关于RF chain 的迷惑
antenna basics
- The digital baseband bits and signals cannot travel through air/space or antennas. The current has to be alternating (AC) at high frequency and analog to leave the wire. This is emitted in terms of electro-magnetic waves that travel at speed of light. Antennas are designed of certain lengths and physical structure to radiate in all directions (omni-directional) or in narrow-directions (directional antennas).
数字基带比特和信号不能通过空气/空间或天线传播。电流必须在高频和模拟以交流电形式离开导线。这是以光速传播的电磁波的形式发出的。天线被设计成一定的长度和物理结构，以向所有方向(全向)或向窄方向(定向天线)辐射。
- For this to happen the signal is moved up in frequency by modulation before it’s fed to filters (for spectral efficiency) and the antenna. This is done by multiplying the signal by a faster signal function (sinusoidal) to raise it higher in frequency for broadband. (in digital it’s baseband/low frequency). Examples of modulation are BPSK, QPSK and QAM.
为了实现这一点，信号在被馈入滤波器(这是为了频谱效率)和天线之前，通过调制实现在频率上的移动。具体方式是将信号（数字信号是基带/低频）乘以一个更快的信号函数(正弦)来提高宽带的频率来实现的。调制的例子有BPSK, QPSK和QAM。
基带信号（即基本频带信号）：像计算机输出的代表各种文字或图像文件的数据信号都属于基带信号。
基带信号往往包含有较多的低频成分，甚至有直流成分，而许多信道并不能传输这种低频分量或直流分量。因此必须对基带信号进行调(modulation)。
带通信号：把基带信号经过载波调制后，把信号的频率范围搬移到较高的频段以便在信道中传输（即仅在一段频率范围内能够通过信道）。
- These modulated signals are then passed to pulse shaping and matching filters for better shaping the signal and optimizing its bandwidth occupancy, then fed to DAC (digital to analog converter) that feeds to the antenna.
这些调制信号然后被传递到脉冲整形和匹配滤波器，以更好地整形信号和优化其带宽占用，然后被馈给DAC(数字-模拟转换器)，馈给天线。
what is the function of DAC in transmit antenna?
From an architecture perspective, the DAC plays a critical role in a radio transmission system. Its speed and performance determine how close to the antenna the digital-to-analog conversion can be performed. The RF DAC extends the scope of digital signal processing beyond the baseband domain out to the antenna. It enables synthesis of the baseband digital signal directly at the final output frequency, which essentially absorbs the analog upconversion operation of the traditional architecture into the digital domain. Digital frequency conversion provides greater flexibility and higher performance in terms of frequency planning and noise. This is especially attractive to the MB-MSR design.
The frequency planning is more flexible using the RF DAC because the digital modulation is ideal and does not generate modulation images that can interfere with signals. The DAC sample clock frequency is the only design variable that needs to be determined in the frequency planning. Figure 2 shows the architecture of direct RF synthesis with an RF DAC and its capability of supporting dual band applications without having issues in the traditional architecture. In this example, the dual band signal is directly synthesized at the final transmission band frequencies. The DAC sample clock frequency is chosen so that the harmonics of the signals fall far out of the band of interest and can be filtered before the signal is fed into the next RF stage.
-
The implementation of
digital beamformingis not suitable for massive MIMO systems becausetraditional beamforming is implemented at the baseband, which helps tocontrol the phase and amplitudeof the signal; -
therefore, it requires the carrier frequency of the processed signal be up-converted after a crossover
RF chain, which includesdigital-to-analogue (D/A) converters, mixers, and power amplifiers. The responses of the RF chain are then combined with the antenna elements. -
In other words, each antenna array element must be reinforced by a dedicated RF chain. This is expensive to realise in massive MIMO systems because a large number of antenna elements is necessary.
-
As noted above,
analogue beamformingis applied in a simple manner usinginexpensive phase shifters. For that reason, analogue beamforming is more cost-effective than digital beamforming. -
Conversely, analogue beamforming exhibits poorer performance compared to digital beamforming because
the amplitudes of the phase shifter are not flexible. To achieve better performance, mixing between digital and analogue beamforming has been proposed and is referred to as hybrid beamforming. -
The digital beamforming portion creates baseband signals, whereas the analogue beamforming portion addresses RF chain effects by reducing the number of ADCs/DACs, which improves the outputs of power amplifiers or changes the architecture of the mixers and hence provides cost savings.
what is the role of RF chain in the signal processing?
Radio frequency (RF) chain circuits play a major role in digital receiver architectures, allowing passband communication signals to be processed in baseband. When operating at high frequencies, these circuits tend to be costly.
An RF chain is a cascade of electronic components and sub-units which may include amplifiers, filters, mixers, attenuators and detectors [Principles of Electronic Communication Systems]. It can take many forms, for example, as a wide-band receiver-detector for electronic warfare (EW) applications, as a tunable narrow-band receiver for communications purposes, as a repeater in signal distribution systems, or as an amplifier and up-converters for a transmitter-driver. In this article, the term RF (radio frequency) covers the frequency range “Medium Frequencies” up to “Microwave Frequencies”, i.e. from 100 kHz to 20 GHz.
Analog beamforming is often implemented using a network of digitally controlled phase shifters. In
this configuration, several antenna elements are connected via phase shifters to a single RF chain, as
illustrated in Fig. 2.
The performance achieved with analog beamforming based on phased arrays is limited by the use of quantized phase shifts and the lack of amplitude adjustment.
This makes it more challenging to finely tune the beams and steer nulls. RF phase shifters may be active or passive. Practical active phase shifters also introduce performance degradation due to phase-shifter loss, noise and non-linearity. Although passive phase shifters have a lower consumption and do not introduce non-linear distortion, they occupy a larger area and incur larger insertion losses. The power consumed by the phase shifters also depends on the resolution of the quantized phases.
一直好奇的 RF chain 到底是什么样子？