architecture
TAP Hardware Requirements

The design of a TAP must support its use in a variety of situations in which it must provide a variety of services. This is clear in the network illustrated in Figure 1. In this example, every TAP provides connectivity to mobile users in its vicinity. Some TAPs rely on high-speed wireless links for their own connectivity (A, C & D, for example). Other TAPs have wired connections to a larger network infrastructure (B & E) but must share this connectivity with non-wired TAPs. Finally, some TAPs must act as wireless routers (C), providing network connectivity to other access points which cannot directly communicate with a wired node. The hardware design of a TAP must provide all of these capabilities.

TAP Network
Figure 1: TAP Network

One of the fundamental premises of the TAP design is its being equipped with multiple radios and antennas which can be used in unison for spectrally efficient links at very high data rates. This requires that the hardware design provide a means for multiple radios to be driven by a common baseband. Further, the physical layer design for TAP-to-TAP links will be entirely custom and will likely not be interoperable with any existing system. Thus, a TAP's radios must be capable of wideband operation in order to support the spectral requirements of these high throughput links but cannot not be tied to any particular standard.

Another requirement imposed by the capabilities described above is the need for multiple air interfaces. An air interface is defined here as the logical abstraction of multiple antennas which act together to provide a particular service. The number of air interfaces required will vary depending on what services a particular TAP is expected to provide. For example, a TAP which provides connectivity to mobile users and uses a high speed wireless link for its own network connection would need at least two interfaces.

A final basic requirement of the TAP hardware is the need for sufficient processing power to implement advanced multiple antenna wireless baseband algorithms. This requirement poses a kind of cart-before-the-horse problem in that some of these algorithms are still being designed. This is especially true for TAP-to-TAP links, whose physical layer design has only recently been started. Consequently, the TAP hardware design should provide as much processing power as possible. Further, it should support some means for supplementing these resources should the base design prove insufficient.

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Transit Access Points (TAPs)
ECE Department, MS 380
Rice University
6100 Main Street
Houston, Texas 77005-1892