Analog Beamforming vs Digital Beamforming | difference between Analog Beamforming and Digital Beamforming

This page compares Analog Beamforming vs Digital Beamforming and mentions difference between Analog Beamforming and Digital Beamforming.

The beamforming is used for directional signal transmission and reception. It has provision to change both amplitude and phase which helps in power variation as well as beam steering in the desired directions respectively. The antenna arrays with separate provision for amplitude/phase variation is used in beamforming for both the transmission as well as reception.

In analog beamforming, amplitude/phase variation is applied to analog signal at transmit end. The signals from different antennas are summed up before the ADC conversion in analog beamforming at receive end.

In digital beamforming, amplitude/phase variation (Wk) is applied to digital signal before DAC conversion at transmit end. The reverse process is done after ADC & DDC operations are performed. As shown the received signals from antennas are first passed from ADC converters and digital down converters (i.e. DDCs) before summation operation.

Analog Beamforming

analog beamforming transmitter

The figure-1 depicts analog beamforming transmitter. As shown baseband signal to be transmitted is modulated first. This radio signal is splitted using power divider and passed through the beamformer which has provision to change amplitude (ak) and phase (θk) of the signals in each of the paths going towards stack of antennas. Power divider depends on number of antennas used in antenna array for example 4 way power divider is needed to address the need of 4 antenna array.

analog beamforming receiver

The figure-2 depicts analog beamforming receiver. As shown in the receiver block diagram, complex weight is applied to the signal from each antenna in the array. Complex weight consists of both amplitude and phase. After these is done, signals are combined into one output. This provides desired directional pattern from array of antennas.

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.

Digital Beamforming

digital beamforming receiver

The figure-3 depicts digital beamforming receiver. As shown in the figure, amplitude scaling, phase shifting of each antenna elements and their sum have been carried out digitally.

• Digital beamforming consists of RF translators, A/D converters, DDCs, complex weight multiplication and summing operation.
• RF Translator converts higher RF signal frequency to lower IF signal frequency. This is done using RF mixer. LO signal is fed to the RF mixer in order to perform RF to IF conversion. Appropriate filters (bandpass and lowpass) are used at the input and output of the RF mixer.
• This IF signal is converted to digital equivalent using A/D converter using appropriate sampling clock.
• The digitized IF signal is passed to the DDC (Digital Down Converter). The DDC using cos(2*π*Fc*t) and sin(2*π*Fc*t) and low pass filtering to convert digital IF into baseband I/Q components or combined I+j*Q signal. The combined signal is designated as s(t) in the figure-3.
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 these baseband signals (s(t)).
• The results from these antenna elements are summed up to produce baseband signal with desired directional pattern.
• The signal after summation is given to demodulator to retrive the information from radio signal.

Following table mentions difference between analog beamforming and digital beamforming.

Analog Beamforming Digital Beamforming
Adaptive transmit/receive weights at RF to form beam Adaptive transmit/receive weights at baseband
One transceiver unit and one RF beam with high antenna gain Each antenna element or antenna port has a transceiver unit, high number (>8) of transceiver units.
"Frequency flat" beam forming "Frequency selective" beam forming
Best for coverage (due to low power consumption & cost characteristics) Best for capacity and flexibility (subject to high power consumption & cost characteristics when bandwidth increases)

Beamforming Related Links

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IEEE 802.11ac beamforming
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PPT on beamforming using Smart Antennas
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