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Direct Sequence Spread Spectrum

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Spread Spectrum Spread power of signal over larger than necessary bandwidth in order to: 1. Reduce interference by signal on other signals 2. Reduce interference on signal by other signals 3. Protection from eavesdropping 4. Add processing gain (error correction), so signal strength can be lower than noise floor

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Spread Spectrum Advantages listed are maximized if: 1. Bandwidth spread is large 2. Bandwidth spread is even, i.e., no spikes at any frequencies

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Direct Sequence Spread Spectrum (DSSS) Spectrum is spread by adding redundant information to signal Narrowband signal requires a minimum BW equal to symbol rate DSSS signal: –↑ number of “symbols” tx’d therefore –↑ “symbol” rate therefore –↑ BW –“symbol” or chip rate > information rate

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DSSS in 802.11b 802.11b uses DSSS in two forms –1 Mbps and 2 Mbps rates use Barker Code encoding –5.5 Mbps and 11 Mbps use Complementary Code Keying

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802.11b 1 and 2 Mbps Barker code blah, blah, blah

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802.11b 5.5Mbps and 11 Mbps Use an 8 bit complementary code keying (CCK) generated number as PN Chipping rate is same – 11 Mchips/sec Both data rates use QPSK modulation technique –Therefore the 11 Mchip/sec rate is actually 22 Mchip/sec (11 Mchip-symbols/sec…see next slide)

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Term Definitions for 802.11b Data bit – a bit of information Symbol – modulated representation of 1 or more bits –BPSK – each symbol represents one bit –QPSK – each symbol represents two bits Chip – a transition of encoded data (i.e., 11 chips per bit for 1 and 2 Mbps 802.11b) Chip Symbol – modulated representaion of 1 or more chips –BPSK – each symbol represents one bit –QPSK – each symbol represents two bits

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802.11b CCK Encoding Description This diagram gives an idea of what is happening, but is not 100% accurate

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1. Input data at 11 Mbps. 2. 8 bits maps to 8 bits -New 8 bits is more random. -Spectrum is more even…no data dependent spikes in frequency spectrum 3. DSSS Encoded data chip rate out = bit rate in 4. Single 11 Mchip/s stream duplicated to introduce redundancy and spread the spectrum 5. DQPSK modulates the two input streams – 2 chips per symbol 6. Output is at 11 Mchip-symbols/sec where each chip-symbol contains 2 chips, and every 2 chips contain 1 data bit

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1. Input data at 5.5 Mbps. 2. 4 input bits map to 8 chips -New 8 bits is more random. -Spectrum is more even…no data dependent spikes in frequency spectrum 3. DSSS Encoded data chip rate out = bit rate in 4. Single 11 Mchip/s stream duplicated to introduce redundancy and spread the spectrum 5. DQPSK modulates the two input streams – 2 chips per symbol 6. Output is at 11 Mchip-symbols/sec where each chip-symbol contains 2 chips and every 4 chips has one data bit

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802.11b CCK Summary 8 chip code used to encode data –8 chip code is data dependent and therefore is constantly changing –Purpose of encoding is to make data appear random Small amount of redundancy introduced (2 to 1 in 11 Mbps, and 4 to 1 in 5.5 Mbps) to spread the spectrum DQPSK is used to modulate and transmit data End result: Frequency spectrum looks the same for all 802.11b rates

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Orthogonal Frequency Division Multiplexing

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OFDM Introduction Used in 802.11a and 802.11g –Supports data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbps Highest data rate and maximum resistance to interference Not technically spread spectrum but result is the same –Low transmit power –Wider than necessary bandwidth

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1) Use many adjacent narrowband channels to transmit data Instead of one stream, use many in parallel –802.11a and g use 52 parallel channels Same principle as in game consoles and PCs: wider parallel “bus” results in faster data transfer Data rate of parallel channels can be slowed down –Reduce error rate

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2) Space adjacent channels “perfectly” so that they do not interfere with each other Channels are very closely spaced with no guard bands between them –Normally, harmonics must be accounted for –OFDM spaces channels so that harmonics cancel each other out

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3) Add Error Correction Codes Add redundant bits specifically to correct errors Technique to do this is called convolutional coding. –Highly efficient, does not require many extra bits to introduce a large amount of redundancy –The more extra bits available, the more redundancy, and the more protection from errors –Actual data rate < bit rate because of redundant bits Changing ratio of (# encoded bits) / (# data bits) will change the actual data rate

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OFDM Block Diagram

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OFDM in 802.11g 802.11g radios must be backwards compatible to 802.11b –Support both OFDM and DSSS 11 channels 22 MHz apart with 5 MHz in between Each channel split into 52 sub-channels of 300 Khz bandwidth –4 for control and monitoring –48 for data Channel rate: 250,000 symbols/sec

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OFDM in 802.11a 3 bands in 5 GHz range –5.150 to 5.250 GHz, 5.250 to 5.350 GHz and 5.725 GHz to 5.825 GHz Each band divided into 4 channels of 20 MHz (20 MHz spare) – 12 channels total Each channel is one OFDM channel and is subdivided into 52 300 kHz subchannels –4 channels carry control and status info –48 channels carry data Modulation is PSK or QAM depending on data rate 250,000 symbols/sec

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Summary of OFDM Encoding Data Rate Modulation Bits/transiti on Ratio of data bits to encoded bits Total bits per transition (48 channels) Data bits encoded per transition 6 Mbps DBPSK1½4824 9 Mbps DBPSK1¾4836 12 Mbps DQPSK2½9648 18 Mbps DQPSK2¾9672 24 Mbps 16QAM4½19296 36 Mbps 16QAM4¾192144 48 Mbps 64QAM62/3288192 54 Mbps 64QAM6¾288216

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802.11 Narrowband Interference Comparison FHSS: jamming or interfering signal will interfere with small number of channels, not all. Some systems are adaptive. DSSS: More susceptible than FHSS because bandwidth is less. More susceptible than OFDM because of less sophisticated encoding. OFDM: Convolutional coding is strong protection against interference, but still not as good as frequency hopping

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802.11 Cost Comparison FHSS: Is pretty much unobtainable DSSS: Still available at significant price reduction 802.11g: Best price/performance due to popularity 802.11a: Slightly more expensive than 802.11g, but it may be worth it to be able to move to the relatively uncrowded 5 GHz bandwidth

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802.11 Co-location Comparison FHSS: Theoretically supports up to 79 co- located channels. Realistically 26. –Total data rate: 26 * 2 Mbps = 52 Mbps DSSS: Can only support 3. –Total data rate: 3 * 11 Mbps = 33 Mbps OFDM: Can only support 3. –Total data rate: 3 * 54 Mbps = 162 Mbps –802.11a channels are non-overlapping therefore, all channels can be co-located

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802.11 Rate Comparison 802.11: 2 Mbps max 802.11b: 11 Mbps max 802.11g: 54 Mbps max, but if it shares with 802.11b, performance takes a hit 802.11a: 54 Mbps max and no worry about backwards compatibility or sharing bandwidth with other rates.

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802.11 Range Comparison 802.11b and 802.11 same range at 1 and 2 Mbps 802.11b range at 5.5 and 11 Mbps is less 802.11g: faster rate = smaller range. –802.11g at 36 Mbps range is same as 802.11 at 11 Mbps 802.11a: higher frequency therefore smaller range.

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802.11 Security Comparison Frequency hopping is secure if you don’t know the hopping sequence –In 802.11, the hopping sequences are known DSSS is secure if you don’t know the PN –In 802.11b, the PNs are known OFDM is not, by default secure Security is only added by adding some kind of encryption

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