Techniques of Millimeter-wave Signal Generation in ROF Systems: A Review

Techniques of Millimeter-wave Signal Generation in ROF Systems: A Review Davinder Singh UIET, Panjab University, Chandigarh Preeti Singh UIET, Panjab University, Chandigarh ABSTRACT Millimeter-wave ROF (Radio Over Fiber) technology has emerged as a competitive candidate that can meet the increasing demands of broadband multimedia services for wireless users. Optical generation of mm-wave signal is one of the most important technologies of millimetre-wave ROF system. Present paper discusses the various techniques of mm-wave generation and their relative pros and cons. Keywords Fiber optic communication, laser, modulator, Millimeter wave, Radio-over-fibre 1. INTRODUCTION Recently there has been observed a huge requirement of Bandwidth in wireless and wired communication with the advent of bandwidth demanding applications like video based interactive and multimedia services. Congestion and limited frequency spectrum has haltered the data rates of current wireless systems to Megabits-per-second(Mbps) only. To achieve high data rates, the viable solution is bandwidth and the most assured path to multigigabit-per-second (Gbps) is the use of millimeter (mm)- wave frequencies which occupies large bands of frequency spectrum [1]. FCC s 60-GHz band that offers a huge bandwidth of 7GHz (57-64 GHz) has gained much attention in recent years [2][3]. Optical fiber is the ideal medium for millimeter-wave s transmission due to its low loss, low cost and wide bandwidth [4]. The resultant technology is called Radio-over-Fibre technology (ROF). Radio-over-Fibre in mm-wave band is the promising technology to meet challenges of next generation communication systems. In the ROF system, generation of optical mm-wave is one of the key techniques. This paper discusses the various techniques to generate optical mm-wave at around 60 to 120 GHz. 2. OPTICAL GENERATION OF MM-WAVE SIGNALS Various techniques have been developed to generate mm-wave signals. Ensuing text briefly discusses the main mm-wave signal generation techniques. 2.1 Direct Intensity Modulation The direct intensity is based on using a mm-wave carrier source to directly modulate a high speed LASER and then the mm-wave signal can be recovered at photodiode by direct detection. The method of direct intensity is shown in Figure 1. In 2003, Hartmannor et al. made use of directly modulated DFB lasers to transmit high data-rate Orthogonal Frequency Division Multiplexing (OFDM) video signals over 1- km Multi-Mode Fiber (MMF) [5]. Figure 2 shows a schematic of experimental set up where the video signal is transmitted from a mobile laptop to a desktop PC. Fig 1: Direct Intensity Modulation Fig 2: A schematic of experimental set up employing Direct Intensity [5] This method is feasible only when the operating RF frequencies are below the cut-off frequency of the laser diode used. So far, the highest cut-off frequency of lasers is reported to be about 40 GHz [6]. Most of the commercially available lasers have frequencies of about 10 GHz or less. Though this method is simple and efficient but is not appropriate to mm-wave bands as the bandwidth of modulating signal is limited by the bandwidth of laser. To generate high frequencies, modulating signal should also be at high frequency. This is not possible due to limited Bandwidth and laser s non-linearity. For fibre transmission of higher frequencies such as millimeter-wave signals, the viable option is optical external. 2.2 External Modulation In this method, the laser operates in CW mode and an external modulator such as Mach-Zehnder modulator (MZM) or electroabsorption modulator (EAM) is used to modulate the intensity of light. Its configuration is simple. But it has some disadvantages P a g e 45

like fibre effects, high insertion loss. It also suffers from distortion due to the intrinsic nonlinearity of the modulators. The scheme of generating mm-wave signal by using MZM as an external modulator was first proposed by O'Rcilly et al. in 1992 [7]. The phenomenon of external intensity using MZM is shown in figure 3. A single DFB laser source is used together with a MZM. By biasing the MZM at V, the half-wave voltage of MZM, the optical carrier at centre wavelength will be suppressed. The beat of upper and lower 1st side-modes will generate the required mm-wave signal with frequency twice to that of the mm-wave signal applied to MZM. Many variants of this method have been proposed [8]-[17]. Fig 3: External Intensity Modulation using MZM Another approach that uses an optical phase modulator to generate a frequency-quadrupled electrical signal wave was proposed in 2003[18]. This approach relies on Fabry-Pérot filter to select the two second-order optical sidebands. An electrical signal with frequency four times to that of the frequency of the electrical drive signal was generated by beating the two second-order sidebands at a photo detector. An optical modulator with a maximum operating frequency of 15 GHz can generate an mm-wave signal up to 60 GHz. The system was complex and costly as it relies on the optical filter to select the two optical sidebands and generate tunable mm-wave signals. 2.2.1 Intensity-Modulation-Based Approach In conventional intensity, the optical carrier is modulated to generate a carrier and double sidebands (DSB). When the signal is sent over fibre, chromatic occurs that causes each spectral component to experience different phase shifts depending on the fibre link distance, frequency, and the fibre parameter. In order to mitigate this problem, a single sideband (SSB) and an optical carrier suppression (OCS) technique has been studied [19] [22]. The OCS scheme possesses the highest receiver sensitivity, the highest spectral efficiency, lowest spectral occupancy, lowest bandwidth requirement for RF signal, electrical amplifier (EA), and optical modulator as well as smallest power penalty over long distance [23]. Figure 4 shows a system to generate and distribute wideband tunable mm-wave signal using an optical external modulator and based on a wavelength-fixed optical filter. No tunable optical filter is needed, which simplifies greatly the system implementation. MZM is biased to suppress the odd-order optical sidebands. The wavelength-fixed optical notch filter is then used to filter out the optical carrier to give two second-order optical sidebands. A mm-wave signal that has four times the frequency of the microwave drive signal is generated by beating the two second order optical sidebands at a photodetector. A stable mm-wave signal tunable from 32 to 50 GHz is obtained by tuning the microwave drive signal from 8 to 12.5 GHz. The millimeter-wave signal is transmitted over a 25-km standard single-mode fiber [8]. P a g e 46 Fig 4: Wideband tunable mm-wave signal generation based on external intensity 2.2.2 Phase-Modulation-Based Approach In order to suppress the even or odd order optical sidebands, the MZM needs to be biased at the minimum or maximum point of the transfer function, which would cause the bias-drifting problem, leading to poor system robustness, or have to employ sophisticated control circuit to minimize the bias-drift. The solution to this problem is the use of Phase Modulator instead of MZM. The advantage of using an optical phase modulator is, it does not need a dc bias. So, it is free from dc bias-drifting problem and provides a stable output. It was experimentally demonstrated that, when the electrical drive signal is tuned from 18.8 25 GHz, two bands of mm-wave signals from 37.6 to 50 GHz and from 75.2 to 100 GHz with high signal quality are generated locally and remotely. This approach does not suffer from the direct current (dc) bias-drifting problem. The generated signals had high-frequency stability and narrow linewidth. It was verified that, after compensation, the integrity of the generated signals was maintained after transmission over a 60-km SMF [24]. Other techniques to generate mm-wave signals in the optical domain include the photonic frequency upconversion by the stimulated Brillouin scattering (SBS)-based frequency tripling method [10], cross-gain in a semiconductor optical amplifier (SOA) for frequency doubling [11]. An approach to generate a frequency-tripled millimeter-wave (mm-wave) signal based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) is experimentally demonstrated in [12].Other methods include a multiband signal generation based on photonic frequency tripling technology for 60 GHz RoF systems [13] and mm-wave signals 32.4 GHz of sextuple microwave source frequency 5.4 GHz with high spectral purity generation using optical carrier suppression (OCS) technology [14]. In all the techniques [8-14], an ultra-narrowband optical filter was required, which made the system unstable and costly. To avoid using an ultra-narrowband optical filter, a technique based on two cascaded MZMs and a tunable electrical phase shifter for frequency quadrupling was proposed [15]. 2.3 Up- and Down-conversion In this technique Intermediate frequency (IF) band signal is transported over optical fibre instead of RF band signal [25][26]. IF-to-RF upconversion is accomplished at the BS level, in which the mm-wave carrier is generated by a local oscillator. The transport of the IF-band optical signal is almost free from the fibre effect; however, the electrical frequency conversion between the IF-band and mm-wave requires frequency mixers and a mm-wave local oscillator, resulting in the additional cost to the BS. Another advantage of this technique is the fact that it occupies

small amount of bandwidth, which is especially beneficial when the system is combined with DWDM. A respective configuration is shown in Figure 5. Fig 5: Representative RoF link configurations for EOM, IF modulated signal 2.4 Optical Heterodyning In optical heterodyning technique, two or more optical signals are simultaneously transmitted and are heterodyned in the receiver. An electrical beat note is then generated at the output of the photodetector with a frequency corresponding to the wavelength spacing of the two optical waves. Using optical heterodyning, very high frequencies can be generated, limited only by the photodetector bandwidth [27]. Heterodyning yields high-detected power (higher link gain) and higher carrier-to-noise ratio (CNR). Remote heterodyning has an advantage concerning chromatic. If only one of the two optical carriers is modulated with data, system sensitivity to chromatic can be reduced greatly. This is not possible in direct intensity based methods, where the two optical sidebands end up both being modulated with data. The major drawback of heterodyning is the strong influence of laser phase noise since the phases of the two optical waves are not correlated, which will be transferred to the generated mm-wave signals. Techniques used to reduce phase noise sensitivity include Optical Phase Locked Loops (OPLL) and Optical Injection Locking (OIL).The two techniques of OPLL and OIL could be combined in a single optical locking system, i.e. Optical Injection Phase Locked Loop (OIPLL) to further improve the signal quality [28]. 40 GHz mm-wave signals by employing optical heterodyne techniques have been obtained in [29]. In this paper, a new mmwave WDM system based on ROF technology was proposed. In this approach a multi-wavelength light source is obtained by supercontinuum (SC) technique. The generation of optical carriers for 6-WDM channels and 40 GHz mm-wave signals by employing optical heterodyne technique were experimentally demonstrated. Low error rate transmission of 2.5 Gbit/s in WDM channels over a distance of 25 km in a G.652 fibre was studied. 3. COMPARISON OF MM-WAVE GENERATION AND TECHNIQUES The advantages and the disadvantages of the four described above are summarised in Table 1. P a g e 47 techniques Table 1: Comparison of Millimeter-wave Generation Techniques Generation Approach Direct Intensity External Modulation Up-and Downconversion Optical Heterodyning Description Advantage Disadvantage Using a mmwave carrier source to directly modulate a high-speed LD Externally of a clean CW laser source using MZM or EAM at mmwave frequencies Transmission of IF band signal and IFto-RF upconversion at the BS level Beating of two optical waves of different wavelength at PD 4. CONCLUSION Simple Efficient Tunable with a mm wave source Simple configuration spectral purity Low noise (depends on mm wave driving source) Tunable with a microwave Source Direct IF Negligible chromatic effects frequency generation capability Full depth Fiber effect free Speed limited by LD bandwidth noise Fiber effect Nonlinear response RIN power consumption, insertion loss cost -freq. EAM cost due to additional components Complicated light source Laser phase noise The demands of broadband multimedia services for wireless users are increasing day by day. Millimeter-wave ROF (Radio Over Fiber) technology is found the latest emerging technology to meet with such demands. This paper basically provides a summary of various techniques of mm-wave generation in ROF systems available to formulate the various shortcomings in the recent

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