Due to ever increasing usage of wireless devices and data hungry applications, it has become necessary to improve the spectral efficiency of existing wireless networks. One way of improving spectral efficiency is to share the spectrum amongst different coexisting networks and serve multiple devices simultaneously. Spectrum sharing mechanisms for coexistence of a licensed network, such as LTE, with an unlicensed network, such as Wi-Fi, are being considered in the recent literature and standardizations. In order to enable the coexistence between licensed and unlicensed users, it is necessary to include interference mitigation techniques to protect the licensed primary users (PUs) from harmful interference. Typical interference mitigation mechanisms are based on spectrum sensing and cognitive radio (CR), wherein unlicensed secondary users (SUs) observe the spectrum and utilize it when licensed PUs are inactive. Thus, the SUs utilize empty time-slots in the shared spectrum to avoid the interference. The spectral efficiency can be further improved if the SUs are allowed to transmit concurrently with PUs by exploiting the spatial dimension provided by multiple antenna techniques.
The underlay CR paradigm allows such coexistence where SUs transmit its signal in the same time-slots as PUs by exploiting the spatial and frequency resources in the network. In order to exploit the spatial dimension, SUs can utilize the location coordinates of PUs to steer its signal away from PUs to mitigate the interference. The SU transmitter can also employ multiple antenna techniques to serve a large number of devices. Further, the SUs can utilize frequency bands occupied by PUs by dynamically selecting the frequency band that provides the highest rate. In this work, we develop techniques for PU location estimation, spatial resource allocation and frequency band selection for SUs in underlay CR networks.
We begin by considering the problem of estimation of PU location coordinates in a network of SUs in the presence of spectrally overlapped interference. A localization algorithm based on cyclostationary properties of the PU signal is proposed in order to mitigate the impact of the interference. The proposed scheme identifies and eliminates the SUs in the vicinity of the interferer thereby improving the localization accuracy.
Next, we propose a low-complexity algorithm to solve a resource allocation and interference control problem in a network where secondary BS (SBS) is equipped with a large antenna array. The proposed algorithm selects maximum number of SUs and allocates power for downlink transmission from the SBS, while keeping the interference to PUs below a specified limit. It has been shown that the proposed low-complexity algorithm provides optimum solution if the number of antennas at the SBS is order of magnitude larger than the number of SUs and PUs in the network.
Finally, we analyze power control and frequency band selection policies for a SU transmitter-receiver pair that can select one out of multiple available frequency band in each time-slot to maximize its achievable rate. We derive an expression for transmit power in a frequency band as a function of interference constraints, PU traffic statistics in the frequency band, and temporal correlation of channels. We show that instead of hopping to a different frequency band in each time-slot, the SU can stay on one frequency band in order to maximize its own rate while keeping the interference toward PUs to a predetermined level.
