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Multi User Detection and Channel Estimation with Rotated Transforms for MC-CDMA Armin Dammann, Stefan Kaiser, Ronald Raulefs, Stephan Sand Institute of Communications and Navigation, DLR Oberpfaffenhofen Gunther Auer DoCoMo Eurolabs, Munich Zurich, 2nd July, 2003. Content.

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  1. Multi User Detection and Channel Estimation with Rotated Transforms for MC-CDMAArmin Dammann, Stefan Kaiser,Ronald Raulefs, Stephan SandInstitute of Communications and Navigation, DLR OberpfaffenhofenGunther AuerDoCoMo Eurolabs, MunichZurich, 2nd July, 2003

  2. Content • Introduction of Rotated Transforms • How to extend the signal space efficiently for low modulation schemes • System Overview of the MC-CDMA System • Results for 4-QAM • Summary/Outlook

  3. 4 -4 -3 -2 -1 1 2 3 Rotated Transforms • Drawback of many orthogonal transforms is ambiguity in one subcarrier Number of sequences “per point”  Ambiguity • “Rotated Transforms” is an add-on technique to overcome this problem Construction: • Let j be the smallest rotation angle (rad), which maps the signal constellation (before spreading) onto itself (e.g. for BPSK: p; for QPSK: p/2) • A “Rotated Transform” is constructed by multiplying the kth data symbol by exp( j·j ·k/N ).

  4. Creation of Non- and Rotated Constellations im re Non-Rotated Rotated 1 -1 exp( j·p ·0/2 ) • 1 • 1 -1 • 1 • 1 -1 exp( j·p ·1/2 ) im 1 im re 1 -1 -2 -1 1 2 re -1

  5. im im re 4 -4 -3 -2 -1 1 2 3 re Comparison of Non- and Rotated Constellations BPSK, Walsh-Hadamard, N = 8

  6. 1 1 … p 1 S/P P/S Spread Rot … … L MOD p Source … OFDM + TG … … 1 … M Rot Spread Q … L Multipath Channel S S Maximum Likelihood Detector with channel state information (CSI) 1 p-1 1 P/S S/P Rot Desp CSI … DMOD p-1 Sink … -TG IOFDM … M Rot Q Desp System Model

  7. MLSE - Rotated • Independent Rayleigh Fading Channel • 4-QAM Modulation • Spreading Length 4 • (Non) Rotated • Uncoded • Perfect Channel Estimation

  8. MLSE - Rotated • 12-Tap Channel • Spreading Length 4 • 4 Active Users • 4-QAM Modulation • (Non) Rotated • Uncoded • Perfect or 2x 1-D Channel Estimation

  9. Summary • Rotated transform extends signal space efficiently • MLSE exploits this even for 4-QAM (3dB gain for full loaded system with 4 Users) • Performance gain applying rotated transforms increases very slightly for real channel estimation • Outlook • Include Coding • Limits for higher modulation schemes or large spreading sequences

  10. Creation of Non- and Rotated Constellations im BPSK alphabet re 1 -1 Rotating Rotated Non-Rotated im Spreading 1 im re 1 -1 -2 -1 1 2 re -1

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