THE 60-GHz millimeter-wave (mm-wave) technology has attracted a growing interest recently in the area of wireless personal area network because of its potential for high-throughput, high-security, and low-interference wireless connectivity. The available license-free bandwidth in 60-GHz band is about 7-GHz. It is a valuable asset for providing extra bandwidth in next-generation wireless communications systems [1], [2]. However, the transmission of 60-GHz signal is limited to a short-range free-space distance due to oxygen absorption and the large loss indicated by Friis transmission equation. Thus, it is difficult to realize a ubiquitous wireless service coverage as WiMax. As a result, radio-over-fiber (RoF) technology is considered as one of the most promising solutions to extend the reach of 60-GHz mm-wave wireless signals [3], [4].
To develop a spectrally efficient 60-GHz RoF system that is compatible with the fiber-to-the-home infrastructures, several schemes have been reported to employ optical mm-wave channels for both wireless and wireline services which are carried by the central carrier and the subcarriers, respectively [5], [6]. However, narrowband optical bandpass filtering for carrier separation at base stations (BSs) is generally required that will increase the component cost and complicate the system operation and reliability. Therefore, in this letter, we propose and experimentally demonstrate a novel hybrid subcarrier modulation (H-SCM) scheme to generate 60-GHz optical mm-wave optical subcarriers with their intensity and phase, respectively, modulated by wireless and wireline data streams, which can be subsequently detected by simple wireless and wireline receivers at the BS without front-end narrowband filtering. To further understand optical beating components at the 60-GHz band, we have also analyzed and experimentally examined the causes of jitter and amplitude fluctuation of the corresponding radio signals.
SECTION II
HYBRID SUBCARRIER MODULATION
A. Operating Principle
To deliver independent wireless and wireline signals on a single optical mm-wave channel with high spectral efficiency, the H-SCM scheme can deliver extra wireline data that is embedded in the phase of both mm-wave subcarriers. In this scheme, only one of the two 60-GHz subcarriers is required to carry an intensity-modulated wireless signal. As shown in Fig. 1, since their phases are modulated at the same time, the two subcarriers are still correlated, which preserves the optical coherency (amplitude addition at the detector rather than power addition). In order to realize H-SCM, two subcarriers of optical mm-wave with 60-GHz spacing are initially split at the interleaving (IL) stage in the central station (CS). The odd subcarrier is then modulated with on–off keying (OOK) while the even one remains constant. After passing through the IL stage, both subcarriers are recombined and transmitted through the next signaling stage, which is responsible for differential phase-shift-keying (DSPK) modulation of wireline data streams. The 60-GHz H-SCM optical mm-wave signal then delivers independent wireline and wireless services to the BSs. For wireless access, both optical subcarriers are detected at the same time, optical–electrical converted to 60-GHz RF OOK electrical signal, and then broadcasted over free space to wireless subscribers. For wireline access, the embedded DPSK bit streams on both subcarriers are simultaneously demodulated, but only the constantly powered even subcarrier will be selected by simply taking advantage of the passband edge of wavelength-division-multiplexing (WDM) thin-film filters, which are integrated in off-the-shelf bidirectional optical transceivers, and the demodulated data can be acquired by wireline end users.
B. Phase Stability Analysis
Before injecting “data” into the optical H-SCM scheme, we first need to assure the stability and quality of RF beating frequency at 60 GHz. Interleaving optical mm-wave subcarriers into two different optical paths during the proposed H-SCM at the CS may cause RF phase instability, and may thus decorrelate the phases of two mm-wave subcarriers. In experiments, the beating RF signals are distorted after adding an intensity modulator (IM) even though these separated optical paths are far shorter than the coherent length approximated by the spectral linewidth of the laser diode. This is mainly because the even and odd subcarriers suffer from statistically independent phase noises in H-SCM. For example, assume all the modulators are zero-chirped and operating in the linear region, the electrical fields of two subcarriers after H-SCM are
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$$\eqalignno{\mathtilde {E}_{e} &= A_{e} e^{j\omega _{e} t + j\phi _{\rm PM} (t)} \cr \mathtilde {E}_{o} &= A_{o} (t)e^{j\omega _{o} t + j\delta + j\phi _{\rm IM} (t) + j\phi _{\rm PM} (t)} &\hbox{(1)}}$$ respectively, where
$A_{o}$ and
$A_{e}$ are the real amplitudes of the odd and even optical subcarriers, respectively, and
$\omega _{o}$ and
$\omega _{e}$ are the corresponding angular frequencies.
$\delta$ is the constant phase shift due to the length difference of the two optical paths. And
$\varphi_{\rm PM}({t})$ is the modulated phases plus phase noise of LiNbO3 phase modulator (PM). During H-SCM,
$\varphi_{\rm IM}({t})$ is the intrinsic phase noise introduced by the IM only added to the odd subcarrier. We also assume that it contains other imbalanced phase noise induced by dispersion [7]. As a result, the detected optical mm-wave signal becomes
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$$\displaylines{\vert A_{o} (t)\vert ^{2} + \vert A_{e} \vert ^{2} + {A_{o} (t)A_{e} \over 2}\cos [(\omega _{0} - \omega _{e})t \hfill \cr \hfill +\, \delta + \phi _{\rm IM} (t)] \quad\hbox{(2)}}$$ which reveals that only
$\varphi _{\rm IM}({t})$, which is dominated by the phase noise of the IM, can affect the generated optical mm-wave signal in the form of timing jitter. Fig. 2 shows the simulated amplitude variation and timing jitter to
$\varphi_{\rm IM}({t})$ of 60-GHz beating optical wave with electrical field amplitude normalized to unity. The insets in Fig. 2 show (a) the simulated and (b) the corresponding measured 60-GHz mm-waves assuming the phase noise is zero-mean normally distributed with variance around 0.8. Two optical mm-wave subcarriers will remain correlated until the phase noise is too large to maintain the beating integrity [8].
SECTION III
EXPERIMENTAL SETUPS AND RESULTS
Fig. 3 shows the proof-of-concept experimental setup of 60-GHz RoF downlink using the proposed H-SCM technique to generate optical mm-wave signals for dual services. In the CS, a distributed-feedback laser at 1550.36 nm is externally modulated by a LiNbO3 optical PM driven by 30-GHz sinusoidal wave, resulting in multiple optical subcarriers with 30-GHz spacing. A 25/50 interleaver is then deployed to select two 60-GHz-spaced first-order subcarriers and to suppress the central carrier [9]. The two optical subcarriers are then separated by a 50/100 interleaver into even and odd channels. The odd subcarrier is intensity-modulated at 2.5 Gb/s by pseudorandom binary sequence whereas the even channel is equipped with a passive polarization controller. Both channels are then recombined with an optical coupler, concurrently passing through another PM to carry 10-Gb/s DPSK wireline signal. After 25-km transmission of single-mode fiber (SMF), the 60-GHz H-SCM optical mm-wave signal is divided by the BS into wireless and wireline access. For wireless access, the signal is directly detected by a 70-GHz photodiode, boosted by a power amplifier, and transmitted through a 60-GHz horn antenna with 25-dBi gain. Due to the low responsivity of the high-speed PD, it is equipped with an erbium-doped fiber amplifier (EDFA) preamplifier to amplify the optical power to 0 dBm. Another 60-GHz horn antenna placed at 4 m away receives the signal, which is down-converted to baseband data by self-mixing and low-pass filtering in the RF receiver. On the other hand, the DPSK signal is retrieved after a 10-GHz Mach–Zehnder delay interferometer. Only the even optical subcarrier is selected and filtered out by using the cutting edge of a WDM thin-film filter for wireline data reception. Signals are sent to a bit-error-rate (BER) tester to evaluate their performance.
Fig. 4(a) illustrates the measured optical spectra of the 60-GHz H-SCM optical mm-wave signal carrying both 10-Gb/s wireline DPSK and 2.5-Gb/s wireless OOK signals, along with the initial 30-GHz-spaced multicarrier lightwave after the first PM in the CS. In addition, Fig. 4(b) shows optical spectra of the selected even optical subcarrier for wireline service and the corresponding WDM thin-film filter. Fig. 5 depicts the BER of both measurements. After 25-km SMF transmission, the power penalties for the received 2.5-Gb/s wireless and 10-Gb/s wireline signals are 1 and 0.2 dB, respectively, at
$10^{- 9}$ BER. The insets in Fig. 5 depict the measured eye diagrams for wireline and wireless signals with and without 25-km SMF, respectively. The BER measurements of DPSK signals imply that the intrinsic phase noise of LiNbO3 PMs are small, which, therefore, have minor influence on the degradation of the RF quality.
We have proposed and demonstrated a novel H-SCM scheme to simultaneously transmit a wireless OOK 60-GHz mm-wave signal at 2.5-Gb/s and a wireline DPSK baseband signal at 10-Gb/s. In this scheme, two optical subcarriers are first separated into two channels and recombined to preserve a coherent 60-GHz channel. The influence of optical path difference and imbalance is analyzed to maintain the quality and stability of RF mm-waves. To further improve the performance of the wireless signals, it is important to minimize the independent phase noise of the even and odd optical-subcarrier channels during modulation. For example, integration of all modulation functions in one photonic lightwave circuit could dramatically reduce the total length and simplifies the H-SCM process, hence cutting down the induced imbalanced phase noise. Otherwise, cascaded two optical carrier suppression to generate the even and odd channels in one optical path can also guarantee the beating frequency quality [10].
In this letter, generation and transmission of 2.5-Gb/s wireless signal and 10-Gb/s wireline signal are successfully demonstrated with H-SCM scheme in the experiment. After 25-km SMF, the power penalties of wireless and wireline signals of 1 and 0.2 dB have been measured, respectively. We have demonstrated that simultaneous wireless and wireline services can be delivered using only a single 60-GHz optical mm-wave channel generated from a single laser source. The desirable property of spectral efficiency also ushers in a scalable, low-cost WDM passive optical network access network systems with 100-GHz channel spacing.