# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 2 Hardware Virtualization  
Hardware (in our case, the RF front-end) virtualization is achieved by means of hypervisors [15; 16]. A hypervisor is a hardware virtualization technique that abstracts, i.e., isolates, multiple concurrent software radio protocol stacks, also known as software-defined radios (SDR), of RATs with correlated numerology from the underlying radio hardware i.e., RF front-end. It allows multiple radio stacks to run on top of a single piece of radio hardware at the same time, i.e., concurrently, such that each protocol stack appears to have its own RF front-end (virtual RF front-end) that can be operated independently. One advantage is that wireless hypervisors help to maximize the effective use of the deployed infrastructure (i.e., servers, RF front ends, etc.). The hypervisor is the mechanism allowing the seamless sharing of a particular resource by meeting three key requirements: abstraction, programability, and isolation. The proposed solution addresses each of these requirements as shown in Figure 2 and briefly described next. * **Abstraction**: this feature hides the underlying hardware characteristics and establishes simplified interfaces for accessing and sharing the hardware resources. This feature allows the clients of the hypervisor architecture to use it with no change to their upper layer radio stack, i.e., from the MAC layer upwards. As shown in Figure 2, the proposed architecture provides concurrent access to multiple virtual RF front ends, which are exposed to clients through several vPHYs. The proposed architecture can be supported by inexpensive RF front ends (e.g., commodity SDR equipment) or expensive wide-band RRHs. The wireless hypervisor ensures that multiple vPHYs can concurrently coexist on top of the same physical RF front-end. The vPHYs are the point of access of the radio stacks to the virtual RF front ends. Along with the hypervisor, the vPHYs provide a computationally efficient way of multiplexing several baseband signals into a wide-band signal, which consequently splits the spectrum bandwidth provided by the physical front-end. The proposed architecture supports the operation of multiple concurrent vPHYs, implementing totally different air-interfaces (as long as they are OFDM-based signals and have subcarrierspacings that are integer multiples of the smallest adopted spacing) with diverse processing constraints, channel bandwidths, medium access schemes, etc. * **Programmability**: the proposed wireless hypervisor has to provide the multiple radio stacks running on top of it the same level of programmability (or configurability) provided by the single physical RF front-end. This feature is addressed by allowing the multiple radio stacks to configure different center frequencies, bandwidths and transmission/reception gains for individual virtual RF front ends, i.e., the virtual RF front ends must provide the same set of functionalities provided by the physical RF front-end. * **Isolation**: the proposed wireless hypervisor must make sure that any configuration or wrong configuration does not affect or cause interference to any other collocated radio stack. Isolation is the fundamental requirement that guarantees fault tolerance, security, and privacy to the multiple radio stacks running (i.e., coexisting) on top of the same physical RF front-end [16]. This feature is enforced across multiple clients by providing them predefined access-points and bandwidths that do not overlap. Figure 2 shows one out of several possible multi-tenancy uses for the proposed architecture, where it is employed in 4G or 5G-like mobile networks. Our proposed approach is generic enough to be applied to both 4G or 5G standards or even to a mix of the two standards, since they both use OFDM-based PHYs. However, it could also be, for example, employed by Internet Service Providers (ISP) aiming at sharing their already deployed Wi-Fi access-points (or hot-spots) at locations such as airports, cafes and common shopping areas.