Multi-channel signal processing, as well as, communications systems employing any kind of diversity represent a challenge full of potential advantages for interference mitigation, anti-jamming, multiplexing gain and diversity enhancement. Specifically the use of multiple antennas, either at the transmitting or the receiving sites proves to be the last frontier to improve the above features without consuming bandwidth neither extra transmission power. Nevertheless, the prize for such advantages is complexity and the use of specific signal processing tools that cannot be directly inferred from one-to-one systems. This work attempts the problem of labelling the overall quality of a multichannel scenario independently of the specific architecture and signal processing techniques currently available. A distance measure, derived directly from the channel sounder is proposed in order to summarize the quality of the scenario regardless of the Tx-Rx space processing to be used. Also an inner class measure of quality is provided in terms of weakly logarithmic majorization. Transmitting and receiving data through multiple channels remains, in many senses, as an open problem from the signal processing point of view, and a risky investment from the technology or the engineering side. Regardless acoustic or electromagnetic propagation and no matter to the existing differences between space diversity and traditional diversities like frequency time and code, the multi-channel scenario is difficult to characterize in a single label, such that it tell us up to what degree it is a good or bad channel. This label has to be as independent as possible of the processing strategies at the transmitting and at the receiving sites. The exis tence of this label is crucial to cope with proper classification of multi-channel scenarios and to fully develop concepts like software defined radio and adaptive modulation. The purpose of this work is to revisit the scope of reported signal processing strategies over multi-channel transmission, which are depicted in the figure, and to determine which features of the channel matrix have major impact in the goodness of the existing strategies. A quality label is proposed and a similarity measure between two multi-channel scenarios is reported as a valid candidate for clustering channels and strategies. Section II deals with the most popular case of global quality (Single stream). Next, the distributed quality techniques are analyzed. Section IV finds out channel features with direct impact on the achievable rate or capacity. Section V analyzes the case of long term BER criteria and the non-CSIR scenario. Finally, Section VI includes the quality label proposed and reports distance criteria which, provides a formal support to the choice made for the goodness label.

BER / RATE
LONG TERM /
SHORT TERM
GLOBAL/DISTRIBUTED
QUALITY(SINGLE
/MULTIPLE STREAMS
Channel State Information
CSIT, CSIR, NO-CSI

Figure . Signal processing strategies for multi-channel scenarios in radio systems represent the solution for an objective which entails specific choices in the four concepts shown.

This paper considers an array of dipoles mounted in front of an electric groundplane. To avoid onset of grating lobes, the interelement spacings are kept to less than ë/2 at the highest frequency. Thus, the length of the dipoles is considerably shorter than normal which necessitates the use of capacitors between adjacent dipole tips. It will be shown that adjustment of these end capacitors will enable us to manipulate the dipole array impedance. More specifically, when we add the groundplane impedance it can be designed to partly compensate for the frequency variation of the dipole array impedance and thereby increase the bandwidth significantly. Further improvement can be obtained by placing a dielectric slab of proper thickness and dielectric constant in front of the dipole elements. Actually, the purpose here is two fold: In addition to increasing the bandwidth we can also obtain a reduction in variation of the impedance with scan angle in the E- as well as the H- plane.