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The use of a distributed antenna system 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 subscriberâ€™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.