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Millimeter-wave band signals have the advantage of a high capacity and have been used in fifth-generation (5G) mobile services. An analog radio over multi-mode fiber (A-RoMMF) employing a directly modulated vertical cavity surface emitting laser is a promising technology for relaying millimeter-wave band signals to its dead area because of the advantages of low latency, low cost, and low power consumption. One of the applications of A-RoMMF is a cascaded intermediate-frequency-over-fiber (IFoF)-based centralized radio access network (C-RAN) mobile fronthaul (MFH) system. We conducted two studies on A-RoMMF to extend the transmission distance and bandwidth to adapt it to the system. First, we evaluated the effect of employing a single-mode vertical cavity surface emitting laser (VCSEL) and a multi-mode VCSEL on the transmission characteristics of A-RoMMF. Next, we evaluated the transmission characteristics of A-RoMMF using a cost-efficient bias-tee consisting of an electrical circuit pattern, the frequency response of which has a slope inverse to that of A-RoMMF at the 28-GHz band, to suppress the channel power difference of two component carrier signals for a wide-bandwidth signal transmission. We conducted these evaluations using A-RoMMF itself and herein demonstrate its adaption to a cascaded IFoF-based C-RAN MFH system at the 28-GHz band. When employing it with a 200-m long multi-mode fiber, the system achieves 400-MHz/CC and two component carrier signal transmissions.

This paper reports an end-to-end demonstration of a fiber-wireless mobile fronthaul (MFH) network based on hybrid intermediate-frequency-over-fiber (IFoF) and radio-over-fiber (RoF) links using a 28-GHz millimeter-wave (MMW) band. We successfully transmitted 24 64-QAM OFDM signals with a total bandwidth of 9.12 GHz over 20-km broadband IFoF, 1-km narrowband IFoF, 500-m RoF, and 10-m free-space links with an aggregate capacity of up to 34.2 Gbps, which complies with the required peak data capacity of IMT-2020. A digital demultiplexer was exploited to flexibly perform channel selection and frequency conversion. The obtained results show the feasibility of implementing fiber-wireless systems with the hybrid IFoF/RoF links for future mobile fronthaul networks.

We present an intermediate frequency-over-fiber (IFoF) transmission system that uses cascaded connections between a broadband link and multiple narrowband links for future mobile fronthaul (MFH). The downlink MFH system employs analog and digital signal processing after broadband and narrowband IFoF transmissions, respectively, for extractions and frequency conversions of IF signals. The bandwidth of the optical components for the downlink MFH system is investigated and compared with that for the subcarrier multiplexed passive optical network (SCM-PON). The results support the satisfactory performance of the MFH system for bandwidth reduction. Two types of our developed digital signal processors for the MFH system are also described in detail: One is for an output of a single radio frequency (RF) stream, whereas the other is for simultaneous outputs of multiple RF streams. In addition, using either of the digital signal processors, we experimentally demonstrate the downlink MFH system's transmission of 64-QAM filtered OFDM (f-OFDM) signals with 360-MHz signal bandwidth and 3.6-MHz guard bands between adjacent IF signals and 64-QAM OFDM signals with 380.16-MHz signal bandwidth and 19.84-MHz guard bands between IF signals. In both experiments, eighteen IF signals are successfully transmitted over 20-km and 1-km fibers, which includes analog and real-time digital signal processing for IF channel extractions and frequency conversions. The results obtained in this study demonstrate the potential of cascaded IFoF links using hybrid signal processing for 5G and future MFH systems.

Analog optical transmission techniques such as radio-over-fiber (RoF) and multiple intermediate-frequency-over-fiber (multi-IFoF) have attracted a great deal of attention for use in 5G mobile fronthaul links. Recently, an IFoF transmission with a record CPRI-equivalent capacity of 1.53 Tb/s has been demonstrated by using the Kramers-Kronig (KK) receiver. However, one major issue associated with the KK receiver is its computational complexity. Also, since the minimum phase condition must be satisfied, the optimal carrier-to-signal power ratio (CSPR) should be large enough; however, this results in a low receiver sensitivity. To address these issues, we have recently proposed and demonstrated a signal-to-signal beat interference (SSBI)-free direct-detection (DD) system employing phase modulation (PM). Since a PM signal has a constant amplitude, its SSBI term becomes just a direct-current (DC) component. Therefore, the SSBI can be perfectly removed using a DC block without resorting to complicated digital-signal-processing (DSP)-based schemes such as the KK algorithm. In this article, we describe the principles of operation of the SSBI-free PM system in detail. We present a numerical study of performance comparisons between the SSBI-free and KK systems. The result reveals that the optimal CSPR is always 0 dB in the SSBI-free system, while it depends on optical-signal-to-noise-ratio (OSNR) levels in the KK system. The dispersion tolerance of the SSBI-free system is also investigated, showing that the SSBI-free system can support a 2-GHz 1024QAM-signal delivery in a typical deployment scenario of mobile fronthaul links. Finally, we present an experimental demonstration of a 2-GHz 1024QAM OFDM transmission with the SSBI-free system. To the best of our knowledge, this is the first demonstration of 1024QAM DD-based transmission for analog mobile fronthaul links.

Thermal-dependent transmission performance and long-term stability of a single-transverse-mode (SM) vertical-cavity surface-emitting laser (VCSEL) with directly pre-leveled 16-quadrature amplitude modulation orthogonal frequency-division multiplexing (16-QAM OFDM) data over 100 m OM4 multi-mode-fiber (MMF) are demonstrated. Although narrowing the oxide-confined aperture to 3 μ m enables the SM VCSEL with low threshold current of 0.21 mA and high modulation bandwidth of 21.4 GHz, this technology also enlarges the insulating oxide area to accumulate the heat for raising core temperature. This induces a harsh environment as the core temperature persistently increases without dissipation even under the water-cooling control at 20 °C. As a result, the SM VCSEL can only guarantee a stable operation for 1.5 h under the encoding of QAM-OFDM data for transmission. After optimization, the SM VCSEL successfully supports a back-to-back transmission capacity as high as 96 Gbit/s. With the assistance of OFDM pre-leveling technique, the 96-Gbit/s QAM-OFDM data transmission is qualified over 100 m OM4 MMF with a receiving power penalty as low as 1.3 dB.

An IF-over-fiber (IFoF)-based analog transport technology for mobile fronthaul applications has recently attracted significant attention. However, most previous studies have employed discrete optical components. For the analog transport technology to be a more cost-effective and power-efficient solution, it is necessary to utilize existing integrated optical transceivers. In this paper, we demonstrate IFoF transmission using a commercial off-the-shelf transmitter optical sub-assembly (TOSA). Although the TOSA was developed for a digital system employing non-return-to-zero (NRZ) signals, we show that it is also possible for the TOSA to support high-capacity analog transmission. As a demonstration, by using the TOSA, we could successfully transmit 64- and 256-ary quadrature-amplitude-modulated (64/256QAM) orthogonal-frequency-division-multiplexed (OFDM) signals with net bit rates of 54.74 and 36.49 Gbps per wavelength, respectively. Since the TOSA has four wavelength channels, the total capacities are 218.94 and 145.98 Gbps, respectively. To the best of our knowledge, these rates are the highest among all the demonstrations using analog transport technology.

This paper describes the potential of analog radio-over-fiber (RoF) and intermediate-frequency-over-fiber (IFoF) transmission technology for the future mobile fronthaul (MFH). The background and approach of the analog RoF-based MFH combined with the beamforming technique are presented. We developed an antenna module by integrating sixteen sets of cascaded photodiode, RF amplifier and a patch antenna as a compact wireless transmitter. Three millimeter-wave (MMW) beams were successfully transmitted from the antenna module with beam separation of 30° by photonic controlled beam steering. The experimental results are presented to show the feasibility of a multi-user multiple-input multiple-output (MIMO) scenario.

This paper first reviews several trial demonstrations during these several years, in which A-RoF and related transmission schemes are applied to 5G mobile communication system. Based on the results, the scalability toward Beyond-5G are discussed.

We propose a novel scheme to generate a wideband PM signal using a photonic Armstrong method, accomplishing both bandwidth expansion and the total elimination of SSBI simultaneously. We experimentally confirm the validity of the concept and demonstrate a 60-GHz-class PM signal generation.

An end-to-end mobile fronthaul system based on hybrid IFoF and analogue Radio-over-fibre/Radio-over-multi-mode-fibre (RoF/RoMMF) links is demonstrated for the first time. With the capacity of 25.65 Gbps, the transmission link complies the required peak data rate of 5G vision of IMT-2020.

We experimentally demonstrate a multi-beam steering system using an optical phase control for a multi-user-MIMO (MU-MIMO) scenario in 5G mobile system. We successfully transmitted three 380-MHz wireless OFDM signals at 28 GHz simultaneously with 30° beam separation over a free space of 5m.

RF signal phase shift method is successfully demonstrated in radio-over-fiber system. Radio wave signal is transmitted with single side band modulation. Optical phases of the optical carrier and the sideband are controlled in planar lightwave circuit, thus attaining RF signal phase shift.

We evaluate and demonstrate radio over fiber (RoF) signal distribution, which employs cost efficient analog radio over multi-mode fiber (A-RoMMF) sub-system with directly modulated single mode vertical cavity surface emitting laser (VCSEL) and multi-mode VCSEL, for cascaded intermediate-frequency-over fiber (IFoF) based centralized radio access network (C-RAN) mobile fronthaul of 28 GHz band.

This paper describes the background, motivation, approach and experimental results of our research and development of mobile fronthaul based on analog radio-over-fiber and related optical transmission technique. Future evolution scenario toward Beyond-5G era is also discussed.

We demonstrate an IFoF transmission using a commercial EML TOSA for the first time. We successfully transmitted 40×400-MHz 256QAM OFDM signals by using the TOSA with a record net bit rate of 128 Gb/s.

A 2-D millimeter-wave beamforming system is demonstrated by using a developed compact 4×4 antenna module integrated with photodiodes and RF chains. We successfully transmitted a 400-MHz 64-QAM signal over a free space of 5 m.

Recently, GaN-based light-emitting diodes (LEDs) find their applications in, other than general lighting, visible light communication (VLC) for high-speed wireless data networks. The pursuit for more bandwidth from the LED can be realized by engineering the LED material structure and device design, or by modulation techniques. In this paper, we proposed GaN-based LEDs with the photonic crystal (PhC) structure for VLC applications. Contradictory to general thought that higher small frequency response translates to better large-signal performance, our results reveal that a PhC LED may not perform well at low data rate due to its relatively poorer signal-to-noise ratio (SNR). However, at higher data rates, the performance of PhC LEDs surpasses conventional LED structure. We showed data transmission capability of PhC LEDs with a bit rate up to 400 Mb/s using on–off keying modulation. To demonstrate the chip capability for large-signal transmission, without using preemphasis or postemphasis, VLC with 16-quadrature amplitude modulation orthogonal frequency-division multiplexing transmission capacity up to 2 Gbit/s is achieved with an error vector magnitude of 17.07%, SNR of 15.35 dB, and bit error rate of $3.3 \times {10}^{-3}$ . Our LEDs embedded with PhCs pass the forward error correction criterion. The results reveal the advantages of LEDs with PhC for achieving higher data rate transmission.

By using a 512-quadrature amplitude modulating-orthogonal frequency division multiplexing (512-QAM OFDM) data stream with universal filtered multicarrier (UFMC) process to directly encode a quasi-color-free laser diode (QCFLD) with dual-mode carrier, a long-reach 28-GHz millimeter-wave over fiber (MMWoF) link over 50-km single-mode fiber (SMF) is demonstrated for future fiber network with 5G wireless link. At the fiber-wired optical transmission stage, the dual-mode QCFLD can support raw data rates up to 54, 42, and 24 Gbit/s after 0, 25, and 50-km SMF transmissions. The data-rate degradation results from chromatic dispersion induced power fading effect. The 28-GHz MMW carrier optically heterodyned at remote node with a carrier-to-noise ratio of 53.5 dB is generated after 25-km SMF propagation, which achieves 6-m free-space transmission at 16 Gbit/s. Lengthening the SMF to 50 km still allows the MMWoF for wireless transmission at 16-Gbit/s over 5 m and 14-Gbit/s over 10 m. To favor the 5G wireless access with high data capacity, the UFMC processed QAM-OFDM was further adopted for noise suppression via sidelobe filtering operation. By lengthening the cyclic prefix and the filter window for the OFDM, the tradeoff between data quality and net-data-rate ratio without and with UFMC process is discussed. With UFMC, the MMWoF transmission capacity can deliver 3-GHz wide 32-QAM UFMC processed OFDM data at 15 Gbit/s over 50 km in SMF and 10 m in free space. Even within a limited subcarrier bandwidth as narrow as 140 MHz for practical 5G users, the 512-QAM UFMC data can be allocated to provide a raw data rate of 1.26 Gbit/s.

In this paper, the optical frequency response of gallium nitride-based blue micro-LEDs with different quantum well numbers in the active region is presented. Single quantum well (SQW) blue micro-LEDs have about 1.6 times higher modulation bandwidth ( $f_{-3\,\text {dB}}$ ) by comparison with triple quantum well blue micro-LEDs. The larger bandwidth enhancement of the SQW micro-LEDs can be attributed to more carriers stored in a quantum well leading to higher recombination rate and lower recombination lifetime. The high $f_{-3\,\text {dB}}$ up to 752 MHz is demonstrated in the SQW micro-LED with a light-emitting area of $10\,\,\mu \text {m} \times 10\,\,\mu \text{m}$ . It suggests that micro-LEDs designed with fewer quantum well numbers may have the potential for the application of visible light communication (VLC) technology.

Single-channel 50-Gbit/s 4-level pulse amplitude modulation (PAM-4) and 62-Gbit/s orthogonal frequency division multiplexed quadrature amplitude modulation (QAM-OFDM) direct encoding of an uncooled coarse wavelength division multiplexer (CWDM) 100-Gbit/s distributed feedback laser diodes (DFBLD) transmitter module at 1310-nm transmitter interdata center transmission over 10-km single-mode fiber (SMF) are demonstrated. To enable preleveled 16-QAM OFDM and PAM-4 data transmissions at 62 and 50 Gbit/s, the 1310-nm channel of the DFBLD CWDM transmitter provides modulation bandwidth of 15 GHz, side-mode suppression ratio of 48 dB, and relative intensity noise of −126 dBc/Hz. For encoding the PAM-4 data, the 1310-nm channel of the CWDM DFBLD 100-Gbit/s transmitter achieves transmission capacities of 58 and 50 Gbit/s after BtB and 10-km SMF transmissions, respectively. Instead of supporting the back-to-back 16-QAM OFDM with the transmission at 62 Gbit/s with forward error correction (FEC) certified BER of 3.3 × 10−3, such a CWDM 100-Gbit/s DFBLD transmitter module can still maintain its highest data rate capacity to indicate a receiving power penalty of 0.35 dB even after 10-km SMF propagation. This is mainly attributed to the selected channel wavelength of the narrow-linewidth DFBLD, which suffers from extremely low chromatic dispersion and RF fading effect.

By changing the transverse-mode spectral linewidth of vertical cavity surface emitting lasers (VCSELs) at 850 nm, the directly encoded four-level pulse amplitude modulation data transmission performance over 100-m-long OM4 multimode fiber (MMF) are demonstrated and compared. The multi-mode VCSEL chip with the largest aperture of $11~\mu \text{m}$ reveals the widest spectral linewidth and the highest optical power, but provides the smallest modulation bandwidth to support only 44- and 28-Gb/s data rates for back-to-back (BtB) and 100-m OM4 MMF transmission cases, respectively. By shrinking the aperture size to reduce transverse-mode number, the few-mode VCSEL with the strongest throughput power enables a BtB transmission capacity as high as 52 Gb/s. However, its modal dispersion induced after OM4 MMF transmission inevitably degrade the data rate to 32 Gb/s. In contrast, the single-mode VCSEL with the smallest aperture of $3~\mu \text{m}$ reveals the highest modulation bandwidth and negligible modal dispersion to show competitive BtB transmission capacity with that of the few-mode VCSEL. In particular, the single-mode VCSEL successfully achieves a data rate of 34 Gb/s with a power penalty as low as 1.4 dB, after 100-m OM4 MMF transmission.

Up to 18-Gbps direct encoding of blue laser diode (BLD) is demonstrated for free-space data transmission. By reshaping the orthogonal frequency multiplexed (16-QAM OFDM) stream with sidelobe filtering, the raw data rate expedites from 17.2 to 18.4 Gbps. Employing an ultrafast p-i-n photodiode with smaller active area diameter and lower noise equivalent power significantly enlarges the data rate by 1.6 Gbps or upgrades the signal-to-noise ratio (SNR) by 0.2 dB. Replacing the 80-mW BLD with the 120-mW one essentially increases the received SNR by 0.4 dB under enhanced modulation throughput. Reinforcing the beam collimation and collection by increasing the numerical aperture with a plano-convex hyper-hemispherical lens further improves the SNR by 0.6 dB. After optimization, the 16-QAM OFDM data with and without sidelobe filtering are respectively delivered at raw data rates of 16.4 and 18 Gbps with spectral-density usage efficiency as high as 4 bit/s/Hz over 16 m in free space, wherein the BLD carried QAM-OFDM data stream remains its capacity after reformation with sidelobe filtering as the superior inter-carrier-interference immunity reinforces.

An 850-nm multi-mode vertical cavity surface emitting laser (VCSEL) bare chip with high-indium-density InGaAs/AlGaAs quantum-well pairs is demonstrated for directly encoded QAM-OFDM transmission in multi-mode fiber (MMF). By directly encoding the 850-nm VCSEL bare chip with a pre-leveled 14-GHz 16-QAM OFDM data, >50-Gb/s transmission over 100-m-long OM4 MMF can be realized without using data recovery circuit. Increasing the bias current of the VCSEL beyond 7.5Ith improves the signal-to-noise ratio (SNR) and the bit error ratio (BER) of received QAM-OFDM data to 15.5 dB and $2.9\times 10^{-3}$ , respectively. The 100-m-long OM4 MMF transmission degrades the SNR with its covered bandwidth reducing to 13 GHz. The OFDM subcarrier pre-leveling technique with a slope of 0.2 dB/GHz ensures the 16-QAM-OFDM data transmission with an error vector magnitude of 17.1% and a BER of $3.4\times 10^{-3}$ .

Wideband signal generation using frequency/phase modulation (FM/PM) is the key fundamental function for various applications such as radar and analog communication systems. It is well known that analog FM/PM communication systems can dramatically improve signal quality by spectral expansion. In classical communication theories, the Armstrong indirect method is one of the most popular methods for bandwidth expansion of FM signals. In the Armstrong method, a narrowband signal is converted to a broadband signal with the help of a nonlinear frequency multiplier. In this paper, we propose a photonic Armstrong method enabled by direct detection. By utilizing the nonlinearity caused by direct detection, we can increase bandwidth so that it is double that of the original signal in the photonic Armstrong method. Also, it completely eliminates signal-to-signal beat interference (SSBI) at the same time. We conducted an experiment to verify the concept and confirmed these advantages experimentally.

A 850-nm single-mode VCSEL with 3-dB bandwidth of 22.1 GHz enables 16-QAM OFDM transmission at nearly 100 Gbit/s over 100-m-Iong OM4 MMF link is demonstrated over 100-m-long OM4 MMF with BER of 3.5×10−3.

Recently, GaN based light-emitting diodes (LEDs) have become a promising candidate for visible light communication (VLC). In this work, we demonstrated 2-Gbit/s data rate transmission using GaN-based LEDs with photonic crystal (PhC) structure.

For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 μm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 μm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.

Through the reduction of mirror-resistance of vertical-cavity surface-emitting lasers, the optical bandwidth of 22.5 GHz is successfully achieved by implementation of zinc-diffusion process. It shows that lower mirror-resistance provides better transmission quality under similar optical bandwidth. With 20 ohm resistance reduction, the bandwidth limitation by electrical parasitics can be effectively improved from 6.4 to 14.71 GHz confirmed by the small-signal extraction results.

Chip-level direct 16-QAM OFDM encoding of a few-transverse-mode 850-nm Zn-diffused VCSEL is employed to transmit 80-Gbit/s data covering 20-GHz analog bandwidth over 100-m-long OM4 MMF with corresponding BER of 3.3×10−3.

TO-can packed GaN LD connected with impedance-matched transmission-line circuit board enhances the UFMC additive 16-QAM OFDM bandwidth to enable 17.6-Gbps VLC at 4.4-bit/s/Hz spectral-usage efficiency with suppressed inter-OFDM-carrier interference and improved BER of 1.07×10−4.

Directly 64-QAM OFDM encoding the colorless laser diode with power-to-frequency pre-leveling is demonstrated for 60-Gbit/s BtB and 55.8-Gbit/s 25-km fiber transmissions with its selectable channel wavelength to construct uniform transmitter for DWDM-PON channels.

A 850 nm single-mode vertical cavity surface emitting laser (VCSEL)-based data transmission is demonstrated, enabling four-level pulse amplitude modulation (PAM-4) at 64 Gbit/s over 100–300 m in an OM4 multi-mode fiber (MMF). By optimizing the bias of the single-mode VCSEL with a differential resistance of 159 Ω, the related electrical return loss of −5.7 dB is determined to provide an analog modulation bandwidth of 18.9 GHz. After pre-emphasizing the waveform of the PAM-4 format, the PAM-4 data stream can be successfully delivered by the single-mode VCSEL at 64 Gbit/s under back-to-back and 100 m long OM4 MMF conditions. Lengthening the transmission distance worsens the signal-to-noise ratio (SNR) of PAM-4 data to the FEC criterion, as the waveform pre-emphasis of the PAM-4 data stream inevitably induces spectral power compensation from low to high frequencies. Therefore, increasing the OM4 MMF distance from 200 to 300 m significantly reduces the peak-to-peak amplitude of data to suppress the SNR and reduce the transmission capacity from 52 to 48 Gbit/s.

This paper is going to review the state-of-the-art of the high-speed 850/940-nm vertical cavity surface emitting laser (VCSEL), discussing the structural design, mode control and the related data transmission performance. InGaAs/AlGaAs multiple quantum well (MQW) was used to increase the differential gain and photon density in VCSEL. The multiple oxide layers and oxide-confined aperture were well designed in VCSEL to decrease the parasitic capacitance and generate single mode (SM) VCSEL. The maximal modulation bandwidth of 30 GHz was achieved with well-designed VCSEL structure. At the end of the paper, other applications of the near-infrared VCSELs are discussed.

A few-mode (FM) vertical cavity surface emitting laser (VCSEL) chip with heavily zinc-diffused contact layer and oxide-confined cross-section is demonstrated for carrying pre-leveled 16-quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) data in OM4 multi-mode fiber (MMF) over 100 m for intra-data-center applications. The FM VCSEL chip, which has an oxide-confined emission aperture of 5 μm, demonstrates high external quantum efficiency, provides an optical power of 2.2 mW at 38 times threshold condition, and exhibits 3 dB direct-modulation bandwidth beyond 22 GHz at a cost of slight heat accumulation. At a DC bias point of 5 mA (22.6Ith) the FM VCSEL chip, with sufficiently normalized modulation output, supports Baud and data rates of 25 and 100 Gb/s, respectively, with forward error correction (FEC) certifying receiving quality after back-to-back transmission. After passing through 100 m OM4 MMF with a receiving power penalty of 4 dB, the FM VCSEL chip demonstrates FEC-certified transmission of the pre-leveled 16-QAM OFDM data at 92 Gb/s.

We demonstrate transmitting 28-GHz band radio signals from cascaded IFoF based C-RAN mobile fronthaul system to inside a vehicle by re-radiation of the cost-effective analog radio over multi-mode fiber employing a directly modulated VCSEL.