Orthogonal frequency division multiplexing (OFDM) can be successfully used in mitigating the effect of multipath in a wireless channel. However, its' performance degrade in the presence of Doppler spreading. In this paper, multiple beamforming using uniform linear arrays (ULA) is used for OFDM systems to reduce Doppler spread and exploit the advantage of diversity. Each of the beams receiver signal which equal Doppler spread, which are combined using equal gain combining (EGC). We also present a method of determining the frequency compensating parameters, for beams pointing in different directions. Finally, our simulation results show that angular diversity in OFDM systems can provide significant improvement as compared to using only single beamforming.

Today's file systems are limited in speed and reliability by memory's vulnerability to operating system crashes. Because memory is viewed as unsafe, systems periodically write modified file data back to disk. These extra disk writes lower system performance and the delay period before data is safe lowers reliability. The goal of the Rio (RAM I/O) file cache is to make ordinary main memory safe for persistent storage by enabling memory to survive operating system crashes. Reliable main memory enables the Rio file cache to be as reliable as a write-through file cache, where every write is safe instantly, and as fast as a pure write-back file cache, with no reliability-induced writes to disk. This paper describes the systematic, quantitative process we used to design and verify the Rio file cache on Intel PCs running FreeBSD and the reliability and performance of the resulting system.

We estimate the availability, reliability, and mean transaction time (response time) for repairable database configurations, centralized or distributed, in which each service component is continuously available for repair. Reliability, the probability that the entire transaction can execute properly without failure, is computed as a function of mean time to failure (MTTF) and mean time to repair (MTTR). Mean transaction time in the system is a function of the mean service delay time for the transaction over all components, plus restart delays due to component failures, plus queuing delays for contention. These estimates are potentially applicable to more generalized distributed systems.

One of the fundamental limits to high-performance, high-reliability file systems is memory's vulnerability to system crashes. Because memory is viewed as unsafe, systems periodically write data back to disk. The extra disk traffic lowers performance, and the delay period before data is safe lowers reliability. The goal of the Rio (RAM I/O) file cache is to make ordinary main memory safe for persistent storage by enabling memory to survive operating system crashes. Reliable memory enables a system to achieve the best of both worlds: reliability equivalent to a write-through file cache, where every write is instantly safe, and performance equivalent to a pure write-back cache, with no reliability-induced writes to disk. To achieve reliability, we protect memory during a crash and restore it during a reboot (a "warm" reboot). Extensive crash tests show that even without protection, warm reboot enables memory to achieve reliability close to that of a write-through file system. Adding protection makes memory even safer than a write-through file system while adding essentially no overhead. By eliminating reliability-induced disk writes, Rio performs 4-22 times as fast as a write-through file system, 2-14 times as fast as a standard Unix file system, and 1-3 times as fast as an optimized system that risks losing 30 seconds of data and metadata.

Memory is commonly viewed as an unreliable place to store permanent data (files) because it is perceived to be vulnerable to system crashes. Yet despite all the negative implications of memory's unreliability, no data exists that quantifies how vulnerable memory actually is to system crashes. This paper quantitatively compares the vulnerability of disk and memory to operating system crashes. We use software fault injection to induce a wide variety of operating system crashes in DEC Alpha workstations running Digital Unix, ranging from bit errors in the kernel stack to deleting branch instructions to C-level allocation management errors. We find that files on disk are rarely corrupted (1.1% corruption rate), which agrees with our intuition. We also find that, surprisingly files in memory are nearly as safe as files on disk. Only 10 of the 650 crashes we observed (1.5%) corrupt any files in memory. Our data contradicts the common assumption that operating system crashes often corrupt files in memory and suggests that memory can be used to store permanent data rather than needing to write it back to disk.