Performance analysis of coherent wireless optical communications with atmospheric turbulence.

Coherent wireless optical communication systems with heterodyne detection are analyzed for binary phase-shift keying (BPSK), differential PSK (DPSK), and M-ary PSK over Gamma-Gamma turbulence channels. Closed-form error rate expressions are derived using a series expansion approach. It is shown that, in the special case of K-distributed turbulence channel, the DPSK incurs a 3 dB signal-to-noise ratio (SNR) penalty compared to BPSK in the large SNR regime. The outage probability is also obtained, and a detailed outage truncation error analysis is presented and used to assess the accuracy in system performance estimation. It is shown that our series error rate expressions are simple to use and highly accurate for practical system performance estimation.

[Responses of litter decomposition and nutrient release to simulated nitrogen deposition in an evergreen broad-leaved forest in southwestern Sichuan].

An in situ experiment was conducted in an evergreen broad-leaved forest in southwestern Sichuan to study the responses of litter decomposition and nutrient release to simulated nitrogen deposition. Four treatments were installed, i.e., null level (CK), low nitrogen level (LN, 50 kg N x hm(-2) x a(-1)), moderate nitrogen level (MN, 100 kg N x hm(-2) x a(-1)) and high nitrogen level (HN, 150 kg N x hm(-2) x a(-1)). The results showed that it would take 4.72-6.33 years to decompose 95% of litter mass, with the highest decomposition rate in CK and the lowest one in HN. After 365 days, the litter decomposition rate in N-amended treatments was lower than that in CK, but significant difference was only observed between HN and CK (P < 0.05). The remained C was higher, and the remained N and K were significantly higher in N-amended treatments than in CK (P < 0.05). The remained P was also higher in N-amended treatments than in CK, but significant difference was only observed between LN and CK (P < 0.05). Compared with CK, all N-amended treatments had a 3.9%-23.7% increase of litter C/N ratio. During litter decomposition, element N featured a pattern of accumulation at early stage and release later, while C, P and K released all the time. Nitrogen deposition inhibited both the nutrient release from the litter and the decomposition of its lignin and cellulose. The effects of nitrogen deposition on litter decomposition in the forest changed from positive to negative as time passed, and the negative effect could be strengthened with the increase of deposited nitrogen concentration.