beamforming basic part-3

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(Qus: what are the types of beam forming?)

In this section let us know what the types of beam forming are. In general beam forming can be implemented in the digital domain, in the analogue domain or in a combination of both digital and analogue domain that is a hybrid domain. 

There are Advantages and disadvantages associated with each approach. Usually, we can say that digital beam forming is applicable to frequency range 1(450 MHz to 6 GHz) while analogue beam forming is applicable to frequency range 2(24.25 GHz to 52.60 GHz). 

(Qus: What is digital beam-forming and its drawback and benefit?) 

In case of digital beam forming the signal is pre coded in baseband processing before RF transmission. The pre coding is applied in the digital domain and there is a requirement to have multiple parallel RF chains. 

This is major drawback for digital beam forming when the number of controllable antenna elements is high as each RF chain is expensive and consumes high quantities of power. 

It becomes less practical to have RF chain for each controllable antenna element when the number of antenna element is high (e.g. when implementing beam forming for mmWave band). 

Multiple beams one per user can be generated simultaneously from the same set of antenna elements. This is possible as the signals for each UE can be separated in the digital domain and different beam forming weights can be applied to each signal. 

If the UE has sufficient spatial separation then they can be allocated the same set of resources blocks to improve both spectral efficiency and end user experience. 

This corresponds to multi user MIMO and this relies on spatial separation rather than frequency or time domain separation. If UE do not have sufficient spatial separation then they can be allocated dedicated sets of resource blocks and so this corresponds to single user MIMO which relies either on frequency domain or time domain separation. 

The primary benefit of digital beam forming is its flexibility. Digital beam forming improves the cell capacity as the same PRBs (physical resource block) can be used to transmit information simultaneously for multiple devices (users). 

(QUS: What is analogue beam-forming and its drawback and benefit?) 

In case of analogue beam forming the beam forming weights that is phase shifts are applied in the analogue domain after the signal has been mixed to RF. 

This approach does not allow different beam forming weights to be applied for different users. All resource blocks and user transmissions are combined into single signal once the IFFT (inverse fast Fourier transform) has been used to generate the CP OFDM waveform prior to digital to analogue conversion. 

Applying the set of beam forming weights to the RF signal generates a single beam which can be directed towards the UE being served at that point in time. Multiple UE can be served simultaneously but those UE must be in same direction. 

The fore most benefit of analogue beam forming is the reduced requirement for RF chains that is it is only required to have one RF chain per beam. Practical deployment is likely to support at least two RF chains and two beams to provide support for 2x2 MIMO. 

The first RF chain and first beam can use +45̊ antenna elements while the second RF chain and second beam can use -45̊ antenna elements. 

Analogue beam forming impacts the radiation pattern and gain of the antenna array, thus improving the coverage. 

In analogue beam forming only beam per set of antenna elements can be formed. The antenna gain boost provided by the analogue beam-forming over comes partly the impact of high path loss in FR2 (mmWave). Thus analog beam forming is considered mandatory for the mmWave frequency range. 

Hybrid beam forming as the name suggest combines both analogue and digital beam forming. It is expected that FR2 (mmWave) base station (gNB) implementations will use some form of hybrid beam-forming. One approach is to use analogue beam forming for coarse beam forming and inside the analogue beam uses a digital beam forming scheme as appropriate, either for MU-MIMO or SU-MIMO.

In Upcoming Part, we will learn more about Beam management and measurement and its interview question and its answer. And for more learning please go through our 5G and LTE page.

Stay tuned for more Update…………………

Pinal Dobariya…


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