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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B18–B25
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Digital predistortion of 75–110 GHz W-band frequency multiplier for fiber wireless short range access systems

Ying Zhao, Lei Deng, Xiaodan Pang, Xianbin Yu, Xiaoping Zheng, Hanyi Zhang, and Idelfonso Tafur Monroy  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B18-B25 (2011)
http://dx.doi.org/10.1364/OE.19.000B18


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Abstract

We present a W-band fiber-wireless transmission system based on a nonlinear frequency multiplier for high-speed wireless short range access applications. By implementing a baseband digital signal predistortion scheme, intensive nonlinear distortions induced in a sextuple frequency multiplier can be effectively pre-compensated. Without using costly W-band components, a transmission system with 26km fiber and 4m wireless transmission operating at 99.6GHz is experimentally validated. Adjacent-channel power ratio (ACPR) improvements for IQ-modulated vector signals are guaranteed and transmission performances for fiber and wireless channels are studied. This W-band predistortion technique is a promising candidate for applications in high capacity wireless-fiber access systems.

© 2011 OSA

1. Introduction

In this paper, we present and demonstrate a novel predistortion technique for a W-band fiber-wireless system based on a sextuple frequency multiplier which translates a carrier frequency from K-band to W-band. The inherent nonlinearity of the frequency multiplier is characterized by pre-measurement of AM-AM and AM-PM distortions. In the digital domain, the recursive least squares (RLS) algorithm is applied to make the digital iterative loop converge to the inverse property of the multiplier distortion. In the experiment, a 99.6GHz fiber-wireless system carrying predistorted quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM) signals with 26km fiber and up to 4m wireless transmission is established. An 18.9dB and a 16.8dB adjacent-channel power ratio (ACPR) improvements for QPSK and 16-QAM signals are achieved respectively, which demonstrates the effective functionality of the predistortion approach and the applicability of the proposed predistortion scheme. To evaluate the transmission performance of the fiber-wireless system, the bit error rate (BER) performances of fiber and wireless channels are investigated for QPSK and 16-QAM signals, respectively.

2. Principle and experimental setup

The experimental setup of the W-band fiber-wireless transmission system is shown in Fig.1. A sextuple frequency multiplier (Agilent E8257DS15) serves as a frequency translator in the W-band wireless transmitter module where the carrier frequency is translated from K-band to W-band. When a modulated bandpass waveform is fed into a frequency multiplier, its output contains harmonics and intermodulation components of the desired order. The output of a n-order frequency multiplier Y(t) can be expressed by
Y(t)=Re{A[r(t)]ej[nω0t+nφ(t)+P[r(t)]]}
(1)
where r(t) and φ(t) are the amplitude and phase of the complex baseband input signal and ω0 is the input carrier frequency. In general, the distortion arising from the frequency-multiplication process can be partly represented by polynomial functions A[·] and P[·], which are corresponding to the nonlinear AM-AM and AM-PM distortions. Another distortion applying n times of φ(t) linear phase deviation is called PM-PM distortion. To counteract the baseband distortion, digital transformation of input complex signal needs to be implemented as an inverse model of the frequency multiplier distortion.

Fig. 1 Experimental setup of a digital predistorted W-band wireless access system. PC: polarization controller PA: power amplifier. LNA: Low noise amplifier.

Since the influences of the multiplier can be represented by definite polynomial-based functions, we can extract the coefficients of A[·] and P[·] by pre-measurement of AM-AM and AM-PM distortions [7

7. Y. Park and J. S. Kenney, “Adaptive digital predistortion linerization of frequency multipliers,” IEEE Trans. Microwave Theory Tech. 51, 2516–2522 (2003). [CrossRef]

]. After characterizing A[·] and P[·], the predistortion processing can be achieved as shown in Fig.2. The digital predistortion is based on two parallel iterative loops, one of which is for amplitude distortion and the other serves for phase deviation. The first 1000 symbols of X(k) are used to drive the predistorters in amplitude iterative loop and phase iterative loop. In the experiment, both amplitude predistorter and phase predistorter are 4-term polynomials so that the 8 coefficients are updated with the iterations. The RLS algorithm is applied to drive the iterative loops converge to a proper predistortion function, in other words, the inverse characteristic of the frequency multiplier. By combining the loop outputs r(k) and φ(k), a fundamental predistorted baseband symbol sequence y(k) can be obtained. Subsequently, by referring the feedback constellation orthogonality information, a re-optimized complex factor C (C ≈ 1) is determined with the conventional hill-climbing algorithm. The predistorted complex baseband signal Y(k) which is the product of C and y(k) is obtained and output from the digital predistortion module.

Fig. 2 Block diagram of the digital signal processing in the digital predistortion module.

3. Experimental results

To characterize the frequency multiplier, pre-measurement is firstly performed by driving the transmitter with an unmodulated 16.6GHz LO. The results show the frequency multiplier has a minimal effective input power of −2dBm due to the switching voltage of Schottky diodes in the multiplier. To avoid the turn-off and the over-saturation effects, the input signal power is clipped from −1dBm to 6dBm, which indicates a limited dynamic range for the input signal. It is also shown from the pre-measurement that the input-output conversion loss is more than 30dB in the W-band since the frequency multiplier is based on high-order nonlinear operation of a passive diode. For nonlinearity evaluation, AM-AM and AM-PM responses are extracted and represented in 4-term polynomials where the 6th-order term dominates.

Figure 3 shows the convergence curves for the amplitude RLS loop and the phase RLS loop for 312.5Mb/s QPSK signal. The normalized amplitude error |[R(k) – xa(k)]/xa(k)| drops by 40dB and the absolute phase error |ϕ(k) – xp(k)| is less than 0.02° after 1000 iterations, which verifies the feasibility of the RLS algorithm for digital predistortion. To accelerate convergence, the parameters of the RLS algorithm need to be further optimized. Due to the pre-measurement error and time-dependent variance of the frequency multiplier, fine adjustment of baseband symbols is performed based on the physical feedback path to further guarantee the predistortion performance. Figure 4 shows the angle deviation from 90° of the demodulated QPSK constellation as a function of the complex angle of the re-optimized factor C for 3dBm input power. It can be seen a 0.5° angle offset from the fundamental predistorted sequence y(k) is induced to achieve optimized orthogonality, which also implies the fine adjustment is able to give an up to 2° orthogonality correction.

Fig. 3 RLS convergence curves for amplitude and phase predistortion loops.
Fig. 4 Orthogonality re-optimization for the QPSK signal.

The inset of Fig.5 shows the constellations of the demodulated 312.5Mb/s QPSK and the pre-distorted symbol sequence Y(k) in the digital predistortion module. For the received QPSK constellation, it qualitatively shows a good magnitude uniformity and angle orthogonality, which implies the effectiveness of the digital predistortion for the sextuple frequency multiplier. The constellation of Y(k) shows that the predistorted clusters are confined in a limited area which implies the inverse nonlinear characteristic of the frequency multiplier. Figure 5 shows the measured 312.5Mb/s QPSK spectra output from the transmitter with and without digital pre-distortion processing. It can be seen that without the predistortion, the signal is spread over 500MHz and it is impossible to be demodulated due to intermodulation distortions. With the digital predistortion, the ACPR quantitatively performs an 18.9dB improvement at 312.5MHz offset from the center frequency. For the 312.5Mb/s 16-QAM signal, the constellations of the demodulated signal and the predistorted sequence for 3dBm input power as well as the spectra with and without predistortion are shown in Fig.6. An ACPR improvement of 16.8dB at 156.25MHz offset is also observed, which verifies the predistortion scheme is applicable for different modulation formats. After predistortion, some residual 6th-order intermodulation distortion still exists and we expect this residual distortion can be further suppressed by treating more terms in the polynomial predistorters.

Fig. 5 QPSK spectra with (red) and without (blue) predistortion. Inset: Predistorted (red) and received (blue) QPSK constellations.
Fig. 6 16-QAM spectra with (red) and without (blue) predistortion. Inset: Predistorted (red) and received (blue) 16-QAM constellations.

Fig. 7 Fiber transmission performances for the 99.6GHz fiber-wireless transmission system. BER vs optical power before the PD with fixed 1m wireless distance.FEC: forward error correction.
Fig. 8 Wireless transmission performances for the 99.6GHz fiber-wireless transmission system. BER vs 99.6GHz wireless transmission distance with fixed 26km fiber transmission. Inset: demodulated constellation of QPSK and 16-QAM signals.

4. Conclusions

This paper has presented a W-band fiber-wireless system that employs digital predistortion technique for eliminating nonlinear distortions in a frequency multiplier. We experimentally proved the feasibility of the predistortion processing in the digital domain. Without need for costly high-frequency optical or MMW components such as W-band photodiode or power amplifier, 312.5Mb/s QPSK and 16-QAM signals carried by 99.6GHz MMW were obtained with detailed transmission performance discussions. 18.9dB and 16.8dB ACPR improvements for QPSK and 16-QAM signals confirmed the capability of the proposed predistortion scheme to counteract nonlinear distortions, which makes the system as a good candidate for future indoor W-band wireless applications.

Acknowledgments

This work was supported in part by National Nature Science Foundation of China (NSFC) under grant No. 60736003, 61025004, 61032005, National 863 Program under grant No. 2009AA01Z222, 2009AA01Z223 and National Basic Research Program of China under grant No 2012CB315603-04.

References and links

1.

“FCC online table of frequency allocations,” www.fcc.gov/oet/spectrum/table/fcctable.pdf.

2.

C. W. Chow, F. M. Kuo, and J. W. Shi, “100 GHz ultra-wideband (UWB) fiber-to-the-antenna (FTTA) system for in-building and in-home networks,” Opt. Express 18, 473–478 (2010). [CrossRef] [PubMed]

3.

J. Marti and J. Capmany, “Microwave photonics and radio-over-fiber research,” IEEE Microw. Mag. 10, 96–105 (2009). [CrossRef]

4.

R. W. Ridgway, D. W. Nippa, and S. Yen, “Data transmission using differential phase-shift keying on a 92 GHz Carrier,” IEEE Trans. Microwave Theory Tech. 58, 3117–3126 (2010). [CrossRef]

5.

A. Hirata, M. Harada, and T. Nagatsuma, “120-GHz wireless link using photonic techniques for generation, modulation, and emission of millimeter-wave signals,” J. Lightwave Technol. 21, 2145–2153 (2003). [CrossRef]

6.

R. Sambaraju, J. Herrera, J. Martí, D. Zibar, A. Caballero, J. B. Jensen, I. Tafur Monroy, U. Westergren, and A. Walber, “Up to 40 Gb/s wireless signal generation and demodulation in 75–110 GHz band using photonic technique,” in 2010 IEEE Topical Meeting on Microwave Photonics (MWP), 1–4, (2010). [CrossRef]

7.

Y. Park and J. S. Kenney, “Adaptive digital predistortion linerization of frequency multipliers,” IEEE Trans. Microwave Theory Tech. 51, 2516–2522 (2003). [CrossRef]

8.

L. C. Chang and Y. L. Lan, “Analysis of amplitude and phase predistortion and polynomial-based predistortion in OFDM systems,” in 2007 6th International Conference on Proceedings of Information, Communications & Signal Processing (ICICS), 1–5 (2007). [CrossRef] [PubMed]

9.

Y. Park, R. Melville, R. C. Frye, M. Chen, and J. S. Kenney, “Dual-band transmitters using digitally predistorted frequendy multipliers for reconfigurable radios,” IEEE Trans. Microwave Theory Tech. 53, 115–122 (2004). [CrossRef]

10.

H. Chang, J. Tsai, T. Huang, H. Wang, Y. Xia, and Y. Shu, “A W-band high-power predistorted direct-conversion digital modulator for transmitter applications,” IEEE Microw. Wirel. Compon. Lett. 15, 600–602 (2005). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(350.4010) Other areas of optics : Microwaves
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Subsystems for Optical Networks

History
Original Manuscript: September 7, 2011
Revised Manuscript: October 16, 2011
Manuscript Accepted: October 17, 2011
Published: November 16, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Ying Zhao, Lei Deng, Xiaodan Pang, Xianbin Yu, Xiaoping Zheng, Hanyi Zhang, and Idelfonso Tafur Monroy, "Digital predistortion of 75–110 GHz W-band frequency multiplier for fiber wireless short range access systems," Opt. Express 19, B18-B25 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B18


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References

  1. “FCC online table of frequency allocations,” www.fcc.gov/oet/spectrum/table/fcctable.pdf .
  2. C. W. Chow, F. M. Kuo, and J. W. Shi, “100 GHz ultra-wideband (UWB) fiber-to-the-antenna (FTTA) system for in-building and in-home networks,” Opt. Express18, 473–478 (2010). [CrossRef] [PubMed]
  3. J. Marti and J. Capmany, “Microwave photonics and radio-over-fiber research,” IEEE Microw. Mag.10, 96–105 (2009). [CrossRef]
  4. R. W. Ridgway, D. W. Nippa, and S. Yen, “Data transmission using differential phase-shift keying on a 92 GHz Carrier,” IEEE Trans. Microwave Theory Tech.58, 3117–3126 (2010). [CrossRef]
  5. A. Hirata, M. Harada, and T. Nagatsuma, “120-GHz wireless link using photonic techniques for generation, modulation, and emission of millimeter-wave signals,” J. Lightwave Technol.21, 2145–2153 (2003). [CrossRef]
  6. R. Sambaraju, J. Herrera, J. Martí, D. Zibar, A. Caballero, J. B. Jensen, I. Tafur Monroy, U. Westergren, and A. Walber, “Up to 40 Gb/s wireless signal generation and demodulation in 75–110 GHz band using photonic technique,” in 2010 IEEE Topical Meeting on Microwave Photonics (MWP), 1–4, (2010). [CrossRef]
  7. Y. Park and J. S. Kenney, “Adaptive digital predistortion linerization of frequency multipliers,” IEEE Trans. Microwave Theory Tech.51, 2516–2522 (2003). [CrossRef]
  8. L. C. Chang and Y. L. Lan, “Analysis of amplitude and phase predistortion and polynomial-based predistortion in OFDM systems,” in 2007 6th International Conference on Proceedings of Information, Communications & Signal Processing (ICICS), 1–5 (2007). [CrossRef] [PubMed]
  9. Y. Park, R. Melville, R. C. Frye, M. Chen, and J. S. Kenney, “Dual-band transmitters using digitally predistorted frequendy multipliers for reconfigurable radios,” IEEE Trans. Microwave Theory Tech.53, 115–122 (2004). [CrossRef]
  10. H. Chang, J. Tsai, T. Huang, H. Wang, Y. Xia, and Y. Shu, “A W-band high-power predistorted direct-conversion digital modulator for transmitter applications,” IEEE Microw. Wirel. Compon. Lett.15, 600–602 (2005). [CrossRef]

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