Effects of Channel on Multi-user CFO Estimation for Interleaved OFDMA Uplink

1. Effects of Channel onMulti-user CFO Estimation for Interleaved OFDMA Uplink Dušan Radović University of Novi Sad Serbia and Montenegro 2nd COST289 Workshop 6-8 July 2005, Antalya, Turkey

2. Outline • Introduction • Multi-user OFDMA Uplink • Uplink Channel Effects • CFO Estimation for Interleaved Scheme • Simulation Results • Conclusions and Discussion

3. Introduction • Future Mobile Radio Systems • Multi carrier systems: OFDM(A), MC-CDMA • Drawback of OFDM(A) system is its sensitivity to synchronization errors • Carrier frequency offset (CFO) in OFDM(A) • Due to oscillator mismatch & user mobility • Time delay or time offset • Due to propagation and asynchronous start

4. y User #k CP User#1, path 1 (nth symbol) ejωkm sk,1(n) sk,2(n) . . . sk,N(n) IFFT CP uk(n) uk(m) r(m) CP FFT y(n) xk(n) r(n) P/S Channel(k) S/P CP User#1, path 2 (nth symbol) Lk . . . Other users n(m) Nx1 Nx1 CP User #1, path L1+1 (nth symbol) . . . CP User #K, path 1 (nth symbol) CP User #K, path 2 (nth symbol) . . . CP User #K, path LK+1 (nth symbol) Multi-user OFDMA Uplink

5. frequency frequency frequency Subcarrier Allocation Schemes • Interleaved • Max. frequency diversity • High ICI (MAI) • Block • No frequency diversity • Low ICI (MAI) • Random (block-random) • Compromise • Low ICI (MAI)

6. Uplink Channel Effects • Different channel types (NLOS, LOS) • Power difference (near-far effect) • Doppler shift - estimation using only one OFDMA symbol • Multiple access interference (MAI) when even one user is not synchronized

7. Channel Model • Non Line Of Sight – NLOS • HiperLAN/2 B channel • Rms delay spread 100 ns • Line Of Sight – LOS, 10 dB Rice factor • HiperLAN/2 D channel • Rms delay spread 140 ns

8. CP |N Nch P P P P ... P 8/16 Structure of Interleaved Signal • Signal of one user is sum of interleaved subcarrires and has periodicity every P samples. • Part of CP with inter symbol interference (Nch) is discarded. • Effective CFO.

9. 1 2 3 4 ... Q+1 Nch P P P P ... P Effective CFO Estimation Based on MUSIC Algorithm • Carrier frequency offset of all K active usersare estimated using signal samples of one OFDMA block as following: • Formulate received signal matrixY (Q+1 x P). • Calculate noise subspaceUnfrom covariance matrix Φ=YYH/P (MUSIC)or • Modified MUSIC • Find the largest Kpeaks of MUSIC cost function that correspond to effective CFO {θ(k)}k=1K • Calculate normalized CFO as Δf(k)=(Qθ(k)-q)Δf. • Base station receiver knows the number of active users and which channel is assigned to each of them. [7] Z. Cao, U Tureli, and Y.-D. Yao, “Deterministic Multiuser Carrier Frequency Offset Estimation for Interleaved OFDMA Uplink,” IEEE Trans. Commun., vol. 52, pp. 1585-1594, Sept. 2004.

10. Simulation Setup • Bandwidth 20 MHz at 5 GHz • Number of subcarriers N = 2048 (9.76 kHz) • QPSK constellation • Maximal number of users K = Q = 16 (P = N/Q = 128) • Sampling period 50 ns • OFDMA symbol duration, ~110 μs (CP + N = Nch+128+2048 = Nch+2176 )

11. Simulation Results, channel type Max. absolute CFO=0.2

12. Simulation Results, near-far Max. absolute CFO=0.2 Power of odd users +20dB (strong) compared to even ones (weak)

13. Cost Function Effective CFO cost function shape and real values for the case of MUSIC underestimation. Maximal absolute normalized CFO is 0.4. SNR = 0 dB. Channel type is AWGN.

14. Underestimation or Mismatch Underestimation occurrence for MUSIC and MMUSIC on the same observation matrix. Maximal absolute value of normalized CFO is 0.4, SNR = 0 dB, 500 Monte Carlo tests

15. Conclusions • The proposed algorithm can be applied in multi-user OFDM system with interleaved subcarrier assignment scheme. • One CFO estimation per user, not subcarrier! • Algorithm is near-far robust (based on MUSIC). • Estimation error depends on SNR (per user) only. • Use Modified MUSIC instead of MUSIC! • The algorithm does not require pilot signals or virtual subcarriers, but may require CP extension.

16. Discussion • RMSE of 1% achieved for SNR = 6 dB (CFO=0.2)! • Max. error of 1% achieved for SNR = 18 dB ! • Underestimation and mismatch ? • Interference sensitivity (co-channel)?

17. YOUR QUESTIONS ! БЛАГОДАРИА – DEKUJI – MERCI – DANKE – EFHARISTÓ – KÖSZÖNÖM – GRAZIE – TAKK – OBRIGADO – MULTUMESC – ХВАЛА – DAKUJEM – GRACIAS – TACK – TESEKKULER – ТHANK YOU .

18. CP |N Nch P P P P ... P 8/16 Structure of Interleaved Signal • Signal of one user is sum of interleaved subcarrires and has periodicity every P samples. • Part of CP with inter symbol interference (Nch) is discarded.