Starting from the Poisson model we present a channel model for optical communications, called the Square Root (SR) Channel, in which the noise is additive Gaussian with constant variance. Initially, we prove that for large peak or average power, the transmission rate of a Poisson Channel when coding and decoding methods for the SR Channel are used, converges to the capacity of the Poisson Channel. Then, we derive bounds and asymptotic expressions for the capacity of the SR Channel, and compare them to those of other optical channel models. Finally, signal-independent noise sources are discussed.

The performance of OFDM systems is compromised in mobile environments due to Doppler spreading. In this paper, we investigate how Doppler spreading due to the mobility of the receiver can be mitigated by using dual antenna. In this context, we propose two simple antenna combining schemes, namely, simple beamforming (SB) and beamforming with frequency offset correction (BFOC), and investigate the ICI mitigating capability of these methods. We show that by using appropriate antenna spacing, beamforming coefficients and frequency offset correction, the performance of the OFDM systems can be greatly improved in high mobility scenarios.

In wireless communications, the receivers have to work properly both in the mobile and static environments. A proper solution might require the development of two parallel receivers modes. In this paper we propose an automatic switch between these two receiver modes. The automatic switch estimates a decision variable based on the temporal correlation of the channel frequency response. Then, it compares the decision variable with a pre-calculated threshold to decide which is the best receiver mode

We present a new method of estimating the maximum delay spread of a channel in an OFDM system. The method differs from other approaches because it uses all active OFDM subcarriers (pilot and data) and therefore avoids the aliasing problem arising from the exclusive usage of pilot subcarriers. The method considers the transmitted data as a random probe signal for obtaining characteristics of a channel. Although the data content is not known at the receiver, it turns out that useful second order statistics can be extracted, from which the maximum delay spread is estimated. The method is shown to be robust to noise and Doppler spread.

In this article, the computation of the entropy rate H(y) of a binary-valued stochastic process (Y/sub 1/, Y/sub 2/,...) which is a function of a stationary, time-invariant and irreducible Markov chain (X/sub 1/, X/sub 2/,..) is considered. The central idea of this article is to replace the summation over all words of length n by a summation over a complete set of prefixes (or prefixset for brevity). A prefixset W is a finite set of words (not necessarily of equal length) containing a unique prefix for each word of sufficient length. The method of prefixsets is of interest beyond computing the entropy rate. For the problem of estimating the next state of a Markov chain from observed output sequences, we can precompute a prefixset W of these sequences and associate a unique estimate of the state with each of the elements of W. The method also has a strong relation with variable-to-fixed length (Tunstall) codes. It replaces the set of all words of a given length by a prefixset of "more typical" words, effectively balancing the contributions of all words in the bounds.

A Diamond code is an error-correcting code obtained from two component codes. As in a product code, any symbol in a word of a Diamond code is checked by both component codes. However, the "code directions" for the component codes have been selected to minimize the memory that is required between successive decoding stages for the component codes. Diamond codes combine the error correcting power of a product code with the reduced memory requirements of the cross interleaved Reed-Solomon code (CIRC), applied in the compact disk system. We discuss encoding, decoding, and minimum distance properties of Diamond codes. Variations on the Diamond code construction are proposed that result in codes that are suited for use in rewritable block-oriented applications.