## Digital predistortion of 75–110 GHz W-band frequency multiplier for fiber wireless short range access systems |

Optics Express, Vol. 19, Issue 26, pp. B18-B25 (2011)

http://dx.doi.org/10.1364/OE.19.000B18

Acrobat PDF (3222 KB)

### 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

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]

## 2. Principle and experimental setup

*Y*(

*t*) can be expressed by where

*r*(

*t*) and

*φ*(

*t*) are the amplitude and phase of the complex baseband input signal and

*ω*

_{0}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.

^{15}– 1 is mapped to a symbol sequence with QPSK/16-QAM modulation format. 1000 training symbols for predistortion and another 100 symbols for synchronization are inserted at the beginning of the symbol sequence, forming the predistortion input sequence

*X*(

*k*) of the digital predistortion module, where

*k*is the symbol index. The structure of the digital predistortion module is presented in detail in the following paragraph. Subsequently, the 1000 train symbols are removed from the output symbol sequence

*Y*(

*k*). After the digital to analog conversion in the AWG, the predistorted baseband waveform is mixed with a 16.6GHz K-band RF local oscillator (LO) using an IQ mixer. The output up-converted signal is modulated on an optical carrier at 1549.3nm in a Mach-Zehnder modulator (MZM), which is followed by a booster EDFA launching the optical signal into a 26km standard single mode fiber (SSMF). After fiber transmission and photodetection, the 16.6GHz RF signal is translated to a 99.6GHz W-band wireless signal in the sextuple frequency multiplier. Subsequently, the signal is radiated through an up to 4m wireless distance using a pair of W-band hone antennas. At the receiver side, the 99.6GHz signal is mixed with the 18

*harmonic of a 5.48GHz LO in a 18-order harmonic mixer (Agilent 11970W) to be down-converted to 960MHz. The bandwidth of the harmonic mixer is ∼600MHz, which is the main limitation to achieve higher data rate in our experiment. To demodulate QPSK/16-QAM data signals, the down-converted signal is digitalized with a 40GSa/s digital oscilloscope and the waveform is processed offline with applicable algorithms. In the digital domain, frequency offset compensation, digital down-conversion and synchronization are performed followed by QPSK/16-QAM phase offset compensation. The constellation of the demodulated QPSK/16-QAM baseband signal after phase offset compensation is evaluated in terms of the IQ orthogonality which is then fed into the digital predistortion module as a auxiliary guide to perform fine adjustments for the predistorted signal. Finally, the BER is obtained in the receiver by using direct error counting of 2 × 10*

^{th}^{5}bits.

*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]

*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.

## 3. Experimental results

*-order term dominates.*

^{th}*R*(

*k*) –

*x*(

_{a}*k*)]/

*x*(

_{a}*k*)| drops by 40dB and the absolute phase error |

*ϕ*(

*k*) –

*x*(

_{p}*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.

*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 6

*-order intermodulation distortion still exists and we expect this residual distortion can be further suppressed by treating more terms in the polynomial predistorters.*

^{th}^{−3}(FEC limit), and another 0.3dB power penalty appears when increasing the data rate from 312.5Mb/s to 625Mb/s due to the bandwidth limitation of the harmonic mixer. For 16-QAM modulation format, the data rate is fixed at 312.5Mb/s. The B2B transmission performs a ∼0.6dB power penalty with respect to the QPSK case. In the 16-QAM case, error bits mainly come from wrong decision between the largest power level (outer 4 clusters) and the secondary power level (middle 8 clusters) due to the nonlinear compression affects high power symbols dominantly, which results in a degraded signal-to-noise ratio (SNR) for outer constellation clusters. After 26km fiber transmission, ∼0.6dB transmission power penalty is observed with respect to the B2B case, which implies the 16.6GHz predistorted 16-QAM signal is more sensitive to fiber dispersion than uniform power level QPSK signal. Figure 8 shows the BER curves versus the wireless transmission distance while the optical power after fiber transmission is fixed at −10dBm. To keep the BER below the 2 × 10

^{−3}threshold, the maximal wireless distances for 312.5Mb/s QPSK, 625Mb/s QPSK and 312.5Mb/s 16-QAM signals are 4m, 2.5m and 1.5m, respectively. It can be seen that due to the intensive path loss, this proof-of-concept experiment supports 4m wireless transmission distance, which is much shorter than the reach of current wireless systems. A quantitative experiment for characterizing the W-band wireless path loss shows 33dB wireless link loss for 4m transmission, which implies that the W-band wireless transmission system is more suitable for short range or indoor access systems.

## 4. Conclusions

## Acknowledgments

## 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 |

3. | J. Marti and J. Capmany, “Microwave photonics and radio-over-fiber research,” IEEE Microw. Mag. |

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. |

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. |

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. |

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. |

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. |

**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

- “FCC online table of frequency allocations,” www.fcc.gov/oet/spectrum/table/fcctable.pdf .
- 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]
- J. Marti and J. Capmany, “Microwave photonics and radio-over-fiber research,” IEEE Microw. Mag.10, 96–105 (2009). [CrossRef]
- 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]
- 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]
- 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]
- Y. Park and J. S. Kenney, “Adaptive digital predistortion linerization of frequency multipliers,” IEEE Trans. Microwave Theory Tech.51, 2516–2522 (2003). [CrossRef]
- 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]
- 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]
- 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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