Lecture #2 - Antennas and Beamforming Flashcards
What is the difference between a transmitter and receiver antenna?
A transmitting (Tx) antenna radiates electromagnetic energy into space.
A receiving (Rx) antenna collects electromagnetic energy from space.
If you were to buy a directional antenna, would you prefer to see its power radiation pattern in dB scale or linear scale? Why?
dB, to have a better understanding of the side lobe level
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?
3dB
What happens if the distance between consecutive elements of an antenna array is made much larger than the operating wavelength?
Grating lobes
How high is the side lobe level of a uniform linear array? How can we reduce the side lobe level of an array?
-13 dBc, by amplitude tapering
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?
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.
Antennas are reciprocal devices. What does this mean?
Tx can be used as Rx and vice versa
Name the three antennas types and explain them
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)
The 3D space surrounding the antenna is divided into how many regions and name them.
Reactive field, radiating near field and far field.
Explain the concept of
Half power beam width
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
Explain the concept of first null beam width
the angular separation in which the magnitude of the radiation pattern reduces from the peak value to - โ 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
Give the definition of Directivity
the ratio of maximum power measured in a particular direction (Pmax) to the average power radiated across all directions (๐ )
Give the definition for Gain (G)
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
Give the definition for Efficiency
The ratio of total radiated power to the input to antenna
What happens when thereโs a gain of 3dBi?
The antennaโs peak radiation is 2x stronger than a lossless isotropic antenna with the same power
Which losses does effiency take into account
Conduction, dielectric, reflection
In antenna impedance exists a real and an imaginary part. What does the real and imaginary part represent?
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.
When is the maximum power transferred from the generator to the antenna?
If the Z_I or Z_E is the complex conjugate of Z_A
Explain the three types of polarisation
Name and explain the three types of polarisation
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
What happens to captured power in terms of conjugate matching.
Half of the captured power is delivered to load and the rest is scattered and lost as heat.
Give the equation for capture area
Capture area = effective area + scattering area + loss area
What is the effective area?
(๐ด๐) times the incident power density of plane wave gives the power available at the Rx terminals of an antenna
Explain the concept of a Uniform Linear Array Antenna
A uniformly-spaced linear array antenna consists of N antennas in a line.
- The antennas are equally spaced
- The antennas are fed with an equal amplitude
- The antennas are fed in phase i.e. ฮ๐ = 00
What are the array factor factors?
The array factor depends on the following factors:
Number of elements (๐): Increasing ๐ reduces the beamwidth & increases the number of nulls.
Element spacing as a function of wavelength (๐/๐): ๐/๐ is fixed at 0.5 for now.
Phase shift between elements (ฮ๐): As ฮ๐ 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 ๐ฮค๐ = 0.5
How is the radiation pattern obtained?
Array factor x individual antenna pattern
Explain the concept of a boresight direction
For a uniform linear array (ULA) antenna, the boresight direction is typically the direction perpendicular to the arrayโs 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 antennaโs 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.
What happens when the beam steers away from the boresight direction
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
In a uniformly-spaced, uniformly-amplitude, the side lobe level is fixed at what dBc and why?
The side lobe level is fixed at -13 dBc, since the array factor resembles a sign function
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 ๐0 =ฬธ 00
How can beam quint be avoided?
Beam squint can be avoided if true time delay units are used instead of phase shifters. Its implementation depends on the beamforming architecture.
What does mm-wave antenna arrays provide?
Higher gain
Give the advantage of mm-waves in antennas.
Mm-wave antennas provide a smaller form factor; hence, larger arrays can be cramped in a limited space, providing a much higher gain.
Why is beamforming necessary?
In a wireless communication system, the spectral efficiency can be improved if:
A base station (BS) can simultaneously communicate with multiple user equipments (UEs). โ serving more number of users simultaneously.
A BS can send multiple data streams to each UE โ 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.
How does beamforming work?
A signal processing technique used at the Tx or Rx side to:
- Direct the main lobe towards the desired direction
- Suppress the side lobes pointing in undesired directions
Explain the functionality of analog beamforming
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.
Explain the functionality of digital beamforming
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.
Explain hybrid beamforming
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