A Sphere-Packing Exponent Approximation
The sphere-packing bound represents a suitable tool for assessing the limiting performance of a digital communication system. In fact, most of the modern transmission techniques, adopting capacity approaching codes, such as turbo codes, or Low Density Parity Check (LDPC) codes, may offer a performance that is within 1 dB from the bound, both for Additive White Gaussian Noise (AWGN) and for the fading channel. Unfortunately, the sphere-packing exponent may be difficult to compute, and this may prevent its adoption as a performance estimator. This paper presents and discusses the accuracy of an approximate expression of the bound, that may be used for evaluating the achievable performance of a communication system, without resorting to cumbersome simulations.
An instruction-level simulator for IBM 3090 with VF (vector facility) has been developed for studying the performance of vector processors and their memory hierarchies. Results of a study of the locality of several large scientific applications are presented. The cache miss ratios of vectorized applications are found to be almost equal to those of their original scalar executions. Moreover, both the spatial and temporal locality of these applications (in scalar and vector executions) are strong enough to show a sufficiently high hit ratio on conventional cache structures.<<ETX>>