Technologies introduced in next wireless generation, such as spatial multiplexing, transmit and receive diversity, and beamforming, are key components for providing higher peak rate and better system efficiency, which are essential for supporting future broadband data service over wireless links. Moreover, the multiple-input multiple-output (MIMO) has been a hot research topic in recent years [1], as it has been widely adopted by wireless standards such as LTE, HSPA, and WiMAX. New tests systems and antenna solutions have been proposed in order to achieve and validate the demanding features of the next generation systems. Major challenges remain in the field of measuring systems as well as in the design of new antennas for MIMO systems. In addition, it is also necessary to pay attention to channel modeling for these systems.

In this special issue, researchers from both academia and industry contribute to innovative research and future trends of this scientific area. These papers promote the exchange of information and the advancement both in measurement techniques as in terminal performance predictions for the next generation of wireless handsets.

This special issue contains 8 papers that gather some of the recent advancements in antenna measurement systems and antenna technology for the next wireless generation.

Regarding system design, the paper entitled “An antenna measurement system based on optical feeding” presents a radiation measurement system by using optical feeding. The system replaces conventional electrical feeding to antennas by the optical feeding which is composed of an electrical/optical (E/O) converter, a graded-index (GI) optical fiber, and an optical/electrical (O/E) converter. The paper entitled “A compact printed quadruple band-notched UWB antenna” presents a novel compact coplanar waveguide- (CPW-) fed ultra-wideband (UWB) printed planar volcano-smoke antenna (PVSA) with four band notches for various wireless applications.

Considering channel prediction and estimation, the paper entitled “Adaptive prediction of channels with sparse features in OFDM systems” presents a time domain channel prediction method exploiting features of sparse channel for orthogonal frequency division multiplexing (OFDM) systems. In this paper, the proposed prediction method is compared with the classical frequency domain method realized at each OFDM subcarrier and demonstrates that the proposed method increases the prediction accuracy and reduces the computational complexity. The paper entitled “Limits on estimating autocorrelation matrices from mobile MIMO measurements” presents a complete study about MIMO channel responses from mobile measurements in an urban microcell and the limits on estimating autocorrelation matrices.

In relation to multiple-input multiple-output over-the-air (MIMO OTA) systems [1], the paper entitled “MIMO OTA testing based on transmit signal processing” proposes a simple methodology that allows over-the-air (OTA) MIMO testing using a MIMO testbed solely, avoiding the use of channel emulators. The method is fully flexible, so it is able to emulate any equivalent baseband narrowband MIMO channel by adequately selecting the weights of the linear combination. To prove its feasibility, the method has been tested over a commercial MIMO testbed. The paper entitled “Over-the-air testing of cognitive radio nodes in a virtual electromagnetic environment” provides an overview of ongoing research in OTA testing for next generation communication and data transmission devices with special consideration of cognitive radio (CR). Existing state-of-the-art techniques and their merits and limitations are discussed.

Finally, regarding pattern optimization and adaptive beamforming the paper entitled “An application of artificial intelligence for the joint estimation of amplitude and two-dimensional direction of arrival of far field sources using 2-L-shape array” proposes some artificial intelligence techniques, based on genetic algorithms [2] and simulated annealing, as global optimizers assisted with rapid local version of pattern search for optimization of the adaptive parameters. The paper entitled “Implementation of a zero-forcing precoding algorithm combined with adaptive beamforming based on WiMAX system” proposes a novel precoding algorithm that is a zero-forcing (ZF) method combined with adaptive beamforming in the Worldwide Interoperability for Microwave Access (WiMAX) system. This adaptive beamforming algorithm is used to achieve the desired SNR gain. It is verified the feasibility of the proposed method for realizing a practical WiMAX base station to utilize the channel resources as efficiently as possible.

Juan F. Valenzuela-Valdes
Sara Burgos
Alfonso Muñoz-Acevedo
Pablo Padilla