《网络通信》Chapter 5 Signal Encoding

Bilingual Course Network Communications 网络通信 ( For Master Students in the Department of Electronic Engineering) Chapter 5 Signal Encoding Instructor: Dr Tianshuang Qiu School of the Electronic and Information Engineering Fall of 2004

1 Bilingual Course Network Communications 网络通信 (For Master Students in the Department of Electronic Engineering) Chapter 5 Signal Encoding Instructor: Dr. Tianshuang Qiu School of the Electronic and Information Engineering Fall of 2004

Data Encoding Digital data, digital signal: In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment Analog data, digital signal: conversion of analog data to digital form permits the use of modern digital transmission and switching equipment. The advantages of the digital approach were outlined in Section 3. 2 Digital data, analog signal: Some transmission media, such as optical fiber and unguided media, will only propagate analog signals Analog data, digital signal: Analog data in electric form can be transmitted as baseband signals easily and cheaply. This is done with voice transmission over voice-grade lines. One common use of modulation is to shift the bandwidth of a baseband signal to another portion of the spectrum. In this way multiple signals each at a different position on the spectrum, can share the same transmission medium. This is know as frequency-division multiplexing 2

2 Data Encoding  Digital data, digital signal: In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment.  Analog data, digital signal: conversion of analog data to digital form permits the use of modern digital transmission and switching equipment. The advantages of the digital approach were outlined in Section 3.2.  Digital data, analog signal: Some transmission media, such as optical fiber and unguided media,will only propagate analog signals.  Analog data, digital signal: Analog data in electric form can be transmitted as baseband signals easily and cheaply. This is done with voice transmission over voice-grade lines. One common use of modulation is to shift the bandwidth of a baseband signal to another portion of the spectrum. In this way multiple signals, each at a different position on the spectrum, can share the same transmission medium. This is know as frequency-division multiplexing

85-1 Digital Data, Digital Signal 0:1:0:01:1:0:0:0:1:1 NRZ-L NRZI Bipolar-AMI Pseudoternary Manchester lUyuN Differential Manchester Figure 5.2 Digital Signal Encoding Formats 3

3 §5-1 Digital Data, Digital Signal

1.几种编码形式 (1) Non Return to Zero(nrz 最简单; 在一个bit周期内电平横定 正负跳变,不回0 0一由高电平表示,1—由低电平表示 优点:易于实现,有效利用带宽; 缺点:有直流成分,缺乏同步能力(若一串1或一串0,则无法同 步)

4 1. 几种编码形式 （1）Non Return to Zero (NRZ) • 最简单； • 在一个bit周期内电平横定； • 正负跳变，不回0； • 0—由高电平表示，1—由低电平表示； • 优点：易于实现，有效利用带宽； • 缺点：有直流成分，缺乏同步能力（若一串1或一串0，则无法同 步）

(2) Nonreturn to Zero inverted(NRZⅠ不归零1制 0—在间隔的起始位置无跳变 1—在间隔的起始位置有跳变 (其他特点同NR (3) Bipolar AMI( Alternate mark inversion,交替信号反转) 0一没有信号 1—正/负电平 优点:很长的串1,不会失去同步; 信号1在正负电平之间跳变,无直流分量; 信号的带宽比较窄 脉冲交替,有利于差错检测 (4) Pseudoternary(伪三进制) ·0—正电平或负电平; 1—没有信号

5 （2）Nonreturn to Zero Inverted (NRZI, 不归零1制) • 0—在间隔的起始位置无跳变 • 1—在间隔的起始位置有跳变 （其他特点同NRZ） （3）Bipolar AMI (Alternate mark inversion, 交替信号反转) • 0—没有信号； • 1—正/负电平 • 优点：很长的串1，不会失去同步； 信号1在正负电平之间跳变，无直流分量； 信号的带宽比较窄； 脉冲交替，有利于差错检测 （4）Pseudoternary（伪三进制） • 0—正电平或负电平； • 1—没有信号

(5) Manchester(曼彻斯特码) 0—在间隔中间负向跳变; 1在间隔中间正向跳变; 优点: 同步好,每bit均有跳变,又称为自对称; 没有直流成分 有助于差错检测; 缺点:带宽是NRZ的2倍 (6)差分曼彻斯特码 ·0—在间隔的起始位置跳变,在中间有跳变; ·1—在间隔的起始位置不跳变,在中间有跳变

6 （5）Manchester（曼彻斯特码） • 0—在间隔中间负向跳变； • 1—在间隔中间正向跳变； • 优点： - 同步好，每bit均有跳变，又称为自对称； - 没有直流成分； - 有助于差错检测； • 缺点：带宽是NRZ的2倍 （6）差分曼彻斯特码 • 0—在间隔的起始位置跳变，在中间有跳变； • 1—在间隔的起始位置不跳变，在中间有跳变；

不同编码方式的频谱特性 a23d10 b1厂aca上5用2 yon n legend 21用O B8Zs HDB3 d: AMI i a Alternate mark inversion SP B8ZS Bipolar with 8 zeros substitution Freque HDB3 ligh-density bipolar--3 zeros NRZ-L NRZL-L= Nonreturn to zero level 1.0 F.NRZI 128 NRZI= Nonreturn to zero inverted R 0.8 aml pseudoternary o d s 1o sungod or 90.6 e wesTener Manchester Differential Mancheste 0.2 0.40.60.81.01.21.4161.8 2.0 Normalized frequency (/R) Figure 5.3 Spectral Density of Various Signal Encoding Schemes

7 不同编码方式的频谱特性

各种编码方法的错误率(理论值) 101 AMI, pseudoternary, 102 ASK FSK 10 NRZ, biphase PSK, QPSK 104 105 106 10 O1=2345678910112131445 (EyNo(dB) Figure 5.4 Theoretical Bit Error Rate for Various Encoding Schemes

8 各种编码方法的错误率（理论值）

不同数字信号编码速率的归一化信号跳变速率 Table 5.3 Normalized Signal Transition Rate of Various Digital Signal Encoding Rates Minimum 101010 Maximum NRZ-L 0(all Os or 1s) 1.0 1.0 NRZI 0(all 0.5 1.0(all 1s)L Bipolar-AMI 0(all Os) 1.0 1.0 Pseudoternary o(all 1s)L 1.0 1.0 Manchester 1.0(010-) 1.0 2.0(all Os or 1s) Differential Manchester 1.0 (all 1s 15 20(l0s)

9 不同数字信号编码速率的归一化信号跳变速率

2. Modulation rate(调制速率) Description: When signal encoding techniques are used, a distinction needs to be made between data rate(expressed in bits per second) and modulation rate (expressed in baud). The data rate, or bit rate, is R-1/Tb, where Tb is bit duration The modulation rate is the rate at which signal elements are generated Consider for example, Manchester encoding. The minimum size signal element is a pulse of one-half the duration of a bit interval. For a string of all binary zeros or all binary ones, a continuous stream of such pulses is generated. Hence the maximum modulation rate for manchester is d=2 /Tb. This situation is illustrated in Figure 5.5(E6), which shows the transmission of a stream of binary ls at a rate of MBps using nrzi and manchester. In general, D=R/b where D= modulation rate. baud R=data rate. bps b=number of bits per signal element

10 2. Modulation Rate（调制速率）  Description: When signal encoding techniques are used, a distinction needs to be made between data rate (expressed in bits per second) and modulation rate (expressed in baud). The data rate, or bit rate, is R=1/Tb, where Tb is bit duration. The modulation rate is the rate at which signal elements are generated. Consider, for example, Manchester encoding. The minimum size signal element is a pulse of one-half the duration of a bit interval. For a string of all binary zeros or all binary ones, a continuous stream of such pulses is generated. Hence the maximum modulation rate for Manchester is D=2/Tb. This situation is illustrated in Figure 5.5（E6）, which shows the transmission of a stream of binary 1s at a rate of 1Mbps using NRZI and Manchester. In general, D=R/b. where  D = modulation rate, baud  R = data rate, bps  b = number of bits per signal element