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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 10 — May. 9, 2011
  • pp: 9881–9889
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Experimental comparison of modulation formats in IM/DD links

Krzysztof Szczerba, Johnny Karout, Petter Westbergh, Erik Agrell, Magnus Karlsson, Peter Andrekson, and Anders Larsson  »View Author Affiliations


Optics Express, Vol. 19, Issue 10, pp. 9881-9889 (2011)
http://dx.doi.org/10.1364/OE.19.009881


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Abstract

We present an experimental comparison of modulation formats for optical intensity modulated links with direct detection. Specifically, we compare OOK, QPSK on an electrical subcarrier and a new modulation format named OOPSK. The OOPSK modulation format is shown to have better sensitivity than the other modulation formats, in agreement with theoretical predictions. The impact of propagation in multimode fiber is also studied and the results show that all modulation formats have similar sensitivity penalties, with respect to the fibre length.

© 2011 OSA

1. Introduction

2. Theoretical background

There is however a constraint on this signal space. Since the optical intensity is modulated, and it cannot be negative, the electrical signal driving the directly modulated laser cannot be negative, after adding the bias current. This means that only a part of the three-dimensional signal space is available for modulation. The problem of admissible signal space for IM/DD channels was solved for generalized case in [21

21. S. Hranilovic and F. R. Kschischang, “Optical intensity-modulated direct detection channels: signal space and lattice codes,” IEEE Trans. Inf. Theory 49, 1385–1399 (2003). [CrossRef]

]. For the basis functions [Eqs. (1)(3)], the admissible signal space is a three-dimensional cone, illustrated in [21, Fig. 2], [8

8. S. Hranilovic, Wireless Optical Communication Systems (Springer, 2005).

, Fig. 4.2]. The signal space of the OOPSK format [23

23. J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]

] consists of four equidistant points on the surface of this cone, as shown in Fig. 1.

Fig. 1 Three dimensional constellation diagram of the OOPSK modulation format inscribed into the available signal space. The admissible signal space, from [21, Fig. 2], [8, Fig. 4.2], is the interior and the surface of the cone.
Fig. 2 Transmitter and receiver structures for the OOPSK format. The H(f) block denotes a rectangular pulse shaping filter and the H r(f) is a matched filter. The mapper maps bits pairs to symbols.

The structure of the OOPSK transmitter is a synthesis of the transmitter structures for PAM and subcarrier PSK. Since there are three basis functions, the transmitter has three branches forming the corresponding part of the final signal. Two branches are generating the PSK signal, using classical IQ modulation and a third branch generates the bias signal, which is the PAM component. A rectangular pulse shaping filter is used in each of the branches. The receiver structure reflects the transmitter structure, with two branches corresponding to the IQ components and a third branch corresponding to the bias signal. Filters matched to the transmitted pulse shape are used. In Fig. 2, these transmitter and receiver designs are shown. They consist of a conventional IQ transmitter and receiver (see, e.g., [24

24. R. G. Gallager, Principles of Digital Communication (Cambridge, 2008).

, Figs. 6.7–6.8]), with the addition of an extra branch to handle the bias. They can also be regarded as a variant of the IM/DD system in [7

7. J. R. Barry, Wireless Infrared Communications (Kluwer, 1994). [CrossRef]

, Fig. 5.6], generalized so that the bias signal too carries data.

The OOPSK modulation format has lower average electrical and optical power for the same minimum symbol distance as the OOK and QPSK, and the result is increased sensitivity of OOPSK compared with OOK and QPSK. A detailed analysis of performance of OOPSK and comparisons with other modulation formats, such as the OOK or QPSK, were carried out in [23

23. J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]

]. Specifically, it was found that the OOPSK format has 0.62 dB better optical sensitivity than OOK and 2.12 dB better sensitivity than subcarrier QPSK.

3. Experimental setup

The three modulation formats were experimentally compared using the same hardware setup. A directly modulated VCSEL was used at the transmitter end and a New Focus 1481-S-50 photodetector was used at the receiver end. The VCSEL was developed in-house and has been reported earlier [1

1. P. Westbergh, J. S. Gustavsson, Å. Haglund, A. Larsson, F. Hopfer, G. Fiol, D. Bimberg, and A. Joel, “32 Gbit/s multimode fiber transmission using high-speed, low current density 850 nm VCSEL,” Electron. Lett. 45, 366–368 (2009). [CrossRef]

]. The modulation bandwidth of this laser is around 20 GHz, the modulation bandwidth of the photodetector is 25 GHz. This high modulation bandwidth ensured, that the components were not the limiting factor in the experiment. No amplifier was used after the photodetector to avoid potential problem with amplifier nonlinearities. OM3+ MMF was used for transmission. The bandwidth-distance product of this fiber is 4700 MHz·km. The tested fiber lengths were 800 m and 1000 m. A back to back (B2B) configuration with a short MMF patchcord was also tested. The test setup is shown in Fig. 3.

Fig. 3 The experimental setup diagram.

The arbitrary waveform generator (AWG) was programmed with the test waveforms for all tested modulation formats. The modulating signal was fed to the VCSEL via a bias-T, which combined the modulating signal with the bias. The VCSEL was biased to keep the same extinction ratio for every modulation format.

The test waveforms generated with the AWG were prepared offline on a personal computer. The sampling rate of the AWG was 10 GS/s and the analog bandwidth was 7.5 GHz. At the receiver, a real time sampling oscilloscope was used, with a sampling rate of 50 GS/s and an analog bandwidth of 16 GHz. The signal was further processed off-line on a computer in order to obtain the BER results. The oscilloscope and the AWG were synchronized to avoid sampling frequencies drift, to provide fair performance comparison. Including the carrier and symbol timing recovery algorithms would mean, that their performance would affect the results. No equalizers were used, because one of the objectives was to observe the impact of propagation in multimode fiber. For similar reasons, rectangular pulse shaping was used, instead of for example raised root cosine or similar pulse shape. The experimental results presented in this paper are intended to show the intrinsic sensitivity results, but also to serve as a benchmark for further work on synchronization, pulse shaping and equalization algorithms.

The modulation formats were compared at the same bit-rate of 5 Gbps. The main-lobe bandwidth of each modulation was the same, 5 GHz for all modulation formats. The OOPSK and QPSK were operated at symbol rate of 2.5 Gbaud, since those modulation formats use 2 bits per symbol, while OOK was operated at 5 Gbaud. In the QPSK subcarrier modulation a single carrier period per symbol was used, in so called single-cycle subcarrier configuration.

4. Experimental results

To compare the sensitivity, the bit error rate (BER) against receiver optical power was measured for each modulation format and for each propagation. The lowest practical measurable BER at this bit rate was around 10–6. Some of the practical limitations were oscilloscope buffer length, the time to transfer the data from the oscilloscope to the computer and the off-line processing time.

It was found, that the main noise contribution is the thermal noise. Theoretically calculated, the shot noise, at the optical power levels used, was around −77 dBm. The measured thermal noise was −50 dBm, this was the main noise contribution in the system.

The BER results for the B2B configuration are presented Fig. 4a, for comparison theoretical performance is in Fig. 4b. Because the signal to noise ratio (SNR) depends on the square of the optical power [17

17. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85, 265–298 (1997). [CrossRef]

], the theoretical BER is plotted against CSNR, where C is proportionality constant depending on the bit rate and photodetector responsivity, analogously to [17

17. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85, 265–298 (1997). [CrossRef]

, eq. (5)]. Theoretically, the OOPSK modulation format has the best sensitivity at BER lower than 10–3, but in experimental B2B case OOPSK is better already below 10–2. The theoretical BER performance was obtained from the union bound in [23

23. J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]

], rescaled against the CSNR. As it is the union bound, it is not accurate at high BER. At BER of 10–4 OOPSK has sensitivity better by around 0.5 dB, than OOK. Theoretically, at asymptotically high SNR, OOPSK should be 0.62 dB better than OOK [23

23. J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]

]. In an ideal case, when real time transmitter and receiver would be available, it would be interesting to operate in the low BER region to take full advantage of the improved sensitivity of OOPSK. The subcarrier QPSK modulation has the worst sensitivity, being around 2 dB worse than OOPSK. This is in agreement with theoretical predictions in [23

23. J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]

].

Fig. 4 Experimental BER in the back to back case (a) and theoretical BER (b). At low BER, the OOPSK format has the the best sensitivity of all the compared modulation formats. The bit rate was 5 Gbps.

The BER values for transmission over 800 m and 1000 m of MMF are shown in Fig. 5a and Fig. 5b. The OOPSK format offers the best sensitivity for all tested fiber lengths, even at high BER, followed by the OOK, the subcarrier QPSK shows the worst sensitivity.

Fig. 5 Experimental BER after propagation over 800 m (a) and 1000 m (b) of MMF. The OOPSK modulation format is still the best one, the subcarrier QPSK has the worst performance.

The effect of the bandwidth limitation on the transmitted signal is most easily seen on the constellation diagrams. Since the OOPSK modulation format uses three basis functions, three spatial dimensions are needed to plot a constellation diagram. Received constellation diagram after back to back is show in Fig. 6. In the upper left there is an isometric projection of the constellation diagram, the rest of are orthogonal projections.

Fig. 6 Experimental constellation diagram of the the OOPSK modulation in the back to back case. The top left diagram is an isometric projection of the three dimensional constellation. The top right diagram is a projection of the constellation on the traditional IQ plane. The two lower diagrams are side projections of the constellation.

A constellation diagram of the signal transmitted over 1000 m of MMF is shown in Fig. 7. The constellation diagrams for propagation over 1000 m show the effect of the bandwidth limitation. It is clear that one of the symbols in the constellation is affected more than the others. The reason for this behavior lies in the fact, that firstly, a rectangular pulse shaping was used, and secondly only one carrier period per symbol is used for the PSK symbols. This means that between some symbols there are steep transitions. This is also true of the the QPSK modulation. It can be seen on the constellation diagrams of the QPSK signal, after B2B, after 800 m and after 1000 m of GI-MMF shown in Fig. 8. The effects of limited bandwidth on QPSK and OOPSK are comparable.

Fig. 7 Comparison of experimental OOPSK constellation diagrams for B2B, transmission over 800 m and 1000 m of MMF. The leftmost column contains projections of the OOPSK constellation diagram for B2B case, the centre one for 800 m and the rightmost one for 1000 m.
Fig. 8 Experimental constellation diagrams of the the subcarrier QPSK modulation after back to back, to the left, transmission over 800 m of GI-MMF in the centre and after 1000 m of MMF to the right.

5. Conclusions and future work

Acknowledgments

This work was supported by the Swedish Foundation for Strategic Research (SSF), the European FP7 project VISIT, the Swedish Scientific Council and the Knut and Alice Wallenberg Foundation.

References and links

1.

P. Westbergh, J. S. Gustavsson, Å. Haglund, A. Larsson, F. Hopfer, G. Fiol, D. Bimberg, and A. Joel, “32 Gbit/s multimode fiber transmission using high-speed, low current density 850 nm VCSEL,” Electron. Lett. 45, 366–368 (2009). [CrossRef]

2.

S. A. Blokhin, J. A. Lott, A. Mutig, G. Fiol, N. N. Ledentsov, M. V. Maximov, A. M. Nadtochiy, V. A. Shchukin, and D. Bimberg, “Oxide-confined 850 nm VCSELs operating at bit rates up to 40 Gbit/s,” Electron. Lett. 45, 501–503 (2009). [CrossRef]

3.

P. Westbergh, J. S. Gustavsson, B. Kögel, Å. Haglund, A. Larsson, A. Mutig, A. Nadtochiy, D. Bimberg, and A. Joel, “40 Gbit/s error-free operation of oxide-confined 850 nm VCSEL,” Electron. Lett. 46, 1014–1016 (2010). [CrossRef]

4.

J. E. Cunningham, D. Beckman, D. Huang, T. Sze, K. Cai, and A. V. Krishnamoorthy, “PAM-4 signaling over VCSELs using 0.13 μm CMOS,” OSA Topical Meeting on Information Photonics , (2005).

5.

F. Breyer, S. C. J. Lee, S. Randel, and N. Hanik, “Comparison of OOK- and PAM-4 modulation for 10 Gbit/s Transmission over up to 300 m polymer optical fiber,” Optical Fiber Communication Conference, OSA Technical Digest (2008), paper OWB5.

6.

F. Breyer, S. C. J. Lee, S. Randel, and N. Hanik, “PAM-4 signalling for gigabit transmission over standard step-index plastic optical fiber using light emitting diodes,” European Conference on Optical Communication , (2008), paper We2A3. [CrossRef]

7.

J. R. Barry, Wireless Infrared Communications (Kluwer, 1994). [CrossRef]

8.

S. Hranilovic, Wireless Optical Communication Systems (Springer, 2005).

9.

A. O. J. Wiberg, B.-E. Olsson, and P. A. Andrekson, “Single cycle subcarrier modulation,” Optical Fiber Communication Conference, OSA Technical Digest, (2009), paper OTuE1.

10.

K. Szczerba, B.-E. Olsson, P. Westbergh, A. Rhodin, J. S. Gustavsson, Å. Haglund, M. Karlsson, A. Larsson, and P. A. Andrekson, “37 Gbps transmission over 200 m of MMF using single cycle subcarrier modulation and a VCSEL with 20 GHz modulation bandwidth,” European Conference on Optical Communication , (2010), paper We7B2. [CrossRef]

11.

B.-E. Olsson and M. Sköld, “QPSK transmitter based on optical amplitude modulation of electrically generated QPSK signal,” Asia Optical Fiber Communication & Optoelectronic Exposition & Conference, OSA Technical Digest, (2008), paper SaA3.

12.

B.-E. Olsson and A. Alping, “Electro-optical subcarrier modulation transmitter for 100 GbE DWDM transport,” Asia Optical Fiber Communication & Optoelectronic Exposition & Conference, OSA Technical Digest, (2008), paper SaF3.

13.

B.-E. Olsson, J. Mårtensson, A. Kristiansson, and A. Alping, “RF-assisted optical dual-carrier 112 Gbit/s polarization-multiplexed 16-QAM transmitter,” Optical Fiber Communication Conference, OSA Technical Digest (2010), paper OMK5.

14.

S. C. J. Lee, F. Breyer, S. Randel, H. P. A. van den Boom, and A. M. J. Koonen, “High-speed transmission over multimode fiber using discrete multitone modulation,” J. Opt. Netw. 7, 183–196 (2008), (Invited paper). [CrossRef]

15.

S. C. J. Lee, F. Breyer, S. Randel, D. Cardenas, H. P. A. van den Boom, and A. M. J. Koonen, “Discrete multi-tone modulation for high-speed data transmission over multimode fibers using 850-nm VCSEL,” Conference on Optical Fiber Communication, OSA Technical Digest, (2009), paper OWM2.

16.

H. Yang, S. C. J. Lee, E. Tangdiongga, C. Okonkwo, H. P. A. van den Boom, F. Breyer, S. Randel, and A. M. J. Koonen, “47.4 Gb/s transmission over 100m graded-index plastic optical fiber based on rate-adaptive discrete multitone modulation,” J. Lightwave Technol . 28, 352–359 (2010). [CrossRef]

17.

J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85, 265–298 (1997). [CrossRef]

18.

R. You and J. M. Kahn, “Average power reduction techniques for multiple-subcarrier intensity-modulated optical signals,” IEEE Trans. Commun . 49, 2164–2171 (2001). [CrossRef]

19.

W. Kang and S. Hranilovic, “Optical power reduction for multiple-subcarrier modulated indoor wireless optical channels,” IEEE International Conference on Communications , (2006), 2743–2748. [CrossRef]

20.

S. Hranilovic and D. A. Johns, “A multilevel modulation scheme for high-speed wireless infrared communications,” in IEEE International Symposium on Circuits and Systems , (1999), 338–341.

21.

S. Hranilovic and F. R. Kschischang, “Optical intensity-modulated direct detection channels: signal space and lattice codes,” IEEE Trans. Inf. Theory 49, 1385–1399 (2003). [CrossRef]

22.

S. Hranilovic (2005), “On the design of bandwidth efficient signalling for indoor wireless optical channels,” Int. J. Commun. Syst. 18, 205–228 (2005). [CrossRef]

23.

J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]

24.

R. G. Gallager, Principles of Digital Communication (Cambridge, 2008).

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.4080) Fiber optics and optical communications : Modulation

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 4, 2011
Revised Manuscript: April 21, 2011
Manuscript Accepted: April 25, 2011
Published: May 5, 2011

Citation
Krzysztof Szczerba, Johnny Karout, Petter Westbergh, Erik Agrell, Magnus Karlsson, Peter Andrekson, and Anders Larsson, "Experimental comparison of modulation formats in IM/DD links," Opt. Express 19, 9881-9889 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-9881


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References

  1. P. Westbergh, J. S. Gustavsson, Å. Haglund, A. Larsson, F. Hopfer, G. Fiol, D. Bimberg, and A. Joel, “32 Gbit/s multimode fiber transmission using high-speed, low current density 850 nm VCSEL,” Electron. Lett. 45, 366–368 (2009). [CrossRef]
  2. S. A. Blokhin, J. A. Lott, A. Mutig, G. Fiol, N. N. Ledentsov, M. V. Maximov, A. M. Nadtochiy, V. A. Shchukin, and D. Bimberg, “Oxide-confined 850 nm VCSELs operating at bit rates up to 40 Gbit/s,” Electron. Lett. 45, 501–503 (2009). [CrossRef]
  3. P. Westbergh, J. S. Gustavsson, B. Kögel, Å. Haglund, A. Larsson, A. Mutig, A. Nadtochiy, D. Bimberg, and A. Joel, “40 Gbit/s error-free operation of oxide-confined 850 nm VCSEL,” Electron. Lett. 46, 1014–1016 (2010). [CrossRef]
  4. J. E. Cunningham, D. Beckman, D. Huang, T. Sze, K. Cai, and A. V. Krishnamoorthy, “PAM-4 signaling over VCSELs using 0.13 μm CMOS,” OSA Topical Meeting on Information Photonics , (2005).
  5. F. Breyer, S. C. J. Lee, S. Randel, and N. Hanik, “Comparison of OOK- and PAM-4 modulation for 10 Gbit/s Transmission over up to 300 m polymer optical fiber,” Optical Fiber Communication Conference, OSA Technical Digest (2008), paper OWB5.
  6. F. Breyer, S. C. J. Lee, S. Randel, and N. Hanik, “PAM-4 signalling for gigabit transmission over standard step-index plastic optical fiber using light emitting diodes,” European Conference on Optical Communication , (2008), paper We2A3. [CrossRef]
  7. J. R. Barry, Wireless Infrared Communications (Kluwer, 1994). [CrossRef]
  8. S. Hranilovic, Wireless Optical Communication Systems (Springer, 2005).
  9. A. O. J. Wiberg, B.-E. Olsson, and P. A. Andrekson, “Single cycle subcarrier modulation,” Optical Fiber Communication Conference , OSA Technical Digest, (2009), paper OTuE1.
  10. K. Szczerba, B.-E. Olsson, P. Westbergh, A. Rhodin, J. S. Gustavsson, Å. Haglund, M. Karlsson, A. Larsson, and P. A. Andrekson, “37 Gbps transmission over 200 m of MMF using single cycle subcarrier modulation and a VCSEL with 20 GHz modulation bandwidth,” European Conference on Optical Communication , (2010), paper We7B2. [CrossRef]
  11. B.-E. Olsson and M. Sköld, “QPSK transmitter based on optical amplitude modulation of electrically generated QPSK signal,” Asia Optical Fiber Communication & Optoelectronic Exposition & Conference , OSA Technical Digest, (2008), paper SaA3.
  12. B.-E. Olsson and A. Alping, “Electro-optical subcarrier modulation transmitter for 100 GbE DWDM transport,” Asia Optical Fiber Communication & Optoelectronic Exposition & Conference , OSA Technical Digest, (2008), paper SaF3.
  13. B.-E. Olsson, J. Mårtensson, A. Kristiansson, and A. Alping, “RF-assisted optical dual-carrier 112 Gbit/s polarization-multiplexed 16-QAM transmitter,” Optical Fiber Communication Conference , OSA Technical Digest (2010), paper OMK5.
  14. S. C. J. Lee, F. Breyer, S. Randel, H. P. A. van den Boom, and A. M. J. Koonen, “High-speed transmission over multimode fiber using discrete multitone modulation,” J. Opt. Netw. 7, 183–196 (2008), (Invited paper). [CrossRef]
  15. S. C. J. Lee, F. Breyer, S. Randel, D. Cardenas, H. P. A. van den Boom, and A. M. J. Koonen, “Discrete multi-tone modulation for high-speed data transmission over multimode fibers using 850-nm VCSEL,” Conference on Optical Fiber Communication , OSA Technical Digest, (2009), paper OWM2.
  16. H. Yang, S. C. J. Lee, E. Tangdiongga, C. Okonkwo, H. P. A. van den Boom, F. Breyer, S. Randel, and A. M. J. Koonen, “47.4 Gb/s transmission over 100m graded-index plastic optical fiber based on rate-adaptive discrete multitone modulation,” J. Lightwave Technol . 28, 352–359 (2010). [CrossRef]
  17. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85, 265–298 (1997). [CrossRef]
  18. R. You and J. M. Kahn, “Average power reduction techniques for multiple-subcarrier intensity-modulated optical signals,” IEEE Trans. Commun . 49, 2164–2171 (2001). [CrossRef]
  19. W. Kang and S. Hranilovic, “Optical power reduction for multiple-subcarrier modulated indoor wireless optical channels,” IEEE International Conference on Communications , (2006), 2743–2748. [CrossRef]
  20. S. Hranilovic and D. A. Johns, “A multilevel modulation scheme for high-speed wireless infrared communications,” in IEEE International Symposium on Circuits and Systems , (1999), 338–341.
  21. S. Hranilovic and F. R. Kschischang, “Optical intensity-modulated direct detection channels: signal space and lattice codes,” IEEE Trans. Inf. Theory 49, 1385–1399 (2003). [CrossRef]
  22. S. Hranilovic (2005), “On the design of bandwidth efficient signalling for indoor wireless optical channels,” Int. J. Commun. Syst. 18, 205–228 (2005). [CrossRef]
  23. J. Karout, E. Agrell, and M. Karlsson, “Power efficient subcarrier modulation for intensity modulated channels,” Opt. Express 18, 17913–17921 (2010). [CrossRef] [PubMed]
  24. R. G. Gallager, Principles of Digital Communication (Cambridge, 2008).

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