Most radios are hardware defined with little or no software control; they are fixed function for mostly consumer items for broadcast reception. They have a short life and are designed to be discarded and replaced.

Software defined radio uses programmable digital devices to perform the signal processing necessary to transmit and receive baseband information at radio frequency. Devices such as digital signal processors (DSP's) and field programmable gate arrays (FPGA's) use software to provide them with the required signal processing functionality. This technology offers greater flexibility and potentially longer life, since the radio can be upgraded very cost effectively with software. [1]

The ideal software radio architecture consists of a digital subsystem and a simple analog subsystem. the analog functions are restricted to those that cannot be performed digitally--that is, antenna, RF filtering, RF combination, receive pre-amplification, transmit power amplification and reference frequency generation. The architecture pushes the analog conversion stage right up as close to the antenna, in this case prior to the power amplifier (PA) in the transmitter and after the low noise amplifier (LNA) in the receiver. The separation of carriers and up/down frequency conversion to baseband is performed by the digital processing resources. Similarly, the channel coding and modulation functions are performed digitally at baseband by the same processing resources.

Software for the ideal architecture is layered so that the hardware is completely abstracted away from the application software. A middleware layer achieves this functionality by wrapping up the hardware elements into objects and providing services that allow the objects to communicate with each other via a standard interface--for example, Common Object Request Broker Architecture (CORBA). Middleware includes the operating system, hardware drivers, resource management, and other non-application-specific software. The combination of hardware and middleware is often termed a framework.[2]

The system architect could choose a hardware specific software architecture and still meet the requirement of implementing a software defined radio. For this case the system software is developed in a native language (to the processor) and the software makes direct calls to the hardware resources, e.g. direct manipulation of registers or I/O. This approach is probably used in conjunction with a structured design method such as Data Flow Diagrams to capture the software design. The resultant architecture is completely non-object oriented and certainly not portable.

Therefore, the ideal SDR should include an emphasis on object orientation (i.e., the radio will be under the control of software and designed to use as much object orientation and software re-use as possible). [3]

[1] Burns, P., "Software Defined radio for 3G", Artech House, 2002, pg 1.
[2] Burns, P., "Software Defined radio for 3G", Artech House, 2002, pg 4.
[3] Burns, P., "Software Defined radio for 3G", Artech House, 2002, pg 127.