Chapter 2 Wireless Communication Technology(Part Two in textbook)

Outline 2.1Antennas and Propagation( 天线与传

输 ) 2.2 Signal Encoding Techniques( 信号编码技术 ) 2.3 Spread Spectrum( 扩频 ) 2.4 Coding and Error Control( 差错控

制 )

2.1Antennas and Propagation

Reading material: [1]Antenna Tutorial

[2]Chapter 5 in textbook

2.1.1 Classifications of Transmission Media (2.4 in textbook)

Transmission Medium( 传输媒介 ) Physical path between transmitter and receiver

Guided Media( 导波介质 ) Waves are guided along a solid medium E.g., copper twisted pair, copper coaxial cable, optical fiber

Unguided Media Provides means of transmission but does not guide

electromagnetic signals Usually referred to as wireless transmission E.g., atmosphere, outer space

Unguided Media Transmission and reception are achieved by

means of an antenna Configurations for wireless transmission

Directional Omnidirectional

无线电波波长与频率

General Frequency Ranges Microwave frequency range

1 GHz to 40 GHz Directional beams possible Suitable for point-to-point transmission Used for satellite communications

Radio frequency range 30 MHz to 1 GHz Suitable for omnidirectional applications

Infrared frequency range Roughly, 3x1011 to 2x1014 Hz Useful in local point-to-point multipoint applications within confined

areas

无线频谱的分配

ISM

无线电 (1)

无线电 (2)

无线电 (3)

无线电先驱—长波 波段 --LF (Low Frequency) 传播特性 -- 白天靠地波，夜晚靠天波 无线电先驱许多无线电通讯的先驱，都是在长波进行试验的。工作频率越高，越不管用 。 应用广泛标帜台或导航电台，标时台 ，地标导航 ，长波广播 ，军事用途 阅读材料：长波及其应用

Broadcast Radio Description of broadcast radio antennas

Omnidirectional Antennas not required to be dish-shaped Antennas need not be rigidly mounted to a precise

alignment Applications

Broadcast radio VHF and part of the UHF band; 30 MHZ to 1GHz Covers FM radio and UHF and VHF television

Microwave

Microwave System

Terrestrial Microwave Description of common microwave antenna

Parabolic "dish", 3 m in diameter Fixed rigidly and focuses a narrow beam Achieves line-of-sight transmission to receiving

antenna Located at substantial heights above ground level

Applications Long haul telecommunications service Short point-to-point links between buildings

Satellite Microwave Description of communication satellite

Microwave relay station Used to link two or more ground-based microwave

transmitter/receivers Receives transmissions on one frequency band (uplink),

amplifies or repeats the signal, and transmits it on another frequency (downlink)

Applications Television distribution Long-distance telephone transmission Private business networks

红外线

多路复用技术 (Multiplexing)(1)

多路复用技术 (2)

多路复用技术 (3)

多路复用技术 (4)

多路复用技术 (5)

多路复用技术 (6)

2.1.2 Introduction to Antennas

天线可以看作一条电子导线和导线系统 (An antenna is an electrical conductor or system of conductors) Transmission - radiates electromagnetic energy into

space Reception - collects electromagnetic energy from space

在双向通信中同一天线既可用于接收也可以用于发送 (In two-way communication, the same antenna can be used for transmission and reception)

辐射模式 (Radiation Patterns) 天线辐射出的功率是全方位的，但各方位上的功率不一定相等。描述天线性能特性的常用方法是辐射模式。 辐射模式（ Radiation pattern ）：

天线的辐射属性的图形化表示 一般被描绘为三维模式的一个二维剽面 (cross

section) 常见的理想化的辐射模式： 各向同性天线（全向天线）、有向天线

接收模式 (Reception pattern) Receiving antenna’s equivalent to radiation pattern

辐射模式 (Radiation Patterns)

各向同性天线（全向天线） 有向天线

天线类型 (Types of Antennas) 等方向性的天线 (idealized)

Radiates power equally in all directions 偶极天线（ Dipole antennas ）

半波偶极天线 (or 赫兹天线 ) 1/4 波垂直天线 (or 马可尼天线 ) ：汽车无线和便携无线中最常见的天线类型

抛物反射天线

偶极天线（ Dipole antennas ）

在一个维上具有一致的或全向的辐射模式。另两个维上具有 8 字形的辐射模式。 天线的长度是可最有效传输信号波长的一半。

偶极天线（ Dipole antennas ） 汽车无线和便携无线中 最常见的天线类型 汽车为什么不能使用长波？

抛物反射天线 (parabolic reflective) 抛物反射天线 : 一种重要的天线类型，常用于地面微波和卫星。 抛物线是由到一固定直线和不在该直线上的某一固定点的距离相等的点的轨迹。固定点叫焦点 (focus) ，固定直线叫准线（ directri

x ）

天线实例—华硕 WL-ANT150 全向天线 产品图

天线实例—华硕 WL-ANT150 全向天线 辐射范围

天线实例—华硕 WL-ANT168 定向天线 产品图案

天线实例—华硕 WL-ANT168 定向天线 产品应用示例

天线实例—抛物天线

天线增益（ Antenna Gain ） 天线增益是天线定向性的度量

天线增益是定义在一特定方向上的功率输出。 在某一特定方向上增加功率是以降低其它方向功率为代价的。 天线增益并不是为了获得比输入功率更高的输出功率，而主要目的是为了定向。

有效面积 Effective area Related to physical size and shape of antenna

天线增益（ Antenna Gain ） 天线增益和有效面积

G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength

2

2

2

π4λπ4

cAf

＝A

G＝ee

天线增益（ Antenna Gain ） 例：一个直径为 2m 的抛物反射天线，工作频率是 12GHz ，它的有效面积和天线增益是多少？ 提示：有效面积为 0.56A ， A 为抛物天线口面积。 答：

A=pi, Ae =0.56A, 波长 =0.025m G=7*pi/(0.025*0.025)=35186 Gdb=l0lg35186=45.46db

2λπ4 eA

G＝

2.1.3 Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example

AM radio

Sky Wave Propagation

Sky Wave Propagation Signal reflected from ionized layer of atmosphere

back down to earth Signal can travel a number of hops, back and forth

between ionosphere and earth’s surface Reflection effect caused by refraction Examples

Amateur radio CB radio

Line-of-Sight Propagation

Line-of-Sight Propagation Transmitting and receiving antennas must be within

line of sight Satellite communication – signal above 30 MHz not reflected

by ionosphere Ground communication – antennas within effective line of

site due to refraction Refraction – bending of microwaves by the atmosphere

Velocity of electromagnetic wave is a function of the density of the medium

When wave changes medium, speed changes Wave bends at the boundary between mediums

Line-of-Sight Equations Optical line of sight

Effective, or radio, line of sight

d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,

rule of thumb K = 4/3

hd 57.3

hd 57.3

Line-of-Sight Equations Maximum distance between two antennas

for LOS propagation:

h1 = height of antenna one h2 = height of antenna two

2157.3 hh

2.1.4 LOS Wireless Transmission Impairments 衰减和失真 (Attenuation and attenuation

distortion) 自由空间损耗 (Free space loss) 噪声 (Noise ） 大气吸收 (Atmospheric absorption) 多径 (Multipath) 折射 (Refraction) 热噪声 (Thermal noise)

衰减 (Attenuation) 信号的强度会随所跨越的任一传输媒介的距离而降低。 对于从事网络传输的工程师来说，必须考虑衰减所带来的三个影响因素：

接收的信号必须有足够的强度 与噪声相比，信号必须维持一种足够高的水平被无误差的接收。 高频下的衰减更为严重，会引起失真。

自由空间损耗 (Free Space Loss) 任一种无线通信中，信号都会随距离发散，因此，具有固定面积的天线离发散天线越远，接收的信号功率就越低。 即使没有其他衰减存在，因为信号随距离的增加会在越来越大的面积范围内散布。这种形式的衰减称为自由空间损耗。 自由空间损耗可以用发射的功率和接收的功率之比来表示。

自由空间损耗 (Free Space Loss) 自由空间损耗 ( 对于理想的全向天线 )

Pt = signal power at transmitting antenna Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (» 3 ´ 10 8 m/s)where d and are in the same units (e.g., meters)

2

2

2

2 )πf4(λ

)π4(c

d＝

d＝P

P

r

t

Free Space Loss Free space loss equation can be recast:

d

PPL

r

tdB

4log20log10

dB 98.21log20log20 d

dB 56.147log20log204log20

df

cfd

自由空间损耗 (Free Space Loss) 自由空间损耗 ( 考虑天线的增益）

Gt = gain of transmitting antenna= Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna

trtrtrr

t

AAfcd

＝AAd

＝GGd

＝PP

2

22

2

22 )()λ(λ

π)4(

2λπ4 eA

G＝

Free Space Loss Free space loss accounting for gain of other

antennas can be recast as

rtdB AAdL log10log20log20

dB54.169log10log20log20 rt AAdf

噪声分类 (Categories of Noise) 热噪声 (Thermal Noise) 互调噪声 (Intermodulation noise ） 串扰 (Crosstalk) 脉冲噪声 (Impulse Noise)

热噪声 (Thermal Noise) 热噪声是由于电子的热搅动而产生的 . 在所有的电子设备和传输媒介中都存在 , 它是温度的一个函数 . 在所跨过的整个频段上均匀分布 , 常被称为白噪声 . 无法被消除 由于卫星地面站所接收到的信号较弱 , 因此在卫星通信中白噪声的影响特别严重 .

热噪声 (Thermal Noise) 在任一设备或导体中 1Hz 的带宽的热噪声是

N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1.3803 * 10-23 J/K T = 温度 ,按开氏温度 (绝对温标 )计算

(W/Hz) k0 T＝N

例：在 T=17 或 290K 的温度下，热噪声的功率 No= 1.3803 * 10-23 *290=4*10-21(W/Hz) = -240(dbw/hz)

热噪声 (Thermal Noise) 热噪声与频率无关 在 B 赫兹的带宽上以瓦特计的白噪声可以表示为

or, 按分贝瓦计TBN＝k

BTN＝ log10 log 10k log10 ++BT log10 log 10dBW 6.228-＝ ++

Noise Terminology 互调噪声 :当不同频率的信号共享相同的传送介质时 ,就会产生互调噪声。例如： f1, f2 ,

f1+f2, f1-f2 串扰 – 多个信号的互相耦合 串扰相对白噪声具有同等的数量级的干扰作用，然而在 ISM 频带上，串扰占主要地位。 脉冲噪声 – 不规则的脉冲或短时间的噪声尖峰

在话音传输中，产生干扰，但不会丢失可理解性。 在数据传输中，是一个主要的错误源。

Expression Eb/N0

Ratio of signal energy per bit to noise power density per Hertz

The bit error rate for digital data is a function of Eb/N0 Given a value for Eb/N0 to achieve a desired error rate,

parameters of this formula can be selected As bit rate R increases, transmitted signal power must

increase to maintain required Eb/N0

TRS

NRS

NEb

k/

00

Other Impairments Atmospheric absorption – water vapor and

oxygen contribute to attenuation Multipath – obstacles reflect signals so that

multiple copies with varying delays are received

Refraction – bending of radio waves as they propagate through the atmosphere

2.1.5 Fading Fading refers to the time variation

of received signal power caused by changes in the transmission medium or path (s).

Multipath Propagation 反射 (Reflection) - occurs when signal

encounters a surface that is large relative to the wavelength of the signal

衍射 (Diffraction) - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

散射 (Scattering) – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

反射

衍射

散射

Multipath Propagation

The Effects of Multipath Propagation Multiple copies of a signal may arrive at

different phases If phases add destructively, the signal level

relative to noise declines, making detection more difficult

Intersymbol interference (ISI) One or more delayed copies of a pulse may

arrive at the same time as the primary pulse for a subsequent bit

多径传播的效果

接收的多径脉冲

接收的直线脉冲

接收的多径脉冲

接收的直线脉冲

脉冲的一个或多个延时副本可能会与主脉冲同时到达 , 这些延时的脉冲对于后来的主脉冲来说就像是一种噪声 . 随着天线的移动 , 次要脉冲的数目、量值和经历的时间也会发生变化。

Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading

差错补偿机制(Error Compensation Mechanisms)

前向纠错 (Forward error correction) 自适应均衡 (Adaptive equalization) 分集技术 (Diversity techniques)

前向纠错 (Forward Error Correction)

可应用于数字传输的应用：所传输的信号是数字数据或数字化的话音或视频数据。 前向：接收器只使用入数字传输数据中的信息来纠正位差错的处理过程。 Transmitter adds error-correcting code to data block

Code is a function of the data bits Receiver calculates error-correcting code from incoming

data bits If calculated code matches incoming code, no error occurred If error-correcting codes don’t match, receiver attempts to

determine bits in error and correct

前向纠错 (Forward Error Correction)

前向纠错技术带来很大的网络开销。 在移动无线应用中，发送的总位数与发送的数据位数的比值为 2~3倍。 卫星通信中，极大的传输延迟会使数据的重传不符合需要。

自适应均衡 ( Adaptive Equalization)

Can be applied to transmissions that carry analog or digital information Analog voice or video Digital data, digitized voice or video

Used to combat intersymbol interference Involves gathering dispersed symbol energy back into

its original time interval Techniques

Lumped analog circuits Sophisticated digital signal processing algorithms

分集技术 ( Diversity Techniques) Diversity is based on the fact that individual

channels experience independent fading events Space diversity – techniques involving physical

transmission path Frequency diversity – techniques where the signal

is spread out over a larger frequency bandwidth or carried on multiple frequency carriers

Time diversity – techniques aimed at spreading the data out over time

2.2 Signal Encoding Techniques

Reading material: [1]Chapter 6 in textbook

Reasons for Choosing Encoding Techniques Digital data, digital signal

Equipment less complex and expensive than digital-to-analog modulation equipment

Analog data, digital signal Permits use of modern digital transmission and

switching equipment

Reasons for Choosing Encoding Techniques Digital data, analog signal

Some transmission media will only propagate analog signals

E.g., optical fiber and unguided media Analog data, analog signal

Analog data in electrical form can be transmitted easily and cheaply

Done with voice transmission over voice-grade lines

2.2.1 Signal Encoding Criteria What determines how successful a receiver will be

in interpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth

An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in

data rate

Factors Used to CompareEncoding Schemes Signal spectrum

With lack of high-frequency components, less bandwidth required

With no dc component, ac coupling via transformer possible

Transfer function of a channel is worse near band edges Clocking

Ease of determining beginning and end of each bit position

Factors Used to CompareEncoding Schemes Signal interference and noise immunity

Performance in the presence of noise Cost and complexity

The higher the signal rate to achieve a given data rate, the greater the cost

2.2.2 Digital data, analog signals

Basic Encoding Techniques Digital data to analog signal

Amplitude-shift keying (ASK) Amplitude difference of carrier frequency

Frequency-shift keying (FSK) Frequency difference near carrier frequency

Phase-shift keying (PSK) Phase of carrier signal shifted

Basic Encoding Techniques

Amplitude-Shift Keying One binary digit represented by presence of

carrier, at constant amplitude Other binary digit represented by absence of

carrier

where the carrier signal is Acos(2πfct)

ts tfA c2cos0

1binary 0binary

Amplitude-Shift Keying Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical

fiber

Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two different

frequencies near the carrier frequency

where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts

ts tfA 12cos tfA 22cos

1binary 0binary

Binary Frequency-Shift Keying (BFSK) Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz)

radio transmission Can be used at higher frequencies on LANs

that use coaxial cable

Multiple Frequency-Shift Keying (MFSK) More than two frequencies are used More bandwidth efficient but more susceptible to error

f i = f c + (2i – 1 – M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L

L = number of bits per signal element

tfAts ii 2cos Mi 1

Multiple Frequency-Shift Keying (MFSK) To match data rate of input bit stream,

each output signal element is held for:Ts=LT seconds

where T is the bit period (data rate = 1/T) So, one signal element encodes L bits

Multiple Frequency-Shift Keying (MFSK) Total bandwidth required

2Mfd

Minimum frequency separation required 2fd=1/Ts

Therefore, modulator requires a bandwidth of

Wd=2L/LT=M/Ts

Multiple Frequency-Shift Keying (MFSK)

Phase-Shift Keying (PSK) Two-level PSK (BPSK)

Uses two phases to represent binary digits

ts tfA c2cos tfA c2cos

1binary 0binary

tfA c2cos tfA c2cos

1binary 0binary

Phase-Shift Keying (PSK) Differential PSK (DPSK)

Phase shift with reference to previous bit Binary 0 – signal burst of same phase as previous

signal burst Binary 1 – signal burst of opposite phase to previous

signal burst

Phase-Shift Keying (PSK) Four-level PSK (QPSK)

Each element represents more than one bit

ts

42cos tfA c 11

432cos tfA c

432cos tfA c

42cos tfA c

01

00

10

Phase-Shift Keying (PSK) Multilevel PSK

Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved

D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2L

L = number of bits per signal element

MR

LRD

2log

Performance Bandwidth of modulated signal (BT)

ASK, PSK BT=(1+r)R FSK BT=2∆F+(1+r)R

R = bit rate 0 < r < 1; related to how signal is filtered ∆F = f2-fc=fc-f1

Performance Bandwidth of modulated signal (BT)

MPSK

MFSK

L = number of bits encoded per signal element M = number of different signal elements

RMrR

LrBT

2log

11

RMMrBT

2log1

Quadrature Amplitude Modulation QAM is a combination of ASK and PSK

Two different signals sent simultaneously on the same carrier frequency

tftdtftdts cc 2sin2cos 21

Quadrature Amplitude Modulation

2.2.3 Analog data, analog signals

Reasons for Analog Modulation Modulation of digital signals

When only analog transmission facilities are available, digital to analog conversion required

Modulation of analog signals A higher frequency may be needed for effective

transmission Modulation permits frequency division

multiplexing

Basic Encoding Techniques Analog data to analog signal

Amplitude modulation (AM) Angle modulation

Frequency modulation (FM) Phase modulation (PM)

Amplitude Modulation

tftxnts ca 2cos1

Amplitude Modulation

cos2fct = carrier x(t) = input signal na = modulation index

Ratio of amplitude of input signal to carrier a.k.a (also known as) double sideband

transmitted carrier (DSBTC)

Spectrum of AM signal

Amplitude Modulation Transmitted power

Pt = total transmitted power in s(t) Pc = transmitted power in carrier

21

2a

ctnPP

Single Sideband (SSB) Variant of AM is single sideband (SSB)

Sends only one sideband Eliminates other sideband and carrier

Advantages Only half the bandwidth is required Less power is required

Disadvantages Suppressed carrier can’t be used for synchronization

purposes

Angle Modulation Angle modulation

Phase modulation Phase is proportional to modulating signal

np = phase modulation index

ttfAts cc 2cos

tmnt p

Angle Modulation Frequency modulation

Derivative of the phase is proportional to modulating signal

nf = frequency modulation index

tmnt f'

Angle Modulation Compared to AM, FM and PM result in a

signal whose bandwidth: is also centered at fc

but has a magnitude that is much different Angle modulation includes cos( (t)) which

produces a wide range of frequencies Thus, FM and PM require greater

bandwidth than AM

Angle Modulation Carson’s rule

where

The formula for FM becomes

BBT 12

BFBT 22

FMfor PMfor

2

BAn

BF

Anmf

mp

2.2.4 Analog data, digital signals

Basic Encoding Techniques Analog data to digital signal

Pulse code modulation (PCM) Delta modulation (DM)

Analog Data to Digital Signal Once analog data have been converted to

digital signals, the digital data: can be transmitted using NRZ-L can be encoded as a digital signal using a code

other than NRZ-L can be converted to an analog signal, using

previously discussed techniques

Pulse Code Modulation Based on the sampling theorem Each analog sample is assigned a binary

code Analog samples are referred to as pulse

amplitude modulation (PAM) samples The digital signal consists of block of n bits,

where each n-bit number is the amplitude of a PCM pulse

Pulse Code Modulation

Pulse Code Modulation By quantizing the PAM pulse, original

signal is only approximated Leads to quantizing noise Signal-to-noise ratio for quantizing noise

Thus, each additional bit increases SNR by 6 dB, or a factor of 4

dB 76.102.6dB 76.12log20SNR dB nn

Delta Modulation Analog input is approximated by staircase

function Moves up or down by one quantization level ()

at each sampling interval The bit stream approximates derivative of

analog signal (rather than amplitude) 1 is generated if function goes up 0 otherwise

Delta Modulation

Delta Modulation Two important parameters

Size of step assigned to each binary digit () Sampling rate

Accuracy improved by increasing sampling rate However, this increases the data rate

Advantage of DM over PCM is the simplicity of its implementation

Reasons for Growth of Digital Techniques Growth in popularity of digital techniques

for sending analog data Repeaters are used instead of amplifiers

No additive noise TDM is used instead of FDM

No intermodulation noise Conversion to digital signaling allows use of

more efficient digital switching techniques

2.3 Spread Spectrum

Reading material: [1]Chapter 7 in textbook

2.3.1 The Concept of Spread Spectrum

扩频技术概述

Spread Spectrum Input is fed into a channel encoder

Produces analog signal with narrow bandwidth Signal is further modulated using sequence of

digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number

generator Effect of modulation is to increase bandwidth of

signal to be transmitted

Spread Spectrum On receiving end, digit sequence is used to

demodulate the spread spectrum signal Signal is fed into a channel decoder to recover

data

Spread Spectrum

Spread Spectrum What can be gained from apparent waste of

spectrum? Immunity from various kinds of noise and

multipath distortion Can be used for hiding and encrypting signals Several users can independently use the same

higher bandwidth with very little interference

扩频通信的发展历史 (1) 有关扩频通信技术的观点是在 1941 年由好莱坞女演员 Hedy

Lamarr 和钢琴家 George Antheil 提出的。 1949 年美国的国家电话电报公司的子公司的联邦电信实验室， Derosa 和 Rogoff 提出设想并生成出伪噪声信号和相干检测的通信系统，成功地工作在 New Jersey 和 California 之间的通信线路上。 1950 年 Basore 首 先 提 出 把 这 种 扩 频 系 统 称 作

NOMACS （ Noise Modulation and Correlation Detection System ）这个名称被使用相当长的时间。

1951 年春天，美国陆军通信协会要求 MIT 电子研究实验室验证一个 NOMACS 系统，目的是在远距离高频无线通信时不再受敌方的人为干扰。后转入 MIT 的林肯实验室。 1952 年由林肯实验室研制出 P9D 型 NOMACS 系统，并进行了试验。

扩频通信的发展历史 (2) 1955 年生产成功并通过了测试。之后，美国海军和空军开始验证各自的扩频系统，空军使用名称为“ Phatom” （鬼怪，幻影）和 “ Hush-Up” （遮掩），海军使用名称为“ Blades” （浆叶），美国海军采用跳频扩频方案。 1976 年第一部扩频通信的概述性专著： Spread

Spectrum Systems 发表。 1978 年在日本举行的国际无线通信咨询委员会 (CCIR) 全会对扩频通信进行专门研究。

扩频通信的发展历史 (3) 1982 年美国第一次军事通信会议展示了扩频通信在军事通信中的主导作用，报告了扩频通信在军事通信各领域的应用，并开始民用扩频通信的调查。 同年第一部扩频通信的理论性专著 Coherent Spread

Spectrum 问世。 1985 年之后民用扩频通信系统发展。 到八十年代，它已经广泛应用于各种战略和战术通信中，成为

电子战中通信反对抗的一种十分重要的手段。

2.3.2 Frequency Hopping Spread Spectrum

Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of

radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input

signal Signal hops from frequency to frequency at fixed

intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is

selected

Frequency Hoping Spread Spectrum Channel sequence dictated by spreading code Receiver, hopping between frequencies in

synchronization with transmitter, picks up message

Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only

at knocking out a few bits

Frequency Hoping Spread Spectrum

FHSS Using MFSK MFSK signal is translated to a new frequency

every Tc seconds by modulating the MFSK signal with the FHSS carrier signal

For data rate of R: duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds

Tc Ts - slow-frequency-hop spread spectrum Tc < Ts - fast-frequency-hop spread spectrum

FHSS Performance Considerations Large number of frequencies used Results in a system that is quite resistant to

jamming Jammer must jam all frequencies With fixed power, this reduces the jamming

power in any one frequency band

2.3.3 Direct Sequence Spread Spectrum

Direct Sequence Spread Spectrum (DSSS) Each bit in original signal is represented by

multiple bits in the transmitted signal Spreading code spreads signal across a wider

frequency band Spread is in direct proportion to number of bits used

One technique combines digital information stream with the spreading code bit stream using exclusive-OR (Figure 7.6)

DSSS Using BPSK Multiply BPSK signal,

sd(t) = A d(t) cos(2 fct)

by c(t) [takes values +1, -1] to gets(t) = A d(t)c(t) cos(2 fct)

A = amplitude of signal fc = carrier frequency d(t) = discrete function [+1, -1]

At receiver, incoming signal multiplied by c(t) Since, c(t) x c(t) = 1, incoming signal is recovered

DSSS Using BPSK

2.3.4 Code Division Multiple Access

Code-Division Multiple Access (CDMA) Basic Principles of CDMA

D = rate of data signal Break each bit into k chips

Chips are a user-specific fixed pattern Chip data rate of new channel = kD

CDMA Example If k=6 and code is a sequence of 1s and -1s

For a ‘1’ bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6>

For a ‘0’ bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6>

Receiver knows sender’s code and performs electronic decode function

<d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code

665544332211 cdcdcdcdcdcddSu

CDMA Example User A code = <1, –1, –1, 1, –1, 1>

To send a 1 bit = <1, –1, –1, 1, –1, 1> To send a 0 bit = <–1, 1, 1, –1, 1, –1>

User B code = <1, 1, –1, – 1, 1, 1> To send a 1 bit = <1, 1, –1, –1, 1, 1>

Receiver receiving with A’s code (A’s code) x (received chip pattern)

User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 -> unwanted signal ignored

CDMA for Direct Sequence Spread Spectrum

2.3.5 Generation of Spreading Sequences

Categories of Spreading Sequences Spreading Sequence Categories

PN sequences Orthogonal codes

For FHSS systems PN sequences most common

For DSSS systems not employing CDMA PN sequences most common

For DSSS CDMA systems PN sequences Orthogonal codes

PN Sequences PN generator produces periodic sequence that

appears to be random PN Sequences

Generated by an algorithm using initial seed Sequence isn’t statistically random but will pass many

test of randomness Sequences referred to as pseudorandom numbers or

pseudonoise sequences Unless algorithm and seed are known, the sequence is

impractical to predict

Important PN Properties Randomness

Uniform distribution Balance property Run property

Independence Correlation property

Unpredictability

Linear Feedback Shift Register Implementation

Properties of M-Sequences Property 1:

Has 2n-1 ones and 2n-1-1 zeros Property 2:

For a window of length n slid along output for N (=2n-1) shifts, each n-tuple appears once, except for the all zeros sequence

Property 3: Sequence contains one run of ones, length n One run of zeros, length n-1 One run of ones and one run of zeros, length n-2 Two runs of ones and two runs of zeros, length n-3 2n-3 runs of ones and 2n-3 runs of zeros, length 1

Properties of M-Sequences Property 4:

The periodic autocorrelation of a ±1 m-sequence is

otherwise

... 2N, N,0, 1

1

τ

NR

Definitions Correlation

The concept of determining how much similarity one set of data has with another

Range between –1 and 1 1 The second sequence matches the first sequence 0 There is no relation at all between the two sequences -1 The two sequences are mirror images

Cross correlation The comparison between two sequences from different

sources rather than a shifted copy of a sequence with itself

Advantages of Cross Correlation The cross correlation between an m-sequence and

noise is low This property is useful to the receiver in filtering out

noise The cross correlation between two different m-

sequences is low This property is useful for CDMA applications Enables a receiver to discriminate among spread

spectrum signals generated by different m-sequences

Gold Sequences Gold sequences constructed by the XOR of two

m-sequences with the same clocking Codes have well-defined cross correlation

properties Only simple circuitry needed to generate large

number of unique codes In following example (Figure 7.16a) two shift

registers generate the two m-sequences and these are then bitwise XORed

Orthogonal Codes Orthogonal codes

All pairwise cross correlations are zero Fixed- and variable-length codes used in CDMA

systems For CDMA application, each mobile user uses one

sequence in the set as a spreading code Provides zero cross correlation among all users

Types Welsh codes Variable-Length Orthogonal codes

Walsh Codes

Set of Walsh codes of length n consists of the n rows of an n ´ n Walsh matrix:

W1 = (0)

n = dimension of the matrix Every row is orthogonal to every other row and to

the logical not of every other row Requires tight synchronization

Cross correlation between different shifts of Walsh sequences is not zero

nn

nnn WW

WWW2

Typical Multiple Spreading Approach Spread data rate by an orthogonal code

(channelization code) Provides mutual orthogonality among all users

in the same cell Further spread result by a PN sequence

(scrambling code) Provides mutual randomness (low cross

correlation) between users in different cells

2.4 Coding and Error Control

Coping with Data Transmission Errors Error detection codes

Detects the presence of an error Automatic repeat request (ARQ) protocols

Block of data with error is discarded Transmitter retransmits that block of data

Error correction codes, or forward correction codes (FEC) Designed to detect and correct errors

2.4.1 Error Detection

Error Detection Probabilities Definitions

Pb : Probability of single bit error (BER) P1 : Probability that a frame arrives with no bit

errors P2 : While using error detection, the probability that

a frame arrives with one or more undetected errors P3 : While using error detection, the probability that

a frame arrives with one or more detected bit errors but no undetected bit errors

Error Detection Probabilities With no error detection

F = Number of bits per frame

011

3

12

1

PPPPP F

b

Error Detection Process Transmitter

For a given frame, an error-detecting code (check bits) is calculated from data bits

Check bits are appended to data bits Receiver

Separates incoming frame into data bits and check bits Calculates check bits from received data bits Compares calculated check bits against received check

bits Detected error occurs if mismatch

Error Detection Process

Parity Check Parity bit appended to a block of data Even parity

Added bit ensures an even number of 1s Odd parity

Added bit ensures an odd number of 1s Example, 7-bit character [1110001]

Even parity [11100010] Odd parity [11100011]

Cyclic Redundancy Check (CRC) Transmitter

For a k-bit block, transmitter generates an (n-k)-bit frame check sequence (FCS)

Resulting frame of n bits is exactly divisible by predetermined number

Receiver Divides incoming frame by predetermined

number If no remainder, assumes no error

CRC using Modulo 2 Arithmetic Exclusive-OR (XOR) operation Parameters:

T = n-bit frame to be transmitted D = k-bit block of data; the first k bits of T F = (n – k)-bit FCS; the last (n – k) bits of T P = pattern of n–k+1 bits; this is the predetermined

divisor Q = Quotient R = Remainder

CRC using Modulo 2 Arithmetic For T/P to have no remainder, start with

Divide 2n-kD by P gives quotient and remainder

Use remainder as FCS

FDT kn 2

PRQ

PDkn

2

RDT kn 2

CRC using Modulo 2 Arithmetic Does R cause T/P have no remainder?

Substituting,

No remainder, so T is exactly divisible by P

PR

PD

PRD

PT knkn

22

QP

RRQPR

PRQ

PT

CRC using Polynomials All values expressed as polynomials

Dummy variable X with binary coefficients

XRXDXXT

XPXRXQ

XPXDX

kn

kn

CRC using Polynomials Widely used versions of P(X)

CRC–12 X12 + X11 + X3 + X2 + X + 1

CRC–16 X16 + X15 + X2 + 1

CRC – CCITT X16 + X12 + X5 + 1

CRC – 32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10 + X8 + X7 + X5 + X4

+ X2 + X + 1

CRC using Digital Logic Dividing circuit consisting of:

XOR gates Up to n – k XOR gates Presence of a gate corresponds to the presence of a

term in the divisor polynomial P(X) A shift register

String of 1-bit storage devices Register contains n – k bits, equal to the length of

the FCS

Digital Logic CRC

2.4.2 Block Error Correction Codes

Wireless Transmission Errors Error detection requires retransmission Detection inadequate for wireless

applications Error rate on wireless link can be high, results

in a large number of retransmissions Long propagation delay compared to

transmission time

Block Error Correction Codes Transmitter

Forward error correction (FEC) encoder maps each k-bit block into an n-bit block codeword

Codeword is transmitted; analog for wireless transmission

Receiver Incoming signal is demodulated Block passed through an FEC decoder

Forward Error Correction Process

FEC Decoder Outcomes No errors present

Codeword produced by decoder matches original codeword

Decoder detects and corrects bit errors Decoder detects but cannot correct bit

errors; reports uncorrectable error Decoder detects no bit errors, though errors

are present

Block Code Principles Hamming distance – for 2 n-bit binary sequences,

the number of different bits E.g., v1=011011; v2=110001; d(v1, v2)=3

Redundancy – ratio of redundant bits to data bits Code rate – ratio of data bits to total bits Coding gain – the reduction in the required Eb/N0

to achieve a specified BER of an error-correcting coded system

Hamming Code Designed to correct single bit errors Family of (n, k) block error-correcting codes with

parameters: Block length: n = 2m – 1 Number of data bits: k = 2m – m – 1 Number of check bits: n – k = m Minimum distance: dmin = 3

Single-error-correcting (SEC) code SEC double-error-detecting (SEC-DED) code

Hamming Code Process Encoding: k data bits + (n -k) check bits Decoding: compares received (n -k) bits

with calculated (n -k) bits using XOR Resulting (n -k) bits called syndrome word Syndrome range is between 0 and 2(n-k)-1 Each bit of syndrome indicates a match (0) or

conflict (1) in that bit position

Cyclic Codes Can be encoded and decoded using linear feedback

shift registers (LFSRs) For cyclic codes, a valid codeword (c0, c1, …, cn-1),

shifted right one bit, is also a valid codeword (cn-1, c0, …, cn-2)

Takes fixed-length input (k) and produces fixed-length check code (n-k) In contrast, CRC error-detecting code accepts arbitrary

length input for fixed-length check code

BCH Codes For positive pair of integers m and t, a (n, k)

BCH code has parameters: Block length: n = 2m – 1 Number of check bits: n – k ≤ mt Minimum distance:dmin ≥2t + 1

Correct combinations of t or fewer errors Flexibility in choice of parameters

Block length, code rate

Reed-Solomon Codes Subclass of nonbinary BCH codes Data processed in chunks of m bits, called

symbols An (n, k) RS code has parameters:

Symbol length: m bits per symbol Block length: n = 2m – 1 symbols = m(2m – 1) bits Data length: k symbols Size of check code: n – k = 2t symbols = m(2t) bits Minimum distance: dmin = 2t + 1 symbols

Block Interleaving Data written to and read from memory in different

orders Data bits and corresponding check bits are

interspersed with bits from other blocks At receiver, data are deinterleaved to recover

original order A burst error that may occur is spread out over a

number of blocks, making error correction possible

Block Interleaving

2.4.3 Convolutional Codes

Convolutional Codes Generates redundant bits continuously Error checking and correcting carried out

continuously (n, k, K) code

Input processes k bits at a time Output produces n bits for every k input bits K = constraint factor k and n generally very small

n-bit output of (n, k, K) code depends on: Current block of k input bits Previous K-1 blocks of k input bits

Convolutional Encoder

Decoding Trellis diagram – expanded encoder diagram Viterbi code – error correction algorithm

Compares received sequence with all possible transmitted sequences

Algorithm chooses path through trellis whose coded sequence differs from received sequence in the fewest number of places

Once a valid path is selected as the correct path, the decoder can recover the input data bits from the output code bits

2.4.4 Automatic Repeat Request

Automatic Repeat Request Mechanism used in data link control and

transport protocols Relies on use of an error detection code

(such as CRC) Flow Control Error Control

Flow Control Assures that transmitting entity does not

overwhelm a receiving entity with data Protocols with flow control mechanism allow

multiple PDUs in transit at the same time PDUs arrive in same order they’re sent Sliding-window flow control

Transmitter maintains list (window) of sequence numbers allowed to send

Receiver maintains list allowed to receive

Flow Control Reasons for breaking up a block of data

before transmitting: Limited buffer size of receiver Retransmission of PDU due to error requires

smaller amounts of data to be retransmitted On shared medium, larger PDUs occupy

medium for extended period, causing delays at other sending stations

Flow Control

Error Control Mechanisms to detect and correct

transmission errors Types of errors:

Lost PDU : a PDU fails to arrive Damaged PDU : PDU arrives with errors

Error Control Requirements Error detection

Receiver detects errors and discards PDUs Positive acknowledgement

Destination returns acknowledgment of received, error-free PDUs

Retransmission after timeout Source retransmits unacknowledged PDU

Negative acknowledgement and retransmission Destination returns negative acknowledgment to PDUs

in error

Go-back-N ARQ Acknowledgments

RR = receive ready (no errors occur) REJ = reject (error detected)

Contingencies Damaged PDU Damaged RR Damaged REJ