Review on the Millimeter-Wave Generation Techniques Based on Photon Assisted for the RoF Network System

With the development trend of wireless and broadband in the communication link and even the whole information industry, the demand of high-frequency microwave bandwidth has been increasing. The RoF network system solves the problem of spectrum congestion in low-frequency band by providing an eﬀective technology for the distribution of high-frequency microwave signals over optical ﬁber links. However, the traditional mm-wave generation technique is limited by the bandwidth of electronic devices. It is diﬃcult to generate high-frequency and low-phase noise mm-wave signals with pure electrical components. The mm-wave communication technology based on photon assisted can overcome the bandwidth bottleneck of electronic devices and provide the potential for developing the low-cost infrastructure demand of broadband mobile services. This paper will brieﬂy explain the characteristics of the RoF network system and the advantages of high-frequency mm-wave. Then we, respectively, introduce the modulation schemes of RoF mm-wave generation based on photon assisted including directly modulated laser (DML), external modulation, and optical heterodyne. The review mainly focuses on a variety of diﬀerent mm-wave generation technologies including multifrequency vector mm-wave. Furthermore, we list several approaches to realize the large capacity data transmission techniques and describe the digital signal processing (DSP) algorithm ﬂow in the receiver. In the end, we summarize the RoF network system and look forward to the future.


Introduction
Facing the development of the ultrahigh definition (UHD) video streaming and the growth of mobile users on social media, people have increasingly higher requirements for communication capability and data transmission throughput. e rapid growth of data traffic is also augmenting the demand for broadband. Wired transmission network technology can provide huge bandwidth for Internet broadband users [1]. However, due to the limitation of an optical fiber or cable, the usability and generality of the products are cut down. Adopting high-speed wireless transmission can get rid of the shackles of fiber and greatly improve the comfort level of user experience [2]. In addition, due to the laying and cost of the line, operators and users prefer to allow roaming connections. erefore, people try to combine the wired fiber transmission network with radio communication technology to propose the RoF network system [2,3]. e RoF network system is a potential solution to increase the capacity and mobility of access networks and could also reduce the cost of access networks. And the current advanced mobile communication 5G and the future 6G technologies are all based on this technology [4]. e basic principle of RoF technology mainly consists of three parts: central stations (CSs), optical fiber, and base stations (BSs) [5]. e CS mainly use modulators to modulate radio frequency (RF) signals on light waves. After generating the mm-wave, the signal is transmitted to the receiver or the BS through optical fibers. And then, the signal is transmitted to the user through the radio antenna. e RoF network system transmits data through optical fibers using radio signals to modulate optical information.
is modulation can be performed directly with RF signals or intermediate frequency (IF) signals. e spectrum of radio signals currently allocated by RoF network systems is very wide usually gigahertz-depending on the application. So, any type of RF signal modulation schemes can be transmitted over optical fibers by the RoF network system theoretically [5,6].
Furthermore, the application of the fiber-wireless integration (FWI) technology is not limited to this. In some special occasions (such as between islands), it is no picnic to lay the optical fiber, and the optical fiber link may be damaged accidentally. At this time, the integration of wireless technology and fiber communication can still play a huge role.
It is worth noting that the realization of the local area network (LAN) transmission rate up to Gbit/s is a big problem in the wireless communication network system [6,7]. e effective method is to increase the carrier frequency to extend the bandwidth of the wireless transmission network system [1]. But the available low-frequency spectrum is very tight. e distribution of its input scope is shown in Table 1.
It can be concluded from Table 1 that the frequency spectrum used by the wireless communication network system is mainly in the frequency band of 800 MHz∼30 GHz. erefore, the available spectrum resources are already very limited which will seriously hinder the further development of mobile communication. If high-frequency mm-wave can be used as carrier signal, the obstacle of spectrum resource shortage could be overcome [9]. High-speed wireless broadband access is also possible, which makes the development of high-frequency regional spectrum more urgent [10].
In recent decades, the development and utilization of mm-wave frequency band has become the main research hotspot of the RoF network system [11]. Such high-rate wireless networks can be as fast as or possibly faster than optical fibers. ere are a lot of existing optical fibers or cables in large modern cities. High-frequency mm-wave can be transmitted in the existing prelaid optical fibers without the need to lay the optical fibers again which greatly cuts down the system cost.
e mm-wave refers to the microwave with a wavelength of 1 mm∼10 mm, and the corresponding frequency range is 30 GHz∼300 GHz [12]. For current wireless communication technologies, the 270 GHz frequency range is a considerable and valuable resource. e most important point is that the 40 GHz∼60 GHz mm-wave signal can be used without authorization internationally [13]. erefore, the research and application of the communication technology based on high-frequency mm-wave have been paid much attention in the world recently. e high-frequency mm-wave wireless communication network system has the following advantages: (a) Millimeter-wave has very high frequency and large broadband, which can realize the transmission of data with superlarge capacity. Its information transfer rate can reach several hundred GB/s, up to 1TGB/s [14]. (b) e BS and radio antennas of the wireless communication system using the mm-wave carrier technology are more miniaturized, which is very helpful for installation, maintenance, and function expansion (c) e beam of mm-wave is very narrow and has good directivity in space propagation. Moreover, there are few interference sources in the propagation for the mm-wave in the high-frequency band. So, the propagation is relatively stable and reliable with the high safety factor.

Photon-Assisted Millimeter-Wave Generation Scheme
Millimeter-wave forward transmission has many advantages including strong capacity, simple deployment, and environmental adaptability. In order to achieve high spectral efficiency, it is necessary to support the RoF network system to realize signaling. In fact, in the recent research work, many RoF network systems based on mm-wave band have been proposed and verified experimentally. e following is a brief introduction to several modulation schemes that can generate high-frequency mm-wave based on photonic assistance. Figure 1 describes the structural principle of the DML scheme. It is a modulation scheme that converts the transmitted information into electrical signals and modulates the optical power, so that the electrical signals can be transmitted on the optical carrier [15]. It mainly superimposes the RF signals on the DC bias of the laser and then modulates them on the light signal through the DML. It generates the electric mm-wave signals after photodiode (PD) detection at the remote end. After filtering out unnecessary noise by the band pass filter (BPF), the generated electric mm-wave signal is amplified by an electrical amplifier (EA) and transmitted to the user through the antenna. Due to it changes the optical intensity information, it is also called intensity modulation (IM) [16,17]. However, the modulation scheme is only divided into two working states with and without light or little light. At the receiver, it is simply a matter of determining the two states. Advances in Condensed Matter Physics e proposed scheme is simple in structure and low in cost. But due to the limited modulation bandwidth of DML, the generated mm-wave frequency is relatively low [18]. Beyond the limit of the baseband signal frequency about 10 GHz, the chromatic dispersion (CD) phenomenon will change significantly [19]. Leading to the broadening of the optical pulse, the pulse will overlap with the next pulse which is called intersymbol interference (ISI) [20]. In addition, as the current change would affect the active region refractive index of the semiguided laser and lead to the change of resonant wavelength, additional phase modulation and frequency chirp will be introduced which will affect the modulation performance [21,22]. Generally, direct modulation is not suitable for high-frequency RoF network systems. And in most cases, it is used to transmit low-frequency systems. In order to overcome the frequency chirp phenomenon and improve the data transmission rate, people naturally turn their attention to the external modulation scheme [23].

External Modulation.
For frequency higher than 10 GHz, the bit error rate (BER) observed by the direct modulation format at the receiver becomes unacceptable. e external modulation associated with the direct modulation could to be selected [23]. It is usually performed by a high-speed external modulator, which generally includes an intensity modulator (IM) [24], a phase modulator (PM) [25], and an electrical absorption modulator (EAM) [26] to produce high quality microwave signals. e principle is shown in Figure 2.
e external modulation scheme uses the connection between the laser source and the external modulator to generate mm-wave. e RF signal drives the external modulator to produce a light band carrying information.
en, the light band generates mm-wave at the beat frequency in PD [27,28]. Usually limited by the low RF frequency generated by the RF source and insufficient optical device bandwidth, the external modulation scheme can employ multiple modulators combined with frequency multiplying technology to generate mm-wave signals [29]. Figure 3 shows a schematic of a cascade of multiple external modulators. e data are modulated by the optical modulator to generate the baseband signal. And then, the second optical modulator is driven by the RF to generate the optical sideband carrying the data [29]. Despite multiple modulator schemes complex structure and higher cost, it can generate higher-frequency stable mm-wave and is no longer limited by the modulation format compared with a single modulator. Unlike the above single external modulation scheme, multiple modulators scheme does not require precoding when generating vector mm-wave signal [30].

Optical Heterodyne.
Besides the DML and external modulation schemes, optical heterodyne modulation is also an efficient method to generate the mm-wave. e heterodyne beat frequency scheme is shown in Figure 4. e modulated signal enters the PD together with the optical signal emitted by the independent laser. e square-law PD is used to beat the frequency of the two signals [31]. e basic idea of optical heterodyne beat frequency is to generate mmwave signal whose carrier frequency is the difference of two lasers frequency [32]. e following is the theoretical analysis of the scheme.
It is assumed that the optical signals output by the two lasers can be expressed as where A 1 and A 2 , respectively, represent the amplitude of two optical signals. f 1 and f 2 represent the optical signal frequency. θ 1 (t) and θ 2 (t), respectively, represent the initial phase information of each laser. After the data information is modulated by laser1, the optical signal can be expressed as where I(t) + jQ(t) represents the modulated baseband vector signal. e two optical signals are then coupled in the OC. e signal is expressed as

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After entering PD, the photocurrent generated by the two photo beat can be expressed as [31][32][33] where R denotes the PD sensitivity, and equation (4) can be simplified as [31][32][33] where f △ is the difference between f 1 and f 2 . θ △ (t) is the difference between θ 1 (t) and θ 2 (t). It can be seen from equation (5) that the first two terms of the current expression are DC components. e last item is the required mm-wave information. e required frequency can be obtained by controlling the optical frequency difference between the two lasers [32,34]. e optical heterodyne scheme has the advantages of simple structure, low cost, and flexible generation of millimeter-wave frequency. However, in this scheme, the independent light sources will cause frequency drift and phase noise. e characteristics of the three mm-wave generation schemes are shown in Table 2.

SSB and DSB Vector Signals Generation Based on DML.
e principle of SSB or DSB vector mm-wave signal generation based on the DML technology is shown in Figure 5.
It is assumed that the laser traveling wave output of the DML is expressed as where A(t) is the amplitude of the traveling wave, and A(t) is the phase angle. e electric vector RF drive signal generated at the offline end with frequency at f s can be expressed as where V m and ω f , respectively, represent the amplitude and phase angle of the driving signal. e drive signal is then converted into a DAC for the DML. Its output can be expressed as [35] where m h � (πV h /V π ), and V h and V π are the RF drive signal voltage and DML half-wave voltage, respectively. J is the first kind Bessel function [36]. e function image is shown in Figure 6. SSB modulation output can be expressed as [35] Here, the modulated signal is transmitted to the remote end via SMF. OSC captures the signal after PD beat frequency. en, the normal electric vector mm-wave signal can be obtained reasonably by offline DSP. It is worth noting that IL is not required when selecting the DSB modulation format [35].
Supposing the output of the laser can be expressed as follows: where E 0 and ω 0 , respectively, represent the amplitude and angular frequency of the optical signal. Both MZM-a and MZM-b are set at the maximum offset point. e electric respectively. V m represents the amplitude of electric vector signal, and θ(t) represents the phase. ω RF represents the central frequency of the RF drive signal. After the output of MZM-b is deflected by 180°phase, the optical signal generated together with MZM-a can be expressed as [30] where m(t) stands for the modulation index. Within the effective range, ignoring higher-order optical sideband greater than J 2 , equation (12) can be simplified as [30] e signal generated after PD beat frequency can be expressed as [30] You can see that the vector signal produced is four times as frequent as the original signal [38,39]. at is to say, we can get a vector mm-wave signal of quadruple frequency. erefore, the vector signal needs to be precoded before modulation. Generally speaking, only phase precoding is required for quadrature phase shift keying (QPSK) signals. For multiple quadrature amplitude modulation (M-QAM) signals, amplitude and phase precoding are both required.

Multifrequency Vector Signals Generation Based on MZM.
Here, we briefly introduce the experiment of generating mmwave with multiple frequency and appropriate amount by selecting sideband signal through wavelength selective switch (WSS) [40]. Its schematic diagram is shown in Figure 8.
MZM works in the carrier suppression mode. e modulated optical signals are suppressed by a carrier wave or by a pair of waves. e unsubdued sideband signals carry vector signals.
en, the symmetric n-order sideband retained by WSS can be selected to generate signals of 2n times RF frequency via PD beat frequency [38,39].
Besides this, it also can generate a photon quadrate vector mm-wave based on a single MZM without WSS. Offsetting MZM to the maximum transmission point, then offline precoding RF signals are used to drive MZM. Properly modulating the drive voltage produces two second-order photon carriers. Other subcarriers are suppressed [38,41]. In this way, after PD beat frequency, quadruple frequency vector mm-wave signal can be generated. Assume that the output signal of MZM can be expressed as [38,41,42] where K 1 is a constant. J 2n (k) is the 2n order Bessel function. e function image is shown in Figure 9. k is the modulation index.
As we can see from the figure, when k increases from 0 to 3, J 0 numeration continues to decrease and J 2 , J 4 , and J 6 continue to increase. When k is set to 2.4048, J 0 goes to 0. In other words, the MZM-modulated voltage is set to the maximum point. At the same time, the center carrier is suppressed. Other subcarriers of order greater than 2th order can be ignored. erefore, the output of MZM is mainly two second-order subcarriers. Equation (15) can be simplified as [41] After the remote PD beat, the RF signal can be expressed as follows: It can be obtained from equation (17) that the frequency of the RF signal is 4 times that of the drive signal. However, the phase will change after the detection of square-law PD. Das et al. [42] describe that normal vector signals can be obtained by precoding technique.

Generation of Multifrequency Vector Millimeter-Wave
Based on a Phase Modulator. Compared with the abovementioned intensity modulator or MZM, the PM has a smaller insertion loss. is results in a higher optical signalto-noise ratio (OSNR) for the optical signal it produces. And its stability is better because there is no DC bias. Figure 10 shows the schematic diagram of generating a photon multifrequency vector mm-wave signal based on a PM and precoding [43].
Here, we take the example of quadruple frequency and briefly introduce the principle of its generation. e offline precoding vector signals with frequency at f s are generated including PRBS, vector mapping, precoding, low-pass filter, and digital upconversion. e signal output by the PM modulator can be represented as follows [43,44]: where K 1 represents the amplitude of the CW light wave. φ(t) stands for the random phase noise. J n is the first Bessel function of n order. k is the modulation index of the phase modulator.
e output carrier signal can be seen from Figure 10 as multiple photon carriers with frequency interval of f c . WSS is then used to select two second-order photonic carriers with frequency intervals of 4f s from the output of the phase modulator. Its output can be written as [25,43,45] e signal selected by WSS generates a 4f s vector mmwave signal with a frequency of 4f s via PD beat. Its current value can be expressed as (except the DC component) e random phase noise is eliminated because the two second-order photon carriers from the same laser source are phase locked with frequency locking. However, the dispersion effect of optical fiber reduces the consistency of the two second-order photon carriers. erefore, random phase noise cannot be eliminated. e performance of the generated mm-wave is reduced. Furthermore, in order to ensure that the generated vector millimeter-wave signal with a frequency of 4f s can obtain the standard modulation format, the parameters should be adjusted to make the amplitude term J 2 2 (n) and phase term θ(t) meet the standard vector modulation rules [43,45,46].
Based on the combined action of the phase modulator and WSS, this technique can theoretically generate vector mm-wave signals with arbitrary photonic multifrequency. Li et al. [43] have successfully generated 3 times optical vector mm-wave signal based on this technique. Using WSS to

Large Capacity Transmission System Structure
In the previous section, we introduced the modulation format of high-frequency mm-wave generated over optical fiber links in the RoF network system. Based on the photonic mm-wave technology, we can reduce the bandwidth required by the system devices by lessening the signal baud rate to further improve the data transmission capacity of the RoF network system. e following would introduce several methods and structures to achieve large capacity transmission based on RoF network systems.

Optical PDM Combined with MIMO.
e polarization of light is the manifestation of the s-wave characteristic of light and has a very good transmission characteristic in the optical communication system. Polarization multiplexing technology can double the transmission capacity of the system [50]. At present, the optical polarization division multiplexing (PDM) technology has been very mature. Due to the maturity of optical PDM technology, many researchers are trying to combine it with the Multiple-Input Multiple-Output (MIMO) technology to realize superlarge capacity data transmission [51]. e following will be combined with its block diagram to briefly introduce the principles of each part. Its block diagram is shown in Figure 11.
It mainly consists of four parts: optical baseband transmitter, optical heterodyne upconverter, radio signal transceiver antennas, and wireless mm-wave signal receiver. e main function of the transmitter is to generate optical modulation PDM signals. en, the signal is generated by a polarization multiplexer. e signal transmitted to the polarization division multiplexer is sent by the modulator. Here, the modulator is driven by electronic data from the transmitter [53]. e principle of polarization multiplexers is described later. en, the optical PDM signal is transmitted to the optical heterodyne upconverter by an optical fiber. e main work of this part is to convert the baseband signal sent by the transmitter into mm-wave frequency band signal. In the Figure 11, the optical frequency difference between the frequency of LO and the baseband signal is the central frequency of mm-wave. en, polarizing beam splitter (PBS) and OC are used to obtain polarized coherent millimeter-wave signals in X and Y directions after upconversion by PD [54]. In the wireless connection part, there are two antennas on each side to form a 2 × 2 MIMO wireless link. At the receiver, the signals need to be converted into baseband signal through downconversion and finally sent to the digital signal processing part, where the signal is optimized to reduce BER and improve the transmission performance of the system. e detailed principle of the optical transmitter part and the basic characteristics of MIMO transmission are described below. At the transmitter, the laser generates a continuous wave (CW), which is then modulated through the I/Q modulator. And the arbitrary waveform generator (AWG) generates the desired modulation signal, which is amplified by the EA to drive the I/Q modulator [55]. Optical PDM technique is realized by the multiplexer. A polarization division multiplexer includes an OC for dividing the signal into two branches, a delay line (DL) for providing symbol delay, and an attenuator (ATT) for balancing the power of the two branches [54,55]. e two branch signals are recombined into a branch signal via PBS and transmitted to a distant base station via SMF. Figure 12 is the schematic diagram of the optical baseband transmitter part.
Besides, we introduced the principle of the MIMO connection part too, and it can be briefly described as follows [55]: where r x r y T is a wireless transmission to receive a signal; s x s y T and n x n y T denote the transmitted signal and the noise. e first term on the right of the equal sign can be described as [55] where the two items on the right side of the equation are the Jones transmission elements of fiber optics and wireless links. In [56], the ROF network transmission of 5 Gb/s based on PDM and MIMO has been successfully verified. In [57], it is confirmed that 224 Gb/s information data can be transmitted.

Advanced Multilevel Modulation.
e optical fiber wireless network system based on the combination of optical PDM and MIMO reception can realize the transmission of ultralarge capacity data, but its upper limit is far larger than that. It is well known that high-order I/Q modulation (such as M-QAM) is used to improve spectrum efficiency [58,59]. erefore, in the signal transmitter part, when the advanced multilevel modulation signal is modulated on the I/Q modulator, the transmission capacity of the ROF network system based on the combination of optical PDM and MIMO can be increased several times. Due to the principle is similar to that in the previous section, we would not elaborate further here.
In addition, it is necessary to introduce the DSP part of the receiver. Compared with the ADC and the basic DSP algorithm in the previous section [56,59,60], the DSP algorithm of this structure is partly complex. Its sequence block diagram is shown in Figure 13.
After the signal is converted by ADC, the two signals (polarization states of X and Y) are restored by the same DSP algorithm. e converted signal is converted into baseband signal by double resampling and downconversion immediately [60]. en, a series of classical algorithms are used to restore the signal. ese include clock recovery, classical constant modulus algorithm (CMA), and frequency offset compensation and phase recovery. After a series of corrections, BER comparisons are made through differential decoding [61,62].

Optical Multicarrier
Modulation. All the above are effective methods based on optical single carrier modulation, while optical multicarrier modulation can effectively reduce the transmission baud rate under the same conditions. It can also realize ultralarge capacity data transmission [63]. Figure 14 shows the schematic diagram of the combination of multicarrier modulation and the RoF transmission network system.
As shown in Figure 14, the system is similar to dense wavelength division multiplexing (DWDM), and its principle is explained by three optical carrier channels (ch1, 2, and 3) combined with optical carrier suppression (OSC) technology [64]. Here, the three-carrier signal sent by the transmitter is divided into two channels after passing through the BPS. Laser2 provides LO frequency to beat frequency or heterodyne to produce mm-wave signals [63][64][65]. e following describes the generation of threecarrier signal principle. Schematic diagram of the optical baseband transmitter is shown in Figure 15.
After passing through the I/Q modulator, the electrical signal is split in two by an OC. One route produces the desired signal (ch2) via a polarization multiplexer and the other is modulated by MZM. en, it is driven by RF signal and offset at zero to achieve optical carrier suppression.

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After the suppression of the signal, through polarization multiplexer, ch1 and ch3 signal finally through AWG and then transmitted by SMF [65,66]. It is worth noting that the subsequent DSP parts of the optical multicarrier modulation technology system are similar to those described in the first two sections, with the only difference being that the front end requires channel demultiplexing. e structure diagram is shown in Figure 16.

Antenna Polarization
Multiplexing. Various modulation schemes over optical fiber links are already capable of transmitting huge amounts of data, but scientists and operators will not be satisfied. In general, the antennas commonly used in our field will show large or small polarization (commonly understood as the direction of the electric field), and the antenna can improve the information transmission efficiency through gain [67]. erefore, we can further improve the transmission capacity from the angle of antenna polarization. Figure 17 shows a schematic diagram of the antenna polarization multiplexing system. ere are two polarization states of antenna, namely, E polarization and H polarization, which most people like to call horizontal polarization (H-pol) and vertical polarization (V-pol).
e MIMO wireless link in the figure consists of four transmitting antennas and four receiving antennas. It can be divided into two parts, one of which is set as horizontal polarization to form an H-polarized HA array to transmit ch1, and the other to form a V-polarized HA array to transmit ch2 [68]. It is worth noting that the signals received by the receiving antenna have to be sent into the OSC and then restored and optimized by a series of DSP part after ADC. Figure 18 shows the schematic diagram.
e signal generation part is still similar to the principle shown above. Here, an additional WSS equivalent to a BPS is added to select the signal spectrum bandwidth. After being selected by WSS, the signal is provided by LO to beat frequency and generate modulated signals [52,67,68].

Conclusion
At the beginning of this paper, people's demand for high broadband and high capacity communication is introduced, and then, the central topic of the fiber optic radio network is introduced. is paper introduces the structure of the RoF network system and the advantages of high-frequency mmwave signal transmission. After that, several modulation formats that generate mm-wave frequency band on RoF based on photon assisted are introduced. Based on these modulation schemes, several vector multifrequency mmwave signal generation techniques are reviewed. Finally, based on the modulation formats mentioned above, several advanced highly complex methods and structures to realize data transmission with very large capacity are listed. To sum up, the RoF network system will continue to be a research hotspot for experts in the field of communication in the next decade. Due to its high broadband and ultrahigh-speed transmission characteristics and its high flexibility in access, its applications are very extensive. However, with the increase of distance and the constant demand of communication capacity, the transmission network system still has more or less shortcomings, and there are still many undiscovered realms to be explored.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.  Advances in Condensed Matter Physics 11