We consider an Amplify-and-Forward cooperative diversity system where a source node communicates with a destination node with the help of one or more relay nodes. The conventional system model assumes all relay nodes participate, with the available channel and power resources equally distributed over all nodes. This approach being clearly sub-optimal, we first present two power allocation schemes to minimize the system outage probability, based on complete channel state information and channel statistics, respectively. We further show that the proposed optimal power allocation methods minimize system symbol error rate as well. Next, we propose a selection scheme where only one "best" relay node is chosen to assist in the transmission. We show that the selection-AF scheme maintains full diversity order, and at reasonable power levels has significantly better outage behavior and average throughput than the conventional all-participate scheme or that with optimal power allocation. Finally we combine power allocation and selection to further improve performance.

We consider molecular communication, with information conveyed in the time of release of molecules.The main contribution of this paper is the development of a theoretical foundation for such a communication system. Specifically, we develop the additive inverse Gaussian (IG) noise channel model: a channel in which the information is corrupted by noise with an inverse Gaussian distribution. We show that such a channel model is appropriate for molecular communication in fluid media -when propagation between transmitter and receiver is governed by Brownian motion and when there is positive drift from transmitter to receiver. Taking advantage of the available literature on the IG distribution, upper and lower bounds on channel capacity are developed, and a maximum likelihood receiver is derived.Theory and simulation results are presented which show that such a channel does not have a single quality measure analogous to signal-to-noise ratio in the AWGN channel. It is also shown that the use of multiple molecules leads to reduced error rate in a manner akin to diversity order in wireless communications. Finally, we discuss some open problems in molecular communications that arise from the IG system model. I. INTRODUCTIONModern communication systems are almost exclusively based on the propagation of electromagnetic (or acoustic) waves. Of growing recent interest, nanoscale networks, or nanonetworks, are systems of communicating devices, where both the devices themselves and the gaps between them are measured in nanometers [1]. Due to the limitations on the available size, energy, and K. V. Srinivas and Raviraj S. Adve are with The Edward S. Rogers Sr. DRAFT 2 processing power, it is difficult for them to communicate through conventional means such as electromagnetic or acoustic waves. Thus, communication between nanoscale devices will substantially differ from the well known wired/wireless communication scenarios.In this paper, we address communication in a nanonetwork operating in a aqueous environment; more precisely, we consider communication between two nanomachines connected through a fluid medium, where messages are encoded in patterns of molecules. In this scheme, the transmitter sends information to the receiver by releasing molecules into the fluid medium connecting them; the molecules propagate through the fluid medium; and the receiver, upon receiving the molecules, decodes the information by processing or reacting with the molecules. This method, known as molecular communication [2], is inspired by biological micro-organisms which exchange information through molecules. Information can be encoded on to the molecules in different ways, such as using timing, concentration, or the identities of the molecules themselves.Molecular communication has recently become a rapidly growing discipline within communications and information theory. The existing literature that can be divided into two broad categories: in the first category, components and designs to implement molecular communication systems are described; f...

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Cooperative diversity schemes significantly improve the performance of wireless networks by transmitting the same information through several nodes. The amplify-and-forward (AF) relaying method is one of the most attractive cooperative diversity schemes due to its low complexity. Selection AF relaying has recently been proven to achieve the same diversity order as and lower outage probability than All-Participate relays. In this letter, we present an asymptotic analysis of the symbol error rates of a selection AF network, and compare it with the conventional all-participate scheme.Index Terms-Symbol error rate, cooperative diversity, wireless cooperative networks.

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