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Welcome to Chapter 2, Duck! ! !
Chapter 2 (4,7,9,14,15,16)
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Data: is the entity that conveys information.
Signal: is the electrical or electromagnetic manifestation of data.
Analog Data: An analog signal that conveys information.
Analog signal: A signal that changes continuously. Baseband signal: The signal from the source.
Baseband signal: The signal from the source. Data signals representing various text or image files output by a computer are all baseband signals.
Bandpass signal: The signal after carrier modulation.
Numeric data: Data whose value is a discontinuous value.
Digital signal: A signal with a limited number of discrete values.
Symbol: When using a time-domain waveform to represent a digital signal, it represents a basic waveform of different discrete values.
Simplex communication: That is, there is only one direction of communication without interaction in the opposite direction.
Half-duplex communication: that is, both parties to the communication can send information, but both parties cannot send information at the same time (of course they cannot receive at the same time). This communication method is that one party sends and the other receives, and then reverses after a period of time.
Full-duplex communication: that is, both parties to the communication can send and receive information at the same time.
Serial transmission: Use one data line to transmit data, one bit at a time, and multiple bits need to be transmitted one by one.
Parallel transfer: Using multiple data wires to transfer multiple bits at once.
analyze
（1）信噪比(dB)=10 log10(S/N)(dB) //S为信号的平均功率，N为噪声的平均功率。
（2）香农公式：C=Wlog2(1+S/N)(bit/s) //C为信道的极限信息传输速率，W是信道带宽
（3）M进制的码元，码元宽度为T秒：C=1/T×log2(M)
（4）无噪:带宽为BHz。 C=2Blog2(M)
（5）有噪:带宽为BHz。 C=Blog2(1+S/N)
（6）最高码元速率R，码元振幅等级n C=Rlog2(n)
（7）频率＝传播速率/波长
（8）频带宽度=最高频率-最低频率
Refer to formula (6) for this question
Solution
C=RLog2(16)=20000×4=80000 bit/s
other
Intersymbol Interference: The frequency range that a specific channel can pass is always limited. Many high-frequency components in the signal often cannot pass through the channel. The sending signal shown in Figure 2-4 is a typical rectangular pulse signal, which contains a lot of high-frequency components. If the high-frequency components in the signal are attenuated during transmission, the leading and trailing edges of the waveform received at the receiving end will become less steep, and the time boundary occupied by each symbol will no longer be very clear. Instead, it dragged the "tail" back and forth. Thus, the signal waveform received at the receiving end loses the clear boundaries between symbols. This phenomenon is called intersymbol interference.
Ney's criterion conclusion: In a low-communication channel with a bandwidth of W (Hz), if the influence of noise is not considered, the highest rate of symbol transmission is 2W (symbol/second). If the transmission rate exceeds this upper limit, serious intersymbol interference will occur, making it impossible for the receiving end to judge (that is, identify) the symbols. For example, if the bandwidth of the channel is 4000Hz, then the highest symbol transmission rate is 8000 symbols per second.
Signal-to-Noise Ratio: Noise is present in all electronic devices and communication channels. Because the noise is randomly generated, its instantaneous value is sometimes very large, so the noise will cause the receiving end to make an error in the judgment of the symbol (1 is misjudged as 0 or 0 is misjudged as 1). But the impact of noise is relative. If the signal is relatively strong, the effect of noise is relatively small. Therefore, the signal-to-noise ratio is very important. The so-called signal-to-noise ratio
信号的平均功率 / 噪声的平均功率is often recorded as S/N. But usually everyone uses decibels (dB) as a unit of measurement. That is: the formula (1) in the analysis of this question
Analysis
Refer to formula (2) in the analysis of 2-07
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Solve
FDM (frequency division multiplexing) frequency division multiplexing, and send multiple signals at the same time. All users can occupy different bandwidth resources at the same time.
TDM (Time Division Multiplexing) time-division multiplexing divides a physical channel into several time slices and uses them in turn for multiple users. Each time slice is occupied by a multiplexed user, and all users occupy the same frequency width at different times. .
STDM (Statistic Time Division Multiplexing) statistical time division multiplexing, an improved time division multiplexing. Unlike time division multiplexing, the time slots are allocated in a fixed way, but the time slots are dynamically allocated on demand.
WDM (Wave Division Multiplexing) wavelength division multiplexing, a perfunctory variant of frequency division multiplexing used on optical channels, that is, optical frequency division multiplexing. The multiplexing method in which light wave signals (often two light wave signals) on different optical fibers are multiplexed onto one optical fiber for long-distance transmission.
DWDM (Dense Wave Division Multiplexing) Dense Wavelength Division Multiplexing, a technology that uses the broadband characteristics of the visible light spectrum to simultaneously transmit multiple light wave signals on a single optical fiber. DWDM can use one optical fiber to transmit multiple wavelengths at the same time. Multiple high-speed signals can be transmitted in the optical fiber medium at the same time, and each signal occupies a different wavelength.
CDMA (Code Wave Division Multiplexing) is a code division multiple access technology that uses spread spectrum. Users can obtain traffic channels according to different codes at the same time and on the same frequency band.
SONETSynchronous Optical Network (Synchronous Optical Network) is an American standard for optical fiber digital transmission at a graded rate from 155Mb/s to 2.5Gb/s. It supports multimedia multiplexing and allows voice, video and data formats to communicate with different transmission protocols. transmitted together on a fiber optic line.
SDH (Synchronous Digital Hierarchy) refers to the international standard synchronous digital hierarchy. SDH simplifies the multiplexing and demultiplexing technology, and can be directly connected to the low-speed branch when necessary, without step-by-step demultiplexing from high-speed to low-speed, and the up and down circuits are convenient.
STM-1 (Synchronous Transfer Module) level 1 synchronous transfer module, the basic rate of SDH is equivalent to the OC-3 rate in the SONET system.
OC-48 (Optical Carrier) is the 48th grade optical carrier, which is the rate representation in the SONET system, corresponding to the STM-16 rate of SDH, and the commonly used approximate value is 2.5Gb/s.
AnswerBecause
when users use CDMA communication, each user uses a specially selected different code pattern to transmit information, and modulates it with a high-speed pseudo-random code whose bandwidth is much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded, and then transmitted through the carrier wave. Modulate and send out. The receiving end uses the same pseudo-random code to perform correlation processing with the received bandwidth signal, and replaces the bandwidth signal with the narrowband signal of the original information book, that is, despreading, to realize information communication. There is no interference between users.
Advantages: high spectrum utilization rate, large capacity; large coverage; strong anti-interference ability, its spectrum is similar to white noise, and the transmitted signal is not easy to be found by the enemy; the use of CDMA can improve the voice quality of communication and the reliability of data transmission Sex, reduce the impact on communication; low network cost; reduce the average transmission power of mobile phones and so on.
Disadvantages: Each station needs to be assigned different mutually orthogonal chip sequences; the region is affected by the line, not every place can be used, and the installation time is long.
analyze
Assuming that there is a station X receiving the data sent by station S, station X uses the obtained chip vector S to perform an inner product operation on the received unknown signal. When station
S sends bit 1, the inner product result is +1.
When station S sends When the bit is 0, the result of the inner product is -1;
when the S station does not send data, the result of the inner product is 0
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