A Review on Radio-Over-Fiber Technology-Based Integrated (Optical/Wireless) Networks

1.2.1 Direct modulation technique

In this method, the intensity of the light source is directly modulated with the RF signal and then direct detection is used at the photodetector (PD) to recover the RF signal. This is the simplest and low-cost method for optically distributing RF signals. It is referred to as intensity modulation direct detection (IMDD). Prior to transmission, the RF signal must be appropriately premodulated. After transmission through the fiber and direct detection on a PD, the photocurrent is a replica of the modulating RF signal applied directly at the head end [5]. Subsequently, at the remote antenna site, the PD and band-pass amplifier convert the received optical signal to an RF signal to be radiated by the antenna (Figures 2 and 3).

Figure 2:

Direct intensity modulation method by laser [6].

Figure 3:

Direct intensity modulation method by external modulator [6].

The main advantage of this method is that it is simple, robust and low cost. Second, the system becomes linear if low dispersion fiber is used together with an external modulator. The transmitter configuration is extremely simple and cost-effective but its performance is severely limited by the laser modulation impairments. RoF systems using direct microwave intensity modulation of a laser diode are commercially available up to limited RFs (up to about 2 GHz, for wireless services such as Global System for Mobile Communication (GSM) and Universal Mobile Telecommunications System (UMTS). Therefore, the major drawback of this technique is that the operation at higher microwave frequencies is prohibited by the restricted modulation bandwidth of the laser diode and by the fiber dispersion, which causes fading of the two modulation sidebands. Such microwave frequencies may only be handled by sophisticated very high-frequency optical analog transmitters and receivers, and careful fiber dispersion compensation techniques [6, 7].

1.2.2 External modulation technique

In order to overcome the drawbacks of the direct modulation, external modulation is the straightforward solution. An optical external modulation technique is a good option to generate optical mm-wave signal with high spectral purity, out of all schemes. External modulation is often used at high RFs (say above 10 GHz). The simplest implementation consists of a continuous wave (CW) laser followed by an external modulator that modulates the laser light with an intermediate frequency (IF) or an mm-wave tone [7]. The operation of the laser in CW mode avoids the excessive chirp of the pulses in external modulation. In this method, high-speed external modulators are used, such as the Mach–Zehnder modulators (MZM) or electroabsorption modulators (EAM) or a phase Modulator (PM), whose output is optically filtered (Figure 4).

Figure 4:

Direct intensity modulation method by external modulator using a Mach–Zehnder modulator [7].

There are two approaches to generate mm-wave signals and they are briefly discussed as follows:

1.2.3 Heterodyne modulation technique

Two or more optical signals are simultaneously transmitted and are heterodyned in the receiver by using this technique. One or more of the heterodyning products is the required RF signal. The optical intensity-modulated signal from an laser diode (LD) is subsequently modulated by an external modulator which is biased at its inflexion point of the modulation characteristic and driven by a sinusoidal signal at half the microwave frequency. Thus, a two-tone optical signal emerges at the modulator output, with a tone spacing equal to the microwave frequency. The desired amplitude-modulated microwave signal is generated after heterodyning. The transmitter may also use multiple LDs, and thus a multi-wavelength RoF system can be realized with a tunable wavelength division multiplexing (WDM) filter to select the desired wavelength radio channel at the antenna site [8]. As phase noise is a major problem in mm-wave transmission, care must be taken to produce a small phase noise only by the heterodyned signals. This approach overcomes chromatic dispersion effect and also offers flexibility in frequency since frequencies from some megahertz up to the terahertz region are possible [7, 8] (Figure 5).

Figure 5:

Remote heterodyning using a filter [8].

1.2.4 Optical frequency/phase-locked loops

This technique is used to reduce phase noise sensitivity. It is able to track small-scale phase perturbations. It does not suppress small-scale frequency variations caused by phase noise. The basic configuration of Optical Frequency Locked-Loops/Optical Phase Locked-Loops (OFLL/OPLL) techniques is shown in Figure 6. It consists of a free-running master laser, a Positive Intrinsic Negative (PIN) photodiode, a slave laser, a frequency or phase detector, an amplifier, a loop filter and a microwave reference oscillator. The combined outputs of the master and slave lasers are split into two parts: one is used in the OPLL/OFLL at the head end while the other part is transmitted to the RAU. To generate a microwave signal, the optical signal at the head end is heterodyned on a PD. This generated signal is then compared with the reference signal. A frequency error signal in the case of the OFLL (and a phase error signal in the case of OPLL) is fed back to the slave laser [8]. Hence, the slave laser is forced to track the master laser at a frequency offset corresponding to the frequency of the microwave reference oscillator. The main advantage of this technique is that it is capable of producing high-quality RF signals with narrow linewidth and also has good temperature-tracking capabilities. In addition, OPLLs exhibit a wide locking range [9]. On the other hand, OFLL techniques have the advantage that they can be realized with standard and fairly inexpensive Distributed Feedback (DFB) lasers [8, 9].

Figure 6:

Principal of optical frequency/phase-locked loops [9].

1.2.5 Dual-mode lasers

The major drawback of optical heterodyning-based techniques is the sensitivity to phase noise of the two heterodyning signals, and the dependence of the RF beat signal on the polarization state difference of the two heterodyning carriers. One way to achieve correlation of optical modes is to remove the phase shift in the DFB laser so that no oscillation occurs at the Bragg frequency which result in a device called the dual-mode laser (DML) as it emits two modes, one on either side of the Bragg frequency [10]. By tuning the grating strength coefficient, the required mode separation can be obtained. The main advantage of this approach is that it does not require complex feedback circuitry. However, because of narrow locking range, this method has limitations regarding tenability [9, 10].

1.3.1 Low attenuation loss

It is a well-known fact that signals transmitted on optical fiber attenuate much less than through other media, especially when compared with wireless medium. By using optical fiber, the signal will travel further, reducing the need of repeaters. With increase in frequency, losses due to absorption and reflection in free space increase, whereas in transmission line, impedance rises with frequency and leads to very high losses [10]. So for electrically distributing high-frequency radio signals over long distances expensive regenerating equipment is required. At each BS, the baseband or IF signals are upconverted to the required mm-wave frequency, amplified and then radiated. Since for upconversion at each BS, high-performance local oscillators (LO) would be required which leads to complex BS with tight performance requirements. However, RoF technology can be used to achieve both low-loss distribution of RF signal and simplification of BS at the same time, since optical fiber offers very low loss [10, 11].

1.3.2 Large bandwidth

Optical fibers offer enormous bandwidth. There are three main transmission windows, which offer low attenuation, namely the 850, 1,310 and 1,550 nm wavelengths. For a single-mode fiber (SMF) optical fiber, the combined bandwidth of the three windows is in the excess of 50 THz. The high optical bandwidth allows high-speed signal processing which is just impossible to do in electronic systems [11]. Thus, some demanding microwave functions such as filtering, mixing, upconversion and downconversion can be easily executed. Subcarrier Multiplexing (SCM) technique is used in analog optical systems including RoF technology in order to increase optical fiber bandwidth utilization.

1.3.3 Easy installations and maintenance

Most RoF techniques eliminate the need for an LO and related equipment at the RAU. In RoF systems, complex and expensive equipment is kept at the head end, thereby making the RAUs simpler. At the CS high-frequency electro-optical modulators and electronics must be avoided due to their high price and power consumption. Similarly complicated implementations of downlink transmission techniques are also avoided due to their high manufacturing and maintenance costs. Modulation and switching equipment is kept in the head end and is shared by several RAUs [12]. This arrangement leads to smaller and lighter RAUs, effectively reducing system installation and maintenance costs. Easy installation and low maintenance costs of BSs are very significant demand for RoF systems, because elevated numbers of BSs are required. Simplicity of the BSs drives the reduction of price associated with energy consumption, site leasing and site acquisition. Therefore, the increasing demand for new services in current cellular system will lead RoF systems to support different traffic characteristics. Hence, a suitable pricing scheme must be selected that allow service providers to assure the uninterrupted quality of service provisioning and simultaneously to be economically survivable [11, 12].

1.3.4 Reduced power consumption

The energy efficiency of a system can also be measured as the energy consumed per bit of data transferred (Joules per bit). The power consumption of a system is one of the important figures of merit that can be expressed by the power consumption per user versus the average access rate (Watts/Mbps) and the power consumption of an access network infrastructure is designed on the basis of network segmentation. The energy consumption of each part of the system for a range of access rates is computed using manufacturer’s data on equipment energy consumption for a range of typical types of hardware. To predict the rise in power consumption because the number of users and access rate per user is increasing rapidly, this perspective provides a better platform. For an RoF system, the power efficiency should account for both optoelectronic and electrical components in the CS [13]. Since, BS accounts for up to 70 % of the total power consumption in commercial cellular systems, therefore, BS design has the most opportunities for saving energy. Since the large numbers of BSs are needed to cover a service, therefore, power consumption of the BSs is of special importance. The BS power consumption varies depending on the transmitting power and traffic load, the higher is the transmitting power or traffic load, higher power will be consumed by the BS. In RoF, the power consumption model for BS includes the mm-wave frequency to be radiated, the expected cell coverage or transmitting power and the transmission schemes for uplink and downlink [13, 14].

1.3.5 Dynamic resource allocation

Dynamic capacity allocation obviates the requirement for allocating permanent capacity, which would be a waste of resources where traffic loads vary frequently. Moreover, the centralized head end enables other signal-processing functions such as macro-diversity transmission and mobility functions [15]. It is possible to dynamically allocate peak times capacity; since modulation, switching and other RF functions are performed at a centralized head end. For example, more capacity can be allocated to an area (e. g. shopping mall) and then re-allocated to other areas (e. g. to populated residential areas in the evenings) when off-peak in an RoF distribution system for GSM traffic. This can be attained by allocating optical wavelengths through WDM technique whenever needed [14, 15].