On the BER of Multiple-Input Multiple-Output Underwater Wireless Optical Communication Systems

In this paper we analyze and investigate the bit error rate (BER) performance of multiple-input multiple-output underwater wireless optical communication (MIMO-UWOC) systems. In addition to exact BER expressions, we also obtain an upper bound on the system BER. To effectively estimate the BER expressions, we use Gauss-Hermite quadrature formula as well as approximation to the sum of log-normal random variables. We confirm the accuracy of our analytical expressions by evaluating the BER through photon-counting approach. Our simulation results show that MIMO technique can mitigate the channel turbulence-induced fading and consequently, can partially extend the viable communication range, especially for channels with stronger turbulence

Entangled states close to the maximally mixed state

We give improved upper bounds on the radius of the largest ball of separable states of an m-qubit system around the maximally mixed state. The ratio between the upper bound and the best known lower bound (Hildebrand, quant.ph/0601201) thus shrinks to a constant c = \sqrt{34/27} ~ 1.122, as opposed to a term of order \sqrt{m\log m} for the best upper bound known previously (Aubrun and Szarek, quant.ph/0503221). We give concrete examples of separable states on the boundary to entanglement which realize these upper bounds. As a by-product, we compute the radii of the largest balls that fit into the projective tensor product of four unit balls in R^3 and in the projective tensor product of an arbitrary number of unit balls in R^n for n = 2,4,8.Comment: 11 pages; v2: n qubit case adde