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ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory

ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory. Zhu Han Department of Electrical and Computer Engineering Class 21 Dec. 2 nd , 2013. Outline. CDMA OFDM. spread-spectrum transmission . Three advantages over fixed spectrum

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ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory

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  1. ECE 4371, Fall, 2013Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 21 Dec. 2nd, 2013

  2. Outline • CDMA • OFDM

  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. 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

  5. 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.

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

  7. 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

  8. 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

  9. Wideband Interference Suppression Transmitter Receiver

  10. Transmitter Receiver Narrowband Interference Suppression

  11. Rake Receiver CDMA

  12. 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

  13. 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

  14. 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)

  15. 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

  16. 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

  17. 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)

  18. 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

  19. Basic Idea • Divide a high bit- rate stream into several low bit- rate streams ( serial to parallel) • Robust against frequency selective fading due to multipath propagation

  20. Orthogonal frequency-division multiplexing • Special form of Multi-Carrier Transmission. • Multi-Carrier Modulation. • Divide a high bit-rate digital stream into several low bit-rate schemes and transmit in parallel (using Sub-Carriers)

  21. OFDM

  22. Transmitted Symbol • To have ISI-free channel, Tsymbol>>τ • In OFDM, each symbol has T =Ts L >> τ • Guard interval between OFDM symbols Tg>> τ ensures no ISI between the symbols.

  23. Guard Time and Cyclic Extension... • A Guard time is introduced at the end of each OFDM symbol for protection against multipath. • The Guard time is “cyclically extended” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval. • Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.

  24. OFDM

  25. OFDM bit loading • Map the rate with the sub-channel condition • Water-filling

  26. OFDM Time and Frequency Grid • Put different users data to different time-frequency slots

  27. OFDM Transmitter and Receiver

  28. OFDM

  29. Multiband OFDM - Simple to implement - Captures 95% of the multipath channel energy in the Cyclic Prefix - Complexity of OFDM system varies Logarithmically with FFT size i.e. - N point FFT  (N/2) Log2 (N) complex multiplies for every OFDM symbol

  30. Pro and Con • Advantages • Can easily be adopted to severe channel conditions without complex equalization • Robust to narrow-band co-channel interference • Robust to inter-symbol interference and fading caused by multipath propagation • High spectral efficiency • Efficient implementation by FFTs • Low sensitivity to time synchronization errors • Tuned sub-channel receiver filters are not required (unlike in conventional FDM) • Facilitates Single Frequency Networks, i.e. transmitter macro-diversity. • Disadvantages • Sensitive to Doppler shift. • Sensitive to frequency synchronization problems • Inefficient transmitter power consumption, since linear power amplifier is required.

  31. OFDM Applications • ADSL and VDSL broadband access via telephone network copper wires. • IEEE 802.11a and 802.11gWireless LANs. • The Digital audio broadcasting systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB. • The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T. • The IEEE 802.16 or WiMax Wireless MAN standard. • The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard. • The Flash-OFDM cellular system. • Some Ultra wideband (UWB) systems. • Power line communication (PLC). • Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.

  32. Applications • WiMax • Digital Audio Broadcast (DAB) • Wireless LAN

  33. Applications • High Definition TV (HDTV) • 4G Cellular Communication systems • Flash -OFDM

  34. The IEEE 802.11a/g Standard • Belongs to the IEEE 802.11 system of specifications for wireless LANs. • 802.11 covers both MAC and PHY layers. • Five different PHY layers. • 802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps • PHY Layer very similar to ETSI’s HIPERLAN Type 2

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