# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 1 Introduction
In this example, two BSs, which are deployed at the same location, use different channels and have totally independent infrastructure (i.e., equipment), are replaced by a single multi-tenancy BS that emulates, through RF front-end virtualization, two BSs where subscribers from the two different networks associate with their corresponding virtual BS. Another way of achieving such demanding rates is through better use of the available and already underused spectrum bands. In general, due to the presence of primary and secondary users in shared licensed bands and of competing (i.e., opportunistic) users in unlicensed bands, the available spectrum for users in a cognitive radio environment is fragmented and its use is intermittent, i.e., the available spectrum is split into non-contiguous chunks and is not used all the time. This intermittent and fragmented spectrum availability calls for a flexible and agile transmission scheme of the desired signals [11]. The concurrent transmission of several narrow orthogonal frequency-division multiplexing (OFDM)-based signals allows for the selective use of the available chunks of spectrum, which enhances the spectrum use, consequently improving the area throughput. Therefore, in this work, we propose a non-intrusive and highly optimized wireless hypervisor architecture for software-defined radios (SDRs) that ensures coexistence, isolation, and programmability for multi-carrier-based systems (i.e., OFDM) with numerologies that are multiple integers of one another, i.e., systems that have subcarrier spacings that are integer multiples of the smallest adopted frequency spacing and cyclic prefix (CP) lengths that are integer multiples of the maximum possible number of multiplexed signals. The proposed architecture is meant for multiplexing concurrent signals of the same RAT or correlated RATs (e.g., LTE/LTE, LTE/NB-IoT, LTE/5G-NR, 5G-NR/5G-NR, NB-IOT/NB-IoT, Wi-Fi/Wi-Fi, ZigBee/ZigBee, etc.). We focus on supporting OFDM-based systems once such waveform is still one of the most used and important ones being used even in 5G standards like the 5G New-Radio (NR) ones [12; 13]. Therefore, we understand that OFDM will still be widely adopted and employed for a long time. The proposed hypervisor is a generic architecture that can be used to serve not only Wi-Fi, LTE and NB-IoT-based sensors, i.e., Machine-type communication (MTC) devices, but also serve other kinds of Human-type communication (HTC) devices. The main idea behind the hypervisor is the ability of having several Wi-Fi routers, LTE/NB-IoT or any other OFDM-based radio access technology (RAT) base stations sharing the same RF front-end. We show that with the proposed architecture, spectrum programmability (i.e., the ability to program/change frequency, bandwidth and gain settings) can be decoupled from physical layer (PHY) processing and delegated to a virtualization layer, (i.e., the wireless hypervisor) which is added between a set of virtual PHYs (vPHYs) and the hardware/physical radio frequency (RF) front-end. The proposed solution can be applied to both approaches mentioned above of achieving higher area throughput. Figure 1: One exemplary use case for RF front-end virtualization: A single BS that emulates multiple virtual BSs. Here, subscribers from different networks associate with their corresponding virtual BS by using the same underlying infrastructure. We demonstrate that spectrum allocation can be decoupled from the PHY (or baseband processing) layer. In this way, the proposed hypervisor architecture supports flexible spectrum bandwidth allocation by creating a new layer located between several vPHYs and the physical RF front-end. The hypervisor dynamically maps the modulated signals of several vPHYs into configurable chunks of spectrum, before sending the resulting multiplexed signal of the vPHYs to the RF front-end.