Multilevel coding in free-space optical MIMO transmission with Q-ary PPM over the atmospheric turbulence channel

I.B. Djordjevic, B. Vasic, M.A. Neifeld
2006 IEEE Photonics Technology Letters  
We propose an efficient coded modulation scheme for multiple input/multiple output free-space optical systems with -ary pulse-position modulation. It is based on multilevel coding and low-density parity-check codes as component codes. The proposed scheme provides excellent coding gain in the regime of strong atmospheric turbulence. Index Terms-Atmospheric turbulence channel, free-space optics, multilevel coding (MLC), multiple input/multiple output (MIMO) processing, pulse-position modulation
more » ... PM). I. INTRODUCTION C URRENT free-space optical (FSO) communication systems [1]-[3] employ intensity modulation with direct detection (IM/DD) and use point-to-point communication between two optical transceivers along a line of sight [1]. FSO communication is an attractive and cost-effective solution for high-rate image, voice, and data transmission [1]-[3]. However, an optical wave propagating through the air experiences fluctuations in amplitude and phase due to atmospheric turbulence [4]. The effect is known as scintillation. It is one of the most important factors that degrade the performance of an FSO communication link, even under the clear sky conditions. An effective way to overcome this problem is to use multiple lasers and multiple photodectors [2], [3] in a fashion analogous to multiple input/multiple output (MIMO) wireless communications. The laser sources and photodetectors are positioned in such a way that intensity fluctuations are statistically independent between any source-detector pair [2], [3]. Previous studies [8] have shown that the performance of an FSO system is degraded most severely by deep fog because of considerable signal attenuation. To increase transmission distance higher laser powers or alternatively power efficient modulation schemes are necessary. The schemes such as pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) that are commonly used in wireless communications provide high spectral efficiency but are not power efficient and are, therefore, not a good choice. The -ary pulse-position modulation (PPM) is an attractive power-efficient modulation scheme for FSO links [2], [3], [5] . The large bandwidth of FSO links (compared to RF links) makes the low spectral efficiency of PPM of little concern [8]. Moreover, as the number of slots in -ary PPM increases, the influence of background radiation becomes less important [2], [3], because the photodiode integration interval
doi:10.1109/lpt.2006.877576 fatcat:oeodersowveedhd354ljwn4vgq