Emad Alsusa & Christos Masouros Dept. of Electrical & Electronic Engineering University of Manchester Adaptive Code Allocation for Interference Exploitation on the Downlink of MC-CDMA Systems

Principles of the Proposed Method  For PSK Modulation, Interference can be Separated to Constructive and Destructive  Interference Depends on Users Crosscorrelations as well as the instantaneous Data  By Reallocating the Codes According to the Current Data, the Crosscorrelations and hence Interference Amongst Users can be Manipulated  By Exploiting Constructive Interference the Effective SINR can be Increased and Performance can be Improved Without the Need to Increase Transmitted per-User Power

MC-CDMA Downlink Employing post-Equalization (K users)  Received Signal at the u-th Mobile Unit (MU) at the i-th symbol period:  Decision Variable:

Constructive - Destructive Interference Separation User-to-User Constructive MAI:Cumulative Constructive MAI:

Constructive - Destructive Interference Separation  Instantaneous per Symbol Effective SINR:

Decision Variables Distributions for p c =8 Different Allocation Patterns for K=5, L=16

Code-to-User Allocation (CUA) Technique (1/9) 1. Create Code Sets 2. Evaluate Code Sets 3. Select Optimum Code Set 4. Spread and Transmit 5. Transmit SI 6. Detect SI and select the correct code 7. Dispread and Detect

Code-to-User Allocation (CUA) Technique (2/9)

Code-to-User Allocation (CUA) Technique (3/9)  Decision Variables pre-Estimation  Code Allocation Selection Criteria

Code-to-User Allocation (CUA) Technique (4/9)  For Correct Dispreading According to the Updated Codes, Transmission of SI bits is Necessary  SI is Common for all Users  If Code Allocation s=7 is [3, 5, 2, 1, 4] then User k=3 Should Employ Code with Index 2 from the reference set for Correct Dispreading

Code-to-User Allocation (CUA) Technique (5/9) Enhanced Received SINR, Improved Reliability Data Detection Very Sensitive to SI Errors

Code-to-User Allocation (CUA) Technique (6/9)  CUA with MRC, EGC, SU- MMSE post-Equalization Performance Improvement of an Order of Magnitude Without Increase in Transmitted per-User Power Efficiency Reduction to 91% due to Transmission of Side Information (SI) Number of paths=4, K=20, L=32, p c =16

Code-to-User Allocation (CUA) Technique (7/9)  CUA with EGC post-Equalization and SIC Detection Number of paths=4, K=20, L=32 Limited Improvement for Increased N C Performance Loss for Low SNR due to Unreliable SI

Code-to-User Allocation (CUA) Technique (8/9)  CUA with EGC Limited Improvement for Increased Number of Available Allocation Patterns (p c ) Number of paths=3, K=16, L=16, SNR=7dB

Code-to-User Allocation (CUA) Technique (9/9)  CUA with pre-decorrelation employing MRC Equalization Significant Performance Improvement Transmission Efficiency of 32/34=94.2% Number of paths=3, K=32, L=32, SNR=7dB

Conclusions  In Conventional Systems Energy Inherent in the System is Wasted due to Data-Code Misalignment  Part of the Existent Interference can be Exploited to Enhance the Received SINR  By Optimizing the Code Allocation Amongst the Users with CUA the Constructive Component of Interference can be Maximized  Improved Received SINR without Transmitted per-User Energy Increase  Application of CUA can Enhance the Performance of a Number of Conventional MultiUser Precoding and Detection Schemes  The Dependency on SI Detection Limits the CUA Overall Performance for Low Transmitted SNR  For High SNR Values Performance Improvement of an Order of Magnitude is Attained

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