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

Institute of Telecommunications2 OFDM-FDMA System (Single Cell) Time Frequency |H| 2 [dB] Bandwidth [MHz] |H| 2 [dB] 1) Frequency Selectivity 2) Additional Path Loss + Shadowing Bandwidth [MHz]

Institute of Telecommunications3 Define User Capacity User Capacity:Number of users per fixed data rate Goal: maximize user capacity maximize number of users at a fixed data rate

Institute of Telecommunications4 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 Telecommunications5 Channel Model Maximum delay 3,2 µs Power delay profile Exp. Number of multipathes 30 OFDM symbol duration 16 µs Bandwidth 20 MHz Subcarriers 256 Fading of subcarriers varies strongly between users Channel variation over bandwidth: WSSUS channel parameters: Frequency [MHz] |H| 2 [dB]

Institute of Telecommunications6 Subcarrier Selection Algorithm Selection criteria: maximize Z usersubc. Selection parameter user subcarrier allocation no allocation under the following constraints: Solution of optimization problem by Hungarian Algorithm

Institute of Telecommunications7 Simulation Results Subcarrierwise selection, QPSK, R=1/2 → Adaptive subcarrier allocation yields a high diversity gain 4 users 8 users 16 users random select. best subc. select. adapt. select. Increasing number of users

Institute of Telecommunications8 Path loss and shadowing cause high variation in average receive power between users Users are uniformly distributed Path loss: Shadowing: Log normal distribution ( ) Cell radius: R = 100m Additional Path Loss and Shadowing New situation in the cell: Consequence:

Institute of Telecommunications9 Frequency [MHz] Received SNR For Different Users High variance of receive power: Different number of subcarriers per user to fulfil QoS Additional PHY-Mode selection Higher complexity of subcarrier allocation SNR [dB] Received SNR for different users

Institute of Telecommunications10 Modified Allocation Algorithm 1) Select one subcarrier for each user by Hungarian Algorithm 2) Determine SNR for each selected subcarrier and choose a suitable PHY-Mode 3) Repeat steps 1) and 2) for every user, until demands for data rate are satisfied Choose PHY for selected subcarriers start end Rate achieved? Select one subcarrier for every user yes no

Institute of Telecommunications11 Allocation Example st iteration 2nd iteration 3rd iteration Selecting subcarriers based on subcarriers users

Institute of Telecommunications12 Subcarrier Allocation & Data Rate Development 256 QAM 128 QAM 64 QAM 32 QAM 16 QAM QPSK Target Data Rate: 3Mbps User Index Number of Subcarriers

Institute of Telecommunications13 Simulation Results Percentage of satisfied users for different data rates Subcarriers allocated by Hungarian Algorithm Subcarrierwise PHY-Mode selection 1020 users 38 users

Institute of Telecommunications14 Comparison Between Considered Models Comparison between uniform und circular user distribution data rate: 3.0 Mbps cell radius: 100m

Institute of Telecommunications15 Conclusions Adaption to frequency selectivity achieves a high diversity gain of maximum 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 Telecommunications16 Thank you for your attention