**4. Radio over fiber network**

The deployment of optical and wireless access network infrastructure is starting to proliferate throughout the world. When these heterogeneous access networks converge to a highly integrated network via a common optical feeder network, network operators can reap the benefits of lowering the operating costs of their access networks and meeting the capital costs of future upgrades more easily. In addition, the converged access network will facilitate greater sharing of common network infrastructure between multiple network operators. Signals received wirelessly and transported over optical fiber (RoF) links will be a possible technology for simplifying the architecture of remote base stations (BSs). By relocating key functions of a conventional BS to a central location, BSs could be simplified into remote antenna units that could be inter-connected with the central office (CO) via a high performance optical fiber feeder network.

Wireless networks typically show considerable dynamics in the traffic loads of their radio access points (RAPs) due to the fluctuations in the number and nature of mobile and wireless services demanded by the networks users. Using the traditional RAPs approach this requires all the wireless nodes to be equipped to cater for the highest capacity likely to be demanded of them which results in the inefficient use of network resources. The design of dynamic reconfigurable micro/pico or femo wireless cells increases network complexity but can significantly increase network efficiency. Similarly, within the optical access network layer WDM PONs allow an extra level of reconfiguration as wavelengths can be assigned either by static or dynamic routing.

The numbers of wireless subscribers are increasing and these subscribers are demanding more capacity for ultra-high data rate transfer at speeds of 1Gbps and up while the radio spectrum is limited. This requirement of more bandwidth allocation places a heavy burden on the current operating radio spectrum and causes spectral congestion at lower microwave frequencies. Millimetre Wave (mm-Wave) communication systems offer a unique way to resolve these problems (Ji, et al. 2009). Radio over fiber (RoF) technology is currently receiving a lot of attention due to its ability to provide simple antenna front ends, increased capacity, and multi wireless access coverage.

An analog RoF (ARoF) also known as RoF is the technique of modulating a radio frequency (RF) sub-carrier onto an optical carrier for distribution over a fiber network. An ARoF link includes optical source, modulator, optical amplifier and filters, optical channel and a photodiode as a receiver, electronic amplifiers and filters; a simple ARoF architecture is shown in Fig. 10. In this system, for a downlink at a central station, a signal received wirelessly is modulated onto an optical carrier generated by a laser diode (LD) and the modulated optical signal is transported over a fiber optic cable. The transported optical signal is detected at base station using a photo diode (PD). The received signal, recovered after performing analog signal processing, is fed to an antenna for wireless transmission. For uplink signal transmission from a base station to the central station, the signal received at an antenna is directed to a low noise amplifier (LNA) and modulated onto an optical carrier that is generated by another LD. The generated optical signal is sent back to the central station for any signal processing and detection. In some cases the RF signal is directly modulated by optical source, but as the laser is usually a significant source of noise and distortion in a radio over fiber link, the laser diode normally exhibits nonlinear behavior. When the LD is driven well above its threshold current, its input/output relationship can be modeled by Volterra series of order 3. Therefore, in high data rate links indirect modulation has better performance. However, an ARoF link suffers from the nonlinearity of both microwave and optical components that constitute the optical link (Al-Raweshidy & Komaki, 2002; Cox, 2004; Li & Yu, 2003). Fig. 11, shows an ARoF link architecture with indirect intensity modulation that uses an electro-optical modulator for modulating an electrical signal representing the information in a wireless signal onto a continuous wave laser source.

Fig. 10. A direct intensity modulation and detection full-duplex ARoF architecture.

Fig. 11. Downlink architecture of a ARoF link with indirect intensity modulation.

An analog RoF (ARoF) also known as RoF is the technique of modulating a radio frequency (RF) sub-carrier onto an optical carrier for distribution over a fiber network. An ARoF link includes optical source, modulator, optical amplifier and filters, optical channel and a photodiode as a receiver, electronic amplifiers and filters; a simple ARoF architecture is shown in Fig. 10. In this system, for a downlink at a central station, a signal received wirelessly is modulated onto an optical carrier generated by a laser diode (LD) and the modulated optical signal is transported over a fiber optic cable. The transported optical signal is detected at base station using a photo diode (PD). The received signal, recovered after performing analog signal processing, is fed to an antenna for wireless transmission. For uplink signal transmission from a base station to the central station, the signal received at an antenna is directed to a low noise amplifier (LNA) and modulated onto an optical carrier that is generated by another LD. The generated optical signal is sent back to the central station for any signal processing and detection. In some cases the RF signal is directly modulated by optical source, but as the laser is usually a significant source of noise and distortion in a radio over fiber link, the laser diode normally exhibits nonlinear behavior. When the LD is driven well above its threshold current, its input/output relationship can be modeled by Volterra series of order 3. Therefore, in high data rate links indirect modulation has better performance. However, an ARoF link suffers from the nonlinearity of both microwave and optical components that constitute the optical link (Al-Raweshidy & Komaki, 2002; Cox, 2004; Li & Yu, 2003). Fig. 11, shows an ARoF link architecture with indirect intensity modulation that uses an electro-optical modulator for modulating an electrical signal representing the information in a wireless signal onto a continuous wave

Fig. 10. A direct intensity modulation and detection full-duplex ARoF architecture.

Fig. 11. Downlink architecture of a ARoF link with indirect intensity modulation.

laser source.

The ARoF technique has been considered a cost-effective and reliable solution for the distribution of future services to wireless devices by using optical fiber with vast transmission bandwidth capacity. An ARoF link is used in remote antenna applications to distribute signals to a Microcell or Picocell base station (BS). The downlink RF signals are distributed from a central station (CS) to a BS known as a Radio Access Point (RAP) through optical fibers. The uplink signals received at RAPs are sent back to the CS for any signal processing. RoF has the following main features: (1) it is transparent to bandwidth or modulation techniques; (2) it only needs simple and small BSs; (3) centralized operation is possible; (4) it supports multiple wired and wireless standards, simultaneously. (5) its power consumption is relatively low. Furthermore, the implementation of the RoF technique faces the following challenges: fiber optic network implementation cost, optical communication components nonlinearity and fiber dispersion. Consequently, in last decade several research projects have sought to develop and discover new solutions to overcome these challenges and broaden the benefits of RoF.

## **4.1 Radio over fiber's link architecture**

The signal that is transmitted over the optical fiber can either be originally an RF, intermediate frequency (IF) or baseband (BB) signal. For the IF and baseband (BB) transmission cases, additional hardware for up converting the signal to the RF band is required at the BS. At the optical transmitter, the RF/IF/BB signal can be modulated onto the optical carrier by using direct or external modulation of the laser light. In an ideal case, the output signal from the optical link will be a copy of the input signal. However, there are some limitations because of non-linearity and frequency response limits in the laser and modulation devices as well as dispersion in the fiber. The transmission of analog signals puts certain requirements on the linearity and dynamic range of the optical link. These demands are different and more exacting than requirements placed on digital transmission systems.

In Fig. 12, typical RoF link configurations are shown, which are classified based on the kinds of frequency bands transmitted over an optical fiber link. In the downlink from the CS to the BS, the information signal from a public switched telephone network (PSTN), an Internet Service Provider (ISP), a mobile telecommunications operator, an Intelligent Transportation System (ITSs) or another CS is fed into the optical network at the CS. The signal that is either RF, IF or BB band modulates an optical signal from a LD. As described earlier, if the RF band is low, it's possible to modulate the LD signal using the RF band signal directly. If the RF band is high, such as the mm-wave band, it's better to use electro- optical modulators (EOMs), like Mach-Zehnder Modulators. The modulated optical signal is transmitted to the BS via optical fibers. At the BS, the RF/IF/BB band signal is recovered by detecting the modulated optical signal by using a PD. The recovered signal, which needs to be upconverted to RF band if an IF or BB signal is to be transmitted to a mobile handset (MHs) via the antenna of the BS.

In the configuration shown in Fig. 12 (a), the modulated signal is generated at the CS in an RF band and directly transmitted to a BS by an EOM, which is called "RF-over-Fiber". At the BS, the modulated signal is recovered by detecting the modulated optical signal with a PD and directly transmitted to a MH. Signal distribution using RF-over-Fiber has the advantage of a simplified BS design but is susceptible to fiber chromatic dispersion that severely limits the transmission distance (Gliese et al., 1996). In the configuration shown in Fig. 12 (b), the modulated signal is generated at the CS in an IF band and transmitted to a BS by an EOM, which is called "IF-over-Fiber". At each BS, the modulated signal is recovered by detecting the modulated optical signal with a PD, up converted to an RF band, and transmitted to a MH. In this scheme, the effect of fiber chromatic dispersion on the distribution of IF signals is much reduced. However, the antennas of the BSs a RoF system incorporating IF-over-Fiber transport require additional electronic hardware such as an mm-wave frequency LO for frequency up- and down conversion.

(a)

*opt*. *IF opt* . *opt*. *IF IF f IF f opt* . *RF f opt* . *IF f RF IF f f RF f* (b)

Fig. 12. Different schemes for signal modulation onto an optical carrier for distribution: (a) Radio over Fiber; (b) IF over Fiber; (c) BB over Fiber.

modulated signal is generated at the CS in an IF band and transmitted to a BS by an EOM, which is called "IF-over-Fiber". At each BS, the modulated signal is recovered by detecting the modulated optical signal with a PD, up converted to an RF band, and transmitted to a MH. In this scheme, the effect of fiber chromatic dispersion on the distribution of IF signals is much reduced. However, the antennas of the BSs a RoF system incorporating IF-over-Fiber transport require additional electronic hardware such as an mm-wave frequency LO

*RF f*

(a)

(b)

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( c )

Fig. 12. Different schemes for signal modulation onto an optical carrier for distribution:

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> *opt*.

*opt*. *RF opt* . *opt*. *RF*

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(a) Radio over Fiber; (b) IF over Fiber; (c) BB over Fiber.

*opt*. *IF opt* . *opt*. *IF* In Fig. 12 (c), the modulated signal is generated at the CS in baseband and transmitted to a BS, which is referred to as "BB-over-Fiber". At the BS, the modulated signal is recovered by detecting the modulated optical signal with a PD, up converted to an RF band through an IF band or directly, and transmitted to a MH. In baseband transmission, the impact of fiber dispersion effects is negligible, but the BS configuration is the most complex. This is especially important when RoF in mm-wave bands is combined with dense wavelength division multiplexing (DWDM). This increases the amount of equipment at the BSs because an up converter for the downlink and a down converter for the uplink are required. In the RF subcarrier transmission, the BS configuration can be simplified only if an mm-wave optical external modulator and a high-frequency PD are implemented in the electric-to-optic (E/O) convertor and the optic-to-electric (O/E) converter, respectively.

Optical links are mainly transmitting microwave and mm-wave signals by applying an intensity modulation technique onto an optical carrier (Al-Raweshidy & Komaki, 2002). Fundamentally, two methods exist for transmission of the microwave/mm-wave signals over optical links with intensity modulation: (1) direct intensity modulation, (2) external modulation.

In direct intensity modulation an electrical parameter of the light source is modulated by the information RF signal. In practical links, this is the current of the laser diode, serving as the optical transmitter. In Fig. 10, the simplest and most cost-effective architecture of intensitymodulation direct-detection (IMDD) is depicted. In this architecture, the detection is performed using a photo diode (PD). In the direct-modulation process a semiconductor laser directly converts an electrical small-signal modulation (around a bias point set by a dc current) into a corresponding optical small-signal modulation of the intensity of the photons emitted (around the average intensity at the bias point). Thus, a single device serves as the optical source and the RF/optical modulator. An important limitation in this architecture for super broadband access are the restrictions placed on the modulation bandwidth by the laser and the mm-wave band while a simple laser's linewidth can be modulated to frequencies of several Gigahertzes. Furthermore, it is reported that direct intensity modulation lasers can operate at up to 40 GHz or even higher, but, these are expensive and are not cost-effective in the commercial market. Therefore, at frequencies above 10 GHz, external modulation rather than direct modulation is applied.

In the external modulation technique, Fig. 11, an unmodulated light source is modulated with an information RF signal using an electro-optical intensity modulator. Because the number of BSs is high in RoF networks, simple and cost-effective components must be utilized. Therefore, in the uplink of a RoF network system, it is convenient to use direct intensity modulation with cheap lasers; this may require down conversion of the uplink RF signal received at the BS. In the downlink either lasers or external modulators can be used.

#### **4.2 Application of WDM in a radio over fiber system**

The application of WDM in RoF networks has many advantages including simplification of the network topology by allocating different wavelengths to individual BSs, enabling easier network and service upgrades and providing simpler network management. Thus, WDM in combination with optical mm-wave transport has been widely studied (Grifin et al., 1999; Toda et al., 2003).

The implementation of WDM in a RoF network is illustrated in Fig. 13, where for simplicity, only downlink transmission is depicted. Optical mm-wave signals from multiple sources are multiplexed and the generated signal is optically amplified, transported over a single fiber and demultiplexed to address each BS concerned separately. A challenging issue is that the optical spectral width of a single optical mm-wave source may approach or exceed WDM channel spacing. Therefore, there have been several reports on dense WDM (DWDM) applied to RoF networks (Grifin et al., 1999; Grifin, 2000; Toda et al., 2003); by utilizing the large number of available wavelengths in the DWDM technique, the lack of free transmission channels for the deployment of more BSs in mm-wave bands can be overcome. Another issue is related to the number of wavelengths required per BS. It is desirable to use one wavelength to support fullduplex operation. In (Nirmalathas et al., 2001), a wavelength reuse technique has been proposed, which is based on recovering the optical carrier used in downstream signal transmission and reusing the same wavelength for upstream signal transmission.

Fig. 13. Schematic illustration of the implementation of WDM in a RoF network.

#### **4.3 Digital radio over fiber**

Digital systems are more flexible, more conveniently interface with other systems, are more reliable and robust against additive noise from devices and channels, and achieve a better dynamic range than analog systems. Analog to digital and digital to analog converters (ADC and DAC, respectively) (Walden R. H., 1999) are the link between the analog world and the digital world of signal processing and data handling. In an analog system the bandwidth is limited by devices performance and parasitic components are introduced.

The implementation of WDM in a RoF network is illustrated in Fig. 13, where for simplicity, only downlink transmission is depicted. Optical mm-wave signals from multiple sources are multiplexed and the generated signal is optically amplified, transported over a single fiber and demultiplexed to address each BS concerned separately. A challenging issue is that the optical spectral width of a single optical mm-wave source may approach or exceed WDM channel spacing. Therefore, there have been several reports on dense WDM (DWDM) applied to RoF networks (Grifin et al., 1999; Grifin, 2000; Toda et al., 2003); by utilizing the large number of available wavelengths in the DWDM technique, the lack of free transmission channels for the deployment of more BSs in mm-wave bands can be overcome. Another issue is related to the number of wavelengths required per BS. It is desirable to use one wavelength to support fullduplex operation. In (Nirmalathas et al., 2001), a wavelength reuse technique has been proposed, which is based on recovering the optical carrier used in downstream signal

transmission and reusing the same wavelength for upstream signal transmission.

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Digital systems are more flexible, more conveniently interface with other systems, are more reliable and robust against additive noise from devices and channels, and achieve a better dynamic range than analog systems. Analog to digital and digital to analog converters (ADC and DAC, respectively) (Walden R. H., 1999) are the link between the analog world and the digital world of signal processing and data handling. In an analog system the bandwidth is limited by devices performance and parasitic components are introduced.

Fig. 13. Schematic illustration of the implementation of WDM in a RoF network.

3 2

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1

**4.3 Digital radio over fiber** 

In a Digital RoF (DRoF) system, an electrical RF signal is digitized by using an Electronic ADC (EADC) (Vaughan et al., 1991). Then, the generated digital data is modulated with a continuous coherent optical carrier wave either using a direct modulation technique or by using an external electro-optical modulator as shown in Fig. 14. The modulated optical carrier is transmitted through the fiber. At the base station, after detecting the optical signal using a photo diode, the detected digital data is converted back to the analog domain using an EDAC. Finally, the analog electrical signal is fed to an antenna (Li et al., 2009; Kuwano, 2006, 2008; Lim et al., 2010). Current EDAC systems experience problems such as jitter in the sampling clock (Stephens, 2004; Hancock, 2004), the settling time of the sample and hold circuit, the speed of the comparator, mismatches in the transistor thresholds and passive component values. The limitations imposed by all of these factors become more severe at higher frequencies. Wideband analog to digital conversion is a critical problem encountered in broadband communication and radar systems (Valley, 2007; Kim et al., 2008). For the future beyond Gigabit/s mobile and wireless end-user traffic rates (Abdollahi et al., 2010) due to the limitations of electronic technology for implementing ultra high-speed, high performance EADC, and the resolution of existing EDAC, the deployment of conventional DRoF links (Li et al., 2009; Kuwano, 2006, 2008; Lim et al., 2010) is not simply achievable.

Fig. 14. Conventional DRoF architecture using EADC (downlink) (Li et al., 2009).

Moreover, if a conventional DRoF link could be achieved for Gigabit/s traffic rates the generated digital traffic creates a new challenge, namely, for this architecture to use more electro-optical modulators and photo diodes to implement the wavelength division multiplexing (WDM) technique to diminish the chromatic dispersion caused by the restrictions on the modulation bandwidth for super broadband access by RoF.
