###### Abstract  
Hybrid free-space optics (FSO) and optical fiber have been viewed as vital transmission techniques to satisfy high bandwidth and extended transmission range requirements under adverse environment conditions in the future last-mile obstruction problem. In this investigation, 80 Gbps data is transmitted on a hybrid FSO and multimode fiber (MMF)-based network using mode division multiplexing of two donut modes, Donut mode 0 and 1, and optical code-division multiplexing (OCDMA) schemes. For the OCDMA schemes, modified new zero-cross-correlation (MNZCC) codes are used, whereas, to add the phases into donut modes, a spiral phase diffuser is used. The purpose of the investigation is to provide an economical, high-speed and advanced last-mile network with adequate resource utilization for hybrid wired/wireless-based systems. The results obtained show achievement of an acceptable BER up to a fixed 100 m FSO link, with the combination of a 385 m MMF link under clear weather conditions. In another case, when the MMF link was fixed at 100 m, an acceptable bit error rate (BER) is achieved at 2.07 km FSO link. Furthermore, the results were obtained in the presence of strong and weak turbulences. A comparison of log-normal and gamma-gamma modeling for scintillations is presented. donut modes; free-space optics (FSO); mode-division multiplexing (MDM); optical code division multiple access (OCDMA) +
Footnote †: journal: _Photonics_  
systems. OCDMA has attracted significant attention for its capacity to enhance bandwidth demand, making it appropriate for designing cross-correlated code patterns to distinguish interfacing and intended signals [5]. Moreover, the optical code, as well as the coding arrangement, needs to be suitable for the provision of quality network services. Recently, researchers have presented several codes for spectral-amplitude coding (SAC)-based OCDMA systems, including optical orthogonal [6], random diagonal (RD) [7], modified frequency hopping (MFH) [8], modified double-weight (MDW) [9], multi-diagonal (MD) [10] codes and others. On the other hand, optical networks use mode-division multiplexing (MDM) to enhance system capacity through multimode channel transmission over a single wavelength, which can efficiently resolve bandwidth and spectrum issues [11]. An MDM-integrated hybrid OCDMA-FSO system was used to improve system capacity and to reduce MAI effects in [12]. In [3], a hybrid orthogonal frequency-division multiplexing (OFDM) FSO link system using a Hermite-Gaussian (HG)-modes-based wavelength MDM scheme over 32 km at 80 Gbps under different weather conditions is described. In [12], an HG-modes MDM-based OCDMA system using RD code over an 11 km multimode link at a 100 Gbps data rate was presented. In [13], the authors proposed a hybrid MDM-OCDM system using linearly polarized (LP) modes, which was able to transmit 80 Gbps data over a 2 km two-mode fiber. In [14], a Laguerre-Gaussian (LG)-modes MDM-FSO system, using SZCC and MD codes over 12 and 10 km, respectively, at 100 Gbps for 10 users was described. In [15], a hybrid LP-modes MDM code-division multiplexing (CDM)-based transmission system, which supported transmission of 80 Gbps over a 42 km fiber distance, was proposed. In [16], an LP-modes MDM-OCDMA-PON system with transmission over 40 km single-mode fiber and 2 km two-mode fiber (TMF) at a 80 Gbps data rate was proposed. In [8], a hybrid MDM-FSO-OCDMA system with a 100 Gbps transmission rate over an 8 km FSO range, under adverse environment conditions, was proposed. In [17], a 40 Gbps MDM-based radio-over-FSO (Ro-FSO) using LG and HG modes over a 50 km transmission distance was described. In [18], the design of a hybrid LG-modes MDM-OCDMA-FSO system operating at 120 Gbps, using an identity-type matrix ZCC over 1.750 km, was reported. In [19], the authors demonstrated an MDM-based PON-FSO using LG modes over a 155 km multimode fiber and a 1000 m FSO.