The key requirement for the future information systems is to offer very high capacity broadband services to end-users. It can be achieved by deploying optical fibers in the access network as FTTC and FTTH networks. However, the liberty and convenience provided by wireless systems have created a sense of autonomy to end-users. Therefore, the most favorable choice for the end-users is to access the high capacities of fixed fiber networks while keeping the autonomy of wireless systems. In this context, the simultaneous support of both technologies over the same physical access infrastructure is the best possible solution for future proof access networks. Moreover, it will also result in both capital and operational cost savings by reducing the number of parallel infrastructures that have to be installed and maintained.
5.1 RoF and PON integration strategies
Radio over Fiber (RoF) technology is referred to extension of wireless coverage by applying the low loss optical fibers. These hybrid wireless/optics systems provide many promising solution for different digital communication systems. Network and service providers are interested to have an integrated network architecture that can offer mobility, bandwidth and range of connectivity options for the customer by combining wireless and wire line solutions -. An integrated optical access network that can support convergence of wired and wireless last mile solutions appears an optimal choice. However, coexistence of the optical and wireless access technologies will be essential to enable such a network. The multiband modulation techniques are highlighted in the chapter-4 to realize integrated multiuser optical access networks that enable baseband and RF technologies to co-exist together in the same fiber. In this chapter, an integration of RoF and PON system is analyzed to achieve the cost effective and efficient radio systems. In order to effectively utilize the bandwidth capabilities of the optical fiber, joint advances in both wired and wireless telecommunication systems are required. An integrated scenario is illustrated in Fig. 5.1, where PON, cellular mobile and fixed wireless base stations are served by a single fiber infrastructure. This needs an optimized resource allocation and also a common resource management. Therefore, This whole chapter is devoted for the study of integrated wireless and fiber access network architectures. This Chapter thus focuses on the investigation of optically modulated BB and RF signals multiplexing together, de-multiplexing and reception of individual signals to achieve an effective integrated optical infrastructure in the access domain. The compatibility of existent fiber access network standards for integration with wireless systems is addressed and an emphasis is laid on the better suited network topologies.
Fig. 5.1. An integrated RoF and PON system to provide different service solutions
5.2 Multiplexing Schemes for integration of RoF with PON
The PON system is considered as the most favorable and desirable solution among all the possible options for fiber based access networks as discussed in chapter-3. That global trend in telecommunications markets is the key factor to consider only PON in the analysis of the convergence of wired and wireless access networks. For all the above stated reasons, the utilization of PON infrastructure to transport radio signals seems the most practical approach. For the transparent exchange of radio signals between base station and central office, distributed radio networks require reliable and inexpensive networks. The PON infrastructure is the simplest and most economical in comparison to all other available options. Therefore, it is the quite cost efficient to transmit radio signals over a PON infrastructure that allow the deployment of a distributed network for the base station. Four multiplexing options are considered to integrate radio waves with PON.
1. Time division multiplexing
2. Coarse wavelength division multiplexing
3. Dense wavelength division multiplexing
4. Subcarrier multiplexing
5. Hybrid multiplexing
5.2.1 A time division multiplexing scheme for integration
Two major standards for TDM PON devised by ITU are known as BPON, GPON and XG-PON, whereas the one formulated by IEEE is called EPON. The wavelength allocation for GPON systems is provided in ITU-T G.983.3 , and is illustrated in Fig. 5.2. On the other hand, EPON also uses the same wavelength windows for downstream.
Fig.5.2. Wavelength allocation plan by ITU and IEEE for GPON/EPON
In GPON systems, for uplink transmission 1260-1360 nm bands is assigned for operation data rates of 155 Mbit/s, 622 Mbit/s, 1244 Mbit/s and 2488 Mbit/s, whereas in 10G-PON (XG-PON) band is limited to 1260-1280 . An Intermediate band is reserved from 1380 to 1460 nm between the uplink and the downlink bands. The 10G-EPON standard aimed at a coexistence with the legacy IEEE 802.3ah is developed in 2009 . This standard is in conformation with the enhanced specifications of GPON systems for wavelength group selection that will enable the massive production of low cost laser sources.
The present wavelength bands allocation for IEEE and ITU-T compliant new systems is conformist to legacy GPON and EPON systems to allow the implementation of different new services over the same PON infrastructure, while retaining the compatibility with the legacy services. Therefore, any additional service can use free bands by avoiding interference with legacy services in order to maintain the quality of these services.
The time division duplex (TDD) scheme is capable of allocating different time intervals for both directions, hence avoiding potential wavelength dearth by using only one optical carrier. The frequency spectrum can be fully utilized to a given direction within its transmission window by sharing the bandwidth in time among all terminals. Mostly, coarse wavelength is used with TDM in upstream and downstream direction. But different wire line and wireless services can be time interleaved in one direction. The capacity can be enhanced appreciably with this technique.
5.2.2 A subcarrier multiplexing scheme for integration
SCM is a very common and efficient way to increase the transmission capacity of a single optical carrier that permits carrying several radio channels using radio frequencies over a given optical carrier as illustrated in Fig 5.3. The analog or digitized radio signals can be transported by SCM depending on the complexity of employed transmitters and receivers. This technique is very useful for transmitting simultaneous wired and wireless services by using the baseband optical carrier and the sub-carriers. The differentiation between the different terminals attached to the PON infrastructure can also be made with different RF carriers using SCM. SCM technique needs stable light sources to prevent the drift from central wavelength that will avoid any interference between channels. SCM also needs very precise filters to separate the RF sub-carriers from the optical carrier. Therefore, overall system performance can be impacted by the beating of the RF sub-carriers with each other and with the optical carrier due to some characteristics of both the laser and the photo-detector. This intermodulation phenomenon depends on the modulation frequency of the sub-carriers and also on the number of RF channels supported by the optical carrier. There is an alternate way also available to apply the SCM technique by down-converting the high radio frequency to an intermediate frequency (IF) that will allow the slower and comparatively cheap electronic devices. However, the number of analogue channels can degrade the transmission performance of the PON data channels due to nonlinear properties of fiber if exceed certain limit .
Fig.5.3. A principle diagram of SCM technique
5.2.3 A coarse wavelength division multiplexing scheme for integration
In 2002 the ITU has standardized a channel spacing grid for use with CWDM using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm . Therefore, CWDM provides the distribution of 18 different channels with each have width of 20 nm from 1260 to 1610 nm. For a bidirectional fiber that channel counts is limited to only 9 due to bidirectional communication for. The CWDM scheme permits the use of comparatively low-cost lasers due to wider channels spacing that didn't require rigid wavelength stabilization. However, 1380-1460 nm bands only can be utilized for small distance communication due to the high attenuation loss in this region. Therefore overall a fewer numbers of channels can be used in CWDM technique but it is very significant and important technique in combination with TDM or subcarrier multiplexing techniques. For Uplink and downlink transmission, an independent coarse wavelength is normally selected, whereas multiple wired and wireless signals are multiplexed by TDM or SCM on that wavelength.
5.2.4 A dense wavelength division multiplexing scheme for integration
A DWDM system supports several tens of channels at a frequency gap of 100GHz, even 50GHz and 25GHz spacing is also possible. Therefore, DWDM provides much greater capacity, but needs complex and costly transmitters. DWDM also needs very strict wavelength drift control due to very small gap of two channels. The choice of a proper wavelength band for radio transmission depends on the variation of attenuation and chromatic dispersion with particular wavelength. For radio transmission, the 1500-1550 nm and 1600-1625 nm bands appears as the most exciting options for wavelength allocation in a DWDM setup. In DWDM converged RoF architecture, mux/demux is normally placed in or near splitting point. The utilization of reflective devices in each base station that receives a carrier wavelength from the downlink and re-modulates it and sends it back in the Uplink is an exciting option. The reducing prices of optical devices and increasing demand of high speed broad systems are creating a greater interest in DWDM integrated systems.
5.2.5 A Novel proposed scheme for hybrid PON integration with 60 GHz mm-wave
The integration of hybrid PON and mm-wave RoF technology is viewed as the most promising solution for the next generation high capacity broadband access networks. This integration offers the vast bandwidth advantages of the optical hybrid networks and mobility characteristics of the wireless networks for the end-users. An integration scenario of a hybrid PON (TDM-WDM PON) and 60 GHz Wi-Fi (802.11ad) hot spots is presented in Fig. 5.4. The purpose of this integration is to improve the end-to-end delay and offer diverse quality of service for different applications through Wi-Fi hot spot domains over high capacity hybrid PON. The present single coarse wavelength based TDM has the limited capacity to offer as a back bone medium to Wi-Fi hotspots. Therefore it is needed to upgrade the single wavelength TDM-PON to multi wavelength PON to satisfy the ongoing broadband demands. This can be achieved by convergence of the TDM-PON with the WDM-PON, a solution to the traditional TDM PON shortages. The WDM PON provides a dedicated wavelength to each ONU instead of shared single wavelength among multiple OUNs in traditional TDM that will greatly enhance the bandwidth capacity of the overall system. Furthermore, multiple Wi-Fi hot spot domains act as the TDM-PON, connected to power splitter. In this proposed setup, each ONU will work as Wi-Fi access point that will convert optically generated 60 GHz radio signal in to the electrical and transmit in the air through horn antenna. The ONU works as the wireless gateway between the wireless and optical worlds and communicates directly with wireless devices.
It is quite difficult to offer adequate end-to-end QoS for different service classes especially for the delay-sensitive applications like voice over IP (VoIP), interactive games, streaming multimedia due to inherent inept of built-in QoS features in 802.1ad. The 802.11ad proposed as the upcoming standard for the IEEE 802.11 enhancement for 60GHz.
Fig. 5.4. An integration scenario of hybrid PON and 60GHz mm-wave
5.3 A novel proposed techniques for integration of 2.5 Gbit/s RoF with XG-PON
A cost effective and simple technique of simultaneous generation and propagation of millimeter wave with recently standardized 10-gigabit passive optical network (XG-PON) is proposed and demonstrated. It is achieved by modulating 2.5 Gbps, 30 GHz radio wave and 10 Gbps based band signal by single electrode Mach Zehnder modulator (MZM).
5.3.1 Concept and theoretical analysis
In our proposed scheme, we have generated and propagated the 60 GHz RF signal with existing and standardized 10G time division multiplexing (XG-PON) baseband signal by applying single electrode MZM for 25-Km bidirectional fiber. The optical field applied to single electrode MZM is given by
We have combined the 10G baseband signal to 2.5G radio signal and then applied it to the MZM. The electrical driving signal sent into the MZM is
The generated optical signals when MZM is biased at null point, can be expressed as
Where [mRF(t)â‰¡Ï€VRF(t)/2VÏ€] and [mBB(t)â‰¡Ï€VBB(t)/2VÏ€] represents modulation indexes "MI" of RF and BB signals respectively. The "Jn" is the Bessel function of the first kind. The optical sidebands with the higher order can be ignored due to weak modulation condition. After square law detection by a photo diode PD, the desired BB and RF signals can be written as
Where "R" is the photo diode responsivity, the frequency of the generated RF vector signal 2Ï‰RF is two times that of the driving signal Ï‰RF. The amplitude and phase information of the generated RF vector signal are (J1)2mRF(t) and 2Î¸(t) respectively. Therefore, the amplitude VRF(t) and phase information Î¸(t) of the driving signal needs to be pre distorted to achieve the desired RF vector signals after square-law photo detection. By properly pre-distortion of driving signals, the proposed scheme can support intensity modulation format.
5.4 Topological architectures for integrated systems
The use of a distributed antenna system (DAS) especially for indoor communications to support next generation wireless systems offers several advantages over conventional wireless architectures. A microcellular network can be easily implemented by using a fiber-fed distributed antenna network. Each antenna unit in this setup, receives RF signals from different peripheral devices and transmits over an optical fiber link to a central base station where all the de-multiplexing and signal processing is done . So, each remote antenna site just consists of an optical transceiver, an electrical amplifier and the antenna. Therefore, the overall cost and complexity of these microcellular antenna sites is quite lower than the traditional base transceiver stations (BTSs). The distributed antenna radio network with the potential of adaptive antenna selection and adaptive channel allocation offers increased spectrum efficiency. Furthermore, the distributed antenna system also offers an infrastructure that takes the radio interface very close to the end-users. Due to less power dissipation and reduced coverage area, it provides quite improved energy efficiency and reliability. Many base stations are interconnected and served by a central office in this setup. The fast deployment of PON also emphasizes the importance of analyzing the fiber optic infrastructure to support a DAS system. This concept can capitalize the increasing penetration of fiber access network all over the world.
In general, a Central office serves a certain geographical area by interconnecting a set of base stations, even though multiple sets of base stations can also be connected to the same CO. It can help to reduce the operational expenditure by using the same space for multi-set BS setup. In this scenario, the CO must have an optical interface for each set of base stations to propagate the radio signals over the optical fiber.
The focus in this chapter is only on the physical connection between the CO and the base stations architectures that addresses the basic requirements imposed by the DAS for the underlying optical fiber infrastructure. To support a DAS network, the optical fiber infrastructure may be planned considering the characteristic of physical topologies like chain/bus, star, tree-and-branch, and ring that are represented in the simplified diagrams of Fig. 5.6. The decision of the most appropriate topology depends on the fiber layout strategy and the number of terminals to be supported by the network. The location of the CO and the base stations normally determines the topology that requires the smallest amount of fiber to offer the required connectivity. For example, the chain topology will be the appropriate choice to connect all the antennas in a building requiring one antenna per floor to minimize the fiber layout. On the other hand, the star will be the better choice if the CO is located in the center of the geographic area, whereas the tree- and-branch is the best option in case the base stations are distributed in a more irregular fashion. For designing of optical fiber network, another important parameter to consider is the active or passive fiber plant. In active optic network, power requiring and costly equipment as switches, routers, and multiplexers/de-multiplexers can be placed within the distribution fiber network requiring environmental enclosures. Where, the passive optical networks only consists of passive splitters/combiners and passive arrayed wavelength grating (AWG) without the need of any powering. Although, the passive optical plants deployment and operational costs are quite low but they lack the flexibility provided by the active approach that allows the reconfiguration of the network to dynamically allocate the resources. Moreover, the active optical network can provide longer reach and large number of subscriber's connection by employing amplification and regeneration. Furthermore, Security is considered an added advantage of active optical networks. However, the passive optical networks can apply encryption mechanisms to minimize the security issues.
The tree topology is normally considered the best solution for implementation of optical access system as discussed in chapter-3. The tree topology allows the minimization of fiber layout and optimization of the resource sharing in most of the geographical situations. Moreover, the passive optical networks are preferred for RoF implementation due to their lower deployment and operational cost when compared to active optical networks. These choices are also in conformation with the standardized PON infrastructures installed all over the world.
Fig 5.6. Topological architectures for a fiber fed DAS (a) bus topology (b) ring topology (c) star topology (d) Tree-and-branch topology
5.4.1 Accompaniment of legacy wireless services in RoF-PON integrated systems
A future access network should be capable of not only provide wired and multi-gigabit wireless services together but also support legacy wireless systems. To make the installation of DAS for multi-gigabit wireless services more attractive for network operators and service providers, the support of legacy wireless services over the same infrastructure has the critical importance. It is necessary to take in to the account the requirement of additional resources for integration of the legacy services while designing the fiber-optic infrastructures. Therefore, the fiber-optic infrastructure should be designed to offer the required resources for simultaneous provision of legacy and new multi-gigabit technologies to reduce the operational cost on the shared infrastructure.
The legacy wireless services do not need high capacity as requires by 60GHz mm-wave or other multi-gigabit services. Therefore, they need quite less bandwidth capacity to transmit their data. Hence, supplying an entire wavelength to legacy service base station or DAS unit represents a significant waste of resources. So, a better approach is to share the each wavelength per multiple base stations of legacy services, separating the services and the antennas using SCM or TDM. A hybrid WDM-TDM or WDM-SCM can be appropriate solution while addressing the deployment of additional wavelengths needed for legacy systems. The choice of the most appropriate solution largely depends upon the number of base stations needed to support legacy service and the required capacity per base station.
Another possible option is to use space division multiplexing (SDM) as the multiplexing technology to support legacy systems over a future DAS network but it requires a duplication of the entire fiber plant infrastructure that will increase the overall capital and operational expenditure. It means two completely independent networks will be required for multi-gigabit wireless and legacy systems. Although, this consideration is quite expensive but make the networks transparent to each other. But, it will not only incur much higher costs for the entire system but also not workable in the cases where fiber plant is already installed.
Therefore, it is more appropriate to choose the better and economical alternatives based on hybrid fiber networks using a multiplexing scheme based on wavelength division and time division since it does not need the duplication of the fiber plant. Moreover, hybrid fiber networks allow the optimization of the resources and can adjust the wavelength capacity to support legacy systems.
5.4.2 Signal transmission options for high capacity wireless networks
In this section, the analysis of the different optical link types will be discussed especially the analog and digital approaches. Presently, the most of the deployed systems are digital including telecommunications and data networks. However, the analog systems will increase in number after the deployment of high speed wireless services.
The two major protocols defined for digital radio networks to support and control the base stations remotely are the common public radio interface (CPRI)  and the open base station architecture initiative (OBSAI) . They are able to manage the 3G and 4G wireless networks by specifying a digitized and serial interface among base stations. These protocols are sufficient to offer all the conventional services to base stations through baseband processing functions. However, they will not able to support high cell throughput and wider channel bandwidth future systems consists of several sectors with multiple-input multiple-output (MIMO) channels.
The implementation of RoF links is a promising alternative that applies less complicated and more energy efficient base stations . The RoF networks allow the placement of all switching, multiplexing, and processing units at one central location. Furthermore, the utilization of optical fibers provides the transparent transmission of the radio signals to/from multiple remote antennas. Therefore, only optical to electrical conversion, filtering and amplification of radio signals are required at remote antenna units with the help of analog RoF link. The RoF analog network is considered as a better choice for the high capacity DAS based wireless systems.
5.4.3 Radio signals propagation over hybrid passive optical networks
As discussed in previous sections, the most attractive solution for future wireless access networks is to interconnect the base station through fiber based passive access network (PON). An economical and consistent network is required to transmit the data between the base station and the baseband module located in a central office. Therefore, the propagation of wireless data over a PON infrastructure is quite lucrative and attractive approach. It allows the deployment of an array of distributed antenna network of base stations. In the wireless network architecture, a baseband module can support multiple base stations quite similar to the hierarchy of PON infrastructure where one OLT serves multiple ONUs. Therefore, the employment of already installed PON infrastructure for wireless network results in the optimization of fiber deployment and additional cost savings.
The standards formulated for PON systems have several differences including the data rate, the transmission protocols and the bandwidth efficiency . GPON is based on a generic encapsulation method (GEM) for layer 2 framing, where each frame supports a maximum payload size of 4095 bytes . GPON system is around 94% bandwidth efficient for both upstream and downstream communication. On the other hand, the second major standard of 1G EPON that released in 2004  only have bandwidth efficiency of 73% for the downlink and ~69% for the uplink. It offers the symmetric line rates of 1.25 Gbit/s and employs 8b/10b line coding with the use of Ethernet framing of a maximum payload size of 1500 bytes. The lower bandwidth efficiency for 1G EPON is due to the use of higher control message overhead. In 2009, IEEE has released the 10G EPON standards  that specify the new system operating at 10.3125 Gbit/s line rates applying Ethernet framing with 64b/66b line coding. In the present system bandwidth efficiency of around 96% is achieved. In hybrid networks, normally a specific wavelength is used to transmit data to multiple base stations by time division multiplexing. Therefore, TDM standards are crucial for mapping radio signals.
In order to transmit the radio data over PON standards need to evaluate the suitability of specific standard. The propagation of radio signals over PON network need to satisfy the tough QoS requirements of both the wireless and optical standards in the integrated network. The possible infrastructures for the optical/wireless integrated networks are specified in previous sections. The previously discussed options CPRI and RoF both need trivial modification in medium access control (MAC). Therefore, only the IP backhauling is considered to be the suitable choice to further investigate the mapping of radio signals over PON. It can satisfy the requirements for integration and can support the frame formatting for transmission over the PON. Therefore, IP backhauling can be applied to ensure minimum delays and bit streams mapping over PON protocols.
5.5 The effectuation of mm-wave RoF architectures
In mm-wave radio systems, cell coverage area is quite limited and normally pico or microcells are specified to provide ultra-high broadband communication to end-users. In general, mm-wave communication is considered more viable for indoor communications. If cells are considered for indoor communication of house, office or small building, called as home access networks (HAN). These home access networks are based on distributed antenna networks and need to combine with PON infrastructure. There are different design approaches to realize these small home networks on the basis of already defined physical topologies.
5.5.1 The point-to-point mm-wave RoF structural design
There are two possible options to achieve point-to-point RoF link, one is with a single hop in the air is called remote antenna, whereas the second option is called optical tunneling. Optical tunneling is used to combine radio cells through optical fibers like a chain topology. It can be helpful to set a direct link between two or more rooms if inter-distance is more than the coverage of mm-wave link.
Fig. 5.7. Point-to-point RoF links (a) remote antenna (b) optical tunnel
These point-to-point mm-wave RoF links consist a distributed antenna unit (RoF transducer) that transpose the incoming RF energy in the optical domain, and the other way around as illustrated in Fig. 5.7. In this scenario, wavelength duplexing is adapted to carry two radio signals over one fiber. Although, it is easy to install but it is more expensive since requires additional components such as optical filters, optical splitters and optical circulators. The other possible option for this infrastructure is to use intermediate frequency (IF) to apply low cost devices. A local oscillator is applied to electrically down-convert mm-wave signal to a lower IF before modulating the laser diode. On other hand, for reception of optical signal, an intensity modulated direct detection is adopted. This structural design is easy to implement and low cost, but not suitable for multi-room environment.
5.5.2 The active star mm-wave RoF structural design
In active star network, several remote antennas are interconnected to a switch that has all the network intelligence and technical complexity as presented in Fig. 5.8. The switch at the central node provides RoF interfaces to the remote antenna units. The antenna units convert the optical radio signals into electrical ones and vice versa.
Fig. 5.8. The active star architecture for point-to-point links connected by a switch
For uplink and down link two different coarse wavelengths will be utilized on a single fiber link between switch and fiber node. The active star structural design can provide the simplified network topology but their costs will very high and very complex circuitry is required for optical switch. Therefore, more cost-effective infrastructures will be preferred for mm-wave home access networks using fewer active components optical media.
5.5.3 The point-to-multipoint mm-wave RoF structural design
This architecture is quite similar to the physical infrastructure of the PON as shown in figure 5.9, splits the signal coming from the fiber node towards the distributed antenna units with 1xN optical splitter. A wavelength duplexing is preferred between splitter and fiber node like in the access network. In this setup, only one point-to-point link between splitter and DAS can be active at one time. Therefore, the overall capacity of the network is very limited. Furthermore, there is no possibility to communicate to other stations located in different radio cells in a direct way. Hence, the throughput is badly degraded for contention-based radio protocols where several stations compete for the channel access . As only one P2P link is possible, the point-to-multipoint architecture is specified for broadcast services like the delivery of the television service.
Fig. 5.9. The point-to-multipoint architecture
5.5.4 The multipoint-to-multipoint mm-wave RoF structural design
To achieve direct exchanges between radio cells, the multipoint-to-multipoint architecture based on NxN optical splitter is recommended. The laser and photo diode of remote antenna are connected to an optical input of the specific splitter as depicted in Fig. 5.10. All these splitters between fiber node and DAS are connected to same number of splitters inside fiber node. In fact, fiber node is combination of power splitters. All the splitter inside fiber node are interconnected with each other in mesh topology. When data is transmitted by one DAS to splitter, at the fiber node, the signal is divided to all the optical outputs of the NxN splitter to further transmit to all the antennas in this setup. All the wireless devices are visible to each other in this infrastructure irrespective of inter-distance among them.
The major advantage of this architecture is that the radio protocol is handling all the communications on the optical infrastructure. The radio access protocols like carrier sense multiple access with collision avoidance (CSMA/CA) can be applied to this infrastructure. The symbol collisions are detected only in the user's devices on this infrastructure after the implementation of the radio protocol. The data rate is now shared among distributed antennas units and the RoF transceivers are works as only physical layer components without any intelligence. As the number of users for this small integrated home networks are not very big, this setup seems the best compromise between cost and efficiency.
The major problem with this network is the challenge to negotiate the high optical losses induced by the splitters. Hence, this structural design is very demanding for the optoelectronic components due to limited power budget.
Fig. 5.10. The optical multipoint-to-multipoint architecture
5.6 Possible impairments during integrated signal transmission in RoF-PON shared architectures
The Transmission of RoF signal over fiber systems comes across many technical and engineering challenges due to the high oscillating frequency at the boundary of electronic and photonic domains. For optical transmission, the inter symbol interference (ISI) due to fiber dispersion plays a major role in the limitations of a ROF system . The dispersion penalty is also introduced by optical filters during recovery of the required signal. Moreover, AWG is normally applied as multiplexer and de-multiplexer in different architecture designs of the optical networks. However, it formed the effective pass-band narrower due to the pass band curvature and ripple transfer functions . Therefore, cascaded stages badly impact the required wavelength stability and accuracy in the integrated networks. The overall network performance severely degrades due to combine effects of signal waveform distortion . Hence, the optical mm-wave radio signals in hybrid network comprising the WDM optical interface will be degraded by the grating dispersion and optical crosstalk .
The development of RoF systems have only been in an early stage for a few years. Unlike conventional single-carrier optical fiber systems, RoF signals suffer from severe chromatic dispersion and therefore require better understanding and need more careful dispersion management schemes. For example, dispersion-induced phase noise, which is only seen in long-haul CO systems, is now observed in a short-distance RoF system because the coherency of multiple optical carriers in RoF systems can be degraded by the dispersive wall-off. As a result, the physical properties of RoF systems, including dispersion, nonlinearity, optical/digital de-multiplexing, frequency-dependent attenuation, etc., are of great interests in the RoF research groups, especially for the fiber distance above 50 km.
The P2P and P2MP architectures also show signal degradation due to the use of multiple lasers simultaneously. The lasers that are transmitting optical power without any input radio signal are only adding noise for the photodiodes that receive data from some other laser. They add the ambient noise by combing RIN noises and induce an excess of optical power at the receiving photodiode. Therefore, the shot noise rises and can cause the saturation of the photodiodes.
To rectify this issue, the access to the media needs to be controlled. It can be achieved by allowing only one laser to be turned on at one time. In fact, the RoF networks need to be built with simple network architectures, and the simplest optical links. The well planned designs incorporating well-known technique also bring the reliability and resiliency against all potential impairments.