Coherent System in Remote Antenna Application

1. Coherent System in Remote Antenna Application Presents by Roland Yuen

2. Overview • Coherent system architecture • Remote antenna: Radio over fiber link • Design issues with coherent system • Potential system • Conclusion • References

3. 3 dB directional coupler with balance detectorOptical combiner with signal detector SMF at 1330 nm or 1550 nm Laser source: DFB, Nd:YAG RF Polarization Control External Modulation Receiver or Local Oscillator Laser Coherent system architecture RF Direct Modulation

4. Spurious Free Dynamic Range (SFDR) • The SNR in which its power due to non-linearity equals to the noise power. [1] Cox et al.

5. Remote antenna: Radio over fiber link Downlink ROF G Air [4] Fernando et al. n_optical n_air Uplink ROF G G Air n_optical n_air • Fiber loss: Converter loss, attenuationAir loss: fading (multipath) and shadowing (surrounding object) • SFDR requirement: 115 dB·Hz2/3 [3]

6. Design issues with coherent system External modulation or direct modulation? • Power consideration – gain in external modulation and loss in direct modulation [1] Cox et al.

7. External modulation or direct modulation? • Bandwidth and intermediate frequency • External modulation and direct modulation have 70 GHz and 20 GHz electrical bandwidth respectively. [2] • Linearization: Optical or electrical • Increase SFDR • Optical: Dual polarization, Multiple cascaded modulator [3] • Electrical: Predistortion circuit [3]

8. Amplitude modulation or angle modulation (PM, FM) • Bandwidth and dynamic range (SFDR) • Electronics – difficult to implement at high intermediate frequency [3] Kalman et al.

9. Relative Intensity Noise (RIN) • Dominate when power are high for it is proportional to the square of the mean optical power. • Balance receiver: Majority of the RIN noise can be cancelled • Balance receiver need to have very close matched photodiodes; 50:50 power split ratio from 3 dB directional coupler.

10. Phase noise (PM, FM) • Phase noise only affects angle modulation system • Optical cancellation: Optical signal sideband (SSB) [6] • Electronics cancellation: Phase noise cancellation circuit, AM-WIRNA [5],[7] • Increase IF: IF limitation [3]

11. Potential systems • Phase Modulation (PM) using phase modulator • Nd:YAG for transmitter and DFB laser local oscillator • Single receiver with optical combiner: RIN does not affect the system • IF amplifier (5-10 GHz) • Phase noise cancellation circuit

12. Conclusion • Coherent system enables angle modulation that has more SFDR • In remote antenna,losses in air channel lead to high SFDR requirement (uplink) • Phase noise cancellation circuit further increase SFDR in angle modulation • Potential system is to employ angle modulation with external phase modulator

13. References [1] An analytic and experimental comparison of direct and external modulation in analog fiber-optic links Cox, C.H., III; Betts, G.E.; Johnson, L.M.; Microwave Theory and Techniques, IEEE Transactions on , Volume: 38 Issue: 5 , May 1990 Page(s): 501 –509 [2] Direct-detection analog optical linksCox, C., III.; Ackerman, E.; Helkey, R.; Betts, G.E.; Microwave Theory and Techniques, IEEE Transactions on , Volume: 45 Issue: 8 , Aug. 1997Page(s): 1375 –1383 [3] Dynamic range of coherent analog fiber-optic links Kalman, R.F.; Fan, J.C.; Kazovsky, L.G.; Lightwave Technology, Journal of , Volume: 12 Issue: 7 , July 1994 Page(s): 1263 –1277 [4] On the design of optical fiber based wireless access systems..Fernando X. N.; Anpalagan A.;WINCORE laboratory, Ryerson University, Toronto [5] Optically coherent direct modulated FM analog link with phase noise canceling circuit Taylor, R.; Forrest, S.; Lightwave Technology, Journal of , Volume: 17 Issue: 4 , April 1999 Page(s): 556 –563 [6] Technique for optical SSB generation to overcome dispersion penalties in fiber-radio systemsSmith, G. H.; Novak D.; Ahmed Z; [7] Phase noise in coherent analog AM-WIRNA optical linkTayor R.; Poor H. V.; Forrest Stephen;