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电气工程代写|数字系统设计作业代写Digital System Design代考|Digital Modulation
The most common form of digital modulation is changing the phase of the carrier frequency in accordance with the digital signal that is being sent. This is known as phase-shift keying (PSK) or direct sequence digital modulation. This digital sequence can be either the digitized data or a combination of digitized data and a spread spectrum sequence. There are many different levels and types of PSK; only basic modulation methods are discussed here. However, the principles can be extended to higher order PSK modulations. The basic form of digital communication is shown in Figure 2-10. This shows the carrier frequency and the digital data being fed into a modulator. The modulator is binary phase-shift keying (BPSK) and changes the phase between $0^{\circ}$ and $180^{\circ}$ according to the digital data being sent.
The BPSK modulator and phasor diagram is shown in Figure 2-11. BPSK is defined as shifting the carrier $0^{\circ}$ or $180^{\circ}$ in phase, depending on the digital waveform. For example, a binary ” $0 “$ gives $0^{\circ}$ phase of the carrier, and a binary ” $1 “$ shifts the carrier by $180^{\circ}$.
To produce the digital waveform, the data or information signal is digitized, encoded, and sent out in a serial bit stream (if not already). The end result is a serial modulating digital waveform representing the data to be transmitted. This digital output contains 0 and 1 and often needs to be changed to $\pm 1$ to directly modulate the carrier frequency in a typical mixer application. However, certain forms of hardware can bypass this step and modulate the mixer directly. The output of the mixer is a BPSK modulated carrier signal that is transmitted and sent over to the receiver for demodulation and detection. As the carrier phase changes $180^{\circ}$, the hardware does not allow for an instantaneous change in phase, so the amplitude goes to zero, which produces $100 \%$ amplitude modulation (Figure 2-11). This is called the zero crossover point and is an unwanted characteristic of BPSK and can cause degradation to the waveform and performance.
A high-speed pseudo-noise (PN) code and modulo-2 adder/exclusive-or functions are added to the basic BPSK modulator to produce a spread spectrum waveform (Figure 2-12). The high-speed code generates a wider spectrum than the data spectrum needed for communications, which is known as a spread spectrum.
Applying minus voltage to an actual RF mixer reverses the current through the balun of the mixer and causes the current to flow in the opposite direction, creating a net $180^{\circ}$ phase shift of the carrier. Thus, the carrier is phase shifted between $0^{\circ}$ and $180^{\circ}$ depending on the input waveform. A simple way of generating BPSK spread spectrum is shown in Figure 2-13. The LO is modulated by this digital sequence producing a $0^{\circ}$ or $180^{\circ}$ phase shift on the output of the mixer. Other devices such as phase modulators and phase shifters can create the same waveform as long as one digital level compared with the other digital level creates a $180^{\circ}$ phase difference in the carrier output.
电气工程代写|数字系统设计作业代写Digital System Design代考|Differential Phase-Shift Keying
The BPSK waveform above can be sent out as absolute phase (i.e., a $0^{\circ}$ phase shift is a ” 1 ” and a $180^{\circ}$ phase shift is a “0”). This type of system is known as a coherent system, or coherent BPSK. Another way to perform this function is to use differential phase-shift keying (DPSK), which generates and detects a change of phase. A change of phase $\left(0^{\circ}\right.$ to $180^{\circ}$ or $180^{\circ}$ to $\left.0^{\circ}\right)$ represents a ” $1 “$ “, and no change $\left(0^{\circ}\right.$ to $0^{\circ}$ or $180^{\circ}$ to $\left.180^{\circ}\right)$ represents a ” 0. .” This scheme is easier to detect because the absolute phase does not need to be determined; just the change of phase is monitored. The differential mode can be applied to various phase shifting schemes and higher order phase shift schemes. DPSK results in a degradation of the signal, compared with using the absolute phase or coherent BPSK. However, it is much easier to demodulate and detect the data and generally is less costly due to the demands of keeping the phase constant over a longer period of time. Note that differential techniques can be applied in higher order PSK systems such as differential quadrature phaseshift keying (DQPSK) and differential 8-level phase-shift keying (D8PSK).
Quadrature phase-shift keying (QPSK) is generated by quadrature phase shifting the LO so that four possible phase states are produced at the output. One method of producing these phasors is by using two channels: one channel containing an LO that is in phase and the output of the mixer is BPSK modulated to produce $0^{\circ}$ or $180^{\circ}$; and the other channel containing the same LO that is shifted by $90^{\circ}$ so that the output of the mixer is BPSK modulated to produce $90^{\circ}$ or $270^{\circ}$. These two channels are then combined to produce the four phase states (Figure 2-13). The two BPSK systems are summed together, which gives four possible resultant phasors $-45^{\circ}, 135^{\circ},-135^{\circ}$, and $-45^{\circ}$-which are all in quadrature, as shown in Figure 2-13. Since the digital transitions occur at the same time, changes hetween any four resultant phasors can necur.
Depending on the input of both bit streams, the phase of the resultant can be at any of the four possible phases. For example, if both bit streams are 0 , then the phasor would be $45^{\circ}$. If both bit streams changed to 1 , then the phasor would be at $-135^{\circ}$, giving a change of carrier phase of $180^{\circ}$. If only the first channel changes to 1 , then the phasor would be at $-45^{\circ}$, giving a change of $90^{\circ}$. Therefore, the four possible phase states are $45^{\circ}, 135^{\circ},-135^{\circ}$, or $-45^{\circ}$. The phasor diagram can be rotated, since it is continually rotating in time and only a snapshot is shown, and then the phasors would be at $0^{\circ}, 90^{\circ},-90^{\circ}$, and $180^{\circ}$ for QPSK generation. QPSK has the capability of $180^{\circ}$ phase shifts depending on the code and the previous phase state, so it often goes through the zero crossover point, which produces $100 \%$ amplitude modulation during the phase transition as shown in Figure 2-13. This causes unwanted characteristic of QPSK and can cause degradation to the waveform and performance.
Usually, the LO contains the $90^{\circ}$ phase shift that is used to provide the two quadrature channels of the BPSK phasors instead of trying to provide the $90^{\circ}$ phase shift of the actual hinary input. Fither way it would provide the $90^{\circ}$ quadrature phase rotation that is required. However, quadrature rotation of the binary input requires phase shifting all of the frequency components by $90^{\circ}$. This requires a more sophisticated and expensive phase shifter that is broadband. Shifting only the carrier to produce the quadrature channels requires a phase shift at one frequency, which is much simpler to build. In fact, in the latter case, a longer piece of cable cut to the right length can provide this $90^{\circ}$ quadrature phase shift.
数字系统设计代考
电气工程代写|数字系统设计作业代写Digital System Design代考|Digital Modulation
最常见的数字调制形式是根据正在发送的数字信号改变载波频率的相位。这称为相移键控 (PSK) 或直接序列数字调制。该数字序列可以是数字化数据，也可以是数字化数据和扩频序列的组合。PSK 有许多不同的级别和类型；这里只讨论基本的调制方法。然而，这些原理可以扩展到更高阶的 PSK 调制。数字通信的基本形式如图 2-10 所示。这显示了载波频率和输入调制器的数字数据。调制器是二进制相移键控 (BPSK) 并改变之间的相位0∘和180∘根据正在发送的数字数据。
BPSK 调制器和相量图如图 2-11 所示。BPSK被定义为移动载波0∘或者180∘同相，取决于数字波形。例如，二进制“0“给0∘载波的相位，以及一个二进制“1“将载体转移180∘.
为了产生数字波形，数据或信息信号被数字化、编码并以串行比特流（如果还没有的话）发送出去。最终结果是表示要传输的数据的串行调制数字波形。此数字输出包含 0 和 1，通常需要更改为±1在典型的混频器应用中直接调制载波频率。但是，某些形式的硬件可以绕过此步骤并直接调制混频器。混频器的输出是一个 BPSK 调制载波信号，它被传输并发送到接收器进行解调和检测。随着载波相位的变化180∘，硬件不允许相位的瞬时变化，因此幅度变为零，从而产生100%幅度调制（图 2-11）。这称为零交叉点，是 BPSK 的一个不需要的特性，可能会导致波形和性能下降。
高速伪噪声 (PN) 码和模 2 加法器/异或函数被添加到基本 BPSK 调制器中，以产生扩频波形（图 2-12）。高速编码产生比通信所需的数据频谱更宽的频谱，这被称为扩频。
向实际的 RF 混频器施加负电压会使通过混频器的平衡不平衡转换器的电流反向，并导致电流沿相反方向流动，从而形成一个网络180∘载波的相移。因此，载波之间的相移0∘和180∘取决于输入波形。生成 BPSK 扩频的一种简单方法如图 2-13 所示。LO 由这个数字序列调制，产生一个0∘或者180∘混频器输出的相移。只要一个数字电平与另一个数字电平相比，其他设备（例如相位调制器和移相器）可以创建相同的波形180∘载波输出的相位差。
电气工程代写|数字系统设计作业代写Digital System Design代考|Differential Phase-Shift Keying
上面的 BPSK 波形可以作为绝对相位发送出去（即0∘相移是“1”和180∘相移为“0”）。这种类型的系统称为相干系统或相干 BPSK。执行此功能的另一种方法是使用差分相移键控 (DPSK)，它生成并检测相位变化。相变(0∘至180∘或者180∘至0∘)代表一个”1““，没有变化(0∘至0∘或者180∘至180∘)代表一个“0..” 这种方案更容易检测，因为不需要确定绝对相位；只是监测相位的变化。差模可以应用于各种相移方案和高阶相移方案。与使用绝对相位或相干 BPSK 相比，DPSK 会导致信号劣化。但是，由于需要在较长时间内保持相位恒定，因此解调和检测数据要容易得多，并且通常成本较低。请注意，差分技术可以应用于更高阶的 PSK 系统，例如差分正交相移键控 (DQPSK) 和差分 8 级相移键控 (D8PSK)。
正交相移键控 (QPSK) 是通过对 LO 进行正交相移来生成的，以便在输出端产生四种可能的相位状态。产生这些相量的一种方法是使用两个通道：一个通道包含同相的 LO，混频器的输出经过 BPSK 调制以产生0∘或者180∘; 另一个包含相同 LO 的通道由90∘使混频器的输出经过 BPSK 调制以产生90∘或者270∘. 然后将这两个通道组合起来产生四个相位状态（图 2-13）。将两个 BPSK 系统相加，得出四个可能的合成相量−45∘,135∘,−135∘， 和−45∘-它们都是正交的，如图2-13所示。由于数字转换同时发生，任何四个合成相量之间的变化都可能发生。
根据两个比特流的输入，结果的相位可以是四个可能相位中的任何一个。例如，如果两个比特流都是 0 ，那么相量将是45∘. 如果两个比特流都变为 1 ，那么相量将在−135∘，给出载波相位的变化180∘. 如果只有第一个通道更改为 1 ，则相量将位于−45∘, 给出一个变化90∘. 因此，四种可能的相位状态是45∘,135∘,−135∘， 或者−45∘. 相量图可以旋转，因为它在时间上不断旋转，并且只显示快照，然后相量将位于0∘,90∘,−90∘， 和180∘用于 QPSK 生成。QPSK 具有以下能力180∘相移取决于代码和先前的相位状态，因此它经常通过零交叉点，从而产生100%相变期间的幅度调制如图 2-13 所示。这会导致 QPSK 出现不需要的特性，并可能导致波形和性能下降。
通常，LO 包含90∘相移用于提供 BPSK 相量的两个正交通道，而不是试图提供90∘实际二进制输入的相移。它将提供的Fither方式90∘需要的正交相位旋转。然而，二进制输入的正交旋转需要将所有频率分量相移90∘. 这需要更复杂和更昂贵的宽带移相器。仅移动载波以产生正交信道需要在一个频率上进行相移，这更容易构建。事实上，在后一种情况下，将一根较长的电缆切割成合适的长度就可以提供这一点90∘正交相移。
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