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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 4 — Feb. 24, 2014
  • pp: 4649–4661
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Experimental investigation on multi-dimensional digital predistortion for multi-band radio-over-fiber systems

Hao Chen, Jianqiang Li, Kun Xu, Yinqing Pei, Yitang Dai, Feifei Yin, and Jintong Lin  »View Author Affiliations


Optics Express, Vol. 22, Issue 4, pp. 4649-4661 (2014)
http://dx.doi.org/10.1364/OE.22.004649


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Abstract

The recently-proposed multi-dimensional digital predistortion (DPD) technique is experimentally investigated in terms of nonlinearity order, memory length, oversampling rate, digital-to-analog conversion resolution, carrier frequency dependence and RF input power tolerance, in both directly-modulated and externally-modulated multi-band radio-over-fiber (RoF) systems. Similar characteristics of the multi-dimensional digital predistorter are identified in directly-modulated and externally-modulated RoF systems. The experimental results suggest implementing a memory-free multi-dimensional digital predistorter involving nonlinearity orders up to 5 at 2 × oversampling rate for practical multi-band RoF systems. Using the suggested parameters, the multi-dimensional DPD is able to improve the RF input power tolerance by greater than 3dB for each band in a two-band RoF system, indicating an enhancement of RF power transmitting efficiency.

© 2014 Optical Society of America

1. Introduction

In this paper, the multi-dimensional digital predistortion method developed in [11

11. Y. Pei, K. Xu, J. Li, A. Zhang, Y. Dai, Y. Ji, and J. Lin, “Complexity-reduced digital predistortion for subcarrier multiplexed radio over fiber systems transmitting sparse multi-band RF signals,” Opt. Express 21(3), 3708–3714 (2013). [CrossRef] [PubMed]

] is experimentally investigated in both directly-modulated and externally-modulated multi-band RoF systems in terms of nonlinearity order, memory length, oversampling rate, digital-to-analog conversion (DAC) resolution, carrier frequency dependence and RF input power tolerance, as not provided in the prior arts. This work is expected to help practical implementation of DPD in multi-band RoF systems.

2. Review of the developed multi-dimensional DPD

Given yi(n) and zi(n), the memory polynomial coefficient ai,k,q(mk) can be extracted from the block labeled “Multi-Dimensional Predistorter training” (Block A) that has yi(n)/Gi as its input and z^i(n) as its output. The actual predistorter that has xi(n) as its input and zi(n) as its output is an exact copy of Block A when |ei(n)|2=|z^i(n)-zi(n)|2is minimized through least-squares (LS) algorithm. In theory, ai,k,q(mk) can be extracted by finding the solution of
zi=Uiai.
(2)
where zi=[zi(0),zi(1),..,zi(N1)]T,
ai=[ai,1,0(1),..,ai,1,Q(1),..,ai,k,0(1),..,ai,k,Q(1),..,ai,k,0(Mk),..,ai,k,Q(Mk),..,ai,K,Q(MK)]T,Ui=[ui,1,0(1),..,ui,1,Q(1),..,ui,k,0(1),..,ui,k,Q(1),..,ui,k,0(Mk),..,ui,k,Q(Mk),..,ui,K,Q(MK)],ui,k,q(mk)=[ui,k,q(mk)(0),..,ui,k,q(mk)(N1)]T,andui,k,q(mk)(n)=yi(n-q)|yi(n-q)|j=1fi=j=1k±fjk|1Gjyj(n-q)|.
The LS-algorithm-based solution of Eq. (2) isa^i=(UiHUi)-1UiHzi, where (▪)H denotes complex conjugate transpose.

3. Experiments for directly-modulated RoF systems

3.1 Experimental setup

3.2 Experimental results

Second, three scenarios were evaluated: the scenario without DPD, the scenario with independent DPD [14

14. L. Ding, G. T. Zhou, Z. Ma, D. R. Morgan, J. S. Kenney, J. Kim, and C. R. Giardina, “A robust digital baseband predistorter constructed using memory polynomials,” IEEE Trans. Commun. 52(1), 159–165 (2004). [CrossRef]

] (i.e. only in-band DPD is performed), and the scenario with multi-dimensional DPD (i.e. both in-band and cross-band DPD are performed). The RF power applied on the LD was 7.5 dBm per band. Figure 4
Fig. 4 Power spectra and constellation diagrams after directly-modulated RoF link. (a) power spectra of Band 1 at 2.3 GHz; (b) constellation diagram of Band 1 without DPD; (c) constellation diagram of Band 1 with independent DPD; (d) constellation diagram of Band 1 with multi-dimensional DPD; (e) power spectra of Band 2 at 2.462 GHz; (f) constellation diagram of Band 2 without DPD; (g) constellation diagram of Band 2 with independent DPD; (h) constellation diagram of Band 2 with multi-dimensional DPD.
shows the measured RF power spectra and constellation diagrams of the output signal from the RoF link. The quantitative results are summarized in Table 1

Table 1. Comparison in ACPR and EVM for directly-modulated RoF systems

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. It is obvious that the independent DPD only provides limited performance improvement, whereas about 5 dB adjacent channel power ratio (ACPR) improvement is observed at both upper sideband (USB) and lower sideband (LSB) for the multi-dimensional DPD. The use of the multi-dimensional DPD reduces the error vector magnitude (EVM) from 3.78% and 4.16% to 1.22% and 1.15% for the two bands, respectively.

Third, the tolerance of the multi-dimensional DPD to the RF power difference between the two bands (P1 and P2) was evaluated. In the experiments, the total RF input power of two bands was kept to be 10.5 dBm while the RF input power difference of the two bands varied from 3 dB to 6 dB with a step of 1 dB. Table 2

Table 2. Tolerance to the RF power difference for directly-modulated RoF systems

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summarizes the experimental results. Given a fixed total RF input power to the RoF link, the band with lower RF power suffers from more nonlinear distortions in terms of EVM, which indicates that the cross-band nonlinearity dominates. However, the multi-dimensional DPD is capable of compensating for the nonlinear distortions in both bands even in the presence of up to 6 dB RF power difference.

Fourth, the performance of the multi-dimensional DPD was analyzed in terms of DAC resolution. The used VSGs in the experiments have a nominal DAC resolution of 14 bits. Therefore, we implemented quantization to I/Q samples of two bands in Matlab before the data was fed to VSGs, through which we simulated different DAC resolutions. The RF power applied on the LD was 7.5 dBm per band.The EVMs of both scenarios with and without multi-dimensional DPD are depicted in Fig. 5
Fig. 5 EVM as a function of DAC resolution for directly-modulated RoF systems. (a) Band 1 at 2.3 GHz; (b) Band 2 at 2.462 GHz.
as a function of DAC resolution bits. For comparison, the EVM of back-to-back (B2B) scenario (i.e. VSG directly connected to VSA without RoF links) is also illustrated in Fig. 5. For all three scenarios, a threshold of 8 bits in DAC resolution can be observed, which is mainly determined by the employed 64 QAM-OFDM format. It can also be concluded that the multi-dimensional DPD puts no more burdens on the DAC in terms of resolution, as compared with the B2B case. In practice the effective number of bits (ENOB) is more important. If taking the ENOB into account, DACs with larger nominal resolution bits are needed.

Fifth, given a fixed carrier frequency of Band 1 at 2.3 GHz, the dependency to the carrier frequency was studied by changing the carrier frequency of Band 2 to 2.412 GHz (IEEE 802.11g Channel 1), 2.427 GHz (IEEE 802.11g Channel 4) and 2.442 GHz (IEEE 802.11g Channel 7). The RF power applied on the LD was 7.5 dBm per band. As shown in Table 3

Table 3. The dependency on carrier frequency for directly-modulated RoF systems

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, the EVM performance maintains as the carrier frequency of Band 2 varies for both cases with and without multi-dimensional DPD.

4. Experiments for externally-modulated RoF systems

4.1 Experimental setup

4.2 Experimental results

Second, three scenarios were evaluated: the scenario without DPD, the scenario with independent DPD (i.e. only in-band DPD is performed), and the scenario with multi-dimensional DPD (i.e. both in-band and cross-band DPD are performed). The RF power applied on the MZM was 1.5 dBm per band. Figure 9
Fig. 9 Power spectra and constellation diagrams after externally-modulated RoF link. (a) power spectra of Band 1 at 2.3 GHz; (b) constellation diagram of Band 1 without DPD; (c) constellation diagram of Band 1 with independent DPD; (d) constellation diagram of Band 1 with multi-dimensional DPD; (e) power spectra of Band 2 at 2.462 GHz; (f) constellation diagram of Band 2 without DPD; (g) constellation diagram of Band 2 with independent DPD; (h) constellation diagram of Band 2 with multi-dimensional DPD.
shows the measured RF power spectra and constellation diagrams of the output signal from the RoF link. The quantitative results are summarized in Table 4

Table 4. Comparison in ACPR and EVM for externaltly-modulated RoF systems

table-icon
View This Table
| View All Tables
. It is obvious that the independent DPD only provides limited performance improvement, whereas about 5 dB ACPR improvement is observed at both upper sideband (USB) and lower sideband (LSB) for the multi-dimensional DPD. The use of the multi-dimensional DPD reduces the EVM from 4.51% and 4.46% to 1.52% and 1.53% for the two bands, respectively.

Third, the tolerance of the multi-dimensional DPD to the RF power difference between the two bands (P1 and P2) was evaluated. In the experiments, the total RF input power of two bands was kept to be 4.5 dBm while the RF input power difference of the two bands varied from 3 dB to 6 dB with a step of 1 dB. Table 5

Table 5. Tolerance to the RF power difference for externally-modulated RoF systems

table-icon
View This Table
| View All Tables
summarizes the experimental results. Given a fixed total RF input power to the RoF link, the band with lower RF power suffers from more nonlinear distortions in terms of EVM, which indicates that the cross-band nonlinearity dominates. However, the multi-dimensional DPD is capable of compensating for the nonlinear distortions in both bands even in the presence of up to 6 dB RF power difference.

Fourth, the performance of the multi-dimensional DPD was analyzed in terms of DAC resolution. The RF power applied on the MZM was 1.5 dBm per band. The EVMs of both scenarios with and without multi-dimensional DPD are depicted in Fig. 10
Fig. 10 EVM as a function of DAC resolution for externally-modulated RoF systems. (a) Band 1 at 2.3 GHz; (b) Band 2 at 2.462 GHz.
as a function of DAC resolution bits. For comparison, the EVM of back-to-back (B2B) scenario (i.e. VSG directly connected to VSA without RoF links) is also illustrated in Fig. 10. For all three scenarios, a threshold of 8 bits in DAC resolution can be observed, which is mainly determined by the employed 64 QAM-OFDM format. It can also be concluded that the multi-dimensional DPD puts no more burdens on the DAC in terms of resolution, as compared with the B2B case.

Fifth, given a fixed carrier frequency of Band 1 at 2.3GHz, the dependency to the carrier frequency was studied by changing the carrier frequency of Band 2 to 2.412 GHz (IEEE 802.11g Channel 1), 2.427 GHz (IEEE 802.11g Channel 4) and 2.442 GHz (IEEE 802.11g Channel 7). The RF power applied on the MZM was 1.5 dBm per band. As shown in Table 6

Table 6. The dependency on carrier frequency for externally-modulated RoF systems

table-icon
View This Table
| View All Tables
, the EVM performance maintains as the carrier frequency of Band 2 varies for both cases with and without multi-dimensional DPD.

5. Discussion on practical implementation of adaptive training by feedback

6. Conclusion

We have experimentally investigated the developed multi-dimensional DPD technique for linearizing multi-band RoF systems. For both directly-modulated and externally-modulated RoF systems, the experimental results show that the cross-band nonlinear distortion is much more distinct than the in-band nonlinear distortion, which necessitates multi-dimensional DPD technique. According to the investigations, memory-free multi-dimensional digital predistorters involving nonlinearity orders up to 5 at 2 × oversampling rate are suggested for practical directly-modulated and externally-modulated multi-band RoF systems. Using the suggested parameters, the multi-dimensional DPD is able to improve the RF input power tolerance by greater than 3dB for each band in a two-band RoF system, indicating an enhancement of RF power transmitting efficiency.

Acknowledgments

This work was supported in part by National 973 Program (2012CB315705), NSFC Program (61271042, 61107058, 61302086, and 61302016), Specialized Research Fund for the Doctoral Program of Higher Education (20130005120007), Program for New Century Excellent Talents in University (NCET-13-0682), and Fundamental Research Funds for the Central Universities.

References and links

1.

K. Andersson and C. Åhlund, “Optimized access network selection in a combined WLAN/LTE environment,” Wirel. Pers. Commun. 61(4), 739–751 (2011). [CrossRef]

2.

M. J. Crisp, S. Li, A. Wonfor, R. V. Penty, and I. H. White, “Demonstration of a radio over fiber distributed antenna network for combined in-building WLAN and 3G coverage,” Optical Fiber Communication Conference 2007, JTh81 (2007).

3.

S. Ghafoor and L. Hanzo, “Radio-over-fiber transmission for distributed antennas radio-over-fiber transmission for distributed antennas,” IEEE Commun. Lett. 15(12), 1368–1371 (2011). [CrossRef]

4.

D. Waken, A. Nkansah, and N. J. Gomes, “Radio over fiber link design for next generation wireless systems,” J. Lightwave Technol. 28(16), 2456–2464 (2010). [CrossRef]

5.

S. Fu, W. D. Zhong, P. Shum, and Y. J. Wen, “Simultaneous multichannel photonic up-conversion based on nonlinear polarization rotation of an SOA for radio-over-fiber system,” IEEE Photon. Technol. Lett. 21(9), 563–565 (2009). [CrossRef]

6.

J. Zhou, S. Fu, F. Luan, J. H. Wong, S. Aditya, P. Shum, and K. E. K. Lee, “Tunable multi-tap bandpass microwave photonic filter using a windowed Fabry-Perot filber-based multi-wavelength tunable laser,” J. Lightwave Technol. 29(22), 3381–3386 (2011). [CrossRef]

7.

X. N. Fernando and A. B. Sesay, “Higher order adaptive filter based predistortion for nonlinear distortion compensation of radio over fiber links,” Proceedings of the International Conference on Communications 2000, 367–371 (2000). [CrossRef]

8.

X. N. Fernando and A. B. Sesay, “Adaptive asymmetric linearization of microwave fiber optic links for wireless access,” IEEE Trans. Vehicular Technol. 51(6), 1576–1586 (2002). [CrossRef]

9.

K. Hayasaka, T. Higashino, K. Tsukamoto, and S. Komaki, “A theoretical estimation of IMD on heterogeneous OFDM service over SCM RoF link,” International Topical Meeting on & Microwave Photonics Conference 2011, 328–330 (2011). [CrossRef]

10.

A. Ferreira, T. Silveira, D. Fonseca, R. Ribeiro, and P. Monteiro, “Highly linear integrated optical transmitter for subcarrier multiplexed systems,” IEEE Photon. Technol. Lett. 21(7), 438–440 (2009). [CrossRef]

11.

Y. Pei, K. Xu, J. Li, A. Zhang, Y. Dai, Y. Ji, and J. Lin, “Complexity-reduced digital predistortion for subcarrier multiplexed radio over fiber systems transmitting sparse multi-band RF signals,” Opt. Express 21(3), 3708–3714 (2013). [CrossRef] [PubMed]

12.

S. A. Bassam, M. Helaoui, and F. M. Ghannouchi, “2-D digital predistortion (2-D-DPD) architecture for concurrent dual-band transmitters,” IEEE Trans. Microw. Theory Tech. 59(10), 2547–2553 (2011). [CrossRef]

13.

Y. J. Liu, W. Chen, J. Zhou, B. H. Zhou, and F. M. Ghannouchi, “Digital predistortion for concurrent dual-band transmitters using 2-D modified memory polynomials,” IEEE Trans. Microw. Theory Tech. 61(1), 281–290 (2013). [CrossRef]

14.

L. Ding, G. T. Zhou, Z. Ma, D. R. Morgan, J. S. Kenney, J. Kim, and C. R. Giardina, “A robust digital baseband predistorter constructed using memory polynomials,” IEEE Trans. Commun. 52(1), 159–165 (2004). [CrossRef]

15.

D. Guo, “Power amplifier and front end module requirements for IEEE 802.11n applications,” High Frequency Electronics (2011).

OCIS Codes
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.4230) Fiber optics and optical communications : Multiplexing
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Optical Communications

History
Original Manuscript: December 19, 2013
Revised Manuscript: February 13, 2014
Manuscript Accepted: February 14, 2014
Published: February 20, 2014

Citation
Hao Chen, Jianqiang Li, Kun Xu, Yinqing Pei, Yitang Dai, Feifei Yin, and Jintong Lin, "Experimental investigation on multi-dimensional digital predistortion for multi-band radio-over-fiber systems," Opt. Express 22, 4649-4661 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-4-4649


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References

  1. K. Andersson, C. Åhlund, “Optimized access network selection in a combined WLAN/LTE environment,” Wirel. Pers. Commun. 61(4), 739–751 (2011). [CrossRef]
  2. M. J. Crisp, S. Li, A. Wonfor, R. V. Penty, and I. H. White, “Demonstration of a radio over fiber distributed antenna network for combined in-building WLAN and 3G coverage,” Optical Fiber Communication Conference 2007, JTh81 (2007).
  3. S. Ghafoor, L. Hanzo, “Radio-over-fiber transmission for distributed antennas radio-over-fiber transmission for distributed antennas,” IEEE Commun. Lett. 15(12), 1368–1371 (2011). [CrossRef]
  4. D. Waken, A. Nkansah, N. J. Gomes, “Radio over fiber link design for next generation wireless systems,” J. Lightwave Technol. 28(16), 2456–2464 (2010). [CrossRef]
  5. S. Fu, W. D. Zhong, P. Shum, Y. J. Wen, “Simultaneous multichannel photonic up-conversion based on nonlinear polarization rotation of an SOA for radio-over-fiber system,” IEEE Photon. Technol. Lett. 21(9), 563–565 (2009). [CrossRef]
  6. J. Zhou, S. Fu, F. Luan, J. H. Wong, S. Aditya, P. Shum, K. E. K. Lee, “Tunable multi-tap bandpass microwave photonic filter using a windowed Fabry-Perot filber-based multi-wavelength tunable laser,” J. Lightwave Technol. 29(22), 3381–3386 (2011). [CrossRef]
  7. X. N. Fernando, A. B. Sesay, “Higher order adaptive filter based predistortion for nonlinear distortion compensation of radio over fiber links,” Proceedings of the International Conference on Communications 2000, 367–371 (2000). [CrossRef]
  8. X. N. Fernando, A. B. Sesay, “Adaptive asymmetric linearization of microwave fiber optic links for wireless access,” IEEE Trans. Vehicular Technol. 51(6), 1576–1586 (2002). [CrossRef]
  9. K. Hayasaka, T. Higashino, K. Tsukamoto, and S. Komaki, “A theoretical estimation of IMD on heterogeneous OFDM service over SCM RoF link,” International Topical Meeting on & Microwave Photonics Conference 2011, 328–330 (2011). [CrossRef]
  10. A. Ferreira, T. Silveira, D. Fonseca, R. Ribeiro, P. Monteiro, “Highly linear integrated optical transmitter for subcarrier multiplexed systems,” IEEE Photon. Technol. Lett. 21(7), 438–440 (2009). [CrossRef]
  11. Y. Pei, K. Xu, J. Li, A. Zhang, Y. Dai, Y. Ji, J. Lin, “Complexity-reduced digital predistortion for subcarrier multiplexed radio over fiber systems transmitting sparse multi-band RF signals,” Opt. Express 21(3), 3708–3714 (2013). [CrossRef] [PubMed]
  12. S. A. Bassam, M. Helaoui, F. M. Ghannouchi, “2-D digital predistortion (2-D-DPD) architecture for concurrent dual-band transmitters,” IEEE Trans. Microw. Theory Tech. 59(10), 2547–2553 (2011). [CrossRef]
  13. Y. J. Liu, W. Chen, J. Zhou, B. H. Zhou, F. M. Ghannouchi, “Digital predistortion for concurrent dual-band transmitters using 2-D modified memory polynomials,” IEEE Trans. Microw. Theory Tech. 61(1), 281–290 (2013). [CrossRef]
  14. L. Ding, G. T. Zhou, Z. Ma, D. R. Morgan, J. S. Kenney, J. Kim, C. R. Giardina, “A robust digital baseband predistorter constructed using memory polynomials,” IEEE Trans. Commun. 52(1), 159–165 (2004). [CrossRef]
  15. D. Guo, “Power amplifier and front end module requirements for IEEE 802.11n applications,” High Frequency Electronics (2011).

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