In this paper, we develop a general reduced-rate orthogonal frequency division multiplexing (OFDM) transmission scheme for inter-subchannel interference (ICI) self-cancellation over high-mobility fading channels. Via transmit and receive processing, we transform the original OFDM system into an equivalent one with fewer subcarriers. By reducing transmission rate, we are able to design a transmitted signal structure with inherent ICI self-cancellation capability without requiring the instantaneous channel state information. We develop a general structure of transmit and receive processing matrices so that all equivalent subchannels in the transformed OFDM system have the same average signal-to-interference ratio (SIR). For the developed structure, we further optimize the transmit and the receive processing coefficients to maximize the SIR based on channel statistics. Numerical and simulation results demonstrate that the developed reduced-rate OFDM transmission achieves an SIR gain of around 5 dB over the existing ICI self-cancellation schemes and significantly reduces the error floor at the receiver.

In this paper, we develop a general reduced-rate orthogonal frequency division multiplexing (OFDM) transmission scheme for inter-subchannel interference (ICI) mitigation in a high-mobility environment. By transmit and receive preprocessing, we transform the original N-subcarrier OFDM system into an equivalent K-subcarrier one with significantly reduced ICI. In particular, we develop a general structure of transmit and receive preprocessing matrices so that the K subchannels in the equivalent OFDM system share a common average signal-to-interference ratio (SIR). Without requiring the instantaneous channel state information, we optimize the preprocessing coefficients to maximize the SIR based on channel statistics. Numerical and simulation results demonstrate that the developed reduced-rate OFDM transmission achieves significant performance improvements over the existing ICI self-cancellation schemes.

In this paper, we consider a two-hop multi-input-multi-output (MIMO) amplify-and-forward (AF) relay system consisting of a source node (SN), a destination node (DN), and a relay node (RN) that simply amplifies and forwards its received signal to the DN without any further processing. In this system, the overall channel from the SN to the DN is a cascade of the backward relay channel over the SN-RN hop, the amplifying matrix at the RN, and the forward relay channel over the RN-DN hop. We investigate the estimation of the two cascaded relay channels at the DN based on the predefined amplifying matrix applied at the RN and the corresponding overall channel obtained through the conventional channel estimation algorithms with the help of pilots transmitted by the SN. In particular, we find necessary and sufficient conditions on the pilot amplifying matrix sequence at the RN to ensure feasible relay channel estimation at the DN. Based on these conditions, we present rules to design diagonal or quasi-diagonal pilot amplifying matrices so that the cascaded relay channels can be estimated with minimum complexity at the RN. In the presence of imperfect overall channel state information at the DN, we further develop the approximate linear least-square estimation of the relay channels based on the designed pilot matrix sequence and demonstrate its performance by simulation results.

Statistics-based inter-subchannel interference (ICI) mitigation schemes are developed for orthogonal frequency division multiplexing (OFDM) transmission over high-mobility channels with line-of-sight components. By utilizing the channel statistics, we develop the Wiener filtering in the downlink and the transmit preprocessing in the uplink for ICI mitigation. Numerical and simulation results demonstrate that the proposed schemes effectively mitigate ICI and lower the error floor, especially in the presence of a strong line-of-sight path.

Spectrum sensing in cognitive radio (CR) typically assumes that the primary user appears only at the beginning of the sensing block. In this paper, we first establish a probability model regarding the appearance of the primary user at any sample of a CR user frame by utilizing the statistical characteristics of the licensed channel occupancy. While conventional spectrum sensing schemes allocate the same weight to each sample, we vary the weight for each sample based on the probability of the presence of the primary user at the corresponding sample and show that such a probability-based spectrum sensing scheme has nearly optimal performance. Based on the assumption that the idle duration of the licensed channel is exponentially distributed, we further investigate how the probability model on the primary user appearance varies from frame to frame in periodic spectrum sensing and show that both the conventional fixed weight and the probability-based dynamic weight energy detection schemes converge to their respective stable detection probability.

This letter proposes a probability-based scheme for combination of spectrum sensing information collected from cooperative cognitive radio users. The proposed scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy and is superior to conventional schemes in terms of detection performance.

In this paper, we are concerned with cross-talk interference from the transmit to the receive antenna of a wireless channel-reuse-relay-station (CRRS) that forwards signals over the same channel as it receives signals. By estimating the coupling channel from the transmit to the receive antenna, the proposed scheme performs cross-talk reconstruction and cancellation at the RS. Different from the conventional coupling channel estimation schemes that require the RS to transmit dedicated pilots, the proposed scheme utilizes the random forwarded signals of the RS as pilots for coupling channel estimation, thus avoiding changing the structure of the RS's transmitted signal. For a general RS with any relay mechanism, we propose a least square coupling channel estimator; for an RS with the decode-and-forward mechanism, we further propose a minimum mean-square error coupling channel estimator. Also, we investigate the performance of the proposed cross-talk cancellation scheme with both analytical and numerical results.

In this paper, we are concerned with a two-hop multi-input-multi-output (MIMO) amplify-and-forward (AF) relay system consisting of a source node (SN), a relay node (RN), and a destination node (DN). Since the simple RN in this system is unaware of the structure of its received signal and incapable of performing complicated signal processing, the interim channels over the SN-RN and the RN-DN hops can not be estimated directly. Therefore, we develop a novel interim channel estimation approach in this paper. Furthermore, we find necessary and sufficient conditions for the pilot amplifying matrix sequence at the RN to ensure successful interim channel estimation at the DN, and present rules to design low-complexity pilot amplifying matrices meeting these conditions.

Most of existing works on spectrum sensing detect primary transmitters while the purpose of spectrum sensing is to avoid interfering with primary receivers (PRs). Therefore, it is more important to detect PRs. In this paper, we propose a proactive spectrum sensing method that detects whether a PR is within the coverage or the interference range of a CR transmitter by exploiting the close-loop power control policy in primary systems. With the proposed scheme, the CR user may still access the spectrum band even though a primary signal is present as long as its transmission does not interfere with the PR. Simulation results show the advantages of the proposed method.

To take advantage of time-varying spectrum opportunities, acognitiveradio(CR) network monitors the dynamic usage of the licensed band through cooperative spectrum sensing. In this paper, we propose a probability-based scheme for combination of spectrum sensing information collected from several CR users. Different from conventional cooperative spectrum sensing schemes that assume synchronous local sensing information, our scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy. In our scheme, the amount of information from each CR user is flexible and a simplified implementation is also feasible under a symmetrical case. Simulation results demonstrate that our scheme is robust and superior in terms of detection performance.

In this paper, we propose a static power allocation algorithm for a two-hop multi-input-multi-output (MIMO) amplify-and-forward (AF) relay system in which the interim channel state information over the first and the second hops is unavailable. Based on the path losses over the first and the second hops, this algorithm performs static power allocation between the source and relay nodes to maximize the equivalent received SNR of the system. We further investigate the optimal location of the relay node when the conventional fixed and the proposed optimal static power allocation schemes are applied. Our comparison between direct transmission and relay-based two-hop transmission indicates that whether the latter outperforms the former depends on a tradeoff between the received SNR gain and the multiplexing loss in the relay-based two-hop transmission scheme.

In this paper, we propose a relay-assisted transmission scheme in cognitive radio (CR) to exploit spatial spectrum holes, which are generated by relay techniques. The proposed scheme enables CR users to coexist with primary users at the same time in the same geographic area and spectrum band. Compared to conventional schemes, a higher spectrum efficiency is achieved by our method. We further analyze the successful communication probability and present numerical results to show advantages of our method.

Cognitive radio allows for usage of licensed frequency bands by unlicensed users. However, these unlicensed (cognitive) users need to monitor the spectrum continuously to avoid possible interference with the licensed (primary) users. Apart from this, cognitive radio is expected to learn from its surroundings and perform functions that best serve its users. Such an adaptive technology naturally presents unique signal-processing challenges. In this paper, we describe the fundamental signal-processing aspects involved in developing a fully functional cognitive radio network, including spectrum sensing and spectrum sculpting.

Spectrum hole (SH) is defined as a spectrum band that can be utilized by unlicensed users, which is a basic resource for cognitive radio (CR) systems. Most of existing contributions detect SHs by sensing whether a primary signal is present or absent and then try to access them so that the CR and primary users use the spectrum band either at different time slots or in different geographic regions. In this paper, we propose a novel scheme with relays or directional relays for CR users to exploit new spectrum opportunity, called spatial SH. It can provide higher spectrum efficiency by coexistence of primary and CR users at the same region, time, and spectrum band. In particular, when the spectrum opportunity of a direct link from a CR transmitter to a CR receiver does not appear, our scheme may still establish the communication through indirect links, i.e., other CR users act as relay stations to assist the communication by using other spatial domains. Furthermore, we analyze the successful communication probabilities of CR users and demonstrate that the spectrum efficiency can be considerably improved by our scheme.

Probability-based strategies are proposed in this letter to determine the optimal inter-sensing duration and power control for cognitive radio (CR). With utilization of the statistics of licensed band occupancy, appropriate inter-sensing duration is determined to capture the recurrence of spectrum opportunity in time when the licensed signal is detected, or to achieve the maximum spectrum efficiency under a certain level of interference with licensed communication when the licensed signal is declared absent. Transmit power is varied dynamically according to the non-interfering probability at each sample so as to increase the transmission rate and decrease the interference power.

In this paper, we propose a cognitive radio (CR) transmission scheme, which enables secondary users to coexist with primary users by exploiting spatial spectrum holes (SSHs) through directional antennas or antenna arrays with beamforming. To ensure reliable CR links and avoid interference to primary users, some CR users may act as relays . We investigate successful communication probability of CR users when this scheme is applied. We further demonstrate that the spectrum efficiency can be greatly improved by multiplexing CR links with directional transmission.

Spectrum sensing in cognitive radio (CR) typically assumes that the primary signal appears before the sensing block. In this paper, we develop a spectrum sensing technique for CR systems where the primary signal may appear at anytime within a sensing block. We first establish the probability model regarding the appearance of the primary signal at each sampling point of a CR user frame. While the conventional spectrum sensing scheme allocates the same weight to each sample, we propose a probability-based energy detection scheme, in which the weight for each sample is based on the probability of the presence of the primary signal at the corresponding sampling point. Numerical results indicate that the probability-based scheme exhibits better performance than the conventional scheme.

In this letter, we consider cooperative spectrum sensing based on energy detection in cognitive radio networks. Soft combination of the observed energies from different cognitive radio users is investigated. Based on the Neyman-Pearson criterion, we obtain an optimal soft combination scheme that maximizes the detection probability for a given false alarm probability. Encouraged by the performance gain of soft combination, we further propose a new softened hard combination scheme with two-bit overhead for each user and achieve a good tradeoff between detection performance and complexity.

In this paper, we consider cooperative spectrum sensing based on energy detection in cognitive radio networks. Soft combination of the observed energy values from different cognitive radio users is investigated. Maximal ratio combination (MRC) is theoretically proved to be nearly optimal in low signal- to-noise ratio (SNR) region, an usual scenario in the context of cognitive radio. Both MRC and equal gain combination (EGC) exhibit significant performance improvement over conventional hard combination. Encouraged by the performance gain of soft combination, we propose a new softened hard combination scheme with two-bit overhead for each user and achieve a good tradeoff between detection performance and complexity. While traditionally energy detection suffers from an SNR wall caused by noise power uncertainty, it is shown in this paper that an SNR wall reduction can be achieved by employing cooperation among independent cognitive radio users.