M. Stemick, S. Olonbayar, H. Rohling Hamburg University of Technology

1. PHY-Mode Selection and Multi User Diversity in OFDM based Transmission Systems M. Stemick, S. Olonbayar, H. Rohling Hamburg University of Technology Institute of Telecommunications

2. OFDM-FDMA System (Single Cell) Frequency Time 1) Frequency Selectivity 2) Additional Path Loss + Shadowing |H|2 [dB] |H|2 [dB] Bandwidth [MHz] Bandwidth [MHz] Institute of Telecommunications

3. Define User Capacity User Capacity: Number of users per cell @ fixed data rate Goal: maximize user capacity maximize number of users at a fixed data rate Institute of Telecommunications

4. Cell Models • Same data rate for all users • Perfect channel knowledge and synchronisation • Downlink situation • Time-invariant channel Single cell with N users at the same distance from base station This scenario is based on a frequency selective channel model Institute of Telecommunications

5. Channel Model Channel variation over bandwidth: WSSUS channel parameters: |H|2 [dB] Frequency [MHz] Fading of subcarriers varies strongly between users Institute of Telecommunications

6. Subcarrier Selection Algorithm Selection criteria: maximizeZ Selection parameter allocation no allocation user subc. under the following constraints: subcarrier user Solution of optimization problem by Hungarian Algorithm Institute of Telecommunications

7. random select. adapt. select. best subc. select. Simulation Results Subcarrierwise selection, QPSK, R=1/2 8 users 4 users 16 users Increasing number of users → Adaptive subcarrier allocation yields a high diversity gain Institute of Telecommunications

8. Additional Path Loss and Shadowing New situation in the cell: • Users are uniformly distributed • Path loss: • Shadowing: Log normal distribution ( ) • Cell radius: R = 100m Consequence: Path loss and shadowing cause high variation in average receive power between users Institute of Telecommunications

9. Higher complexity of subcarrier allocation Received SNR For Different Users Received SNR for different users High variance of receive power: • Different number of subcarriers per user to fulfil QoS SNR [dB] • Additional PHY-Mode selection Frequency [MHz] Institute of Telecommunications

10. Select onesubcarrier for every user Modified Allocation Algorithm start 1) Select one subcarrier for each user by Hungarian Algorithm 2) Determine SNR for each selected subcarrier and choose a suitable PHY-Mode Choose PHYfor selectedsubcarriers 3) Repeat steps 1) and 2) for every user, until demands for data rate are satisfied Rate achieved? no yes end Institute of Telecommunications

11. Allocation Example Selecting subcarriers based on users subcarriers 3rd iteration 2nd iteration 1st iteration Institute of Telecommunications

12. Subcarrier Allocation & Data Rate Development Target Data Rate: 3Mbps 1 256 QAM 2 128 QAM 3 4 64 QAM User Index 5 32 QAM 6 7 16 QAM 8 QPSK 9 10 7 4 5 1 6 2 3 8 9 10 Number of Subcarriers Institute of Telecommunications

13. 38 users 10 20 users Simulation Results Percentage of satisfied users for different data rates • Subcarriers allocated by Hungarian Algorithm • Subcarrierwise PHY-Mode selection Institute of Telecommunications

14. Comparison Between Considered Models Comparison between uniform und circular user distribution data rate: 3.0 Mbps cell radius: 100m Institute of Telecommunications

15. Conclusions • Adaption to frequency selectivity achieves a high diversity gain ofmaximum 7 dB • Simultaneous adaption to path loss and frequency selectivity achieves high user capacity • OFDM-FDMA + PHY-Mode selection allows flexible adaption to various QoS demands and system loads Institute of Telecommunications

16. Thank you for your attention Institute of Telecommunications