## 4 Application of the SIW LWA for Radio-Over-Fiber-Based 5G Hot-Spots
### Radio-Over-Fiber Point-To-(Multi)Point Data Transmission Experiments  
The fabricated SIW LWA was utilized for fiber-wireless data transmission experiments in the V-band with the described setup. The employed commercial telecom laser diodes are integrable tunable laser assemblies (ITLAs; Pure Photonics PPLC200, San Jose, CA, USA), which can be tuned flexibly across the whole C-band (191.5-196.25 THz). They are external cavity lasers with a low linewidth <100 kHz, which use additional Etalon filters to obtain monomode operation. Since they already contain a thermo-electric cooler (TEC) and a laser diode driver to adjust their output power from 7 to 15.5 dBm, they allow compact packaging. The lasers are controlled via an RS-232 serial interface. Here, an optical carrier at \(\lambda\) = 1550 nm is externally OOK modulated by employing a Mach-Zehnder modulator (MZM; Fujitsu, Tokyo, Japan) and a pulse pattern generator, which generates a pseudorandom binary sequence (PRBS) with a word length of 2\({}^{31}-\)1. The optical baseband signal is amplified by an Erbium-doped fiber amplifier (EDFA; Calmar Laser, Pablo Alto, CA, USA) to +10 dBm and transmitted over 10 km single mode fiber (SMF) to a remote photodiode with a 70 GHz bandwidth and a V-type interface (Finisar, Sunnyvale, CA, USA). The generated RF signal is then amplified by an amplifier with 30 dB gain up to a maximum power of +18 dBm and fed to the SIW LWA. An optical LO is added to generate the 61 GHz RF carrier by optical heterodyning. A 23 dBi horn antenna receives the signal at the mobile unit after a 2 m wireless distance. An LNA with a 35 dB gain and a 4.5 dB noise figure is employed before the V-band SBD with a responsivity of 2000 V/W. After downconversion and amplification of the baseband signal, an error detector is utilized to evaluate the received signal quality by its bit error rate (BER). A variable optical attenuator (VOA; Anritsu, Atsugi, Japan) is employed at the RAU to decrease the optical signal power and consequently the RF power to yield different signal-to-noise ratios for BER measurements. The OOK data rate was modified from 0.5 to 2.5 Gbit/s in 0.5 Gbit/s steps, and the measured BERs with respect to received RF power are shown in Figure 11. As can be seen, error-free (BER < 10\({}^{-9}\)) transmission was achieved up to 2.0 Gbit/s. For 2.5 Gbit/s, forward-error correction (FEC) would have been required. This was traced back to the coherence bandwidth of the SIW LWA. Based on the  
Figure 10: Proposed radio-over-fiber (RoF) based architecture for serving multiple mobile users via an optical dense WDM scheme and the presented SIW leaky-wave antenna. rise and fall times of the measured eye diagrams (see inset in Figure 11), the bandwidth of the detected signals was determined to be 1.3 GHz. Considering that the transmission bandwidth is twice as large for OOK, this corresponds well to the determined coherence bandwidth of the antenna. Of course, data rates can be increased using complex modulation formats, as reported in [22], where 6 Gbit/s was achieved for a single beam using the same SIW LWA and 64-QAM-OFDM modulation (quadrature amplitude modulation with orthogonal frequency-division multiplexing). However, even for simple OOK modulation, the presented SIW LWA supports multiple Gbit/s transmission rates, which is essential for 5G communications. The large overall bandwidth of the SIW LWA can be exploited in synergy with its 1D beam steering capabilities to support multiple users simultaneously. To demonstrate this, three 1 Gbit/s OOK optical baseband channels with a ultra-dense channel spacing of 3 GHz were generated at the CBU. To enable comparable results, identical lasers and MZMs were used to enable comparable results. The optical baseband signals were then multiplexed using two 3-dB optical couplers, and their power was equalized by adjusting the laser powers. The signals were amplified by an EDFA before being transmitted over 10 km SMF to the RAU.