Ahmad Abu Al Haija

A half-duplex cooperative scheme with partial decode-forward relaying

In this paper, we present a new cooperative communication scheme consisting of two users in half-duplex mode communicating with one destination over a discrete memoryless channel. The users encode messages in independent blocks and divide the transmission of each block into 3 time slots with variable durations. Cooperation is performed by partial decode-forward relaying over these 3 time slots. During the first two time slots, each user alternatively transmits and decodes, while during the last time slot, both users cooperate to send information to the destination. An achievable rate region for this scheme is derived using superposition encoding and joint maximum likelihood (ML) decoding across the 3 time slots. An example of the Gaussian channel is treated in detail and its achievable rate region is given explicitly. Results show that the proposed half-duplex scheme achieves significantly larger rate region than the classical multiple access channel and approaches the performance of a full-duplex cooperative scheme as the inter-user channel quality increases.

Spectral Efficiency and Outage Performance for Hybrid D2D-Infrastructure Uplink Cooperation

We propose a time-division uplink transmission scheme that is applicable to future-generation cellular systems by introducing hybrid device-to-device (D2D) and infrastructure cooperation. Compared with existing frequency-division schemes, the proposed time-division scheme achieves the same or better spectral efficiency and outage performance with simpler signals and shorter decoding delay. These advantages come from sending a different message in each transmission block without block Markovity as in existing schemes. Using time-division, the proposed scheme divides each transmission frame into three phases with variable duration. The two user equipment units (UEs) partially exchange their information in the first two phases and then cooperatively transmit to the base station (BS) in the third phase. We further formulate the end-to-end outage probabilities, considering outages at both UEs and the BS. We analyze this outage performance in Rayleigh fading environment assuming full channel state information (CSI) at receivers and limited CSI at transmitters. Results show that user cooperation improves the achievable rate region even under half-duplex transmission. Moreover, as the received SNR increases, this uplink cooperation significantly reduces outage probabilities and achieves the full diversity order despite additional outages at the UEs. These characteristics make the proposed scheme appealing for deployment in future cellular networks.

Outage Analysis for Coherent Decode-Forward Relaying Over Rayleigh Fading Channels

We analyze the outage performance of coherent partial decode-forward (pDF) relaying over Rayleigh fading channels in both half- and full-duplex transmissions. In coherent DF relaying, the relay either partially or fully decodes the source message, then coherently forwards the decoded message with the source to the destination. This coherent transmission creates a beamforming gain from the source and relay to the destination which improves the transmission rate. We analyze the impact of this beamforming gain on the outage performance, considering outage events at both the relay and the destination. We derive analytical expressions for the overall outage probability, assuming full CSI at receivers and only limited CSI at transmitters. We further show that at high SNR, coherent pDF relaying converges to full DF relaying and achieves the full diversity order of 2. These analyses provide a fundamental understanding of the reliability of coherent pDF relaying and form the basis for analyzing performance in larger network settings. Numerical results show that partial decoding at the relay outperforms full decoding for low SNR and high target rates in the half-duplex mode. Comparison with existing results further shows that source-relay coherent transmission and joint decoding at the destination both improve the outage performance.

Rate Maximization for Half-Duplex Multiple Access with Cooperating Transmitters

We derive the optimal resource allocation of a practical half-duplex scheme for the Gaussian multiple access channel with transmitter cooperation (MAC-TC). Based on rate splitting and superposition coding, two users transmit information to a destination over 3 phases, such that the users partially exchange their information during the first 2 phases and cooperatively transmit to the destination during the last one. This scheme is near capacity-achieving when the inter-user links are stronger than each user-destination link; it also includes partial decode-forward relaying as a special case. We propose efficient algorithms to find the optimal resource allocation for maximizing either the individual or the sum rate and identify the corresponding optimal scheme for each channel configuration. For fixed phase durations, the power allocation problem is convex and can be solved analytically based on the KKT conditions. The optimal phase durations can then be obtained numerically using simple search methods. Results show that as the inter-user link qualities increase, the optimal scheme moves from no cooperation to partial then to full cooperation, in which the users fully exchange their information and cooperatively send it to the destination. Therefore, in practical systems with strong inter-user links, simple decode-forward relaying at both users is rate-optimal.

Link Regime and Power Savings of Decode-Forward Relaying in Fading Channels

In this paper, we re-examine the relay channel under the decode-forward (DF) strategy. Contrary to the established belief that block Markov coding is always the rate-optimal DF strategy, under certain channel conditions (a link regime), independent signaling between the source and relay achieves the same transmission rate without requiring coherent channel phase information. Furthermore, this independent signaling regime allows the relay to conserve power. As such, we design a composite DF relaying strategy that achieves the same rate as block Markov DF but with less required relay power. The finding is attractive from the link adaptation perspective to adapt relay coding and relay power according to the link state. We examine this link adaptation in fading under both perfect channel state information (CSI) and practical CSI in which nodes have perfect receive and long-term transmit CSI, and derive the corresponding relay power savings in both cases. We also derive the outage probability of the composite relaying scheme, which adapts the signaling to the link regime. Through simulation, we expose a tradeoff for relay placement showing that the relay conserves the most power when closer to the destination but achieves the most rate gain when closer to the source.

Outage Analysis and Power Savings for Independent and Coherent Decode-Forward Relaying

In this paper, we consider a composite independent and coherent decode-forward (DF) relaying scheme and analytically evaluate its outage performance over Rayleigh fading channels. We examine the link-state regimes in which the optimal strategy is either direct transmission, independent DF (IDF), or coherent DF (CDF), depending on the relation among the links. Outage probabilities at both the relay and destination are determined in order to analytically derive the overall outage performance. With full channel state information (CSI), the relay can save power without affecting the overall outage performance. Specifically, by using only a portion of its maximum power for transmission, the relay conserves power in the IDF regime while still achieving the maximum transmission rate. Even with just statistical CSI, the relay can employ these link regime results to reduce transmit power significantly while maintaining the same outage performance as that of using full power.

Outage analysis for uplink mobile-to-mobile cooperation

We consider the uplink transmission over Rayleigh fading channels of two mobiles cooperating to send their information to a base station. We propose a cooperative transmission scheme, derive its achievable rate region and analyze its outage performance. By dividing each transmission block into 3 phases with variable durations, the two mobiles partially exchange their information in the first two phases, then cooperatively transmit to the base station in the third phase. Assuming full CSI at the receivers and limited CSI at the transmitters, we formulate the common and individual outage probabilities for the proposed cooperative scheme. Numerical results show that, in spite of additional outages at the mobiles, cooperation reduces the overall outage probability and achieves the full diversity order of 2, which makes mobile-to-mobile cooperation appealing for next generation cellular systems.

An asymptotically capacity-achieving scheme for the Gaussian relay channel with relay-destination cooperation

This paper shows that the capacity of the relay channel can be asymptotically achieved by allowing relay-destination cooperation. Consider a scheme with rate splitting and second-order superposition block Markov encoding at the source, partial decode-forward (PDF) relaying and joint decoding over two consecutive blocks at the relay, and quantize-forward relaying and backward decoding at the destination. This scheme includes as special cases the classical PDF scheme and the destination feedback scheme of the relay channel. In Gaussian channels, the proposed scheme outperforms noisy network coding scheme except for weak source-relay link. For all channel gains, it asymptotically achieves the capacity by reaching the cutset bound as the destination power approaches infinity.

Outage analysis for half-duplex partial decode-forward relaying over fading channels

We analytically evaluate the outage performance of the half-duplex partial decode-forward (PDF) relaying over Rayleigh fading channels. In PDF relaying, the source splits its information into cooperative and private parts, and the relay decodes only the cooperative information then forwards it to the destination coherently with the source. Assuming full CSI at the receivers and limited CSI at the transmitters, we analyze the outage performance by jointly considering outages of the cooperative and private parts at both the relay and the destination. In spite of additional outage at the relay and limited CSI at the transmitters, we show that PDF relaying achieves the full diversity order of 2 at high SNR. Numerical results confirm the analysis and show the advantage of the considered PDF scheme, with coherent transmission and joint decoding at the destination, over the existing DF and PDF schemes.

Joint typicality analysis for half-duplex cooperative communication

We propose a half-duplex cooperative scheme for a discrete memoryless channel (DMC) consisting of two users communicating with one destination. The half-duplex constraint is satisfied by performing the communication over 3 time slots with variable durations in each code block. Each user alternatively transmits and receives during the first 2 time slots, then both of them transmit during the last one. Different from [1], here we use joint typicality instead of maximum likelihood (ML) decoding to derive the rate region. The main contribution is in the joint decoding and proof techniques that concurrently combine code segments of different lengths.