Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

A broad-band semiconductor optical amplifier (SOA) that achieves both a high chip saturation output power and a low chip noise figure (NF) was developed by using a thin multiquantum well with low internal loss. The SOA exhibited a high chip saturation output power of >+19.6 dBm and a low chip NF of <4.5 dB over a 3-dB gain bandwidth of 120 nm (1450-1570 nm). For the amplification of optical signals modulated at 40-Gb/s nonreturn-to-zero format, a penalty-free amplification was confirmed up to an average chip output power of +18.1 dBm.

A broad-band MQW semiconductor optical amplifier with high saturation output power and low noise figure

A directly modulated dual-mode laser diode (DMLD) with third-order intermodulation distortion (IMD3) suppression is proposed for a 60-GHz millimeter-wave over fiber (MMWoF) architecture, enabling new fiber-wireless communication access to cover 4-km single-mode-fiber (SMF) and 3-m wireless 16-QAM OFDM transmissions. By dual-mode injection-locking, the throughput degradation of the DMLD is mitigated with saturation effect to reduce its threshold, IMD3 power and relative intensity noise to 7.7 mA, -85 dBm and -110.4 dBc/Hz, respectively, providing huge spurious-free dynamic range of 85.8 dB/Hz(2/3). This operation suppresses the noise floor of the DMLD carried QPSK-OFDM spectrum by 5 dB. The optical receiving power is optimized to restrict the power fading effect for improving the bit error rate to 1.9 × 10(-3 )and the receiving power penalty to 1.1 dB. Such DMLD based hybrid architecture for 60-GHz MMW fiber-wireless access can directly cover the current optical and wireless networks for next-generation indoor and short-reach mobile communications.

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60-GHz Millimeter-wave Over Fiber with Directly Modulated Dual-mode Laser Diode.

Hybrid MMW-over fiber/OFDM-FSO transmission system based on doublet lens scheme and POLMUX technique

The increased bandwidth demand has motivated the exploration of fiber-wireless integration (FWI) for future broadband 5G+ cellular communication networks. FWI offers ultra-wideband (UWB) wireless delivery with low interference, which will be prospective for 5G/5G+ mobile communication wireless access, military application, disaster emergency communication, broadband communication at home, and so on. As an effective carrier, millimeter-wave (mm-wave) frequencies between 30 GHz and 300 GHz are a new frontier for FWI that offers the promise of orders of magnitude greater bandwidths. In this paper, we summarize all kinds of enabling technologies for FWI, including the photonic vector mm-wave generation scheme, the integration of various multi-dimensional multiplexing techniques, radio-frequency-transparent (RF-transparent) photonic demodulation technology for fiber-wireless-fiber network, and low-complexity high-efficiency digital signal processing (DSP). Based on DSP for UWB high-spectrum-efficiency signal coherent detection, we have made great progress in the field of the mm-wave-band (from Q- to D-band) broadband signal generation and long-distance transmission. These experimental results show that FWI with large-capacity, long-distance, and high-spectrum-efficiency has important scientific and practical significance for the development of the future 5G+ wireless communication.

Tutorial: Broadband fiber-wireless integration for 5G+ communication

By using a nully biased MZM to modulate an incoming single-mode light into a double sideband (CCA-DSB or CCS-DSB) master with preserved or suppressed central carrier, the directly OFDM encoded dual-mode colorless laser diode is performed for a successful fusion between wired and wireless links to establish a 5G-based MMWoF system. The dual-mode L-band optical carrier successfully delivers a 36-Gb/s OFDM data with a BER of $3.2 \times 10^{-3}$ , while the stabilized 39-GHz mm-wave carrier can provide wireless 4-Gb/s 16-QAM OFDM data with 16.6-dB SNR. The in-situ 39-GHz mm-wave carrier can be synthesized by optically heterodyne mixing the dual-mode carrier at remote node. When injection-locking with the CCS-DSB master, the dual-mode slave colorless laser diode improves its central carrier suppression ratio and RIN to 38 dB and −104 dBc/Hz, respectively. In comparison, the dual-DFBLD master injection-locking mixed mm-wave carrier is relatively unstable due to the individual DFBLDs at free-running condition. With the CCS-DSB master, the mm-wave carrier self-beat from the dual-mode optical carrier exhibits a narrow linewidth of <3 kHz with high purity and stability. Such a dual-mode colorless laser diode-based MMWoF link is capable of fusing the fiber wired and the 5G wireless links demanded in the near future.

39-GHz Millimeter-Wave Carrier Generation in Dual-Mode Colorless Laser Diode for OFDM-MMWoF Transmission

With the use of central carrier suppressed master which contains a double sideband carrier, a directly encoded dual-color laser diode with low coherence based millimeter-wave (MMW) carrier generation for hybrid wired and wireless MMW over fiber (MMWoF) system is demonstrated. The maximal transmission capacities for optical wired band at 36 Gb/s and for MMW wireless band at 12 Gb/s are achieved in the proposed MMWoF link. Three MMW carrier frequencies of 28, 39, and 47 GHz are synthesized for implementing the fifth generation (5G) mobile network and satellite communication. The dual-color injection-locking effectively suppresses the central carrier power to –37 dBm, which provides the dual-color optical carrier with a central carrier suppression ratio of 38 dB. The 47-GHz double sideband carrier injection has shown its capability to carry 64-QAM OFDM data at 36 Gb/s with the lowest bit-error-rate (BER) of 2.1 × 10–3 over 25-km single-mode fiber, as its weakest four-wave-mixing modes greatly suppress the fiber chromatic dispersion. After optical baseband transmission, the photonic mixed 28-, 39-, and 47-GHz MMW carriers are heterodyne beat from the dual-color carrier to show similar spectral linewidth of 41 dB. The 28-GHz MMW carrier can wireless transmit 12-Gb/s 16-QAM OFDM data over 1.6-m free-space to show an average signal-to-noise ratio of 15.3 dB, an error vector magnitude of 17.1%, and a receiving BER of 3.3 × 10-3.

Millimeter-Wave Carrier Embedded Dual-Color Laser Diode for 5G MMW oF Link

A two-color, laser-diode-based, full-duplex fiber-wired and millimeter-wave (MMW)-wireless orthogonal frequency-division multiplexing (OFDM) transmission link is performed. With modal control on the two-color laser diode, a single-wavelength optical carrier is used as both the downstream and upstream transmitters to replace the dual-mode one with an optical baseband, which effectively suppresses the chromatic dispersion that occurs in fiber. The proposed system demonstrates a carrier-reused, full-duplex 28-GHz MMW dense wavelength division multiplexing passive optical network (DWDM-PON) system, providing OFDM transmission with 54 Gb/s downstream, 36 Gb/s upstream and 16 Gb/s wireless data rates. The single-wavelength optical carrier transmits data from an optical line terminal (OLT) to a remote node (RN) and then transfers into a dual-wavelength carrier for optically heterodyne beating an MMW carrier, which further wirelessly transmits the data to an optical network unit (OUN). In addition, the upstream data is carried by another slave colorless laser diode injection-locked by reusing the downstream carrier without additional data erasing, which avoids the wavelength selection problem that resulted from identifying or addressing upstream and downstream channels. Among three laser transmitters with different cavity lengths, the 600-μm embedded MMW wireless carrier can provide the lowest bit error rate (BER) after 25 km of fiber and 1.6-m free-space transmissions at 16 Gb/s, because its highest external quantum efficiency supports the optimization on self-heterodyne transferring the MMW carrier to enable the low-noise and long-distance wireless transmission.

Two-color laser diode for 54-Gb/s fiber-wired and 16-Gb/s MMW wireless OFDM transmissions

The four-wave-mixing (FWM) suppression of a master-to-slave injection-locked two-wavelength Fabry–Perot laser diode (FPLD) pair is investigated from the viewpoint of integrating fiber-optic wired and millimeter-wave (MMW) wireless networks for mobile and satellite communications. This is achieved by shifting the FWM side modes away from the cavity mode to avoid FWM enhancement induced by cavity resonance. Mode-deviated two-wavelength injection from a 900-μm master FPLD to a 600-μm slave FPLD successfully suppresses the resonant FWM side modes to –31 and –35 dBm, which enables the transmission of 18-Gb/s 64-quadrature amplitude modulation orthogonal frequency-division multiplexing (QAM OFDM) data with error vector magnitude (EVM), signal-to-noise ratio (SNR), and bit error rate (BER) values of 6.3%, 24 dB, and 1.6 × 10-4, respectively. The FWM-suppressed 600-μm slave shows lower chromatic dispersion than that of the 750-μm slave. In addition, a 47-GHz MMW carrier remotely beat by the two-wavelength-injected slave enables the transmission of passband 2-Gb/s 4-QAM OFDM data with EVM, SNR, and BER values of 36.3%, 8.8 dB, and 2.9 × 10−3, respectively, after 25-km single-mode fiber wired and 1.6-m free-space wireless transmission.

Four-Wave-Mixing Suppression of Master-to-Slave Injection-Locked Two-Wavelength FPLD Pair for MMW-PON

By delivering a reference carrier through adjacent dense-wavelength-division-multiplexing (DWDM) channel to remotely beat with the downstream transmitted optical carrier, a full-duplex millimeter-wave DWDM passive-optical-network (MMW-DWDM-PON) is demonstrated to transmit 36- ${\text{Gb/s}}$ data over 25-km single-mode fiber (SMF) and to remotely deliver a 28-GHz MMW carrier for wireless 6- ${\text{Gb/s}}$ data transmission. Such a remotely recombined dual-mode beating scheme provides baseband 36- ${\text{Gb/s}}$ data with corresponding bit error rate (BER) of 2.7 × 10 −3, which is better than the directly dual-mode injection-locked colorless Fabry-Perot laser diode (FPLD) does because of the less fiber chromatic dispersion during 25-km SMF transmission. In a hybrid 25-km SMF-wired and 1.6-m wireless link, it successfully transmits 4- ${\text{Gb/s}}$ data to fit the forward error correcting criterion. By maintaining the free-space distance at 1.6 m, the allowable raw data rate for MMW wireless transmission can be increased up to 6 ${\text{Gb/s}}$ with BER of 1.9×10−3. For optical baseband upstream transmission, the 36- ${\text{Gb/s}}$ data is also directly encoded onto the upstream colorless FPLD injection-locked by reusing the downstream pure carrier delivered through adjacent DWDM channel, and the received data reveals a BER of 2.9 × 10 −3.

Chung-Yu Lin

Papers

39-GHz Millimeter-Wave Carrier Generation in Dual-Mode Colorless Laser Diode for OFDM-MMWoF Transmission

Millimeter-Wave Carrier Embedded Dual-Color Laser Diode for 5G MMW oF Link

Two-color laser diode for 54-Gb/s fiber-wired and 16-Gb/s MMW wireless OFDM transmissions

Four-Wave-Mixing Suppression of Master-to-Slave Injection-Locked Two-Wavelength FPLD Pair for MMW-PON

Adjacent Channel Beating With Recombined Dual-Mode Colorless FPLD for MMW-PON