When expanding digital signal processing of mobile communications terminals toward the antenna while making the terminal more wideband in order to be able to cope with different mobile communications standards in a software-radio-based terminal, the designer is faced with strong requirements such as bandwidth and dynamic range. Many publications claim that only reconfigurable hardware such as FPGAs can simultaneously cope with such diversity and requirements. Starting with considerations of the receiver architecture, we describe key functionalities of the digital front-end and highlight how signal characteristics of mobile communications signals and commonalities among different signal processing operations can be exploited to great advantage, eventually enabling implementations on an ASIC that, although not reconfigurable, would empower the software radio concept.

Software radio terminals must be able to process different communications standards which are generally based on different master clock rates and thus employ different bit/chip-rates. A straightforward solution to cope with this diversity of master clock rates in one terminal is to employ dedicated master clocks for each standard of operation. Being too costly in most cases, this kind of solution moreover limits the applicability of a once realized and thus fixed terminal. The smart solution to this problem is to run the terminal on a fixed clock rate, and to perform digital sample rate conversion that can be controlled by software and thus, empowers the software radio concept.

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Reconfigurable radio terminals must cope with a multitude of master clock rates of diverse mobile communications air interfaces. Digital sample rate conversion (SRC) is an elegant way to enable the processing of signals with sample rates incommensurate to the clock rate of a nonsynchronized analog-to-digital converter. SRC is a process of resampling and thus, requires anti-aliasing filtering. The well-known Farrow-structure provides a means of implementing digital SRC on a parameterizable hardware platform enabling the adaptation to different rate change factors. Still, the Farrow-structure can only implement filters with poor anti-aliasing characteristics. The transposed Farrow-structure introduced in this article overcomes these problems and thus, represents a perfect means for SRC in reconfigurable radio terminals. ÜbersichtRekonfigurierbare Funkendgeräte müssen in der Lage sein, Signale verschiedenster Mobilfunkstandards zu verarbeiten. Diese Signale basieren auf unterschiedlichen Systemtaktraten. Die digitale Abtastratenwandlung schafft auf elegante Art und Weise die Voraussetzung, Signale zu verarbeiten, deren zugrunde liegender Systemtakt nicht mit der Abtastrate des Analog-Digital-Wandlers im Funkendgerät synchronisiert ist. Da die Abtastratenwandlung ein Abtastprozeß ist, muß ein Anti-Aliasing Filter vorgesehen werden. Mit Hilfe der bekannten Farrow-Struktur kann die digitale Abtastratenwandlung auf einer parameterisierbaren Hardware-Plattform implementiert werden, die unabhängig vom Abtastratenwandlungsfaktor ist und somit beliebige Abtastratenverhältnisse realisieren kann. Der Nachteil dieser Struktur ist, daß auf ihr nur Filter mit sehr schlechten Anti-Aliasing Eigenschaften implementiert werden können. Im Gegensatz dazu können mit der in diesem Artikel vorgestellten Transponierten Farrow-Struktur Filter mit sehr guten Anti-Aliasing Eigenschaften realisiert werden. Somit ist die Transponierte Farrow-Struktur ausgezeichnet zur Abtastratenwandlung in rekonfigurierbaren Funkendgeräten geeignet.Für die Dokumentation Software Radio / Abtastratenwandlung / Interpolation / Anti-Aliasing / Polyphasen-Filter / Polynom-Filter Frequenz 55 (2001) 5-6