# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 4 Use Cases
### Network Densification  
The capacity of an additive white Gaussian noise (AWGN) channel is given by the following equation [9]  
\[R<C=m\left(\frac{W}{n}\right)\log_{2}\left(1+\frac{S}{I+N}\right), \tag{12}\]  
where \(W\) is the BS allocated bandwidth, \(n\) denotes the BS load factor (i.e., the number of users sharing the given BS), \(m\) is the spatial multiplexing factor (i.e., it denotes the number of spatial data streams connecting the BS and devices), \(S\) gives the signal power and \(I\) and \(N\) represent the interference and power noise, respectively, experienced at the receiver side. After analyzing the equation, it is possible to see that the capacity can be increased by decreasing the BS load factor, which can be attained through cell splitting. Cell splitting involves deploying a larger number of BSs and making sure that the user traffic is evenly distributed among all the deployed BSs [9]. A possible consequence of cell  
Figure 6: Snapshot of the spectrum usage in the TV whitespace. splitting/densification is that it might improve the signal power, as the users could now be closer to the new deployed BS and consequently experiencing a reduced path-loss. The dense deployment of infrastructure is a precondition for cutting the BS load factor \(n\) down in (12). However, the deployment of additional BSs entails significant costs and detailed site survey/planning [35]. Therefore, another use case, which represents a very important application for the proposed architecture, is instantiating the proposed architecture as an RF front-end multiplexer, where several vPHYs share a single physical RF front-end. This could be employed in multi-tenancy cases [21], where a cellular network infrastructure provider shares their owned infrastructure and/or spectrum with mobile virtual network operators (MVNOs) or vertical markets such as energy, automotive, city management, food and agriculture, health-care, government, public transportation, manufacturing, etc. In this case, the infrastructure provider shares its already deployed infrastructure, which ranges from baseband processing units (BBUs) to the RF front ends (also known as Remote Radio Head - RRH). By employing multi-tenancy schemes, operators, MVNOs and verticals can decrease their capital expenditure (CAPEX) and operational expenditure (OPEX). Multi-tenancy makes the deployed infrastructure more energy efficient/greener by allowing a reduced number of BS sites, and, therefore, largely reducing the power consumption of air conditioning and other site support pieces of equipment. The infrastructure provider can, for example, split its owned spectrum band into smaller chunks and lease it to MNVOs or verticals. Another possible example is instead of splitting its own spectrum into smaller chunks, is the possibility of providing other operators access to their own spectrum bands due to the wide-band capabilities of the current RRHs, ranging from 10 to 250 MHz of useful instantaneous bandwidth [36; 37; 38]. The virtualization of the physical RRHs is an important step in the direction of multi-tenancy networks as being studied by the 3 GPP [39]. This possible application/instantiation of the proposed architecture enables the Radio Access Network (RAN) to be made available as a service (RANaaS) to MVNOs and verticals. Virtual PHYs create new ways for infrastructure providers to monetize their owned spectrum bandwidth and deployed infrastructure. In this way, infrastructure providers can offer virtual PHYs as a service (vPHYaaS) in order to provide isolated and independent virtual networks to MNVOs and/or verticals sitting on the top of a shared physical infrastructure [40].