Direct Sequence Spread Direct Sequence Spread SpectrumSpectrum

Spread SpectrumSpread Spectrum

Spread power of signal over larger than Spread power of signal over larger than necessary bandwidth in order to:necessary bandwidth in order to:

1.1. Reduce interference by signal on other signalsReduce interference by signal on other signals

2.2. Reduce interference on signal by other signalsReduce interference on signal by other signals

3.3. Protection from eavesdroppingProtection from eavesdropping

4.4. Add processing gain (error correction), so Add processing gain (error correction), so signal strength can be lower than noise floorsignal strength can be lower than noise floor

Spread SpectrumSpread Spectrum

Advantages listed are maximized if:Advantages listed are maximized if:

1.1. Bandwidth spread is largeBandwidth spread is large

2.2. Bandwidth spread is even, i.e., no spikes Bandwidth spread is even, i.e., no spikes at any frequenciesat any frequencies

Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum(DSSS)(DSSS)

Spectrum is spread by adding redundant Spectrum is spread by adding redundant information to signalinformation to signalNarrowband signal requires a minimum Narrowband signal requires a minimum BW equal to symbol rateBW equal to symbol rateDSSS signal: DSSS signal: – ↑ ↑ number of “symbols” tx’d thereforenumber of “symbols” tx’d therefore– ↑ “↑ “symbol” rate therefore symbol” rate therefore – ↑ ↑ BWBW– ““symbol” or chip rate > information ratesymbol” or chip rate > information rate

DSSS in 802.11bDSSS in 802.11b

802.11b uses DSSS in two forms802.11b uses DSSS in two forms– 1 Mbps and 2 Mbps rates use Barker Code 1 Mbps and 2 Mbps rates use Barker Code

encodingencoding– 5.5 Mbps and 11 Mbps use Complementary 5.5 Mbps and 11 Mbps use Complementary

Code KeyingCode Keying

802.11b 1 and 2 Mbps802.11b 1 and 2 Mbps

Barker code blah, blah, blahBarker code blah, blah, blah

802.11b 5.5Mbps and 11 Mbps802.11b 5.5Mbps and 11 Mbps

Use an 8 bit complementary code keying Use an 8 bit complementary code keying (CCK) generated number as PN(CCK) generated number as PN

Chipping rate is same – 11 Mchips/secChipping rate is same – 11 Mchips/sec

Both data rates use QPSK modulation Both data rates use QPSK modulation techniquetechnique– Therefore the 11 Mchip/sec rate is actually 22 Therefore the 11 Mchip/sec rate is actually 22

Mchip/sec (11 Mchip-symbols/sec…see next Mchip/sec (11 Mchip-symbols/sec…see next slide)slide)

Term Definitions for 802.11bTerm Definitions for 802.11bData bit – a bit of informationData bit – a bit of informationSymbol – modulated representation of 1 or Symbol – modulated representation of 1 or more bitsmore bits– BPSK – each symbol represents one bitBPSK – each symbol represents one bit– QPSK – each symbol represents two bitsQPSK – each symbol represents two bits

Chip – a transition of encoded data (i.e., 11 Chip – a transition of encoded data (i.e., 11 chips per bit for 1 and 2 Mbps 802.11b)chips per bit for 1 and 2 Mbps 802.11b)Chip Symbol – modulated representaion of Chip Symbol – modulated representaion of 1 or more chips1 or more chips– BPSK – each symbol represents one bitBPSK – each symbol represents one bit– QPSK – each symbol represents two bitsQPSK – each symbol represents two bits

802.11b CCK Encoding Description802.11b CCK Encoding Description

This diagram gives an idea of what is happening, but is This diagram gives an idea of what is happening, but is not 100% accuratenot 100% accurate

1.1. Input data at 11 Mbps.Input data at 11 Mbps.2.2. 8 bits maps to 8 bits8 bits maps to 8 bits

- New 8 bits is more random.New 8 bits is more random.- Spectrum is more even…no data dependent spikes in frequency spectrumSpectrum is more even…no data dependent spikes in frequency spectrum

3.3. DSSS Encoded data chip rate out = bit rate inDSSS Encoded data chip rate out = bit rate in4.4. Single 11 Mchip/s stream duplicated to introduce redundancy and Single 11 Mchip/s stream duplicated to introduce redundancy and

spread the spectrumspread the spectrum5.5. DQPSK modulates the two input streams – 2 chips per symbolDQPSK modulates the two input streams – 2 chips per symbol6.6. Output is at 11 Mchip-symbols/sec where each chip-symbol contains Output is at 11 Mchip-symbols/sec where each chip-symbol contains

2 chips, and every 2 chips contain 1 data bit2 chips, and every 2 chips contain 1 data bit

1.1. Input data at 5.5 Mbps.Input data at 5.5 Mbps.2.2. 4 input bits map to 8 chips4 input bits map to 8 chips

- New 8 bits is more random.New 8 bits is more random.- Spectrum is more even…no data dependent spikes in frequency spectrumSpectrum is more even…no data dependent spikes in frequency spectrum

3.3. DSSS Encoded data chip rate out = bit rate inDSSS Encoded data chip rate out = bit rate in4.4. Single 11 Mchip/s stream duplicated to introduce redundancy and Single 11 Mchip/s stream duplicated to introduce redundancy and

spread the spectrumspread the spectrum5.5. DQPSK modulates the two input streams – 2 chips per symbolDQPSK modulates the two input streams – 2 chips per symbol6.6. Output is at 11 Mchip-symbols/sec where each chip-symbol contains Output is at 11 Mchip-symbols/sec where each chip-symbol contains

2 chips and every 4 chips has one data bit2 chips and every 4 chips has one data bit

802.11b CCK Summary802.11b CCK Summary8 chip code used to encode data8 chip code used to encode data– 8 chip code is data dependent and therefore 8 chip code is data dependent and therefore

is constantly changingis constantly changing– Purpose of encoding is to make data appear Purpose of encoding is to make data appear

randomrandom

Small amount of redundancy introduced (2 Small amount of redundancy introduced (2 to 1 in 11 Mbps, and 4 to 1 in 5.5 Mbps) to to 1 in 11 Mbps, and 4 to 1 in 5.5 Mbps) to spread the spectrumspread the spectrumDQPSK is used to modulate and transmit DQPSK is used to modulate and transmit datadataEnd result: Frequency spectrum looks the End result: Frequency spectrum looks the same for all 802.11b ratessame for all 802.11b rates

Orthogonal Frequency Orthogonal Frequency Division MultiplexingDivision Multiplexing

OFDM IntroductionOFDM Introduction

Used in 802.11a and 802.11gUsed in 802.11a and 802.11g– Supports data rates of 6, 9, 12, 18, 24, 36, 48 Supports data rates of 6, 9, 12, 18, 24, 36, 48

and 54 Mbpsand 54 Mbps

Highest data rate and maximum Highest data rate and maximum resistance to interferenceresistance to interferenceNot technically spread spectrum but result Not technically spread spectrum but result is the sameis the same– Low transmit powerLow transmit power– Wider than necessary bandwidthWider than necessary bandwidth

1) Use many adjacent narrowband 1) Use many adjacent narrowband channels to transmit datachannels to transmit data

Instead of one stream, use many in Instead of one stream, use many in parallelparallel– 802.11a and g use 52 parallel channels802.11a and g use 52 parallel channels

Same principle as in game consoles and Same principle as in game consoles and PCs: wider parallel “bus” results in faster PCs: wider parallel “bus” results in faster data transferdata transferData rate of parallel channels can be Data rate of parallel channels can be slowed downslowed down– Reduce error rateReduce error rate

2) Space adjacent channels “perfectly” so 2) Space adjacent channels “perfectly” so that they do not interfere with each otherthat they do not interfere with each other

Channels are very closely spaced with no Channels are very closely spaced with no guard bands between themguard bands between them– Normally, harmonics must be accounted forNormally, harmonics must be accounted for– OFDM spaces channels so that harmonics OFDM spaces channels so that harmonics

cancel each other outcancel each other out

3) Add Error Correction Codes3) Add Error Correction Codes

Add redundant bits specifically to correct errorsAdd redundant bits specifically to correct errors

Technique to do this is called convolutional Technique to do this is called convolutional coding.coding.– Highly efficient, does not require many extra bits to Highly efficient, does not require many extra bits to

introduce a large amount of redundancyintroduce a large amount of redundancy– The more extra bits available, the more redundancy, The more extra bits available, the more redundancy,

and the more protection from errorsand the more protection from errors– Actual data rate < bit rate because of redundant bitsActual data rate < bit rate because of redundant bits

Changing ratio of (# encoded bits) / (# data bits) will change Changing ratio of (# encoded bits) / (# data bits) will change the actual data ratethe actual data rate

OFDM Block DiagramOFDM Block Diagram

OFDM in 802.11gOFDM in 802.11g

802.11g radios must be backwards 802.11g radios must be backwards compatible to 802.11bcompatible to 802.11b– Support both OFDM and DSSSSupport both OFDM and DSSS

11 channels 22 MHz apart with 5 MHz in 11 channels 22 MHz apart with 5 MHz in betweenbetweenEach channel split into 52 sub-channels of Each channel split into 52 sub-channels of 300 Khz bandwidth300 Khz bandwidth– 4 for control and monitoring4 for control and monitoring– 48 for data48 for data

Channel rate: 250,000 symbols/secChannel rate: 250,000 symbols/sec

OFDM in 802.11aOFDM in 802.11a3 bands in 5 GHz range3 bands in 5 GHz range– 5.150 to 5.250 GHz, 5.250 to 5.350 GHz and 5.725 5.150 to 5.250 GHz, 5.250 to 5.350 GHz and 5.725

GHz to 5.825 GHzGHz to 5.825 GHz

Each band divided into 4 channels of 20 MHz Each band divided into 4 channels of 20 MHz (20 MHz spare) – 12 channels total(20 MHz spare) – 12 channels total

Each channel is one OFDM channel and is Each channel is one OFDM channel and is subdivided into 52 300 kHz subchannelssubdivided into 52 300 kHz subchannels– 4 channels carry control and status info4 channels carry control and status info– 48 channels carry data48 channels carry data

Modulation is PSK or QAM depending on data Modulation is PSK or QAM depending on data raterate

250,000 symbols/sec250,000 symbols/sec

Summary of OFDM EncodingSummary of OFDM EncodingData RateData Rate ModulationModulation Bits/Bits/

transitiontransitionRatio of Ratio of data bits data bits to to encoded encoded bitsbits

Total bits Total bits per per transition transition (48 (48 channels)channels)

Data bits Data bits encoded encoded per per transition transition

6 Mbps6 Mbps DBPSKDBPSK 11 ½½ 4848 2424

9 Mbps9 Mbps DBPSKDBPSK 11 ¾¾ 4848 3636

12 Mbps12 Mbps DQPSKDQPSK 22 ½½ 9696 4848

18 Mbps18 Mbps DQPSKDQPSK 22 ¾¾ 9696 7272

24 Mbps24 Mbps 16QAM16QAM 44 ½½ 192192 9696

36 Mbps36 Mbps 16QAM16QAM 44 ¾¾ 192192 144144

48 Mbps48 Mbps 64QAM64QAM 66 2/32/3 288288 192192

54 Mbps54 Mbps 64QAM64QAM 66 ¾¾ 288288 216216

802.11 Narrowband Interference 802.11 Narrowband Interference ComparisonComparison

FHSS: jamming or interfering signal will interfere FHSS: jamming or interfering signal will interfere with small number of channels, not all. Some with small number of channels, not all. Some systems are adaptive.systems are adaptive.

DSSS: More susceptible than FHSS because DSSS: More susceptible than FHSS because bandwidth is less. More susceptible than OFDM bandwidth is less. More susceptible than OFDM because of less sophisticated encoding.because of less sophisticated encoding.

OFDM: Convolutional coding is strong protection OFDM: Convolutional coding is strong protection against interference, but still not as good as against interference, but still not as good as frequency hoppingfrequency hopping

802.11 Cost Comparison802.11 Cost Comparison

FHSS: Is pretty much unobtainableFHSS: Is pretty much unobtainableDSSS: Still available at significant price DSSS: Still available at significant price reductionreduction802.11g: Best price/performance due to 802.11g: Best price/performance due to popularitypopularity802.11a: Slightly more expensive than 802.11a: Slightly more expensive than 802.11g, but it may be worth it to be able 802.11g, but it may be worth it to be able to move to the relatively uncrowded 5 GHz to move to the relatively uncrowded 5 GHz bandwidthbandwidth

802.11 Co-location Comparison802.11 Co-location Comparison

FHSS: Theoretically supports up to 79 co-FHSS: Theoretically supports up to 79 co-located channels. Realistically 26.located channels. Realistically 26.– Total data rate: 26 * 2 Mbps = 52 MbpsTotal data rate: 26 * 2 Mbps = 52 Mbps

DSSS: Can only support 3.DSSS: Can only support 3.– Total data rate: 3 * 11 Mbps = 33 MbpsTotal data rate: 3 * 11 Mbps = 33 Mbps

OFDM: Can only support 3.OFDM: Can only support 3.– Total data rate: 3 * 54 Mbps = 162 MbpsTotal data rate: 3 * 54 Mbps = 162 Mbps– 802.11a channels are non-overlapping 802.11a channels are non-overlapping

therefore, all channels can be co-locatedtherefore, all channels can be co-located

802.11 Rate Comparison802.11 Rate Comparison

802.11: 2 Mbps max802.11: 2 Mbps max

802.11b: 11 Mbps max802.11b: 11 Mbps max

802.11g: 54 Mbps max, but if it shares 802.11g: 54 Mbps max, but if it shares with 802.11b, performance takes a hitwith 802.11b, performance takes a hit

802.11a: 54 Mbps max and no worry 802.11a: 54 Mbps max and no worry about backwards compatibility or sharing about backwards compatibility or sharing bandwidth with other rates.bandwidth with other rates.

802.11 Range Comparison802.11 Range Comparison

802.11b and 802.11 same range at 1 and 802.11b and 802.11 same range at 1 and 2 Mbps2 Mbps

802.11b range at 5.5 and 11 Mbps is less802.11b range at 5.5 and 11 Mbps is less

802.11g: faster rate = smaller range.802.11g: faster rate = smaller range.– 802.11g at 36 Mbps range is same as 802.11 802.11g at 36 Mbps range is same as 802.11

at 11 Mbpsat 11 Mbps

802.11a: higher frequency therefore 802.11a: higher frequency therefore smaller range.smaller range.

802.11 Security Comparison802.11 Security Comparison

Frequency hopping Frequency hopping isis secure if you don’t secure if you don’t know the hopping sequenceknow the hopping sequence– In 802.11, the hopping sequences In 802.11, the hopping sequences areare known known

DSSS DSSS is is secure if you don’t know the PNsecure if you don’t know the PN– In 802.11b, the PNs are knownIn 802.11b, the PNs are known

OFDM is not, by default secureOFDM is not, by default secure

Security is only added by adding some Security is only added by adding some kind of encryptionkind of encryption