# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 4 Use Cases
### Dynamic Spectrum Access  
The Federal Communications Commission (FCC) has reported that licensed spectrum bandwidths are greatly underused [24]. One of the examples mentioned in the report is the TV spectrum, which has one of the lowest use rates. That spectrum band is in most cases left totally unused in areas not so populated (e.g., rural areas), and due to the fact that it is referred to as TV whitespace. Based on this under-use of spectrum originally meant only for TV broadcasting, regulators are opening up this previously licensed spectrum for unlicensed use [25]. TV whitespaces for unlicensed use brought about a revolution to cognitive radio, spectrum sensing as well as to dynamic spectrum access [26]. TV whitespaces allow for the opportunistic use of vacant TV channels for data communications [27; 28].  
Therefore, based on the opportunistic use of vacant spectrum, one of the use cases of the proposed wireless hypervisor is instantiating it as a Non-Contiguous Orthogonal Frequency-Division Multiplexing (NC-OFDM) PHY, where one or several vPHYs could, based on spectrum availability/occupancy, have their data mapped into subcarriers that do not interfere/overlap with primary users in a Dynamic Spectrum Access (DSA) scheme, and thereby enabling efficient use of the spectrum [11]. For this use case, an NC-OFDM PHY node would employ the proposed hypervisor architecture as a wide-band OFDM modulator, where the subcarrier mapping module depicted in Figure 4 maps the modulated symbols only into subcarriers that correspond to vacant spectrum, i.e., not being used by the primary or other users. At the receiver side, the signal is demodulated and decoded as a wide-band NC-OFDM signal where some subcarriers had their values set to zero. The PHY receiver for this signal is depicted in the bottom right corner of Figure 4, and is nothing but a generic OFDM demodulator/decoder where the subcarrier demapper module needs to know beforehand which subcarriers are active during a given transmission interval [11].  
Figure 6 depicts a possible snapshot of the spectrum usage in the TV whitespace when the proposed architecture is used as a dynamic packet-based PHY layer with allocated bandwidth being dynamically changed according to vacant spectrum. In this example, the PHY layer establishes concurrent/simultaneous communications with several nodes. As can be seen, the channel's center frequency and allocated transmission bandwidth are changed throughout time, depending on spectrum availability and/or traffic load.  
Another use case for the proposed architecture is in the implementation of a spectrum-sharing scheme between wireless operators and Fixed Satellite Services (FSS) in the Citizens Broadband Radio Service (CBRS) band. FSS stations must share spectrum with new entrant wireless operations, while the entrant networks must ensure that the interference that they introduce to the incumbent FSS remains below a specified threshold [29; 30]. The aim with the launch of CBRS in the USA is that wireless systems should dynamically share the spectrum among them [30]. In this use case, the proposed architecture could easily adapt its transmission bandwidth to the available (either continuous or discontinuous) spectrum at any specific time. This use case is similar to the one described by Figure 6. Mainly aligned with this use case, DARPA, the Defense Advanced Research Projects Agency from the United States, has started the Spectrum Collaboration Challenge with the aim to encourage research and development of smarter/more intelligent coexistence and collaboration techniques of heterogeneous networks in the same wireless spectrum bands [31; 32]. One of the examples they have been advocating for is the adoption of such spectrum-sharing technologies in the CBRS band [33; 34].