Spectrally efficient transmitter diversity scheme for optical satellite feeder links

Abstract

There is an ever-growing demand for increasing the data traffic in the order of Tb/s to the geostationary (GEO) satellites. It will help connect numerous users on the ground who do not have access to internet service. This high throughput can be achieved using multiple laser beams in the uplink and combining them with the dense wavelength division multiplexing technique. However, optical signals propagating through the turbulent atmosphere to GEO satellites suffer from the intensity and phase fluctuations. Additionally, atmospherically induced beam wander leads to pointing errors at the satellite resulting in deep fades, hence loss of signal power which can fall below the receiver sensitivity making the communication impossible. The problem of photon scarcity can be tackled by using advanced power-efficient coherent modulation formats which are highly sensitive, but they come at the expense of increased system complexity. Therefore, in this thesis, only an intensity modulation and direct detection scheme called non-return-to-zero on-off keying is considered, which is relatively easier to implement in free-space optical communications. To mitigate the atmospheric fades, a transmitter diversity technique called MISO is considered for GEO feeder links for reliable signal reception at the satellite. It requires multiple laser beams to propagate through uncorrelated channels, which can be achieved by having a physical separation between the transmitting telescopes greater than the atmospheric coherence length. This thesis is divided into two main parts: The first part includes the quantitative analysis of the MISO scheme with no spectral overlapping between the neighboring signals. Here, the fading consists of log-normal scintillation and residual beam pointing jitter. The bit error rate (BER) for the single-input single-output and MISO systems is obtained using the fading statistics of the atmosphere and considering the receiver model of a commercially available 10Gb/s photoreceiver with an avalanche photodiode. For the given atmospheric conditions and residual beam pointing jitter, the transmit power of each beam is optimized to minimize the overall power scintillation index and maximize the BER gain. The second part of the thesis aims at increasing the spectral efficiency of the transmission system where SSB signals are generated using optical filters to achieve the desired BER performance. A laboratory experiment with a 32Gb/s system is performed in a back-to-backup setup to optimize the SSB signals using a passive filtering technique. Here, the filter bandwidth and the center frequency from the carrier are optimized to get the error-free performance. Finally, simulations are performed where the optimized upper sideband and lower sideband from the respective double-sideband signals are obtained, and then they are propagated through the atmospheric channel, which consists of log-normal scintillation effects and phase piston. The carrier separation between the two signals is selected such to emulate constructive and destructive interference due to the slowly varying phase piston. A diversity gain of 2.3dB is achieved, which shows the efficacy of using transmitter diversity in a GEO uplink channel.