# Study and Analysis of Interference Signals of the LTE System of the GNSS Receiver
## 2 Criteria for Evaluating Solutions of Noise Elimination in a GNSS Receiver
### Diagram of the Processing of Navigation Signals in a GNSS Receiver
In addition to the identification of the signals, the Doppler deviations of the signal carrier frequency and the relative time shifts of the \(C/A\) spreading sequences are determined in the acquisition block. These are the parameters that allow replicas of signals to be produced. Due to the variability of these parameters over time, it is required to constantly update them in the receiver. It is the tracking block that performs the simultaneous update of the Doppler frequency and the phase of the \(C/A\) sequence. At this point, the signals are multiplied by their replicas, and carrier phase jumps \(0\pm\pi\) due to a change in the sign of the message bit are detected. Thus, it can be concluded that focusing of the signal spectrum takes place in the tracking phase. Pseudo-distance differences from individual satellites \(w^{T}\) may be determined based on the detection of bit sign change moments that define the start of a navigation message frame. The orbital parameters of the satellites and the pseudo-range information are passed to the block, calculating the position and speed of the receiver and the current system time. The determined navigation parameters constitute information for the presentation block, which can display them, e.g., \(w^{T}\) in the form of a text or a graphic object applied to the map background. The method of implementing the anti-noise system in the form proposed by the authors requires the use of an antenna array instead of a single antenna  
Figure 1: Block diagram of the GPS receiver. (Figure 2). The signal from the output of each receiving antenna is frequency filtered, amplified, and frequency converted, as is done in a standard GPS receiver. The signals processed in this way are fed to the noise detection and spatial filtering blocks. The detection procedure determines if noise is active, and if so, which signals are the most \"noisy\" and what the phase delays are measured between the elements of the antenna array. The noise presence information can be used to control the position of the \(A\) and \(B\) switches. Depending on their configuration, the GPS receiver works in standard mode or in anti-noise mode [40; 41]. The determined phase delays, as well as the signal powers from individual antennas, are transferred to the spatial filtration block, which determines the form of the weight vector, according to the Formula (1). Spatial filtration is accomplished by multiplying the vectors of the signal samples received by different antennas with the weight vector. The filtration process is described by the following Formula (1):  
\[ \begin{array}{l}s_{fp}[t_{n}]=s[t_{n}]\cdot w^{T}[t_{n}]=\\ =[s_{1}[t_{n}]s_{2}[t_{n}]\end{array} \quad\ldots\;s_{M}[t_{n}]]\cdot\big{[}\; \;w_{1}[t_{n}]\quad w_{2}[t_{n}]\quad\ldots\quad w_{M}[t_{n}]\;\;\big{]},^{T}\end{array} \tag{1}\]  
where \(s_{fp}[t_{n}]\) is the signal sample value after spatial filtration; \(s_{M}[t_{n}]\) is the signal sample value from the output of the \(m\)-th antenna element; \(w_{M}[t_{n}]\) is the current value of the \(m\)-th weighting factor, and \(a^{T}\) means transposition. If the anti-noise system is an integral part of the GNSS receiver, as shown in the next figure (Figure 3), the signal from the spatial filtering block output is directly passed to the signal acquisition and tracking blocks. It is also possible to implement the anti-noise system as an independent system. In such a case, the output of the spatial filter is converted back to analog form and transferred to the L1 frequency band. In this form, it can be given at the HF entrance. (external antenna input) of any GNSS receiver. The structure of the spoofing detection block, which is the first module of the anti-spoofing system, is shown in the figure above. First, it performs  
Figure 3: Schematic of noise detection block from the LTE signal. Figure 2: Block diagram of a GPS receiver with an anti-noise system. signals, which is carried out in the same way as in a standard receiver.