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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, 2010

Cost-Effective Optical Millimeter Technologies and Field Demonstrations for Very High Throughput Wireless-Over-Fiber Access Systems Jianjun Yu, Senior Member, IEEE, Fellow, OSA, Gee-Kung Chang, Fellow, IEEE, OSA, Zhensheng Jia, Arshad Chowdhury, Ming-Fang Huang, Hung-Chang Chien, Yu-Ting Hsueh, Wei Jian, Cheng Liu, and Ze Dong (Invited Paper)

Abstract—The broadband penetration and continuing growth of Internet traffic among residential and business customers are driving the migration of today’s end user’s network access from cable to optical fiber and superbroadband wireless systems The integration of optical and wireless systems operating at much higher carrier frequencies in the millimeter-wave (mm-wave) range is considered to be one of the most promising solutions for increasing the existing capacity and mobility, as well as decreasing the costs in next-generation optical access networks. In this paper, several key enabling technologies for very high throughput wireless-over-fiber networks are reviewed, including photonic mm-wave generation based on external modulation or nonlinear effects, spectrum-efficient multicarrier orthogonal frequency-division multiplexing and single-carrier multilevel signal modulation. We also demonstrated some applications in wireless-over-fiber trials using these enabling techniques. The results show that the integrated systems are practical solutions to offer very high throughput wireless to end users in optically enabled wireless access networks. Index Terms—External modulation, field trial, millimeter-wave (mm-wave), multilevel modulation, orthogonal frequency-division multiplexing (OFDM), radio over fiber (ROF).

I. INTRODUCTION ESPITE decades of progress in broadband access technology, the demand for emerging applications by end users continues to stress the data rates and flexible ability of existing wireless and wired broadband access solutions. Accordingly, there is a promising opportunity to evolve from hybrid fiber–copper (HFC) solutions and exclusively fiber-based passive optical network (PON) to converged wireless-over-fiber architecture. In such systems, users can benefit from abundant bandwidth and convenience in roaming by combing radio and photonics technologies. New wireless-over-fiber systems, like

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Manuscript received November 17, 2009; revised December 29, 2009; accepted December 30, 2009. Date of publication February 17, 2010; date of current version August 06, 2010. J. Yu and M.-F. Huang are with NEC Laboratories America, Princeton, NJ 08540 USA (e-mail: [email protected]). Z. Jia, A. Chowdhury, M.-F. Huang, H.-C. Chien, Y.-T. Hsueh, W. Jian, C. Liu, and Z. Dong are with Georgia Institute of Technology, Atlanta, GA 30332 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2010.2041748

Fig. 1. Enabling technologies of wireless-over-fiber networks.

wireless local area network (WLAN), worldwide interoperability for microwave access (WiMAX), and third-generation cellular wireless systems, have been developed and deployed shopping center or Olympic Games [1]–[3]. As more applications spring up, overcrowding and inference at high end of the microwave region push operating frequencies toward millimeter-wave (mm-wave) band, such as 60 GHz and 70–94 GHz band allocation. Higher frequency operation provides larger instantaneous bandwidth for greater transfer of information, and offer new capabilities for more detailed imaging and sensing with reduced dimensions for antennas and other components. Since propagation characteristics limit the range of mm-wave wireless links, exchanging high data rate can appear between rooms or indoor buildings after fiber distribution. These applications include a wireless equivalent of gigabit Ethernet (GigE), high-speed vehicle wireless sensor and communications, elimination of the cable connecting video sources and projectors, delivery of high-definition uncompressed baseband audio and video without high-definition multimedia interface (HDMI) wires, practical high-bandwidth wireless links to either high-capacity or low-power computing or storage devices such as hard drives or digital video cameras. Fig. 1 depicts the building blocks of the wireless-over-fiber system and the enabling technologies identified for providing the capability of very high throughput (VHT) to end users. Implementing mm-wave over fiber systems presents technical and engineering challenges due to the high oscillating frequency at the boundary of electronic and photonic domains. In order to reduce the bandwidth of the optical and electrical components, many cost-effective and advanced techniques

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YU et al.: OPTICAL MILLIMETER TECHNOLOGIES AND FIELD DEMONSTRATIONS FOR WIRELESS-OVER-FIBER ACCESS SYSTEMS

based on photonic beating between in-phase optical harmonic spectral components generated in optical modulation have been developed with the use of square-law photodetection [4]–[72]. In addition to shifting the carrier technologies of current microwave photonics to mm-wave photonics, another way to achieve faster transmission speed wirelessly is to use advanced multicarrier [73]–[88] and multilevel single-carrier modulations [89], [90] for higher spectral efficiency (SE) and better transmission performance. As shown in Fig. 1, there are other issues regarding architecture design for the integration of existing passive optical networks and full-duplex connection, which have been studied in the past experimental work [91]–[112]. This paper will provide the recent progress in wireless-over-fiber technologies mainly focusing on photonic mm-wave generation and spectrum-efficient modulation for VHT applications, and also present field-trial demonstration. The paper is structured as follows. Section II investigates various technologies shown in Fig. 1 for photonic mm-wave generation. These include higher mm-wave frequency generation based on external intensity/phase modulation or nonlinear effects. Multiband service and arbitrary carrier generation at integer multiples of a base frequency for flexible reconfiguration system are also reviewed in this section. Section III presents 60 GHz orthogonal frequency-division multiplexing (OFDM) wireless signals over fiber systems using convolutional coding and optimal equalization. Section IV provides multilevel modulation techniques for high SE, and shows the results of record 15 Gb/s 60 GHz carriers modulated by quadrature amplitude modulation (QAM) and 10 Gb/s by DB format. Section V reports the field-trial demonstration of integrated photonic 60 GHz wireless-over-fiber systems for transmitting uncompressed high-definition video signals and existing WiFi services in campus fiber network at Georgia Tech. II. COST-EFFECTIVE GENERATION OF OPTICAL MM-WAVE SIGNALS Generating mm-wave frequencies using electrical devices is challenging due to the limited frequency response of electronic components. The more attractive solution is to use optical means. Recently, many groups have developed optical mm-wave generation, upconversion, and transmission techniques for radio-on-fiber (ROF) systems. Three traditional methods are used for the generation of mm-wave signals over optical links with intensity modulation: direct intensity modulation, external modulation, and remote heterodyning [4]–[67]. Although direct modulation is by far the simplest, due to the limited modulation bandwidth of the laser, it is difficult for 60 GHz mm-wave bands [42], [43]. For optical heterodyning technique, two or more optical signals are simultaneously transmitted and heterodyned in the receiver. However, it requires either a precisely biased electro-optic modulator or a complex laser to reduce the severe phase noise, which greatly adds to the cost and complexity of the system [44]. Recently, several approaches for generation of optical mm-wave have been reported. These techniques, based on nonlinear effects in waveguide device, exhibit low conversion efficiency and need very high input optical power [45]. The

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Fig. 2. Principle of optical carrier suppression technology. DFB-LD: distributed feedback laser diode, IM: intensity modulator. The frequency of the RF signal is f , and the IM is dc biased at null point.

scheme based on cross-gain modulation (XGM) in semiconductor optical amplifier (SOA) [46] requires a large input power to saturate the gain of SOA. The scheme, by using cross-phase modulation (XPM) in SOA Mach–Zehnder interferometer (SOA-MZI) [47], loosens the requirement for the input power; however, the complicated conversion structure and nonlinear crosstalk among multiple channels is a major hurdle to greatly limit the signal quantity of wireless end users. The scheme based on electro-absorption modulator (EAM) has small saturation input power; hence, it cannot be used for multichannel upconversion [48], [49]. Since external modulator has wide bandwidth, small power consumption, and large saturation power for input signals, it is a good option to be used for optical mm-wave generation. Utilizing some novel enabling techniques, the bandwidth for the external modulator and electrical amplifier (EA)/driver can be reduced [6]–[17]. Double-sideband (DSB), single-sideband (SSB), and optical-carrier-suppression (OCS) optical mm-wave signals can be generated when the external modulator is operated at different bias and driving voltage [7]. Optical DSB mm-wave signal can be easily generated by an intensity modulator (IM) driven by single-arm electrical signal with low RF voltage (only around half-wave voltage). The main problem for DSB mm-wave generation is the high-frequency bandwidth requirement for all optical and electrical components. For 60 GHz optical mm-wave, the bandwidth of optical modulator and RF GHz. The generated mm-wave signal amplifier has to be by DSB modulation exhibits the fading effect in fiber; therefore, it cannot be used to realize long-distance transmission over high-speed optical wireless signal. SSB optical mm-wave signal can overcome fading effect; however, it has low receiver sensitivity and also needs high bandwidth for optical and electrical components [7]. Single-sideband signal can be generated by push–pull external modulator driven by two electrical RF signal with 90 phase shifter [4], and it can also be generated by DSB optical signal cascaded one optical filter to remove one sideband [5]. OCS can be realized when an IM is biased at null point, as shown in Fig. 2. If we assume that the modulator is driven by an RF sinusoidal wave signal with a frequency of . After the

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IM, two subcarriers with frequency spacing of 2 will be generated from a continuous-wave (CW) lightwave by carrier suppression scheme. The generated two lightwaves have the same polarization direction, optical power, and locked phase. The frequency spacing is exactly controlled by the RF of the sinusoidal wave on the modulator. In order to fully drive the modulator, the RF voltage should be double of the half-wave voltage of the modulator. The first report on OCS technique to generate optical mm-wave is published in [6]. We compared the optical mm-wave performance generated by DSB, SSB, and OCS in [7], and found that the generated optical mm-wave by OCS technique has the highest receiver sensitivity, the highest SE, and a small power penalty over long-distance delivery. Moreover, this scheme has a simple configuration and low-frequency bandwidth requirement for both electrical and optical components. By means of this scheme, we demonstrated that a 32-channel dense wavelength-division multiplexing (DWDM) signals at 2.5 Gb/s per channel have been upconverted to a 40 GHz carrier simultaneously. We demonstrated to deliver 100 Gb/s baseband and 10 Gb/s optical wireless signals at 60 GHz mm-wave generated by OCS technique in [27], which is the largest capacity for wireline and wireless signals in one wavelength channel. The bandwidth of optical and electrical components can be reduced when OCS technique is employed. For 60 GHz optical mm-wave, the requirement bandwidth of the optical and electrical components is only 30 GHz. Recently, there are a lot of advanced and cost-effective techniques to reduce the bandwidth of the optical and electrical components to generate optical mm-wave, and we will review these techniques in the following sections.

A. Optical mm-Wave Generation Using Frequency-Quadrupling Techniques 1) Scheme Based on External Intensity Modulation: To further reduce the bandwidth requirement for the external modulator, Qi et al. reported a scheme to generate optical mm-wave with quadrupling frequency [8]. We experimentally demonstrated optical mm-wave signal generation by this scheme for the first time in [9] and [10]. Fig. 3 illustrates the principle of the proposed frequency-quadrupling scheme. A LiNbO IM is employed to generate optical mm-wave with low-frequency RF. To realize an optical mm-wave carrier with four times of local oscillator (LO) frequency, the modulator needs to be dc biased at the maximum transmission point [9], [10]. If the frequency of the RF microwave source is , the frequency spacing between the second-order sidebands is equal to 4 , while the first-order sidebands are suppressed. As an example, the output optical spectrum shown in Fig. 3 is for the case of a 10-GHz modulation frequency. From the figure, it can be seen that the frequency spacing of the second-order sidebands is 40 GHz. After passing an interleaver (IL), the wireless and wired signal can be separated and delivered to customer units. Taking the advantages of this property can dramatically lower the bandwidth requirements for the optical modulator and allows the use of a much lower frequency electrical drive signal.

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, 2010

Fig. 3. Principle of frequency-quadrupling scheme. Resolution of the optical spectrum is 0.01 nm. LD: laser diode; LO: local oscillator; IL: interleaver.

This can greatly reduce the cost of the system and makes it more practical to use. The experimental setup of 40 GHz optical mm-wave generation by utilizing quadrupling frequency scheme to provide downstream wireless transmission is shown in Fig. 4. In central office (CO), a CW lightwave is generated by a DFB laser diode (DFB-LD) and fed into an IM. 2.5 Gb/s signals are mixed with a 10 GHz RF microwave signal (LO signal). The electrical waveform of the mixed signals is shown in Fig. 4. The optical spectrum with a 0.01 nm resolution after modulation is shown in Fig. 4(a). It can be seen that the first-order sidebands are almost suppressed and the power of optical carrier is 18 dB larger than the second-order sidebands after modulation. The subcarriers are generated and separated by 40 GHz (0.32 nm), four times of LO, for optical mm-wave carrier, while the original carrier is modulated by 2.5 Gb/s nonreturn-to-zero (NRZ) signals. After 20 km of standard single-mode fiber (SMF-28) transmission, an optical IL is used to separate optical carrier and two sidebands. The optical spectra received at the output port of the IL are shown in Fig. 4(b) and (c). In the wireless part, one optical–electrical (O/E) converter is used, and the converted electrical mm-wave signal is then amplified by an EA. A 40-GHz electrical LO signal is mixed to downconvert the electrical signal to its baseband form. Fig. 4(i) illustrates the eye diagram at measured point (i) in Fig. 4. On the basis of the bit-error-ratio (BER) measurement, the wireless data are obresulted from served a 0.8 dB power penalty at the chromatic dispersion of the 20 km SMF-28. 2) Scheme Based on Cascaded External Modulator and Nonlinear Fiber: The previous scheme based on external modulator to generate optical mm-wave with quadrupling frequency has a simple architecture. However, polarization is an issue due to the external modulator resulting from its characteristic of polarization sensitivity. When the baseband signals need to be frequency upconverted, a common external modulator cannot be employed because of the polarization-sensitive issue [51], [52]. To solve this issue, we proposed and experimentally demonstrated a scheme to generate optical mm-wave with frequency quadrupling and polarization insensitivity [12]. The proposed scheme to realize all-optical upconversion is shown in Fig. 5.

YU et al.: OPTICAL MILLIMETER TECHNOLOGIES AND FIELD DEMONSTRATIONS FOR WIRELESS-OVER-FIBER ACCESS SYSTEMS

Fig. 4. Experimental setup and measured results by frequency-quadrupling scheme (RF=10 GHz). IM: intensity modulator, EA: electrical amplifier, IL: interleaver, O/E: optical–electrical converter, TOF: tunable optical filter, and BERT: bit-error-rate taster.

Fig. 5. All-optical upconversion based on polarization-insensitive FWM in nonlinear medium. DSB: double sideband, OCS: optical carrier suppressed.

It can be seen that the two converted new peaks (signals) with channel spacing of 4 can be obtained after four-wave mixing (FWM) process in the nonlinear medium. In this scheme, the optical signals similar to a DSB signal are achieved, and the generated signals including two converted signals and the original signal after wavelength conversion (WC). However, when the original signal is removed by an optical filter or an IL, the optical signals similar to OCS upconverted signals carried by 4 optical carrier are achieved. Both DSB and OCS optical RF signals can be generated in this proposed architecture. Since OCS optical signals generated in [7] have higher receiver sensitivity without fading effect, it is a good candidate in future ROF network. Fig. 6(a) shows the experimental setup of all-optical upconversion based on FWM in the highly nonlinear fiber (HNLF). The 30 WDM channels are generated by DFB-LDs from 1533.47 to 1556.55 nm with 100 GHz channel spacing. The signals are combined by an array waveguide grating (AWG) and sent to the IM1 driven by 7.5 Gb/s electrical signal to generate regular NRZ ON–OFF keying (OOK) signals and transmitted over 20 km SMF-28. In the setup, the 20 km fiber

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is used to emulate the baseband optical signals coming from the core or metro network. Another lightwave from the external cavity lasers (ECLs) at 1561.32 nm is generated by IM2 with 15 GHz sinusoidal wave. The modulator is biased at the null points to realize OCS technique. Therefore, two pumps with 30-GHz channel spacing and the same polarization direction are accomplished. One phase modulator (PM) driven by 100 MHz sinusoidal wave is used to broaden the spectrum and reduce the stimulated Brilloum scattering (SBS) effect. Before the WDM coupler combines the OOK signals and the 30 GHz OCS carriers, one 50/100 GHz spaced IL is employed to enhance carrier suppression function of the pump signals. The total launched power of the WDM 7.5 Gb/s NRZ-OOK signals and copolarized pump signals to the HNLF are 3 and 21 dBm, respectively. The HNLF has 1 km length, a zero dispersion wavelength at 1561 nm, dispersion slope of 0.02 ps/nm /km, a /km. loss of 0.4 dB/km, and a nonlinear coefficient of 10 After FWM in the HNLF, the DSB signals with 30 GHz spaced between two sidebands are created from 7.5 Gb/s OOK signals, as shown in Fig. 6. It can be seen that the optical SNR (OSNR) of the converted signals are identical, more than 35 dB. In order to enhance high carrier suppression ratio, another WDM coupler with the identical performance as the first one and a 25/50 GHz spaced IL is used to suppress the optical carrier. In this way, the optical mm-wave signal with 7.5 Gb/s data carried by the 60 GHz optical carrier is realized. After the tunable optical filter (TOF) with 3 dB bandwidth of 0.5 nm, which is set to remove the accumulated amplified spontaneous emission (ASE) noise and choose one desired channel, the optical mm-wave signals, as shown in inset (i) in Fig. 6, is detected by a standard mm-wave receiver. After transmission over the 20 km SMF-28, the average receiver sensitivity for dBm. The inset (ii) in Fig. 6 shows these 30 channels is the optical eye diagram after upconversion at the point (ii) in Fig. 6. It can be clearly seen that the 7.5 Gb/s signal is carried by 60 GHz optical mm-wave. The inset (iii) in Fig. 6 is obtained at point (iii) in Fig. 6, which is the eye diagram in baseband form of the 30th channel. We also measure the performance when only one channel is converted. The receiver sensitivity of dBm, while the BER is . the converted signal is In the previous section, we have demonstrated the all-optical upconversion for ROF system based on FWM effect in 1 km HNLF [12]. However, the bulky and stable issues arising from such long fiber for all-optical upconversion should be considered. Also, the optical mm-wave signal is OCS modulation in the earlier experiment; however, because of fiber dispersion, the transmission distance is limited to 2 km. Recently, we generated SSB-modulated signal for all-optical upconversion through 2 m bismuth oxide-based fiber to mitigate severe fiber dispersion for optical mm-wave signals. As opposed to limited transmission distance by the OCS scheme, 10 Gb/s data carried by 60-GHz mm-wave in SSB signal can be transmitted over 20 km SMF-28 without much distortion. We experimentally demonstrate the all-optical 60 GHz upconversion with polarization-insensitive feature based on SSB technique and dual-pump FWM in compact bismuth fiber. The proposed scheme to realize all-optical upconversion based on FWM in bismuth fiber is shown in Fig. 7, which is

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Fig. 6. Experimental setup for 30 channels upconversion in a 60 GHz ROF system. AWG: array-waveguide grating, ECL: external cavity laser, HNLF: highly nonlinear fiber, LPF: low-pass filter.

Fig. 7. Polarization-insensitive all-optical upconversion based on FWM in the bismuth oxide-based fiber. The frequency of the RF signal is f , and the IM is dc biased at the null point. ECL: external cavity laser, OC: optical coupler, WSS: wavelength-selective switch.

quite similar to Fig. 6. The only difference is that we use optical filter to suppress one sideband. In this way, we can generate SSB signal. SSB optical mm-wave signal can tolerant the fiber dispersion and can be used to realize long-distance transmission [7]. The dual-pump light signals are generated by an external IM biased at null point to realize OCS. The CW lightwave generated by ECL is modulated via a single-arm IM driven by an RF sinusoidal wave signal with a frequency . After OCS, two subcarriers with wavelength spacing of 2 would be acted as dual-pump signals. Therefore, these pump signals will maintain the same polarization direction, optical power, and coherent phase relationship. In the meantime, the baseband OOK signal is generated by another IM and combined with the

Fig. 8. Experimental setup and results for all-optical upconversion based on FWM in the 2 m bismuth oxide-based fiber. ECL: external cavity laser, TOF: tunable optical filter, EA: electrical amplifier. (a) Optical spectrum after 2 m bismuth fiber. (b) Optical spectrum of the SSB signals after WSS. (c) 60 GHz mm-wave of the SSB signals before transmission. (d) After downconversion before transmission. (e) 60 GHz mm-wave of the SSB signals after 20 km SMF-28 transmission. (f) After downconversion after transmission.

pump signals via an optical coupler (OC), and lunched into the bismuth fiber. After WC, two converted signals with channel spacing of 4 , the same polarization direction and locked phase can be realized. One wavelength-selective switch (WSS) is employed to select the optical carrier and one sideband signals with 2 spacing for the wireless carrier. In order to improve the receiver sensitivity, the optical carrier is also suppressed by the WSS. Fig. 8 shows the experimental setup of the proposed scheme for mm-wave generation in the ROF system. In the CO, the CW

YU et al.: OPTICAL MILLIMETER TECHNOLOGIES AND FIELD DEMONSTRATIONS FOR WIRELESS-OVER-FIBER ACCESS SYSTEMS

lightwave from ECL1 at 1550 nm is modulated by a single-arm and driven by a 30 GHz RF signal LN-MOD IM1 biased at of the to achieve OCS modulation. The half-wave voltage IM1 is 5.2 V. After OCS, the frequency of 60 GHz can be obtained and the carrier suppression ratio is more than 25 dB. Another lightwave at 1556.3 nm from ECL2 is modulated via IM2 driven by 10 Gb/s data to generate regular NRZ-OOK optical baseband signals. Then the 10 Gb/s optical signal and the two pump signals with 60 GHz spacing are combined by a 3 dB OC and lunched into the 2 m bismuth fiber. The 2 m bismuth fiber, which induces 6 dB insertion loss including two connecps/nm/km dispersion at 1550 nm and the nonlinear tors, km , is used to realize the all-opcoefficient of 1360 tical upconversion. The optical power of two pumps and signal before the bismuth fiber is 21 and 18 dBm, respectively. The optical spectrum after bismuth fiber is shown in Fig. 8(a). It can be seen that the channel spacing between two sidebands is 120 GHz resulting from the upconversion. The optical power ratio of the optical carrier over one sideband is 25 dB. A programmable WSS with any bandwidth spacing (smallest bandwidth of 4 GHz) is used to select the optical carrier (baseband signal) and one converted signal; therefore, SSB signal can be obtained at this point. In order to improve the receiver sensitivity, the optical carrier is partly suppressed. After the WSS, the optical power ratio of the optical carrier over one sideband is 7 dB. The selected optical spectrum is shown in Fig. 8(b), and the optical SSB signals are detected by an O/E converter after transmission over 20 km SMF-28. The eye diagrams of the 60-GHz optical mm-wave signals carried 10 Gb/s baseband signals before and after 20 km SMF-28 are shown in Fig. 8(c) and (e), respectively. The corresponding eye diagrams after downvonversion are shown in Fig. 8(d) and (f). In the base station (BS), after the filtering of ASE noise by a TOF with 3 dB bandwidth of 1 nm, the mm-wave signal with the down-link data is detected by O/E converter with a 3 dB bandwidth of 100 GHz and amplified by a narrowband EA. One LO with 60 GHz RF signal is used to downconvert the optical mm-wave signals to the baseband format. Therefore, the BER performance is evaluated by a BER tester. The receiver sensitivity at a BER of for the converted signals is dBm in this proposed scheme. The power penalty is less than 3 dB, mostly because of dispersion effect on the high-bit-rate baseband signals.

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Fig. 9. Optical upconversion using a frequency multiplication technique for WDM ROF systems. MZ: Mach–Zehnder modulator, EDFA: erbium-doped fiber amplifier, OSA: optical spectrum analyzer, ESA: electrical spectrum analyzer.

arm of the main modulator (MZ-c). The optical field at the input of the integrated MZM is defined as

(1) and are the amplitude and the angular frequency where of the optical carrier. MZ-a and MZ-b are both biased at the full point. Electrical driving modulation signals sent into MZ-a and and , respectively. The odd sidebands MZ-b are are suppressed after passing through MZ-a and MZ-b. Since the MZ-c is biased at the null point, the optical carrier is suppressed when the lightwave passes through this integrated modulator and only the second sidebands are left; hence, a quadrupling mm-wave signal is generated. The integrated modulator shown in Fig. 9 includes two parallel IMs. In fact, if the IMs are cascaded, it can also be used to generate quadrupling frequency optical mm-wave without optical filter. Zhao et al. experimentally demonstrated 40 GHz optical mm-wave generation by cascaded two IMs with OCS operation [13]. C. Optical mm-Wave Generation Using Frequency Sextupling Techniques

B. Quadrupling Without Optical Filter The previous schemes need optical filter to suppress optical carrier to generate frequency quadrupling. Lin et al. show one scheme to generate optical mm-wave without optical filtering by using an integrated modulator with frequency quadrupling [54]. A conceptual diagram of optical-carrier-suppressed mm-wave signal generation using a frequency-quadrupling technique without any optical filter is shown in Fig. 9. An external integrated Mach–Zehnder modulator (MZM) that consists of three sub-MZMs is key to generate optical mm-wave signals. One sub-MZM (MZ-a or MZ-b) is embedded in each

Wang et al. employed two cascaded optical modulators and FWM effect in SOAs to generate RF optical signal with 4 times, 8 times, and 12 times microwave source frequency with high spectral purity and stability, and an mm-wave of 42 GHz (12 fundamental) is obtained [14]. Lu et al. [15] have numerically and experimentally investigated how to generate high-frequency mm-wave using multicascaded IMs based on OCS scheme. They have found the rule how to generate the high-frequency mm-wave by adjusting the frequency of the RF signals and the phase relation between the RF signals on the IMs. On the basis of this rule, they have experimentally demonstrated to generate over 80 GHz optical mm-wave (eight fundamental).

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Fig. 10. Conceptual diagram of multiple optical RF signal generation based on MLL and programmable filter.

D. Arbitrary mm-Wave Carrier Generation at Integer Multiples of a Base Frequency

As we can see that all aforementioned schemes are used to generate one frequency optical mm-wave. However, in some cases, arbitrary frequency optical mm-wave signals are necessary. The following three schemes can be used to generate optical mm-wave with arbitrary frequency. 1) Generation by a Short Pulse Laser: The schematic diagram of mode-locked laser (MLL)-based RF generation is shown in Fig. 10. The output of an MLL is a train of short pulses in the time domain and a comb of phase-locked frequency bins spaced at the pulse repetition rate in the frequency domain. The frequency bins are initially filtered to obtain the desired flat wavelength bands and then modulated by the wireless baseband signals. At the base station, the optical mm-wave channels are divided into two branches. One part, used for the wired link, is passed through a low-speed photodiode for wired connection. To produce the wireless signal, the second branch is injected to rapid reconfigurable ring-resonator-based filter to perform the flexible selection of frequency bins for RF carrier generation. As shown in Fig. 11(a), a fourth-order filter cell occupies an on-chip area of 100 m 400 m, allowing a large number of filter cells on a chip. Each filter is independently tunable in wavelength and each passband represents a frequency bin. In this setup, the bins are spaced by 10 GHz with each passband having a 3 dB bandwidth of 8 GHz. The center wavelength can be tuned via a thermoelectric cooler (TEC). The optical path lengths from the input to the output are the same for all wavelengths, and hence, the original phase relationships are maintained for all wavelengths when the phase heaters are not activated. Fig. 11(b) shows the filtered optical spectrum and optical eye diagrams from 30 to 60 GHz. The resulting suppression ratio is larger than 35 dB for all cases. This ring-resonator-based filter is ultracompact and

offers several key advantages such as ultrahigh-frequency resolution with programmable, stable, and accurate wavelength control, all of which combine to make it viable for realistic application. Moreover, this scheme can be scalable to integrate with current PON access with reconfigurable function for multiple wireless band options. 2) Supercontinuum: If a short pulse laser is injected into the nonlinear fiber with proper input power, an optical supercontinuum can be generated. By using optical filter, we can select optical mm-wave with arbitrary frequency. Kuri et al. and Kawanishi et al. [16], [17] investigate the mm-wave with carrier frequency of n 25 GHz, including to qualitatively evaluate phase-noise characteristics of RF carriers of a supercontinuum generator, and prove to be promising as the multiwavelength light source for WDM mm-wave-band ROF systems. 3) Cascaded Phase Modulator and IM: The principle of the proposed architecture is shown in Fig. 12. A single-mode CW lightwave is generated by an ECL and modulated by a cascaded PM and IM driven by a sinusoidal RF source with a frequency of . With proper driving voltage on this cascaded PM and IM, a CW lightwave carried by multiple subchannels or peaks can be generated in a fixed frequency spacing and equal amplitude [18]. Fig. 12(a) illustrates the sketched optical spectrum after cascaded PM and IM at the corresponding point (a) within the setup. Here, 12 wavelengths with fixed frequency spacing can be generated at the same amplitude level. After the multiwavelengths have been produced, one WSS with one input and four output ports is employed to select the arbitrary channel. In the meantime, the arbitrary optical mm-wave signals from to 12 can be achieved. Fig. 12(b)–(e) shows the selected arbitrary frequency for spacing of , 2 , 4 , and 12 after the WSS. If the frequency we used is 25 GHz, the arbitrary optical mm-wave from 25 to 300 GHz can be accomplished by using this proposed scheme. The configuration of multiwavelengths generation on ROF system is illustrated in Fig. 13. The CW lightwave with linewidth smaller than 100 kHz at 1554.81 nm is modulated by

YU et al.: OPTICAL MILLIMETER TECHNOLOGIES AND FIELD DEMONSTRATIONS FOR WIRELESS-OVER-FIBER ACCESS SYSTEMS

a cascaded PM and IM driven by a 25 GHz sinusoidal RF source with a peak-to-peak voltage of 7.8 V. The half-voltage of the PM is smaller than 4 V, and IM is biased at the null point to generate phase. The optical spectrum with 0.01 nm resolution for multiwavelengths generation is displayed in Fig. 13(a). Among these multiwavelengths, it can be seen that 12 wavelengths with 25 GHz spacing are flat with power difference of smaller than 2 dB. One WSS with one input and four output ports and 25 GHz channel spacing is used to select arbitrary frequency in this scheme. The optical spectrum after the WSS is exhibited in Fig. 14. The adjacent channel can be selected at haphazard with 25 GHz spacing, as shown in Fig. 14(a). The electrical spectrum for this 25 GHz signal is shown in Fig. 13(b). Furthermore, the optical mm-wave from 25 to 325 GHz can be achieved in sequence, as shown in Fig. 14(a)–(f). Any four of the arbitrary optical mm-wave signals can be realized in the meanwhile at different output ports of the WSS. Because of the limitation of the experimental equipments, only the electrical eye diagrams of 25, 50, and 75 GHz optical mm-wave signals are shown in Fig. 14(a)–(c). After multifrequencies generation, the selected 50 GHz optical mm-wave signal would be fed into another IM after a TOF with 3 dB bandwidth of 1 nm. The 50 GHz optical mm-wave is used to carry 3.125 Gb/s signals with a pseudo. After random binary sequence (PRBS) word length of 20 km of SMF-28 transmission, one O/E converter is used, and the converted electrical mm-wave signal is then amplified by an EA. A 50 GHz electrical LO signal is mixed to downconvert the electrical signal to its baseband form. The eye diagrams of 3.125 Gb/s data carried by the 50 GHz mm-wave before and after downconversion are explicitly observed in Fig. 13(c) and (d), respectively. Regarding the BER, it is observed that the power after over 20 penalty is less than 0.2 dB at a given BER of km SMF-28. E. Multiband Services Generation It is much desired that one optical carrier, i.e., one wavelength, transports multiple optical subcarriers or RF signals to further leverage the cost. It has been shown that one optical carrier can transport multiple RF signals using optical subcarrier modulation such as single-sideband modulation [19], [20] (note photonic frequency upconversion using optical carrier suppression is not obtained). A technique of an optical-carrier transmitting two RF signals using optical carrier suppression has been proposed and experimentally demonstrated in [25]. A single optical MZM is used for both OCS and signal modulation, and OCS modulation is also used for frequency conversion of RF signals. In this experiment, one used two RFs at 6 and 18 GHz transmitting two 750 Mb/s signals. According to the recent draft standardization [26], the 60–GHz unlicense band is divided into four frequency subbands (at center frequency of 58.32, 60.48, 62.64, and 64.80 GHz) with frequency separation of 2.160 GHz and symbol rate of 1.728 GS/s. Thus, it is expected that future ROF network should be able to transmit multiband multiservice radio signals at 60-GHz mm-wave band in order to facilitate the seamless convergence with future-proof VHT wireless personal area network (VHT-WPAN) with optical wireless access. Recently,

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we experimentally demonstrated all-optical simultaneous upconversion of two separate 1 Gb/s services at 60 and 64 GHz mm-wave carried by one single lightwave for in-building ROF-based optical wireless access system. We evaluate the optical wireless transmission performance of the 60 GHz band after 10 km of SMF-28 with or without the presence of the 64-GHz band signal. Fig. 15 shows the proposed system architecture of the multiservice 60 and 64 GHz mm-wave ROF system using one single lightwave. At the main hub, service A and service B, each carrying separate set of data destined to two users: user 1 and user 2, are mixed with 15 and 17 GHz LO signals, respectively, before driving an IM. A CW lightwave at carrier frequency of is injected to the IM to realize carrier-suppressed subcarrier modis used after IM to eliminate ulation. An optical filter any residual unsuppressed carriers. The output of the OF1 is the carrier-suppressed optical signal with four newly generated as shown in inset (ii) of Fig. 15. sidebands A PM, driving by 15 GHz LO, is used to generate further higher and lower frequency sidebands. The output of the PM is shown is used to eliminate is inset (iii). Another optical filter unwanted peaks and will allow sidebands of interest, as shown and ) in inset (iv). The frequency spacing between ( is 60 GHz and between ( and ) is 64 GHz. The resultant optical signal is received by a broadband PD. The coherent and ) will result 60 GHz mm-wave, beating between ( and ) will produce 64 GHz while beating between ( mm-wave. These multiband signals are now wirelessly broadcast using a 60 GHz mm-wave broadband antenna. The remote users receive the broadcasted multiband signals simultaneously and select the band of interest accordingly. Fig. 16 shows the experimental setup. Two sinusoidal clock signals at 15 and 17 GHz from two LOs are mixed with the data and data bar output from a 1 Gb/s PRBS generator. A tunable delay is used to decorrelate between the data and data bar. An electrical coupler is used to combine the mixed signals before driving an IM with 20 GHz bandwidth. A CW lightsource at 1554.7 nm is injected to the IM, which is biased at transmission null in order to realize carrier-suppressed subcarrier modulation. The output of the IM is passed through the OF , which consists of one 50/100 GHz IL1 and an MZI filter (MZIF). Fig. 17(a) shows the optical spectrum after IM and the resultant passband of OF . The original carrier at 1554.7 nm is suppressed with over 18 dB and produces first-order sidebands at 15 and 17 GHz away from the central carrier, both in the lower and higher wavelength directions. Fig. 17(b) shows the optical spectrum after OF . It is clear that the original carrier is now suppressed over 30 dB, while the second-order sidebands are suppressed over 25 dB. A 40 GHz PM followed by optical filers (OF ) is used to complete the multiband upconversion process. The OF consists of one inline 33/66 GHz optical IL2 and another 50/100 GHz IL. Fig. 17(c) shows the output of the PM and the passband of the OF . The output of PM contains other higher order sidebands, which can be suppressed over 20 dB, as shown in Fig. 17(d). The output of OF is transmitted over 10 km SMF-28 to the remote antenna unit (RAU). At the RAU, the multiband 60 and 64 GHz mm-wave optical wireless signal is received by a 60 GHz photodetector (PD)

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Fig. 11. Fourth-order ring-resonator-based photonic IC, spectral response, output of MLL, spectrum, and optical eye diagrams.

and amplified by a power amplifier (PA) with bandwidth of . Fig. 16 (inset) about 7 GHz centered at 60 GHz and 3.55 shows the multicarrier tones at 60 and 64 GHz mm-wave. The received multiband signal is them broadcasted through a double-ridge guide rectangular horn antenna with a gain of 15 dBi, frequency range of 50–75 GHz, and 22 E/H-plane 3 dB beamwidth. The optical input power to the PD is about 3 dBm and the PA electrical output power is about 5 dBm, giving about 20 dBm equivalent isotropically radiated power (EIRP), which is well below the 40 dBm Federal Communications Commission (FCC) power limit for in-building 60 GHz radio. At the end user terminal, the broadcasted multiband wireless signal is received by another 60 GHz horn antenna with 15 dBi gain at frequency range of 50–75 GHz. The received signal is amplified before mixing with 60 GHz RF clock using a V-band balanced mixer for direct signal downconversion. A low-pass filter (LPF) with 933 MHz bandwidth is used to recover the 1 Gb/s baseband data after the downconversion. We measure the BER performance of the 60 GHz band with or without the presence of 64 GHz band after 10 km SMF-28 with various wireless distances, and the downconverted eye diagram for 1 Gb/s signal of 60 GHz band is exhibited in Fig. 17(e). The results present that the 60 GHz channel in multiband requires over 2 dB additional optical power compared to single-band transmission after 10 km SMF-28 and 6 m wireless transmisBER. Similarly, for a fixed optical power of sion at dBm, the achieved wireless distance of the multiband transmission is only 2 m compared to 5 m in the single-band BER. case at F. Optical Generation of mm-Wave by External Phase Modulator In addition to the intensity modulation, external phase modulation is also utilized to produce downstream optical mm-waves in optical-wireless networks. Chowdhury et al. [27] proposed a solution to upconvert signals on 11 GHz carrier by using a phase modulator. After that, the bandwidth of optical mm-wave has been explored to 30, 40, even 60 GHz. We had demonstrated

the applications of ROF system by phase modulation to generate 30 GHz mm-wave in [28], and utilized OCS to realize 40 GHz carrier [29], [30]. Recently, ROF network for 60 GHz wireless and the applications has been a hot research topic by using modulation formats [31]–[33]. III. BROADBAND OFDM-ROF TECHNIQUE OFDM-ROF system exhibits such a good potential in future broadband access network and has attracted numerous interests. As OFDM has been widely deployed in wireless communication system to get high SE, OFDM-ROF techniques are adopted to combine optical access networks and nowadays low-frequency wireless access networks, such a WiMAX, WLAN [67], [68]. However, the growth of broadband access work is spurred by the consumer desire for unrestricted access to information and entertainments using gigabit wireless techniques and HDMI. A 16 Gb/s superbroadband OFDM-ROF system on 24 GHz microwave carrier is presented in [69], and a 1 Gb/s OFDM signal transmitting over 80 km SMF-28 on 40 GHz mm-wave carrier is presented in [70]. Since an abundance of widely available spectrum around the 60 GHz operating frequency has the ability to support these high-rate, unlicensed wireless communications, there are many surveys of 60 GHz OFDM-ROF communication system. For example, a novel scheme for seamless integration of ROF with OFDM-WDM-PON system is provide in [71], and a 100 km long-reach 60 GHz OFDM-ROF system is demonstrated in [72]. Recently, a series of current literatures is convergent on multiband-OFDM ultra-wideband radio-signal-over-fiber transmission system, which is considered as a solution for high-capacity wireless access network [73]–[87]. Also, because of the advantages of multimode fiber and polymer fiber, such as easy handling, and ability to resist modal dispersion of OFDM signal, people pay more and more attention on techniques of 60 GHz OFDM signal over multimode fiber and polymer fiber [78]–[80]. The wireless transmission of 60 GHz OFDM-ROF system, as a particular critical portion of seamless integrated broadband access network, is also experimentally setup in [81].

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Fig. 12. Principle of the proposed architecture of arbitrary frequency optical mm-wave generation (WSS: wavelength-selective switch).

On the other hand, although OFDM-ROF system provides the combination of wireless and optical communication techniques, unfortunately, the drawbacks of this kind of system are also integrated and even more complicated. As a result of the reasons aforementioned, the optimization techniques for OFDM-ROF system are expected to be clarified [82]–[87] and promoted. We will present the 60 GHz OFDM-ROF system performance by using coding and optimal equalization techniques. A. Electrical OFDM Signal Generation and Detection The typical generation and detection procedures of OFDM signals based on signal processing are shown in Fig. 18. The transmitted PRBS data are transformed through serial/parallel processes, and then are modulated by digital modulation format as quadrature phase-shift keying (QPSK) or 16 QAM. The modulated signal is processed by inverse fast Fourier transform (IFFT) algorithm to fulfill the orthogonality between subcarriers. Afterward, the transformed data are converted back to serial sequence. Training sequence and cyclic prefix are added ahead to the data. Training sequence is used for time synchronization and signal equalization, and cyclic prefix is used to resist multipath fading. The signal is then sampled and transmitted. At the receiver side, received waveform is sampled and digitized, and delivered to digital signal processing. After the reverse processing, received data are demodulated. Here, a series of advanced experiments have been successfully demonstrated to evaluate the performances of 60 GHz OFDM-ROF system, such as 16-QAM-OFDM signal transmission using convolutional code, and the specific difference of equalization techniques in 60 GHz OFDM-ROF Optical-wireless system are also studied. B. Performance Improvement of 60 GHz OFDM-ROF Optical-Wireless System Using Convolutional Codes Because of the applications of high-level digital modulation formats, severe noises and interferences of wireless channel,

digital techniques of error correction, and antijamming are indispensable to 60 GHz ROF systems. Convoluntional codes are used extensively in numerous applications in order to achieve reliable data transfer. These codes are often implemented in concatenation with a hard-decision code. Prior to turbo codes, such constructions were the most efficient, coming closest to the Shannon limit. On the other hand, the power of transmitter in 60 GHz wireless indoor access network is a key factor that people are concerning on. When the signal is transmitted at a lower power, a series error bits, which are not only caused by external noises and interferences, but also by power insufficient can be detected and corrected with convolutional codes, which means convolutional codes make system achieve the BER performance with a lower power efficiency. Therefore, the performance of 60 GHz wireless indoor access network, especially the 60 GHz RF transmitter, can be enhanced. At the transmitter, binary series are converted by a convolutional shifting register. Fig. 19(a) depicts a convolutional shifting encoder and corresponding generator matrix. This has kind of convolutional coder of two stages and three outputs with a constraint length 2. At the receiver, a maximum-likelihood sequence estimator (MLSE) based on Viterbi algorithm is adopted for soft-decision decoding. The decoding trellis diagram with four retained is illustrated in Fig. 19(b). We transmitted paths 750 Mb/s 16QAM-OFDM signal over 1 km SMF-28 and 6 m wireless distance based on 60 GHz mm-wave ROF technique using convolutional code, which is shown in Fig. 20. A CW lightwave is generated by a tunable laser at 1555.2 nm and modulated by an MZM. The 750 Mb/s OFDM baseband signal PRBS is generated offline with MATLAB program. A data stream is converted by a shifting convolution coder to resist severe wireless channel loss and noises. Also, channel interleaving is accomplished for dispersing influences of noises and interferences. OFDM subcarriers are mapped with inter-

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Fig. 13. Experimental setup of multiwavelengths generation and the application on ROF system. (a) Received optical spectrum with 0.1 nm resolution for multiwavelengths generation. (b) Electrical spectrum for 25 GHz ROF. (c) and (d) Eye diagrams before and after downconversion.

leaved data using 16 QAM format. The fast Fourier transform (FFT) size is 1024 and a 32-cyclic-prefix (CP) length (61.5 ns) has been applied to resist the chromatic dispersions. One particular preamble frame is added into every 128 OFDM frames for time synchronization and calculating the channel response for frequency equalization at the receiver. Finally, the OFDM baseband signal is uploaded to Tektronix AWG operating at 250 MSample/s to drive the modulator. A 7.5 GHz RF sinusoidal wave is used to combine with a 1:4 frequency multiplier and produce a 30 GHz electrical LO signal to drive the PM. The DSB filtering is achieved by an optical IL with a periodicity of 66 GHz. The optical spectra after PM and interleave are shown in Fig. 20(c) and (d), respectively. At the wireless transmitter, the optical signal is detected and converted to mm-wave signal by using 60 GHz bandwidth PIN photodiode after 1 km SMF-28 transmission, and then the 60 GHz RF signals are broadcasted over 6 m of wireless distance at a transmitting RF power of 2.3 dBm. At the wireless receiver, the 60 GHz mm-wave signal is received by a horn antenna, amplified by an EA with 10 GHz bandwidth, and then downconverted to retrieve the baseband signals. Electrical out-of-band noise is blocked by a 1 GHz LPF. After that, the downconverted 16-QAM-OFDM baseband signal is sampled and recorded by Tektronix oscilloscope operating at a sampling rate of 250 MS/s, which is shown in Fig. 20(f). The received data are demodulated and processed offline. A shifting register based on Viterbi algorithm is adopted to decode and recover the original PRBS data. A total of 6 m wireless transmissions after back-to-back and 1 km SMF-28 (with/without convolutional code) are evaluated for BER testing. The BER performances of 60 GHz 16 QAM-OFDM-ROF optical-wireless system with a transmitting power of 2.3 dBm is measured and shown in of coded Fig. 21. The RF receiver sensitivities at signal are 7.5 dBm (B-t-B) and 10.3 (1 km SMF) with a 6

m wireless transmission. The system development in back to back is one order of magnitude improved by convolutional %. It can be seen that all curves code with become sharp deterioration after the 6 m wireless transmission, because of SNRs of the received signals are lower than that of the error-tolerant threshold. C. Equalization Techniques Optimizations for OFDM-ROF Optical-Wireless System Since we successfully transmitted 16-QAM-OFDM signal with 60 GHz ROF techniques, there are some differences of equalization techniques between conventional low-frequency wireless system and 60 GHz OFDM-ROF system. OFDM signal is well known to be tolerant to multipath fading and chromatic dispersion. At the same time, OFDM signal is very sensitive to frequency-selective fading because of its multisubcarriers. Therefore, equalization techniques are very critical for OFDM systems to mitigate channel impairments, especially in wireless frequency-selective fading transmission environment. Equalization techniques for OFDM systems are mainly referred to training sequences and pilot tones. By using training sequence and pilot information, we are able to estimate the channel frequency response and equalize received signal according to calculated channel information. The training sequence is a known string of symbols that occupies a whole set of OFDM frame to obtain frequency responses at each subcarrier frequency. The principle of training sequence is shown in Fig. 22. By comparing the value on training sequences before and after transmission, the channel frequency response matrix H could be obtained. Therefore, by applying the conjugated matrix of H on received signal, we could attain recovered data. Pilots are inserted symbols with known values to evaluate the phase shift between training sequence and current data frame. As shown in Fig. 23, the value of pilot tones on certain

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Fig. 14. Received optical spectra after the WSS for arbitrary frequency generation (0.01 nm resolution) and the corresponding eye diagrams.

Fig. 15. System architecture of the simultaneous upconversion of multiservice multiband 60 and 64 GHz mm-wave using single lightwave. PM: phase modulator; IM: intensity modulator; OF: optical filter; PD: photodetector; PA: power amplifier.

subcarrier is designed to be same as the value on corresponded training sequence subcarrier. By comparing the values on pilot tones and training sequence of received signal, the information of phase shift between training sequence and current data frame could be attained. Therefore, each data frame is corresponded to its own modified channel response matrix AH, BH, etc. After inverse transform, received signal will be recovered. Training sequence and pilots work together to attain more accurate channel estimation and better data recovery for received signal. IV. HIGH SE MULTILEVEL MODULATION The technique of microwave/mm-wave signals for broadband wireless and wired access networks using photonic techniques

Fig. 16. Experimental setup of the simultaneous generation of multiservice multiband optical wireless at 60 GHz mm-wave band.

has received much attention recently [89]–[98]. More efficient electrical modulation techniques such as QAM scheme are employed as an effect solution to reduce the utilized requirement of electrical bandwidth. Photonic vector modulation (PVM) schemes have been proposed to generate QAM signals by photonic processing [99], [100], compared to their complex electrical control circuits to translate binary data information into optical modulated signals and limited data bit rate, and some PVM architectures with electrical QAMs from 4-QAM up to 16-QAM and 3.6 Gb/s data rate have been experimentally demonstrated [101], [102]. However, in these electrical QAM PVM architecture, a high level of electrical carrier existing at the photodiode output plays an important limitation to the system performance. This undesired electrical carrier level will reduce the maximum available signal power in power amplifier.

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Fig. 17. Optical spectra at various points after: (a) IM, (b) OF , (c) PM, and (d) OF . The resolution bandwidth is 0.01 nm. (e) Downconverted eye diagram of 1 Gb/s signal in 60-GHz band.

Fig. 18. OFDM signal (a) generation and (b) detection.

Rakesh et al. [103] have proposed a PVM architecture to generated pure QAM signals at microwave-mm-wave frequencies from its corresponding baseband in-phase (I) and quadrature (Q) components, a pure 1.25 Gb/s 4-QAM signals has been experimentally generated at a 42 GHz carrier frequency. In [104], a PVM architecture for the generation of 16-QAM signals carried on mm-wave by using dual-drive MZM has been demonstrated. In this architecture, 5 Gb/s 4-amplitide shift-keying (4-ASK) and 10 Gb/s 16-QAM signals are carried on 42 GHz mm-wave.

Multilevel, multidimensional coding combined with DSP-based coherent detection has shown to be an effective method to increase SE, and therefore overall fiber capacity [103]. Therefore, on one hand, the advent of 60 GHz mm-wave technologies is revolutionizing the consumer electronics market on a wide range of proliferating applications expecting multiGigabit wireless connectivity. VHT 60 GHz single-carrier chip-to-chip transmission has been demonstrated within the 7 GHz unlicensed bandwidth using well-known multilevel modulation formats in wireless communication such as QPSK at 7 Gb/s and 16 QAM at 15 Gb/s. On the other hand, since the 60 GHz radio has extremely high atmospheric loss, ROF is considered a required and promising means to offer both extended reach and ubiquitous in-building coverage for 60 GHz mm-wave through centralized, simplified remote access units [92]. However, since mm-wave is more light-of-sight oriented, ROF system at 60 GHz band may open the possibility of leveraging these potential wireline modulation formats in such seamless broadband wireline and wireless convergence, which has not been studied yet. Therefore, in this section, we propose and demonstrate a VHT 60 GHz ROF link delivering spectral-efficient DB signal from end to end at 10 Gb/s. DB modulation format is famous for its spectral compression capability and high tolerance to intersymbol interference (ISI) in fiber-optic communication. More importantly, since direct detecting an optical DB signal is easily achievable, no decoders

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Fig. 20. Experimental setup of 60 GHz 16 QAM-OFDM-ROF system transmission over 1-km SMF and 6 m air link using convolutional code. TL: tunable laser, PM: Phase modulator [(a), (e), (b), and (f) 16-QAM constellations, OFDM baseband waveform before and after transmission, respectively; (c) and (d) Opticalcarrier spectra before and after carrier suppression].

Fig. 21. Experimental results: BER curves for 6 m air link plus back-to-back or 1 km SMF with/without coding.

Fig. 19. Convolutional shifting encoder. (a) Generator matrix. (b) Trellis diagram.

or complicated demodulators are required in the 60 GHz receivers. As shown in Fig. 24, we propose and experimentally demonstrate a record ROF system with 15 Gb/s QAM signal carried by 60 GHz. The 60 GHz optical carrier is generation by cascading one PM, one MZM, and a WSS, which is similar

to Fig. 13. The single-mode CW lightwave is generated by an ECL and modulated by cascaded PM and IM, both driven by a sinusoidal RF source with a repetitive of 20 GHz with a peak-to-peak voltage of 6 and 12 V, respectively. The optical spectrum with 0.01 nm resolution for multiwavelengths is shown in Fig. 24 (i). It can be seen that 11 flat peaks with 20 GHz frequency spacing are generated. One polarization maintaining EDFA is utilized to boost the optical power before the multipeaks are sent to another single-arm IM driven by four-level electrical signals. The four-level electrical signal is generated by an electrical coupler, which is used to combine the two 7.5 Gb/s binary data, one of which has been introduced time delays (for decorrelation). The clear eye diagram of the four-level electrical signal is shown in Fig. 24(iii). The 60 GHz

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Fig. 22. Principle of training sequences (yellow represents training sequence).

Fig. 23. Principle of pilot tones (yellow represents training sequence, green represents pilots).

Fig. 24. Multilevel signaling for 60 GHz ROF link.

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Fig. 26. Proof-of-concept VHT 60 GHz optical-wireless test link that delivers DB bit streams at 10 Gb/s and beyond.

Fig. 25. Electrical spectra of binary and DB signals.

optical mm-wave signals are selected by the programmable WSS, and the corresponding optical spectrum is shown in Fig. 24(ii). After O/E conversion by using one 60 GHz PD and the downconversion by using an electrical mixer with a 60 GHz LO signals, the four-level 15 Gb/s signals can be retrieved. The eye diagram of the demodulated binary signals by one high-speed sampling oscilloscope is show in Fig. 24(iv), and it can be seen clearly that the eye diagram is open completely. The measured BER after computer offline processing from the data recorded by a 10 GHz real-time oscilloscope is smaller based on data, which is far less than the than . forward error correction (FEC) limitation of Except for BPSK modulation, DB signal is less level modulation format compared to other QAM modulation. Fig. 30 illustrates the conceptual diagram of the proposed VHT mm-wave optical-wireless access system with signal spectrum squeezed through DB line coding. The optical DB signal can shrink the required bandwidth by 50%. Fig. 25 shows the electrical spectra of binary and DB signals. As we can see that DB signal at the same bit rate has much narrow spectrum compared to that of binary signal. More im-

portantly, the DB signal can be directly detected without complicated decoders and demodultors at the receiver, and the oppose electrical fields between two adjacent one bits will help to reduce ISI. After all-optical upconversion, the optical mm-wave carrying multigigabit DB bit stream will be delivered over fiber to RAU, which simply detects downlink mm-wave signal and transmits it over the air using O/E and antenna modules, respectively. For wireless subscribers, the mm-wave receiver will just work on a bits-in, bits-out basis, and again no decoder or demodulator is required that highly simplifies hardware implementation and considerably saves power consumption. Fig. 26 shows the proof-of-concept experimental setup to generate and deliver 60 GHz optical mm-wave carrying DB signals at a line rate over 10 Gb/s. Between headend and RAU, a 5 km SMF-28 was designated to emulate a campus-wide access network environment. In the headend office, a 10 Gb/s PRBS signal is considered a precoded bit sequence, and can be converted into a three-level electrical DB signal by passing it through a Bessel electrical LPF with 3 dB bandwidth of 2.8 GHz, which is about 25% of the bit rate. To realize E/O and DB to binary conversion at the same time, a DFB-LD at 1553.2 nm was externally modulated by a LiNbO optical IM, which was driven by the three-level electrical DB signal with bias around 3.2 V at its transmission null. An EA with voltage saturation output power of 7.8 V was prearranged before the LPF to boost the driving electrical DB signal and ensure that the IM can be driven at full swing (2 V ) to maximize the extinction ratio (ER) of the converted two-level optical DB signal. It was then fed into an optical PM driven by 30 GHz sinusoidal wave for all-optical upconversion, and the central optical carrier is further suppressed by using a 33/66 GHz optical IL to double the beating frequency of the generated optical mm-wave. After 5 km fiber transmission, the 60 GHz optical mm-wave carrying 10 Gb/s DB signal was directly detected by a 60 GHz PD, boosted by a PA, and radiated to free space through a 60 GHz horn antenna with 15 dBi gain and 22

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Fig. 28. Field-trial demonstration setup of the SD/HD video content delivery using 2.4 and 60 GHz mm-wave ROF in the GT campus fiber network.

the experiment since the 60 GHz PA and linear amplifier (LNA) had limited passband bandwidth of 8 GHz, which can be further addressed by comparing the measured baseband spectra of NRZ and DB signals at 10 and 20 Gb/s, respectively, in Fig. 25 (inset). Fig. 27 shows the BER performance of the 10 Gb/s downconverted DB signals at launched EIRP of 15 dBm as a functional of wireless propagation distance with 5 km optical fiber transmission. Error-free transmission over a combined distance of 5 km wireline and 6 ft in-building wireless links was observed at 15 dBm EIRP. One typical eye diagrams of downconverted 10 Gb/s DS signal is shown in Fig. 27.

Fig. 27. BER performance of the downconverted 10-Gb/s DB signals at launched EIRP of 15 dBm versus different wireless propagation distance with 5-km fiber transmission.

E/H-plane 3 dB beam width. The optical input power to the PD is about 2 dBm and the PA electrical output power is about 0 dBm, giving EIRP around 15 dBm, which is well below the 40 dBm FCC power limit for in-building 60 GHz radio. A 60 GHz receiver was placed at several meters away within a room, and its mm-wave front-end consists of a 60 GHz horn antenna, a low noise amplifier (LNA), a V-band balanced mixer for direct signal downcconversion, a 60 GHz LO from the combination of a 15 GHz synthesizer and a 1 4 multiplier (4 ), and a 7.5 GHz LPF. Fig. 26 (inset) illustrate the measured eye diagrams of the three-level electrical [see Fig. 26(a)] and two-level optical DB signals at 10 Gb/s [see Fig. 26(b)], the 60 GHz optical mm-wave carrying 10 Gb/s DB signal before [see Fig. 26(c)] and after 5 km fiber transmission [see Fig. 26(d)], and the electrical 60 GHz mm-wave signal before wireless transmission [see Fig. 26(e)]. Fig. 26(f) shows the feasibility of upconverting a 20 Gb/s DB signal to 60 GHz band using the same generation scheme in the headend office; however, the required 20 GHz mm-wave channel bandwidth made it difficult for wireless delivery during

V. FIELD DEMONSTRATION OF 60-GHZ WIRELESS-OVER-FIBER NETWORKS There are some demonstrations of 60 GHz mm-wave optical wireless network in recent years. In [104], IPHOBAC constructed broadband 60 GHz photonic-wireless systems for home area networks and access networks. Their systems are capable to deliver data rates up to 12.5 Gb/s over short- to medium-range wireless spans. At the ICT 2008 fair, they have been demonstrating the wireless transmission of uncompressed broadband HDTV channels. In recent years, we have setup two demonstrations for 60 GHz mm-wave network [105]–[108]. The first demonstration is to use separated devices and the next one is to use integrated 60 GHz mm-wave CMOS transceiver. Fig. 28 shows the system implementation of the first field demonstration of delivering dual service uncompressed 270 Mb/s standard definition (SD) and uncompressed 1.485 Gb/s high definition (HD) video content using 2.4 microwave and 60 GHz mm-wave radio signals, respectively, over Georgia Institute of Technology (GT) Campus fiber backbone network from Centergy Research Laboratory at 10th Street to Aware Home Residential Laboratory at 5th Street [105]. The transmission distance is 2.5 km SMF-28. At the transmitter (Centergy building), all-optical upconversion method is used to perform simultaneous generation of 60 GHz mm-wave and upconversion of 1.485 Gb/s HD signals at wavelength 1554.0 nm. The all-optical mm-wave generation at 60 GHz is realized by using an optical phase modulator driven by

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Fig. 30. Test bed setup of in-building bidirectional superbroadband 60 GHz mm-wave ROF network interoperates with multigigabit 60 GHz CMOS IC wireless transceiver.

Fig. 29. Optical wireless transmitter and receiver of the 3-screen, dual-service 2.4-GHz and 60-GHz RoF carrying 270-Mb/s SD and 1.485-Gb/s HD video content. (a) Transmitter at Centergy lab at 5th Street, (b) 60-GHz test bed setup at residential laboratory at 10th Street, and (c) 3-screen receiver at residential laboratory.

30 GHz sinusoidal electrical clock signal and an optical de-IL as described in the previous section. The HD signal at 1.485 Gb/s is generated from the component output of commercially available Sony Blue-Ray Disc player and an analog-to-digital converter. For 2.4 GHz radio signal, we used electrical mixing and double-sideband optical modulation to upconvert 270 Mb/s real-time SD video content generated from a commercially available Canon Camcorder before optically transmitted at wavelength of 1550 nm. At the receiver (Aware home), direct detection of optical60 GHz mm-wave signal is performed by a 60 GHz bandwidth PIN photodiode to realize O/E conversion. The converted electrical mm-wave signal is then amplified by

an EA with 5 GHz bandwidth centered at 60 GHz before it is broadcasted through a double-ridge guide rectangular horn antenna with a gain of 25 dBi, frequency range of 50–75 GHz, and 3 dB beamwidth of 7 . After the wireless transmission, the 60 GHz mm-wave signal is received by the end mobile terminal in order to perform the downconversion and recover the 1.485 Gb/s HD video signal. The downconversion is performed by a 60 GHz balanced mixer using self-reflective mixing technique. Similarly, the 2.4 GHz radio signal is received by a 2.5 GHz PIN receiver at the BS and distributed over the wireless to the receiver antenna. The 270 Mb/s SD signal is then recovered by downconversion process. Fig. 29 shows the transmitter and receiver modules at two separate locations. We did not measure any BER in the field trial, since we do not have any available electrical clock recovery module that is required to recover the clock at the distantly located receiver. However, the video quality displayed at the remote receiver TV screens located at the residential laboratory (2.5 km away from the transmitter at Centergy building) indicates the very good BER performance of the received signal. The development of single-chip low-cost IC solutions at 60 GHz mm-wave with integrated RF front-end functionalities, baseband processing capabilities, and low-power multigigabit modulation/demodulation techniques makes mobile high-speed Gb/s) wireless access a reality in the near future [106]. ( It is assumed that, within near future, the end-terminal mobile devices will be equipped with such 60 GHz mm-wave CMOS transceiver. Thus, it is important to examine the interoperability between the ROF-based 60 GHz mm-wave optical wireless system and the 60 GHz mm-wave CMOS wireless transceiver modules. We setup another field-trial test bed of in-building ROF access system that supports bidirectional 60 GHz mm-wave optical wireless providing 1.485 Gb/s uncompressed high-definition video connectivity between the optical headend gateway and end user’s mobile terminals at the GT campus [105]. At the headend gateway, we used all-optical upconversion and receiving of the downstream and upstream signals, respectively. At the mobile end user terminal, we

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Fig. 31. BER performance of the interoperable system.

used the integrated mm-wave multigigabit CMOS wireless transceiver unit developed by Georgia Tech. Fig. 30 shows the experimental test bed setup of in-building bidirectional superbroadband 60 GHz ROF network system deployed in Technology Square Research Building (TSRB) at the GT campus. The headend gateway (HE) of the in-building ROF network is located at TSRB room 109, while the remote base station and mobile end user terminals (MT) are located in TSRB room 104. At the HE, the 1.485 Gb/s uncompressed HD video signal from a Blue-ray player is upconverted to 60 GHz mm-wave using all-optical upconversion method realized by an optical PM and a 33/66 GHz optical IL. The generated optical mm-wave signal is then amplified by an EDFA and transmitted over SMF-28 link to the base station located at TSRB room 104. Fig. 30 [(i) and (ii)] shows the optical eye diagram at two locations. At the base station, direct detection of optical mm-wave signal is performed by a 60 GHz bandwidth PIN photodiode for optical-to-electrical conversion. The converted electrical mm-wave signal is amplified by an EA with 5 GHz before it is broadbandwidth centered at 60 GHz and 3.55 casted through a double-ridge guide rectangular horn antenna with a gain of 25 dBi, frequency range of 50–75 GHz, and 3 dB beam width of 7 . After the wireless transmission, the 60 GHz mm-wave RF signal is received by the MT, using 90 nm CMOS IC receiver module (Rx). The single-chip receiver first amplify the received 60 GHz RF signal using mm-wave low-noise amplifier then down-converted to an intermediate frequency (IF) of 8–9 GHz. The resulting IF signal is mixed with the quadrature voltage-controlled oscillator (QVCO) outputs, which is set by the fixed PLL. Fig. 31 shows the achievable BER for various wireless transmission distances and corresponding electrical eye diagrams after 25 km SMF-28 wireline transmission. Again, at the mobile terminal, a 60-GHz single-chip 90 nm CMOS radio transmitter is used to upconvert the upstream 1.485 Gb/s HD video signal to 60 GHz mm-wave RF signal before wirelessly broadcast to the BS. In the transmitter chain, the baseband signal is first upconverted to IF using a double-balanced quadrature Gilbert cell mixer and a QVCO controlled by a fixed PLL. An active differential-to-single-ended converter and common source digitally controlled variable-gain

Fig. 32. Testbed modules. (a) Headend gateway at TSRB room 109. (b) BS and mobile terminal at TSRB room 104.

amplifier are driving a mm-wave resistive upconverter mixer to ensure good stability and good linearity. At the BS, the 60 GHz upstream wireless signal is electrically downconverted to 1.485 Gb/s upstream data stream using another set of CMOS radio Rx module before optically transmitted to the headend gateway. At the headend, the upstream HD signal is received by the 2.5 Gb/s APD photodiode before fed into an HD display unit. Fig. 32(a) and (b) shows the transmitter and receiver modules of both downstream and upstream signal at two separate locations. VI. CONCLUSION mm-Wave wireless-over-fiber systems with VHT has gained much interest in recent years. Enabling technologies for such applications were reviewed in this paper, ranging from advanced photonic generation to spectrum-efficient modulation technologies. Several cost-efficient optical transport schemes have been developed for higher frequency carrier generation using low-cost and low-frequency components. These are based on external intensity or phase modulation, or nonlinear effect in HNLF or bismuth oxide fiber with or without optical filter. Flexible multiband service provision and arbitrary carrier generation at integer multiples of a base frequency were also introduced for future agile wireless-over-fiber configuration. In

YU et al.: OPTICAL MILLIMETER TECHNOLOGIES AND FIELD DEMONSTRATIONS FOR WIRELESS-OVER-FIBER ACCESS SYSTEMS

addition to pursuing higher carrier frequency, multicarrier and multilevel modulation formats were also investigated for further increasing the throughput in wireless-over-fiber systems. We evaluated the generation and detection of OFDM wireless signals over fiber using convolutional coding and optimal equalization to enhance the transmission performance. As well as single carrier with multilevel modulation, we studied the DB and QAM format, showing the record 15 Gb/s transmission using 60-GHz wireless-over-fiber links. Furthermore, we demonstrated a field trial for on-campus access network capable of delivering 1.485-Gb/s uncompressed HD video signals with 60-GHz ICs on optical platform.

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