Wireless interfaces are fast becoming the norm for almost every handheld electronic system, from games to cell phones, and from PDAs to instruments. The wireless interface employed depends on the application—typically it’s either Bluetooth, 802.11a, or 11b (plus one or more extensions, such as 802.11g), or a combination that provides both Bluetooth and 802.11. Of course, other options are out there, such as ultra-wideband or a proprietary wireless link. But no matter which interface you want to integrate into your system, most of the design guidelines are the same. So any of these cases can illustrate the typical design decisions and tradeoffs.
Depending on the point in the design cycle where the wireless interface is incorporated, several system architectural approaches can be taken. The different approaches hinge on the amount of processing power available in the host system being designed, the available space, the power budget, the time-to-market, the total cost, and still other factors.
System Partitioning
For example,in almost all systems where an embedded processor only handles the basic system control functions,no extra CPU cycles will be available to deal with the protocol processing functions and data-handling operations of a wireless interface.Such a system would typically only provide simple data-transfer control and a data bus. Thus,the wireless subsystem must include all of the protocol processing and data extraction,as well as the wireless transmit/receive functions (Fig.1a).The only communication going back and forth would be over a simple data bus and control interface. Alternatively,if the host CPU has processing cycles to spare,it can handle some of the protocol and baseband processing functions with software.Or,if the system has a custom-designed chip (an ASIC)that includes the CPU or supports the CPU,the time-critical protocol processing can be integrated into the system ASIC,which offloads the CPU.Either approach can reduce the complexity and cost of the wireless subsystem (Fig.1b).
If all baseband processing functions are incorporated into the host system,then the RF subsystem can be simplified and cost-reduced to a simple RF transmitter and receiver.Table 1a sums up some of the issues and tradeoffs for selecting the best approach, depending on the performance of the host system CPU.Expanding the available choices and approach decisions,Table 1b adds factors,such as time-to-market and cost,with some of the attendant pros and cons when deciding on the best wireless design approach.
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