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.

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)

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.

Noise

◆ Noise is the main source of a signal being corrupted

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

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

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)。

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，