An important challenge for Femtocells is optimising their radio coverage area dynamically. The goal is to achieve a desired level of performance for mobile transmission, avoid the undesired interference and reduce power consumption. Providing optimal Femtocell signal coverage is important to improve UEs’ mobile usage experience as well as reducing service cost expenditure [Ma et al., 2015]. In outdoor enterprise environment, a number of Femtocells may be deployed together to achieve joint coverage. This is also done to cover a large area while balancing the UEs load, minimising coverage gaps and overlaps between multiple Femtocells. Hence, researchers have begun to consider the problem of coverage optimisation of Femtocells in LTE networks as an important aspect that influences the amount of generated interference on UEs performance [Lu et al., 2012]. However, most of the previous works focused on the coverage optimisation of Fixed-Femtos rather than on the coverage optimisation of Mobile-Femtos as the mobility of Mobile-Femtos is considered a challenge in this research area.
Hence, the purpose of this section is to find an efficient Femtocells coverage planning solution that involves identifying locations for installing Femtocells that are either Fixed or Mobile in LTE and 5G Networks. Femtocells locations play a vital role in achieving a good trade-off between coverage and interference. Poor choices of Fixed or Mobile Femtocells locations may result in coverage holes. Coverage hole occurs when Femtocells are either placed or moving distant apart from each other, while the maximum transmission power-level is not sufficient to cover certain regions in the outdoor environment. Moreover, incorrect locations such as placing or moving Femtocells too close to each other may result in excessive interference to Mobile or Fixed Femtocells’ UEs and Macro UEs. Therefore, in the case of Mobile-Femto, identifying the bus path that the Mobile-Femto will be installed in will
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be a reasonable solution to avoid the interference issue between any nearby Mobile-Femtos. However, there is always a worst-case scenario and this will be discussed later.
Thus, the goal of the coverage planning process is to have good radio conditions everywhere within the outdoor area (especially the cell-edges) or in the vehicular environment so UEs can acquire, initiate and sustain voice calls, as well as overcome the penetration loss and path-loss issues. Good coverage performance is addressed by limiting the maximum path-loss, which can be mitigated by deploying Fixed-Femtos and Mobile-Femtos in the right positions inside the Macrocell and added to this a proper Fixed-Femtos and Mobile-Femtos power calibration is needed.
Therefore, it is important to state that the used path-loss model in this research is the Microcell NLOS path-loss, which is based on the COST 231 Walfish-Ikegami NLOS model. The COST 231 Walfish-Ikegami model is an evolution of the Ikegami model [Alqudah, 2013]. It is developed for urban areas and it takes into consideration obstructing building height and street width, as well as other factors related to the urban environment. Therefore, in order to calculate the Microcell NLOS path-loss, thefollowing parameters are needed: BS antenna height 12.5m, building height 12m, building to building distance 50m, street width 25m, Mobile Station (MS) antenna height 1.5m, orientation 30deg for all paths, and selection of metropolitan center. And the distance between two BSs is less than 1Km [Naguib, 2007]. Hence, based on the previous parameters, the NLOS path-loss equation simplifies to:
This is resulting that the path-loss at 1900 MHz is given by:
Thus, it is equal to (5.1) (5.3) (5.2)
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Where L is the distance in meters and it is at least 20m.
On the other hand, the Microcell LOS path-loss is based on the COST 231 Walfish-Ikegami street canyon model with the same parameters as in the NLOS case [Naguib, 2007]. The LOS path-loss is given by
This is resulting that the path-loss at 1900 MHz is equal to:
Thus, it is equal to
Hence, it becomes easier to classify the achieved path-loss equations among the direct and access links of eNB and Femtocells in LTE network. The used path-loss on the direct transmission between the eNB and the vehicular UEs is the 3GPP Spatial Channel Model (SCM) urban NLOS Microcells model as shown in equation (5.3). The same NLOS model has been used for the Fixed-Femto for both; the backhaul link between the eNB and the Fixed-Femto and the access links between the Fixed-Femtos and vehicular UEs. Therefore, based on equation (4.5) the received SNR at the NLOS receiver Rx can be modified to
. It is to be mentioned that the path-loss model has been included in all the developed equations because of its negative impact on the signal quality together with the penetration loss and the noise power issues in the SNR case. Based on Figure 4.3, the L distance here is changeable according to the distance between the transmitter and the receiver, which could be either x in the case of the direct transmission from the eNB or x-d in the case of the Fixed-Femto transmission.
On the other hand, in the case of the Mobile-Femto, the backhaul link between the Mobile- Femto and the eNB has used the same previous NLOS model. Whereas, for the LOS access link between the Mobile-Femto and the vehicular UEs in the same bus a Constant Path-Loss
(5.4)
(5.6) (5.5)
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has used, which is a free space loss when there is no obstacle against the transmitted and received signals. The free space loss can be very small or sometimes unity when the distance between the receiver and the transmitter is very small and does not exceed few meters. However, there is always a constant power loss of -84.55 [Masui et al., 2002] and based on equation (4.8), the received SNR at the LOS receiver Rx is given by
.
Thus, identifying the used path-loss model between the links of two BSs or between the links of BSs and vehicular UEs has helped in reducing the interference issue by allowing the operator to know where the Fixed and Mobile Femtocells are needed to be placed or moving based on the used path-loss model. In the case of the Mobile-Femto, identifying the path-loss is an important factor for the network operator because it helps the operator to indicate the Mobile-Femto paths, which are the bus paths. This is due to the fact that, as much as it is important to reduce the path and penetration losses between the vehicular UEs and the serving Femtocell, it is more important to maintain the backhaul path-loss between the eNB and Mobile-Femto, so it can serve those vehicular UEs without losing the connection with the mother BS (eNB). Having strong connection between the Mobile-Femto and the eNB as well as between the Mobile-Femto and the vehicular UEs in the same bus has helped in mitigating the generated interference. This strong connection can be achieved by having an efficient coverage planning technique together with choosing the right path-loss model.
Hence, after discussing the chosen path-loss model in the case of vehicular environment and its impact on mitigating the interference, it is important now to discuss the effect of the chosen transmission power on the generated interference as the following section shows.