# Fault Injection Emulation for Systems in FPGAs: Tools, Techniques and Methodology, a Tutorial
## 2 Reliability Assurance for Communication System Modules  
Most of the communications system modules, such as equalizers, filters, mixers or demodulators, are widely used in radiation environments which can be affected by SEUs, as happens with satellite transceivers. One of the most classic examples that can be found in literature is the superheterodyne receiver (Figure 1). The digital signal processing (DSP) units implemented to process the received signals include different algorithms from FIR and infinite impulse response (IIR) filters, to direct digital synthesizers (DDS) obtained with CORDIC units and other modules, such as equalizers, to adjust the gain of the signal in different radio spectrum. All these modules, implemented with analog components in the past, are now designed and integrated in digital devices, first application-specific integrated circuits (ASICs) and now FPGAs. These modern architectures support reconfiguration when a change in the specifications is required. Also, FPGA devices allow a high integration and extensibility on the functionality of the communication modules. For example, new algorithms can be integrated with these devices without the requirement of changing the chips onboard, only by modifying the configuration file, i.e., changing the digital demodulators or the error correction codes applied for the transmissions. As a first approach for space communications, space-grade rad-hard FPGAs were employed. However, their cost and reduced efficiency in terms of power and area, encouraged researchers to look for alternatives to protect the DSP units included in the different communication modules. One of these solutions is using an ad-hoc protection. The correct  
Figure 1: Basic components of a receiver. and efficient protection of these modules is totally critical since the effect of just one error (which in FPGAs is a permanent error, as we will see in the next sections of the paper) can totally modify the receiver operation mode and with this, the electromagnetic spectrum of the received signal. As an example, in the following we will show the impact of one single error in the behavior of a digital receiver. Figure 2 shows how S1, which is the desired frequency, is processed with other incoming signals which need to be filtered like S2 (1 st graph). These filtering processes are shown in both the second and fifth graphics. For example, the RF filter (2 nd graph) removes any signal such as S2 at the image frequency local oscillator (LO)--intermediate frequency (IF). The remaining signal is applied to the mixer which implements a CORDIC processor where a sine or cosine wave with a frequency oscillator (3 rd graph) is added. At this point the signal S1 is combined with the LO frequency to create a heterodyne at the difference between these frequencies, the IF, at the mixer output (4 th graph). Finally, it passes through the IF bandpass filter with and without an SEU (5 th graph left and right respectively) and is amplified and demodulated. As it can be noticed through this receiver example, the filtering processes are present in most of the stages of the communication module, so it is important to know what the impact of a single error in one filter is. In the next Figure 3, it can be seen the distortion that a tone suffers when it goes through a filter that has just one SEU (result of a real simulation). Comparing the output without the effect of radiation with the output with the radiation effects, the spectrum of the output signal is totally distorted including not only multiple harmonics but also a change in the phase, that will alter the received signal in a way that will make the message transmitted impossible to recover. The IF output of Figure 2, which just one error would be similar to Figure 3.  
Figure 2: Operation mode in presence of an SEU (**right**) and without SEU (**left**).