Performance improvement of OFDM-FSO multi-user communication system with combined transmit frequency diversity and receive space diversity

• OFDM-FSO link gives better performance but still has ICI due to loss of orthogonality.
• It is further improved with transmit frequency diversity and receive space diversity.
• OFDM architecture combined with spreading code in electrical domain is used.
• Analytical model is analyzed in presence of turbulent atmosphere, MUI, timing jitter.
• CDM-OFDM-FSO link model with photo-detector space diversity and MRC is analyzed.
• In the analysis, Gamma-Gamma distribution is considered for atmospheric turbulence.
• The performance of OFDM-FSO link and CDM-OFDM-FSO link is compared.

Orthogonal frequency division multiplexed (OFDM) based free space optical (FSO) communication link gives improved performance because of narrow-band interference, improved robustness against fading and high bandwidth efficiency. It is further improved using transmit frequency diversity and space diversity at the receiver. In this paper, we propose to use OFDM architecture combined with spreading code in electrical domain, referred as code division multiplexed-orthogonal frequency division multiplexing (CDM-OFDM) which provides frequency diversity at the transmitter and using more than one receiver to get receive diversity. Analytical model of CDM-OFDM-FSO communication system with photo-detector space diversity using maximal ratio combining (MRC) is analyzed in the presence of turbulent atmosphere, multi-user-interference (MUI) and timing jitter. The error performance is computed in terms of receiver sensitivity and bit-error-rate (BER). In the analysis, Gamma–Gamma distribution is considered for atmospheric turbulence. The performance of OFDM-FSO link and CDM-OFDM-FSO link is compared. It is seen that for multiple users, CDM-OFDM-FSO link with transmit and receive diversity gives improved performance as compared to OFDM-FSO link.