Analog vs. Digital Beamforming: Key Differences Explained

This page compares Analog Beamforming vs. Digital Beamforming, highlighting the key differences between the two techniques. Beamforming is used for directional signal transmission and reception. It allows for changes in both amplitude and phase, enabling power variation and beam steering in desired directions. Antenna arrays with separate amplitude/phase variation capabilities are used in beamforming for both transmission and reception.

In analog beamforming, amplitude/phase variation is applied to the analog signal at the transmit end. At the receive end, signals from different antennas are summed up before the Analog-to-Digital Conversion (ADC).

In digital beamforming, amplitude/phase variation (Wk) is applied to the digital signal before the Digital-to-Analog Conversion (DAC) at the transmit end. The reverse process occurs after ADC and Digital Down Conversion (DDC) operations at the receiver. Received signals from antennas are first passed through ADCs and DDCs before being summed.

Analog Beamforming

Analog Beamforming Transmitter

Figure 1: Analog Beamforming Transmitter

As shown above, the baseband signal to be transmitted is modulated first. This radio signal is then split using a power divider and passed through the beamformer. The beamformer changes the amplitude (a_k) and phase (θ_k) of the signals in each of the paths leading to the antenna array. The power divider’s configuration depends on the number of antennas used in the antenna array. For example, a 4-way power divider is needed for a 4-antenna array.

Analog Beamforming Receiver

Figure 2: Analog Beamforming Receiver

The receiver block diagram shows that a complex weight is applied to the signal from each antenna in the array. This complex weight incorporates both amplitude and phase adjustments. After the weights are applied, the signals are combined into a single output, providing the desired directional pattern from the antenna array.

The complex weight (W_k) can be represented as:

W_k = a_k * e^jsin(θk)

Which can also be expressed as:

W_k = a_k * cos(θk) + j * a_k sin(θk)

Where:

• W_k represents the complex weight for the k^th antenna in the array.
• a_k is the relative amplitude of the weight.
• θ_k is the phase shift of the weight.

Digital Beamforming

Digital Beamforming Receiver

Figure 3: Digital Beamforming Receiver

As shown in Figure 3, amplitude scaling and phase shifting of each antenna element, along with their summation, are carried out digitally.

Digital beamforming typically consists of the following components:

• RF Translators: Convert higher RF signal frequencies to lower IF (Intermediate Frequency) signals. This is achieved using an RF mixer with a Local Oscillator (LO) signal. Appropriate filters (bandpass and lowpass) are used at the input and output of the RF mixer.

• A/D Converters: Convert the IF signal to its digital equivalent using an appropriate sampling clock.

• DDCs (Digital Down Converters): Use cos(2πFct) and sin(2πFct) and low pass filtering to convert the digital IF signal into baseband I/Q components or a 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 Weight Multiplication: Complex weights are applied to these baseband signals (s(t)).

• Summing Operation: The results from each antenna element are summed to produce a baseband signal with the desired directional pattern.

• Demodulator: The summed signal is then passed to a demodulator to retrieve the information from the radio signal.

Analog vs. Digital Beamforming: Key Differences

The following table summarizes the key differences between analog and digital beamforming: