15-441 Computer NetworkingFrom Signals to PacketsOutlineSlide 4Diversity TechniquesTime DiversitySpace DiversitySpread Spectrum and CDMAFrequency Hopping SSExample: Original 802.11 StandardDirect Sequence Spread SpectrumPropertiesSpectrogram: Original FSK SignalSpectrogram: DSSS-encoded SignalExample: Original 802.11Example: Current 802.11bDiscussionSlide 22MAC LayerMAC Layer (Cont.)Supporting Multiple ChannelsFrequency Division MultiplexingFDM Example: AMPSTime Division MultiplexingFrequency versus Time-division MultiplexingTDM Example: GSMCode Division Multiple AccessCDMACDMA DiscussionCDMA ExampleSupporting Bursty Data TrafficMedium Access ControlExample MAC Protocols“Wireless Ethernet”Hidden Terminal ProblemPossible Solution: RTS/CTSCollision Detection & ReliabilitySimple Solution802.11 Frame PrioritiesSIFS/DIFS802.11 RTS/CTSIEEE 802.11Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Readings115-441 Computer NetworkingLecture 3 – Physical Layer1-23-06 Lecture 3: Physical Layer 2From Signals to PacketsAnalog Signal“Digital” SignalBit Stream0 0 1 0 1 1 1 0 0 0 1Packets0100010101011100101010101011101110000001111010101110101010101101011010111001Header/BodyHeader/BodyHeader/BodyReceiverSenderPacketTransmission1-23-06 Lecture 3: Physical Layer 3Outline•RF introduction•Modulation•Antennas and signal propagation•Equalization, diversity, channel coding•Multiple access techniques•Wireless systems and standards1-23-06 Lecture 3: Physical Layer 4Outline•RF introduction•Modulation•Antennas and signal propagation•Equalization, diversity, channel coding•Dynamic equalization•Diversity in space, frequency, and time•Multiple access techniques•Wireless systems and standards1-23-06 Lecture 3: Physical Layer 5Diversity Techniques•Distribute signal over multiple “channels”.•Channels experience independent fading •Reduces the error, i.e. only part of the signal is affected•Time diversity: spread data out over time.•Useful for bursty errors, e.g. slow fading•Space diversity: use multiple nearby antennas and combine signals.•Can be directional•Frequency diversity: spread signal over a multiple frequencies.•For example, spread spectrum1-23-06 Lecture 3: Physical Layer 6Time Diversity•Spread blocks out over time.•Can use FEC or other error recovery techniques to deal with burst errors.A1 A2 A4A3 B1 B2 B4B3 C1 C2 C4C3A1 A2D1 A3B1 B2 D2 B3C1 C2 D3C31-23-06 Lecture 3: Physical Layer 8Space Diversity•Use multiple antennas that pick up the signal in slightly different locations.•If there is no direct path (Raleigh), chances are that the signals are mostly uncorrelated•Antennas should be separated by ½ wavelength or more•If one antenna experiences deep fading, chances are that the other antenna has a strong signal•Can use more than two antennas!•Multiple space diversity reception methods:•Selection diversity: pick antenna with best SNR•Feedback/scanning: only switch is signals becomes weak•Maximal ratio combining: combine signals with a weight that is based on their SNR•MIMO: multiple in multiple out.•Also have multiple transmitting antennas1-23-06 Lecture 3: Physical Layer 9Spread Spectrum and CDMA•Basic idea: Use a wider bandwidth than needed to transmit the signal.•Why??•Don’t put all your eggs in one basket!•Resistance to jamming and interference•If one sub-channel is blocked, you still have the others•Good for military•Minimize impact of a “bad” frequency•Pseudo-encryption•Have to know what frequencies it will use•Two techniques for spread spectrum…1-23-06 Lecture 3: Physical Layer 10Frequency Hopping SS•Pick a set of frequencies within a band•At each time slot, pick a new frequency•Ex: original 1Mbit 802.11 used 300ms time slots•Each frequency has the bandwidth of the original signal•Dwell time is the time spent using one frequency•Spreading code determines the hopping sequence•Must be shared by sender and receiver (e.g. standardized)•Usually frequency determined by a pseudorandom generator function with a shared seedTimeFrequency1-23-06 Lecture 3: Physical Layer 11Example: Original 802.11 Standard•Used frequency hopping.•96 channels of 1 MHz (only 78 used in US).•Each channel carries only ~1% of the bandwidth•The dwell time is 390 msec.•transmitter/receiver must be synchronized!•Standard defined 26 orthogonal hop sequences.•Transmitter used a beacon on fixed frequency to inform the receiver of the hop sequence that will be used.•Can support multiple simultaneous transmissions – use different hop sequences.1-23-06 Lecture 3: Physical Layer 14Direct Sequence Spread Spectrum•Each bit is encoded as multiple bits, called chips.•Each chip is XORed with a “random” bit sequence called a spreading or chipping code.•The resulting bit sequence is used to modulate the signal.10 0 11 1 000 1 10 1 110 0 11 1 001 1 01 1 001 0 11 0 110 1 01 0 1Original SignalSpreading CodeTransmitted ChipsXORModulated Signal1-23-06 Lecture 3: Physical Layer 15Properties•Since each bit it sent as multiple chips, you need more bps bandwidth to send the signal.•Number of chips per bit is called the spreading ratio•Given the Nyquist and Shannon results, you need more spectral bandwidth to do this.•Spreading the signal over the spectrum•Advantage is that is transmission is more resilient.•DSSS signal will look like noise in a narrow band•Can lose some chips in a word and recover easily•Multiple users can share bandwidth (easily).•Follows directly from Shannon (capacity is there)•Use a different chipping sequence1-23-06 Lecture 3: Physical Layer 16Spectrogram: Original FSK SignalTimeFrequency1-23-06 Lecture 3: Physical Layer 17Spectrogram: DSSS-encoded SignalTimeFrequency1-23-06 Lecture 3: Physical Layer 18Example: Original 802.11•The DS PHY used an 11-to-1 spreading ratio and a Barker chipping sequence.•Barker sequence has low autocorrelation properties – why?•Receiver decodes by counting the number of “1” bits in each word•6 “1” bits correspond to a 0 data bit•Chips were transmitted using B-PSK modulation.•Data rate was 1 Mbps (i.e. 11 Mchips/sec)•Extended to 2 Mbps by using a Q-PSK modulation•Requires the detection of a ¼ phase shift1-23-06 Lecture 3: Physical Layer 19Example: Current 802.11b•(Maximum) data rate is 11 Mbs.•Uses Complementary Code Keying (CCK).•Complementary means that the
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