An architecture for large scale low power sensor network is pro-posed. Referred to as sensor networh with mobile agents (SENMA). SENMA exploit node redundancies by introducing mobile agents that communicate opportunistically with a largefreld of sensors. The addition of mobile agents shifIs computationally intensive task awayfrom primitive sensors to more powerful mobile agents, which enables energy effcient operations under severely limited power constraints. An opportunistic ALOHA random access cou-pled with a direct sequence spread spectrum physical layer is pro-posed. A comparison ofSENMA with apor ad hocsensor network shows a substantial gain in energy efficiency.

We consider the problem of determining rates of growth for the maximum stable throughput achievable in dense wireless networks. We formulate this problem as one of finding maximum flows on random unit-disk graphs. Equipped with the max-flow/min-cut theorem as our basic analysis tool, we obtain rates of growth under three models of communication: (a) omnidirectional transmissions; (b) "simple" directional transmissions, in which sending nodes generate a single beam aimed at a particular receiver; and (c) "complex " directional transmissions, in which sending nodes generate multiple beams aimed at multiple receivers. Our main finding is that an increase of 54 54 in maximum stable throughput is all that can be achieved by allowing arbitrarily complex signal processing (in the form of generation of directed beams) at the transmitters and receivers. We conclude therefore that neither directional antennas, nor the ability to communicate simultaneously with multiple nodes, can be expected in practice to effectively circumvent the constriction on capacity in dense networks that results from the geometric layout of nodes in space.

The effect of Multipacket Reception (MPR) on stability and delay of slotted ALOHA based random access systems is considered. A general asymmetric MPR model is introduced and the MAC capacity region is specified. An explicit characterization of the ALOHA stability region for the two user system is given. It is shown that the stability region undergoes a phase transition from a concave region to a convex region bounded by lines as the MPR capability improves. It is also shown that after this phase transition, slotted ALOHA is optimal i.e., the ALOHA stability region coincides with the MAC capacity region. Further, it is observed that there is no need for transmission control when ALOHA is optimal i.e., ALOHA with transmission probability one is optimal. These results are extended to a symmetric N > 2 user ALOHA system, where it is shown that for a large class of symmetric MPR channels no transmission control is optimal from a stability viewpoint. This finding suggests that if the physical layer is even reasonably good, there is no need for sophisticated Medium Access Control protocols.

We consider random access and coding schemes for sensor networks with mobile access (SENMA). Using an orthogonal code-division multiple access (CDMA) as the physical layer, an opportunistic ALOHA (O-ALOHA) protocol that utilizes channel state information is proposed. Under the packet capture model and using the asymptotic throughput as the performance metric, we show that O-ALOHA approaches the throughput equal to the spreading gain with an arbitrarily small power at each sensor. This result implies that O-ALOHA is close to the optimal centralized scheduling scheme for the orthogonal CDMA networks. When side information such as location is available, the transmission control is modified to incorporate either the distribution or the actual realization of the side information. Convergence of the throughput with respect to the size of the network is analyzed. For networks allowing sensor collaborations, we combine coding with random access by proposing two coded random access schemes: spreading code dependent and independent transmissions. In the low rate regime, the spreading code independent transmission has a larger random coding exponent (therefore, faster decay of error probability) than that of the spreading code dependent transmission. On the other hand, the spreading code dependent transmission gives higher achievable rate.

Abstract—We study the stability and capacity problems in regular wireless networks. In the first part of the paper, we provide a general approach to characterizing the capacity region of arbitrary networks, find an outer bound to the capacity region in terms of the transport capacity, and discuss connections between the capacity formulation and the stability of node buffers. In the second part of the paper, we obtain closed-form expressions for the capacity of Manhattan (two-dimensional grid) and ring networks (circular array of nodes). We also find the optimal (i.e., capacity-achieving) medium access and routing policies. Our objective in analyzing regular networks is to provide insights and design guidelines for general networks. The knowledge of the exact capacity enables us to quantify the loss incurred by suboptimal protocols such as slotted ALOHA medium access and random-walk-based routing. Optimal connectivity and the effects of link fading on network capacity are also investigated. Index Terms—Capacity, multipacket reception, optimal connectivity, regular topology, scheduling, slotted ALOHA, stability, transport capacity, wireless networks. I.

Abstract — Distributed opportunistic scheduling is studied for wireless ad-hoc networks, where many links contend for one channel using random access. In such networks, distributed opportunistic scheduling (DOS) involves a process of joint channel probing and distributed scheduling. It has been shown that under perfect channel estimation, the optimal DOS for maximizing the network throughput is a pure threshold policy. In this paper, this formalism is generalized to explore DOS under noisy channel estimation, where the transmission rate needs to be backed off from the estimated rate to reduce the outage. It is shown that the optimal scheduling policy remains to be threshold-based, and that the rate threshold turns out to be a function of the variance of the estimation error and be a functional of the backoff rate function. Since the optimal backoff rate is intractable, a suboptimal linear backoff scheme that backs off the estimated signal-to-noise ratio (SNR) and hence the rate is proposed. The corresponding optimal backoff ratio and rate threshold can be obtained via an iterative algorithm. Finally, simulation results are provided to illustrate the tradeoff caused by increasing training time to improve channel estimation at the cost of probing efficiency. I.

In wireless fading channels, multiuser diversity can be exploited by scheduling users so that they transmit when their channel conditions are favorable. This leads to a sum throughput that increases with the number of users and, in certain cases, achieves capacity. However, such scheduling requires global knowledge of every user’s channel gain, which may be difficult to obtain in some situations. This paper addresses contention-based protocols for exploiting multiuser diversity with only local channel knowledge. A variation of the classic ALOHA protocol is given in which users attempt to exploit multi-user diversity gains, but suffer contention losses due to the distributed channel knowledge. We characterize the growth rate of the sum throughput for this protocol in a backlogged system under both short-term and long-term average power constraints. A simple “fixed-rate ” system is shown to be asymptotically optimal and to achieve the same growth rate as in a system with a centralized scheduler. Moreover, asymptotically, the fraction of throughput lost due to contention is shown to be 1/e. Also, in a system with random arrivals and an infinite user population, a variation of this ALOHA protocol is shown to be stable for any total arrival rate, given that users can estimate the backlog. I.

We propose a novel distributed medium access control scheme called opportunistic ALOHA for reachback in sensor networks with mobile agents. Each sensor transmits its information with a probability that is a function of its channel state (propagation channel gain). This function called transmission control is then designed under the assumption that orthogonal CDMA is employed to transmit information. The gains achieved in the throughput by use of transmission control are analyzed and evaluated numerically. The variation of the average number of transmitting users with distance from the collecting agent is analyzed. The proposed reachback protocol can be used in a variety of sensor network applications. We end by giving two examples of how the reachback protocol can be used by the sensor network to transmit information reliably to the collecting agent. The maximum rate at which the information can be reliably transmitted with the proposed schemes is evaluated as a function of the performance parameters of the reachback protocol.

The effect of multipacket reception (MPR) on stability and delay of slotted ALOHA based random-access systems is considered. A general asymmetric MPR model is introduced and the medium-access control (MAC) capacity region is specified. An explicit characterization of the ALOHA stability region for the two-user system is given. It is shown that the stability region undergoes a phase transition from a concave region to a convex polyhedral region as the MPR capability improves. It is also shown that after this phase transition, slotted ALOHA is optimal i.e., the ALOHA stability region coincides with the MAC capacity region. Further, it is observed that there is no need for transmission control when ALOHA is optimal i.e., ALOHA with transmission probability one is optimal. Next, these results are extended to a symmetric P user ALOHA system. Finally, a complete characterization of average delay in capture channels for the two-user system is given. It is shown that in certain capture scenarios, ALOHA with transmission probability one is delay optimal for all stable arrival rates. Further, it is also shown that ALOHA with transmission probability one is optimal for stability and delay simultaneously in the two-user capture channel.

We study the stability and the capacity problems in packetized wireless networks. Communication medium is modelled using probability density functions that determine the packet reception probabilities. The model subsumes several previous models as spe-cial cases, and it is suitable for networks with time-varying topology and channels. Our main result is a characterization of the stability and the capacity regions using network flows. We also introduce a class of control policies sufficient to achieve every rate inside these regions. In the second part of the paper, we apply the proposed policies and the flow analysis to regular networks. We obtain closed-form expressions for the capacity of Manhattan networks (two-dimensional grid) and ring networks (circular array of nodes). We analyze the performance loss due to suboptimal medium access and routing. We also investigate the impact of link fading, link state information, and variable connectivity on achievable rates in Manhattan networks.