Analog vs. Digital Beamforming
Beamforming as a Buzzword
Beamforming is used for directional signal transmission and reception with the versatility to change both amplitude and phase to help regulate power needs and steer the beam in the intended direction. Bandwidth from 6 to 100 GHz, or millimeter Wave (mmWave), is likely an integral part of future mobile broadband as 5G communication systems are introduced in the global market. Concepts like beamforming and analog vs. digital become part of the discussion when the topic turns to “What’s next?” In high frequency mmWave transmission, large path loss during signal propagation limits the transmission range; to overcome this obstacle, directional antennas with beamforming abilities are used in transmission and reception. Beamforming directs the antenna beams at the transmitter and receiver so that the transmission rate is maximized with minimal loss.
Analog vs. Digital
When working with electronics, both analog and digital signals have to be understood and integrated in meaningful ways in order for our electronic systems have perform as intended. While analog signals may be limited to a range of maximum and minimum values, there are an infinite number of possible values within that range. The waves of a time-versus-voltage graph of an analog signal are smooth and continuous. Conversely, digital signals have a finite set of possible values and are one of two values such as either 0V or 5V, for example, and timing graphs of these signals look like square waves. To identify whether a signal is analog or digital, compare how the signal appears; a time-versus-voltage graph of an analog signal should be smooth and continuous while digital waves are stepping, square, and discrete. Most basic electronic components like resistors, capacitors, inductors, diodes, transistors, and amplifiers are analog. Digital circuits use digital, discrete signals using a combination of transistors, logic gates, and microcontrollers. An analog to digital converter (ADC) allows a microcontroller to connect to an analog sensor to read in an analog voltage. A digital to analog converter (DAC) allows a microcontroller to produce analog voltages. A digital down converter (DDC) preserves information in the original signal and is often used to convert analog radio frequency or intermediate frequency down to a complex baseband signal.
In analog beamforming, a single signal is fed to each antenna element in the array by passing through analog phase-shifters where the signal is amplified and directed to the desired receiver. The amplitude/phase variation is applied to the analog signal at transmit end where the signals from different antennas are added before the ADC conversion. At present, analogue beamforming is the most cost-effective way to build a beamforming array but it can manage and generate only one signal beam.
In digital beamforming, the conversion of the RF signal at each antenna element into two streams of binary baseband signals cos and sin, are used to recover both the amplitudes and phases of the signals received at each element of the array. The goal of this technology is the accurate translation of the analog signal into the digital realm. Matching receivers is a complex calibration process with each antenna having its own transceiver and data converters that generate multiple beams simultaneously from one array. The amplitude/phase variation is applied to digital signal before DAC conversion at transmit end. The received signals from antennas pass from ADC converters and DDC converters.
Digital Beamforming Challenges
< Amount of data generated – The data rate out of the ADC effects the digital interface and processing power requirements and the question becomes how to handle the data when electronic systems want increased resolution and higher sampling rate for increased bandwidth.
< Power consumption – Processors require lots and lots of power. Because of the limitation in data bandwidth, there is a practical limit on the number of elements in the array which requires waveform generators at each element.
< Loss – The losses high frequency mmWave transmission incur include high free space path loss, absorption from atmospheric gases and rainfall, and non-line of sight propagation
< Expense – The overall expensive of implementing digital beamforming systems includes but is not limited to the physical size of the electronics and the high cost of a large number of ADCs operating at high sampling frequencies.
Possible solutions for some of these challenges in 5G mobile communications are forthcoming in the research and tend to dominate discussions on the future of beamforming. It appears that, at present, digital beamforming is the future in communication systems but not without its challenges. To mitigate these challenges, it appears evident that the first 5G mobile systems will integrate a combination of analogue and digital beamforming systems.