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and other distortions, such as frequency shifts and the spreading of signals over time. Signals that travel over the air are also more vulnerable to jamming and interception than are those transmitted through wires or fibers. These limitations are often addressed with a combination of sophisticated signal processing techniques and antennas. However, these solutions add to the complexity of portable communications devices and increase power requirements.

Wireless systems pose two types of power challenges. First, when power is radiated from an antenna, very little of it typically reaches the receiver, a phenomenon known as path loss. This problem can be partly overcome with increased transmit power, special types of antennas, and other solutions. Second, wireless terminals often carry their own power supplies in the form of batteries. Battery life is limited and is influenced by many aspects of terminal design as well as the technology of the network infrastructure. Scarce power constrains the signal processing capabilities and transmit power of the mobile terminal, motivating efforts to keep these units as simple as possible. However, a low-power design cannot accommodate the most sophisticated techniques available to cope with the vagaries of the wireless channel and support the network protocols of mobility management. In the absence of research breakthroughs that simplify these techniques, the only solution is to increase the complexity of the network, which needs to compensate for the simplicity of portable communications devices.

The challenges related to mobility, connectivity, and energy have stimulated a high level of R&D activity in the telecommunications industry and academia. Still, a chasm remains between the capabilities of wired and wireless communications systems. Even as commercial wireless systems evolve, additional features will be needed to meet military requirements for untethered communications. Military applications introduce additional challenges because the systems need to be rapidly deployable on mobile platforms in any one of a diverse range of operating environments; they need to interoperate with other systems; and they need protection against enemy attempts to jam, intercept, and alter information.

This chapter provides the technical basis for the analysis of military-commercial synergy in Chapter 3 by examining the challenges of mobility, connectivity, and energy and the technologies devised to overcome them. The discussion refers to the various layers of a network as defined in the Open Systems Interconnection (OSI) model (see Box 2-1). Section 2.1 is a tutorial on the wireless channel, its capacity limits, techniques for overcoming channel impairments, and the access and operational issues that arise when multiple users share the same channel. The next three sections address network, system, and hardware issues with an emphasis

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