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ECE 4331, Fall, 2009

ECE 4331, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 25 Nov. 17 th , 2009. spread-spectrum transmission . Three advantages over fixed spectrum

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ECE 4331, Fall, 2009

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  1. ECE 4331, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 25 Nov. 17th, 2009

  2. 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.

  3. PN Sequence Generator Pseudorandom sequence Randomness and noise properties Walsh, M-sequence, Gold, Kasami, Z4 Provide signal privacy

  4. 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.

  5. Code Division Multiple Access (CDMA) In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term. Message consists of symbols Has symbol period and hence, symbol rate Spreading signal (code) consists of chips Has Chip period and and hence, chip rate Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence) PN sequence is called a codeword Each user has its own cordword Codewords are orthogonal. (low autocorrelation) Chip rate is oder of magnitude larger than the symbol rate. The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code The sent signal is recovered by despreading process at the receiver.

  6. CDMA Advantages Low power spectral density. Signal is spread over a larger frequency band Other systems suffer less from the transmitter Interference limited operation All frequency spectrum is used Privacy The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit Reduction of multipath affects by using a larger spectrum Random access possible Users can start their transmission at any time Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity Higher capacity than TDMA and FDMA No frequency management No equalizers needed No guard time needed Enables soft handoff

  7. Direct Sequence Spread Spectrum 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: 6.134, 6.135 • Processing Gain: • Despreading • Spread signal is multiplied by the spreading code • Polar {±1} signal representation

  9. Direct Sequence Spreading

  10. Spreading & Despreading

  11. CDMA – Multiple Users One user’s information is the other’s interferences If the interference structure can be explored, multiuser detection Match filter Decorrelator MMSE decodor Successive cancellation Decision feedback

  12. CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1 One bit period (symbol period) 1 1 Data 0 1 1 1 0 1 0 1 1 1 1 1 0 1 0 1 1 Coded Signal Chip period Input to the modulator (phase modulation)

  13. Processing Gain • Main parameter of CDMA is the processing gain that is defined as: Gp: processing gain Bspread: PN code rate Bchip: Chip rate R: Data rate • IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. • 1.228 Mhz chipping rate • 1.25 MHz spread bandwidth

  14. Near Far Problem and Power Control • At a receiver, the signals may come from various (multiple sources. • The strongest signal usually captures the modulator. The other signals are considered as noise • Each source may have different distances to the base station • In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. • Therefore the receiver power at the base station for all mobile users should be close to eacother. • This requires power control at the mobiles. • Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power. B pr(M) M M M M

  15. DSSS Transmitter Baseband BPF sss(t) Message + m(t) Transmitted Signal p(t) PN Code Generator Oscillator fc Chip Clock

  16. DSSS Receiver Phase Shift Keying Demodulator IF Wideband Filter Received Data Received DSSS Signal at IF Synchronization System PN Code Generator

  17. Spectra of Received Signal Spectral Density Spectral Density Interference Signal Interference Signal Frequency Frequency Output of Correlator after dispreading, Input to Demodulator Output of Wideband filter

  18. CDMA Example R Receiver (a base station) Data=1011… Data=0010… A B Transmitter Transmitter (a mobile) Codeword=101010 Codeword=010011 Data transmitted from A and B is multiplexed using CDMA and codewords. The Receiver de-multiplexes the data using dispreading.

  19. 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

  20. 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

  21. 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

  22. 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!

  23. 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, bluetooth

  24. Hybrid Spread Spectrum Techniques FDMA/CDMA Available wideband spectrum is frequency divided into number narrowband radio channels. CDMA is employed inside each channel. DS/FHMA The signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency.

  25. Hybrid Spread Spectrum Techniques Time Division CDMA (TCDMA) Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range. Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users. Time Division Frequency Hopping At each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence. Employed in severe co-interference and multi-path environments. Bluetooth and GSM are using this technique.

  26. 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

  27. 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

  28. 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)

  29. 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

  30. 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

  31. 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)

  32. 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

  33. 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)

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

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

  36. 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.

  37. OFDM Transmitter and Receiver

  38. 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.

  39. 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.

  40. 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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