Efficient Optimal Minimum Error Discrimination of Symmetric Quantum States

This paper deals with the quantum optimal discrimination among mixed quantum states enjoying geometrical uniform symmetry with respect to a reference density operator $\rho_0$. It is well-known that the minimal error probability is given by the positive operator-valued measure (POVM) obtained as a solution of a convex optimization problem, namely a set of operators satisfying geometrical symmetry, with respect to a reference operator $\Pi_0$, and maximizing $\textrm{Tr}(\rho_0 \Pi_0)$. In this paper, by resolving the dual problem, we show that the same result is obtained by minimizing the trace of a semidefinite positive operator $X$ commuting with the symmetry operator and such that $X >= \rho_0$. The new formulation gives a deeper insight into the optimization problem and allows to obtain closed-form analytical solutions, as shown by a simple but not trivial explanatory example. Besides the theoretical interest, the result leads to semidefinite programming solutions of reduced complexity, allowing to extend the numerical performance evaluation to quantum communication systems modeled in Hilbert spaces of large dimension.Comment: 5 pages, 1 Table, no figure

Maximizing Outage Capacity of OFDM Transmit Diversity Systems

Performance analysis of open-loop transmit diversity schemes for orthogonal frequency-division-multiplexing (OFDM) systems is addressed by investigating the outage capacity limits in frequency-selective fading channels. It is shown that the variance of the mutual information provides details on the amount of available diversity, and it suggests guidelines on transmitter design. The optimal delays, in the outage capacity maximization sense, are found for linear and cyclic delay diversity (LDD and CDD) schemes. It is proved that the outage rates for subcarrier diversity (SD) systems are the same as for the optimized CDD case. The effect of spatial correlation on system performance for a uniform linear transmit array is discussed and numerically evaluated

Outage Behavior for Transmit Diversity OFDM-Based Systems

he following transmit diversity schemes for OFDM-based systems are analyzed: Linear Delay Diversity (LDD), Cyclic Delay Diversity (CDD), Subcarrier Diversity (SD), and Time-Switched Transmit Diversity (TSTD). The overall system performance is investigated by computing the theoretic outage rates achievable over the composite radio channels induced by the different transmission strategies. In particular, the analysis points out that a convenient exploitation of the available diversity branches leads to lower outage probabilities and, consequently, to higher outage rates. Incidentally, the ergodic capacity limit for the considered diversity schemes remains equal to the single transmit antenna case. On the other hand, transmit diversity reduces the statistical dispersion of the mutual information about its mean. As a result, the optimal selection of the delay for both LDD and CDD is addressed. Nevertheless, in many cases of interest the failure probability is the same with either SD, LDD or CDD. It is also mentioned that SD and TSTD are equivalent if the propagation medium is sufficiently slow time-varying, while SD outperforms TSTD over highly time-selective radio channels

On the Impact of the Radio Channel on the Performance of Space-Time Bit-Interleaved Coded OFDM Systems

This paper reports theoretical and practical remarks useful for the analysis of space-time bit-interleaved coded OFDM (orthogonal frequency division multiplexing) modulated transceivers. The system performance is derived by means of pairwise error probability pointing out the achievable diversity order and the coding gain for channels with both time and space correlations. The tradeoff between available and exploitable diversities in time, frequency and space domains is emphasized. For a given transmitter the expected coding gain can be deduced either from the eigenvalue spread or from a full analysis of the autocorrelation matrix of the composite radio channel. Simulation results that corroborate this approach are reported taking into account for different transmission scenarios and outer codes

Improved Nyquist Pulses

In this letter, we propose two new Nyquist (intersymbol interference free) pulses that exhibit better error probability performance in the presence of sampling errors than the popular raised-cosine and a recently proposed pulse by Beaulieu, Tan, and Damen. The new pulses are also robust to the root and truncation operations