Abstract-Cooperative diversity has been recently proposed as a way to form virtual antenna arrays that provide dramatic gains in slow fading wireless environments. However, most of the proposed solutions require distributed space-time coding algorithms, the careful design of which is left for future investigation if there is more than one cooperative relay. We propose a novel scheme that alleviates these problems and provides diversity gains on the order of the number of relays in the network. Our scheme first selects the best relay from a set of available relays and then uses this "best" relay for cooperation between the source and the destination. We develop and analyze a distributed method to select the best relay that requires no topology information and is based on local measurements of the instantaneous channel conditions. This method also requires no explicit communication among the relays. The success (or failure) to select the best available path depends on the statistics of the wireless channel, and a methodology to evaluate performance for any kind of wireless channel statistics, is provided. Information theoretic analysis of outage probability shows that our scheme achieves the same diversity-multiplexing tradeoff as achieved by more complex protocols, where coordination and distributed space-time coding for relay nodes is required, such as those proposed by Laneman and Wornell (2003). The simplicity of the technique allows for immediate implementation in existing radio hardware and its adoption could provide for improved flexibility, reliability, and efficiency in future 4G wireless systems.Index Terms-Coherence time, fading channel, network cooperative diversity, outage probability, wireless networks.

This paper presents a design principle that helps guide placement of functions among the modules of a distributed computer system. The principle, called the end-to-end argument, suggests that functions placed at low levels of a system may be redundant or of little value when compared with the cost of providing them at that low level. Examples discussed in the paper include bit error recovery, security using encryption, duplicate message suppression, recovery from system crashes, and delivery acknowledgement. Low level mechanisms to support these functions are justified only as performance enhancements.

We propose a simple distributed method to find out the "best" end-to-end path between source and destination, among M possible relays. The method requires no explicit communication among the relays, assumes no prior knowledge of network geometry and is based on instantaneous wireless channel measurements and reciprocity. The success (or failure) to select the "best" available path, depends on the statistics of the wireless channel, and a methodology to evaluate performance for any kind of wireless channel statistics, is provided. Benefits of cooperative diversity are increased with increased number of cooperating relays, even though a single relay transmits. The method simplifies required space-time coding and coordination among the cooperating terminals.

Synchronization of accesses to shared data and recovering the state of such data in the case of failures are really two aspects of the same problem--implementing atomic actions on a related set of data items. In this paper a mechanism that solves both problems simultaneously in a way that is compatible with requirements of decentralized systems is described. In particular, the correct construction and execution of a new atomic action can be accomplished without knowledge of all other atomic actions in the system that might execute concurrently. Further, the mechanisms degrade gracefully if parts of the system fail: only those atomic actions that require resources in failed parts of the system are prevented from executing, and there is no single coordinator that can fail and bring down the whole system.

ford, and Fred Young. We also thank John Corman for keeping our hardware running. We have drawn from many sources in the design of the Thoth Machine described in Section 2. The main influence has been other operating systems, particularly: Brinch-Hansen's RC 4000 system, Multics, Data General's RTOS, Honeywell's GCOS, and BeU Laboratories' UNIX.A note on the name: in Egyptian mythology [9], Thoth ruled Egypt for 3226 years. He was endowed with complete knowledge and wisdom, inventing all arts and sciences including arithmetic, geometry, astronomy, soothsaying, magic, medicine, drawing, and writing. In some stories, creation was accomplished by the sound of his voice. After his death, Thoth went to the skies where he became god of letters, god of wisdom, messenger for the gods, upholder of justice, and searcher after truth. He measured time, divided the world, kept divine archives, and was patron of history. When Egyptians died, Thoth weighed their hearts and proclaimed them "guilty" or "not guilty." He then revealed the magic formulae needed to traverse the underworld in safety. References(Note. References [3] and [5] are not cited in the text.) 1. Braga, R.S.C. Eh ref. manual.Synchronization of concurrent processes requires controlling the relative ordering of events in tbe processes. A new synchronization mechanism is proposed, using abstract objects called eventcounts and sequencers, tbat allows processes to control tbe ordering of events directly, ratber than using mutual exclusion to protect manipulations of shared variables that control ordering of events. Direct control of ordering seems to simplify correctness arguments and also simplifies implementation in distributed systems. Tbe mechanism is defined formally, and tben several examples of its use are given. Tbe relationship of tbe mecbanism to protection mechanisms in the system is explained; in particular, eventcounts are shown to be applicable to situations wbere confinement of information matters. An implementation of eventcounts and sequencers in a system witb sbared memory is described.