Samuli Tiiro

Optimum welch FFT segment size for duty cycle estimation in spectrum awareness system

In order to realize practical dynamic spectrum access (DSA), implementing spectrum sensing with reasonably low cost is very challenging due to the required detection accuracy and quickness. For this issue, we have introduced an extended DSA, smart spectrum access (SSA), where any useful information in terms of spectrum utilization, such as statistics regarding spectrum utilization, is used not only for accomplishing the requirements of spectrum sensing but also for enhancing the performance of the DSA. In this approach, obtaining the statistics of the spectrum utilization is an important issue for which we have developed a spectrum awareness system prototype. One issue in the spectrum awareness is the Welch FFT segment size design which is used for spectrum analysis. This issue involves a trade-off between the spectrum usage detection accuracy and available frequency resolution. For this issue, we derive an optimum segment size based on analysis and show that the optimum segment size depends on signal-to-measurement bandwidth ratio and duty cycle. The optimum segment size derived by the analysis is validated with numerical simulation and experimental results.

Power control method for MIMO-based cognitive radio with imperfect channel state information

Cognitive radio (CR) systems aim for more efficient spectrum utilization by having so called secondary user (SU) terminals transmit on a frequency band reserved for licensed primary user (PU) systems. The secondary transmissions are allowed provided that no harmful interference will be caused to the PUs. Equipping the SU terminals with multiple antennas enables them to use transmit precoding combined with power control in order to control the interference levels. In most of the previous works, perfect interference channel state information (CSI) has been assumed. With imperfect CSI, however, the SU is no longer able to guarantee that the interference constraint can be satisfied. In this paper, we investigate the problem of spectrum sharing with estimated CSI in CR systems using multiantenna terminals. Due to the randomness of the estimation error, we set a probabilistic interference constraint and, in order to satisfy it, analyze the interference power statistics. In addition, we provide a framework for transmit power control for satisfying this constraint. Moreover, we briefly cover the estimation-throughput tradeoff and numerically confirm that an optimal value for the estimation period can be obtained.

A study on Welch FFT segment size selection method for spectrum awareness

We investigate Welch FFT segment size selection method for spectrum awareness in the context of smart spectrum access (SSA), in which spectrum usage information of primary users (PUs), such as duty cycle (DC), will be exploited by secondary users (SUs). Energy detector (ED) based on the Welch FFT can detect the presence of PU signal in a broadband environment efficiently and DC can be estimated based on the detection results. There is a trade-off between the detection performance and the frequency resolution in terms of the Welch FFT segment size. The optimum segment size depends on signal-to-noise ratio (SNR), and therefore practical segment size setting is difficult. In this paper, we will show two practical segment size selection methods: the first one can achieve accurate spectrum awareness while it requires relatively high computational complexity since it employs exhaustive search, and the second one can achieve reasonable spectrum awareness performance with low computational complexity since limited search is used. Numerical evaluations show that the method with limited search can achieve comparable spectrum awareness performance to the method of exhaustive search while requiring significantly lower complexity.

Cooperative source number estimation for cognitive radio networks

In this paper, we consider a cognitive radio (CR) network operating on a spectrum reserved for licensed primary user (PU) by employing spatial spectrum sharing (SS). When the secondary users (SUs) are equipped with multiple antennas, they can perform SS without interfering with the PU by orthogonalizing the secondary transmission with the interference channel . In order to avoid interference and to maximize the capacity of the secondary transmissions, it is essential that the SUs determine the dimension of the orthogonal transmission space. In CR networks where no cooperation is provided by the PU, this information can be obtained by estimating the number of primary transmit streams at the secondary terminals. We study this issue when the secondary network performs cooperative estimation by using the minimum description length (MDL) algorithm to obtain the independent estimates and propose a decision fusion rule which takes into account the estimation bias of MDL. We will show that the proposed method can outperform conventional decision fusion rule and that it provides additional protection for the PU in the case of estimation error.