A Simulation Study of Cooperative Communications over HF Channels

The High Frequency (HF) band lies within 2-30 MHz of the electromagnetic spectrum. In this part of the spectrum, propagation via direct wave, surface wave, and ionospheric refraction mechanisms provides means of communications from line-of-sight to beyond-line-of-sight ranges. The characteristics of ionospheric channel impose fundamental limitations on the performance of HF communication systems. The major impairment is fading which results in random fluctuations in the received signal level and affects the instantaneous signal-to-noise ratio. This requires the deployment of powerful diversity techniques to mitigate the degrading effects of fading on the performance. The range of wavelengths in HF band unfortunately restricts the use of spatial diversity (i.e., deployment of multiple antennas) for most practical purposes. This thesis focuses on an alternative method to exploit the spatial dimension of the HF channel. Specifically, we aim to extract distributed spatial diversity through relay-assisted transmission. Towards this main goal, we consider multi-carrier HF communication and investigate the performance of cooperative OFDM over HF channels

Efficient lead sorption from wastewater by carbon nanofibers

Wastewater from many industries contains toxic metal ions, which should be removed before water is released into the environment. Many adsorbents have been developed to remove metals, but their efficiency should be improved by, e.g., increasing surface area. Here, we tested the use of carbon nanofibers synthesized on the surface of activated carbon, to remove lead ions (Pb2?). We studied the effect of carbon nanofiber dose, pH, contact time and agitation speed on the sorption capacity. The best conditions were with nanofiber dose of 0.25 g/L, pH of 5.5, contact time of 60 min and agitation speed of 200 rpm. The nanocomposite product was also used for the removal of lead from industrial wastewater. We also found a successful removal of 67 % of lead ions from semiconductor wastewater sample