In this paper we present results from the study of optical signal amplification using Raman assisted fiber optical parametric amplifier with considerable benefits for S-band telecommunication systems where the use of widely used erbium-doped fiber amplifier is limited. We have created detailed models and performed computer simulations of combined Raman and fiber optical parametric amplification in a 16-channel 40 Gbps/channel wavelength division multiplexed transmission system. Achieved gain bandwidth, as well as transmission system parameters—signal-to-noise ratio and bit-error-ratio—were analyzed by comparing the Raman assisted fiber optical parametric amplifier to the single pump fiber optical parametric amplifier. Results show that the 3 dB gain bandwidth in the case of combined amplification is up to 0.2 THz wider with 1.9 dB difference between the lowest and highest gain.

Space-division multiplexing (SDM) attracts attention to cladding-pumped optical amplifiers, but they suffer from a low pump power conversion efficiency. To address this issue, ytterbium (Yb³⁺) and erbium (Er³⁺) co-doping is considered as an effective approach. However, it changes the gain profile of Er³⁺-doped fiber amplifiers and induces the gain difference between optical wavelengths in the C-band, significantly limiting the effective band of the dense wavelength-division multiplexed (DWDM) system. This paper is devoted to a detailed study of a cladding-pumped Er³⁺/Yb³⁺ co-doped fiber amplifier (EYDFA) through numerical simulations aiming to identify a configuration, before assembling a similar EYDFA in our laboratory premises that ensures the desired performance. The simulation model is based on a commercial double cladding EYDF whose parameters are experimentally extracted and fed to the EYDFA setup for the system-level studies. We investigate the wavelength dependence of the amplifier’s characteristics (absolute gain, gain uniformity, noise figure) and bit error rate (BER) performance for several DWDM channels and their optical power. The obtained results show that a 7 m long EYDF and co-propagating pump direction is preferable for the EYDFA with a 3 W pump source at 975 nm and with the given gain medium characteristics for WDM applications. For instance, it ensures a gain of 19.7–28.3 dB and a noise figure of 3.7–4.2 dB when amplifying 40 DWDM channels with the input power of −20 dBm per channel. Besides, we study EYDFA gain bandwidth and the maximum output power when operating close to the saturation regime and perform a sensitivity analysis showing how the doped fiber’s absorption and emission cross-sections impact the amplification process through energy transfer from Yb³⁺ to Er³⁺. Finally, we quantify the power penalty introduced by the EYDFA; the results show that it is not higher than 0.1 dB when amplifying 40 × 10 Gbps non-return-to-zero on-off keying signals from −20 dBm/channel.

Monitoring of optical signal polarization state in optical fiber networks can provide a variety of information for infrastructure owners. There is a rather limited choice of available techniques for monitoring the change in a polarization state of an optical signal over the transmission line. Mostly, as the typical measurement, the total optical line polarization mode dispersion (PMD) is determined, or the instantaneous change in polarization state at the line input or output is measured. But that does not allow determining the location and cause of sudden polarization changes in an optical fiber. The authors propose a polarization monitoring technique using a traditional commercially-used optical time-domain reflectometer (OTDR) with additional circuitry to generate polarization-dependent measurement traces. Such a technique can potentially be used for continuously tracking a physical impact on an optical fiber link.

Forward error correction (FEC) codes combined with high-order modulator formats, i.e., coded modulation (CM), are essential in optical communication networks to achieve highly efficient and reliable communication. The task of providing additional error control in the design of CM systems with high-performance requirements remains urgent. As an additional control of CM systems, we propose to use indivisible error detection codes based on a positional number system. In this work, we evaluated the indivisible code using the average probability method (APM) for the binary symmetric channel (BSC), which has the simplicity, versatility and reliability of the estimate, which is close to reality. The APM allows for evaluation and compares indivisible codes according to parameters of correct transmission, and detectable and undetectable errors. Indivisible codes allow for the end-to-end (E2E) control of the transmission and processing of information in digital systems and design devices with a regular structure and high speed. This study researched a fractal decoder device for additional error control, implemented in field-programmable gate array (FPGA) software with FEC for short-reach optical interconnects with multilevel pulse amplitude (PAM-M) modulated with Gray code mapping. Indivisible codes with natural redundancy require far fewer hardware costs to develop and implement encoding and decoding devices with a sufficiently high error detection efficiency. We achieved a reduction in hardware costs for a fractal decoder by using the fractal property of the indivisible code from 10% to 30% for different n while receiving the reciprocal of the golden ratio.

This paper investigates the quality of mobile network coverage along the Riga–Tukums railway corridor with a focus on the performance of 4G and 5G technologies. Ensuring reliable mobile connectivity along suburban railway corridors remains a significant technical challenge due to mixed forest–urban propagation conditions, macro-cell-dominated LTE infrastructure, mobility-induced channel variability, and fluctuating passenger density. Unlike high-speed railway environments that are extensively studied in dedicated 5G-R scenarios, suburban railway systems often rely on existing macro-cell deployments, where coverage continuity, signal quality stability, and capacity constraints must be addressed simultaneously. This study presents a measurement-based evaluation of 4G and 5G radio performance along the Riga–Tukums railway corridor under real operational conditions (50–90 km/h). Classical propagation models (Okumura–Hata and COST231-Hata) are quantitatively validated using MAE and RMSE metrics, followed by correlation analysis between RSSNR and QoS indicators. A theoretical Doppler sensitivity assessment (80–200 km/h) is conducted to evaluate mobility robustness across LTE and 5G frequency bands. Mobility transition regions and handover-related time windows are geometrically estimated, and passenger density-based capacity modeling is applied to assess throughput degradation under peak occupancy scenarios. Based on these results, a multi-layer network planning strategy integrating 700 MHz macro coverage, 1700 MHz capacity enhancement, and 3500 MHz 5G NR deployment is proposed. The optimization strategy resulted in an estimated 22–28% increase in stable service coverage in previously weak-signal zones and demonstrated that propagation model deviations remain within ranges comparable to recent railway studies (≈15–25 dB RMSE). These findings provide a structured framework for suburban railway communication optimization and support the gradual modernization of railway infrastructure toward FRMCS-ready architectures. The study illustrates the applicability of modern modelling tools for assessing and improving mobile communication systems and contributes to the broader development of digital infrastructure within Latvia’s transport sector.

Pēdējo gadu laikā notiek strauja informācijas tehnoloģiju nozares attīstība un novērojams straujš pārraidāmās informācijas apjoma palielinājums. Pieaug pieprasījums pēc lielākām telekomunikāciju tīkla kapacitātēm, tāpēc ir nepieciešams palielināt kanālu skaitu un pārraides ātrumu viļņgarumdales blīvētajās pārraides sistēmās. Ir nepieciešams meklēt jaunus pastiprināšanas risinājumus, kas ļautu paplašināt pastiprinājuma spektru un palielināt sasniedzamo optiskā signāla pastiprinājumu. Promocijas darbā ir veikts pētījums par EDFA, Ramana, FOPA un kombinēto optisko pastiprinātāju izmantošanu viļņgarumdales blīvēšanas sistēmās. Darbā īpašs uzsvars ir likts uz erbija un iterbija leģējuma šķiedras ar apvalka pumpēšanas tehnoloģijas optiskā pastiprinātāja datormodeļa pētījumu.