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

The final goal of the development of the communication network is to realize the real-time, high-speed, and reliable information access everywhere and all the time.^1–3 In order to achieve this goal, both wireless communication^1,2 and optical fiber communication, complementary to each other,³ are important. Optical fiber communication has ultra-large bandwidth and capacity, but it has less mobility and cannot realize seamless coverage.³ Wireless communication in theory can cover everywhere, but it is limited by insufficient frequency resources and affected by different kinds of impairments, which leads to limited communication bandwidth and transmission distance.^1,2 To meet the ever-increasing communication bandwidth requirement, the future broadband access networks need to balance and seamlessly integrate wireless communication and optical fiber communication, which is the so-called fiber-wireless integration technology.^4–38 At the same time, ultra-high wireless transmission rates (>40 Gb/s), which are equivalent to or even exceed the fiber transmission rates, have emerged.^39–41 However, it is difficult to generate such ultra-high-speed wireless signals simply relying on bandwidth-limiting electronic devices and therefore the photonics-aided millimeter-wave (mm-wave) technology came into being and has been intensively investigated.^42–56 

The fiber-wireless integration access system effectively integrates the advantages of optical fiber communication in terms of communication bandwidth and transmission distance as well as the advantages of wireless communication in terms of mobility and seamless coverage.^57–69 It is a significant development trend of future broadband access networks. The fiber-wireless integration access system can make full use of the photonics-aided mm-wave technology with a simple structure and low cost to overcome the bandwidth bottleneck of electronic devices and to generate ultra-high-speed wireless mm-wave signals.⁴⁸ It can also make full use of various kinds of multiplexing technologies, including spatial multiplexing, frequency band multiplexing, and antenna polarization multiplexing, to reduce signal baud rate and bandwidth requirements for optoelectronic devices and to greatly increase wireless transmission capacity.^70–78 Advanced digital signal processing (DSP) technology can be further applied to the fiber-wireless integration access system to effectively compensate for various kinds of linear and nonlinear impairments caused by fiber-wireless integration transmission links and to achieve high-spectrum-efficiency high-receiver-sensitivity fiber-wireless integration transmission.^79–101 

The fiber-wireless integration access transmission system is expected to provide gigabit-class mobile data transmission, which can satisfy high-speed data transmission and communication in many different application scenarios.⁷⁶ In terms of commercialization, several international advanced chip companies, such as Intel of the United States, Panasonic of Japan, and Samsung of South Korea, have deployed 8k × 8k and 4k × 4k super-high-definition (SHD) cameras in smart phones, which require the transmission rates of uncompressed SHD video images up to 60 Gb/s and 30 Gb/s, respectively.¹⁰² Obviously, it is impractical to install high-definition multimedia interfaces (HDMIs) or optical cables in such thin and light mobile terminals, and therefore the ultra-broadband fiber-wireless integration access system is expected to be used in this scenario. In terms of information transmission security, in the event of an earthquake or other natural disasters, such as the disasters caused by the tsunami in Northeast Japan and the earthquake in Sichuan of China in 2008, the optical fiber may be destroyed. In this case, if we can use a wireless link to transmit the signal delivered by the optical fiber, the interrupted communication can be recovered. So the key is to realize the seamless fiber-wireless integration communication system and wireless signal transmission at a rate comparable to the fiber transmission rate (100 Gb/s or higher).⁴⁰ In terms of national defense construction and military demand, W-band (75-110 GHz) mm-wave with higher frequencies can penetrate the clouds to achieve the all-time communications, and W-band multi-dimensional multiplexing communication technology based on photonics-aided mm-wave generation can achieve the link performance (high-speed, stable, and large-capacity) required by future new-type space communications.⁴ In the frequency bands for satellite communications, the traditionally applied C-band (4-8 GHz) and Ku-band (12-18 GHz) are currently very crowded and increasingly unable to meet the market demand.^103–107 At present, the Ka-band (26.5-40 GHz) with a relatively large communication bandwidth has become the preferred frequency band for broadband multimedia services throughout the world. However, with the development of the big data and Internet of things, as well as the emergence of the applications such as 4k × 4k or 8k × 8k video, the Ka-band has also gradually been saturated.

In order to solve the problem of high-speed communication in the situation of natural disasters, the Japanese government promoted a special project to carry out 100-Gb/s fiber-wireless signal transmission at W-band.^16–18 For the need of military communication, in the United States, Defense Advanced Research Projects Agency (DARPA) achieved the 100-Gb/s wireless signal transmission by the 100-GHz mm-wave carrier.⁴ Europe also has carried out special research on fiber-wireless integration access. In 2010, the FUTON project of Europe put forward a fiber carrying strategy for future wireless cellular network, which uses the fiber-wireless-integration method to realize remote radio and miniaturization of the cellular network, in order to further enhance the capacity of the existing radio access.²⁰ In the same year, the Future Optical Wireless Access Network project of Europe clearly proposed that the future wireless access network should be based on the existing fiber access bearing system and the ultra-broadband signal should be introduced to the fiber-wireless integration access network to achieve ultra-broadband access over fiber. The international advanced chip companies, such as Intel and IBM in the United States, Panasonic in Japan, and Samsung in South Korea, are all developing the chips which employ mm-wave carriers to transmit high-definition radio signal. For example, the chip at 3-Gb/s wireless rate has been implemented and commercially available.¹⁰² 

Fiber-wireless integration technology can also be used for satellite communication between the earth and satellites. In this case, the extremely high frequency (EHF) bands, such as the Q-band (33-50 GHz), V-band (40-75 GHz), and W-band, have attracted increasing attention and become a hot research area. At the same time, the free-space optical (FSO) communication has the advantages of high transmission rate, large available bandwidth, high security (reliability), and strong confidentiality, as well as terminal equipment with small size, light weight, and low power consumption. The FSO communication is a promising solution to increase communication capacity. In long-distance transmission, the FSO communication shows the equivalent capacity to optical fiber communication and it is expected to play an important role in the space-based data backbone communication network. However, for the space and ground communication, the FSO communication is easily affected by the atmosphere, which causes degraded communication quality and reduces the efficiency of network transmission. Compared with the FSO communication, the higher-frequency W-band mm-wave is less affected by adverse environmental conditions¹⁰⁷ and it can penetrate dust, clouds, fog, and other environments. What is more, it can overcome the deficiencies faced by infrared and laser systems, realizing medium-range multiplexing communication. It can also support the distribution of large-capacity media streams. Compared with the traditional satellite microwave band, the W-band has the following advantages. First, the W-band can achieve a higher information transmission rate to meet the requirements of the current high-rate multimedia services. Second, the mm-wave antenna gain increases with mm-wave carrier frequency. So transmitting signals at W-band has the advantages of lower transmitting power and higher directionality. Third, for the antenna of the same size, the higher the frequency, the narrower the beam width. So the W-band has better quality for anti-interception and anti-interference. Fourth, due to the reciprocal relationship between the antenna aperture and frequency, the W-band antenna and device can be designed with a smaller size. Fifth, the size of satellites and launch vehicles can be made smaller if the W-band carrier frequencies are employed, reducing the design cost and the cost of launching satellites. Therefore, the study of W-band satellite communications, whether military or civilian, plays an important role in occupying the commanding heights of future science and technology.

The future 5G/5G+ mobile communication technology also needs to meet the requirements of ultra-large capacity, ultra-high reliability, and accessibility anytime and anywhere, in order to solve the “traffic storm” and many other issues. It requires the integration of a variety of new wireless access technologies on the basis of existing wireless access technologies, while fiber-wireless integration access systems can provide ultra-broadband low-interference mm-wave wireless routing that can be used in future 5G/5G+ networks. In recent years, large-capacity fiber-wireless integration communication and transmission technologies that can be applied in different scenarios are attracting more and more researchers at home and abroad. The following part of the Introduction will show the important research work in fiber-wireless integration access technologies from both international and domestic perspectives.

In terms of novel network architectures for fiber-wireless integration access, in 2010, the ITEAM Research Institute of Spain proposed a low-cost wireless transmitter scheme for fiber-wireless integration based on wide spectrum phase modulation and introduced the coarse wavelength division multiplexing (CWDM) mechanism to implement the channel segmentation of the wide spectrum. Based on this, a multi-channel fiber-wireless integration access system with ∼20-GHz carrier frequency and ∼500-MHz electrical bandwidth was experimentally demonstrated.⁷ In the same year, people compared the performance of the broadband radio-frequency (RF) signals in the fiber-wireless integration access links under three different kinds of modulation schemes, namely, direct modulation, external modulation, and reflective semiconductor optical amplifier (SOA) modulation, and discussed the optimal solution under different scenarios.⁸ In 2011, Chowdhury et al. realized the indoor communication experiment of a 60-GHz bi-directional fiber-wireless integration access system using a complementary metal oxide semiconductor (CMOS) transceiver and achieved 1.485-Gb/s high-definition video and data signal transmission.⁹ In the same year, Wang et al. put forward a high-speed indoor fiber-wireless integration access communication system based on wavelength division multiplexing (WDM), which can provide flexible bandwidth allocation according to different application requirements.¹⁰ In 2012, Lim et al. put forward a fiber-wireless integration access network combined with Fiber to the Home (FTTH) and promoted a new idea of the future Long-Term-Evolution (LTE) fiber-wireless integration access. In this new architecture, the new regional assignment plays an important role in the promotion of future access capacity.¹¹ Kellerer et al. also put forward a dynamic allocation structure for the fiber-wireless-integration signal, adopting a reconfigurable SOA array for RF channel routing, and their experiment also verified the feasibility of the architecture.¹² In the same year, Ho et al. proposed a 60-GHz fiber-wireless integration access system based on orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) and achieved a corresponding experimental demonstration with 50-Gb/s data rate and 8-bit s⁻¹ Hz⁻¹ spectrum efficiency.¹⁴ Shao et al. proposed a 60-GHz fiber-wireless integration access structure integrated with the WDM passive-optical-network (WDM-PON) using a single Mach-Zehnder modulator (MZM) to implement parallel phase modulation and the system can be compatible with the ECMA387 standard mm-wave and 2.5-Gb/s baseband signal. They also studied the influence of RF modulation on the baseband signal.¹⁵ In 2013, Kanno et al. put forward a coherent detection structure based on DSP to realize the seamless access of fiber-wireless integration. At the same time, they also used the MIMO technology to improve system capacity and achieved 74.4-Gb/s fiber-wireless-integration signal transmission.¹⁶ 

In terms of the key techniques and algorithms for fiber-wireless integration access, Kanno et al. realized 20-Gb/s mm-wave signal transmission using quadrature phase shift keying (QPSK) modulation;¹⁷* Kanno et al. also utilized 16-ary quadrature amplitude modulation (16QAM) to accomplish 40-Gb/s mm-wave signal wireless transmission and the wireless distance is 3 cm.¹⁸ Zibar et al. experimentally demonstrated the generation of 40-Gb/s wireless mm-wave signal based on all-optical OFDM modulation and the digital coherent demodulation technique.¹⁹ Pang et al. experimentally demonstrated a 100-Gb/s fiber-wireless integration access system using polarization-division-multiplexing 16QAM (PDM-16QAM) modulation and achieved the wireless transmission distance of 1.2 m.²⁰ Kanesan et al. proposed a nonlinear optical fiber transmission optimization scheme aimed at the fiber-wireless integration access system based on LTE and improved the chirp effect of the distributed feedback (DFB) laser and the system nonlinear threshold.²⁴ They also carried out the experimental demonstration of this scheme for enhanced LTE based station coverage and the error vector magnitude (EVM) and the transmitted optical power of the 64QAM signal were both improved.²⁵ Zhu et al. utilized the coherent detection technology to achieve the ultra-dense wavelength division multiplexing (UDWDM) fiber-wireless integration access system and increased the flexibility of bandwidth allocation and simultaneously reduced the system cost.²⁶ Cao et al. put forward a signal synchronization method based on digital frequency division multiplexing used for 60-GHz indoor fiber-wireless integration access networks, which adopted the Nyquist shaping to suppress the inter-carrier interference (ICI) and the corresponding experiment achieved 12.7-Gb/s throughput.²⁷ Pang et al. realized 25-Gb/s wireless mm-wave signal transmission based on QPSK modulation and it can be applied to in-building wireless networks.²⁸ Lim et al. put forward a flexible Quality of Service (QoS) partition aimed at the baseband and RF part of the fiber-wireless integration access, promoted a new method to determine the allocation of resources, and distributed the LTE QoS Class Identifier (QCI) into baseband data group.²⁹ Koenig et al. achieved the generation of 100-Gb/s fiber-wireless-integration signal with the carrier frequency of 237.5 GHz and wireless transmission distance of more than 20 m. The transmitter used heterodyne optical in-phase/quadrature (I/Q) modulator, and the receiver used monolithic microwave integrated circuit (MMIC) technology to directly mix the 237.5-GHz RF signal to the baseband.³⁰ In Refs. 31 and 32, the techniques of optical polarization multiplexing and antenna MIMO were applied to the fiber-wireless integration access system and the transmission bit rate was effectively improved. In Refs. 34–37, the fiber-wireless integration access system, combined with optical multi-carrier modulation or electrical multi-carrier modulation, realized the multiplexing of multiple mm-wave carrier frequencies and made the subcarrier optimization possible. Jiang et al. put forward a frequency-domain equalization algorithm to improve the beating noise of the 60-GHz fiber-wireless integration access system and has increased the signal-to-noise ratio (SNR) by 5.5 dB.⁴⁷ Since 2012, we have carried out a series of experiments on fiber-wireless integration and have created several world records on mm-wave transmission.^{39–41,71–75}

In terms of ultra-high-speed mm-wave space transmission, in October 2011, in the United States, Viasat, Inc. successfully launched the satellite ViaSat-1, which marks the broadband satellite with the world’s highest capacity, i.e., a total throughput of 140 Gb/s, began to operation. The satellite applied Ka-band multipoint beam and frequency reuse technology and can provide mobile broadband service for millions of people.¹⁰³ However, because it cannot yet satisfy the needs of the rapid growth of the data traffic, Viasat, Inc. launched the second satellite with a higher throughput of 300 Gb/s, namely, ViaSat-2, in 2017. In February 2014, the American Media Development Investment (AMDI) Fund officially released the Outernet plan (Extranet Plan), and it is expected to launch hundreds of satellites to the near-earth orbit. The satellites will continuously release the Internet signal oriented to the earth, becoming the world’s free Wi-Fi source. In November 2014, the SPACE Company under the management of Tesla CEO and WORLDVU satellite company reached cooperation to jointly research and develop the satellite which can provide mobile Internet access services for the world. The Military Strategy and Tactical Relay (Milstar) satellite system and Advanced Extremely High Frequency (AEHF) satellite system of the United States are the typical representatives of the Q/V-band satellite communication systems; also, the digital audio-video interactive distribution-data collection experiment (DAVID-DCE) and the W-band analysis and evaluation (WAVE) carried out by the Italian Space Agency (ASI) are the typical representatives of the W-band satellite communication systems.^104–106 In addition, the Quasi-Zenith Satellite System (QZSS) project of the Japan Aerospace Exploration Agency (JAXA) recommended to use 84 GHz and 74 GHz as the carrier frequencies of the uplink and downlink, respectively, and it can provide good transmission performance for urban mobile telecommunications services.¹⁰⁷ 

When the mm-wave signal is delivered over the fiber-wireless integration transmission link, both the optical fiber and the air will play the role of the transmission medium. The mm-wave signal delivery in the optical fiber and in the air displays different characteristics, which will be introduced in this section.

Using the optical fiber as the transmission medium can realize long-distance large-capacity mm-wave signal delivery. The optical fiber cable has a small weight as well as a very low transmission loss and insertion loss with respect to the copper cable. To be specific, the transmission loss of the optical fiber cable is about 0.25 dB/km at 1550-nm operating wavelength. As we know, the 1550-nm operating wavelength corresponds to 193.4145-THz operating frequency. Since 100 GHz is much smaller than 193.4145 THz, we can consider that the transmission loss of the optical fiber cable is also about 0.25 dB/km for any optical mm-wave signal within the mm-wave carrier frequency range from 33 GHz to 100 GHz if we employ an optical local oscillator (LO) source operating at 1550-nm wavelength. Moreover, the weight of the optical fiber cable is about 1.7 kg/km, corresponding to <0.5-dB insertion loss, while that of the copper cable is about 567 kg/km, corresponding to about 360-dB insertion loss.⁸ Although the vector mm-wave signals will be affected by the walk-off effect due to the chromatic dispersion in optical fiber transmission, which, however, can be addressed by single sideband (SSB) modulation¹¹ or electrical dispersion compensation (EDC).²³ Moreover, the transmission and insertion losses caused by the optical fiber cable can be compensated by the optical fiber amplifier, and therefore it is easy to realize the optical fiber transmission of the vector mm-wave signal over a distance up to 100 km.²³ In addition, considering the security performance of the optical fiber cable, the WDM technology can be used to achieve higher mm-wave signal transmission capacity.¹⁰ 

Relative to the optical fiber, when we use the air as the transmission medium of the mm-wave signal, the transmission distance is usually very short, mainly due to the atmospheric loss and free-space path loss.

Figure 1 shows an example of the atmospheric loss versus the frequency for a terrestrial link without significant humidity, rain, clouds, or fog. We can see that only a few spectral windows have relatively low atmospheric loss. For example, the atmospheric loss at 80-GHz W-band frequency is about 0.35 dB/km, which will limit the transmission distance of the terrestrial transmission link carried by 80-GHz carrier frequency within 20 km, even under the best atmospheric conditions. However, when one end of the transmission link at 80 GHz is at a high altitude and a large part of the transmission link goes through a thin atmosphere, the transmission distance will be much longer than the case of a terrestrial link, and even in the presence of clouds, a transmission distance of 80 km can be achieved. The path loss and the total loss (atmospheric loss + path loss) versus transmission distance at different frequencies are shown in Fig. 2.⁴ V-band at 60 GHz has very large atmospheric loss caused by oxygen, and therefore after 400-m wireless delivery, the atmospheric loss at 60 GHz is dominative as shown in Fig. 2(b).

In addition, the atmospheric loss also includes the attenuation caused by severe atmospheric conditions. According to the ITU-T rain attenuation model, Fig. 3 shows the attenuation of the horizontally polarized electromagnetic waves with different carrier frequencies subjected to different rain rates. We can see that the attenuation caused by rain at carrier frequencies below 10 GHz is negligible, but at W-band, the attenuation caused by rain is severe. For example, in the case of a rain speed of 50 mm/h, an attenuation of 20 dB/km can be achieved at W-band.

As mentioned in Sec. II, the mm-wave signal delivery in the optical fiber and in the air has different characteristics and therefore enabling techniques are required to realize the seamless integration of the optical fiber link and the wireless air link, i.e., the fiber-wireless integration. In this section, we will introduce the enabling techniques for the fiber-wireless integration, including simple and cost-effective photonic mm-wave vector signal generation techniques, multi-dimensional multiplexing techniques, optimized antenna structures, RF-transparent photonic demodulation techniques, and low-complexity high-efficiency DSP.

Simple and cost-effective photonic mm-wave vector signal generation techniques are the key for the practical implementation of the fiber-wireless integration systems and networks. Various kinds of advanced techniques have been proposed to realize mm-wave vector signal generation, which can be classified into three types.

The first type is based on the heterodyne beating of two continuous-wavelength (CW) lightwaves from two independent frequency-unlocked lasers.^16–23 This type can generate mm-wave signals with a large signal-to-noise ratio (SNR) and has the advantages of simple structure and adjustable carrier frequency. However, at present, many of the requirements envisioned by the mm-wave transmission in fifth-generation (5G) mobile systems, such as higher transmission speed, larger bandwidth, and steady carrier frequency, are already daunting. The mm-wave generation based on this type of techniques will limit the 5G development due to its inherent frequency instability problem.

The second type of techniques used to generate mm-wave signal is based on the optical multi-carrier source.^60,92 Here, an optical filter is needed to select two optical subcarriers with a certain frequency spacing from the multiple optical subcarriers generated by the optical multi-carrier source. One of the selected optical subcarriers is modulated by the transmitter data, while the other is kept unmodulated, before the two selected optical subcarriers are mixed in a photodiode (PD). The advantage of this type of techniques is that wideband vector mm-wave generation can be realized using optoelectronic devices with lower bandwidth. But this type of techniques also has the disadvantages of a relatively complicated structure and reduced SNR due to frequency multiplication.

The third type of techniques is based on a single external intensity modulator (IM) and transmitter pre-coding, which has been intensively investigated recently.^80–82,87 The advantage of this type of techniques is that the target two optical subcarriers are from the same laser source and therefore they are frequency-locked and phase-locked, which effectively suppresses the phase noise. However, the frequency adjustment is not flexible because of the employment of transmitter pre-coding. Figure 4 shows the principle of the photonic mm-wave vector signal generation scheme based on a single external IM and transmitter pre-coding. The vector RF signal driving the IM at the transmitter can be amplitude and phase pre-coded in advance, and we can get multiple optical subcarriers after the IM. Then two subcarriers with a certain frequency spacing can be filtered by a programmable wavelength selective switch (WSS), and we can obtain a vector mm-wave signal with correct amplitude and phase information after PD detection. In particular, the vector signal frequency multiplication (i.e., 2×, 4×, 6×, 8×) can be achieved when the IM is biased at different points (i.e., the maximum transmission point and the minimum transmission point) and the WSS is programmed with two different bandpass channels. Moreover, the amplitude and phase pre-coding algorithms should change with different vector modulation formats, including QPSK, 8QAM, 16QAM, 64QAM, 128QAM, and so on. Figure 5 shows QPSK and 8QAM constellations of the transmitted RF signals before and after pre-coding.⁸⁰ 

The realization and coordinative integration of various kinds of multi-dimensional multiplexing techniques can significantly reduce the signal baud rate and increase the system transmission capacity of the fiber-wireless integration access systems. Figure 6 summarizes the typical multi-dimensional multiplexing techniques used for the fiber-wireless integration access systems, including multiple-input multiple-output (MIMO) spatial multiplexing, high-order modulation format multiplexing, antenna polarization multiplexing, and mm-band multiplexing.^70–76 

Within the Rayleigh range, MIMO signals are independent with each other and in this case they can be received without a mass of signal processing. However, it is difficult to maintain the signal independence out of the Rayleigh range and hence the additional signal processing should be employed to decompose the correlated signal. Figure 7 gives the Rayleigh range versus the carrier frequency when the antenna spacing changes from 1 cm to 10 m. It can be seen that the Rayleigh range is close to 100 km when the mm-wave carrier frequency approaches 100 GHz and the antenna spacing is almost 10 m.

For the large-capacity fiber-wireless integration systems based on multiple multi-dimensional multiplexing techniques including spatial multiplexing, mm-band multiplexing, and antenna polarization multiplexing, the antenna structure needs to be further optimized to completely integrate multiple multi-dimensional multiplexing techniques.⁷⁰ For the antenna structure design based on antenna polarization multiplexing, we can increase the cross polarization discrimination and optimize the parameters of the antenna structure to reduce the depolarization effect of the propagation path on the transmitted signal. Figure 8 shows the schematic diagram of MIMO spatial multiplexing. The compromise between the antenna size, the antenna weight, and the transmission distance can be achieved by choosing the number of antennas and antenna spacing at both the transmitter and receiver ends. For example, ultra-long wireless transmission distance up to kilometers can be realized with the employment of small-beam width and large-gain W-band Cassegrain antennas.⁷¹ Moreover, the W-band phased array antenna has the advantages of higher aperture efficiency as well as smaller physical size and weight, which can further improve the SNR of the received signals and the ability to receive weak signals after long distance transmission in a more cost-effective and stable way.

The conversion from high-speed wireless mm-wave signal into optical signal can be realized by employing RF-transparent photonic demodulation technique, and then the converted optical signal can be delivered over a long-distance fiber link. Figure 9 gives the principle of the RF-transparent photonic demodulation technique based on the Mach-Zehnder modulator (MZM).¹⁰⁸ First, the baseband polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) optical signal (λ₁) can be obtained at the transmitter central office and then it is heterodyne beat with the local oscillator (λ₂) at the transmitter base station to generate wireless PDM mm-wave signal (f_RF = c|1/λ₁ − 1/λ₂|). Second, after 2 × 2 MIMO wireless transmission, the generated PDM wireless mm-wave signal can be converted via two parallel MZMs at the receiver base station into an optical mm-wave signal, which has an optical carrier and two PDM-QPSK modulated sidebands spaced by λ_RF (λ_RF = c/f_RF) from the optical carrier. After filtering out one sideband and the optical carrier, only one sideband as an optical baseband signal is delivered over the fiber to the receiver central office. Finally, the original transmitted data can be recovered by homodyne coherent detection and DSP. At the receiver base station, the analog down conversion before electro-optical conversion effectively reduces the carrier frequency of the regenerated optical mm-wave signal, which significantly reduces the bandwidth requirement for the MZMs. It is worth nothing that the down conversion will be more compatible with the next 5G/5G+ mobile communication network occupying lower mm-wave frequency bands. The fiber-wireless-fiber integration systems and networks based on this RF-transparent photonic demodulation technique can potentially replace the interrupted large-capacity long-distance fiber link during the natural disasters including earthquakes and tsunamis to provide the emergency communication services.

Low-complexity high-efficiency DSP is also critical for fiber-wireless integration systems, since it can effectively mitigate or compensate for various kinds of linear and nonlinear impairments caused by imperfect components and transmission links, to significantly improve the system performance.⁷⁶ 

Figure 10 gives an example of the wideband wireless receiver based on heterodyne detection and DSP. First, the optimal sampling point can be achieved by using clock extraction and clock recovery algorithms. Second, after down conversion and chromatic dispersion compensation, constant-modulus-algorithm (CMA) equalization can be used to realize polarization de-multiplexing and wireless interference suppression at the same time. Third, the carrier recovery algorithms including frequency and phase offset compensation can be applied to adaptive blind equalization. Finally, considering the compatibility and efficiency of different kinds of algorithms for different modulation formats, the feed-forward and feedback equalization can be integrated to achieve the best overall system performance.

In this section, we will introduce the great research progress we have made in the field of the high-speed/long-distance fiber-wireless integration transmission systems, particularly at W-band and D-band, which is enabled by the enabling techniques introduced in Sec. III.

For the first time, we have realized 108-Gb/s multi-dimensional multi-level vector mm-wave signal transmission over a fiber-wireless integration transmission link at 100 GHz as shown in Fig. 11.²³ The optical polarization multiplexing and heterodyne beating are used to generate PDM vector mm-wave signals with a rate of 108 Gb/s at W-band. The mm-wave carrier frequency is 100 GHz and the modulation format is PDM-QPSK. The fiber transmission distance is 80 km and the wireless air transmission distance is 1 m. At the wireless receiver side, two-stage down conversion is implemented for the X- and Y-polarization received signal components. The first stage is in the analog domain based on balanced mixers and sinusoidal RF signals and the second stage is in the digital domain based on DSP techniques. The polarization de-multiplexing is implemented by the CMA equalization based on DSP in heterodyne coherent detection. The insets in Fig. 11 also give the measured optical spectrum after polarization-diversity splitting as well as the received X- and Y-polarization QPSK constellations.

Furthermore, we have realized the record 432-Gb/s multi-dimensional multi-level vector mm-wave wireless signal delivery at W-band as shown in Fig. 12.⁷² The remote heterodyne beating, optical polarization multiplexing, and antenna polarization multiplexing are applied to generate the multi-dimensional multi-level vector mm-wave signals at a rate of 432 Gb/s and a mm-wave carrier frequency of 94 GHz. The modulation format is PDM-16QAM, and the spectrum efficiency is as high as 11.4 bits s⁻¹ Hz⁻¹. The wireless air transmission distance is 2 m. Both electrical pre-equalization at the transmitter and long-tap cascaded multi-modulus algorithm (CMMA) equalization at the receiver are adopted based on DSP. Figure 12(a) gives the measured optical spectrum after polarization-diversity splitting. The bit-to-error ratio (BER) after 2-m 4 × 4 MIMO wireless delivery using four pairs of antennas can be less than the hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10⁻³ as shown in Fig. 12(b). This is the first time to demonstrate 400 G wireless delivery at W-band.

In 2014, we achieved a record wireless transmission distance of up to 1.7 km for W-band signal.¹⁰⁹ The remote heterodyne beating, optical polarization multiplexing, and antenna polarization multiplexing are applied to realize the generation of the multi-dimensional multi-level vector mm-wave signal at a rate of 20 Gb/s at W-band as shown in Fig. 13. The mm-wave carrier frequency is 85.5 GHz and the modulation format is PDM-QPSK. Figure 13(a) gives the measured optical spectrum after polarization-diversity splitting. The 1.7-km 2 × 2 MIMO wireless link employs two pairs of CAs with large gain and small beam width. The employment of two parallel W-band low-noise power amplifiers at the wireless transmitter side significantly extends the wireless transmission distance and their gain performance is given by Fig. 13(b). After the 20-Gb/s PDM-QPSK signal is transmitted over the 1.7-km 2 × 2 MIMO wireless link at 85.5 GHz, the BER is less than the HD-FEC threshold of 3.8 × 10⁻³. Figure 13(c) gives the electrical spectrum after analog down conversion. Figure 14 gives the map display of the 1.7-km wireless transmission link.

Moreover, we have demonstrated a large-capacity long-distance high-spectrum-efficiency photonic-aided fiber-wireless integration transmission system, which can realize bidirectional delivery over the wireline link and wireless link as shown in Fig. 15.⁷¹ Advanced DSPs with pre-distortion and decision-directed least-mean-square (DD-LMS) equalization are employed to significantly improve the system performance. For both K-band uplink and W-band downlink, the fiber transmission distance is 20 km and the wireless transmission distance is 2500 m. For W-band downlink, the modulation format is PDM-8QAM and the mm-wave carrier frequency is 94 GHz. Two pairs of W-band CAs are used for 2500-m wireless transmission link and each W-band CA has ∼45-dBi gain, ∼0.8° 3-dB beam width, <1-ft diameter, >35-dB cross polarization discrimination, and 75-110-GHz operating frequency range. For K-band uplink, the modulation format is PDM-16QAM and the mm-wave carrier frequency is 23 GHz. The 2 × 2 MIMO wireless link at K-band is consisted of two pairs of K-band CAs and each K-band CA has ∼34-dBi gain, ∼2.9° 3-dB beam width, <1-ft diameter, and >30-dB cross polarization discrimination and an operating frequency range of 21.2-23.6 GHz. To our best knowledge, this is the first demonstration of multi-band bidirectional >2-km wireless signal delivery, the first demonstration of >2-km K-band wireless signal delivery with 16QAM, and the first demonstration of >2-km W-band wireless signal delivery with 8QAM.

Recently, we realized the record >1-Tb/s vector signal delivery over the fiber-wireless integration transmission link at D-band, as shown in Fig. 16.⁷⁵ The modulation format is Probabilistic shaping (PS) 64QAM, the fiber transmission distance is 10 km, and the wireless transmission distance is 3.1 m. We simultaneously employ two different D-band mm-wave carrier frequencies, i.e., 124.5 GHz and 150.5 GHz. The employment of advanced DSP techniques, including PS, Nyquist shaping, and look-up-table algorithm, significantly improves the transmission capacity and system performance. Two dual-subcarrier PDM-64QAM-PS5.5 modulated mm-wave signals with a total baud rate of 2 × 2 × 24 = 96 Gbaud and a total bit rate of 96 × 5.5 × 2 = 1.056 Tb/s can be delivered over 3.1-m wireless distance with a BER under the soft-decision forward-error-correction (SD-FEC) threshold of 4 × 10⁻². After removing the SD-FEC and PS overhead, the corresponding net bit rate is ∼762.2 Gb/s.

Fiber-wireless integration can possess the advantages of both fiber and wireless transmission, in particular, the large bandwidth of fiber and ultra-wide bandwidth in the mm-wave (from Q- to D-band). We have touched upon the importance of fiber-wireless integration for future 5G+ wireless communication. In this paper, fiber-wireless integration techniques have been actively pursued and investigated and they include photonic vector mm-wave generation techniques, the integration of multiple multi-dimensional multiplexing techniques, RF-transparent photonic demodulation technique for fiber-wireless-fiber network, and low-complexity high-efficiency DSP. Based on the combination of these approaches, we have demonstrated the implementation of large-capacity and long-distance fiber-wireless integration transmission systems. We have realized a record wireless transmission capacity up to 432 Gb/s at W-band, as well as a record wireless transmission distance up to 1.7 km for W-band signal. Moreover, employing photonic-aided technology, wireless MIMO, PS, and other advanced DSP techniques including Nyquist shaping and look-up-table algorithm, we experimentally demonstrated a wireless transmission of 4 × 4 MIMO PS 64QAM mm-wave signals at D-band over 3.1-m distance with a total bit rate of 1.056 Tb/s. To our best knowledge, this is the first time to realize >1-Tb/s mm-wave signal wireless delivery. Our experimental results verified that the advances in fiber-wireless integration technology and movement to higher carrier frequencies will greatly promote the development of future 5G/5G+ mobile communication.

This work was partially supported by NNSF of China (Nos. 61325002, 61250018, 61527801, 61675048, 61720106015, 61835002, and 61805043).