Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 105

The model in the previous section and illustrated in Figure 6.7 considers transmission capacity in a given area in a general way. The model sheds light on the metrics a 5G DSM design would consider. However, 5G uses small cells that overlay on existing cellular networks to improve capacity and coverage. Indoor users specifically make use of small 5G cells such as femtocells. The best model for using 5G cells on top cellular networks is the two‐tier model where one tier is a base station and the other is the cells. This overlay model is represented in Figure 6.8 where end users are randomly located. In this model, the density of the 5G cells can be expressed as l_C, while the density of the end users can be expressed as l_EU. Naturally, l_EU ≫ l_C. One can extend the model in the previous section to consider other factors. For example, one can state the following:

1 The interference at an end user can be impacted by the interference from neighboring base stations and from the randomly placed small cells.

2 The interference at a small cell can be impacted by all the uplink connections from the end users within a certain vicinity of the small cell.

3 End users transmitting to a base station can use relatively higher power than end users transmitting to a small cell.

4 The aggregate calculated SIR may consider the impact of small cells, base stations, base station users, and cell users.

Figure 6.8 5G cell overlay over cellular base station.

Notice in Figure 6.8 that it is possible for an end user to be in the coverage of a 5G cell but use the base station not the cell because of the cell limited capacity or because of a required transmission rate and quality of service (QoS).

A DSM technique can consider the density of end users in a base station coverage area to create a metric for the transmission capacity needed and to point out the need for more cell deployment within the base station coverage area. Other factors that can be used are the transmission outage estimated from Equation (6.4) and actual measurements of events such as connection denial to an end user can be utilized to increase the cell density.¹⁴

Cellular infrastructure pre 5G is fixed. 5G has no fixed infrastructure since cells can be deployed anywhere where demand is needed. Spatial modeling, covered in Section 6.2, is essential to DSM for both the planning and runtime aspects. After deployment, the model can be used for finding out what new features and capabilities a 5G infrastructure needs in order to increase spectrum efficiency.

There is another layer of overlay that can complicate this spatial modeling. If and when 5G deploys nonterrestrial infrastructure, there will be a satellite or a high altitude platform (HAP) that will have a nonterrestrial base station overlaid on top of multiple ground base stations areas. This model, however, is far in the future and beyond the scope of DSM in this chapter.