# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 5 Related Work  
In this section, we describe and discuss some related pieces of work on virtualization. The related works on virtualization can be split into two main categories, time- or frequency-multiplexing. The time-multiplexing approaches achieve virtualization by splitting the access time to a common Wi-Fi access-point PHY layer [41; 42]. This is possible due to the fact that Wi-Fi is a packet-based wireless network, where access points do not transmit data all the time, and therefore, being able to allow virtual Wi-Fi radio stacks to use its idle time. The frequency-multiplexing approaches can be further divided into two sub-categories based on the type of the virtualized resource, which can be the spectrum bandwidth through the virtualization of the RF front-end [21; 22; 23; 27; 43] or the time-frequency resource grid through the virtualization of the resource blocks (RB) provided by Orthogonal Frequency-Division Multiple Access (OFDMA)-based radio technologies such as LTE and WiMax [44; 45; 46; 47]. Out of all the compared related works, only a few are technology agnostic, i.e., can provide virtual radios to any RAT [21; 22; 23; 27; 43]. However, the issue with these works is that they exchange optimized performance for being generalized so that they can operate with different RATs. In most of these works [21; 22; 23; 43], in order to be agnostic, the hypervisor layer re-implements operations (e.g., FFT, IFFT, Subcarrier Mapper, etc.) that are already performed by the PHY layers of the individual RATs, creating an extra overhead that decreases the performance of the solution. An important comparison point is the dynamic creation and destruction of virtual radios without interfering with or stopping the hypervisor or other already instantiated virtual radios. Out of all works, only a few do not support such feature [21; 22; 23; 27; 44], where the number of instantiated virtual radios and their respective bandwidth allocations must be configured before running the hypervisor. Another interesting point of discussion is the maximum possible number of instantiated virtual radios concurrently running during the experiments. The compared works were experimentally tested with the number of concurrently running virtual radios ranging from 1 to 4 virtual radios; however, on the other hand, our prototype has been experimentally tested with 12 concurrently running vPHys. Finally, it is also important to add that independent gain configuration is an important feature to be exposed to the virtual radios once, for example, devices might be at totally different locations; however, it seems that independent gain configuration is not a major concern to most of the related works, once only a very few mention its support [23; 27; 43]. Differently from the other compared works, the virtualization architecture proposed in this work was designed to be highly optimized for sharing the same underlying physical RF front-end among several concurrent multi-carrier-based virtual radios (i.e., vPHys). It provides dynamic access to several concurrent and configurable virtual RF front ends (e.g., frequency gain, frequency location, and bandwidth) that are accessed through multiple vPHys. The vPHys can be instantiated in real time without interfering with running vPHys and without the necessity to stop the hypervisor. The majority of the compared related works do not make their source code available [27; 41; 42; 44; 45; 46; 47]; however, we believe that research on this field can only progress if the different implementations are made available for comparison and a better understanding of their functionalities and features. Therefore, we make our proposed architecture prototype available at GitHub [14]. The source code includes some examples to measure the prototype's performance, a Graphical User Interface (GUI) for visualizing the transmitted spectrum and a channel emulator that can be used to run experiments without the necessity of having a dedicated piece of physical RF front-end.