In cognitive radio networks, imperfect spectrum sensing causes harmful interferences or low utilization of unused bands, depending on an access policy of the secondary system. Since these losses have a trade-off relationship, it is hard to decide proper sensing and access policies for the licensed bands. To deal with this problem, we propose a joint algorithm for sensing adaptation and opportunistic resource allocation. The joint optimization problem minimizes the total expected cost of the losses and utilities which are likely generated for the secondary system. To protect the primary system and ensure the secondary system's utility, we employ an average interference threshold constraint and a QoS constraint for the secondary system. Two types of average secondary utilities are considered for the QoS constraint; one is averaged over only transmitted frames and the other is averaged over total frames. Based on energy detection, we derive feasible ranges of sensing threshold to satisfy the constraints. Simulation results show that the proposed scheme dynamically adapts sensing threshold depending on the network environment and minimizes the total cost of the secondary system. In addition, we investigate the performance of the proposed scheme with the two types of the secondary utility.

In this paper, we propose power budget allocation algorithms for bidding of auctions in cooperation-based spectrum leasing. The cooperation-based spectrum leasing allows secondary users to dynamically access licensed bands while providing the benefit of cooperative diversity to primary users as a reward. For efficient allocation of the licensed bands to the secondary users, a second-price based multiple auction mechanism is considered. In this mechanism, the secondary users select bands which they want to use and calculate bids for those bands, under a limited transmit power. For the optimal solution of the bidding problem, however, computational complexity increases exponentially with the number of bands. To overcome this problem, we find feasibility conditions for band selection and propose a suboptimal algorithm. The suboptimal algorithm makes bids consisting of the maximum achievable rates for cooperative transmissions. Simulation results show that the primary and secondary users achieve significantly increased achievable rates through spectrum leasing with the proposed algorithms. It is also shown that the performance of the suboptimal algorithm approaches that of the optimal solution as the invested power budget increases.

This paper proposes spectral efficient relay-assisted transmit diversity techniques, i.e., space-time and space-frequency block codes (STBC/SFBC), for single carrier frequency-domain equalization (SC-FDE). The proposed systems achieve spatial diversity over frequency selective channels in a distributed fashion without cyclic prefix (CP), which increases spectral efficiency of conventional relay-assisted systems. In practical mobile applications, the CP-less distributed STBC (D-STBC) SC-FDE suffers from the time selectivity of wireless fading channels, resulting in a deviation from the basic assumption of Alamouti codeword. In order to obtain a reliable performance over doubly selective channels, the CP-less D-SFBC SC-FDE and its efficient implementation are presented. Further, the performance of these two distributed SC transmissions is analytically compared over fast fading (i.e., time selective) environments. For evaluation of mean square error over fast fading channels, we present a channel model that captures the time-varying nature of wireless channels. It is shown through analysis that the CP-less DSFBC SC-FDE outperforms the CP-less D-STBC SC-FDE when there exists a severe Doppler spread. Timing synchronization and highly accurate channel estimation of the proposed systems are also addressed for practical implementations. Simulation results show that the proposed systems approach the lower bound of full-CP systems, thus increasing the spectral efficiency.

Spectrum sensing is a key enabling technology of cognitive radio. Reliable detection increases access opportunity to temporarily unused bands and prevents harmful interference to the licensed users. Due to the receiver noise, signal attenuation, and multi-path fading effect, however, it is usually not possible to determine the existence of primary signal with absolute certainty. By extracting a global decision from shared local sensing results, cooperative sensing achieves high reliability over fading channels. In this paper, we assume that the traffic statistic of primary system is logged into the radio environment map (REM) and can be accessed by the secondary systems. The threshold of each energy detector is dynamically adapted according to the utility values and a priori information from REM. Then, decision results and corresponding operating points are collected by a fusion center, which makes a global decision with high confidence.

Spectrum sensing is a key enabling technology of cognitive radio. Reliable detection increases access opportunity to temporarily unused bands and prevents harmful interference to the licensed users. Due to the receiver noise, signal attenuation, and multi-path fading effect, however, it is usually not possible to determine the existence of primary signal with absolute certainty. Without the information of primary user activity, Neyman-Pearson criterion has been commonly used to minimize the missed detection probability for a given false alarm rate. In this paper, we assume that the traffic statistic of primary system is logged into the radio environment map (REM) and can be accessed by the secondary system. Considering sensing errors, Bayes criterion is adopted for total utility maximization of primary and secondary systems. The threshold of energy detector is adapted according to the utility values and a priori information from REM, i.e., both false alarm and detection probabilities are dynamically adjusted.

Peak-to-average power ratio (PAPR) regrowth after clipping is one disadvantage of space-frequency block coded orthogonal frequency-division multiplexing (SFBC-OFDM). In this letter, we propose an effective PAPR reduction technique of SFBC-OFDM for multinode cooperative transmission. To reduce PAPR at the source (mobile equipment), the relay applies SFBC encoding, which enables the source to transmit clipped single-input single-output (SISO)-OFDM signals without any increase of PAPR. Simulation results show that the clipped signals of proposed scheme are effectively recovered, and the proposed scheme achieves the diversity of SFBC without the complexity of multiple antennas at the source.

Space-time block coded (STBC) single carrier (SC) transmission was extended in a distributed fashion for practical implementation of user cooperation. However, STBC was designed under the assumption that the channel is static over the duration of a space-time codeword. Thus, the distributed STBC SC system suffers from the time selectivity of wireless fading channels. To achieve a reliable performance over fast fading channels, we propose a distributed space-frequency block coded (D-SFBC) SC transmission. This paper analytically compares the performance of these two distributed SC transmissions over fast fading environment. For evaluation of mean square error (MSE) over fast fading channels, we present a channel model that captures the time-varying nature of wireless channels. It gives an insight into the characteristics of inter-carrier interference and simplifies the evaluation of MSE. Using this model, it is proven that the D-SFBC SC transmission outperforms the D-STBC SC transmission when there exists a severe Doppler spread. Simulation results are also provided to validate our analysis and to compare two distributed single carrier transmission schemes.

This paper proposes a relay-based transmit diversity technique for single carrier frequency-domain equalization (SC-FDE). The proposed system achieves spatial diversity over fading channels in a distribution fashion without cyclic prefix (CP), which increases spectral efficiency of conventional relay- based systems. The destruction of channel cyclicity due to the lack of CP is recovered at the input of relay and destination. In order to obtain spatial diversity, the transmit sequence of relay is efficiently generated in time domain, realizing space-frequency block code (SFBC). Corresponding destination structure using FDE, timing synchronization, and highly accurate channel estimation for the proposed system are also presented. Simulation results show that the proposed system approaches the lower bound of full-CP system.

This paper proposes a relay-assisted space- frequency block code (SFBC) for single carrier frequency-domain equalization (SC-FDE) in a distributed fashion. The proposed scheme achieves spatial diversity over uplink fast fading channels without the complexity of multiple antennas. The source (mobile equipment) of the proposed system has a very simple transmitter structure with constant amplitude transmit sequences. In order to obtain spatial diversity, the transmit sequence of relay is efficiently generated in the time domain, which is equivalent to the SFBC. The corresponding destination structure and frequency domain equalization are also presented. Simulation results show that the proposed system considerably outperforms the distributed space-time block code (D-STBC) SC-FDE over fast fading channels.