Hybrid Beamforming basics | Hybrid Beamforming types
This page describes hybrid beamforming basics and types. It compares hybrid beamforming types viz. partially connected and fully connected. It mentions advantages and disadvantages of hybrid beamforming types. Difference between architectures of all the beamforming types viz. analog beamforming, digital beamforming and hybrid beamforming are covered.
Introduction: The purpose of beamforming is to achieve directional signal transmission and reception as well as power variation. This has become possible due to provision of amplitude and phase variation in beamforming system. Antenna arrays with provision of amplitude/phase variation are employed for this purpose.
Following are the types of beamforming:
• Analog Beamforming
• Digital Beamforming
• Hybrid Beamforming
Before we jump into hybrid beamforming, let us understand basic concepts used in analog beamforming and digital beamforming types.
Analog Beamforming architecture
In analog beamforming, amplitude/phase variation is applied to analog signal at transmit end. At the receive end, the signals from different antennas are summed up before the ADC conversion in analog beamforming. The figure-1 depicts transmitter and receiver parts used in analog beamforming.
Wk= ak *ejsin(θk)
Wk= ak*cos(θk) + j* ak sin(θk)
Wk represents complex weight for kth antenna in the array.
ak is relative amplitude of weight.
θk is phase shift of weight.
➨Weights are being applied to analog signals in analog beamforming.
Refer Analog beamforming vs Digital beamforming>> for block diagram based explaination on analog beamforming and digital beamforming types.
As mentined weights are applied to analog signal at RF frequencies or at IF frequencies. It is restricted to one RF chain even with large number of antenna arrays are being used.
Digital Beamforming architecture
In digital beamforming, amplitude/phase variation (Wk) is applied to digital signal after ADC/DDC conversion at the transmit end. At the receive end, the received signals from antennas are first passed from ADC converters and digital down converters (i.e. DDCs) before summation operation. The figure-2 depicts transmitter and receiver parts used in digital beamforming.
s(t) = x(t) +j*y(t)
Where, s(t) -> complex baseband signal
x(t) -> i(t) i.e. real part (I)
y(t) ->-q(t) i.e. imaginary part (Q)
j = SQRT(-1)
➨Complex weights are being applied to baseband signals (s(t)) in digital beamforming.
Unlike analog beamforming, digital beamforming supports multiple RF chains proportional to antenna elements in use. Here beamforming or precoding is applied to digital baseband signal.
What is Hybrid Beamforming?
This beamforming type utilizes advantages of both analog beamforming and digital beamforming type. Hence the name hybrid beamforming. Here precoding is applied to both analog domain and digital domains i.e. it employed precoding/beamforming at both radio frequency(RF) and baseband. Due to this, it has been adopted in millimeter wave radio based next generation mobile networks including 5G.
Hybrid Beamforming types | Advantages and disadvantages
Following are the two hybrid beamforming types.
• Partially connected hybrid beamforming (HB)
• Fully connected hybrid beamforming
The figure-3 depicts partially connected HB type. This architecture type uses separate antenna array (known as "sub-array") for RF beamformer of individual AFE/RF chain. Due to the architecture, this type is also known by the name sub-array hybrid beamforming.
The figure-4 depicts fully connected HB type. In this architecture type, RF beamformer of each of the AFE/RF chain make use of all the antennas. Here RF signals of different AFE/RF chains are combined before transmission through antenna. This is done for each of the antenna in the system. Due to the architecture, this type is also known by the name full-array hybrid beamforming.
In fully connected architecture type, additional components used to combine RF signals will incur signal attenuations and power losses. This is a real challenge in mmwave radio systems.
Partially connected architecture on the other hand, allows AFE/RF chain access to less number of antennas. This leads to serios drawbacks such as wide beamwidth, less directivity and strong interference from other chains. Inspite of these disadvantages, advanced MIMO techniques help achieve mitigation of interference in partially connected hybrid beamforming type. Moreover the partially connected architecture offers simple circuit design and less losses compare to fully connected architecture type.
The use of appropriate architecture is decided based on applications, hardware and software complexities, technologies etc. The reconfigurable hybrid beamforming is also evolving to leverage many benefits in single user MIMO based systems.
This basic short note on hybrid beamforming will help one understand beamforming types and their architectures. For more information, one can refer technology/standard specific design aspects.