ASK, FSK, PSK和QAM
There are three major classes of digital modulation techniques used for transmission of digitally represented data:
- Amplitude-shift keying (ASK)
- Frequency-shift keying (FSK)
- Phase-shift keying (PSK)
key应该是这个意思：roughen a surface so that plaster or paint will stick to it, 把表面弄粗糙以便附着灰浆或颜料。
复数和复平面 So the complex number 3 + 4i can also be shown as distance (5) and angle (0.927 radians):
QAM的中文译名是“正交振幅调制”。要是词典里说Quadrature=正交就好了。可是：
In signal processing:
- Quadrature amplitude modulation (QAM), a modulation method of using both an (in-phase) carrier wave and a 'quadrature' carrier wave that is 90° out of phase with the main, or in-phase, carrier
- Quadrature phase, oscillations that are said to be in quadrature if they are separated in phase by 90° (π/2, or λ/4)
- Quadrature filter, the analytic signal of a real-valued filter
- Quadrature phase-shift keying (QPSK), a phase-shift keying of using four quadrate [a square or cube] points on the constellation diagram, equispaced around a circle
In mathematics:
- Quadrature (mathematics), drawing a square with the same area as a given plane figure (squaring) or computing that area
- Quadrature of the circle
- Numerical integration is often called 'numerical quadrature' or simply 'quadrature'
- Gaussian quadrature, a special case of numerical integration
- Formerly, a synonym for "integral" 积分是曲线下的面积，无数个x方向和y(正交)方向的小矩形？
W236: THE DISCRETE HILBERT TRANSFORM: A BRIEF TUTORIAL (dspguru.com)
为啥高中数学用i表示根号-1，信号处理又用j? 为啥高中用弧度表示相位，信号处理又用角度？上文的one后面少了个of，因为后面是mathematicians，即one不能替换为a.
哦，据www.merriam-webster，释义有且仅有2条:
- a configuration in which two celestial bodies (such as the moon and the sun) have an angular separation of 90 degrees as seen from the earth
- the process of finding a square equal in area to a given area
In the case of PSK, the phase is changed to represent the data signal. There are two fundamental ways of utilizing the phase of a signal in this way:
- By viewing the phase itself as conveying the information, in which case the demodulator must have a reference signal to compare the received signal's phase against; or
- By viewing the change in the phase as conveying information – differential schemes, some of which do not need a reference carrier (to a certain extent).
A convenient method to represent PSK schemes is on a constellation diagram. This shows the points in the complex plane where, in this context, the real and imaginary axes are termed the in-phase and quadrature axes respectively due to their 90° separation. Such a representation on perpendicular axes lends itself to straightforward implementation. The amplitude of each point along the in-phase axis is used to modulate a cosine (or sine) wave and the amplitude along the quadrature axis to modulate a sine (or cosine) wave. By convention, in-phase modulates cosine and quadrature modulates sine.
A constellation diagram displays the signal as a two-dimensional xy-plane scatter diagram in the complex plane at symbol sampling instants. The angle of a point, measured counterclockwise from the horizontal axis, represents the phase shift of the carrier wave from a reference phase. The distance of a point from the origin represents a measure of the amplitude or power of the signal.
头条有文章说QAM的解调既有趣又容易，哎呦我去。我觉得最容易理解的是ASK(不懂就问:-))，想提高传输率就提高频率。但是频带资源不是想占用就占用、还有高频电磁波走直线等等具体困难。sin(2x)=2sin(x)cos(x)。cos(2x)=cos2x - sin2x，QAM好像提高了频率又好像没提高？载波的频率没变。忘了在哪里看到过: 理论上幅度的变化还可以表示0~65535而不是0~1，模拟而不是数字信号的level就更多了。“双通道、125MHz 转换速率、8bi的高速DA芯片” [link] TI声称的The world's highest-speed ADCs [link] 还有Claiming to be the industry's fastest 16-bit DAC converter, TI's DAC38J84 and DAC38J82 support the JEDEC JESD204B serial interface standard for data converters up to 12.5Gbps. TI claims that the 4-channel, 2.5-GSPS DAC38J8 is 66% faster than competing devices, whilst the 2-channel, 16-bit DAC38J82 is 22% faster than exisitng 16-bit dual DACs. 在2021-06-07开幕的OFC 上，多家中国光模块厂商发布了800G光模块产品。[link]
Transformer是谷歌大脑在2017年底发表的论文attention is all you need中所提出的seq2seq模型…… 论文中使用了sin和cos函数的线性变换来提供给模型位置信息。
