# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 1 Introduction  
According to [1], the average mobile connection speed in 2016 was 6.8 Mbps and it is forecast to grow at a compound annual growth rate (CAGR) of 24.4%, reaching nearly 20.4 Mbps by 2021. By 2020 it is forecast that there will be 11.6 billion mobile-connected devices, including Machine to Machine (M2M) modules [1]. Reports like [2] state that future mobile networks will need to support 1 million connections per square kilometer and up to a total of 100 billion connections in total. The research and development of future mobile networks (i.e., next generation mobile networks) has been derived by several novel breakthroughs in a myriad of areas. The recent advancements of technologies such as licensed assisted access, carrier aggregation, massive multiple-input-multiple-output (massive MIMO), cooperative spectrum sensing, beamforming techniques, compressive sensing, machine learning, etc., have provided exciting and encouraging approaches to address the forecast increase in number of connections and their respective speeds [3; 4; 5; 6; 7]. These demanding requirements pose daunting challenges to the design of both base stations and user equipment. In particular, at the user equipment side, these new requirements bring a series of daring implementation issues due to limited design budget and hardware resources imposed to the implementation of those devices [6; 8]. To support the expected growth in capacity, the forecast for the future mobile networks, the area throughput, i.e., bit/s/km\({}^{2}\), needs to be increased. Some ways to improve area throughput in cellular networks include densification of existing cellular networks using extra-small cells, provision of peer-to-peer (P2P) communications, multi-tier heterogeneous networks, full-duplex communication, massive MIMO, millimeter-wave technologies, cognitive radios, beam division multiple access (BDMA), cloud-based radio access networks (CRAN) and wireless networks virtualization (WNV) [3; 4; 5; 6]. One of the key features to achieve the forecast high data rates is the dense deployment of remote radio heads (RRHs). RRH is the name given to the RF front-end in mobile networks and encompasses the BS's RF circuitry plus analog-to-digital/digital-to-analog converters and up/down converters. The dense deployment of RRHs can be achieved, with a relatively lower cost when compared to the deployment of several physical RRHs, by multiplexing several different signals at a single RRH and transmitting them over several virtual RRHs. Carrier aggregation is one of the usage examples, where a base station (BS) increases its capacity by allocating more spectrum bandwidth. Infrastructure sharing is another usage example, where densification is achieved by sharing already deployed pieces of equipment, such as the RRH, among several wireless or mobile networks [8; 9]. Infrastructure sharing has the advantage of reducing capital expenditure. It is well known that the deployment of cellular or wireless networks is expensive, and raising the capital for that effort is quite difficult as the operators always want to make the most out of the already deployed infrastructure. Therefore, in order to obtain a better return on the costs related to installation and maintenance of mobile or wireless infrastructure, it would be less expensive to share the already (or newly) deployed infrastructure with other operators than to build overlapping and concurrent infrastructure [10]. This way, RF front-end virtualization becomes very useful for flexible spectrum access, once it allows several physical layers (PHYs) to concurrently share a single physical wide-band RF front-end. Some direct consequences of RF front-end virtualization are the reduction in the required physical space for deployment (only a single wide-band RRH is required), energy consumption and price (reduction in redundancy) for the infrastructure provider. Figure 1 shows an example of a use case where RF front-end virtualization is used in a multi-tenancy application.