Beamforming is an effective technique that enables precise directional coverage while significantly minimizing interference and noise from other directions.
Beamforming and MIMO are crucial aspects of modern communication systems in 5G, where significant challenges like increased path loss and throughput are demanded.
Massive MIMO and beamforming techniques are extensively used to enhance coverage, capacity, and spectral efficiency.
Antenna array patterns
Antenna elements spacing
The physical size of an antenna array depends on the spacing between the individual antenna elements and the number of AE which are important to maximize gain and directivity.
A linear array will consist of equally spaced antenna elements (AE) on horizontal or/and vertical straight so considering the distance between AE about half of the wavelength it is considered to be directly tied to the operating frequency :
FR1 --> for example on 3 Ghz --> λ= c/f = 10 cm. --> λ/2= 5 cm .
FR2 --> for example for 30 Ghz -->λ= c/f = 10 mm --> λ/2= 5 mm .
Increasing the distance between antenna elements will result in higher gain and performance; however, increasing the spacing between antenna elements
above > λ/2 results in additional lobes called grating lobes.
Generally, having a λ/2 spacing between antenna elements is a good way to balance coverage and interference, however, using a larger spacing like 2λ could be a viable option in certain situations. This would help reduce the half-power beam width and increase the gain of the main lobe with the trade-off of spreading power more widely away from the main lobe (interference to neighboring cells for example). This approach may be suitable, for example, in MU-MIMO scenarios with fewer antenna elements narrower beams are essential for efficient beam steering and UE discrimination.
Beamforming relies on constructive interference between the transmitted signals by each antenna element, and an array composed of a higher number of antenna elements provides additional gain and relatively narrow main lobes
The beamforming array gain could be modeled in the case of the lossless array as :
Array Gain (dB) = Element gain + 10 log10 (N) where N: represents the number of array elements.
we have simulated the following linear array of dipole antenna using the Matlab antenna toolbox ( Figure 1 ) which shows the change of pattern, gain, and directivity as the number of antenna elements increases for 4, 8,16, and 32.
Figure 1: Plot using different antenna element configuration
Beam Scanning
Assuming a phased array of a fixed number of antenna elements (AEs) positioned fixedly;
by manipulating the RF signal phases and amplitudes supplied to these elements, it becomes feasible to dynamically modify and mold the array factor (which reflects the collective impact of signals from all antenna elements). This modification leads to a particular radiation pattern that concentrates energy in preferred directions while reducing it in others this is the foundation of beamforming, where the signal directionality is dynamically adjusted without physically moving the antennas. Instead, signal steering is achieved by precisely controlling the phases and amplitudes at each antenna element.
However, for beam scanning 2 characteristics to be considered :
Scan Loss: As the beam of a phased array antenna is steered away from the boresight direction Θ = 0 the antenna's effective aperture gain decreases, this is known as scan loss which follows the cosine law , k a numeric value, typically in the 1.3 range as described by David Corman in this article :
So, for around 30-degree scanning, for example, a drop in directivity of about 0.9 dB is expected and considered for SSB sweeping.
Massive MIMO AE mapping
Antenna elements can be used for beamforming or transmitting multiple streams, or a combination of both, as described in Figure 3 with a reduced number of antenna elements available for beamforming (multiple subarrays). Each polarization, vertical and horizontal, in a cross-polarized antenna operates independently. This enables the antenna to transmit and receive signals in two perpendicular planes, which enhances signal diversity, minimizes interference, and enables MIMO functionality.
Note that only horizontal beamforming will be possible in the case of a one-dimensional configuration example 1Vx32H.
Figure 3: Massive MIMO 32 AE mapping
However, when the antenna element number is large relying on simple digital beamforming may not be an appropriate option due to cost power consumption and design complexity so hybrid analog-digital beam-forming as shown below could be an excellent option to reduce the cost and complexity of 5G equipment RF Chain will not be one to one mapping this is achieved by mapping each RF Chain only to the antenna elements belonging to the same subarray and this is achieved by dividing antenna elements into multiple subarrays as described by commscope
Analog beamforming
Digital beamforming
Hybrid beamforming
The hybrid beamforming system could be modeled before rx beamforming as described in Figure 4 :
Y=H*P*W*S+n
S: Modulated symbole or data stream
W: Represents digital precoding matrix applied in the baseband,
P: Represents analog beamforming matrix applied via phase shifters
H: Represents the wireless channel coefficient matrix.
n: noise added to the received signal
Figure 4: Hybrid beamforming
To tackle the beamforming challenges associated with rising costs and energy usage, the earlier approach combined digital beamforming at the baseband level with analog beamforming in the RF domain. Each RF chain is digitally controlled and mapped to a single subarray.
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