Software Defined Radio
Radio communication technologies are widely spread in many areas such as commercial, military, and meteorological. The traditional radio communication systems are built with hardware dedicated to specific applications that require different frequencies, bandwidths, modulation modes and coding protocols. As communication technology keeps evolving, the hardware-based implementation method is presenting noticeable disadvantages in terms of cost, production cycle, and compatibility. In order to avoid these disadvantages, a new conception called software-defined radio (SDR) technology has been introduced.
SDR technology enables a general-purpose hardware platform to realize compatibility with different wireless communication systems by updating software configurations. It brings flexibility to radio systems, allowing adding new functions and upgrading system easily.
SDR systems could be implemented based on either RF sampling or IF sampling. The former sampling method directly converts RF signal into digital one so that the analog circuitry section could be reduced as much as possible. However this method results in high implementation difficulty because RF sampling needs extreme-high-speed A/D converters and DSPs. The latter sampling method is the popular one at present. It firstly converts RF signal down to IF signal, which is then sampled for digitalization. Although the method is compromised in terms of flexibility, the requirements on devices’ performance are greatly reduced and the implementation becomes much easier.
The hardware of a SDR system basically consists of antenna, RF front-end, ADCs, DACs, and a DSP. In order to cover a wider frequency band, a wide-band antenna or multiple antennas could be used. The RF front-end conducts a series of work including filtering, up and down conversion, power amplification, and low-noise amplification. The ADC operates in the receiving chain of the system to perform analog-to-digital conversion, while the DAC is located in the transmission chain for digital-to-analog conversion. Both ADC and DAC must feature sufficient operation bandwidth and speed to satisfy Nyquist sampling rate. In order to relieve the work load of the DSP, a DDC (Digital Down Converter) device can be used to convert the output of ADC into base-band so as to reduce data transfer speed. The similar device, DUC (Digital Up Converter), could also be used in the transmission chain for the same purpose. Another option for this functionality is using a FPGA as the substitute for DDC and DUC. The DSP is responsible for base-band signal processing such as modulation/demodulation, anti-interference and FEC (Forward Error Correction).
Moreover, modern communication systems usually adopt non-constant envelope modulation which commonly requires power amplifiers running in their linear regions, resulting in lower efficiency. Running the amplifiers in their non-linear regions could obtain higher efficiency, but a dedicated chip or a FPGA should be added before power amplifier to implement CFR (Crest Factor Reduction) and DPD (Digital Pre-Distortion) on signals.