On the Influence of IQ-Imbalance on Carrier Recovery in WDM CO-OFDM Systems

Transkript

1 On the Influence of IQ-Imbalance on Carrier Recovery in WDM CO-OFDM Systems Workshop der ITG-Fachgruppe Simon Ohlendorf, Dennis Clausen und Werner Rosenkranz Christian-Albrechts-Universität zu Kiel (CAU)

2 Outline 1. CO-OFDM System 2. Influence of Carrier Frequency Offset 3. Methods for Carrier Recovery a. Pilot tone based b. TS based 4. Transmitter IQ-Imbalances a. Impact b. Compensation 5. Results 6. Summary and Conclusion -2-

3 Serial/Parallel Parallel/Serial OFDM Transmission System OFDM: Orthogonal Frequency Division Multiplexing Bandwidth is divided into subbands A f f SC 1/TS Parallel data streams htx rect t / T N S orthogonal subcarriers with frequencies f0,..., f( N 1) Modulation: IDFT (IFFT), Demodulation: DFT (FFT) f fn 1 f n fn 1 OFDM Transmitter s () t 0 h Tx OFDM Receiver h Rx r () t 0 Data s () t 1 h Tx j 2 f0t e j 2 f1t e Channel e e j2 f t 0 j2 f t 1 h Rx r () t 1 Data sn () 1 t h Tx h Rx rn () 1 t j2 fn 1t e e j f t 2 N1-3-

4 Sync CO Rx ADC Rx DSP Synchronisation - CP FFT Zero Forcing EQ DATA CO-OFDM Transmission System CFO: fc fc flo f LO f c ECL IQ-MOD LPF LPF DAC ECL Tx DSP +CP (GI) IFFT (N FFT ) +Sync +EQ S/P DATA (4-QAM) f 1 OFDM-frame: S OFDM-symbols overhead EQ data load data N D OFDM-data symbols N TS OFDM training symbols t -4-

5 Outline 1. CO-OFDM System 2. Influence of Carrier Frequency Offset 3. Methods for Carrier Recovery a. Pilot tone based b. TS based 4. Transmitter IQ-Imbalances a. Impact b. Compensation 5. Results 6. Summary and Conclusion -5-

6 Effects of Carrier Frequency Offset on CO-OFDM A f Received symbol k of n th subcarrier Frequency offset f c Relative freq. offset c f f c SC carrier freq. offset subcarrier spacing Amplitude FFT grid size N Intercarrier interference N n fsc Symbol k of n th subcarrier after FFT (Rx): 2 sin sin ( mn) r k e s k e s k e N 1 j2 k j j ( m n) n n m c N sin m 0 N sin N N c m n nm NNCP N1 N1 c N c c N c c N f Phase rotation Morelli, M., et al.(2007): Proceedings of the IEEE 95, Nr.7 Attenuation Phase rotation -6- Intercarrier interference

7 Outline 1. CO-OFDM System 2. Influence of Carrier Frequency Offset 3. Methods for Carrier Recovery a. Pilot tone based b. TS based 4. Transmitter IQ-Imbalances a. Impact b. Compensation 5. Results 6. Summary and Conclusion -7-

8 PSD [db] CO Rx ADC Rx DSP Carrier Recovery Synchronisation - CP FFT Zero Forcing EQ DATA CO-OFDM Transmission System Pilot Tone CR CFO: fc fc flo f LO f c ECL ECL IQ-MOD LPF LPF DAC Tx DSP +CP (GI) IFFT (N FFT ) + Pilot Tone +Sync +EQ +IQ DATA (4-QAM) f [GHz] -8-

9 EVM [%] PSD [db] Pilot Tone Based CFO and PN Compensation ADC Carrier Recovery Sync Pilot tone LP LP FFT BPF LP Δf c argmax(.) conj(.) BP data gap f [MHz] OSNR: 10 db OSNR: 20 db Pilot-Signal-Ratio [db] OSNR: 10 db OSNR: 20 db Gap [SC] 30 GAP: 3 SC GAP: 5 SC 20 GAP: 21 SC GAP: 41 SC B BP [MHz] 80 Jansen, S. L., et. al (2008). Journal of Lightw, Techn.. -9-

10 CO-OFDM Transmission System TS Based CFO and PN Compensation CO Rx ADC Sync Rx DSP Synchronisation CR (pre FFT) - CP FFT CR (post FFT) Zero-Forcing EQ DATA CFO: fc fc flo f LO f c ECL ECL IQ-MOD LPF LPF DAC Tx DSP +CP (GI) IFFT (N FFT ) + Pilot subcarriers f EQ data +Sync +EQ DATA (4-QAM) t -10-

11 TS Based CFO and PN Compensation ADC Sync & Carrier Recovery 1 S/P Carrier -GI FFT Recovery EQ S&C TS with identical halves CFO causes phase rotation Φ Timing metric ( ) for synchronisation phase offset Max. CFO: Subcarrier spacing ˆ arg ( k 0) fractional CFO T A f f c 2z T T TS with differential data on even SC Estimation of integer CFO after FFT 2z T f Schmidl, T., & Cox, D. (1997)., IEEE Transactions on Communications -11-

12 TS Based CFO and PN Compensation ADC Sync & Carrier Recovery 1 S/P Carrier -GI FFT Recovery EQ Phase Noise Track along complete signal 3. Pilot subcarriers Extract N p pilot subcarriers Phase rotation for every OFDM-symbol N p ˆ 1 ( k) arg n( ) arg n( ) N p n 1 r k s k Compensation sˆ ( ) ( )exp 2 ˆ n k rn k j ( k) Yi, X. (2007), Australian Conference on Optical Fibre Technology, 19(12). -12-

13 CO-OFDM WDM Simulation Model Ch. 2 ECL IQ- Modulator LPF c, nm IQ- Modulator LPF 3 db EDFA c, nm 3 db ECL c, nm Att./PM x2 100km ECL EDFA BPF Coherent Receiver OSNR 10 GS/s DAC Transmitter DSP 10 GS/s DAC Transmitter DSP Preamble: 1 Sync-TS, 2 EQ-TS Load: 50 Data-Symbols Ch. 1 & 3 N FFT = 512 N = 354 (70%) N CP = 24 Gap = 41 subcarriers 5.6 Gbaud/ch 11.2 Gbit/s/ch ADC LPF Receiver DSP Ch

14 Carrier Recovery Simulation Results ideal CR Pilot CR S&C CR f c = 200 MHz linewidth = 100 khz l = 200 km BER OSNR [db] BER = 10-3 : Ideal: 9,1 db Pilot tone: 10 db S&C: 11,4 db -14-

15 Outline 1. CO-OFDM System 2. Influence of Carrier Frequency Offset 3. Methods for Carrier Recovery a. Pilot tone based b. TS based 4. Transmitter IQ-Imbalances a. Impact b. Compensation 5. Experimental Results 6. Summary and Conclusion -15-

16 Transmitter IQ-Imbalance Impairments Tx IQ-Imbalance: Loss of power balance between I- & Q-branch amplitude imbalance Loss of orthogonality between I- & Q-branch phase imbalance Data symbols of n th SC: r k h G s k h G s k * n( ) n 1, n n( ) n 2, n n( ) Imbalance ICI Tx g Tx I Q 90 Tx G 1, n 1 g Tx, n 2 e j Tx, n, G 2, n 1g Tx, n 2 e j Tx, n. A f -2 0, g 1 8, g 0.8 Tx Tx -2 Tx Tx f n f n f 1 OSNR = 16 db OSNR = 16 db Schenk, T. (2008), RF-Imperfections in high-rate wireless systems: impact and digital compensation. -16-

17 Transmitter IQ-Imbalance Compensation Training symbols for compensation & equalization: 1 st Channel estimation 2 nd IQ-Imbalance estimation s s n n (1) (2) n subcarrier n Compensation & EQ * sˆ ( k) ˆ ( ) ˆ ˆ ( ) ˆ ( ) n h k G h k G n 1, n n 2, n r k n ˆ* ˆ ˆ* ˆ * sˆ ( k) ( ) ( ) ( k) n h k G h k G r n 2, n n 1, n n EVM [%] , g 0.8 Tx 8 IQ-imb.-comp. ZF-EQ OSNR [db] Tx Realteil Realteil Imaginärteil Imaginärteil

18 CO-OFDM WDM Experimental Setup Ch. 2 ECL Avanex Q50 IQ- Modulator Waveshaper EDFA P Tx = 1 dbm Att./PM 100 km Att./PM EDFA OSA ECL 3 db Ch. 1 & 3 Amplifier IQ- Modulator Codeon Amplifier x2 P Rx = -10 dbm BPF Att./PM LPF LPF PBS 10 GS/s AWG Offline DSP Transmitter 10 GS/s AWG Preamble: 1 Sync-TS, 2 IQ-TS or 2 EQ-TS Load: 50 Data-Symbols N FFT = 512 N = 354 (70%) N CP = 24 Gap = 41 subcarriers -18- ECL PBS 5.6 Gbaud/ch 11.2 Gbit/s/ch X Coherent Receiver 50 GS/s Scope Offline DSP Receiver Coherent Receiver Chair for Communications Y

19 EVM [%] EVM [%] Phase Imbalance Variation (B2B) B2B, OSNR = 40 db I Variation of bias voltage from phase modulator Comparison between ZF-Equalizer and IQ-imbalance compensation Q 90 Tx Pilot tone approach: ZF-EQ IQ-Imb. Compensation S&C TS approach: Tx[ ] Tx[ ]

20 CFO Variation (200km) Pilot tone approach S&C TS approach OSNR = 20 db Pilot EQ Pilot IQ S&C EQ S&C IQ OSNR = 20 db EVM [%] EVM [%] , g 1.06 Tx CFO [MHz] Tx Only ZF-EQ: 18.4% w IQ-Imb. Comp.: 17.5% 15 Mean EVM: CFO [MHz] Only ZF-EQ: 18.4% w IQ-Imb. Comp.: 18.2% -20-

21 OSNR Variation (200km) Pilot EQ Pilot IQ S&C EQ S&C IQ f c 2.65 g Tx Tx 150 MHz 1.16 IQ-Imb. Comp. BER ZF: OSNR [db] BER = 10-3 : Ideal EQ (Simulation): 9,1 db Pilot EQ: 11,75 db S&C EQ: 14,6 db -21-

22 Summary + Conclusion Summary Experimental verification for CFO + PN compensation methods Experimental evaluation of an IQ-imbalance compensation method Conclusion Pilot tone based CFO + PN compensation: Robust, IQ-imbalances can be compensated TS based CFO + PN compensation (pilot subcarriers): Degraded performance with IQ-imbalances Thank you for your attention! -22-