Lecture 16: Wireless NetworkingOutline• Wireless physical layer challenges- Signal, noise, modulation- A little bit of EE goes a long way• Wireless link layers- Hidden terminals, exposed terminals- CSMA/CA- RTS/CTS• Wireless routing and throughputEthernet: 802.3• Dominant wired LAN technology- 10BASE5 (vampire taps)- 10BASE-T, 100BASE-TX, 1000BASE-T• Frame format:PreambleType/LenPayload7 x 10101010SFD10101011Src6 bytes 2 bytes 46-1500 bytesCRC4 bytesGap96 ns,960 ns,9600 nsPhysical Link LinkLayer 3Dest6 bytesPhysical Layer (Layer 1)• Responsible for specifying the physical medium- Category 5 cable (Cat5): 8 wires, twisted pair, RJ45 jack- WiFi wireless: 2.4GHz• Responsible for specifying the signal- 100BASE-T: 5-level pulse amplitude modulation (PAM-5)- 802.11b: Binary and quadrature phase shift keying(BPSK/QPSK)• Responsible for specifying the bits- 100BASE-T: 4-to-6 bit-to-chip encoding, 3 chip symbols- 802.11b: Barker code (1-2Mbps), complementary codekeying (5.5-11Mbps)Wireless is Different• Variable: signal attenuates over space• Interference: other RF sources can interfere withsignal• Multipath: signal can self-interfere• Distributed: nodes cannot detect collisions• To address these differences, wireless link layersuse slightly different mechanisms• Also, can’t just abstract away the physical and linklayers: need a brief introduction to underlying EEAttenuation Over Space• Signal weakens as distance from transmitterincreases• Reflections, obstructions, etc. complicate theattenuation• Depending on the antenna, not uniform in alldirections• Much more complex than the wired modelSignal Strength Over SpaceDirectional AntennasInterference• In unlicensed bands (e.g., 802.11), there are lots oftransmitters- 802.11 cards- 802.15.1 (Bluetooth)- 802.15.4 (ZigBee)- 2.4GHz phones- Microwave ovens• This interference can be stronger or weaker thanthe signal, and can prevent successful receptionAnalog SignalsAmplitudeWavelengthSpecifying the Signal: ModulationOn-Off Keying(OOK)1 0 1Amplitude ShiftKeying (ASK)1 0 1Modulation, ContinuedFrequency ShiftKeying (FSK)1 0 1Phase ShiftKeying (PSK)1 0 1I/Q Modulation• I: in-phase, Q: quadrature• Sum of two sines is a sine• Show what the carrier looks like compared to asimple, unmodulated signal• Use I/Q because this is how it’s actually done inhardwareI+QModulation in I/Q PlotsQ Q Q QIQOOKASK FSK BPSK QPSKExample measurements from 16-QAMSignal, Noise, and Interference• Signal: energy of desired transmission• Noise/Noise floor: energy of hardware thermaleffects• Interference: energy of other transmitters• Usually measured in dBm/dBW: 0dBm = 1mW,0dBW = 30dBm = 1W- Note dB is a logarithmic scale: 10dBm = 10mW, 20dBm =100mWSignal Plus NoiseSINR• Signal to Interference-and-Noise Ratio• Measured in dB:|S||N +I|- S = -50dBm, N+I = -95dBm, SINR = 35dB- S = -89dBm, N+I = -93dBm, SINR = 4dB• SINR is particularly critical in wireless because ofattenuation over spaceBit Error Rates• There is a theoretical limit on how muchinformation a channel can carry (Shannon limit)• Bit error rate depends on the SINR and themodulation• This is why wireless link layers use more complexchip/bit encoding- If signal is strong (high SINR), have few chip errors, can uselow encoding- If signal is weak (low SINR), have many chip errors, usehigher encoding to recover from errorsExample Theoretical Bit Error Rates2-3After the signal is received and digitized, it is fed through aseries of adaptive delay stages which are summed togethervia feedback loops. This technique is particularly effective inslowly changing environments such as transmission overtelephone lines, but is more difficult to implement in rapidlychanging environments like factory floors, offices and homeswhere transmitters and receivers are moving in relation toeach other. The main drawback is the impact on system costand complexity. Adaptive equalizers can be expensive toimplement for broadband data links.Spread spectrum systems are fairly robust in the presenceof multipath. Direct Sequence Spread Spectrum (DSSS)systems will reject reflected signals which are significantlydelayed relative to the direct path or strongest signal. This isthe same property which allows multiple users to share thesame bandwidth in Code Diversity Multiple Access (CDMA)systems. Frequency Hopping Spread Systems (FHSS) alsoexhibit some degree of immunity to multipath. Because aFHSS transmitter is continuously changing frequencies, itwill always hop to some frequencies which experience littleor no multipath loss. In a severe fading environment,throughput of an FHSS system will be reduced, but it isunlikely that the link will be lost completely. The performanceof DSSS systems in the presence of multipath is describedfurther in a separate section below.Modulation TechniqueModulation technique is a key consideration. This is themethod by which the analog or digital information isconverted to signals at RF frequencies suitable fortransmission. Selection of modulation method determinessystem bandwidth, power efficiency, sensitivity, andcomplexity. Most of us are familiar with AmplitudeModulation (AM) and Frequency Modulation (FM) becauseof their widespread use in commercial radio. PhaseModulation is another important technique. It is used inapplications such as Global Position System (GPS)receivers and some cellular telephone networks.For the purposes of link budget analysis, the most importantaspect of a given modulation technique is the Signal-to-Noise Ratio (SNR) necessary for a receiver to achieve aspecified level of reliability in terms of BER. A graph of Eb/Novs BER is shown in Figure 4. Eb/No is a measure of therequired energy per bit relative to the noise power. Note thatEb/No is independent of the system data rate. In order toconvert from Eb/No to SNR, the data rate and systembandwidth must be taken into account as shown below:where:Eb= Energy required per bit of informationNo= thermal noise in 1Hz of bandwidthR = system data rateBT= system bandwidthSpread Spectrum RadiosThe term “spread spectrum” simply means that the energyradiated by the transmitter is spread out over a wider amountof the RF spectrum than would otherwise be used. Byspreading out the energy, it is far less likely that two userssharing the same spectrum will interfere with each other.This is an important consideration in an unlicensed band,which why the regulatory authorities imposed
View Full Document
Unlocking...