for as much of the signal Conversion gain chain as possible, digitizing and reconstructing as close to the antenna as possible, which allows digital techniques In addition to RF tuning, a transceiver must include the to perform functions traditionally done by analog com- ability to take advantage of one or more of these char- ponents as well as others not possible in the analog acteristics to be considered an SDR. domain. There are limits to this, however. Despite the fact that an ADC or DAC connected directly to an antenna is a desirable end goal, there are issues with Aspects of SDR selectivity and sensitivity that an analog front end (AFE) can remedy. The alternative to digitizing at the antenna As the preceding list indicates, there are a number of is the use of a completely flexible AFE capable of characteristics that an SDR possesses. Although it is not Communications Engineering Desk Reference; ISBN: 9780123746481 Copyright © 2004 Elsevier Inc. All rights reserved CHAPTER 2.3 Software defined radio required that an SDR have all of these characteristics, filter, or one where the radio has the ability to process having one or more of them is. Additionally, the cate- different modes that require different data rates. An gories above can be further broken down as detailed in example of a multi-rate radio would be one that can the following sections. It should be kept in mind that process GSM at 270.833 kSPS or CDMA at 1.2288 since software-defined implies a high degree of flexibility MCPS. As with other characteristics, this can be done and variability, this discussion is not all encompassing and sequentially or at the same time on different carriers. is subject to change over time. Nevertheless, it will serve as a starting point for understanding the different facets Variable bandwidth of what SDR can be. Variable bandwidth is another aspect of multimode. A traditional radio determines the channel bandwidth with Multiband a fixed analog filter such as an SAW or ceramic filter. A SDR, however, determines the c