As a true 4G cellular network, LTE-A and its proposed self-organisation should fulfil the network necessities to reach the target pick data rate and scalable system bandwidth. The additional functionalities are only applicable on the system, if there is a reliable network platform available from previous releases and network planning.
2.2.1. Existing LTE Characteristics and Compatibility
The conventional cellular network deployment is typically launched based on homogeneous architecture by applying a macro-centric planning process, in which the base stations are operating within a planned layout and serving the user terminals. The values of receiver noise floor, transmit power levels and antenna radiation patterns are similar for all the base stations in homogeneous networks.
16
Furthermore, all the base stations in homogenous networks prepare similar backhaul connectivity to the data network and unrestricted access to the user terminals in the network, while serving roughly the same number of users [6]. Therefore the additional carriers are required to overcome the capacity shortcomings when the traffic demands are growing. Nevertheless, as a main target of 3GPP LTE-A systems to improve the ITU requirements, the new releases of LTE-A systems are compatible to and share the frequency bands with the first LTE release.
2.2.2. Network Heterogeneity and Heterogeneous Architecture
Throughout this network heterogeneity investigation, two types of base stations are being considered within different HetNet architectures. One is the long-range macrocell base station, called as evolved Node-B (eNB), and the other is one or more short-range nodes, which in case of femtocell nodes are called Home evolved Node-B (HeNB). As a technical consideration, these two sub-networks could be assumed to contain two cooperating network layers, or tiers, which are linked through the core system [4]. By providing the indoor area coverage devices like femtocells, this will support a variety of services by using these low power access points, which can provide the higher data rate of several Mbps for the indoor areas [19,20].
The successful co-existence of both macro and femto nodes in an LTE network requires considered research for obtaining efficient and elegant solutions. Since the radio resource management protocols for coexisting macro and femto nodes are not specified by the standards (e.g. 3GPP’s UMTS LTE) [4], a solution could be found by applying network organisation and cognition schemes into the existing systems.
2.2.3. Base Station Application and Deployment
Small-cell concept is referred to networks with smaller size of implementation, shorter communication range, lower transmission power and lower deployment costs. To provide network users with the experience of a ubiquitous network with improved QoS at both the cell-centre and the cell-edge areas, a heterogeneous network solution should propose cooperative spectral efficiency algorithms by
17
using a range of small-cell nodes. Figure 2-2 shows the various types of nodes in HetNet architectures, based on their coverage radius (rough distances).
Figure 2-2: Variety of nodes in heterogeneous network architecture
2.2.3.1. Macrocell
Macrocell nodes provide wide coverage area up to about 40 Kilometres by a high transmission power of about 40 to 100 Watt. The number of users per base transceiver station (BTS) depends on deployment cell, but this is usually between 200 to more than 1000 users [21]. In LTE systems, the macro layer acts as the main support for small-cell nodes although facing several spectrum challenges e.g. interference and overloading.
2.2.3.2. Microcell
Microcell node has been used in outdoor areas to offload users from the macrocell nodes by its maximum cell radius of about 2 Kilometres and maximum transmission power of 2 to 10 Watts [21]. Microcells have also been used for 3G cellular systems as well as LTE releases, because of their capability to cooperate in outdoor areas.
2.2.3.3. Picocell
Picocells have substantially lower transmission power compared to macro nodes and are mostly deployed in an ad-hoc way in the network [22,23]. Furthermore, the networks using picocell nodes are expected to experience lower signal to
18
interference ratio, because of their unplanned deployment on the network, which results in a challenging RF channel for control channel transmissions to the cell- edge users. An important point regarding macro-pico deployment is the large difference of the transmit power between macro and pico nodes in the network, which causes the smaller downlink coverage of a picocell compared to the macro. However, this is not the same case for uplink, which uses the same transmit power strength from user terminal to all the base stations, because this only depends on the user terminal’s transmit power [6
]. Picocell transceivers could be used either in indoor or outdoor areas, but their coverage radius is up to 200 meters only, which should be carefully considered while planning the network infrastructure. Figure 2-3 shows a macro-pico scenario in which the pico nodes are applied to enable range-extension to support additional number of UEs.
Figure 2-3: Range-extension for macro-pico network
2.2.3.4. Relay Node
Relay Stations (RSs) are planned to forward user information from the neighbouring user equipment (UE)/mobile station (MS) to a local eNode-B (eNB)/base station (BS) [24]. The RSs can enhance the total throughput of the system by extending the signal and service coverage of an eNB. The selection of the relay types and relay partners (collaborative strategy) play a great role on the performance of relay transmission. There are two types of rely nodes defined in 3GPP LTE-Advanced and 802.16j standards: type 1 (non-transparency), which could help a remote UE located far away from eNB to access to the eNB. This type of relay nodes need to transmit the common reference signal and the control information for the eNB, and its main aim is to extend the signal coverage and
19
services. On the other hand, type 2 (transparency) could help a local UE, which is located within the coverage of eNB and has a direct communication link with eNB to improve the link capacity and service quality. So it does not transmit the common reference signal and the control information and its main aim is just to increase the overall system capacity, by achieving the transmission gain and multipath diversity for the local UEs [25]. Therefore, the general application of relay nodes in combination with macrocell is to pass transmission for out of range mobile nodes, as well as improving the existing connections.
2.2.3.5. Femtocell
Femtocell is introduced as an intelligent access point to support 3G and 4G mobile devices, which use cellular air interface, e.g. CDMA2000, UMTS, LTE and LTE-A. Femto nodes are tightly integrated with the existing macro networks, and so their use and switching between macro and femto are seamless for the users in particular. The femtocell network architecture and its specifications allow the ordinary users to install them with plug-and-play simplicity [26]. In case of using of femtocell within closed mode HetNet architecture, only the registered subscribers of femtocell are allowed to access. Hence, the nearby users, either from the neighbouring femto, or general macro node are likely to face severe interference caused by the femtocell [6]. Therefore, when deploying femto sub- network, which is an indoor application, there is a need to consider an appropriate control strategy to receive the maximum support from this cooperation.
There is an additional focus on femtocell in HetNet architecture, compared to the other small-cells, because of its low-power, low-complexity and compatibility with the existing core network mobile network operators (MNOs), while promoting different ranges of tariffs for home broadband.