The study of joint transmit-receive design over multi-input multioutput (MIMO) channels representing a wireless channel with spatial diversity at both ends is nowadays receiving a significant attention. The theoretical closed-form solutions obtained so far are given in the continuous-frequency domain corresponding in general to IIR filters. When those filters are to be implemented in a discrete-time domain, FIR filters are highly desired for many reasons. In a realistic implementation, the filter order is generally constrained to a few taps for computational reasons.In this paper, we propose practical approximations of the IIR filters using FIR filters constrained to a few taps by efficiently utilizing the degrees of freedom inherent to the filter solutions, i.e., the possibility of freely choosing arbitrary phases appearing in the structure of the filters.

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Abstract-The cross-layer concept originated almost ten years ago with the aim of taking the most advantage from the difficult wireless media to break the barriers imposed by the layered transmission. One of the domains where cross-layer design has been more investigated is Radio Resource Allocation, since current and future networks need to provide wireless connectivity to heterogeneous users, offering many different data traffic types. Nonetheless, new paradigms are emerging in the field of wireless communications, like cognitive radios, wireless systems with relays and Multiple Input Multiple Output (MIMO) systems, where the potential advantages of cross-layer scheduling are still largely unknown. Moreover, in spite of a large literature on cross-layer, in the most cases different focuses and perspectives, biased by the application(s), are addressed, thus, there is lack of a general framework. The main goal of this paper is not to provide a state of the art on cross-layer scheduling and resource allocation, since this would result maybe in a heterogeneous list of scientific contributions. Rather, we are interested in performing a cataloguing of the work so far, trying to identify common tools and general frameworks used for cross-layer resource allocation, justify them by means of specific network examples and highlight open issues and challenges to be faced. Nevertheless, a preliminary objective of this work is to introduce a set of definitions which can be hopefully agreed by the scientific community.

The spread spectrum modulation presents a more robust interference rejection capability than non-code-based modulations. However, the increase of the communication services has augmented the power of the interference signals. One example is unlicensed bands such as the 2.4-2.5 GHz one, where the DS-CDMA communications of the IEEE 802.11b standard are interfered with by other Industrial Scientific and Medical (ISM) services. One way of improving the interference robustness of the DS-CDMA receivers is introducing an interference canceler (IC) before the despreading stage. The optimum criterion to estimate or detect the interference is the maximum-a-posteriori (MAP) one. However, it requires a high computational burden and a complete knowledge of the statistical model, which reduce its feasibility. To overcome these drawbacks this article proposes an interference canceler based on Fuzzy Logic that allows (1) suppressing both analog and digital interferences, and (2) easily controlling the complexity versus performance trade-off. In other words, the design of the fuzzy canceler follows a down/ top strategy; that is, a first low complex design is possible just departing from linguistic information and, in a second stage, we refine it, incorporating the available statistical information into its structure. Finally, to assess the proposed fuzzy canceler we have derived asymptotic performance bounds.

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