Telecoms Systems

introduction

repeater作用

When a signal travels through a channel, it suffers attenuation, distortion and noise contamination. Since their negative effects increase with the distance, special equipment called repeaters are inserted along the way.

由于存在损耗,在线路上传输的信号功率会逐渐衰减,衰减到一定程度时将造成信号失真,因此会导致接收错误。中继器就是为解决这一问题而设计的。它完成物理线路的连接,对衰减的信号进行放大,保持与原数据相同。

在模拟系统中,传输的是连续变化(continuously- varying)的波形。为了保持发射波形,中继器主要对信号进⾏滤波、均衡和放⼤。(filter, equalize and amplify the signal)

  • analog systems: repeaters essentially filter, equalize and amplify the signal.
  • In digital systems sequences predefined waveforms (symbols) are transmitted. In this case, repeaters regenerate such waveforms

Transmission impairment

媒介中信号传输会有 impairment(损失)其中包括:

attenuation 衰减, distortion 失真 and noise 噪声.

image-20220920085443941

如何去让信号的传输更加稳定可靠,就是本课程的目的。我们关注信号的处理方式,以及信道的设计等等,以尽可能的减少Impairment,让通信过程更加可靠。

Attenuation

loss of energy for overcoming the **resistance **of medium(克服介质的阻⼒).

amplifiers(放大器) are used to boost the signal back up to its original level(compensate for energy loss)

image-20220920085432843

Distortion

signal changes in its form or shape.

Typically effects complex or composite signals: the composite signal carrying different frequencies suffers from the delay of some of these frequencies

Each frequency component has its own propagation attenuation through a medium.

image-20220920085852630

Noise

◆ Noise is the main source of a signal being corrupted

image-20220920090119278 image-20220920090203227

可以看出analog信号在传输过程中比digital更容易失真

WIRELESS COMMUNICATIONS

Types of wireless network

◆ WPAN (Wireless Personal Area Network)

– typically operates within about 30 feet

无线个人网:主要用于个人用户工作空间,典型距离覆盖几米,可以与计算机同步传输文件,访问本地外围设备,如打印机等。目前主要技术包括蓝牙(Bluetooth)和红外(IrDA)。

◆ WLAN (Wireless Local Area Network)

– operates within 300 yards

无线局域网:主要用于宽带家庭、大楼内部以及园区内部,典型距离覆盖几十米至上百米。目前主要技术为802.11系列。

◆ WMAN (Wireless Metropolitan Area Network )

– operates within tens of miles

◆ WWAN (Wireless Wide Area Network )

– operates over a large geographical area, mobile phone,

无线城域网和广域网:覆盖城域和广域环境,主要用于Internet/email访问,但提供的带宽比无线局域网技术要低很多。

Why wireless?

◆ No more cables

◆ Mobility and convenience

◆ Flexibility

◆ Scalability

INFORMATION CONVERSION

Formatting Data

image-20220920091038908

Different sources of information need different methods to transform the source information to a digital format( minimise bitrate but maintain quality

– Text – ASCII (used to be others)

Each alphanumeric character is transformed into binary by character coding. Most popular character coding method is ASCII

– Voice (PSTN) – Pulse Code Modulation (G711a/u) 64kps

– Voice (GSM) – GSM codec (13kbps) EFR (improved quality)

– 3G WCDMA – AMR (adaptive Multi Rate)

– Picture – JPEG …..

– Video – MPEG2, MPEG4, H264

可靠的通信系统传输信息

image-20220920125903361

pulse shaping : 脉冲整形是改变传输脉冲波形的过程。其目的是使发射的信号更适合其目的或通信信道,通常是通过限制传输的有效带宽。

Multiplex: 传输媒体的带宽或容量往往会大于传输单一信号的需求,为了有效地利用通信线路,希望一个信道同时传输多路信号,这就是所谓的多路复用技术(Multiplexing)。

image-20220920124632755

信源输出可以为 模拟 OR 数字;信源编码器把analog or digital signal转成二进制符号表示(被称为信息序列);信道编码器以受控的方式向信息序列中提供某种冗余,以克服噪声和信道干扰对信道造成的失真;数字调制器把二进制序列映射成信号波形,是与信道的接口。

数字解调器把传输波形映射为信号编码,判断波形是0/1,