Interference avoidance in MIMO cognitive radio networks with estimation errors

In recent years, due to the explosive growth of the number of new mobile wireless systems with increasingly high data rate requirements, we are facing the problem of spectrum scarcity. To facilitate new wireless services, frequency bands need to be allocated within the usable radio frequency (RF) spectrum. Furthermore, larger and larger bandwidths are required to cater the high data rate communications. However, the existing RF spectrum is already reserved for various government and commercial services. Because of this the introduction of new wireless services is difficult without changing the existing frequency allocation policies. The spectrum scarcity issue has directed the research of future wireless communications towards more flexible and dynamic spectrum access techniques. The new wireless systems can take advantage of the licensed spectrum that is left unused by licensee or they can share the spectrum provided that they do not interfere with the licensed users. One of the techniques that has potential to achieve spectrum coexistence are cognitive radios (CRs) which have gained a significant amount of interest in the research community. CR terminals are able to sense the surrounding RF spectrum and adjust their operating parameters accordingly due to which they are a natural candidate for dynamic spectrum access (DSA) technology. Several techniques based on CR and DSA methodologies have been proposed with a promise to achieve more efficient spectrum utilization. However, the gap between theory and practice still remains. While the concepts are simple, implementing them in practice is still hindered by several challenges. The goal of this work is to take one step closer to more practical applications. In this work we focus on the issue of spectrum sharing and the coexistence of primary and secondary users. We consider multiple antenna based terminals which have become a mainstay in modern commercial wireless systems. By taking advantage of the spatial multiplexing capabilities of the multiantenna terminals, it is possible to achieve simultaneous primary and secondary transmissions by interleaving them in the spatial domain. In practical applications, the difficulty of performing spatial spectrum sharing while protecting the primary users is due to the fact that the secondary network can only obtain information on the primary users by estimating it without any cooperation provided by the primary network. If no additional considerations are taken into account, this will result in prohibitive amount of interference to the primary users due to the secondary transmissions. In this work, two sources of potential interference are tackled. First, channel estimation errors are considered which are the main cause of interference due to the fact that without perfect channel state information (CSI), the secondary users are not able to perfectly orthogonalize their transmit signals with the interference channel. The second issue is incorrect allocation of spatial spectrum resources by the secondary users. If the secondary network is not able to accurately determine the number of transmit streams used by the primary system, it is possible that the secondary system incorrectly allocates too many transmit streams, some of which will overlap with the primary user’s signal space and therefore will result in interference. For the issue of spatial resource allocation, at first, cooperative estimation scheme is investigated in order to improve the estimation accuracy in the low signal-tonoise ratio region. A new decision fusion rule is proposed which is able to both improve the probability of correct estimation as well as to avoid interference to the primary user in the case of an error. To further avoid interfering with primary system when an error actually occurs or when only estimated CSI is available, a transmit power control method is proposed for the secondary users. In order to achieve more realistic characterization for the interference, it is modeled as a random variable and a probabilistic protection constraint is applied in order to guarantee desired interference outage probability for the primary user.