# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 3 Multiplexing Concurrent OFDM Signals
### Discussion  
In this section, we discuss the proposed architecture, its functionalities, features, possible uses and limitation. As mathematically shown earlier, the proposed wireless hypervisor provides a virtual and discretized (in steps of \(\Delta_{f}=R_{\text{hyper}}/N\)) baseband spectrum abstraction layer to the several vPHY layers sitting on top of it. Its key function is the multiplexing, in the frequency domain, of several narrow baseband signals. It receives concurrent sets of modulated signals (e.g., BPSK, QPSK, M-QAM, etc.) from several vPHYs and maps these sets into continuous or non-contiguous subcarriers (i.e., spectrum) that will then be converted into time-domain by the \(N\)-point IFFT module and have a proper \(N_{CP}\) samples long CP added to it. The digital signal processing carried out by the spectrum hypervisor transforms the sets of modulated vPHY signals into a wide baseband waveform signal that is appropriate for transmission, while keeping the concurrent sets of vPHY signals unchanged and isolated from each other. The hypervisor supports data flows from multiple concurrent vPHYs and provides each one of them a virtual RF front-end, which can have the following settings configured independently: frequency-domain gain (\(\rho_{k}\)), frequency location (given by the mapping of the sub-band into the hypervisor's IFFT) and bandwidth of the vPHY (\(N_{\text{vPHY}}\)). Regarding the gain settings for each vPHY, as there is only one physical RF amplifier, the gain of the amplifier is set to a reasonable level (a level that avoids saturating the signal) and the independent vPHYs can set what we called frequency gain (\(\rho_{k}\)), which is a gain applied to the frequency-domain signal and corresponds to a percentage (i.e., 0 up to 100 %) of the RF amplifier's current gain. Access to the wireless hypervisor is exposed through the vPHYs. The vPHYs work the same way as regular OFDM-based PHY layers (e.g., Wi-Fi, WiMAX, LTE, 5G-NR, NB-IoT, etc.) being the only exception the removal of the OFDM modulation part. In the proposed architecture (see Figure 4), OFDM modulation is now executed by the wireless hypervisor, which efficiently multiplexes the baseband signal of several vPHYs into a single wide baseband signal. By removing and transferring the OFDM modulation of all PHYs to the hypervisor, we optimize the whole baseband processing performance by avoiding redundant/unnecessary, for example FFT and IFFT, operations that would be required to multiplex several concurrent regular PHY signals into a wide baseband signal [21; 22; 23]. The down-side of our approach is that it can only operate with multi-carrier-based (i.e., OFDM) signals. At the receiver side, different receiving approaches might be followed, depending on how the proposed hypervisor is employed to generate the wide baseband signal. The first approach refers to a communications connection between two devices (i.e., a point-to-point connection). In this approach, as a first step, the received wide baseband signal coming from the RF front-end is split by a channelizer into equally or unequally (i.e., non-uniform) spaced spectrum chunks and then, the down-converted and down-sampled narrow sub-band signals are fed into the narrow-band PHY receivers running on the device. The second approach is similar to the first one, where the only difference is that the signal coming from the RF receiver front-end is not split into narrow sub-bands. Here the wide-band received signal is fed directly into the OFDM demodulator of a corresponding wide-band vPHY. In the third approach, several independent, distributed and narrow-band devices have their RF receiver front ends tuned to the center frequency and transmission bandwidth of each one of the transmitted vPHY signals. The fourth approach represents a mix of the previous three approaches, where there could be wide-band devices tuning to more than one transmitted narrow-band vPHY signal while other narrow-band devices would only be tuning to individual transmitted narrow-band vPHY signals.