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Multi-carrier CDMA

Multi-carrier CDMA. Outlines. Introduction Family of multi-carrier CDMA Systems MC-CDMA System Multi-carrier DS-CDMA System Multi-tone CDMA System System features comparison Differences between OFDM and MC-CDMA. Introduction. 1/3. Transmit high data rate in a mobile environment

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Multi-carrier CDMA

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  1. Multi-carrier CDMA

  2. Outlines • Introduction • Family of multi-carrier CDMA Systems • MC-CDMA System • Multi-carrier DS-CDMA System • Multi-tone CDMA System • System features comparison • Differences between OFDM and MC-CDMA

  3. Introduction 1/3 • Transmit high data rate in a mobile environment • Multi-carrier transmission • Multi-carrier transmission and OFDM • Multi-carrier transmission • Multi-carrier concept • Orthogonality • OFDM • Using FFT device without increasing the transmitter and receiver complexities • High spectral efficiency due to minimally densely subcarrier spacing

  4. Introduction 2/3 • Orthogonality • Definition Time domain Frequency domain • Bandpass signal where is the equivalent lowpass signal of xm(t) if , n is a non-zero integer, i.e. , then .

  5. Introduction 3/3 • Multi-carrier concept Orthogonal Orthogonal, n=3 Orthogonal, n=2 (OFDM) Orthogonal, n=1 Non-orthogonal

  6. Family of Multi-carrier CDMA Systems 1/27 • Three types of multiple access schemes • Combination of CDMA and multi-carrier modulation techniques • MC-CDMA System • Multi-carrier DS-CDMA System • Multi-tone CDMA System

  7. Family of Multi-carrier CDMA Systems 2/27 • Frequency domain spreading + multi-carrier modulation • MC-CDMA scheme • Time domain spreading + multi-carrier modulation • Multi-carrier DS-CDMA scheme • MT-CDMA scheme Frequency domain and time domain spreading

  8. MC-CDMA System 3/27 • MC-CDMA System • Frequency domain spreading • The resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimum frequency separation. • In a (synchronous) down-link mobile radio communication channel, we can use Hadamard Walsh codes as an optimum orthogonal set. • It can be implemented via OFDM technique. • It’s a potential candidate for the 4th wireless communication system.

  9. MC-CDMA System 4/27 • MC-CDMA System

  10. MC-CDMA System 5/27 • MC-CDMA System

  11. Multi-carrier DS-CDMA System 6/27 • Multi-carrier DS-CDMA system • Time domain spreading • The resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimum frequency separation. • This scheme can lower the data rate in each subcarrier so that a large chip time make it easier to synchronize the spreading sequences. • The multicarrier DS-CDMA scheme is originally proposed for a up-link communication channel, because this characteristic is effective for establishment of a (quasi-) synchronous channel. • When setting the number of subcarriers to be one, Multi-carrier DS-CDMA becomes equivalent to a normal DS-CDMA scheme.

  12. Multi-carrier DS-CDMA System 7/27 • Multi-carrier DS-CDMA system

  13. Multi-carrier DS-CDMA System 8/27 • Multi-carrier DS-CDMA system

  14. Multi-Tone CDMA System 9/27 • Multi-tone CDMA (MT-CDMA) system • Time domain spreading • The resulting spectrum of each subcarrier no longer satisfies the orthogonality condition. • The MT-CDMA scheme uses longer spreading codes in proportion to the number of subcarriers, as compared with a normal DS-CDMA scheme. • The MT-CDMA system can accommodate more users than the DS-CDMA system.

  15. Multi-Tone CDMA System 10/27 • Multi-tone CDMA (MT-CDMA) system

  16. Multi-Tone CDMA System 11/27 • Multi-tone CDMA (MT-CDMA) system

  17. System Features Comparison 12/27 • System features comparison

  18. System Features Comparison 13/27 • System features comparison • MC-CDMA • Multi-carrier DS-CDMA • MT-CDMA

  19. System Features Comparison 14/27 • Diversity and combining schemes • Diversity • Combiner • Selection diversity • Equal gain combining • Maximum ratio combining • Orthogonality restoring combining • MMSE combining • ……….

  20. System Features Comparison 15/27 • Receiver structures • DS-CDMA scheme • MC-CDMA scheme • Non-cancellation ORC, CE, EGC/MRC, MMSEC, MLMUD • Cancellation – SIC(Successive Interference Cancellation) PIC(Parallel IC) EGC-EGC MUD, ORC-MRC MUD,CE-ML MUD, DIC and MMSEIC • Multicarrier DS-CDMA • Compose of Ncnormal coherent (non-Rake) receivers • MT-CDMA • Compose of Nc rake combiners • Detection strategies comparison

  21. System Features Comparison 16/27 • Receiver structures • MC-CDMA system- Non-cancellation • Orthogonality restoring combining (ORC) • Choosing the gain qm as • Noise enhancement where zm is the complex envelope of the m-th subcarrier

  22. System Features Comparison 17/27 • Receiver structures • MC-CDMA system- Non-cancellation • Controlled equalization (CE) • This method suppress the excessive noise amplification effect in the orthogonality resorting detection.

  23. System Features Comparison 18/27 • Receiver structures • MC-CDMA system- Non-cancellation • Equal gain combining (EGC) and maximum ratio combining (MRC) • In the case of one user, the maximum ratio combining method can minimum the BER.

  24. System Features Comparison 19/27 • Receiver structures • MC-CDMA system- Non-cancellation • Minimum mean square error combining (MMSEC)

  25. System Features Comparison 20/27 • Receiver structures • MC-CDMA system - Non-cancellation • Maximum likelihood multi-user detection (MLMUD) • When all the users know and for and this method finds a set of to minimize the following likelihood function Λ

  26. System Features Comparison 21/27 • Receiver structures • MC-CDMA system – Cancellation: SIC,PIC • EGC-EGC multi-user detection • In a downlink channel, this method first using EGC to estimate a set of • And then estimates the by EGC after removing the MAI from the received signal • This method requires knowledge of the spreading codes assigned to all the active users.

  27. System Features Comparison 22/27 • Receiver structures • MC-CDMA system – Cancellation: SIC,PIC • ORC-MRC multi-user detection (O-M MUD) • In a downlink channel, this method first estimates a set of by the orthogonality restoring selection, and then estimates by the maximum ratio combining method after removing the multiple user interference component from the received signal. • If the decisions for other users are correct, this method can minimum the BER. • This method also requires knowledge of the spreading codes assigned to all active users.

  28. System Features Comparison 23/27 • Receiver structures • MC-CDMA system – Cancellation: SIC,PIC • CE-ML multi-user detection (C-M MUD) • In a downlink channel, this method first estimates , by the controlled equalization, and then estimates a different set of based on the maximum likelihood criterion. • This method needs to know the spreading codes assigned to all the active user, and requires the estimation of .

  29. System Features Comparison 24/27 • Receiver structures • MC-CDMA system – Cancellation: SIC,PIC • Decorrelating and MMSE interference canceller (DIC and MMSEIC) • This method cancels MAI by adaptively estimating it based on the decorrelation and MMSE criteria, respectively. • The DIC tries to decorrelate each canceler output from all the other users bit estimate, while the MMSEIC to minimize the mean square error. • They are applicable to both uplink and downlink channels.

  30. System Features Comparison 25/27 • Receiver structures • Multi-carrier DS-CDMA scheme • Multi-carrier DS-CDMA receiver is composed of normal coherent (no-Rake) receivers

  31. System Features Comparison 26/27 • Receiver structures • MT-CDMA system • MT-CDMA receiver is composed of Rake receivers

  32. System Features Comparison 27/27 • Detection strategies comparison

  33. 1/4 Differences between OFDM and MC-CDMA • MC-CDMA System • OFDM-CDMA • It can be implemented with OFDM technique

  34. 2/4 Differences between OFDM and MC-CDMA • MC-CDMA spreads the signal in the frequency domain according to the spread code (PN code), it can view as employing the frequency diversity method. • MC-CDMA performs better than DS-CDMA in Downlink level, but its performs even worse in Uplink level. • MC-CDMA has gained much attention, because the signal can be easily transmitted and received using the fast Fourier transform (FFT) device without increasing the transmitter and receiver complexities and it is potentially robust to channel frequency selectivity with a good frequency use efficiency.

  35. References • [1] Richard van Nee Ramjee Prasad, OFDM Wireless Multimedia Communication, Artech House Boston London • [2] Ahmad R.S.Bahai and Burton R. Saltzberg, Multi-carrier Digital Communications Theory and Applications of OFDM, Kluwer Academic/Plenum Publishers. • [3] L. Hanzo, W. Webb and T. Keller, Single- and multi-carrier quadrature amplitude modulation – Principles and applications for personal communications, WLANs and broadcasting, John Wiley & Sons, Ltd, 2000. • [4] Prasad, R. and Hara, SP., “An overview of multi-carrier CDMA,” Spread Spectrum Techniques and Applications Proceedings, IEEE 4th International Symposium on, Vol.1, pp. 107 –114,1996

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