Chapter 2 Wireless Communication Technology (第2章[1], or Part Two in [2])

Outline 2.1Antennas, Propagation and Propagation Characteristics (天线、传输、传输特性) 2.2 Spread Spectrum(扩频)

2.1Antennas and Propagation Reading material: 1.Antenna Tutorial 2.Chapter 5 in [2]

2.1.1 Classifications of Transmission Media (2.4 in [2]) 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

交通传输介质：铁路、公路、河流、天空

无线电波波长与频率

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

Microwave

Microwave System

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

全向天线辐射图

定向天线辐射图

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（directrix）

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

LTE/4G天线

Atacama Large Millimeter Array

全向天线与毫米波波束

天线增益（Antenna Gain） 天线增益是天线定向性的度量 有效面积Effective area 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

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

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

Line-of-Sight Equations Maximum distance between two antennas for LOS propagation: h1 = height of antenna one h2 = height of antenna two

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)

Free Space Loss Free space loss equation can be recast:

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

Free Space Loss Free space loss accounting for gain of other antennas can be recast as

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 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

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

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 接收的直线脉冲 接收的直线脉冲 接收的多径脉冲 接收的多径脉冲 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

Propagation Characteristics Path Loss (includes average shadowing) Shadowing (due to obstructions) Multipath Fading Fast Slow Very slow Pr/Pt Pt Pr v d=vt d=vt

Path Loss Modeling Maxwell’s equations Free space and 2-path models Complex and impractical Free space and 2-path models Too simple Ray tracing models Requires site-specific information Simplified power falloff models Main characteristics: good for high-level analysis Empirical Models Don’t always generalize to other environments

Free Space (LOS) Model Path loss for unobstructed LOS path d=vt Path loss for unobstructed LOS path Power falls off : Proportional to 1/d2 Proportional to l2 (inversely proportional to f2) This is due to the effective aperature of the antenna

Two Ray Model Path loss for one LOS path and 1 ground (or reflected) bounce Ground bounce approximately cancels LOS path above critical distance Power falls off Proportional to d2 (small d) Proportional to d4 (d>dc) Independent of l (fc) Two-path cancellation equivalent to 2-element array, i.e. the effective aperature of the receive antenna is changed.

General Ray Tracing Models signal components as particles Reflections Scattering Diffraction Requires site geometry and dielectric properties Easier than Maxwell (geometry vs. differential eqns) Computer packages often used Reflections generally dominate

Simplified Path Loss Model Used when path loss dominated by reflections. Most important parameter is the path loss exponent g, determined empirically.

mmWave: What’s the big deal? All existing commercial systems fit into a small fraction of the mmWave band

mmWave Propagation (60-100GHz) Massive MIMO Channel models immature Based on measurements, few accurate analytical models Path loss proportion to l2 (huge) Also have oxygen and rain absorbtion l is on the order of a water molecule mmWave systems will be short range or require “massive MIMO”

Empirical Channel Models Cellular Models: Okumura model and extensions: Empirically based (site/freq specific), uses graphs Hata model: Analytical approximation to Okumura Cost 231 Model: extends Hata to higher freq. (2 GHz) Multi-slope model Walfish/Bertoni: extends Cost 231 to include diffraction WiFi channel models: TGn Empirical model for 802.11n developed in the IEEE standards committee. Free space loss up to a breakpoint, then slope of 3.5. Breakpoint is empirically-based. Commonly used in cellular and WiFi system simulations

Main Points Path loss models simplify Maxwell’s equations Models vary in complexity and accuracy Power falloff with distance is proportional to d2 in free space, d4 in two path model Main characteristics of path loss captured in simple model Pr=PtK[d0/d]g mmWave propagation models still immature Path loss large due to frequency, rain, and oxygen Empirical models used in simulations

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 Spread Spectrum Reading material: [1]Chapter 7 in [2]

2.2.1 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。

2.2.2 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

2.2.3 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 get s(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

2.2.4 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

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