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Third-generation (3G) and beyond 3G mobile communication systems must provide interoperability with the Internet, increase throughput for mobile devices, and optimize their operation for multimedia applications. The limited ability of traditional layered architectures to exploit the unique nature of wireless communication has fostered the introduction of cross-layer design solutions that allow optimized operation for mobile devices in the modern heterogeneous wireless environment. In this article we present the major cross-layer design solutions that handle such problems, and discuss cross-layer implementations with a focus on functional entities that support cross-layer processes and the respective signaling. In addition, we consider the associated architectural complexity and communication overhead they introduce. Furthermore, we point out the major open technical challenges in the cross-layer design research area. Finally, we conclude our article with a summary of cross-layer approaches developed thus far and provide directions for future work.