Abstract
With the remote beating of two mutually incoherent laser carriers, the local-oscillator-free long-reach millimeter-wave over fiber (MMWoF) link at 60-GHz band is demonstrated. The unique schemes of the proposed MMWoF are the wavelength-locked colorless laser diode (CLD) modulator, the mutually incoherent optical carrier for heterodyne MMW generation, and the square-law power envelope detection at receiving end. By directly encoding the single-mode with the CLD modulator, the single-carrier modulated QAM-OFDM data is achieved to release the RF power fading after fiber transmission. The mutually incoherent laser beating enables the optical heterodyne MMW generation with two independent optical carriers, which provides the advantages of local-oscillator-free operation and rules out the requirement of dual-mode optical carrier delivery from central office. At the wireless receiving end, the received QAM-OFDM data is self-down-converted to the baseband by employing the square-law power envelope detection. This eliminates the requirement of local oscillator and rules out the influence of the MMW carrier frequency fluctuation between two mutually incoherent lasers (used at central office and remote node), which effectively provides the MMW carrier immunity against the down-conversion instability caused by clock jitter or carrier incoherence. This architecture ensures the transmission of 16.5-Gbit/s 32-QAM OFDM data over 50 km in SMF and 3 m in free-space with the FEC certificated error vector magnitude of 12%, signal-to-noise ratio (SNR) of 18.4 dB, and bit error rate of 3.8 × 10−3. For multi-channel DWDM-PON applications, the proposed local-oscillator-free MMWoF link can successfully perform 11 DWDM channels of 32-QAM OFDM data access at 16.5 Gbit/s per channel via the wavelength controlling of the CLD modulation stage and the detuning of the beating carrier at remote node.
Highlights
Nowadays, the 5th generation (5 G) wireless mobile networks with the highly adaptive access capability have been comprehensively investigated to take over the current 4 G mobile networks in near future, which can support the real-time multimedia services at 100 M-1 Gbit/s for metropolitan areas with enhanced spectral efficiency, reduced latency, and improved network coverage for mobile users[1,2]
To synthesize the typical dual-mode optical carrier for the optical heterodyne millimeter wave (MMW) over fiber (MMWoF) systems, the central-carrier suppressed double-sideband (CCS-DSB) optical carrier with high coherence feature is usually employed as representative candidate, which can be generated by externally modulating a continuous-wave (CW) laser with a nully-biased Mach-Zehnder modulator (MZM)[20,21]
For quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM) encoding onto a specific DWDM channel, a directly modulated slave colorless laser diode (CLD) is employed as the optical modulator and mode-selective amplifier (Fig. 1(b)) via the master controlled wavelength-locking (Fig. 1(c))
Summary
The 5th generation (5 G) wireless mobile networks with the highly adaptive access capability have been comprehensively investigated to take over the current 4 G mobile networks in near future, which can support the real-time multimedia services at 100 M-1 Gbit/s for metropolitan areas with enhanced spectral efficiency, reduced latency, and improved network coverage for mobile users[1,2]. The mutually incoherent optical carriers with two wavelengths delivered by two independent lasers are employed to establish the local-oscillator-free long-reach MMWoF link at wireless frequency of 60 GHz. The MMW carrier is synthesized by remotely combining the down-stream single-carrier transmitted data with another independent single-mode carrier at the optical receiving end. As the incoherence between two optical carriers from CO and remote node inevitably fluctuates the optically heterodyned MMW carrier frequency, it induces tremendous down-conversion frequency instability at wireless receiving end This issue is released by employing the square-law power envelope detection technique, which makes the MMW central carrier and its carried QAM-OFDM data self-down-convert to baseband without disregarding the carrier frequency fluctuation. To confirm the multi-channel DWDM flexibility, the allowable channel number availably offered by such a mutually incoherent dual-wavelength optical carriers for the MMWoF link is surveyed
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