A novel millimeter-wave radio over fiber (MMW-RoF) link at carrier frequency of 35-GHz is proposed with the use of remotely beating MMW generation from reference master and injected slave colorless laser diode (LD) carriers at orthogonally polarized dual-wavelength injection-locking. The slave colorless LD supports lasing one of the dual-wavelength master modes with orthogonal polarizations, which facilitates the single-mode direct modulation of the quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data. Such an injected single-carrier encoding and coupled dual-carrier transmission with orthogonal polarization effectively suppresses the cross-heterodyne mode-beating intensity noise, the nonlinear modulation (NLM) and four-wave mixing (FWM) sidemodes during injection locking and fiber transmission. In 25-km single-mode fiber (SMF) based wireline system, the dual-carrier under single-mode encoding provides baseband 24-Gbit/s 64-QAM OFDM transmission with an error vector magnitude (EVM) of 8.8%, a bit error rate (BER) of 3.7 × 10−3, a power penalty of <1.5 dB. After remotely self-beating for wireless transmission, the beat MMW carrier at 35 GHz can deliver the passband 16-QAM OFDM at 4 Gbit/s to show corresponding EVM and BER of 15.5% and 1.4 × 10−3, respectively, after 25-km SMF and 1.6-m free-space transmission.
© 2016 Optical Society of America
The demand for higher data transmission rates has abruptly increased because of the widespread adoption of high-definition videography and smart phones. Although fourth-generation (4G) mobile communication technology have been deployed, they are likely to be inadequate for meeting the user demand for transmission capacity in the near future, and utilizing the millimeter-wave (MMW) frequency band for data transmission is the most promising approach to realize gigabit wireless communication network to meet this demand. Several transmission architectures for commercial MMW applications have emerged in the past few decades. The Federal Communications Commission allocated the 57-64 GHz MMW band for unlicensed use; the 60-GHz carrier-based wireless local area networks (LANs) are primarily intended for use in short-range and single-room environments because of the high propagation and penetration losses in MMW network . Recently, fifth generation (5G) wireless network with novel technical issues and frameworks have been attracted and investigated [2–6]. To fulfill the 5G mobile services with increasing diversity, the 5G network needs to fit targets of data rate up to 10 Gbit/s, less than 1-ms latency and supporting several billions of applications . Although the MMW carrier frequency of the 5G network has not been clearly specified currently, it ranged from 3 to 300 GHz shows potential for 5G applications . In addition, the currently available information for the requested distance of 30-200 m has been defined for the 5G MMW wireless network environment [9–11]. More recently, a 35-GHz MMW carrier was recently implemented to develop 5G mobile communication systems for commercial applications . A converged network for fusing 5G technologies with the current optical access network architectures in the near future is illustrated in Fig. 1. Remotely generating MMW carriers and extending their transmission distance in wireless LANs is essential for developing cost-effective next-generation MMW-over-fiber (MMWoF) . Furthermore, fusing MMWoF with dense wavelength-division-multiplexed passive optical network (DWDM-PON) is essential for supporting an independent wavelength for each optical network unit (ONU) [14, 15], which can increase transmission capacity. Moreover, MMWoF with DWDM-PON can meet the expected demands of future wireless access networks because of their wide service coverage and large cost-effective capacity per user. In general, MMW carriers can be optoelectrically beat through optical heterodyning, wherein two optical carriers with different wavelengths are transmitted to a photodetector for generating the MMW carrier . Assume two optical signals Êw1·cos(2πfw1t + ϕw1) and Êw2·cos(2πfw2 t + ϕw2), where Êw1 and Êw2, fw1 and fw2, and ϕw1 and ϕw2 denote the electric field vectors, frequencies, and phases of two optical signals, respectively. After transmission through the photodetector, the output current is obtained as Iout = RPD × (Êw1 + Êw2*)·(Êw1* + Êw2) = RPD × (2Ew1Ew2)cos[2π(fw1−fw2)t + (ϕw1−ϕw2)], where RPD is the responsivity of the photodetector , and the MMW carrier with a central frequency of fw1−fw2 is generated.
However, the peak power of an MMW carrier depends strictly on the phase difference between the two beating carriers. At early stage, a conventional Fabry–Perot laser diode (FPLD) injection locked by two highly coherent laser sources was used to generate an MMW carrier with phase correlation . The phase difference between two injection sources can be suppressed to enhance both the dual-wavelength optical power and beat MMW carrier power by utilizing the high cavity resonance of the FPLD. Although injection locking can slightly decrease the phase difference between two free-running carriers, it is not easily achieved in a typical FPLD, and a weak-resonant alternative must be considered [19–21]. Besides, the beat MMW carriers remain unstable in the absence of a phase-locking system. The alternative approach is to externally modulate a highly coherent LD by using a microwave synthesizer  at half the frequency of the desired MMW carrier, which outputs a double sideband (DSB) dual-wavelength carrier with a residual central carrier that affects the purity of the generated MMW carrier. Central carrier suppression requires an additional interference scheme with a delay line  or an interleaver , which inevitably attenuates the modal power of the carrier-suppressed DSB (CS-DSB) carrier. An optical amplifier compensates the power loss but induces additional intensity noise, which degrades the signal-to-noise ratio (SNR) of beat MMW carriers. When employing the CS-DSB optical carrier for MMWoF transmission, the data must be modulated electronically through up-frequency mixing  or optically through external modulation .
On the other hand, the fiber based chromatic dispersion also affects the dual-wavelength modulated CS-DSB optical carriers after single-mode fiber (SMF) transmission, which induces severe waveform distortion and degrades the data receiving and decoding performances. In particular, when the phase difference between two CS-DSB optical carriers becomes π after long distance propagation in single-mode fiber, a destructive interference could be induced due to the slight difference on propagation constant and dispersion coefficient between two carriers at deviated wavelengths. This would attenuate the heterodyne beating MMW carrier power and degrade the delivered data amplitude , which needs to be solved by acquiring a cost-ineffective dispersion compensating element in the MMW-RoF system. To avoid such a problem, versatile single-sideband (SSB) modulation techniques, such as parallel electro-optic modulation , cross-gain modulation, cross-polarization modulation  and dual-mode injection-locked distributed feedback laser diode (DFBLD)  have been successively proposed as alternatives for reducing dispersion-induced distortion in MMWoF system. However, these dispersion compensating solutions inevitably require specific and high-cost components to increase the cost of the MMW-RoF system.
In this work, an MMW-RoF architecture based on a parallel/orthogonally polarized dual-wavelength injection-locked colorless LD for converging wireline and wireless transmissions was demonstrated. The dual-wavelength polarizations were orthogonally adjusted for mitigating the chromatic dispersion and power fading effect, which helps to improve the transmissions of 24-Gbit/s 64-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) for wireline and 4-Gibt/s 16-QAM OFDM for wireless. The intensity of four-wave-mixing (FWM) sidemodes was characterized, and the wavelength-spacing dependent transmission quality was investigated. For wireline transmission, the back-to-back (BtB) and 25-km SMF transmission performances of the 64-QAM OFDM data such as SNR, error vector magnitude (EVM) and bit error rate (BER) were analyzed. After remote self-beating at user end, the peak power and carrier-to-noise ratio (CNR) of the self-beat MMW carriers from the dual-wavelength optical carriers were analyzed. For the forward error correction (FEC) requirement, the maximal wireless distance of the 16-QAM OFDM data carried by the MMW carrier was obtained. After 25-km SMF and 1.6-m free-space transmissions, the power fading effect on the transmitted data was found to be minimized. Moreover, crucial parameters, such as SNR, EVM and BER, of wireless transmission in both systems were compared.
2. Experimental setup
Twe types of dual-wavelength optical carriers for MMWoF transmission, namely dual-wavelength injection-locking slave colorless LD [31, 32] with (A) parallel polarized CS-DSB master and (B) orthogonally polarized CS-DSB master, were demonstrated. The detailed operation of each structure is as follows.
2.1 Dual-wavelength injection-locking slave colorless LD with parallel polarized CS-DSB
The schematic architecture of the parallel polarized dual-wavelength CS-DSB carrier injection-locked colorless LD for both wireline and wireless transmissions is illustrated in Fig. 2. At the central office, the dual-wavelength master was constructed using a DFBLD and null-point biased MZM with a Vπ of 4.2 V; the DFBLD was driven by a DC current source of 12 mA, and the MZM was driven by a local oscillator (LO) signal with a power of 22.5 dBm and frequency of 17.5 GHz. To obtain sufficient injection power, the dual-wavelength master was amplified by employing an Erbium-doped fiber amplifier. In addition, an optical bandpass filter (OBPF, JDSU-TB9) with a 3-dB linewidth of 0.46 nm was used for suppressing the amplified spontaneous emission noise induced during optical amplification. A 64-QAM OFDM with 86-subcarriers covering a 4-GHz bandwidth for delivering 24-Gbit/s raw data rate and a 16-QAM OFDM with 22 subcarriers covering 1-GHz bandwidth with a row data rate of 4-Gbit/s were employed for both wireline and wireless transmissions through a 24-GSa/s arbitrary waveform generator (AWG, 70001A), respectively. The FFT size and cyclic prefix of used QAM-OFDM data were 512 and 1/32, respectively.
After power amplification by using an electrical pre-amplifier (Picosecond, Model 5828A), the amplified OFDM data was directly modulated onto the slave colorless LD injection locked by the dual-wavelength master . The slave colorless LD with 1% front-facet reflectance and 900-μm cavity length exhibited a broadened laser mode spectrum of approximately 25 nm and a considerable amount of dense longitudinal modes with a mode spacing of 0.373-nm . During the experiment, the colorless LD was biased at 54 mA (2.5 times of its Ith) to facilitate high-power and high-coherence operation. For wireline transmission after 25-km SMF, the dual-wavelength injection-locked slave colorless LD with encoded 64-QAM OFDM data was directly received using a photodetector (Nortel, PP-10G), and its waveform was instantly retrieved using a digital signal analyzer (DSA, Tektronix, 71640C). For wireless transmission in free space, the remotely beat 35-GHz MMW carrier with the up-converted 16-QAM OFDM data was received using a high-speed photodetector (New Focus, 1014), which was then amplified using an electrical amplifier (Quinstar, QLW-24403336) and was delivered in free space through a pair of antennas (A-INFO, LB-22-20). Finally, the 16-QAM OFDM data waveform in the time domain was retrieved using the same DSA after a frequency down-conversion by using a balanced mixer (Quinstar, QMB-FBFBAS) and a post amplification by using a low-noise amplifier (LNA, New Focus, 1422).
2.2 Dual-wavelength injection-locking slave colorless LD with orthogonally polarized CS-DSB
The slave colorless LD injection-locked by the parallel polarized dual-wavelength CS-DSB carrier exhibited a marked FWM effect, which generated pedestal modes beneath both sides of the dual-wavelength carriers carrying the delivered OFDM data. After 25-km SMF, the OFDM data was delivered from the dual-wavelength carriers, and FWM sidemodes inevitably interfered with each other to cause the OFDM waveform distortion in the time domain.
Figure 3 presents the schematic architecture of the slave colorless LD injection-locked by the orthogonally polarized dual-wavelength master to suppress the FWM effect for improving the transmission performance of the dual-wavelength optical carrier. With orthogonal polarization injection, only one of the injected modes (TE mode) was supported in the colorless LD so that the FWM effect induced by the dual modes can be suppressed for improving the transmission performance. This resulted in a single-mode slave, and part of the orthogonally polarized dual-wavelength master was coupled into the PON network for the dual-wavelength beating to generate the MMW carrier at the ONU part. The same CS-DSB source was used as the dual-wavelength master carrier. However, the dual modes were transferred from mutually parallel polarization to mutually orthogonal polarization, which is approached by applying a delay interferometer (DI, Optoplex-10G) with mode spacing and 3-dB linewidth of 10 GHz and 0.041 nm, respectively. After separating two modes from the dual-wavelength master with the DI, a polarization beam combiner (PBC) was inserted for combining these two modes with controlled orthogonal polarization. After injection-locking the colorless LD with the orthogonally polarized dual-wavelength master, the single-longitudinal-mode output was available by exploiting the TE-mode selectivity of the colorless LD. To achieve the remotely dual-wavelength beating for wireless transmission at the ONU part, the master source was first separated into the injection and reference components by using an optical coupler (OC). The single-mode colorless LD output was combined with the reference master to form the dual-wavelength output with another OC for delivering through 25-km SMF. The same receiving and decoding procedures were employed for wireline and wireless transmissions.
Figure 4 shows the polarization dependency between the dual-wavelength CS-DSB master and the injection-locked slave colorless LD. Initially, the dual-wavelength master with mutually parallel polarization is delivered by a DSB modulating a single-mode optical carrier with a null-point driven MZM at a sinusoidal-wave LO signal frequency of f0/2. The MZM is operated at its null point to suppress the central-wavelength carrier.
The mode spacing of the sinusoidally modulated DSB at λ1 and λ2 is therefore f0, as shown in Fig. 4(a). To create mutually orthogonal polarization between λ1 and λ2, a DI is employed for splitting λ1 and λ2, and the polarizations between the two longitudinal modes are adjusted using individual PCs before recombining them into a PBC for ensuring mutually orthogonal polarization, as shown in Fig. 4(b). The dual-wavelength master is divided into two components: one was used as an injection-locking source and the other as the reference source. After injection-locking by using a PC, the slave LD is supported by only one of the injected dual modes (λ1), which enables the preferred polarization of the colorless LD. Another mode (λ2) is absorbed after entering into the waveguide of the colorless LD because its polarization is orthogonal to that of the preferred mode, as illustrated in Fig. 4(c). This considerably suppresses the dual-wavelength lasing induced FWM effect in the colorless LD. To remotely beat for generating a MMW carrier by a dual-wavelength carrier, the dual-wavelength master reference and single-mode injection-locked slave are combined using an OC prior to the transmission through SMF. A λ1 component persists in the reference master; this component overlaps that from the injection-locked colorless LD. To avoid crosstalk between these λ1 components, their relative polarization is set orthogonal by using another PC, as shown in Fig. 4(d). The polarizations of the λ1 from the injection-locked slave and λ2 from the reference master are mutually parallel and can directly beat to deliver a MMW carrier for wireless applications in the remote node.
4. Results and discussions
4.1 Conventional dual-wavelength injection-locking slave colorless LD with parallel polarized CS-DSB
To realize the injection-locking throughput of the dual-wavelength colorless LD slave, the mode space between two modes from the CS-DSB master was detuned for characterization. In this study, the short-wavelength master was fixed to injection-lock the selected principle mode of the colorless LD slave, whereas the long wavelength master was detuned to analyze the induced FWM strength under mode-matched or mode-deviated injection-locking condition. First, the power of the resonant FWM modes in the injection-locked colorless LD was plotted as a function of dual-wavelength spacing, as shown in Fig. 5(a). The FWM suppression ratio is defined as the power ratio of the injection-locked mode to the short-/long-wavelength FWM mode. At the master, with its wavelength-spacing coincident with the slave mode-matched condition (Δλ = 0.373 nm), the dual-wavelength resulted in a marked FWM effect [35–37]. By slightly adjusting the master wavelength-spacing, the FWM suppression ratio was weakened, which weakened the slave FWM modes. When adjusting the mode spacing from 0.373 to 0.32 nm, the lowest pedestal FWM modal powers of −26.6–−41 dBm were observed for the corresponding FWM suppression ratios of 23.7–38 dB, as shown in Fig. 5(b). The peak FWM powers increased because of their resonant amplification in the colorless LD cavity, which considerably degraded the BtB transmitted BER of the carried 24-Gbit/s 64-QAM OFDM data and the peak power of the remotely beat MMW carrier, as shown in Fig. 5(c).
By appropriately increasing the master mode spacing, the transmitted BER was considerably improved by more than five orders of magnitude because the FWM modes were suppressed. Furthermore, such a mode-deviated operation not only increases the frequency of the beat MMW carrier but also increases its RF power. When the master mode spacing was increased to more than 0.2 nm to provide a beat MMW carrier frequency higher than 24 GHz, the dual-wavelength optical transmitted baseband QAM-OFDM BER was even lower than that in the single-mode injection case with a BER of 8.4 × 10−6. The second injection-locking mode can help suppress spontaneous emission from the injected colorless LD for carrying the QAM-OFDM data with larger modulation depth, which further suppresses the relative intensity noise (RIN) and thus enhances the SNR of the QAM-OFDM data and reduces its BER. At an optimized mode spacing of 0.32 nm, the transmitted BER and RF power of the beat MMW carrier can be improved to 1.3 × 10−6 and −43 dBm, respectively. Further increasing the master mode spacing to 0.373 nm to coincide with the slave mode spacing inevitably enlarged the FWM modes to conversely degrade the transmitted BER as well as attenuate the beat MMW carrier power. The master mode spacing of 0.2–0.36 nm with the corresponding beat MMW carrier frequency of 24–47 GHz is a tolerant range for detuning the dual-wavelength output of the slave colorless LD under master injection locking and outperforms the same device operating in single-mode injection locking. Because a typical single-mode carrier from the semiconductor LD possesses a nearly degenerate FWM, the generated FWM carrier usually exhibits a conjugated phase because of the nearly degenerate FWM effect compared with the original carrier [38, 39]. In addition, the remotely beat MMW carrier from the dual-wavelength main carriers is also phase conjugating with each other. Therefore, the modulated QAM-OFDM data carried by the original and dual-wavelength induced FWM carrier were relatively conjugating, resulting in power fading between the QAM-OFDM data of the original and FWM carriers after distant transmission. Therefore, the induced FWM carrier easily degraded the BER performance of the dual-wavelength injection-locked colorless LD in not only the baseband but also the MMW wireless transmission.
4.2 Comparison of BtB transmission performances of dual-wavelength orthogonally polarized optical carrier with or without residual injection reference
The schematic of the polarization evolutions before and after combining the injected slave and reference master in the orthogonally polarized CS-DSB-based system are illustrated in Fig. 6(a). Both the injected slave and reference master provided the short-wavelength mode (λ1) and combined with each other during relative detuned parallel or orthogonal polarization. Without proper polarization control, a crosstalk occurred after coupling because the λ1 components from the injected slave and reference master completely overlapped in the case of mutually parallel polarization. The effect of relative polarization orientation between λ1 components on the RIN spectra is plotted and both parallel and orthogonal polarization conditions are compared in the right-hand side of Fig. 6(b).
As shown in Fig. 6(b), severe intensity noise was observed in the case of orthogonally polarized dual-wavelength injection-locking (λ1, ref. // λ1, inj.). This considerably degraded the SNR after remotely self-beating the injected and transmitted CS-DSB dual-wavelength carriers. The intensity noise can be suppressed after adjusting the relative polarization between the reference master and the injected slave under the orthogonal polarization condition (λ1, ref.⊥λ1, inj.). The RIN level in a low frequency region was considerably increased by only 1–1.5 dB from DC to 2 GHz after coupling with the reference master.
The transmission performance of the 24-Gbit/s 64-QAM OFDM data modulated on one mode of the orthogonally polarized dual-wavelength optical carrier with different polarizations of the injected slave and the reference master is illustrated in Fig. 7(a). In the case of parallel injection-locking, the average SNR and EVM were 25.8 dB and 5.1% with a BER of 1.3 × 10−6, respectively. In the case of orthogonally polarized CS-DSB injection locking (λ1, ref. // λ1, inj.), the average SNR, EVM, and BER were degraded to 21.9 dB, 8.1%, and 2 × 10−3, respectively, because the severe intensity noise distorted the QAM OFDM data. Because of the decreased intensity noise in the orthogonally polarized dual-wavelength injection-locking case (λ1, ref.⊥λ1, inj.), the transmission performances of the QAM OFDM data can be improved by increasing the average SNR, EVM, and BER to 23.8 dB, 6.9% and 4.5 × 10−4, respectively. Moreover, the completely overlapped modes at λ1 from both the master and slave resulted in severe background noise from DC to 0.3 GHz after self-beating, as shown in the lower panel in Fig. 7(b). Such a cross-heterodyne intensity noise can be suppressed by implementing the orthogonal polarizations between the reference master and the injected slave at λ1. The cross-heterodyne intensity noise was considerably reduced by more than 10 dB, as shown in the upper panel of Fig. 7(b). Although the FWM sidemodes and mode-beating noise induced by the injected slave and reference master with parallel polarizations can be suppressed by setting their polarizations as orthogonal, a punished SNR decay by 2 dB was still observed because of the insertion loss of the added components. Furthermore, even if the two carriers at the same wavelength are mutually orthogonal, the RIN continues to increase to distort the OFDM data waveform modulated on the slave carrier. The environmentally induced residual polarization fluctuation reveals a high correlation with the enlarged RIN in the low-frequency region. Such SNR degradation must be improved by up-shifting the QAM OFDM data bandwidth from DC to >0.3 GHz.
4.3 Comparison of the wireline transmission performances of dual-wavelength coupled carrier with parallel and orthogonal polarizations
In parallel CS-DSB injection with the peak power of each master mode set at −3 dBm, as shown in Fig. 8(a), the central carrier suppression ratio of the master was 28 dB, and the peak powers of the short- and long-wavelength high-order modes (owing to nonlinear modulation) were −36 and −38 dBm, respectively. The short-wavelength main mode was set as the major injection-locking mode for the slave colorless LD, whereas the longer wavelength main mode with almost was adjusted for the deviated injection-locking case. The dynamic output of the LD in the mode-spacing mismatched master is shown in the right-side panel of Fig. 8(a). The side-mode suppression ratio was higher than 40 dB; however, the difference between the peak powers of the dual main modes was 1.6 dB because of the deviated mode-spacing master injection between the two longitudinal modes of the slave. With resonant amplification in the slave, the peak power of the short- and long-wavelength high-order modes increased to −29 and −37 dBm, respectively, because of the strong FWM effect in the resonant cavity of the slave colorless LD during dual-wavelength injection-locking. By contrast, the orthogonally polarized CS-DSB dual-wavelength carrier was employed to reduce the FWM effect in the resonant cavity of the colorless LD owing to its polarization selectivity (TE mode preferred lasing), as shown in the left-side panel of Fig. 8(b). Each main-mode peak power was −3 dBm and the high-order peak powers remained between −38.8 and −37.6 dBm. After injection locking, the peak powers of these higher-order modes were suppressed to −48 dBm, as shown in the right-side panel of Fig. 8(b) (red line). The slave LD functions as a mode selector for a single-mode laser such that the long-wavelength main mode is suppressed in the waveguide because of its improper polarization. This operation effectively degraded the resonant FWM efficiency in the slave colorless LD. After combining the injection-locked single-mode slave and the reference master through an OC, the dual-wavelength optical carrier was formed; its spectrum is shown in the right-side panel of Fig. 8(b) (blue line).
The peak power of the injection-locking mode was reduced by 3 dB because of the additional insertion loss from the OC. The transmission performance of the parallel and orthogonal CS-DSB master-based injections was compared using the SNR spectra and constellation plots of the 24-Gbit/s 64-QAM OFDM data with corresponding EVM and BER after BtB and 25-km SMF transmissions, as shown in Figs. 9(a) and 9(b). The parallel and orthogonally polarized dual-wavelength transmission results were compared at the same receiving power of −3.5 dBm. During the BtB transmission, the parallel polarized CS-DSB exhibited the highest performance of the carried 64-QAM OFDM data, including the highest subcarrier SNR ratio with the EVM of 5.4% and the BER of 1.5 × 10−5. During the orthogonally polarized dual-wavelength CS-DSB injection, the injection-locked slave LD carrier under the λ1, ref. // λ1, inj. condition delivered the 64-QAM OFDM data with its subcarrier SNR degraded by more than 5 dB, which corresponds to an enlarged EVM of 8.1% and a degraded BER of 2 × 10−3. Such degradation is attributed not only to the self-beating but also to the increased insertion loss of the inserted OC. When detuning to the λ1, ref.⊥λ1, inj. case, the injected slave LD carrier successfully transmitted the 64-QAM OFDM data with an intensity noise suppressed to <2 dB, providing a reduced EVM of 6.9% and an improved BER of 4.5 × 10−4.
After 25-km SMF transmission, severe SNR decay of the 64-QAM OFDM data carried by the parallel polarized CS-DSB injected system can be observed at high-frequencies. Because both subcarriers of the dual main modes carried the OFDM data and each mode received different time delays after SMF transmission by their different wavelengths, the received 64-QAM OFDM data combined from the parallel polarized dual-wavelength carrier would lead to power fading effect to decline its high-frequency subcarrier transmission performance . Moreover, the strong FWM modes in the parallel polarized CS-DSB injected system distorted the waveform of the modulated 64-QAM OFDM data, reducing the average SNR from 25.4 to 21.3 dB with corresponding EVM of 8.6% and BER of 3.4 × 10−3 after 25-km SMF transmission. The orthogonally polarized CS-DSB injected system contained the dual-wavelength optical carrier with only the short-wavelength main mode carrying the 64-QAM OFDM data. In addition, the high-order nonlinearly modulated modes with the CS-DSB master carrier were relatively immune to the modulation power redistribution as well as the OFDM data distortion. As expected, because of the weak FWM and NLM sidemodes accompanied with the orthogonally polarized CS-DSB injected system, the 64-QAM OFDM data carried can effectively restrict the chromatic dispersion and power fading effect after 25-km SMF transmission. The λ1, ref. // λ1, inj. case cannot be compared with the other case because it exhibited severe intensity noise and could not pass the FEC requirement after 25-km SMF transmission. In the λ1, ref.⊥λ1, inj. case, the average SNR of the received 64-QAM OFDM data carried by the orthogonally polarized dual-wavelength carrier decreases less than 1 dB with almost unaltered EVM of 8.8% and BER of 3.7 × 10−3 after transmission. Moreover, the two transmission systems were compared the decaying trend of the BER and related power penalty for the transmitted 24-Gbit/s 64-QAM OFDM data with increased receiving power after BtB and 25-km SMF transmissions, as plotted in Fig. 9(c). During the BtB transmission, the required receiving power sensitivities to meet the FEC requirement of 3.8 × 10−3 were −10.5 and −5.1 dBm for parallel and orthogonally polarized CS-DSB injected systems, respectively. After 25-km SMF transmission, the 64-QAM OFDM data delivered from the parallel polarized CS-DSB injected system revealed a strong power fading between the dual main modes to provide the highest power penalty of 6.9 dB for the FEC requirement. Conversely, the orthogonally polarized CS-DSB injected system with a suppressed high-order NLM modes and reduced FWM sidemodes for single-mode modulation can meet the demand of FEC required BER with the receiving power sensitivity higher than −3 dBm and power penalty reduced to less than 1.5 dB for BtB and 25-km transmissions.
4.4 Comparison of the wireless transmission performances of the MMW carrier remotely self-beat from the dual-wavelength optical carrier with parallel and orthogonal polarizations
The peak power of the injection-locked mode was set to −14 dBm for both the transmission systems for comparing the transmission performance of the MMW carrier by remotely self-beating the parallel and orthogonally polarized CS-DSB dual-wavelength optical carriers individually at the receiving part. After self-beating in a high-speed photodetector, the RF spectrum of the beat MMW carrier for each system is illustrated in Fig. 10(a). The measured peak powers of the beat MMW carrier from the parallel- and orthogonal-polarization CS-DSB injected carriers were −51.3 and −50.2 dBm with CNRs of 33.3 and 27 dB, respectively. During parallel-polarization CS-DSB injection locking, the peak power of the long-wavelength main mode reference was lower than that of the short-wavelength injection-locked mode because the long-wavelength main mode was located away from the resonant mode of the slave colorless LD. In addition, the generated strong FWM sidemodes were conjugated in phase with the dual main modes.
The weakest peak power of the remotely beat MMW carrier was observed in the parallel-polarization CS-DSB injected system and can be attributed to the aforementioned two drawbacks. In the orthogonal-polarization CS-DSB injected system, a residual phase noise within the offset frequency ranged from 0 to 4 MHz around the central frequency of 35 GHz is produced after self-beating, because the variation in environmental conditions during remote self-beating resulted in residual frequency and phase fluctuations, which cannot be suppressed during a short-term operation. To avoid the QAM-OFDM data waveform distortion and SNR degradation induced by destructive interference from the fluctuated phase noise, the frequency to spectrally allocate the OFDM subcarrier in the frequency domain was initiated from 300 MHz. This arrangement effectively precludes possibilities of distorting the OFDM data waveform to preserve the beating efficiency and the receiving SNR after 25-km SMF transmission.
After transmitting for 0.2 m in free space and down-conversion frequency mixing, the spectra of the 4-Gbit/s 16-QAM OFDM delivered by the remotely beat MMW carrier from the parallel/orthogonally polarized dual-wavelength optical carrier were compared, as shown in Fig. 10(b). The peak power of all the OFDM subcarriers were improved by 5 dB when using the MMW carrier self-beating from the orthogonally polarized dual-wavelength optical carrier, which ensures the enhancement of the average SNR of the OFDM data by 1.2 dB at least. Figure 11(a) displays the received BER performance of the 4-Gbit/s 16-QAM OFDM down-converted from the 35-GHz MMW carrier by propagating through different wireless distances after 25-km optical transmission in SMF. In the parallel-polarization CS-DSB injected system, both the dual main modes carried the OFDM data, which suffered from different phase delays each other after 25-km SMF transmission to distort the passband QAM OFDM data after it was received by the photodetector. In comparison, the dual-wavelength carrier with only one mode carrying the QAM OFDM data minimized such distortion by using the orthogonal-polarization CS-DSB injected system. Although the MMW carrier remotely self-beat from the orthogonally polarized dual-wavelength optical carrier produced low intensity noise after remote self-beating, such an optical carrier still exhibited the best MMW wireless transmission performance for 4-Gbit/s 16-QAM OFDM data at a wireless distance of 1.6 m in free space. After 25-km SMF and 1.6-m free-space transmissions with the remotely self-beat MMW carrier at 35 GHz, the SNR spectrum and constellation plot of the received 4-Gbit/s 16-QAM OFDM data modulated on one mode with the corresponding EVM and BER for the dual-wavelength optical carriers during parallel/orthogonal polarization were analyzed, as shown in Fig. 11(b). Because of the strong FWM effect that caused severe power fading when delivering the QAM OFDM data by the parallel-polarized dual-wavelength optical carrier, the lowest average SNR of 20.9 dB accompanied with the largest EVM and highest BER of 17% and 3.2 × 10−3 was obtained. As expected, the highest SNR with the smallest EVM and lowest BER of 15.5% and 1.4 × 10−3, respectively, was obtained from the orthogonally polarized dual-wavelength optical carrier because the suppressed FWM and NLM effects provide the highest beating efficiency and single-mode modulation for attenuating the chromatic dispersion during optical transmission.
In this approach, the power pre-leveling technology which individually adjusts the peak amplitude of OFDM subcarrier can improve the SNR for the data delivered by different subcarriers. The basic principle of power pre-leveling is very similar to that of power loading. The power loading technology reallocates the peak powers of OFDM subcarriers to compensate the degraded power and SNR of data delivered by OFDM subcarriers [41, 42]. The only difference in between is that the power pre-leveling simply employs a specific curve for compensating the power degradation, which increases the high-frequency subcarrier power by sacrificing low-frequency subcarrier power. Alternatively, the bit loading technology can also improve the systematic performance by individually reallocating the QAM-level of each OFDM subcarrier for adapted modulation [43, 44]. Since the difference of the SNR criterion between adjacent QAM-levels is about 3 dB, the bit loading would be more efficient for an OFDM transmission with throughput SNR deviation of larger than 3 dB. In view of SNR spectra from Figs. 7(a) and 11(b), the bit loading technology may not play an important role on enhancing the transmission performance, as both wireline and wireless transmissions exhibit the subcarrier SNR fluctuations below 3 dB. The parametric comparison on wireless transmission performance among several distinguished works using SSB modulation method is listed in Table 1.
Since different data formats were employed in different works, it is unfair to directly compare the transmitted BER owing to their different SNR criterions. In contrast to this work, most approaches require additional modulator for data encoding [29, 45, 46]. The external modulator enables the frequency chirp suppression at increasing systematic cost. Among these approaches, the dual-mode DFBLD was proposed as a cost-effective scheme for achieving data encoding and optical amplification concurrently . Nevertheless, the highly coherent DFBLD can hardly be injection-locked for dual-wavelength generation. That is why the proposed colorless LD with weak resonance can be the potential alternative for the remote heterodyne MMW-RoF system when comparing with others.
In addition, the comparison between published and current works on the MMWoF transmission performance after 25-km SMF transmission is listed in Table 2. For our previous work with a parallel polarized dual-mode CS-DSB master carrier , the wireless communication after 25-km SMF transmission was not implemented then. In addition, the data distributed on two transmitted optical carriers at different wavelengths suffer from chromatic dispersion to degrade the data receiving quality after SMF transmission. Moreover, the inevitably generated FWM modes also decrease the transmitted SNR. Another previous work with parallel polarized master-to-slave (M-to-S) injection  reveal allowable wireless data rate of only 2 Gbit/s after 25-km SMF and 1.6-m wireless transmissions. This method also suffers from the same problem with the CS-DSB injection method. By employing the orthogonally polarized dual-mode CS-DSB master carrier in this work, the colorless LD becomes the best candidate to avoid aforementioned problems in previous works. Therefore, the 4-Gbit/s 16-QAM OFDM data can successfully be delivered after 25-km SMF and 1.6-m wireless transmission.
The proposed MMW-RoF is capable of handling either the 5G mobile or the indoor MMW wireless network after passing through the metropolitan optical network. Under the specified data rate for indoor wireless network of 1 Gbit/s or larger , the proposed approach is able to meet the required transmission distance of up to 3 m. However, the currently available wireless transmission within 1.6 m is somewhat inadequate for the requested distance of 30-200 m defined by the 5G MMW wireless network environment with [9–11]. In this approach, the allowable wireless distance of 1.6 m is mainly limited by the insufficient baseband receiving power, the relatively broadened modal linewidth of two carriers, and the residual central mode originated from injection master. These factors affect the throughput power of heterodyne beating MMW carrier, the background noise and SNR of the delivered data-stream, and the waveform distortion of the received data symbol by group velocity dispersion. To lengthen the transmission distance, the additional optical amplifier can considerably be added for enhancing the baseband receiving power, the dual-wavelength optical transmitter with reduced modal linewidth should be employed, and the residual central mode must be suppressed or entirely eliminated for least dispersion and distortion during transmission. With these mandatory improvements, either the longer distance or the larger capacity data transmission can be enabled to meet the demand of 5G wireless networks.
By injection-locking a slave colorless LD with a dual-wavelength master carrier at parallel or orthogonal polarization for the use of remote self-beating MMW generation, a novel MMW-RoF link with suppressed NLM and FWM effects is demonstrated for converging the wireline and wireless transmission access services. By dual-wavelength injection-locking the slave colorless LD with parallel polarized dual-wavelength CS-DSB master carrier, the optical baseband BER of 24-Gbit/s 64-QAM OFDM can marginally meet the FEC at 3.4 × 10−3 after 25-km SMF transmission. With a mode spacing set as 0.32 nm between two wavelengths, the MMW carrier with RF power of −43 dBm at 35 GHz can be generated after remotely self-beating. In contrast, the reference dual-wavelength master with orthogonally polarized dual wavelengths only supports single-mode carrier lasing in the injected slave LD carriers. Such an injected single-carrier encoding and coupled dual-carrier transmission with orthogonal polarization effectively suppresses the cross-heterodyne mode-beating intensity noise over 10 dB, and attenuates the NLM and FWM sidemodes by more than 15 dB during injection locking and fiber transmission. This design facilitates the single-mode direct modulation of the better 64-QAM OFDM transmission than that obtained with parallel polarization case. Under BtB transmission with the dual-wavelength orthogonally polarized CS-DSB injection, dual-wavelength carrier under the λ1, ref.⊥λ1, inj. condition and single-carrier modulation essentially improves the EVM, SNR and BER of the 64-QAM OFDM data at 24 Gbit/s from 8.1% to 6.9%, from 21.9 dB to 23.8 dB and from 2 × 10−3 to 4.5 × 10−4, respectively. For wireline transmission through 25-km SMF, the orthogonal CS-DSB injected dual-wavelength carrier exhibits superiorities including suppressed high-order NLM modes, reduced FWM sidemodes, and the least chromatic dispersion owing to single-carrier encoding, which only degrades the EVM to 8.8% and BER to 3.7 × 10−3 with minimized power penalty of <1.5 dB when comparing with the BtB transmission case. For wireless transmission, the 35-GHz MMW carrier self-beat by employing the orthogonally polarized dual-wavelength optical carrier effectively enlarges the peak power of all OFDM subcarriers by 5 dB at least, which guarantees the enhancement on average SNR of received data by more than 1.2 dB. After passing through 25-km SMF and 1.6-m free-space transmission, the orthogonally polarized dual-wavelength optical carrier with single-mode encoding scheme provides the lowest NLM and FWM sidemodes and the highest self-beating MMW power efficiency. Such an optimized dual-wavelength carrier self-beats itself to remotely generate the MMW carrier for wireless transmitting the 4-Gbit/s 16-QAM OFDM data with the smallest EVM of 15.5%, the highest SNR of 16.3 dB and the lowest BER of 1.4 × 10−3.
This work was supported by the Ministry of Science and Technology, Taiwan, R.O.C., under grants MOST 104-2221-E-002-117-MY3, MOST 103-2221-E-002-042-MY3, MOST 103-2218-E-002-017-MY3 and MOST 104-2218-E-005-004-, and Excellent Research Projects of National Taiwan University under grants NTU-ERP-105R89081.
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