Ad Conversion and Channelization for Multi-mode Terminals

This paper presents an architecture for multi-mode terminals, strictly speaking multi-mode receivers, exploiting IF sampling with Sigma-Delta ADCs. It is shown that Sigma-Delta ADCs are not only an efficient means of digitizing signals but are a nearly perfect fit to the task of analog-to-digital conversion in multi-mode terminals. For further processing the digitized signal, i.e. channel filtering for FDMA systems as well as decorrelation for spread-spectrum systems, a common hardware is presented. INTRODUCTION In today’s heterogeneous mobile communications world there is a demand for terminals being able to cope with different standards, namely multi-mode terminals. It was not until recently, that dual-mode terminals were realizable. However, these dual-mode terminals basically combine two separate receivers/transmitters in one housing. While this is not an elegant approach it is unfeasible if one goes further to multi-mode terminals. When trying to combine more and more standards into one terminal, one cannot bypass the Software Radio approach. Based on the idea running any standard-specific software on a common standard-independent platform, the Software Radio concept is a candidate worth considering when designing multi-mode terminals. Using this approach we derive basic requirements of selected parts of the hardware-platform of a Software Radio receiver, namely the analog-to-digital converter (ADC) and the channelizer, i.e. a general digital filter which performs the channel-filtering and the decorrelation, respectively the different standards. However, before doing so the characteristics of the signals to be processed in a multi-mode receiver have to be investigated, and secondly, different receiver architectures are compared with respect to realizability and suitability for implementing Software Radio. RECEIVER ARCHITECTURE Hetero or Homo, that is the question. Which means, that the two fundamental receiver architectures are heterodyne receivers, employing several stages of down-conversion and thus one or several intermediate frequencies (IF), and secondly homodyne receivers, also called direct conversion receivers, employing only one stage of down-conversion at which the signal is directly converted to base-band. The difference between those two is that some serious problems encountered with the direct conversion approach, such as Rx-Tx coupling and dynamic DC offset, as investigated in Abidi [1], are circumvented with the heterodyne approach at the cost of additional hardware. Since additional hardware should be avoided, however, the problems of the direct conversion approch as well, we suggest a heterodyne receiver with one fixed IF at which the signal is digitized. The down-conversion to base-band is done digitally. This architecture is sketched in figure 2. Thus the hardware effort in analog components is comparable to the homodyne approach by simultaneously avoiding the inherent problems of this solution.