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Cognitive radio is widely expected to be the next Big Bang in wireless communications. Spectrum sensing, that is, detecting the presence of the primary users in a licensed spectrum, is a fundamental problem for cognitive radio. As a result, spectrum sensing has reborn as a very active research area in recent years despite its long history. In this paper, spectrum sensing techniques fromthe optimal likelihood ratio test to energy detection, matched filtering detection, cyclostationary detection, eigenvalue-based sensing, joint space-time sensing, and robust sensing methods are reviewed. Cooperative spectrum sensing with multiple receivers is also discussed. Special attention is paid to sensing methods that need little prior information on the source signal and the propagation channel. Practical challenges such as noise power uncertainty are discussed and possible solutions are provided. Theoretical analysis on the test statistic distribution and threshold setting is also investigated. [ABSTRACT FROM AUTHOR]

This paper considers the wireless downlink transmissions in a single cell environment, for which the base station (BS) is assumed to schedule its transmission to each mobile station (MS) on a time-slot basis. Only one MS is selected for transmission during each time-slot and the selected MS possibly changes from one time-slot to another. This transmission scheme is thus referred to as dynamic time-division multiple-access (D-TDMA). Random transmit beam forming with the feedback of effective signal-to-noise ratio (ESNR) was proposed by Viswanath and Tse [IEEE Transactions on Information Theory, Vol. 48, No. 6, pp. 1277-1294, 2002] for D-TDMA-based systems in which multiple transmit antennas are equipped at the BS but only single receive antenna is equipped at each MS, or the so-called "MISO" systems. It was also shown in [Viswanath and Tse, 2002] that when the number of MSs in the system becomes large, the system throughput achieved by random transmit beamforming converges to that by coherent transmit beamforming which, however, requires the complete channel state information (CSI) of each MS at the BS. This paper extends upon the work in [Viswanath and Tse, 2002] to a more general scenario for which multiple transmit antennas and multiple receive antennas are equipped at the BS and each MS, respectively, or the so-called "MIMO" systems. We also consider several linear and nonlinear receiver structures and propose novel power allocation schemes to further improve the achievable system throughput. The throughput performance of the proposed receivers and power allocations schemes is compared through computer simulations and their fast convergence to the system throughput by coherent transmit beamforming is demonstrated. [ABSTRACT FROM AUTHOR]

Antenna diversity is able to alleviate multipath fading in a wireless communications system. Traditionally, multiple down-conversion chains have to be used in order to implement antenna receive diversity, making it difficult for implementation in mobile terminals, where the physical size, power consumption and implementation cost are three major limiting factors. RF combining, which combines the received signals at RF level, is well applicable for mobile terminals as it requires only one down-conversion chain. This paper investigates RF combining techniques for improving downlink performance by applying a two-element array at the mobile terminal. A new phase adjustment algorithm is suggested in order to combine the RF signals while minimizing noise effect. Also, a new channel estimation method based on the suggested phase adjustment algorithm is proposed, and further applied to systems with transmit diversity. Computer simulations have shown that with the new channel estimation method together with the new phase adjustment algorithm, the RF combining techniques can improve the BER performance considerably, especially for fast fading environment. [ABSTRACT FROM AUTHOR]

In this paper, we evaluate the capacity performance of multirate multicell DS-CDMA wireless local loop system with narrowbeam antenna and SINR based power control. Multiple access interference from the surrounding cells in the multicell environment is firstly studied for both uplink and downlink. It can be seen from the results that the capacity can be improved effectively by directional antenna equipped at the subscriber side. More significant improvement can be achieved if multielement antenna array is employed at the base station. In addition, the analysis is also extended to the situation of multimedia multirate transmission. The results show that, regardless of the voice to data user population ratio, the bit rate traffic supported by the system is roughly constant for a set of predetermined parameter values. [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented which discusses articles published within the issue including a paper on the techniques that can be used for spectrum sensing, another on multicarrier modulated (MCM) signals and its characteristics and one regarding the estimation of time delay in cognitive radio.