## 2 System Architecture, Spectrum Sharing Technique, and Interference Management
### Co-primary Shared Access
#### Static Spectrum Sharing by Renting  
In general, the density of UEs of an MNO within a building constitutes a fraction of the total number of UEs served simultaneously at any time by the network. This rationale underpins the fact that the whole spectrum of MNO 2 may not be necessary to share with the average number of small cells per building of MNO 1. Saying it another way, a fraction of the whole shared spectrum would be enough to serve small cell UEs per building. Hence, unlike DySP, instead of sharing the whole spectrum of MNO 2 subject to the interference management with UEs of MNO 2, a portion of the spectrum of MNO 2 would be shared with small cells per building for each TTI. UEs of MNO 2 are served only within the remaining portion of the spectrum of MNO 2. Such kind of static allocation of the spectrum of one MNO through renting for a certain period of time by another is termed as StSR. Over the period of agreement between MNOs, one cannot get access to the spectrum of the other even though one of the MNOs rents the spectrum from the other. A noticeable advantage of this spectrum sharing policy is that no control signaling for the TD co-ordination is needed to exchange in order to update the number of ABSs per APP dynamically for both MNOs per building by CoRS, which helps overcome the requirement of spectrum usage for exchanging control signaling overhead over the backhauls between MNOs. Hence, based on the above discussion, in StSR, by exploiting the external wall penetration loss of a 3D building, MNO 1 typically asks for renting certain portion of the spectrum of MNO 2 to use exclusively only within the building such that the co-channel interference with outdoor UEs of MNO 2 can be avoided (Figure 4b). In StSR, both TD and FD schedulers of each MNO schedule their respective UEs. As the LTE system bandwidth can be allocated to an MNO in various amounts, e.g., 1.5, 5, 10, 20, 40, and 100 MHz, StSR gives a flexibility in renting spectrum from MNO 2 such that an appropriate amount of spectrum can be rented to serve the user demand per building to improve the spectral utilization of MNO 1. Note that the rented spectrum can be flexibly operated with the existing spectrum of MNO 1 without any significant network modification. For example, following the contiguous/non-contiguous carrier aggregation techniques, the shared spectrum of MNO 2 can be aggregated to that of MNO 1's own spectrum. Further, the shared spectrum is rented by MNO 1 by a fixed amount and does not vary with the time irrespective of the load demand of small cells. Hence, no coordination signaling is required. Furthermore, for the multiband enabled small cells, the whole spectrum of an MNO itself can be shared with its in-building small cells within each building to avoid additional spectrum licensing fee. However, because of different operating mechanisms for sharing the spectrums of different MNOs to address issues such as interference management, both spectrums of different MNOs cannot be operated by the same transceiver. This requires an additional transceiver on top of the existing transceiver  
Figure 4: Techniques for MNO 1 and MNO 2: (a) DySP; (b) StSR. operating at the spectrum of MNO itself resulting in each small cell is enabled with dual transceivers. Hence, in such dual-transceiver enabled small cells, one of the transceivers of each small cell operates at its own MNO spectrum and the other transceiver operates at a portion or the whole spectrum of the other MNO. Hence, for the small cells of MNO 1, transceiver 1 of a small cell operates at the spectrum of MNO 1 itself. Since all macro UEs and small cell UEs are served at the same MNO 1 spectrum, whenever a macro UE is present in the building within the coverage of any SBS, such an indoor macro UE causes co-channel interference to small cells and their UEs.