Common sense in signal transmission

Due to professional requirements, common sense in signal transmission is required. This blog post is a note and will be updated from time to time.

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1. Quick knowledge improvement

• 1. Baud can be understood as how many symbols of data a device sends (or receives) in one second. It is a measure of the symbol transmission rate and represents the number of symbols transmitted per unit time. (Transmission symbol rate). Multiple bits of information can be loaded on one symbol through different modulation methods.
• 2. Bit rate refers to the amount of information transmitted through the channel per second (also called bit transmission rate), that is, the number of binary digits transmitted per second, which is used to indicate the transmission rate of effective data.
• 3. Bandwidth refers to the ability of the communication line to transmit data, and initially means the width of the electromagnetic wave frequency band. The commonly used unit is bps (bit per second)
• 4. Carrier-to-noise ratio (signal-to-noise ratio) is a standard measurement scale used to indicate the relationship between carrier and carrier noise, usually denoted as CNRor C/N(dB).
• 5. The difference between signal-to-noise ratio and carrier-to-noise ratio is that the power of the modulated signal in the carrier-to-noise ratio includes the power of the transmission signal and the power of the modulated carrier, while the signal-to-noise ratio only includes the power of the transmission signal.

2. Basic concepts

1. Porter

Porter (Baud, unit symbol: Bd) is named after the French telecommunications engineer Emile Baudot (1845-1903). He was one of the pioneers of digital communications and was one of the pioneers of digital communications. Biography and the inventor of the Hakata-style telegraph.

In the information transmission channel, the signal unit that carries the data information is called the symbol, and the number of symbols transmitted through the channel per second is called the symbol transmission rate, referred to as the baud rate, and its unit is Baud (symbol/s) , The baud rate is an indicator of the bandwidth of the transmission channel. "Baud" is already a rate, so there is no need to write Baud Rate (Rate is a superfluous word). The unit "baud" itself already represents the number of modulations per second. It is a common mistake to use "baud per second" as the unit, but in general oral Chinese communication, the "baud rate" is often used. Describe "Baud".

The baud rate can be understood as how many symbols of data a device sends (or receives) in one second. It is a measure of the symbol transmission rate and represents the number of symbols transmitted per unit time (symbol rate ). Multiple bits of information can be loaded on one symbol through different modulation methods. In computer network communication, the baud rate refers to the rate of the single-chip microcomputer or computer in the serial communication, and refers to the change of the signal within a unit time after being modulated, that is, the number of times the carrier parameter changes per unit time. The waveform is measured by the number of oscillations per second. If the data is not compressed, the baud rate is equal to the number of data bits transmitted per second. If the data is compressed, then the number of data bits transmitted per second is usually greater than the modulation rate, so that the exchange of baud and bits per second occasionally produces error.

The information transmission rate, that is, the bit rate is numerically related to the baud rate:
$I=S\cdot lo{g}_{2}N$

Among them: I is the transmission rate, S is the baud rate, N is the amount of information carried by each symbol, $lo{g}_{2}N\; is$ in bits.

Baud rates and bit rates may also be converted into: 比特率=波特率*单个调制状态对应的二进制位数. For example, the data transfer rate is 240b/s, and each character format contains 10 bits (1 start bit, 1 stop bit, 8 valid data bits), then the baud rate 240 baud(baud), bit rate (valid data The bit transfer rate) is 240*10=2400 bps(bits per second). (But the general modulation rate is greater than the baud rate, such as Manchester encoding).

The relationship between baud rate and bit rate:

• The baud rate (in fact, it should be called baud) is the number of symbols transmitted per unit time (symbol transmission rate), and multiple bits of information can be carried on one symbol through different modulation methods. The bit rate bps (bit per second) is a measure of the information transmission rate (transmission rate).

• Bit rate refers to the amount of information transmitted through the channel per second (also called bit transmission rate), that is, the number of binary digits transmitted per second, which is used to indicate the transmission rate of effective data, using b/s, bit/s, bit /Sec, read as: bits per second, commonly used units are bits per second (bps), kilobits per second (Kbps) or megabits per second (Mbps) (where K and M are respectively 1000 and 1000000 instead of 1024 and 1048576 when referring to computer memory capacity).

• 1 baud refers to the transmission of 1 symbol symbol per second (multiple bits of information can be loaded on a symbol symbol through different modulation methods), and 1 bit per second refers to the transmission of 1 bit per second (bit).

• The baud rate generally refers to the communication speed of the modem, that is, the number of times the line status changes. Only when each signal matches one bit of the transmitted data, it is equal to the number of bits per second. In order to communicate with each other, modems must operate at the same baud rate. If the baud rate of the modem is set to be higher than the baud rate of other modems, the faster modem will usually change its baud rate to match the slower modem.

• Strictly speaking, the baud rate is generally not used in the field of communication transmission. The baud rate describes the ability to modulate a signal per unit time, and the signal modulated by it is transmitted in the form of bits. In other words, if the signal is to undergo digital-to-analog conversion during transmission, modulation is required, then In addition to the transmission time consumed in other fields, it is also consumed in the modulation process and the transmission process in the channel. The baud rate is used to describe the signal modulation capability, and the bit rate is used to describe the signal transmission capability.

2. Bandwidth

Bandwidth is applied in many fields, and it can be used to identify the data transmission capacity of signal transmission, the amount of data passing through the link per unit time, and the display ability of the display. In the analog signal system, it is also called bandwidth, which refers to the amount of data that can be transmitted at a fixed time, that is, the ability to transmit data in the transmission pipeline. It is usually Hzexpressed in transmission cycles per second or Hertz ( ). In digital equipment, bandwidth refers to the amount of data that can pass through the link per unit time. Generally bps(bit per second)expressed, i.e. the number of bits can be transmitted per second.

The term bandwidth originally refers to the width of the electromagnetic wave frequency band, that is, the difference between the highest frequency and the lowest frequency of a signal. At present, it is more widely used in digital communication to describe the theoretical maximum rate of data transmission on a network or line. This is not its academic definition, but it is used by extension.

• The bus bandwidth refers to the total amount of data that the bus can transmit in a unit time, which is equal to the product of the bus bit width and the operating frequency. For example: For the front side bus of 64bit, 800MHzits data transfer rate is equal to64bit×800×1000×1000Hz÷8(Byte)÷1024÷1024÷1024≈6.0GB/s
• Memory bandwidth refers to the data transmission capacity that the memory bus can provide. For example: DDR400the data transmission frequency of the memory is 400MHz, then a single module has the bandwidth 64bit×400×2(ddr is the upper and lower edge collection signal) ×1000×1000Hz÷8(Byte)÷1024÷1024÷1024≈3.0GB/s.

Generally speaking, bandwidth is bitexpressed in (bits), and operators such as China Telecom, China Unicom, and China Mobile often ignore this unit when promoting it. The normal conversion situation is as follows:

• 1Mbit=128KB
• 2Mbit=256KB
  (and so on)

The converted speed is your real Internet speed. That is to say, if the bandwidth you subscribed from your carrier is 10M, then substitute the calculation formula, and use the above-converted 1M to measure: (1M=1024K)

• 1M/128K=1024/128=8
• 10/8=1.25M

In other words, if you enable 10M bandwidth, you can reach a speed of up to 1.25M. Generally speaking, 4M bandwidth is sufficient for a computer to watch movies, play games, etc. But if you need to download large files frequently, it is recommended to use a higher bandwidth

When bandwidth represents the ability of a communication line to transmit data, it means the "highest data rate" that can be passed from one point to another in the network within a unit time. For the concept of bandwidth, a more vivid analogy is the highway. The amount of data that can be transmitted on the wire per unit time, the commonly used unit is bps (bit per second). The bandwidth of a computer network refers to the highest data rate that the network can pass through, that is, how many bits per second.

Strictly speaking, the bandwidth of a digital network should be expressed in baud rate (baud), which means the number of pulses per second. Bit is the unit of information. Since digital equipment uses binary system, the amount of information carried by each bit level is the logarithm of base 2. If it is quaternary, it is the logarithm of base 2. The amount of information carried by each bit level is 2. Therefore, in terms of value, the baud is the same as the bit. Since people are not very clear about the two concepts, bit rate is often used to express rate. It is also because there are too many people using bits, so bit rate has become a de facto standard name for bandwidth.

3. Carrier-to-noise ratio and signal-to-noise ratio

Carrier-to-noise ratio (signal-to-noise ratio) is a standard measurement scale used to indicate the relationship between carrier and carrier noise, usually denoted as CNRor C/N(dB). A high carrier-to-noise ratio can provide a better network reception rate, better network communication quality, and better network reliability. In the carrier-to-noise ratio, the power of the carrier is Pcdenoted by and the power of the noise is Pndenoted by. Then the decibel unit formula of the C/N = 10 lg(Pc/Pn)carrier-to-noise ratio is expressed as: the carrier-to-noise ratio is similar to the signal-to-noise ratio as a measure of network channel quality. But the signal-to-noise ratio is usually used in practical applications. The carrier-to-noise ratio is used in satellite communication systems. The best antenna arrangement can get the best carrier-to-noise ratio value.

In communications, another commonly used carrier-to-noise ratio is C/no, the unit is dBHz, and the calculation formula is (C/N)*B, where Bis the system bandwidth, which is the ratio of signal power to noise power density.

The difference between the signal-to-noise ratio and the carrier-to-noise ratio is that the power of the modulated signal in the carrier-to-noise ratio includes the power of the transmission signal and the power of the modulated carrier, while the signal-to-noise ratio only includes the power of the transmission signal. 因此对同一个传输系统而言，载噪比要比信噪比大，两者之间相差一个载波功率。Of course, the carrier power is usually very small compared to the transmission signal power, so the carrier-to-noise ratio and the signal-to-noise ratio are very close in value. 在调制传输系统中，一般采用载噪比指标；而在基带传输系统中，一般采用信噪比指标. DVBThe broadcast transmission system is a modulation transmission system, so the carrier-to-noise ratio index is adopted. The carrier-to-noise ratio represents the strength of the noise interference relative to the modulation signal. The absolute strength of the noise interference is meaningless, because the average power of the signal of different transmission systems is different, and the carrier-to-noise ratio directly reflects the relative strength of the modulation signal and the noise interference. DVBThe reliability of the final information transmission of the transmission system is determined by the carrier-to-noise ratio.

The improvement coefficient Iis defined as the difference between the signal-to-noise ratio and the carrier-to-noise ratio, namelyI=S/N－C/N