1University of Electronic Science and Technology of ChinaWireless Sensor NetworksChapter 4: Physical Layer2University of Electronic Science and Technology of ChinaWireless Sensor Networks物 理 层数据链路层网 络 层传 输 层应 用 层能 量 分 配移 动 管 理应 用 优 化无线传感器网络协议Application LayerTransport LayerNetwork LayerLink LayerPhysical LayerPower Management PlaneMobility Management PlaneTask Management Plane3University of Electronic Science and Technology of ChinaWireless Sensor Networks 物理层的功能物理层定义了物理接口的机械、电气、功 能和过程（同步）特性提供信号的编码和解码、时钟提取、发送 和接收比特流、载波检测等功能指定了传输速率、基带传输或宽带传输、 拓扑结构、传输介质和连接设备物理层的媒介包括平衡电缆、光纤、无线 信道等。4University of Electronic Science and Technology of ChinaWireless Sensor Networks 物理层的核心功能信道选择信道能量检测（energy detect ,ED）；空闲信道评估（clear channel assessment ,CCA）；无线信道收发数据（PHY Protocol Data Unit）；接收包链路质量（link quality indication, LQI）的检测5University of Electronic Science and Technology of ChinaWireless Sensor Networks 典型数字通信系统6University of Electronic Science and Technology of ChinaWireless Sensor Networks 无线传感器网络物理层物理层为设备之间的数据通信提供传输媒体及互连设备，为数据传输提供可靠的环境物理层的媒体包括平衡电缆、光纤、无线信道等。在传感器网络中，主要的挑战性工作是确定调制方式和收发机的体系结构，使之具有简单、低成本的特性，并且能够提供所需要的足够稳健的服务。无线传感器网络物理层主要负责数据的调制与解调、发送与接收，是决定WSN的节点体积、成本以及能耗的关键无线传感器网络物理层是WSN的研究重点之一7University of Electronic Science and Technology of ChinaWireless Sensor Networks OverviewSignal distortion wireless channelsFrom waves to bitsChannel modelsModulationFrequency bandsPhysical layer design8University of Electronic Science and Technology of ChinaWireless Sensor Networks Wireless channel propagationAttenuation (衰减): As the signal wave propagates through air, the signal strength is attenuated. Proportional to the distance traveled over the airResults in path loss for radio wavesReflection(反射) and refraction(折射): When a signal wave is incident at a boundary between two different types of material a certain fraction of the wave bounces off the surface, reflection. a certain fraction of the wave propagates through the boundary, refraction. 9University of Electronic Science and Technology of ChinaWireless Sensor Networks Wireless channel propagationDiffraction(衍射): When signal wave propagates through sharp edges such as the tip of a mountain or a building, the sharp edge acts as a source, New waves are generatedSignal strength is distributed to the new generated waves.Scattering(散射): In reality, no perfect boundaries. When a signal wave is incident at a rough surface, it scatters in different directions10University of Electronic Science and Technology of ChinaWireless Sensor Networks Wireless ChannelWireless transmission distorts any transmitted signalWireless channel describes these distortion effectsSources of distortionAttenuation Signal strength decreases with increasing distanceReflection/refraction Signal bounces of a surface; enter material11University of Electronic Science and Technology of ChinaWireless Sensor Networks Non-line-of-sightBecause of reflection, scattering, , radio communication is not limited to direct line of sight communicationEffects depend strongly on frequency, thus different behavior at higher frequencies Line-of-sight pathNon-line-of-sight path12University of Electronic Science and Technology of ChinaWireless Sensor Networks Non-line-of-sightDifferent paths have different lengths = propagation timeResults in delay spread of the wireless channel signal at receiverLOS pulsesmultipath pulses13University of Electronic Science and Technology of ChinaWireless Sensor Networks Multi-pathBrighter color = stronger signalSimple (quadratic) free space attenuation formula is not sufficient to capture these effects14University of Electronic Science and Technology of ChinaWireless Sensor NetworksTo take into account stronger attenuation than only caused by distance (e.g., walls, ), use a larger exponent 2 is the path-loss exponent Rewrite in logarithmic form (in dB):Generalizing the Attenuation Formula15University of Electronic Science and Technology of ChinaWireless Sensor NetworksObstacles, multi-path, etc?Experiments show can be represented by a random variableEquivalent to multiplying with a lognormal distributed r.v. in metric units ! lognormal fading Generalizing the Attenuation Formula16University of Electronic Science and Technology of ChinaWireless Sensor Networks Log-normal Fading Channel modelTransmit PowerReceived Power Path lossPath loss exponentLog-normal Shadow fading17University of Electronic Science and Technology of ChinaWireless Sensor Networks OverviewSignal distortion wireless channelsFrom waves to bitsChannel modelsModulationFrequency bandsPhysical layer design18University of Electronic Science and Technology of ChinaWireless Sensor Networks 加性噪声加性噪声一般指热噪声、散弹噪声等，它们与信号的关系 是相加，不管有没有信号，噪声都存在加性噪声可以分为四类无线电噪声。来源于各种用途的外台无线电发射机工业噪声。它来源于各种电气设备，如电力线、点火系统、电车 、电源开关、电力铁道、高频电炉等天电噪声。它来源于闪电、大气中的磁暴、太阳黑子以及宇宙射 线（天体辐射波）等。可以说整个宇宙空间都是产生这类噪声的 根源内部噪声。它来源于信道本身所包含的各种电子器件、转换器以 及天线或传输线等,又可称为随机噪声r(t) = s(t) + n(t)s(t)n(t)19