Tim Hentschel

The digital front-end of software radio terminals

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 the 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.

Sample rate conversion for software radio

Software radio terminals must be able to process many various communications standards. These standards are generally based on different master clock rates and thus employ different bit/chip rates. The most obvious solution to cope with the diversity of master clock rates in one terminal is to provide a dedicated master clock for each standard of operation. Not only too costly, this kind of solution limits the applicability of a realized terminal. Hence, it is much more elegant to run the terminal on a fixed clock rate, and perform digital sample rate conversion controlled by software.

Channelization for software defined base-stations

Channelization is the task of channel selection in a communications receiver. Usually it comprises down-conversion of the signal to base-band and channel filtering. To be able to influence the characteristics of channelization by means of software it is advantageous to realize as much as possible of the channelization functionality with digital signal processing. In base-stations several channels have to be received in parallel. An obvious approach to meet this task is to have a separate channelizer for each channel. This so-called per-channel approach does not look very elegant. An alternative is to employ filter banks. This promises to save much effort since the necessary filter is implemented only once for all channels. Both approaches are introduced and discussed in detail. Finally, they are compared with respect to their multiplication rate. The result of this comparison is somewhat surprising.RésuméLa canalisation désigne la tâche du récepteur de sélectionner un canal désiré. En général, cette tâche est réalisée par un mixage du signal en bande de base et un filtrage. Afin qu ’on puisse ajuster les caractéristiques de la canalisation, il est avantageux de réaliser cette fonctionnalité autant que possible par un traitement de signal numérique. Aux stations de base il faut recevoir plusieurs canaux en parallèle, ce qui peut être réalisé évidemment par une unité de canalisation pour chaque canal. Cette méthode, nommée perchannel approach, n ’est pas très élégante. Alternativement, on peut utiliser un banc de filtre ce qui promet d’économiser beaucoup d’efforts en réalisant seulement un filtre pour tous les canaux. Ces deux méthodes sont présentées et discutées en détail. Finalement, elles sont comparées l’une avec l’autre relatif à vos taux de multiplication. Ce résultat de comparaison est un peu surprenant.

Software Radio Receivers

From the experiences made one can easily extrapolate that the foreseeable mobile communications market as well as the communications devices will allow for heterogeneous plurality, i.e. there will be no common standard. At least today’s standards will continue as new standards are introduced. Different operators will deploy different standards in different areas of the world, always in order to try to exploit the forces of market to their own profit. We cannot expect a unification of the mobile communications market organized by the network operators. On the other hand there is a demand for unification of the mobile communications market from the equipment manufacturer’s and user’s point-of-view. In the future few users of mobile communications services will accept to carry dedicated terminals for different services in different networks. Moreover, the equipment manufacturers can reduce the cost of their products by unifying the hardware platform.

Continuous-Time Digital Filters for Sample-Rate Conversion in Reconfigurable Radio Terminals

Abstract 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.

Time-variant CIC-filters for sample rate conversion with arbitrary rational factors

Sample rate conversion (SRC) with rational factors can be realized by interpolation followed by decimation, where CIC-filters can be chosen for either. However, the necessary increase of the sample rate that goes with the interpolation is not feasible in most RF applications. Therefore a time-variant implementation of CIC-filters is presented which circumvents the high intermediate sample rate. This time-variant implementation results in a linear periodically time-variant system (LPTV) which is completely equivalent to its original linear time-invariant system (LTI) consisting of the interpolator and the decimator. Thus well-known methods of system analysis can be used by analysing the LTI system, while implementing the system as an LPTV system, avoiding the high intermediate sample rates of the LTI system. The advantage of CIC-filters not having stored the coefficients of the impulse response but rather the description of the impulse response, enabling an implementation which is independent of the interpolation- as well as the decimation-factor, is preserved with the LPTV system. In contrast to Lagrange interpolators cancelling only the image components of the interpolated signal, time-variant CIC-filters also cancel the aliasing components, which is important in applications, where antialiasing is more important than anti-imaging.

Reduced complexity comb-filters for decimation and interpolation in mobile communications terminals

In mobile communications systems complexity and efficiency are issues of paramount importance. Therefore when implementing integer factor sample-rate conversion, comb-filters-especially cascaded-integrator-comb (CIC) filters-are a good choice of realizing linear-phase filters with low complexity. However, neither the direct implementation of comb-filters as transversal filters nor their implementation as CIC-filters are minimal realizations. Since both, decimators and interpolators are time-variant systems, the minimal realization is certainly time-variant also. Based on an equivalent block-processing structure of the original system such time-variant realizations can be found. In the case of comb-filters this results in halving the number of registers though at the cost of multipliers.