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

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

5. Remote antenna: Radio over fiber link
Downlink
ROF G
Air
[4] Fernando et al.
n_optical
n_air
Uplink G
ROF G
Air
n_optical
n_air
Fiber loss: Converter loss, attenuation Air 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 Page(s): 501 –509
[2] Direct-detection analog optical links Cox, C., III.; Ackerman, E.; Helkey, R.; Betts, G.E.; Microwave Theory and Techniques, IEEE Transactions on , Volume: 45 Issue: 8 , Aug Page(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 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 Page(s): 556 –563
[6] Technique for optical SSB generation to overcome dispersion penalties in fiber-radio systems Smith, G. H.; Novak D.; Ahmed Z;
[7] Phase noise in coherent analog AM-WIRNA optical link Tayor R.; Poor H. V.; Forrest Stephen;