Search

An offset encoding technique is presented that improves sliding-window decoding with decode-and-forward for K-user multiple-access relay channels. The technique offsets user transmissions by one block per user and achieves the corner points of the destination's backward decoding rate regions but with a smaller delay. As a result, one achieves boundary points of the best known decode-and-forward rate regions with a smaller delay than with backward decoding. Index Terms--Cooperative systems, encoding, multiple-access communication, relaying.

A half-duplex relay channel consisting of two relays in a diamond topology is studied. In contrast to full-duplex systems, it is shown that time should be optimally allocated among different states. In contrast to the classical three-node relay channel, it is shown that spatial reuse can he utilized to allow communication without interference. Both achievable rate and upper bound on capacity arc studied. Several communication schemes such as moltihop with spatial reuse, scale-forward, broadeast-multiaccess with common message, compress-forward, as well as hybrid ones are characterized, some of which are novel. It is also shown that simple multihop with spatial reuse achieves the capacity of certain symmetric diamond channels. Index Terms--Cooperation, half-duplex, relay channel, relay network, Shannon capacity.

In cooperative relaying, multiple nodes cooperate to forward a packet within a network. To date, such schemes have been primarily investigated at the physical layer with the focus on communication of a single end-to-end flow. This paper considers cooperative relay networks with multiple stochastically varying flows, which may be queued within the network. Throughput optimal network control policies are studied that take into account queue dynamics to jointly optimize routing, scheduling and resource allocation. To this end, a generalization of the Maximum Differential Backlog algorithm is given, which takes into account the cooperative gains in the network. Several structural characteristics of this policy are discussed for the special case of parallel relay networks. Index Terms--Backpressure algorithm, cooperative relaying, wireless networks, wireless resource allocation.

This correspondence deals with the outage capacity of relay networks in the low signal-to-noise ratio (SNR) regime. This work is motivated by the fact that in relay channels, unlike multi-antenna point-to-point links, the transmitters, i.e., source and relays, may not be co-located and therefore their channel statistics can be quite different. It has been recently shown that bursty amplify and forward (BAF) is outage optimal when all the links have a Rayleigh distribution. In this correspondence, it is shown that BAF is in fact outage optimal for a wide class of independent channels with smooth distribution functions. Optimality of BAF is further generalized to a special class of dependent channels, namely, the case where we opportunistically use only the best relay (out of N relays). It turns out that relative to the strategy where all the N relays are used, this opportunistic best relay strategy uses significantly smaller average power (independent of N) while suffering negligible increase in outage. This holds out potential for significant gains in an ad hoc network scenario where minimizing interference to possibly other users as well as conserving power are important considerations. Index Terms--Cooperation, diversity, fading channels, outage capacity, relay channel.

The relay channel with private messages (RCPM) is a generalized relay channel model where in addition to the traditional communication from source to destination (assisted by relay), the source has a private message for the relay, and the relay has a private message for the destination. This paper develops coding strategies for this channel based on decode-and-forward and compress-and-forward schemes. Achievable rate regions as well as outer bounds on the capacity region are obtained for the discrete memoryless relay channel with private messages. Then, the Gaussian versions of this channel are studied and achievable rate regions are characterized. Numerical results are provided that give insights into the trade-offs between private messaging and relayed messaging in this hybrid three-node network. Index Terms--Channel capacity, channel coding, relay channels, wireless networks.

n source and destination pairs randomly located in an area want to communicate with each other. Signals transmitted from one user to another at distance r apart are subject to a power loss of [r.sup.-[alpha] as well as a random phase. We identify the scaling laws of the information-theoretic capacity of the network when nodes can relay information for each other. In the case of dense networks, where the area is fixed and the density of nodes increasing, we show that the total capacity of the network scales linearly with n. This improves on the best known achievability result of [n.sup.2/3] of Aeron and Saligrama. In the case of extended networks, where the density of nodes is fixed and the area increasing linearly with n, we show that this capacity scales as [r.sup.2-[alpha]/2] for 2 [less than or equal to] < 3 and [square root]n for [alpha] [greater than or equal] 3. The best known earlier result of Xie and Kumar identified the scaling law for a > 4. Thus, much better scaling than multihop can be achieved in dense networks, as well as in extended networks with low attenuation. The performance gain is achieved by intelligent node cooperation and distributed multiple-input multiple-output (MIMO) communication. The key ingredient is a hierarchical and digital architecture for nodal exchange of information for realizing the cooperation. Index Terms--Ad hoc wireless networks, capacity scaling, digital communications, distributed multiple-input multiple-output (MIMO), hierarchical cooperation.

This paper describes an efficient implementation of binning for decode-and-forward (DF) in relay channels using lowdensity parity-check (LDPC) codes. Bilayer LDPC codes are devised to approach the theoretically promised rate of the DF relaying strategy by incorporating relay-generated parity bits in specially designed bilayer graphical code structures. While conventional LDPC codes are sensitively tuned to operate efficiently at a certain channel parameter, the proposed bilayer LDPC codes are capable of working at two different channel parameters and two different rates: that at the relay and at the destination. To analyze the performance of bilayer LDPC codes, bilayer density evolution is devised as an extension of the standard density evolution algorithm. Based on bilayer density evolution, a design methodology is developed for the bilayer codes in which the degree distribution is iteratively improved using linear programming. Further, in order to approach to the theoretical DF rate for a wide range of channel parameters, this paper proposes two different forms of bilayer codes: the bilayer-expurgated and bilayer-lengthened codes. It is demonstrated that the rate of a properly designed bilayer LDPC code can closely approach the theoretical DF limit. Finally, it is shown that a generalized version of the proposed hilayer code construction is applicable to relay networks with multiple relays. Index Terms--Binning, decode-and-forward, density evolution, low-density parity-check (LDPC) codes, relay channel.

This paper addresses network coding in wireless networks in conjunction with medium access control (MAC). It is known that coding over wired networks enables connections with rates that cannot be achieved by routing. However, the properties of wireless networks (e.g., omnidirectional transmissions, destructive interference, single transceiver per node, finite energy) modify the formulation of time-varying network coding in a way that reflects strong interactions with underlying MAC protocols and deviates from the classical approach used in wired network coding. To perform network coding over conflict-free transmission schedules, predetermined network realizations are separately activated by a time-division mechanism and the content of network flows is derived through network coding to optimize performance measures such as achievable throughput and energy costs. A systematic method is presented to construct linear wireless network codes and interactions with MAC schedules are discussed under wireless assumptions. Network coding is also extended to operate with arbitrary (random or scheduled access based) MAC protocols. Alternatively, conflict-free transmission schedules are jointly constructed with network codes by decomposing wireless networks into subtrees and employing graph coloring on simplified subtree graphs. Finally, network coding and plain routing are compared in terms of throughput, energy and delay performance under different MAC solutions. Index Terms--Cross-layer design, energy efficiency, graph theory, medium access control (MAC), multiuser information theory, network coding, wireless ad hoc networks.

In this paper, a low-complexity cooperative protocol that significantly increases the average throughput of multihop upstream transmissions for wireless tree networks is developed and analyzed. A system in which transmissions are assigned to nodes in a collision free, spatial time division fashion is considered. The suggested protocol exploits the broadcast nature of wireless networks where the communication channel is shared between multiple adjacent nodes within interference range. For any upstream end-to-end flow in the tree, each intermediate node receives information from both one-hop and two-hop neighbors and transmits only sufficient information such that the next upstream one-hop neighbor will be able to decode the packet. This approach can be viewed as the generalization of the classical three node relay channel for end-to-end flows in which each intermediate node becomes successively source, relay and destination. The achievable rate for any regular tree network is derived and an optimal schedule that realizes this rate in most cases is proposed. Our protocol is shown to dramatically outperform the conventional scheme where intermediate nodes simply forward the packets hop by hop. At high signal-to-noise ratio (SNR), it yields approximately 66% throughput gain for practical scenarios. Index Terms--Fairness, relay channel, time division multiaccess, user cooperation, wireless networks.

In this paper, we propose a stochastic geometric model to study the energy burdens seen in a large scale hierarchical sensor network. The network makes use of aggregation nodes, for compression, filtering, and/or data fusion of locally sensed data. Aggregation nodes (AGNs) then relay the traffic to mobile sinks. While aggregation may substantially reduce the overall traffic on the network, it may have the deleterious effect of concentrating loads on paths between AGNs and the sinks--such inhomogeneities in the energy burden may in turn lead to nodes with depleted energy reserves. To remedy this problem, we consider how one might achieve a more balanced energy burden across the network by spreading traffic, i.e., using a multiplicity of paths between AGNs and sinks. The proposed model reveals, how various aspects of the task at hand impact the characteristics of energy burdens on the network and in turn the lifetime for the system. We show that the scale of aggregation and degree of spreading can be optimized. Additionally, if the sensing activity involves large amounts of data flowing to sinks, then inhomogeneities in the energy burdens seen by nodes around the sinks will be hard to overcome, and indeed the network appears to scale poorly. By contrast, if the sensed data is bursty in space and time, then one can reap substantial benefits from aggregation and balancing. Index Terms--Boolean model, data fusion, sensor networks, stochastic geometry.

A joint source-channel hybrid digital-analog (HDA) vector quantization (VQ) system is presented. The main advantage of the new VQ-based HDA system is that it achieves excellent rate-distortion-capacity performance at the design signal-to-noise ratio (SNR) while maintaining a "graceful improvement" characteristic at higher SNRs. It is demonstrated that, within the HDA framework, the parameters of the system can be optimized using an iterative procedure similar to that of channel-optimized vector quantizer design. Comparisons are made with three purely digital systems and one purely analog system. It is found that, at high SNRs, the VQ-based HDA system is superior to the other investigated systems. At low SNRs, the performance of the new scheme can be improved using the optimization procedure and using soft decoding in the digital part of the system. These results demonstrate that the introduced scheme provides an attractive method for terrestrial broadcasting applications. Index Terms--Broadcasting, hybrid digital-analog coding, joint source-channel coding, robust transmission, source coding, vector quantization (VQ).

Two parties, each holding one input of a two-variable function, communicate in order to determine the value of the function. Each party wants to expose as little of its input as possible to the other party. We prove tight bounds on the minimum amount of information about the individual inputs that must be revealed in the computation of most functions and of some specific ones, and show that a computation that reveals little information about the individual inputs may require many more message exchanges than a more revealing computation.