正交幅度调制(QAM):这是什么?| Electrical4U

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What is QAM (Quadrature Amplitude Modulation)?

QAM（正交幅度调制）被定义为阶段和阶段组合的调制技术amplitude调制载波进入单个通道。换句话说，QAM通过改变载波的幅度和相位来发送信息，从而加倍有效带宽。QAM也称为“正交载体multiplexing”.

在QAM信号中，涉及正交中的载波的直接调制。作为名称“正交”表示两个载波之间的相位差为90度，但每个载波差异频率。

One signal is called the in-phase “I” signal, and the other is called the quadrature “Q” signal. Mathematically, one of the carrier signals can be represented by aS.ine wave（IE。 $sin \ omega t$ 另一个可以由余弦波表示（即 $cos \omega t$ ).

两个调制载波信号在源和目的地处一起发送，这些两个载波信号被独立地解调（即分离）。为了解调使用信号相干检测方法。

QAM技术的波形如下所示。

In Phase Signal and Quadrature Signal Component — In-Phase Signal and Quadrature Signal Component

Analog vs Digital QAM

Analog QAM

模拟QAM通常用于允许多个信号在单个载波上承载。它与AM（幅度调制）相同，具有两个载波信号的相同频率，但与90度不同的相位。

Analog QAM is used in the transmission of chroma (color) information in PAL and NTSC analog video television systems. Where the I (in-phase) and Q (Quadrature) signals carry the components of chroma (color) information.

PAL stands for Phase Alternating Line is the video standard which is mostly used in the European and Asian countries and NTSC stands for National Television Standards Committee is the analog color television standard which is mostly used in South America and North America.

Digital QAM

Digital QAMs are usually described as a “Quantized QAM” and they are commonly used in radio communications systems ranging from cellular technology to Wi-Fi. Digital QAM can carry higher data rates compared to both amplitude and phase-modulated scheme.

In digital QAM schemes, to define the values of phase and amplitude different points can be used. This is known as a constellation diagram. Thus a constellation diagram is the set of possible message points.

QAM.can be realized by using a constellation diagram. In the constellation diagram, the constellation points are arranged in a square grid with equal horizontal and vertical distance. The minimum distance between the constellation points is known as a Euclidean distance.

In the digital communications, data is usually in a binary form and it has two states 0 or 1, so the number of constellation points in the grid is usually a power of 2 i.e. 2, 4, 8, 16, 32………… the most common formats of QAM are16-QAM (2^4.), 32-QAM (2^5.），64-QAM（2^6.），128-QAM（2^7.）和256-QAM（2^8.).

16-QAM的比特序列映射在星座图中示出。图示示出了与16 QAM信号的不同位置相关联的二进制值。可以看出，连续比特流可以表示为序列，并在四个象限中的每一个中被分成四个组。

Constellation Diagram for 16 QAM — Constellation Diagram for 16-QAM

Normally a 16-QAM is considered as the lowest order QAM because 2-QAM is considered the same as for BPSK (i.e. Binary Phase Shift Keying) and 4-QAM is the same as QPSK (i.e. Quadrature Phase Shift Keying). In addition to the error-rate performance of 8-QAM is almost the same as that of 16-QAM hence it is not widely used.

What Are QAM Channels?

QAM.channels are the variety of different formats of QAMs which are used in many radio communications and data delivery applications. Some specific variants of QAMs channel are used in some specific applications and standards.

让我们假设我们希望通过QAM发送由4位组成的符号。这意味着n = 4，有2个^4.= 16 different possible symbols. Hence the QAM system can generate 16 different distinguishable signals. It is known as a 16-QAM. By using 16-QAM we can modulate the carrier signal into any of 16 different phase and amplitude states.

Examples of some other QAM formats or channels are 16-QAM, 32-QAM, 64-QAM,128-QAM, and 256-QAM.

QAM符号或状态的数量由数量决定binarybits per second.

The number of possible symbols for QAM channels is given by

$\ begin {对齐*} m = 2 ^ n \ neg {alight *}$

这里，术语“符号”是指幅度和相位的组合。

Depending on the number of bits per message or symbol the QAM signals are classified as follows:

名称	Bits per symbol	符号数量
16 QAM	4.	2^4.= 16
32 QAM	5.	2^5.= 32
64 QAM.	6.	2^6.= 64
128 QAM.	7.	2^7.= 128.
256 QAM.	8.	2^8.= 256.

16 QAM

16 QAM is one of the different formats of QAM in which the carrier signal is modulated into any of 16 different phase and amplitude states.

How many bits per symbol can be transmitted using 16-QAM?

By using 16-QAM 4 bits per symbol can be transmitted.

32 QAM

32QAM是其中载波信号的不同格式之一，其中载波信号被调制成32个不同的相位和幅度状态中的任何一个。

可以使用32-QAM传输每符号的每个符号多少位？

通过使用每个符号的32-QAM 5位。

64 QAM.

64 QAM的不同格式的QAM which the carrier signal is modulated into any of 64 different phase and amplitude states.

可以使用64-QAM传输每符号的数位？

通过使用每个符号的64-QAM 6位。

128 QAM.

128 QAM.is one of the different formats of QAM in which the carrier signal is modulated into any of 128 different phase and amplitude states.

How many bits per symbol can be transmitted using 128-QAM?

By using 128-QAM 7 bits per symbol can be transmitted.

256 QAM.

256 QAM.is one of the different formats of QAM in which the carrier signal is modulated into any of 256 different phase and amplitude states.

How many bits per symbol can be transmitted using 256-QAM?

By using 256-QAM 8 bits per symbol can be transmitted.

QAM系统的带宽

QAM系统的功率谱密度的曲线图如下图所示。

频谱的主要部分从-f延伸_S.to +f_S.。Therefore the bandwidth of QAM system is given by,

$\ begin {align *}带宽（bw）= f_s - （-f_s）= 2f_s = \ frac {2} {t_s}（作为f_s = \ frac {1} {t_s}）\ ent {align *}$

$\ begin {align *} = \ frac {2} {n t_b}（作为t_s = n t_b）\结束{align *}$

$\ begin {aligne *} bw = \ frac {2f_b} {n}（作为f_b = \ frac {1} {t_b}）\ neg {align *}$

QAM.vs QPSK vs BPSK vs OOK

The comparison of various modulation techniques has been discussed in the table below.

范围	QAM.	QPSK.	BPSK	OOK or BASK
信息是 transmitted by change in	Amplitude 和阶段	Phase	Phase	Amplitude
Number of bits (N) 每个符号	N	n = 2	n = 1	n = 1
Number of possible symbol $M=2^N$	$M=2^N$	Four	二	二
Minimum bandwidth in Hz	$\ frac {2f_b} {n}$	$F_B.$	$2F_B.$	$2F_B.$
Symbol duration $T_s$	$NT_B.$	$2T_b$	$T_b$	$T_b$
最小欧几里德距离	$\ sqrt {0.4} e_s$ for M=16	$2 \ sqrt {e_b}$	$2 \ sqrt {e_b}$	$\sqrt{E_b}$

注意，欧几里德距离是星座图中的两个点之间的距离。

QAM.can transmit the data at four times the bit rate over the same bandwidth as BPSK. Therefore, QAM is considered to be a bandwidth-efficient modulation technique.

Advantages of QAM

QAM的一些优点包括：

Noise immunity of QAM’s is very high hence noise interference is very less.
QAM.has a low probability of error value.
QAM支持高数据速率。这样比特数可以由载波信号承载。因此，它主要用于无线通信系统。
QAM.has a doubling the effective bandwidth.
By using both sine wave and cosine wave into single-channel the communication channel capacity is doubled compared to the use of only one sine wave or one cosine wave.

Disadvantages of QAM

QAM的一些缺点包括：

In QAM, amplitude changes are susceptible to noise.
当相位或频率调制信号被放大时，不必在无线电发射器中使用线性放大器，而是由于QAM中存在幅度分量，必须使用线性放大器以保持线性度。这些线性放大器效率较低并占用更多power。
It is possible to transmit more bits per symbol but in higher-order QAM formats the constellation points are closely spaced which is more susceptible to noise and produces errors in the data.
Also in higher-order QAM formats, there is a difficulty for the receiver to decode the signal appropriately. In other words, there is reduced noise immunity. So the higher-order QAM formats are only used when there is a high signal to noise ratio.

Applications of QAM

QAM的一些应用包括：

由于比特数据速率的增加，QAM技术广泛用于无线电通信领域。
QAM用于applications ranging from short-range wireless communications to long-distance telephone systems.
QAM用于微波和电信系统以传输信息。
The 64 QAM and 256 QAM are used in digital cable television and cable modem.
QAM用于光纤S.ystems to increase bit rates.
It is used in many communication systems like Wi-Fi, Digital Video Broadcast (DVB), and WiMAX.