Lecture #2 - Antennas and Beamforming

Question 1

What is the difference between a transmitter and receiver antenna?

Answer 1

A transmitting (Tx) antenna radiates electromagnetic energy into space.

A receiving (Rx) antenna collects electromagnetic energy from space.

Question 2

If you were to buy a directional antenna, would you prefer to see its power radiation pattern in dB scale or linear scale? Why?

Answer 2

dB, to have a better understanding of the side lobe level

Question 3

For a line-of-sight link between a linearly-polarized transmit antenna and a circularly-polarized receive antenna, how much power (in dB) will be lost due to polarization mismatch?

Answer 3

3dB

Question 4

What happens if the distance between consecutive elements of an antenna array is made much larger than the operating wavelength?

Answer 4

Grating lobes

Question 5

How high is the side lobe level of a uniform linear array? How can we reduce the side lobe level of an array?

Answer 5

-13 dBc, by amplitude tapering

Question 6

If the beam of a phased array made up of elements shown on slide 23 is steered away from the boresight direction, how does it impact the main lobe?

Answer 6

The amplitude of main lobe reduces and its HPBW increases. Note, if the radiation pattern of the element is constant (isotropic/omnidirectional radiator), the main lobe width will not change.

Question 7

Antennas are reciprocal devices. What does this mean?

Answer 7

Tx can be used as Rx and vice versa

Question 8

Name the three antennas types and explain them

Answer 8

Isotropic: ideal antenna that radiates evenly in all directions

Omnidirectional: the radiation pattern is isotropic in a single plane e.g. dipole

Directional: the bulk of antenna radiation is transmitted or received along a specific direction e.g. parabolic reflector ( dish)

Question 9

The 3D space surrounding the antenna is divided into how many regions and name them.

Answer 9

Reactive field, radiating near field and far field.

Question 10

Explain the concept of
Half power beam width

Answer 10

the angular separation in which the magnitude of the radiation pattern reduces by 3 dB (i.e. one-half in linear scale) with respect to the peak of the main lobe

Question 11

Explain the concept of first null beam width

Answer 11

the angular separation in which the magnitude of the radiation pattern reduces from the peak value to - ∞ in dB & zero in linear scale (in theory).

Here, we are focused on where the signal has mass attenuation. Therefore, not at the main lobes but at the side lobes

Question 12

Give the definition of Directivity

Answer 12

the ratio of maximum power measured in a particular direction (Pmax) to the average power radiated across all directions (𝑃 )

Question 13

Give the definition for Gain (G)

Answer 13

Directivity shows the ability an antenna to focus radiated energy in a particular direction. Gain shows how well this ability is realised in practice, taking the antenna losses into considerstion

Question 14

Give the definition for Efficiency

Answer 14

The ratio of total radiated power to the input to antenna

Question 15

What happens when there’s a gain of 3dBi?

Answer 15

The antenna’s peak radiation is 2x stronger than a lossless isotropic antenna with the same power

Question 16

Which losses does effiency take into account

Answer 16

Conduction, dielectric, reflection

Question 17

In antenna impedance exists a real and an imaginary part. What does the real and imaginary part represent?

Answer 17

The real part represents the power I.e. radiated or absorbed in the antenna.

The imaginary part represents the owner I.e. stored in a the non-radiating near field.

Question 18

When is the maximum power transferred from the generator to the antenna?

Answer 18

If the Z_I or Z_E is the complex conjugate of Z_A

Question 19

Explain the three types of polarisation

Question 20

Name and explain the three types of polarisation

Answer 20

Linear polarization: There is only one component of E-field vector, whose tip traces a horizontal, vertical or slant line in time

Circular polarization: There are two components of E-field vector with equal magnitude & 900 phase difference. The tip of their sum traces a right- or left-hand circle in time

Elliptical polarization: There are two components of E-field vector with unequal magnitude & 900 phase difference. The tip of their sum traces a right- or left-hand ellipse in time

Question 21

What happens to captured power in terms of conjugate matching.

Answer 21

Half of the captured power is delivered to load and the rest is scattered and lost as heat.

Question 22

Give the equation for capture area

Answer 22

Capture area = effective area + scattering area + loss area

Question 23

What is the effective area?

Answer 23

(𝐴𝑒) times the incident power density of plane wave gives the power available at the Rx terminals of an antenna

Question 24

Explain the concept of a Uniform Linear Array Antenna

Answer 24

A uniformly-spaced linear array antenna consists of N antennas in a line.

1. The antennas are equally spaced
2. The antennas are fed with an equal amplitude
3. The antennas are fed in phase i.e. Δ𝜑 = 00

Question 25

What are the array factor factors?

Answer 25

The array factor depends on the following factors:

Number of elements (𝑁): Increasing 𝑁 reduces the beamwidth & increases the number of nulls.

Element spacing as a function of wavelength (𝑑/𝜆): 𝑑/𝜆 is fixed at 0.5 for now.

Phase shift between elements (Δ𝜑): As Δ𝜑 increases, the beam steers away from the boresight direction and the beamwidth widens. A beam angle of 300 requires a phase shift of 900 between elements for 𝑑Τ𝜆 = 0.5

Question 26

How is the radiation pattern obtained?

Answer 26

Array factor x individual antenna pattern

Question 27

Explain the concept of a boresight direction

Answer 27

For a uniform linear array (ULA) antenna, the boresight direction is typically the direction perpendicular to the array’s alignment. In a ULA, if the antenna elements are arranged in a straight line, the boresight direction would be perpendicular to that line.

The boresight direction is significant because it represents the antenna’s main lobe, which is the region of maximum radiation intensity or sensitivity. By steering the boresight direction, the antenna can focus its energy in a specific direction or towards a particular target of interest. This allows for increased gain, improved communication range, or enhanced radar detection capabilities.

Question 28

What happens when the beam steers away from the boresight direction

Answer 28

As the beam steers away from the boresight direction:
The mainlobe width and amplitude reduces compared to the array factor
The sidelobe level along the boresight remains the same
The sidelobe level reduces with an increasing angular separation from the boresight direction

Question 29

In a uniformly-spaced, uniformly-amplitude, the side lobe level is fixed at what dBc and why?

Answer 29

The side lobe level is fixed at -13 dBc, since the array factor resembles a sign function

Question 30

The beam squint problem refers to a phenomenon that occurs in phased array antennas when the main lobe of the radiation pattern shifts away from the desired direction as the frequency of operation changes. This shift in the main lobe direction is known as beam squint.

The beam angle is steered by changing the phase shift between array elements, but phase shift is a function of wavelength/frequency. As frequency changes, the phase shift changes causing a beam squint.

Beam squint occurs in phase-shifter based array antennas that radiate at an angle of 𝜃0 ≠ 00

Question 31

How can beam quint be avoided?

Answer 31

Beam squint can be avoided if true time delay units are used instead of phase shifters. Its implementation depends on the beamforming architecture.

Question 32

What does mm-wave antenna arrays provide?

Answer 32

Higher gain

Question 33

Give the advantage of mm-waves in antennas.

Answer 33

Mm-wave antennas provide a smaller form factor; hence, larger arrays can be cramped in a limited space, providing a much higher gain.

Question 34

Why is beamforming necessary?

Answer 34

In a wireless communication system, the spectral efficiency can be improved if:

A base station (BS) can simultaneously communicate with multiple user equipments (UEs). – serving more number of users simultaneously.

A BS can send multiple data streams to each UE – providing higher data rate, more number of services to each user

This means that we need multiple beams to be formed in 3D space where each beam shows a connection between BS and UE.

Question 35

How does beamforming work?

Answer 35

A signal processing technique used at the Tx or Rx side to:

1. Direct the main lobe towards the desired direction
2. Suppress the side lobes pointing in undesired directions

Question 36

Explain the functionality of analog beamforming

Answer 36

Simple implementation. The digital development effort is limited since fewer ADCs are used

Single RF chain and ADC/DAC lead to low power consumption and low cost.

Preferred in low-cost, low beam count systems. However, realizing high-performance RF phase shifters is difficult in CMOS. Phase & amplitude error with frequency and phase variation vs control voltage makes precise spectral adjustment difficult.

Amplitude weighting and phase shifting is applied in RF domain after every antenna.

Signals from multiple antennas are fed into a single RF Tx/Rx chain

The power-combined RF signal is converted into digital domain by using a single ADC

Single beam pointing in a specific direction is formed. Even if multiple RF chains are used to form multiple simultaneous beams, the signal power will be split up between these RF chains, thus decreasing the SNR.

Question 37

Explain the functionality of digital beamforming

Answer 37

Amplitude weighting and phase shifting is applied in digital baseband

Each antenna has a dedicated RF Tx/Rx chain, thus multiple RF chains are needed

The RF downconverted signals from multiple antennas are individually converted into digital domain by using multiple ADCs

Multiple beams pointing in different directions can be formed simultabeously. Digital processing enables duplication of data and independently- programmable parallel processing.

Challenging implementation due to large volume of digital data, synchronization, and physical size constraints for the electronics needed behind every radiating element.

Larger number of RF chains and power-hungry ADCs and DACs cause high power consumption and high cost.

Preferred when simultaneous beams in different directions are needed. Practical realization requires a trade-off between the number of beams and the bandwidth per beam while maintaining a constraint on the data rates required for the system.

Question 38

Explain hybrid beamforming

Answer 38

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