QAM stands for Quadrature Amplitude Modulation and is the most common modulation the modern radios use to encode information onto an RF wave.

QAM 是 Quadature Amplitude Modulation（正交調幅）的縮寫，是現代無線電最常用的調製方式，用於將資訊編碼到射頻波上。

The RF waves have three main properties we can control to encode information onto them the amplitude, frequency, and phase.

我們可以通過控制射頻波的振幅、頻率和相位這三個主要特性來對其進行資訊編碼。

Digital communication systems work with 1s and 0s, which are easy to encode.

數字通信系統使用 1 和 0，易於編碼。

With amplitude modulation, a simple on-off switch is enough to encode 1 and 0.

使用振幅調製時，一個簡單的開關就足以對 1 和 0 進行編碼。

With frequency modulation, signals with two different frequencies do the job.

通過頻率調製，兩種不同頻率的信號就能完成工作。

And with phase modulation, it's shifting the signal by 180 degrees.

而相位調製則是將信號移動 180 度。

The QAM is a combination of amplitude and phase modulation.

QAM 是振幅和相位調製的組合。

Here is the simplest QAM modulator scheme.

下面是最簡單的 QAM 調製器方案。

At the input is digital data processed by groups of bits, or symbols.

輸入端是按比特組或符號組處理的數字數據。

At the other input of the modulator is the RF carrier signal.

調製器的另一個輸入端是射頻載波信號。

These input signals are combined in the modulator which controls the amplitude and the phase of the resulting output signal, which lets us encode more information onto the signal compared to any other modulation alone.

這些輸入信號在調製器中進行組合，調製器控制輸出信號的振幅和相位，與其他單獨的調製方式相比，我們可以將更多的資訊編碼到信號中。

The length of the symbol determines the QAM modulation depth and maximum number of symbols.

符號長度決定 QAM 調製深度和最大符號數。

The symbols can be mapped into so-called constellation diagram.

這些符號可以映射到所謂的星座圖中。

The QPSK has 4 possible symbols with 2 information bits per symbol.

QPSK 有 4 個可能的符號，每個符號有 2 個資訊比特。

The 16-QAM has 16 symbols with 4 information bits per symbol.

16-QAM 有 16 個符號，每個符號有 4 個資訊比特。

64-QAM has 64 symbols, and so on.

64-QAM 有 64 個符號，以此類推。

Because of the noise, the transmitted symbols fluctuate around the ideal value.

由於存在噪聲，傳輸的符號會在理想值附近波動。

The noise limit of QAM of given depth is given by the point where the acceptable noise level areas for each symbol start to overlap.

給定深度 QAM 的噪聲限值由每個符號的可接受噪聲級區域開始重疊的點給出。

Once they do, the transmitted symbols within the overlapping area can be wrongly identified causing errors in data transfers.

一旦出現這種情況，重疊區域內的傳輸符號就會被錯誤識別，導致數據傳輸出錯。

With growing QAM depth, we packed more information into the same signal but also shrink the space between the symbols, which naturally decreases the acceptable noise level and is the reason why high MCS rates need high signal-to-noise ratio levels to work.

隨著 QAM 深度的增加，我們在同一信號中包含了更多的資訊，但同時也縮小了符號之間的空間，這自然會降低可接受的噪聲水準，這也是高 MCS 速率需要高信噪比水準才能工作的原因。

So considering the 802.11ac standard and 20 MHz channel, we need at least 5 dB SNR for

是以，考慮到 802.11ac 標準和 20 MHz 的信道，我們需要至少 5 dB 的信噪比來實現

QPSK, 11 dB SNR for 16-QAM, 18 dB SNR for 64-QAM, and so on.

QPSK，16-QAM 的信噪比為 11 dB，64-QAM 的信噪比為 18 dB，以此類推。

For practical use, the MCS rates of the 802.11ac standard combine information about the modulation depth, number of spatial channels, and coding rate, which says what part of the transferred data is user data.

在實際使用中，802.11ac 標準的 MCS 速率結合了調製深度、空間信道數和編碼率（即傳輸數據中哪一部分是用戶數據）等資訊。

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