We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss

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Spectral efficient protocols for half-duplex fading relay channels

Traditionally, interference is considered harmful. Wireless networks strive to avoid scheduling multiple transmissions at the same time in order to prevent interference. This paper adopts the opposite approach; it encourages strategically picked senders to interfere. Instead of forwarding packets, routers forward the interfering signals. The destination leverages network-level information to cancel the interference and recover the signal destined to it. The result is analog network coding because it mixes signals not bits.So, what if wireless routers forward signals instead of packets? Theoretically, such an approach doubles the capacity of the canonical 2-way relay network. Surprisingly, it is also practical. We implement our design using software radios and show that it achieves significantly higher throughput than both traditional wireless routing and prior work on wireless network coding.

/pdf/embracing-wireless-interference-analog-network-coding-1vuvzsncu8.pdf

Embracing wireless interference: analog network coding

Interference is usually viewed as an obstacle to communication in wireless networks. This paper proposes a new strategy, compute-and-forward, that exploits interference to obtain significantly higher rates between users in a network. The key idea is that relays should decode linear functions of transmitted messages according to their observed channel coefficients rather than ignoring the interference as noise. After decoding these linear equations, the relays simply send them towards the destinations, which given enough equations, can recover their desired messages. The underlying codes are based on nested lattices whose algebraic structure ensures that integer combinations of codewords can be decoded reliably. Encoders map messages from a finite field to a lattice and decoders recover equations of lattice points which are then mapped back to equations over the finite field. This scheme is applicable even if the transmitters lack channel state information.

Compute-and-Forward: Harnessing Interference Through Structured Codes

In a wireless network with a single source and a single destination and an arbitrary number of relay nodes, what is the maximum rate of information flow achievable? We make progress on this long standing problem through a two-step approach. First, we propose a deterministic channel model which captures the key wireless properties of signal strength, broadcast and superposition. We obtain an exact characterization of the capacity of a network with nodes connected by such deterministic channels. This result is a natural generalization of the celebrated max-flow min-cut theorem for wired networks. Second, we use the insights obtained from the deterministic analysis to design a new quantize-map-and-forward scheme for Gaussian networks. In this scheme, each relay quantizes the received signal at the noise level and maps it to a random Gaussian codeword for forwarding, and the final destination decodes the source's message based on the received signal. We show that, in contrast to existing schemes, this scheme can achieve the cut-set upper bound to within a gap which is independent of the channel parameters. In the case of the relay channel with a single relay as well as the two-relay Gaussian diamond network, the gap is 1 bit/s/Hz. Moreover, the scheme is universal in the sense that the relays need no knowledge of the values of the channel parameters to (approximately) achieve the rate supportable by the network. We also present extensions of the results to multicast networks, half-duplex networks, and ergodic networks.

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Wireless Network Information Flow: A Deterministic Approach

A main distinguishing feature of a wireless network compared with a wired network is its broadcast nature, in which the signal transmitted by a node may reach several other nodes, and a node may receive signals from several other nodes simultaneously. Rather than a blessing, this feature is treated more as an interference-inducing nuisance in most wireless networks today (e.g., IEEE 802.11). The goal of this paper is to show how the concept of network coding can be applied at the physical layer to turn the broadcast property into a capacityboosting advantage in wireless ad hoc networks. Specifically, we propose a physical-layer network coding (PNC) scheme to coordinate transmissions among nodes. In contrast to “straightforward” network coding which performs coding arithmetic on digital bit streams after they have been received, PNC makes use of the additive nature of simultaneously arriving electromagnetic (EM) waves for equivalent coding operation. PNC can yield higher capacity than straightforward network coding when applied to wireless networks. We believe this is a first paper that ventures into EM-wavebased network coding at the physical layer and demonstrates its potential for boosting network capacity. PNC opens up a whole new research area because of its implications and new design requirements for the physical, MAC, and network layers of ad hoc wireless stations. The resolution of the many outstanding but interesting issues in PNC may lead to a revolutionary new paradigm for wireless ad hoc networking.

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Physical Layer Network Coding

We study the two-way communication problem for the relay channel. Hereby, two terminals communicate simultaneously in both directions with the help of one relay. We consider the restricted two-way problem, i.e., the encoders at both terminals do not cooperate. We provide achievable rate regions for different cooperation strategies, such as decode-and-forward based on block Markov superposition coding and compress-and-forward based on Wyner-Ziv source coding. We also evaluate the regions for the special case of additive white Gaussian noise channels. We show that a combined strategy of block Markov superposition coding and Wyner-Ziv coding achieves the cut-set upper bound on the sum-rate of the two-way relay channel when the relay is in the proximity of one of the terminals.

Achievable Rate Regions for the Two-way Relay Channel

We study two-hop communication protocols where one or two relay terminals assist in the communication between two transceiver terminals. All terminals operate in half-duplex mode, i.e., may not receive and transmit simultaneously at the same time and frequency. This leads to a loss in spectral efficiency due to the pre-log factor 1/2 in corresponding expressions for the achievable rate (capacity). We propose and analyze two relaying protocols that avoid the pre-log factor 1/2 but still work with halfduplex relays. Firstly, we consider a relaying protocol where two half-duplex relays, either amplify-and-forward (AF) or decodeand-forward (DF), alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that the protocol can recover a significant portion of the halfduplex loss. Secondly, we propose a relaying protocol where a bidirectional connection between two transceiver terminals is established via one half-duplex AF or DF relay (two-way relaying). It is shown that the sum rate of the two-way half-duplex AF relay channel achieves the rate of the one-way full-duplex AF relay channel, whereas the sum rate of the two-way half-duplex DF relay channel achieves the rate of the one-way full-duplex DF relay channel only in certain cases.

/pdf/spectral-efficient-signaling-for-half-duplex-relay-channels-56tasqzah6.pdf

Spectral Efficient Signaling for Half-duplex Relay Channels

We study the impact of multiple linear amplifyand-forward relays on the capacity of rank-deficient MIMO channels. We derive a general system model for wireless networks with one source/destination pair and several linear amplify-and-forward relay nodes which assist the communication between source and destination. All nodes may be equipped with multiple antennas. For a given allocation of gain factors at the relay nodes we give an analytical expression of the capacity by generalizing the results in [1], [2]. We compare the performance of a relay assisted MIMO link in a line-of-sight environment with a MIMO link without relay nodes. Our results show that the proposed cooperative signaling scheme solves a fundamental problem of MIMO systems: the rich scattering requirement.

Impact of Cooperative Relays on the Capacity of Rank-Deficient MIMO Channels

Spatial multiplexing is mandatory to achieve the extreme bandwidth efficiency of future gigabit/sec WLAN. Both distributed antenna systems (DAS) at the access point and cooperative relaying (the infrastructureless counterpart) have been recognized as means to meet coverage/range requirements and to enable spatial multiplexing in a low scattering environment. In this paper we evaluate three candidate schemes under a two-hop (relay) traffic pattern: (i) DAS with decode and forward in the access point (DDAS), (ii) DAS with linear processing in the access point (LDAS) and (iii) linear relaying without any information exchange between the relay nodes. We give lower bounds on the capacity of LDAS and DDAS. A main contribution of this paper is a systematic derivation of local gain allocation strategies for linear relaying with multi-antenna source and destination nodes, which are based on large system analysis and do not require global channel knowledge at the relays. We derive approximate expression for the ergodic capacity. We show for a source and destination with M antennas, that asymptotically (large number of relays) linear relaying with MN support nodes performs similar to LDAS with M distributed antenna elements. Finally we propose a zero forcing gain allocation, which enables spatial multiplexing of multiple single antenna source/destination pairs based on a small number of autonomous relays. The theory is supported by comprehensive performance results.

/pdf/distributed-antenna-systems-and-linear-relaying-for-gigabit-4l0b49sa3e.pdf

B. Rankov

Papers

Spectral efficient protocols for half-duplex fading relay channels

Achievable Rate Regions for the Two-way Relay Channel

Spectral Efficient Signaling for Half-duplex Relay Channels

Impact of Cooperative Relays on the Capacity of Rank-Deficient MIMO Channels

Distributed antenna systems and linear relaying for gigabit MIMO wireless