Chapter 4 WDM Fiber-Radio Network Implementation
4.4 Experimental setup and results
4.4.1 Reflective optical spectra from FBGA
In experiment we use optical spectrum analyzer from Advantest (Q8384) to observe the reflective optical spectra from FBGA. Fig. 4-4(a) displays the optical spectrum of four carrier wavelengths after optical multiplexer .This FBGA is dropped a fixed wavelength 1547.32 nm (i.e. λ2) shown in Fig. 4-4(b) to the RAU1 and RAU2, and a dynamic wavelength 1551.32 nm (i.e. λ8)shown in Fig. 4-4(c) to the
stand-by subscribers. It can be seen from Fig. 4-4 that it is more 30 dB optical power of the reflected wavelengths (λ2 & λ8) than those of the transmitted wavelengths (λ1 & λ3).
Fig. 4-4 Reflective optical spectra from FBGA (a) four wavelengths input (b) only λ2 reflected(c) λ2 and λ8 reflected.
4.4.2 Spectrum of injection-locked FP-LD
Experimental results showed that, under suitable operation conditions, the injection-locked of the FP-LD largely suppressed the original downstream data stream, allowing reuse of optical power and simultaneous direct modulation of upstream data.
Fig.4-5 displays the optical spectra of the FP laser before and after the injection mode-locked. It is shown the side-mode suppression ratio (SMSR) is greatly
-100
31 dB
30 dB
λ1 λ2 λ3 λ8
0
-50
Wavelength (nm)
Intensity (dBm)
improved from 5 dB to 34 dB and average output power is –7 dBm. The injection-locked FP-LD offered singlemode operation and thus greatly reduced the fiber-dispersion-included penalty.
Fig. 4-5 (a) free running; and (b) Injection- locked FP laser spectra
4.4.3 BER measurement
In experiment, a 110 Mb/s NRZ 231-1 PRBS data stream is generated from an Anritsu pattern generator (MP1763) and detected by 10GHz PIN detector. In a complete radio/fiber network system, the detector output would be transmitted through an RF antenna, to the mobile stations; however, in our experiment we have concentrated on the optical part of system, hence the RF carrier data signals take place after the photodiode. By unitizing Anritsu error analyzer (MP1764C) we can detect the bit-error rate of the received signals. Figure 4-8 shows the bit error rate (BER)
performance of downstream and re-modulated upstream 110 Mb/s NRZ 231-1 PRBS data stream as functions of received optical power. Error-free (< 10-9 BER) operation was up to an optical power of –12.9 dBm for downstream and –11.8 dBm for upstream, respectively.
Fig. 4-6 BER measurements on downstream and upstream traffic
4.4.4 Comparison performance between experiment and simulation Fig. 4-8 gives a comparison of BER measurement for experiment and simulation.
Since the laser diode used in the experiment is not stable enough compared with the simulation model and would be influenced by temperature and other factors. Thus,
10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3
-20 -18 -16 -14 -12 -10 -8
downstream upstream
Received Optical Power (dBm)
B E R
(110 Mb/s NRZ
2
31-1 PRBS
data stream)
there is some deviation between experiment and simulation. The differences of received optical power between experiment and simulation for downstream and upstream are 0.4dB and 0.6dB for error-free (10-9) as shown in Table 4-2.
Fig. 4-7 comparison for BER performance
Table 4-2 Comparison of experiment and simulation for downstream and upstream (received optical power at BER 10-9)
Downstream Upstream Difference(up-down)
experiment -12.9dBm -11.8dBm 1.1dB
simulation -13.3dBm -12.4dBm 0.9dB
Difference(exp-sim) 0.4dB 0.6dB X
Chapter 5 Conclusion
In this study, we numerically simulate and experimentally verify the proposed hybrid fiber-radio network based on the re-modulation injection locked scheme for upstream traffic and a dynamic wavelength allocation technique.
Firstly, we simulate the overall network by software VPI which is a powerful tool for system simulation. We find that under suitable condition(injection power greater than -13.038dBm), injection-locked scheme not only improve the SMSR(5.6dB to 45dB) of FP-LD but also increase the magnitude of RF signal, diminish the relative intensity noise of FP-LD and the intermodulation distortion due to multicarrier application. After verifying the characteristic of injection mode-locked FP-LD, we simulate the system performance for downstream and upstream including BER measurement and eye pattern measurement. Simulation results show that 2.4dB improvement was achieved for error-free (10-9) when injection-locked scheme is applied and the power penalty of downstream and upstream is around 0.9dB.
Numerically presentation for CNDR is provided to compute the noise and distortion characteristics and estimate optimum OMI for different demands.
In experiment, we use the fiber Brag grating array (FBGA) as wavelength add-drop multiplexer (WADM) to achieve the dynamic wavelength allocation and the corresponding SMSR is up to 30dB. For injection mode-locked of FP-LD, under proper situation, the SMSR is improved from 5 dB to 34dB and the singlemode operation enabled much better tolerance to the fiber dispersion. At last, we measured BER for both downstream and upstream. The received optical powers for error-free (10-9) are -12.9dBm and -11.8dBm, respectively.
The directly modulated injection mode-locked FP-LD that replaces the relative high cost laser source (such as DFB-LD) or external optical modulator (such as electric absorption modulator (EAM)) is used to transmit radio signals in the radio access unit (RAU) in a low-cost regime for acceptance of subscribers. And dynamic wavelength allocation technique can provide a cost-effective access network for large and burst wireless terminals.
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