# Ultra-Wideband Analog Radio-over-Fiber Communication System Employing Pulse-Position Modulation
## 1 Introduction
It provides an overview of various UWB modulation schemes for use in ARoF systems, such as on-off keying (OOK), bi-phase modulation, pulse amplitude modulation, pulse shape modulation, and PPM. The authors have achieved error-free data transmission over a 20 km long SMF and 5 GHz wireless channel. In the paper [22], authors provide a performance evaluation of UWB signal transmission over SMF by using double-sided UWB and quasi single-sideband UWB (QSSB-UWB) monocycle pulses and analyzing the power spectral density of the signal at multiple transmission distances. The authors have found that QSSB-UWB pulses are more suitable for use with OOK and PPM. In paper [23], a low-complexity IR-UWB pulse generation method based on a linear combination of two monocycle pulses is presented, and transmission over SMF is evaluated. Error-free performance was achieved over 25 km long fiber at the speed of 2 Gb/s. The PPM modulation scheme is widely used in RF and optical IR-UWB communication systems [24; 25]. This modulation scheme encodes data as a time interval and allows ultra-short pulses to transmit data, increasing the power efficiency [26]. There are many types of PPM, such as coherent PPM, differential PPM, TR-PPM [27; 28], variable PPM [29], overlap PPM and more [30]. Paper [31] proposes a nested PPM scheme for use in visible light communication systems, which simplifies the transmitter and receiver structure, therefore making nested PPM an attractive choice for a wide range of applications where energy efficiency and low cost are crucial. Another application of PPM is in joint radar communication, which was experimentally validated with the employment of chirp sequences [32]. PPM transceiver can be set up as a joint radar communication system, using the receiving antenna unit as a radar. In this implementation, the transmitted signal will be locally used for symbol synchronization of the received waveform. Depending on the received reflections and pulses from other transmitters, distances and velocities of objects and receivers can be measured. This theoretical PPM communication system with radar capabilities will work as an ISAC system, adjusting, for example, the power of transmitted pulses, depending on the distance to the receiver, for power conservation purposes. In deep space communications, the position of a directional antenna may be adjusted, depending on the received pulses' power [33]. In terrestrial applications, PPM pulses may also be used for the localization of mobile objects, essentially working as a radar on top of the PPM communication [34]. This research presents a novel, experimentally validated communication system employing TR-PPM transmission over ARoF. In this research implementation, the central office is equipped with a local analog RoF transmitter; in turn, the base station consists of an remote antenna unit (RAU) serving as the RF transmitter in the access network. The demodulation of PPM signal is performed by TDC based on a commercial event timer. The presented system functions as a testbed for the investigation of IR-UWB communication and future use cases of UWB ISAC. The manuscript is organized as follows: Section 2 discusses the use of PPM in ARoF applications, Section 3 describes the experimental setup used in this research, Section 4 shows the results of the conducted experiments, and finally, Section 5 provides conclusions about the presented research.