(zslcn周生烈编译摘注评)
矩形QAM 奇数-kQAM 星座图 FFT AWGN
http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
This page was last modified on 6 November 2011 at 02:39
Quadrature amplitude modulation正交幅度调制
From Wikipedia, the free encyclopedia
"QAM" redirects here. For other uses, see QAM (disambiguation).
This article is about a modulation technique. For the digital television standard, see QAM (television).
| Passband modulation v · d · e |
| FSK · MFSK · ASK · OOK · PSK · QAM |
| See also: Demodulation, modem, |
Quadrature amplitude modulation (QAM) (( /kwa:m/ or /kæm/ or simply "Q-A-M") "Q-A-M") is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK), or (in the analog case) of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digital telecommunication systems. Spectral efficiencies of 6 bits/s/Hz can be achieved with QAM.[1]
正交幅度调制（QAM）（其读为 ( /kwa:m/ or /kæm/ or 直接读 "Q-A-M") 既是模拟调制方案,也是数字调制方案。它改变（调制）两个载波的振幅，来传递两个模拟消息信号，或两个数字位流；改变的方法是通过使用数字调制方案的幅移键控（ASK），或模拟调制方案的调幅（AM）。两个载波，相位通常差90 °，因而被称为正交载波，或称为正交分量--因此也就是该方案的名称。两个(正交的调幅)调制波相加，从而合成的波形是相移键控（PSK）和幅移键控（ASK）两者的结合，或者（在模拟情况下）是相位调制（PM）和调幅相结合。在数字QAM方案的情况下，使用有限数量的相位（至少有两个）和振幅（至少有两个）。 PSK调制的设计,通常也使用QAM原理，但不作为QAM调制考虑，因为其载波信号的幅度是不变的。QAM作为数字通信系统的调制方案,被广泛使用着。 QAM的频谱效率,可以达到6比特位/信符/赫兹(即每个周期波形中可以安排一个6比特位信符(64种信符)的调制信息)。[1]
QAM modulation is being used in optical fiber systems as bit rates increase – QAM16 and QAM64 can be optically emulated with a 3-path interferometer.[2]
由于位速率增加,QAM调制已被使用于光纤系统中 - QAM16和QAM64在光学上可以采用三路径干涉仪仿真。[2]
| Contents [hide] 1 Digital QAM 数字QAM 2 Analog QAM 模拟QAM 2.1 Fourier analysis of QAM QAM傅立叶分析 3 Quantized QAM QAM量化 3.1 Ideal structure 理想结构 4 Quantized QAM performance 已量化QAM的性能 4.1 Rectangular QAM 矩形QAM 4.1.1 Odd-k QAM 奇数-k QAM 4.2 Non-rectangular QAM 非矩形QAM 5 Interference and noise 干扰与噪声 6 See also 参见 7 References 参考 8 External links 外部链接 |
1. [edit] Digital QAM [编辑]数字QAM
Like all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal. Amplitude modulating two carriers in quadrature can be equivalently viewed as both amplitude modulating and phase modulating a single carrier.
像所有的调制方案一样，QAM通过改变载波信号或载波波形（通常是正弦波形）的某些方面,以与数据信号相对应,来传递相应的数据。在QAM中,两个相位差彼此为90o（正交）的波形(注意：是向量),采用改变自己的幅值(注意：是幅值，可正可负,以同时实现向量的同相或反相；而不是幅度。幅度只有正值。)（称为调制或键控）,来表示数据信号。调制两个正交载波的幅值,可以等效看作为一个单一载波同时调制幅度和相位。
Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the magnitude of the modulating signal is a constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded as a special case of phase modulation.
相位调制（模拟PM）和相移键控（数字PSK），可以作为QAM的一个特殊情况,来对待,也就是,调制信号的幅度是固定的,而只是改变相位。这也可以扩展到调频（FM）和频移键控（FSK），它们可以看作为相位调制的特殊情况（相位调制扩展到±2π之外）。
2. [edit] Analog QAM [编辑]模拟QAM
Analog QAM: measured PAL colour bar signal on a vector analyser screen.
模拟QAM：在一个矢量分析仪屏幕上所测量的PAL彩色条信号。
When transmitting two signals by modulating them with QAM, the transmitted signal will be of the form:
当发送两个通过采用QAM来调制它们的信号时，被发送信号的形式是：
,
where I(t) and Q(t) are the modulating signals and f0 is the carrier frequency. 其中, I(t) 和Q(t)是调制信号，f0是载波频率。
At the receiver, these two modulating signals can be demodulated using a coherent demodulator. Such a receiver multiplies the received signal separately with both a cosine and sine signal to produce the received estimates of I(t) and Q(t) respectively. Because of the orthogonality property of the carrier signals, it is possible to detect the modulating signals independently.
在接收端，这两种调制信号,可使用相干解调器解调。这种收信机,将所接收的信号,分别乘以余弦和正弦信号,以产生I(t)和Q(t)的估值。由于载波信号的正交属性，它们各自检出调制信号是可能的。
In the ideal case I(t) is demodulated by multiplying the transmitted signal with a cosine signal:
在理想的情况下，通过将传输信号乘以一个余弦信号,来解调I(t)：
Using standard trigonometric identities, we can write it as:
使用标准的三角恒等式，我们可以写为：
Low-pass filtering ri(t) removes the high frequency terms (containing 4πf0t), leaving only the I(t) term. This filtered signal is unaffected by Q(t), showing that the in-phase component can be received independently of the quadrature component. Similarly, we may multiply s(t) by a sine wave and then low-pass filter to extract Q(t).
低通滤波ri(t)移除高频项（含4πf0t）,只留下I(t)项。滤波后的信号不受Q(t)的影响，表明同相分量可以独立于正交分量而被接收。同样，我们可以对s(t)乘以一个正弦波,然后低通滤波,以提取Q(t)。
The phase of the received signal is assumed to be known accurately at the receiver. If the demodulating phase is even a little off, it results in crosstalk between the modulated signals. This issue of carrier synchronization at the receiver must be handled somehow in QAM systems. The coherent demodulator needs to be exactly in phase with the received signal, or otherwise the modulated signals cannot be independently received. For example analog television systems transmit a burst of the transmitting colour subcarrier after each horizontal synchronization pulse for reference.
假定在收信机端,接收到的信号的相位是精确地已知的。如果解调的相位 哪怕是一点点的偏离,都会导致调制信号之间的串扰。在QAM系统中,这种收信机端载波同步的问题 是必须作某种处理的。相干解调器需要准确地与接收到的信号同相位，否则,被调制的信号就不能独立接收到。因此,例如模拟电视系统,就是传送一个猝发的彩色副载波,跟在每个水平同步脉冲的后面作为参考,来实现的。
Analog QAM is used in NTSC and PAL television systems, where the I- and Q-signals carry the components of chroma (colour) information. "Compatible QAM" or C-QUAM is used in AM stereo radio to carry the stereo difference information.
模拟QAM用于NTSC和PAL电视系统，其中的I信号和Q信号还承载色度（颜色）信息分量。 “兼容的QAM”,或称 C-QUAM,用于AM立体声收音机,以携带立体声差分信息。
2.1 [edit] Fourier analysis of QAM [编辑]QAM的傅里叶分析
In the frequency domain, QAM has a similar spectral pattern to DSB-SC modulation. Using the properties of the Fourier transform, we find that:
在频域，QAM有一个类似于DSB-SC(Double-sideband suppressed-carrier 双边带载波抑制) 调制的频谱模式。利用傅里叶变换的特性，我们发现：
where S(f), MI(f) and MQ(f) are the Fourier transforms (frequency-domain representations) of s(t), I(t) and Q(t), respectively.
其中，S(f), MI(f) 和MQ(f)分别是 s(t), I(t),和Q(t), 的傅里叶变换（频域表示）。
3. [edit] Quantized QAM [编辑]被量化的QAM
Digital 16-QAM with example constellation points.
数字,16 - QAM例如星座点。
Like many digital modulation schemes, the constellation diagram is a useful representation. In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible (e.g. Cross-QAM). Since in digital telecommunications the data are usually binary, the number of points in the grid is usually a power of 2 (2, 4, 8 ...). Since QAM is usually square, some of these are rare—the most common forms are 16-QAM, 64-QAM and 256-QAM. By moving to a higher-order constellation, it is possible to transmit more bits per symbol. However, if the mean energy of the constellation is to remain the same (by way of making a fair comparison), the points must be closer together and are thus more susceptible to noise and other corruption; this results in a higher bit error rate and so higher-order QAM can deliver more data less reliably than lower-order QAM, for constant mean constellation energy.
像在许多数字调制方案中一样，在QAM中,星座图是一个很有用的表示形式。QAM星座点通常安排在一个相等的纵向间隔和横向间隔的方形栅格中，虽然也可以有其他配置（如交叉QAM）。由于在数字通信中的数据通常是二进制，栅格点的数目一般是2的乘方（2，4，8 ...)。因为QAM通常是正方形的，最常见的形式是16-QAM ,64-QAM和256-QAM，其它的形式用得不多，有些更是罕见。通过移向更高阶的星座，就有可能传输更多的每信符比特位数。但是，如果星座的平均能量保持不变（一个公平的比较方式），星座点必然会挨得更紧密，因而更易遭受噪声和其它侵袭的干扰；这就会导致更高的比特误码率。所以,对于恒定平均星座能源而言,高阶QAM可以提供更多的数据，但可靠性低于低阶QAM。
If data-rates beyond those offered by 8-PSK are required, it is more usual to move to QAM since it achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. The complicating factor is that the points are no longer all the same amplitude and so the demodulator must now correctly detect both phase and amplitude, rather than just phase.
如果超出8-PSK提供的数据速率是必要的，更常见的做法是移用到QAM，因为它能通过更均匀地分布星座点,实现在I-Q平面中邻近点间更大的距离。其复杂因素在于，所有的点不再是相同的幅度，所以此刻解调器必须同时正确检测出相位和幅度，而不仅仅是相位。
64-QAM and 256-QAM are often used in digital cable television and cable modem applications. In the United States, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable (see QAM tuner) as standardised by the SCTE in the standard ANSI/SCTE 07 2000. Note that many marketing people will refer to these as QAM-64 and QAM-256. In the UK, 16-QAM and 64-QAM are currently used for digital terrestrial television (Freeview and Top Up TV) and 256-QAM is planned for Freeview-HD.
64-QAM 和256-QAM通常用于有线数字电视和电缆调制解调器的应用中。在美国，64-QAM 和256-QAM由SCTE标准化为ANSI/SCTE 07 2000标准,被授权在数字有线电视中,作为强制执行的调制方案（见QAM 调谐器）。请注意，许多营销人员称呼的64-QAM 和256-QAM,指的就是这个标准。在英国,64-QAM 和256-QAM目前用于地面数字电视(Freeview 和 Top Up TV)，而256-QAM 计划用于高清的Freeview-HD中。
Communication systems designed to achieve very high levels of spectral efficiency usually employ very dense QAM constellations. One example is the ITU-T G.hn standard for networking over existing home wiring (coaxial cable, phone lines and power lines), which employs constellations up to 4096-QAM (12 bits/symbol). Another example is VDSL2 technology for copper twisted pairs, whose constellation size goes up to 32768 points.
为了实现高水平频谱效率的通信系统,通常采用十分密集的QAM星座图。其中一个例子是ITU-T G.hn标准。它用于在现有家居布线（同轴电缆，电话线和电源线）上建网，采用了4096-QAM（12位/信符）的星座。另一个例子是用于双绞铜线的VDSL2技术，其星座的大小上升到32768点(同时使用了OFDM技术)。
3.1 [edit] Ideal structure [编辑]理想结构
3.1.1 [edit] Transmitter [编辑]发信机
The following picture shows the ideal structure of a QAM transmitter, with a carrier frequency f0 and the frequency response of the transmitter's filter Ht(f):
下图显示了一个载波频率为f0,发信机滤波器频率响应为 Ht(f),的QAM发信机的理想结构：
First the flow of bits to be transmitted is split into two equal parts: this process generates two independent signals to be transmitted. They are encoded separately just like they were in an amplitude-shift keying (ASK) modulator. Then one channel (the one "in phase") is multiplied by a cosine, while the other channel (in "quadrature") is multiplied by a sine. This way there is a phase of 90° between them. They are simply added one to the other and sent through the real channel.
首先,将要传输的比特流分成两个相等的部分：这个过程会产生两个要传输的独立信号。他们分别被编码，就像他们在幅移键控（ASK）调制器中那样。然后一个通道（‘同相’的那个）乘以余弦函数，而另一个通道（“正交”的那个）乘以正弦函数。这种方法使它们之间有一个90 °的相位差。再直接将它们加在一起，然后通过实际通道发送。
The sent signal can be expressed in the form:
发送的信号可以用下列形式表示：
where vc[n] and vs[n] are the voltages applied in response to the nth symbol to the cosine and sine waves respectively.
其中vc[n] 和 vs[n]分别相应于 施加到第n个信符中的 余弦波和正弦波的电压，。
3.1.2 [edit] Receiver [编辑]收信机
The receiver simply performs the inverse process of the transmitter. Its ideal structure is shown in the picture below with Hr the receive filter's frequency response :
收信机简单地执行发信机的逆过程。它的理想结构示于下面的图中,并带有频率响应为Hr的接收滤波器：
Multiplying by a cosine (or a sine) and by a low-pass filter it is possible to extract the component in phase (or in quadrature). Then there is only an ASK demodulator and the two flows of data are merged back.
接收信号乘以余弦函数（或正弦函数），并乘以低通滤波器函数，就能够提取同相分量（或正交分量）。然后只通过一个ASK解调器,将两个数据流合并在一起，回到数据的原始状态。
In practice, there is an unknown phase delay between the transmitter and receiver that must be compensated by synchronization of the receivers local oscillator, i.e. the sine and cosine functions in the above figure. In mobile applications, there will often be an offset in the relative frequency as well, due to the possible presence of a Doppler shift proportional to the relative velocity of the transmitter and receiver. Both the phase and frequency variations introduced by the channel must be compensated by properly tuning the sine and cosine components, which requires a phase reference, and is typically accomplished using a Phase-Locked Loop (PLL).
实际上,发信机和收信机之间有一个未知的相位延迟，必须通过收信机本地振荡器的同步来补偿，也就是上图中的正弦函数和余弦函数。在移动应用中，通常会有频率的相对偏移，同时,由于可能存在一个多普勒频移，它比例于发信机和收信机间的相对速度。通道所引入的相位和频率变化,都必须通过妥善调整正弦和余弦分量来补偿，这就需要一个参照相位；通常使用一个锁相环（PLL）来完成。
In any application, the low-pass filter will be within hr (t): here it was shown just to be clearer.
在任何应用中，低通滤波器应该是放在hr (t)内的；这里,它被特地显示出来,只是为了更清楚些。
4. [edit] Quantized QAM performance [编辑]量化QAM性能
The following definitions are needed in determining error rates:
下面的定义,对于确定错误率是必须的：
· M = Number of symbols in modulation constellation
M = 在数字调制星座中的信符数
· Eb = Energy-per-bit 每位能量
· Es = Energy-per-symbol = k·Eb with k bits per symbol
每信符能量= k·Eb k是每个信符的比特数
· No = Noise power spectral density (W/Hz) 噪声功率谱密度(W/Hz)
· Pb = Probability of bit-error 比特错误的概率
· Pbc = Probability of bit-error per carrier每个载波比特错误的概率
· Ps = Probability of symbol-error 信符错误的概率
· Psc = Probability of symbol-error per carrier
Psc = 每个载波信符错误的概率
·
Q(x) is related to the complementary Gaussian error function by:
Q(x)与互补型高斯误差函数的关系,是由下式确定：
, which is the probability that x will be under the tail of the Gaussian
PDF(Probability density function) towards positive infinity.
Q(x)是概率，是高斯概率密度函数,从x一直到正无穷大的积分(概率)。
The error rates quoted here are those in additive white Gaussian noise (AWGN). 引述在这里的错误率,是指在加性高斯白噪声（AWGN）中的那些。
Where coordinates for constellation points are given in this article, note that they represent a non-normalised constellation. That is, if a particular mean average energy were required (e.g. unit average energy), the constellation would need to be linearly scaled.
星座点的坐标在本文中给出。请注意,它们表示了一个非规范化的星座。也就是说，如果需要一个特定的平均值能量（如单位平均能量），该星座将需要进行线性缩放。
4.1 [edit] Rectangular QAM [编辑]矩形QAM
Constellation diagram for rectangular 16-QAM. 16-QAM矩形星座图。
Rectangular QAM constellations are, in general, sub-optimal in the sense that they do not maximally space the constellation points for a given energy. However, they have the considerable advantage that they may be easily transmitted as two pulse amplitude modulation (PAM) signals on quadrature carriers, and can be easily demodulated. The non-square constellations, dealt with below, achieve marginally better bit-error rate (BER) but are harder to modulate and demodulate.
一般说来,矩形QAM星座 并没有在指定能量下 使星座点间隔最大化。然而,它们便于作为正交载波上两个脉冲幅度调制(PAM)来发送,且易于解调,是它们具有的相当大的优点。下面将要涉及到的非正方形星座,能达到更小的比特误码率（BER），但调制和解调更为困难。
The first rectangular QAM constellation usually encountered is 16-QAM, the constellation diagram for which is shown here. A Gray coded bit-assignment is also given. The reason that 16-QAM is usually the first is that a brief consideration reveals that 2-QAM and 4-QAM are in fact binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK), respectively. Also, the error-rate performance of 8-QAM is close to that of 16-QAM (only about 0.5 dB better[citation needed]), but its data rate is only three-quarters that of 16-QAM.
通常首先遇到的矩形QAM星座是16-QAM，其星座图已显示在此，同时也给出了一个格雷编码的位分配。首先采用16-QAM的理由是,稍加考虑就不难看出,2-QAM和4-QAM 其实分别是二进制相移键控（BPSK）和正交相移键控（QPSK）,而8- QAM的误码率性能接近16-QAM（只有约0.5分贝的改善[需要引证]），然而其数据传输速率只有16-QAM的四分之三。
Expressions for the symbol-error rate of rectangular QAM are not hard to derive but yield rather unpleasant expressions. For an even number of bits per symbol, k, exact expressions are available. They are most easily expressed in a per carrier sense:
矩形QAM的信符误码率的表达式不难推导，但是所得到的表达式并不直观、简洁。对于每个信符的位数是偶次K，确切的表达式是可用的。在每单个载波中,他们最容易表达为：
,
so 所以
.
The bit-error rate depends on the bit to symbol mapping, but for and a Gray-coded assignment—so that we can assume each symbol error causes only one bit error—the bit-error rate is approximately
比特误码率取决于比特对信符的映射，但只对于，以及一种格雷编码的安排而言---这样我们可以假定,每个信符错误只引起一位错误---其比特误码率近似为
.
Since the carriers are independent, the overall bit error rate is the same as the per-carrier error rate, just like BPSK and QPSK.
由于载波是独立的，每载波的错误率与整体比特误码率是一样的，就像BPSK和QPSK是相同的。
.
4.1.1 [edit] Odd-k QAM [编辑]每个信符的位数是奇次-K QAM
For odd k, such as 8-QAM (k = 3) it is harder to obtain symbol-error rates, but a tight upper bound is:
对于位数是奇次,k,如 8-QAM（K = 3），这很难获得信符误码率，但一个很接近的上限是：
.
Two rectangular 8-QAM constellations are shown below without bit assignments. These both have the same minimum distance between symbol points, and thus the same symbol-error rate (to a first approximation).
两个矩形8-QAM星座,没有注明比特位安排,显示在下面。这两个都具有相同的最小信符点间距，因而有相同的信符误码率（一级近似）。
The exact bit-error rate, Pb will depend on the bit-assignment.
确切的误码率, Pb ,取决于比特位安排。
Note that both of these constellations are seldom used in practice, as the non-rectangular version of 8-QAM is optimal. Example of second constellation's usage: LDPC and 8-QAM.
请注意，这两个星座在实践中很少使用，虽然作为8-QAM的非矩形型式是最佳的。第二个星座的用法示例是：LDPC 和 8-QAM。
Constellation diagram for rectangular 8-QAM. 8-QAM矩形星座图。
Alternative constellation diagram for rectangular 8-QAM.
另一种8-QAM矩形星座图。
4.2 [edit] Non-rectangular QAM [编辑]非矩形QAM
Constellation diagram for circular 8-QAM. 8-QAM圆形星座图。
Constellation diagram for circular 16-QAM. 16-QAM圆形星座图。
It is the nature of QAM that most orders of constellations can be constructed in many different ways and it is neither possible nor instructive to cover them all here. This article instead presents two, lower-order constellations.
圆形星座反映了QAM的内在性质，它们可以用许多不同的方法 来构建大多数阶次的星座。要在这里全部包括它们,既不可能,也没有指导意义。本文代之以介绍两种低阶星座。
Two diagrams of circular QAM constellation are shown, for 8-QAM and 16-QAM. The circular 8-QAM constellation is known to be the optimal 8-QAM constellation in the sense of requiring the least mean power for a given minimum Euclidean distance. The 16-QAM constellation is suboptimal although the optimal one may be constructed along the same lines as the 8-QAM constellation. The circular constellation highlights the relationship between QAM and PSK. Other orders of constellation may be constructed along similar (or very different) lines. It is consequently hard to establish expressions for the error rates of non-rectangular QAM since it necessarily depends on the constellation. Nevertheless, an obvious upper bound to the rate is related to the minimum Euclidean distance of the constellation (the shortest straight-line distance between two points):
两个用于8-QAM 和16-QAM的圆形QAM星座图已经显示如上。圆形8-QAM星座被称为最佳的8-QAM星座,这是指：在一个给定的最小欧氏距离下,所需要的最小平均功率而言。图中的16-QAM星座是次佳的，虽然可以沿着建造最佳8-QAM星座的相同路线构成16-QAM星座。圆形星座突出了QAM和PSK之间的关系。其他阶次的星座可以沿着相似的（或十分不同的）建造路线。因此难以建立非矩形QAM的误码率表达式，因为它必然取决于星座。然而，对于与星座的最小欧氏距离相关的误码率,其上界的显式（两点之间的最短直线距离）为：.
Again, the bit-error rate will depend on the assignment of bits to symbols. 另外，该比特误码率取决于信符的比特位安排。
Although, in general, there is a non-rectangular constellation that is optimal for a particular M, they are not often used since the rectangular QAMs are much easier to modulate and demodulate.
虽然,在一般情况下,对于一个特定的M,有一个最佳的非矩形星座，但他们却不是经常使用的，因为矩形QAM的调制和解调要容易得多。
5. [edit] Interference and noise [编辑]干扰和噪声
In moving to a higher order QAM constellation (higher data rate and mode) in hostile RF/microwave QAM application environments, such as in broadcasting or telecommunications, multipath interference typically increases. There is a spreading of the spots in the constellation, decreasing the separation between adjacent states, making it difficult for the receiver to decode the signal appropriately. In other words, there is reduced noise immunity . There are several test parameter measurements which help determine an optimal QAM mode for a specific operating environment. The following three are most significant:[3]
当移向高阶QAM星座（更高的数据速率和模式）时,在竞争的射频/微波QAM的应用环境中，如通播或电信，多径干扰通常会增加。在星座中,这是一种扩展了的星座点,减少了相邻状态间的分离，使收信机难以适当地解码信号。换句话说，降低了噪声免除率。有几种测试参数的测量方法，它们有助于确定一个优化的QAM模式,应用于特定的运行环境。下面是最有用的三个：[3]
· Carrier/interference ratio 载波/干扰比
· Carrier-to-noise ratio 载波噪声比
· Threshold-to-noise ratio 阈值信噪比
6. [edit] See also 参见
· Modulation for other examples of modulation techniques
· Amplitude and phase-shift keying or Asymmetric phase-shift keying (APSK)
· Carrierless Amplitude Phase Modulation (CAP)
· QAM tuner for HDTV
7. [edit] References 参考
1. ^ UAS UAV communications links
2. ^ Kylia products, dwdm mux demux, 90 degree optical hybrid, d(q) psk demodulatorssingle polarization
3. ^ Howard Friedenberg and Sunil Naik. "Hitless Space Diversity STL Enables IP+Audio in Narrow STL Bands". 2005 National Association of Broadcasters Annual Convention. Retrieved April 17, 2005.
The notation used here has mainly (but not exclusively) been taken from
John G. Proakis, "Digital Communications, 3rd Edition",
8. [edit] External links 外部链接
| |
Wikimedia Commons has media related to: Quadrature amplitude modulation |
· How imperfections affect QAM constellation[dead link]
· Microwave Phase Shifters Overview by Herley General Microwave
· QAM Baseband Modem - EPN-online.co.uk
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