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Basic knowledge of CDMA

Basic knowledge of CDMA. Contents. World Trends The Cellular Concept and Multiple Access Techniques Principles of CDMA CDMA Major Features Comparison with Other Technologies. GSM and CDMA Coverage. Evolution of CDMA. Technologies of Mobile Communications.

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Basic knowledge of CDMA

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  1. Basic knowledge of CDMA

  2. Contents • World Trends • The Cellular Concept and Multiple Access Techniques • Principles of CDMA • CDMA Major Features • Comparison with Other Technologies

  3. GSM and CDMA Coverage

  4. Evolution of CDMA

  5. Technologies of Mobile Communications

  6. CDMA: Past, Present, and Future • The origins of spread spectrum are in military field and navigation systems • In 1949, John Pierce wrote a technical memorandum where he described a multiplexing system in which a common medium carries coded signals that need not be synchronized. This system can be classified as a time hopping spread spectrum multiple access system • In 1949, Claude Shannon and Robert Pierce introduced the basic ideas of CDMA by describing the interference averaging effect and the graceful degradation of CDMA • In 1950, De Rosa-Rogoff proposed a direct sequence spread spectrum system and introduced the processing gain equation and noise multiplexing idea

  7. CDMA: Past, Present, and Future • In 1956, Price and Green filed for the anti-multipath "RAKE" patent . Signals arriving over different propagation paths can be resolved by a wideband spread spectrum signal and combined by the RAKE receiver. • In 1961 ,The near-far problem (i.e., a high interference overwhelming a weaker spread spectrum signal) was first mentioned by Magnuski • For cellular application spread spectrum was suggested by Cooper and Nettleton in 1978 • During the 1980s Qualcomm investigated DS-CDMA techniques, which finally led to the commercialization of cellular spread spectrum communications in the form of the narrowband CDMA IS-95 standard in July 1993 • Commercial operation of IS-95 systems started in 1996.

  8. CDMA: Past, Present, and Future • Multiuser detection (MUD) has been subject to extensive research since 1986 when Verdu formulated an optimum multiuser detection for the additive white Gaussian noise (AWGN) channel, maximum likelihood sequence estimator (MLSE) • During the 1990s ,wideband CDMA techniques with a bandwidth of 5 MHz or more have been studied intensively throughout the world, and several trial systems have been built and tested • Based on the above description, the CDMA era is divided into three periods: 1. the pioneer CDMA era 2. the narrowband CDMA era 3. the wideband CDMA era

  9. Pioneer Era 1949 John Pierce: time hopping spread spectrum 1949 Claude Shannon and Robert Pierce: basic ideas of CDMA 1950 De Rosa-Rogoff: direct sequence spread spectrum 1956 Price and Green: antimultipath "RAKE" patent 1961 Magnuski: near-far problem 1970s Several developments for military field and navigation systems Narrowband CDMA Era 1978 Cooper and Nettleton: cellular application of spread spectrum 1980s Investigation of narrowband CDMA techniques for cellular applications 1986 Formulation of optimum multiuser detection by Verdu 1993 IS-95 standard Wideband CDMA Era 1995 Europe:FRAMES FMA2 WCDMA Japan: Core-A USA :cdma2000 Korea :TTA I TTA II 2000s Commercialization of wideband CDMA systems Table 1. CDMA era

  10. Evolution of Mobile communication system N-BAND W-BAND Analog Digital

  11. Evolution from 2G to 3G WCDMA GSM GPRS TD-SCDMA EDGE cdma2000-3x 1X-EVDV IS-95A cdma2000-1x IS-95B HRPD

  12. CDMA Concepts- Multiple Access Techniques - Description of CDMA - Two Types of CDMA - How does CDMA work? - DSSS Spreading: Time-Domain View - Spreading from a Frequency-Domain View - CDMA Spread Spectrum Payoff - CDMA’s Nested Spreading Sequences

  13. FDMA Power Frequency Time TDMA Power Frequency Time CDMA Power Frequency Time Multiple Access Techniques • FDMA:Frequency division multiple access Feature:each user is allocated a unique frequency band or channel TACS AMPS • TDMA:Time Division Multiple Access Feature:Radio spectrum is divided into time slots,and in each slot only one user is allowed to either transmit or receive. GSM DAMPS • CDMA:Code division multiple access Feature:In CDMA each user is assigned a unique code sequence it uses to encode its information-bearing signal IS-95/CDMA2000 /WCDMA/TD-SCDMA

  14. Description of CDMA • The spreading signal is a pseudo-noise code sequence that has a chip rate which is greater than the data rate of the message. • All CDMA users occupy the same frequency at the same time! Frequency and time are not used as discriminators. • In cdma systems,the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. • CDMA operates by using CODING to discriminate between users. • Each user has its own pseudorandom codeword which is approximately orthogonal to all other codewords. • CDMA interference comes mainly from nearby users • The receiver performs a time correlation operation to detect only the specific desired codeword.All other codewords appear as noise due to decorrelation.

  15. Two Types of CDMA

  16. How does CDMA work? • Sender combines data with a fast spreading sequence, transmits spread data stream • Receiver intercepts the stream, uses same spreading sequence to extract original data

  17. Direct Sequence CDMA For example: Data rate 240 kb/s Spreading factor 16 Chip rate 3.84 Mchip/s Data signal: bit Code signal: chip

  18. DSSS Spreading: Time-Domain View

  19. Spreading from a Frequency-Domain View

  20. CDMA Spread Spectrum Payoff

  21. CDMA’s Nested Spreading Sequences

  22. Principle of spread-spectrum multiple access • Code signal consists of a number of code bits called "chips" that can be either +1 or ­1. • Chip rate of the code signal must be much higher than the rate of the information signal. • In this figure, 10 code chips per information symbol are transmitted (the code chip rate is 10 times the data rate) so the processing gain is equal to 10. Block diagram of a DS-SS transmitter Generation of a BPSK-modulated SS signal

  23. Spreading the Spectrum Original narrowband bit rate information Fc: Carrier frequency rb: Bit rate rc: Chip rate Encoded wideband signal at symbol rate Frequency Fc Fc + rc Fc + rb

  24. Receiver of a DS-SS signal Receiver of a DS-SS signal • The receiver uses coherent demodulation to despread the SS signal, using a locally generated code sequence. • To be able to perform the despreading operation, the receiver must not only know the code sequence used to spread the signal, but the codes of the received signal and the locally generated code must also be synchronized. • This synchronization must be accomplished at the beginning of the reception and maintained until the whole signal has been received. • The code synchronization/tracking block performs this operation • After despreading a data modulated signal results, and after demodulation the original data can be recovered.

  25. ACF Code sequence #1 Code sequence #2 CCF Spreading Sequences • Sequence properties: • High ACF peak • Low ACF sidelobe inter-symbol interference (ISI) • Low CCF multi-user interference (MUI)

  26. Block of DS-SS communication system B A C Information Demodulation Spreading Modulation Information Modulation Despreading b(t) C(t) fc Interference fc PN PN A Point B Point C Point

  27. Spreading spectrum Sequences(PN) - Property of PN - m-sequences -Walsh Code

  28. Property of PN • Correlation - The Rule of Sequence design • The ideal Sequence: • The side peak value of auto-correlation(ACF) is zero • The value of cross-correlation(CCF) is zero • Ideal Sequence can eliminate co-channel interference(MAI) Unfortunately,We can’t find the ideal sequence So,our target is to find such sequence: The smaller of the side peak value of auto-correlation and the value of cross-correlation ,the better of the sequence. Auto-correlation Function Cross-correlation Function

  29. Definition of ACF and CCF Periodic sequence The Periodic ACF: The Periodic CCF:

  30. m-sequences • Good periodic ACF properties • Bad periodic CCF properties • CDMA operates by using different offset of the same m-sequence to discriminate between users for the bad periodic CCF properties • In IS-95 and IS2000 standard ,two kinds of m-sequences are used: short code and long code • The m-sequences are generated using shift register • The normative ACF of m-sequence:

  31. Spreading / Despreading Despreading of mobile 1 Terminal transmission Base station reception 31

  32. Interference Rejection Before despreading After despreading 32

  33. An Example of periodic ACF of m-sequence • length 15 m-sequence (- - - -+ - + - - + + - + + +) here , - means 1,+means -1

  34. Introduction of the short code

  35. Introduce of the long code

  36. Walsh Code- orthogonal sequence Walsh codes are generated by applying Hadamard transform upon 0 repeatedly. Hadamard transform is given by • most important feature: Orthogonal Walsh Sequence is Orthogonal when synchronized. ACF and CCF of Walsh Sequence are not ideal when not synchronized. • Walsh function Set is self-contained. Walsh Sequence with Length n= can constitute n= Sequences orthogonal one another.

  37. Features of CDMA- Multiple Access Capability- Protection Against Multipath Interference- Privacy- Interference Rejection- Anti-Jamming Capability, Especially Narrowband Jamming- Low Probability of Interception(LPI)

  38. Multiple Access Capability • If multiple users transmit a spread-spectrum signal at the same time, the receiver will still be able to distinguish between the users provided each user has a unique code that has a sufficiently low cross-correlation with the other codes. • Correlating the received signal with a code signal from a certain user will then only despread the signal of this user, while the other spread-spectrum signals will remain spread over a large bandwidth. • Within the information bandwidth the power of the desired user will be larger than the interfering power provided there are not too many interferers, and the desired signal can be extracted. • At the receiver 1 only the signal of user 1 is "despread" and the data recovered. Principle of spread-spectrum multiple access

  39. Protection Against Multipath Interference • The signals of the different paths are all copies of the same transmitted signal but with different amplitudes, phases, delays, and arrival angles. • Adding these signals at the receiver will be constructive at some of the frequencies and destructive at others. In the time domain, this results in a dispersed signal. • If the code sequence has an ideal autocorrelation function, then the correlation function is zero outside the interval [­Tc,Tc], where Tc is the chip duration. This means that if the desired signal and a version that is delayed for more than 2Tc are received, coherent demodulation will treat the delayed version as an interfering signal, putting only a small part of the power in the information bandwidth.

  40. Privacy • Privacy -- The transmitted signal can only be despread and the data recovered if the code is known to the receiver.

  41. Interference Rejection • Cross-correlating the code signal with a narrowband signal will spread the power of the narrowband signal thereby reducing the interfering power in the information bandwidth. • The spread-spectrum signal (s) receives a narrowband interference (i). At the receiver the SS signal is "despread" while the interference signal is spread, making it appear as background noise compared to the despread signal. Interference rejection

  42. Anti-Jamming Capability • This is more or less the same as interference rejection except the interference is now willfully inflicted on the system. It is this property, together with the next one-LPI, that makes spread-spectrum modulation attractive for military applications.

  43. Low Probability of Interception(LPI) S（f） Signal • Because of its low power density, the spread-spectrum signal is difficult to detect and intercept by a hostile listener. Signal f0 f f0 f Signal Frequency Before SS Signal Frequency after SS S（f） S（f） Signal Noise Noise Signal f0 f f0 f Signal Frequency Before Decoding Signal Frequency After Decoding Signal Pulse Noise Other Noise

  44. Features of cdma2000-1X • Reverse Pilot Support: Function:Phase reference,Coherent demodulation • For each cdma2000 user, either Turbo or Convolutional codes can be used. • Fast 800 Hz forward and reverse link power control.The reverse power control subchannel can be divided into two independent power control channel,the power control rate can be 400/400bps or 200/600bps for FCH/SCH. • Double the capacity vs. IS-95-A/B • Provide higher data rates more efficiently (up to 307.2 kbps)

  45. Features of cdma2000-1X • Support for Quasi Orthogonal Functions (QOF) increasing available forward channels • Quick paging channel. This allows the mobile to wake up for a shorter period of time before entering sleep mode, thereby increasing the standby time of phones. • OTD(Orthogonal Transmit Diversity) and STS(Space Time Spreading) transmit diversity

  46. Power Control • Near-far ambiguity • Transmit just enough power to achieve min. SIR • Open loop : path loss and shadow fading • Inner closed loop: fast adjustment to achieve target SIR, compensates some fast fading (Rayleigh) and lower required SIR 46

  47. Soft and Softer Handovers • CDMA has the ability to talk to multiple cells simultaneously… a “soft handover*” • When you’re in a soft handover, it’s likely that there’s at least one good link into the system… nice! • Question: What two characteristics of CDMA make this possible? Soft handover region Freq1 Freq1 * “Softer” handover is when a terminal talks to two sectors of the same cell Answer: Universal Frequency Re-use and Rake Receivers

  48. Soft/ Softer Handover • Soft Handover • The terminal is connected to (at least) two cells • The signals are combined in the RNC by means of e.g. selection combining using CRC • Softer Handover • The terminal is connected to two sectors within one cell • More efficient combining in the uplink is possible like maximum ratio combining (MRC) in the BTS.

  49. NodeB 1 NodeB 2 UE Soft Handover Principle soft handover area Measurement Quantity CPICH 1 CPICH 2 time Link to 1 Link to 1 & 2 Link to 2

  50. Powerreceived Multipath and Fading • Multipath signals can cancel each other, causing “fading” • Two receive antennas largely takes care of this problem because different places have different fading profiles • Fine for the uplink, but you’ve still got a problem on the downlink Transmitter Receiver R Ant0 R Ant1 X X Distance Base Station Combiner To receiver

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