Introduction
The recent timeline of the lifestyle of the human beings to use the gadgets shows the growth of the use of small and portable devices like notebooks, laptops, tabs, mobile phones etc. And if we see the trend of their usage and the ease that it gives to the world it is clear that the future use of these devices is strong and in fact it will continue to grow. While all these devices are getting its popularity, their growth also poses a problem: these devices are hogging the already congested lower microwave frequency region of the wireless spectrum (the 2.4 GHz band, for example). So to solve these issues a solution is to use the unlicensed 60 GHz frequency band, which has 5-mm wavelengths and this band is known as the Extremely High Frequency band (EHF) instead of using the heavily used lower microwave regions which have frequencies of 2-4 GHz with wavelengths of 7.5-15cm [12]. But the use of the EHF is not without challenges. As these 60GHz signals have high propagation losses so they are targeted towards short-range, in-building, high-speed applications. So for this we need a large number of base stations (BS) to cover a service area. But to install so many BS will incur cost. So this requirement has led to the development of a system where functions such as signal routing/processing, handover and frequency allocation are carried out at a central control station (CS) instead of at the BS. The use of CS also requires that the sensitive equipment needs to be located in safer environments and the cost of expensive components to be shared among several BS [8]. An alternative approach is linking the CS with BS in EHF network via an optical fiber network [8]. Since such a system uses both optical fibres and mm-wave wireless transmission, the technology is called “ fiber-wireless” (Fi-Wi).

Network Architecture

Figure1.2a is a typical hybrid fibre-wireless network where an optical head end or CO acts as the gateway to the optical metropolitan backbone (red colour) while serving a large number of widely distributed antenna BSs and remote nodes (RNs). The base stations wirelessly transmit 5-mm-wave (60 GHz) signals to customers. Within buildings and homes, the short-range wireless signals can provide high-speed connectivity (faster than 1 Gb/s) for a variety of wireless, high-bandwidth communication devices.

Figure 1.2a mm-wave fibre wireless network [12]

There are three possible approaches for transporting mm-wave wireless signals over optical fibres. The simplest scheme, called RF-over-fibre.In ROF network a CS is connected to numerous functionally simple BSs via an optic fibre as in figure 1.2b. The main function of BS is to convert optical signal to wireless one and vice versa. Almost all processing including modulation, demodulation, coding, routing is performed at the CS. That means, ROF networks use highly linear optic fibre links to distribute RF signals between the CS and BSs. At a minimum, an ROF link consists of all the hardware required to impose an RF signal on an optical carrier, the fibre-optic link, and the hardware required to recover the RF signal from the carrier. The optical carrier’s wavelength is usually selected to coincide with either the 1.3 mm window, at which standard single-mode fibre has minimum dispersion, or the 1.55 mm window, at which its attenuation is minimum.

Figure 1.2b: General radio over fibre system [8]

 Impairments in optical Fiber Transmission

Although the optical distribution of radio signals at mm-wave frequencies has the potential to simplify the BS design, the signals are susceptible to a number of impairments along the link that may degrade the overall system performance. These impairments include low optical-to-electrical conversion efficiency, inefficient optical spectral usage, fibre chromatic dispersion and also nonlinearity along the link. Due to the nonlinear characteristics of optical intensity modulators, there is only a very narrow window available for linear operation which results in the weak modulation of the mm-wave radio signals onto the optical carrier. To improve the modulation efficiency, the mm-wave radio signals can be electrically amplified before modulating the optical source; however this may lead to increased inter-modulation distortions (IMDs) at the optical frontend which limits the overall system dynamic range [9] and this condition may further worsen due to other fibre nonlinearities as the radio signals propagate through the fibre links [12].To improve the optical frontend many solutions were introduced. However this approach may lead to increased IMDs at the receiver or even damage the receiver due to too large an optical power incident on the optical detector. Apart from low modulation efficiency, the optical distribution of mm-wave radio signals is also susceptible to fibre chromatic dispersion that severely limits the transmission distance. Due to the nonlinear characteristics of the external modulator, the mm-wave wireless signals are typically weakly modulated onto the optical resulting in very low modulation efficiency. In addition the external modulator nonlinear characteristics generate inter-modulation products which contribute to the overall signal degradation [8]. In addition, the optical spectral usage for the distribution of the mm-wave radio signal is highly inefficient, considering that the amount of useful information that is being transported (< 3 Gb/s) is only a fraction of the occupied spectrum (> 40 GHz). In a long-reach scenario, the optical signal may experience signal degradation due to fiber non-linearities if the optical signal power is optically amplified to overcome link losses and the amplified optical power is large enough to trigger the fibre nonlinear effects. Another impairment that the signals may experience in a long-reach environment is the phase decorrelation between the optical carrier and the wireless signals which may introduce an addition penalty [8]. As Fi-Wi involves optical fibre so a sharp bend in a fibre can cause significant losses as well as the possibility of mechanical failure. If the fibre gets too cold, the outer layers will shrink and get shorter. If the core/cladding shrinks at a slower rate, it is likely to kink and cause a micro bend.
 Integration Option

The requirement for fibre-wireless systems to be integrated with the existing optical infrastructure is very important. There are three possible approaches for transporting mm-wave wireless signals over optical fibres.
a. The simplest scheme, called RF-over-fibre, involves a data-carrying RF (Radio Frequency) signal with a high frequency (usually greater than 10 GHz) is imposed on a light wave signal before being transported over the optical link. Therefore, wireless signals are optically distributed to base stations (BS) directly at high frequencies and converted to from optical to electrical domain at the base stations before being amplified and radiated by an antenna. As a result, no frequency up/down conversion is required at the various base station, thereby resulting in simple and rather cost-effective implementation is enabled at the base stations [12].

b. The second method involves an IF (Intermediate Frequency) radio signal with a lower frequency (less than 10 GHz) which is used for modulating light before being transported over the optical link. Therefore, wireless signals are transported at intermediate frequency over the optical [12].
c. The third method, called baseband-over-fibre, involves transporting the wireless signals as very low-frequency baseband signals over optical fibre from the central office to the base station, and then up converting the information to the mm-wave frequency at the base station [12].
d. Fibre-Wireless systems can take the advantage of current technologies such as wavelength-division-multiplexing (WDM), which combines multiple signals on a single optical fibre by using different wavelengths. WDM in combination with optical mm-wave transport has been widely studied. Optical mm-wave signals from multiple sources are multiplexed and the composite signal is optically amplified, transported over a single fibre, and de multiplexed to address each BS.[9]
e. Furthermore, there have been several reports on dense WDM (DWDM) applied to ROF networks. Though a large number of wavelengths are available in the modern DWDM technologies, since mm-wave bands ROF networks may require even more BSs wavelength resources should be efficiently utilized. [9]
f. Even in some places modified millimetre wave wireless system uses orthogonal frequency division multiplexing (OFDM) technique, integrated with optical fibre as a feeder network [9].
g. The simplest method for optically distributing RF signals is simply to directly modulate the intensity of the light source with the RF signal itself and then to use direct detection at the photo detector to recover the RF signal. There are two ways of modulating the light source. One way is to let the RF signal directly modulate the laser diodes current. The second option is to operate the laser in continuous wave (CW) mode and then use an external modulator such as the Mach-Zehnder Modulator (MZM), to modulate the intensity of the light [11][3].
h. Most RoF systems, including IM-DD RoF systems, use SMFs for distribution. However, the use of the IM-DD RoF technique to transport RF signals over multimode fibre, by utilising the higher order transmission pass bands, has also been demonstrated for WLAN signals below 6 GHz[11] [3].
i. Most RoF techniques rely on the principle of coherent mixing in the photodiode to generate the RF signal. These techniques are generally referred to as Remote Heterodyne Detection (RHD) techniques [6].
Another option to integrate the wireless in optical fibre to multiplex and amplify all downlink and un-modulated uplink channels in the CS. These channels are then fed into the fibre backbone of the ring network. At each BS, a pair of down- and uplink wavelengths is dropped through an OADM (Optically Add Drop Multiplexer) to the EAT (Electro Absorption Transceiver), which simultaneously detects and modulates the down- and uplink channel, respectively. The modulated uplink channels are added to the backbone again, looped back to the CS, where they are de multiplexed and detected. [9]
 Technical and Financial Implications

As the use of small, portable communication devices continues to grow the mm-wave fibre wireless system has the potential to widen the wireless spectrum. This system can widely be used for last mile data transmission to customers as well as in building networking as it has the faster speed and has lower costs. So these systems can be more used in densely populated areas and for disaster recovery environments [12] where wired communication lines are unavailable. Considering the RoF system which is a Fi-Wi system, it involves analogue modulation, and detection of light, it is fundamentally an analogue transmission system. Therefore, signal impairments such as noise and distortion, which are important in analogue communication systems, are important in these systems as well. These impairments tend to limit the Noise Figure (NF) and Dynamic Range (DR) of the RoF links [7]. DR is a very important parameter for mobile (cellular) communication systems such as GSM because the power received at the BS from the MUs varies widely (e.g. 80 dB [10]). That is, the RF power received from a MU which is close to the BS can be much higher than the RF power received from a MU which is several kilometres away, but within the same cell. The RoF way of Fi-Wi technology is generally unsuitable for system applications, where high Spurious Free Dynamic Range (SFDR = maximum output signal power for which the power of the third-order inter-modulation product is equal to the noise floor) is required, because of the limited DR. This is especially true of wide coverage mobile systems such as GSM, where SFDR of > 70 dB (outdoor) are required. However, most indoor applications do not require high SFDR. For instance, the required (uplink) SFDR for GSM reduces from >70 dB to about 50 dB for indoor applications [7]. RoF systems are also attractive for other present and future applications where high SFDR is not required [10]. Another application area is in Fixed Wireless Access (FWA) systems, such as WiMAX, where RoF technology may be used to optically transport signals over long distances bringing the significantly simplified RAUs closer to the end user, from where wireless links help to achieve broadband first/last mile access, in a cost effective way.
In Fi-Wi since the RF waves needs to be transmitted in the optical fibre so there is low loss of signals. Key challenges for Fi-Wi include minimising the energy consumption, maintaining or improving link linearity and reducing signal conversion times. This requires substituting, as much as possible, electronic processing with the corresponding optical processing. Such high-speed, photonic-driven (and integrated) signal processing systems may support high bandwidth at high carrier frequencies from a much larger number of antennas than with conventional network nodes. They still require more good algorithms addressing user mobility, traffic routing and delivering, energy consumption, congestion, and capacity optimization, to cope with the considerably higher attenuations at higher bands.
Here special skills are needed for installation, field splicing and field termination. It involves high cost of installation higher than the copper wire and LAN environments. It also has high cost of transceiver equipment but these costs are coming down. These may be vulnerable as very high information in one fibre as too many eggs in one basket.
Prior to the widespread deployment of reconfigurable radio -over-fibre networks over a wide area the following important architectural issues must be resolved:
1) The need for a full duplex networked focused, approach to radio-over-fibre architecture definition and design;
2) The need for radio-over-fibre systems to coexist on the same fibre infrastructure with other systems or services;
3) The need for WDM radio-over-fibre systems to inherently support and facilitate the flexible reconfiguration of the wireless network. This requires the design of a vertically and horizontally integrated network ensuring that future wireless networks and system have radio-over-fibre network capability. Detailed schemes and protocols need to be devised to efficiently manage and control these highly reconfigurable networks. This management capability need to be vertically integrated across all layers to provide optimum performances, as it requires control of all levels of the network from the optical path, through the sub-carrier assignment up to the radio protocol. However, for operational or cost reasons, introduction of additional complexity in the distribution network may not be desirable. Careful planning will be required to balance the functionality increase possible with the implementation costs.
 Conclusion

For the future provision of broadband, interactive and multimedia services over wireless media, some typical characteristics are required which are-1) to reduce cell size to accommodate more users and 2) to operate in the microwave/millimetre wave (mm-wave) frequency bands to avoid spectral congestion in lower frequency bands. It demands a large number of base stations (BSs) to cover a service area, and cost-effective BS is a key to success in the market. This requirement has led to the development of system architecture where functions such as signal routing/processing, handover and frequency allocation are carried out at a central control station (CS), rather than at the BS. Furthermore, such a centralized configuration allows sensitive equipment to be located in safer environment and enables the cost of expensive components to be shared among several BSs. An attractive alternative for linking a CS with BSs in such a radio network is via an optical fibre network, since fibre has low loss, is immune to EMI and has broad bandwidth. The transmission of mm-wave over fibre, with simple optical-to electrical conversion, followed by radiation at remote antennas, which are connected to a central CS, has been proposed as a method of minimizing costs. The reduction in cost can be brought about in two ways. Firstly, the remote antenna BS or radio distribution point needs to perform only simple functions, and it is small in size and low in cost. Secondly, the resources provided by the CS can be shared among many antenna BSs.
Although the optical distribution of radio signals at mm-wave frequencies has the potential to simplify the BS design, the signals are susceptible to a number of impairments along the link that may degrade the overall system performance.
These impairments include low optical-to-electrical conversion efficiency, inefficient optical spectral usage, fibre chromatic dispersion and also nonlinearity along the link, Macro bends, Micro bends, etc . Therefore it is of great importance that these various impairments that the signals experience along the link be mitigated to improve the signal quality and overall performance of mm-wave hybrid fibre wireless links [9].
Fi-Wi networks form a powerful platform that provides a number of advantages. By simultaneously providing wired and wireless services over the same infrastructure, Fi-Wi networks are able to consolidate (optical) wired and wireless access networks that are usually run independently of each other, thus potentially leading to major cost savings. Though fibre-based networks can easily support the rapid growth in bandwidth demands, they carry high initial deployment costs and take longer to deploy. In most cases the Return-On-Investment (ROI) in fibre installations can only be expected in the long term, making it hard for operators to achieve lower costs per bit and earn profits in the foreseeable future.

Author:Anshuman Biswal

References
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