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ECE 6332, Spring, 2012 Wireless Communications

ECE 6332, Spring, 2012 Wireless Communications. Zhu Han Department of Electrical and Computer Engineering Class 22 April 23 th , 2012. Outline. Chapter 13 CDMA, Spread Spectrum FHSS DSSS Multiuser case. spread-spectrum transmission. Three advantages over fixed spectrum

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ECE 6332, Spring, 2012 Wireless Communications

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  1. ECE 6332, Spring, 2012Wireless Communications Zhu Han Department of Electrical and Computer Engineering Class 22 April 23th, 2012

  2. Outline • Chapter 13 CDMA, Spread Spectrum • FHSS • DSSS • Multiuser case

  3. spread-spectrum transmission • Three advantages over fixed spectrum • Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background. • Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter. • Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

  4. Pseudo Random Sequence Generator • Pseudorandom sequence • Randomness and noise properties • Walsh, M-sequence, Gold, Kasami, Z4 • Provide signal privacy

  5. Example: Linear Congruential Generators

  6. Frequency Hopping Spread Spectrum • Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. • Military

  7. Frequency Hopping Example

  8. Who is Bluetooth? • Harald Blaatand “Bluetooth” II • King of Denmark 940-981 AC • Harald Bluetooth was first Christian king of Denmark • He united Denmark under his rule in the mid-900's • Similarly, Bluetooth seeks to unite personal computing devices wirelessly • In 1994 – need for low power consumption wireless devices to substitute for cable • Ericsson – driving force behind Bluetooth • Pre-Cell phone • 1998, Ericsson, Nokia, IBM, Toshiba, Intel formed the Bluetooth Special Interest Group (SIG) • 1999 – Release of Bluetooth protocol • 2002 – IEEE adopted Bluetooth standard, 802.15 working group

  9. Adaptive Frequency-hopping spread spectrum In Bluetooth • Works like this … • During a connection, radio transceivers hop from one channel to another • One packet is sent on a channel, two devices then retune their frequencies (hop) to send the next packet on a different channel. • So, if one frequency channel is blocked, limited disturbance to the Bluetooth communication • Allows several Bluetooth networks to run concurrently without interrupting one other • Link rate: 1 Mbps, but with overhead, this reduces to 721 kbps • Range for Bluetooth: • 10m, can reach up to 100m depending on the power class of the device • Bluetooth version 2.0 + EDR uses an enhanced technology called: Adaptive Frequency Hopping (AFH) • AFH allows Bluetooth devices to measure quality of wireless signal • Determines if there are bad channels present on specific frequencies due to interference from other wireless devices. • If bad channels present on a specific frequency, Bluetooth device will adjust its hopping sequence to avoid them • As a result, the Bluetooth connection is stronger, faster, and more reliable

  10. Bluetooth radio and baseband parameters Topology Up to 7 simultaneous links Modulation Gaussian filtered FSK RF bandwidth 220 kHz (-3 dB), 1 MHz (-20 dB) RF band 2.4 GHz ISM frequency band RF carriers 79 (23 as reduced option) Carrier spacing 1 MHz Access method FHSS-TDD-TDMA Freq. hop rate 1600 hops/s S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks

  11. Frequency hopping spread spectrum (1) Bluetooth technology operates in the 2.4 GHz ISM band, using a spread spectrum, frequency hopping, full-duplex signal at a nominal rate of 1600 hops/second. Time 1 MHz The signal hops among 79 frequencies (spaced 1 MHz apart) in a pseudo-random fashion. 83.5 MHz 2.4000 GHz 2.4835 GHz S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks

  12. Frequency hopping spread spectrum (2) The adaptive frequency hopping (AFH) feature (from Bluetooth version 1.2 onward) is designed to reduce interference between wireless technologies sharing the 2.4 GHz spectrum. Time Interference e.g. due to microwave oven => this frequency in the hopping sequence should be avoided. 2.4000 GHz 2.4835 GHz S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks

  13. Frequency hopping spread spectrum (3) In addition to avoiding microwave oven interference, the adaptive frequency hopping (AFH) feature can also avoid interference from WLAN networks: 22 MHz (802.11b) 16.5 MHz (802.11g) 79 FHSS frequencies WLAN channel ... ... 2.4 GHz 2.48 GHz 2.4 GHz 2.48 GHz S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks

  14. Direct Sequence (DS)-CDMA • It phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. • It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

  15. System Block Diagram • Unique code to differentiate all users • Sequence used for spreading have low cross-correlations • Allow many users to occupy all the frequency/bandwidth allocations at that same time • Processing gain is the system capacity • How many users the system can support

  16. Spreading & Despreading • Spreading • Source signal is multiplied by a PN signal • Processing Gain: • Despreading • Spread signal is multiplied by the spreading code • Polar {±1} signal representation

  17. Direct Sequence Spread Spectrum Example

  18. CDMA Example – transmission from two sources 1 0 1 1 A Data 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 A Codeword 1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0 1 0 1 1 0 0 A Signal 0 0 1 0 B Data 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 B Codeword 1 0 1 0 1 0 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 B Signal Transmitted A+B Signal

  19. CDMA Example – recovering signal A at the receiver A+B Signal received A Codeword at receiver Integrator Output Comparator Output 0 1 0 0 Take the inverse of this to obtain A

  20. CDMA Example – recovering signal B at the receiver A+B Signal received B Codeword at receiver Integrator Output Comparator Output 1 1 0 1 Take the inverse of this to obtain B

  21. CDMA Example – using wrong codeword at the receiver A+B Signal received Wrong Codeword Used at receiver Integrator Output Comparator Output X 0 1 1 Noise Wrong codeword will not be able to decode the original data!

  22. Wideband Interference Suppression Transmitter Receiver

  23. Transmitter Receiver Narrowband Interference Suppression

  24. Rake Receiver CDMA

  25. CDMA (IS 95 A) IS 95 B GSM GPRS W-CDMA 3X UWC-136 TDMA EDGE cdmaOne IS-95A 1X No 3X Road Map 1XRTT/3XRTT cdma2000 1999 2000 2001 2002 IS-95B 3G Phase 1 3G Phase 2 2G 2.5G

  26. 2G: IS-95A (1995) • Known as CDMAOne • Chip rate at 1.25Mbps • Convolutional codes, Viterbi Decoding • Downlink (Base station to mobile): • Walsh code 64-bit for channel separation • M-sequence 215 for cell separation • Uplink (Mobile to base station): • M-sequence 241 for channel and user separation

  27. 2.5G: IS-95B (1998) • Increased data rate for internet applications • Up to 115 kbps (8 times that of 2G) • Support web browser format language • Wireless Application Protocol (WAP)

  28. 3G Technology • Ability to receive live music, interactive web sessions, voice and data with multimedia features • Global Standard IMT-2000 • CDMA 2000, proposed by TIA • W-CDMA, proposed by ARIB/ETSI • Issued by ITU (International Telecommunication Union) • Excellent voice quality • Data rate • 144 kbps in high mobility • 384 kbps in limited mobility • 2 Mbps in door • Frequency Band 1885-2025 MHz • Convolutional Codes • Turbo Codes for high data rates

  29. 3G: CDMA2000 (2000) • CDMA 1xEV-DO • peak data rate 2.4 Mbps • supports mp3 transfer and video conferencing • CDMA 1xEV-DV • Integrated voice and high-speed data multimedia service up to 3.1 Mbps • Channel Bandwidth: • 1.25, 5, 10, 15 or 20 MHz • Chip rate at 3.6864 Mbps • Modulation Scheme • QPSK in downlink • BPSK in uplink

  30. 3G: CDMA2000 Spreading Codes • Downlink • Variable length orthogonal Walsh sequences for channel separation • M-sequences 3x215 for cell separation (different phase shifts) • Uplink • Variable length orthogonal Walsh sequences for channel separation • M-sequences 241 for user separation (different phase shifts)

  31. 3G: W-CDMA (2000) • Stands for “wideband” CDMA • Channel Bandwidth: • 5, 10 or 20 MHz • Chip rate at 4.096 Mbps • Modulation Scheme • QPSK in downlink • BPSK in uplink • Downlink • Variable length orthogonal sequences for channel separation • Gold sequences 218 for cell separation • Uplink • Variable length orthogonal sequences for channel separation • Gold sequences 241 for user separation

  32. Near/Far Problem • Performance estimates derived using assumption that all users have same power level • Reverse link (mobile to base) makes this unrealistic since mobiles are moving • Adjust power levels constantly to keep equal 1 k

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