Although SDR concepts have been used in military applications for several years, it is only recently that designers of cost-sensitive products such as car radios and mobile phones have been able to think seriously about SDR. Moore's Law and the resulting ability to harness millions of transistors to fabricate highly computational intensive signal processing chips is one important development.
Raw processing power alone is not sufficient, however, because in the real world radio performance is also judged by other metrics as well, such as power consumption, chip size, and the flexibility of the processing core's architecture so that it can accommodate new standards.
This article shows how this automotive qualified multi-standard digital radio receiver concept can be applied to seven digital radio standards:
HD-radio FM
Where it is possible and efficient, the receiver uses fully embedded software for each of the digital radio standards to implement digital signal processing techniques such as filtering, demodulation, synchronization, error correction, audio-decompression, de-multiplexing, and signaling.
This automotive multi standard digital radio receiver can support AM/FM reception plus concurrent scenarios of the seven different digital radio standards. In the current implementation, NXP is using the co-processor concept for digital radio reception. In other words, the SDR is realized as a co-processor that works with NXP's automotive DSP family, which is used for AM/FM reception and audio post processing.
These dedicated Car DSPs include advanced processing algorithms, such as phase and scanning diversity, enhanced multipath cancellation, advanced adjacent channel suppression, delay time management, sound enhancing features (e.g. adaptive ultra bass, music elevation), and a wide range of additional audio features (e.g. NXP Pure Studio compression restorer). Also important are integrated IF processing that significantly reduces external filter circuitry, full reproducibility for mass manufacture, sustained quality in poor reception areas, and immunity to external noise and the high temperature changes commonly experienced within the hostile car environment.
This co-processor concept with the automotive/car DSP family is shown below.
The co-processor concept extends the AM/FM reception of NXP's automotive DSP family by adding efficient digital radio reception. This combination delivers a high-performance, flexible, and efficient SDR. In addition to its multi-standard capabilities, the co-processor concept also supports multiple receivers of the same digital radio standards for back-ground scanning, multiple audio/data applications, and video.
The rest of this article focuses on the seven previously mentioned standards in some detail to illustrate the commonalities and difference between them.
Digital Radio Mondiale (DRM)
DRM is a digital radio system for short-wave, medium-wave, and long-wave radio for frequencies below 30 MHz. It delivers near-FM sound quality and the ease-of-use that comes from digital transmissions. The improvement over AM is immediately noticeable.
The DRM system uses Coded Orthogonal Frequency Division Multiplex (COFDM). All of the data produced from the digitally encoded audio and associated signaling data are distributed for transmission across a large number of closely spaced carriers.
All carriers are contained within the transmission channel. Time and frequency interleaving is applied to mitigate fading from multi-path disturbances. Various parameters of the OFDM and coding can be varied to allow DRM to operate successfully in many different propagation environments.
The maximum bit rate for DRM is 72 kbit/sec. A block-diagram of the DRM channel encoder is shown below.
View a full-size image
The DRM system uses MPEG 4 High Efficiency Advanced Audio Compression (HE AAC+ v2) to provide high audio quality at low data rates. In addition, Code Excited Linear Prediction (CELP) and Harmonic Vector eXcitation Coding (HVXC) speech compression algorithms provide speech-only programming at even lower data rates. These DRM source encoders are shown below.
DRM+
This designation denotes an ongoing development of DRM, and is a standard for digital radio transmissions in Band I and Band II (FM-Band). Also here, OFDM provides a highly efficient usage of spectrum and offers undisturbed mobile reception with no interference. With its bandwidth of 95 kHz DRM+ fits into the 100 kHz FM pattern used in Europe and can thus be transmitted within the respective gaps in Band ll.
The maximum effective data rate is up to 186 kbit/sec per multiplex. Using the MPEG 4 HE AAC+ audio compression permits the integration of up to four different audio streams including additional data services or even video streams on one DRM+ Multiplex. The system overview is the same as that of DRM, shown by the two preceding figures (i.e. DRM+ integrates "smoothly" into DRM).
