in wireless systems to improve the reliability of data transmission. Wireless channels are prone to errors due to noise, fading, and interference. FEC adds redundant information to the transmitted data, allowing the receiver to detect and correct errors without requiring retransmission.

22. How is the data rate calculated for broadband technologies viz, WLAN and WiMAX based on OFDM?

The data rate for broadband technologies like WLAN and WiMAX based on OFDM depends on several factors:

• Number of Subcarriers: The more subcarriers, the higher the potential data rate.
• Modulation Order: Higher-order modulation schemes (e.g., 64-QAM) allow more bits to be transmitted per symbol.
• Coding Rate: The coding rate of the FEC code.
• Bandwidth: The wider the bandwidth, the higher the data rate.
• MIMO: The number of spatial streams used in MIMO.

The formula for calculating data rate is generally proportional to:

23. Why is BPSK preferable for the critical part of the frame to be transmitted in a communication system?

BPSK (Binary Phase Shift Keying) is often preferred for critical parts of the frame because it is the most robust modulation scheme against noise and interference. It has the lowest spectral efficiency but is very reliable. This makes it ideal for transmitting control information or synchronization signals where reliability is paramount.

24. What is the difference between BPSK and QPSK modulation schemes?

• BPSK (Binary Phase Shift Keying): Encodes one bit per symbol by shifting the phase of the carrier signal by either 0 or 180 degrees.
• QPSK (Quadrature Phase Shift Keying): Encodes two bits per symbol by shifting the phase of the carrier signal by 0, 90, 180, or 270 degrees. QPSK has twice the spectral efficiency of BPSK but is more susceptible to noise.

25. What is the difference between QPSK and OQPSK?

• QPSK (Quadrature Phase Shift Keying): The in-phase (I) and quadrature (Q) components of the signal can change simultaneously, which can result in large phase transitions of 180 degrees.
• OQPSK (Offset Quadrature Phase Shift Keying): Introduces a half-symbol delay between the I and Q components. This limits the maximum phase transition to 90 degrees, reducing amplitude fluctuations and improving performance in nonlinear channels.

26. Why is a scrambler used in a wireless system?

A scrambler is used in wireless systems to randomize the data being transmitted. This helps to:

• Prevent long sequences of 0s or 1s: These sequences can cause problems with clock recovery and synchronization at the receiver.
• Improve spectral characteristics: Scrambling helps to spread the signal’s energy more evenly across the spectrum.
• Reduce inter-symbol interference (ISI): By making the data more random.

RF System Components

27. Can you explain the function of AGC in a wireless system?

AGC stands for Automatic Gain Control. It’s a circuit that automatically adjusts the gain of the receiver to maintain a constant signal level at the output, regardless of the input signal strength. This prevents the receiver from being overloaded by strong signals and ensures that weak signals are amplified sufficiently.

28. Can you explain the effect of I-Q DC offset in the transmitter?

I-Q DC offset in the transmitter occurs when there is a DC voltage offset in the in-phase (I) or quadrature (Q) components of the modulated signal. This can result in:

• Carrier Leakage: The DC offset can cause a portion of the carrier signal to leak through the modulator, resulting in unwanted spectral components.
• Degraded Modulation Quality: The DC offset can distort the constellation diagram and increase the EVM, reducing the data rate and increasing the BER.

29. Explain Dynamic range of ADC

The dynamic range of an Analog-to-Digital Converter (ADC) is the ratio between the largest and smallest signals that the ADC can accurately convert. It is usually expressed in decibels (dB). A larger dynamic range means the ADC can handle a wider range of signal levels.

OSI Model and Wireless Layers

30. Explain the OSI layer and function of each layer in brief

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven abstraction layers:

1. Physical Layer: Deals with the physical transmission of data over a communication channel (e.g., voltage levels, data rates, physical connectors).
2. Data Link Layer: Provides error-free transmission of data between two directly connected nodes (e.g., MAC addressing, framing, error detection).
3. Network Layer: Handles routing of data packets between different networks (e.g., IP addressing, routing protocols).
4. Transport Layer: Provides reliable and ordered delivery of data between applications (e.g., TCP, UDP).
5. Session Layer: Manages connections between applications (e.g., authentication, authorization).
6. Presentation Layer: Handles data formatting and encryption (e.g., data compression, encryption).
7. Application Layer: Provides network services to applications (e.g., HTTP, SMTP, FTP).

31. What is the basic function of the Physical (PHY) layer in a wireless system?

The Physical (PHY) layer in a wireless system is responsible for:

• Modulation and Demodulation: Converting digital data into analog signals for transmission and vice versa.
• Channel Coding and Decoding: Adding redundancy to the data to protect against errors and correcting errors at the receiver.
• Signal Processing: Performing tasks like equalization, filtering, and synchronization.
• RF Transmission and Reception: Transmitting and receiving radio frequency signals through the air.

32. What is the basic function of the MAC layer in a wireless system?

The MAC (Medium Access Control) layer in a wireless system is responsible for:

• Controlling access to the shared wireless medium: Preventing collisions and ensuring fair access for all devices.
• Addressing and Framing: Adding MAC addresses to data packets and creating frames for transmission.
• Error Detection and Correction: Detecting errors in received frames and requesting retransmission if necessary.
• Prioritization: Prioritizing certain types of traffic over others.

33. What is DAMA?

DAMA stands for Demand Assigned Multiple Access. It’s a technique used in satellite communication where bandwidth is allocated to users on demand, rather than being pre-assigned. This allows for more efficient use of satellite resources.

34. What is the difference between FDMA and TDMA?

• FDMA (Frequency Division Multiple Access): Divides the available bandwidth into multiple frequency channels, and each user is assigned a specific frequency channel.
• TDMA (Time Division Multiple Access): Divides the available time into multiple time slots, and each user is assigned a specific time slot.

35. What is the function of OMT in VSAT?

OMT stands for Orthomode Transducer. In a VSAT (Very Small Aperture Terminal) system, the OMT is used to separate the transmit and receive signals, which typically use orthogonal polarizations (e.g., vertical and horizontal). This allows the VSAT to transmit and receive simultaneously using the same antenna.