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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 1 — Jan. 14, 2013
  • pp: 270–275
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Synchronized signaling delivery for broadband 60 GHz in-building optical wireless network based on digital frequency division multiplexing and digital Nyquist shaping

Z. Cao, F. Li, C. M. Okonkwo, H.P.A. van den Boom, M. Chen, B. Yang, S. Zou, E. Tangdiongga, L. Chen, and A.M.J. Koonen  »View Author Affiliations


Optics Express, Vol. 21, Issue 1, pp. 270-275 (2013)
http://dx.doi.org/10.1364/OE.21.000270


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Abstract

A simple and low-cost synchronized signaling delivery scheme has been proposed for a 60 GHz in-building optical wireless network with 12.7Gbps throughput based on digital frequency division multiplexing and digital Nyquist shaping.

© 2013 OSA

1. Introduction

2. Operation principle

2.1 Digital frequency division multiplexing (D-FDM) and digital Nyquist shaping

The principle of D-FDM is depicted in Fig. 1
Fig. 1 Principle of D-FDM for: (a) time domain and (b) frequency domain.
. Low speed Polar non-return-to-zero (Polar-NRZ) signal for control signaling is coupled with OFDM signal by simply adding their digital values in time domain before digital-to-analog conversion shown in Fig. 1(a). This process will synchronize the signaling with OFDM signal, which is extremely important to time-varying application like optical routing. To reduce ICI, as shown in Fig. 1(b), the OFDM subcarriers overlapping main lobe of Polar-NRZ spectrum are reserved (carrying no bits). This is because the main power of Polar-NRZ signal is within its main lobe. To further reduce ICI, the digital Nyquist shaping is employed. Nyquist shaping is used to cut off or roll off unwanted spectrum without additional inter-symbol interference at sampling instants. The digital Nyquist shaping is achieved by convolving normalized sinc function f(t/T) = sin(πt/T)/(πt/T) with Polar-NRZ signal. Its cutoff frequency is fcutoff = 0.5 × T−1,where T is the period of symbol rate of Polar-NRZ signal For practical implementation of the sinc function, its tails are truncated to reduce the number of discrete samples. The sinc function is chosen since it can completely suppress unwanted spectrum and introduce little distortion to the remained part. The waveforms and RF spectrum with and without Nyquist shaping are shown in Fig. 2
Fig. 2 Process of Nyquist shaping: (a) without and (b) with Nyquist shaping
.

Without Nyquist shaping, the waveform of Polar-NRZ signal is rectangular and its spectrum is extended far beyond 1st null. With Nyquist shaping, the spectrum of Polar-NRZ signal is well confined between 0 Hz - 1st null. To avoid complicated convolution process, waveform of convolved “1” is placed in the look-up table and recalled when “1” appears. Its inverted waveform is recalled when “-1” appears. This process is for downlinks, and analog FDM can be used for uplinks.

2.2 Low frequency detection for RoF signal

In this section, a mathematic model will be built to analyze the low frequency detection (LFD) of the RoF signal. The electrical field of optical mm-wave with OFDM signal modulated can be expressed as [5

5. Z. Cao, J. Yu, M. Xia, Q. Tang, Y. Gao, W. Wang, and L. Chen, “Reduction of intersubcarrier interference and frequency-selective fading in OFDM-ROF systems,” J. Lightwave Technol. 28(16), 2423–2429 (2010). [CrossRef]

]: E(t) = [A+1 × cos(ωo + ωm)t + A-1 × cos(ωom)t] × (1 + γS(t)), where A+1 and A-1 denote amplitudes of upper and lower sidebands, respectively. The ωo and ωm denote angular frequency of optical carrier and driving LO. Optical modulation index is denoted by γ. S(t) is the presentation of OFDM signal, which can be expressed as
S(t)=k=1N(akcoskΩt+bksinkΩt)
(1)
where N denotes the number of subcarriers in a OFDM symbol, Ω is the 1st sub-carrier frequency,kΩis the frequency of kth subcarrier.akandbkrepresent complex symbols of in-phase and quadrature components for the kth subcarrier. After square-law detection of photo-diode, the output current can be expressed as [5

5. Z. Cao, J. Yu, M. Xia, Q. Tang, Y. Gao, W. Wang, and L. Chen, “Reduction of intersubcarrier interference and frequency-selective fading in OFDM-ROF systems,” J. Lightwave Technol. 28(16), 2423–2429 (2010). [CrossRef]

]:
I(t)=12μ(A+12+A12)[1+2γS(t)+γ2S2(t)]+μA+1A1cos(2ωmt)[1+2γS(t)+γ2S2(t)]
(2)
It is obvious that the baseband (BB) OFDM is identical to OFDM at mm-wave. Thus we can detect the BB OFDM to get signaling data. As discussed in section 2.1, the signaling data is inserted in the lower frequency part of BB OFDM and thus a low speed (100MHz level) PD can detect the signaling data.

The principle of low frequency detection is illustrated in Fig. 3
Fig. 3 Beating process and low frequency detection: (a) optical spectrum of data modulated optical mm-wave; (b) RF spectrum of detected BB signal; (c) RF spectrum of detected 60 GHz signal.
. The optical spectrum of 60 GHz optical mm-wave with OFDM signal modulated is principally depicted in Fig. 3(a). The two vertical black arrows denote two sidebands of optical mm-wave. Colored arrows around them denote subcarriers of modulated OFDM signal. The beating process includes two parts, namely, self-beating and inter-beating. Self-beating refers to beating between one sideband and the subcarriers around it, which generates baseband (BB) OFDM signal shown in Fig. 3(b). The self-beating process allows low frequency detection for OFDM signal on 60 GHz optical mm-wave. Inter-beating refers to beating between one sideband and subcarriers around the other sideband which generates 60 GHz OFDM signal shown in Fig. 3(c). Since signaling (Polar-NRZ signal) is inserted in the low frequency part of BB OFDM as shown in Fig. 3(b), PD bandwidth required for detection is only a little higher than signaling bandwidth. The detected signaling can be further processed using low speed logical circuit. Thus high-speed PD and logical circuit, and down conversion scheme can be avoided.

3. Experimental setup, results and discussion

Figure 4
Fig. 4 Experimental setup and measured results: (a)-(c): measured optical spectrum; (d)-(e): measured RF spectrum.
shows the experimental setup to demonstrate proposed signaling insertion and detection scheme in a 60 GHz OFDM-RoF system. The OFDM signal is generated from the Tektronix AWG 7122B at 10 GSa/s. The size of IFFT is 256 and the subcarriers used for data, pilots and high frequency guard interval are 192, 8, and 56, respectively. The subcarriers are arranged to satisfy the Hermitian symmetry with respect to their complex conjugate counterparts to allow the real-valued IFFT output. The period of OFDM symbol is 25.6 ns. The symbol rate for each subcarrier is 39 MSps. 16QAM is employed in subcarrier modulation. The cyclic prefix is 1/8 of an OFDM period, which corresponds to 32 samples in every OFDM symbol. One training symbol is inserted in front of 64 data OFDM symbols for timing and channel estimation. The net bit rate of OFDM signal is 12.72 Gbps. The bit rate of Polar-NRZ signal is 100 Mbps and one Polar-NRZ symbol lasts for 10ns. The fcutoff of sinc function is set to 100MHz and its digital waveform is sampled at f = 100 × fcutoff agreeing with the sample rate of OFDM signal. The truncation size of sinc function is 10 × T. The three lowest frequency subcarriers are closed reserving 117 MHz bandwidth.

The amplitude (Vpp) of Polar-NRZ signal is set to 30 mV while the amplitude (Vpp) of OFDM signal is 1 V. The RF spectrums of the combined signal of Polar-NRZ and OFDM are depicted in Fig. 4(a). DFB-laser with 1 dBm launch power, is modulated to generate 60 GHz optical mm-wave via a MZM (MZ-a) biased at null point with 30 GHz electrical local oscillator (LO). A MZM (MZ-b) is used for OFDM signal modulation. The optical spectrum of OFDM modulated optical mm-wave is depicted in Fig. 4(b). The optical signal is amplified to 0 dBm before launched to 4.5 km single mode fiber (SMF). after transmission, its received optical power (ROP) is −1.5 dBm with optical spectrum shown in Fig. 4(c). An variable optical attenuator (VOA) is used to control ROP for measurement. Two optical receivers are constructed for both signaling node, and antenna point (RAP) and terminal users (TU). The signaling node receiver comprises a 10 GHz photo-diode (PD, PIN-1) and a 6 GHz low noise amplifier (LNA1). The signal after LNA1 is filtered by a 4-order Butterworth LPF (LPF1) with 100 MHz 1 dB pass bandwidth and then sampled at 1.25 GSample/s by a real time oscilloscope (DPO). The RAP-TU receiver contains a preamplifier (EDFA-2) and a 75 GHz PD (PIN-2). After PD, electrical 60 GHz OFDM is generated and amplified by an 8 GHz narrow band amplifier centralized at 60 GHz. The 60 GHz OFDM is then down converted via a 60 GHz electrical mixer with 60 GHz LO. The 60 GHz LO is generated from an electrical frequency quadruple multiplier cascaded with a 15 GHz microwave source. An electrical phase shifter (PS) is used to adjust the phase of 60GHz LO. A LNA (LNA2) is followed by a 6GHz low-pass filter (LPF2) to retrieve BB OFDM signal which is further sampled at 25Gsample/s by a DPO and processed off-line. The system evaluation is carried out to assess the followings: (i) The impact to OFDM signal induced by signaling insertion; (iii) Evaluation of low-pass filters for signaling detection; (ii) The EVM performance of OFDM before and after fiber transmission; (iv) The Q factor of signaling data before and after fiber transmission. The constellations of OFDM signal over 4.5 km SMF are depicted in Fig. 5
Fig. 5 Constellations of OFDM signals (a) without and (b) with Polar-NRZ, (c) with Nyquist shaped Polar-NRZ
with ROP of −16 dBm, which include OFDM signal (a) without Polar-NRZ, (b) with no-shaped Polar-NRZ and (c) with Nyquist shaped Polar-NRZ. Comparing Fig. 6(a)
Fig. 6 The eye diagrams of the signaling inserted OFDM signal at 60 GHz optical mm-wave over 4.5 km SMF with different detection low pass filters.
-6(b), it is evident that the inserted Polar-NRZ introduce interference to the OFDM signal whereas for the Nyquist shaped Polar-NRZ, the interference is not obvious.

The EVMs of OFDM signal with and without inserted Polar-NRZ signal are measured for optical BTB and over 4.5 km SMF. Plots are shown in Fig. 7(a)
Fig. 7 Measured system performance for BTB and 4.5 km SMF transmission: (a) EVM for user data, (b) Q factor for signaling data
. No high pass filter is implemented to suppress the Polar-NRZ signal. The Polar-NRZ insertion induced power penalties for EVM of 11.2% are both 0.8 dB for BTB and 4.5 km transmission demonstrating that this signaling insertion and detection scheme does not increase system penalty considerably. The power penalty for EVM of 11.2% is 1.7 dB after 4.5 km transmission for Polar-NRZ inserted OFDM. This is mainly due to the frequency selective fading for double sideband modulation. The curves of Q factor of Polar-NRZ signal versus ROP for BTB and 4.5 km SMF transmission are depicted in Fig. 7(b). The power penalty at Q = 5.5 (corresponding BER is about 1.95 × 10−8) is 1.2 dB for 4.5 km SMF transmission.

4. Conclusion

We demonstrated a signaling insertion and detection scheme based on digital frequency multiplexing and Nyquist shaping for 12.7 Gbps throughput 60 GHz in-building optical wireless networks. The power penalty of signaling insertion is less than 0.8 dB. Based on achieved results, we believe the proposed scheme can provide reliable and low-cost signaling delivery channel for 60 GHz in-building optical wireless network.

Acknowledgment

This work has been partly supported by The Netherlands Organization for Scientific Research NWO under the project grant Smart Optical-Wireless In-home Communication Infrastructure (SOWICI).

References and links

1.

T. Kuri, H. Toda, J. Olmos, and K. Kitayama, “Reconfigurable dense wavelength-division-multiplexing millimeter-waveband radio-over-fiber access system technologies,” J. Lightwave Technol. 28(16), 2247–2257 (2010). [CrossRef]

2.

J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. Huang, H. Chien, Y. Hsueh, W. Jian, C. Liu, and Z. Dong, “Cost-effective optical millimeter technologies and field demonstrations for very high throughput wireless-over-fiber access systems,” J. Lightwave Technol. 28(16), 2376–2397 (2010). [CrossRef]

3.

W. Jiang, C. Lin, A. Ng'oma, P. Shih, J. Chen, M. Sauer, F. Annunziata, and S. Chi, “Simple 14-Gb/s short-range radio-over-fiber system employing a single-electrode MZM for 60-GHz wireless applications,” J. Lightwave Technol. 28(16), 2238–2246 (2010). [CrossRef]

4.

Y. Hsueh, Z. Jia, H. Chien, A. Chowdhury, J. Yu, and G. K. Chang, “Multiband 60-GHz wireless over fiber access system with high dispersion tolerance using frequency tripling technique,” J. Lightwave Technol. 29(8), 1105–1111 (2011). [CrossRef]

5.

Z. Cao, J. Yu, M. Xia, Q. Tang, Y. Gao, W. Wang, and L. Chen, “Reduction of intersubcarrier interference and frequency-selective fading in OFDM-ROF systems,” J. Lightwave Technol. 28(16), 2423–2429 (2010). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Subsystems for Optical Networks

History
Original Manuscript: October 2, 2012
Revised Manuscript: November 10, 2012
Manuscript Accepted: November 11, 2012
Published: January 4, 2013

Virtual Issues
European Conference on Optical Communication 2012 (2012) Optics Express

Citation
Z. Cao, F. Li, C. M. Okonkwo, H.P.A. van den Boom, M. Chen, B. Yang, S. Zou, E. Tangdiongga, L. Chen, and A.M.J. Koonen, "Synchronized signaling delivery for broadband 60 GHz in-building optical wireless network based on digital frequency division multiplexing and digital Nyquist shaping," Opt. Express 21, 270-275 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-1-270


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