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SUMMARY In this paper, we propose two new power loading schemes for an OFDM cognitive radio (CR). The novelty of this paper is that the CR has no knowledge about the interference power introduced by primary users (PUs). In the first scheme, we maximize the total received signal power on all CR subcarriers as the objective function with the two constraints: (i) keeping the interference to the PUs below a given threshold, and (ii) allocating less than peak transmit power as the total power levels on all CR subcarriers. The second proposed scheme is similar to the first one with one more constraint, which is more power loading to the CR subcarriers experiencing higher ratio of channel power gain to the noise power. This third constraint results in increasing CR throughput (similar to the water-filling scheme). Performances of these schemes are analyzed by numerical and simulation results, which illustrate that the two proposed schemes achieve relatively close to the optimal scheme (in which the secondary transmitter has full knowledge about PUs’ interference power) and outperform the scheme based on estimation of this interference power with an estimation error higher than a given threshold. Copyright © 2012 John Wiley & Sons, Ltd.

To apply a beamforming algorithm with a linear array of N antennas in real wireless communication systems, two main issues should be addressed: Algorithm complexity and number of antennas. In this paper, we propose three simple beamforming algorithms for direct sequence code division multiple access systems. We first propose a new framework to obtain the updating equation for the weight vector using PCA. Then, using this framework, four different objective functions are used as examples to illustrate how to obtain new adaptive algorithms. We demonstrate that these objective functions lead to three novel blind algorithms, with two of them resulting in the same algorithm. Moreover, they have considerably low complexities, requiring only 2N-1 complex additions and either 2N+1 or 2N complex multiplications for a single run. We evaluate the performances of these algorithms in estimating the angle-of-arrival of the desired signal and in producing the array beam pattern, and in alleviating the BER in Rayleigh fading and AWGN channels. In summary, these new beamforming algorithms obtained based on the proposed framework operate as blind algorithms, have low computational complexities, and offer desirable performances even if N is restricted.Copyright © 2012 John Wiley & Sons, Ltd.

SUMMARY In this paper, a new practical dirty paper coding scheme and its extension is proposed for interference mitigation of a single TV broadcast station in uplink and downlink of a cognitive radio network using both a single secondary user (SU) and multiple SU scenarios. In the single SU scenario, which is called interference cognitive radio channel, derived simulation results show that the transmission rate of the SU archives the capacity of an AWGN channel with the cost of a 2.5 dB extra signal-to-noise ratio. In the sequel, the proposed scheme is extended to a multiple SU scenario using direct sequence spread spectrum technique in both uplink and downlink considering a TV band. Derived simulation results show that the number of supportable SUs in our proposed scheme increases to a fully loaded scenario of the same multiuser direct sequence spread spectrum system. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, a two-phase algorithm for the spectrum sensing and power/rate control of a secondary user (S-user) or cognitive radio is proposed. In the first phase, the primary base station (P-BS), which is conscious of both the number and the data rate of primary active users (P-user), broadcasts theitusage capacity percentage (UCP) of its cell. Since knowing only the UCP is not enough to guarantee that the total load (of P-users and S-users) is less than a maximum permissible load, the S-user must measure the total interference received from both the P-users and other S-users. In this direction, using both the UCP and measurement of the interference received from the P-users and the S-users by the S-user or secondary base station (S-BS), we mathematically derive an equation for issuing data transmission permission, which if it is held then the second phase of the algorithm: the transmit power/rate control starts. In this phase, the S-user and the S-BS look for feasible values for the transmit power level and transmission rate. If there are feasible values, it starts its transmission at these feasible transmit power and rate. Since both the location of the S-user and the channel condition vary in time, the whole algorithm is iterated periodically with a period faster than the coherence time of the channel. Furthermore, we consider the down link of the above system with cooperation among neighboring S-users to overcome fading channels and we investigate the amount of improvement in the reliability of the issuing data transmission permission. As well, we consider the uplink of the system with multiple antennas in the S-BS to investigate the improvement in the same parameter over spatially correlated and independent fading channels. Theoretical analysis is validated using computer simulations. Both theoretical analysis and computer simulations show that the proposed cooperative spectrum sensing algorithm performs properly at SNR = −5dB in flat Nakagami-m fading channels with m = 1 even in correlated fading channels. We also address the improvement of the reliability of the issuing data transmission permission in the uplink in case of using multiple antennas only in the S-BS. Copyright © 2011 John Wiley & Sons, Ltd.