Basics of Software Defined Radio, Part 1

Aspects of Software-Defined Radio
As the preceding list indicates, there are a number of characteristics that an SDR possesses. Although it is not required that an SDR have all of these characteristics, having one or more of them is. Additionally, the preceding categories can be further broken down as detailed in the following sections. It should be kept in mind that since software-defined implies a high degree of flexibility and variability, this discussion is not all encompassing and is subject to change over time. Nevertheless, it will serve as a starting point for understanding the different facets of what SDR can be.

Multiband
Most traditional radio architectures operate on a single band or range of frequencies. There are many applications where multiple frequencies of operations are desired. These include cellular communications, government and nongovernment agencies, and intelligence collection, to list a few. Where these situations exist, the norm is to utilize multiple radios, each designed to operate in one specified band. A multiband radio has the ability to operate on two or more bands either sequentially or simultaneously, as in the case of a base station that may be linking handsets from different bands.

Multicarrier
A multicarrier or multichannel radio has the ability to simultaneously operate on more than one frequency at a time. This may be within the same band or, in the case of a multiband radio, in two different bands at the same time. Quite often, multicarrier applies to a base station that may be servicing many users at once, but it can also apply to a user terminal that may be processing both voice and data on different RF carriers.

Multimode
Multimode implies the ability to process several different kinds of standards. Examples of standards include AM, FM, GMSK, and CDMA but are limited to none of these. An SDR has the ability to work with many different standards and be continuously reprogrammed. Therefore, a better term than multimode, which implies a discrete number of modes, may be variable mode, which implies a continuously changeable mode of operation. As with other characteristics, these modes may be sequential or simultaneous, in the case of a multicarrier radio.

Multirate
Multirate is closely related to multimode. A multirate radio is one that either processes different parts of the signal chain at different samples rates, as in a multirate filter, or one where the radio has the ability to process different modes that require different data rates. An example of a multirate radio would be one that can process GSM at 270.833 kSPS or CDMA at 1.2288 MCPS. As with other characteristics, this can be done sequentially or at the same time on different carriers.

Variable Bandwidth
Variable bandwidth is another aspect of multimode. A traditional radio determines the channel bandwidth with a fixed analog filter such as an SAW or ceramic filter. An SDR, however, determines the channel bandwidth using digital filters that can be altered. While a series of switched analog filters could be used to change the channel bandwidth in a traditional receiver, only a small number would be practical. Additionally, digital filters have the potential to implement filters not possible in the analog domain. Lastly, digital filters can be tailored to both adapt around interferers and compensate for transmission path distortion, both features that analog filters are hard pressed to accomplish.

Part 2 examines architectures for SDR, focusing on the receiver.

References
1. J. H. Reed, Software Radio: A Modern Approach to Radio Engineering, Prentice Hall, Upper Saddle River, NJ, 2002. 2. J. Mitola, III, "Software Radio"Cognitive Radio," http://ourworld. compuserve. com/homepages/jmitola/.

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