Buffer-Aided Relaying

Project Members

Current members:

  • Prof. Dr.-Ing. Robert Schober
  • Vahid Jamali, M.Sc.

Former members: 

  • Diomidis Michalopoulos, Ph.D.
  • Nikola Zlatanov, M.Sc.
  • Heba Shoukry (Master Thesis)
  • Noha Waly (Master Thesis)
  • Wayan Wicke (Bachelor Thesis)
  • Renato Simoni, M.Sc.

 

Buffer-aided relaying is a new paradigm for wireless cooperative networks. In particular, buffers enable half-duplex relays in a cooperative network to adaptively choose whether to receive or transmit a packet in a given time slot based on the instantaneous quality of the receiving and transmitting channels. In contrast, in conventional relaying, the half-duplex relays have a predefined schedule of when to receive and when to transmit independent of the instantaneous quality of the receiving and transmitting channels. Usually, in conventional relaying, the relay receives in one time slot and forwards the received information to the destination in the following time slot.  We illustrate the benefits of buffer-aided relying for a simple example. However, many extensions are possible, see below.

System Model

Consider the basic cooperative network. It consists of a source S, a half-duplex relay R, which is equipped with a buffer, and a destination D.

Three node relay network.

 

Assume the direct source-destination link is not available and therefore the source can communicate with the destination only through the relay. Assume that the network operates in a wireless environment. Thereby, the source-relay and relay-destination links are affected by slow flat time-varying fading, i.e., the channel quality significantly varies over time. Let the time be divided into slots of equal lengths such that the fading over a single time slot can be considered constant. Assume that the source and relay transmit packets which span one time slot. Hence, each packet transmitted by the source or the relay sees a constant channel. Furthermore, assume that source and relay employ capacity-achieving codes, such as LDPC or polar codes, so that error-free communication is possible provided that the data rate does not exceed the link capacity. Since the fading is constant during each time slot i, the capacities of the S-R and R-D channels, denoted by CSR(i) and CRD(i), respectively, are also constant during each time slot i. For simplicity, we assume identically distributed fading on the S-R and R-D channels, i.e., E{CSR(i)} = E{CRD(i)} holds, where E{·} denotes expectation. Note that this restriction is introduced for simplicity of illustration only.

Buffer-Aided Protocol with Adaptive Link Selection

The optimal buffer-aided protocol maximizing the throughput for the considered simple network is as follows:

  1. If CSR(i) ≥ CRD(i), the source transmits a packet to the relay. The relay receives and decodes the packet. Then, it stores the information contained in the packet in its buffer.
  2. If CSR(i) < CRD(i), the relay transmits a packet to the destination. Thereby, the relay extracts information from its buffer, maps it to a packet, and sends the packet to the destination. The destination receives and decodes the packet.

Performance of the Buffer-Aided Protocol

For the considered simple cooperative network, employing the buffer-aided protocol, the following ergodic rate is achieved

Rergodic,BA = 0.5 E{max (CSR(i), CRD (i))}
On the other hand, the conventional relaying protocol achieves for the considered cooperative network the following ergodic rate

Rergodic,Conv = 0.5 E{min (CSR(i), CRD (i))}y/div>

 The figures below show the ergodic rates and outage probabilities, respectively, achieved with conventional and buffer-aided relaying.

 

Ergodic rate comparison.

 

Outage probability comparison.

Research Challenges and Open Problems

There are many open research problems for buffer-aided relaying. In the following, we list some of these problems.

 

A) Deriving Optimal Buffer-Aided Relaying Protocols for More Complex Networks

The optimal buffer-aided relaying protocol has only been derived for simple relay networks such as the one-way and two-way three-node relay networks without source-destination link. The optimal buffer-aided relaying protocols for more complex networks are unknown.

B) Performance Loss Due to Non-Ideal Fading and Channel State Information (CSI)

Buffer-aided relaying protocols require CSI of the transmitting and receiving links. For most of the current work, only the case of independent and identically distributed fading with perfect CSI has been considered and the degradation due to non-ideal CSI is unknown.

C) Optimal Delay-Limited Buffer-Aided Protocol

The optimal buffer-aided relaying protocol introduces unbounded delay. An optimal delay limited buffer-aided protocol even for the simplest networks is not known. Currently, bounding the delay is done by heuristically modifying the optimal buffer-aided relaying protocol for unbounded delay. However, the heuristic delay-limited buffer-aided protocols are typically only applicable for slot-by-slot uncorrelated fading.

D) Application of Buffer-Aided Protocols in 5G Communication Systems

The next generation of wireless communications requires more complex protocol to meet with the quality-of-service (QoS) requirements. Therefore, buffer-aided relaying protocols can be well-adopted to enhance the performance of the 5G communication systems. To this end, new buffer-aided relaying protocols should be developed with respect to other candidate setups for 5G, e.g., cognitive radio systems, free-space optical systems, mmWave systems, etc.

Relevant Literature

In the following, we provide a brief (and incomplete) overview of the existing literature on buffer-aided relaying (note that each journal paper is included only in one category due to space constraints, however, it may cover multiple categories).

  • Survey Paper
    • N. Zlatanov, A. Ikhlef, T. Islam and R. Schober
      Buffer-Aided Cooperative Communications: Opportunities and Challenges.
      IEEE Communications Magazine, vol. 52, no. 4, pp. 146-153, April 2014.
    • N. Nomikos et al.
      A Survey on Buffer-Aided Relay Selection.
      IEEE Communications Surveys & Tutorials, vol. 18, no. 2, pp. 1073-1097, Secondquarter 2016.
  • Protocol Design 
    • One-Way Relaying
      • B. Xia, Y. Fan, J. Thompson, and H. V. Poor
        Buffering in a Three-Node Relay Network.
        IEEE Transactions on Wireless Communications, vol. 7, no. 11, pp. 4492–4496, Nov. 2008.
      • N. Zlatanov, R. Schober, and P. Popovski
        Buffer-Aided Rlaying with Adaptive Link Selection.
        Accepted for publication in IEEE J. Select. Areas Commun., 2012. Available [Online]
      • N. Zlatanov and R. Schober
        Buffer-Aided Relaying With Adaptive Link Selection – Fixed and Mixed Rate Transmission.
        IEEE Trans. Inform. Theory, vol. 59, pp. 2816–2840, 2013.
      • N. Zlatanov and R. Schober
        Capacity of the State-Dependent Half-Duplex Relay Channel Without Source-Destination Link.
        Submitted to IEEE Trans. Inform. Theory, 2014. Available [Online]
    • Two-Way Relaying
      • H. Liu, P. Popovski, E. de Carvalho, and Y. Zhao
        Sum-Rate Optimization in a Two-Way Relay Network with Buffering.
        IEEE Commun. Letters, vol. 17, pp. 95–98, 2013.
      • V. Jamali, N. Zlatanov, and R. Schober
        Achievable Rate Region of Bidirectional Buffer-Aided Relay Channel with Block Fading.
        IEEE Trans. Inform. Theory, 2014.
      • V. Jamali, N. Zlatanov, and R. Schober
        Buffer-Aided Bidirectional Relay Networks with Fixed Rate Transmission – Part I: Delay-Unconstrained Case.
        IEEE Trans. Wireless Commun., 2014.
    • Multi-Hop (Cascaded) Relaying
      • R. Wang, V. K. N. Lau and H. Huang
        Opportunistic Buffered Decode-Wait-and-Forward (OBDWF) Protocol for Mobile Wireless Relay Networks.
        IEEE Transactions on Wireless Communications, vol. 10, no. 4, pp. 1224-1231, April 2011.
      • V. Jamali, H. Shoukry, N. Zlatanov, and R. Schober
        Achievable Rate of the Half-Duplex Multi-Hop Buffer-Aided Relay Channel with Block Fading.
        IEEE Trans. Wireless Commun, vol. 14, no. 11, pp. 6240-6256, Nov. 2015.
      • Z. Tian; Y. Gong; G. Chen; Z. Chen; J. Chambers
        Buffer-aided Link Selection with Network-coding in Multi-hop Networks.
        IEEE Transactions on Vehicular Technology. 2016.
    • Diamond (Parallel) Relaying
      • A. Ikhlef, D. Michalopoulos, and R. Schober
        Max-Max Relay Selection for Relays with Buffers.
        IEEE Trans. Wireless Commun., vol. 11, pp. 1124 – 1135, Mar. 2012.
      • N. Nomikos, T. Charalambous, I. Krikidis, D. Skoutas, D. Vouyioukas, and M. Johansson
        A Buffer-aided Successive Opportunistic Relay Selection Scheme with Power Adaptation and Inter-Relay Interference Cancellation for Cooperative Diversity Systems.
        IEEE Trans. Commun., vol. 63, no. 5, pp. 1623-1634, May 2015.
      • Z. Tian, G. Chen, Y. Gong, Z. Chen and J. A. Chambers
        Buffer-Aided Max-Link Relay Selection in Amplify-and-Forward Cooperative Networks.
        IEEE Transactions on Vehicular Technology, vol. 64, no. 2, pp. 553-565, Feb. 2015.
      • N. Zlatanov, V. Jamali, and R. Schober
        Achievable Rates for the Fading Half-Duplex Single Relay Selection Network Using Buffer-Aided Relaying.
        IEEE Transactions on Wireless Communications, vol. 14, no. 8, pp. 4494-4507, Aug. 2015.
      • R. Simoni; V. Jamali; N. Zlatanov; R. Schober; L. Pierucci; R. Fantacci
        Buffer-Aided Diamond Relay Network with Block Fading and Inter-Relay Interference.
        IEEE Transactions on Wireless Communications, 2016.
  • Performance Analysis
    • N. B. Mehta, V. Sharma and G. Bansal
      Performance Analysis of a Cooperative System with Rateless Codes and Buffered Relays.
      IEEE Transactions on Wireless Communications, vol. 10, no. 4, pp. 1069-1081, April 2011.
    • I. Krikidis, T. Charalambous, and J. Thompson
      Buffer–Aided Relay Selection for Cooperative Diversity Systems Without Delay Constraints.
      IEEE Trans. Wireless Commun., vol. 11, no. 5, pp. 1957–1967, May 2012.
    • D. Chen, L. L. Yang and L. Hanzo
      Multi-Hop Diversity Aided Multi-Hop Communications: A Cumulative Distribution Function Aware Approach.
      IEEE Transactions on Communications, vol. 61, no. 11, pp. 4486-4499, November 2013.
    • C. Dong, L. L. Yang, J. Zuo, S. X. Ng and L. Hanzo
      Energy, Delay, and Outage Analysis of a Buffer-Aided Three-Node Network Relying on Opportunistic Routing.
      IEEE Transactions on Communications, vol. 63, no. 3, pp. 667-682, March 2015.
    • P. Xu; Z. Ding; I. Krikidis; X. Dai
      Achieving Optimal Diversity Gain in Buffer-Aided Relay Networks with Small Buffer Size.
      IEEE Transactions on Vehicular Technology, 2016.
  • Non-Ideal Fading and CSI
    • V. Jamali, N. Waly, N. Zlatanov and R. Schober
      Optimal Buffer-Aided Relaying With Imperfect CSI.
      IEEE Communications Letters, vol. 20, no. 7, pp. 1309-1312, July 2016.
    • S. Huang; J. Cai
      An Analysis Framework for Buffer-aided Relaying under Time-correlated Fading Channels.
      IEEE Transactions on Vehicular Technology, 2016.
    • T. Islam, D. S. Michalopoulos, R. Schober and V. K. Bhargava
      Buffer-Aided Relaying With Outdated CSI.
      IEEE Transactions on Wireless Communications, vol. 15, no. 3, pp. 1979-1997, March 2016.
  • Delay-Limited Protocols
    • T. Islam, A. Ikhlef, R. Schober and V. K. Bhargava
      Diversity and Delay Analysis of Buffer-Aided BICM-OFDM Relaying.
      IEEE Transactions on Wireless Communications, vol. 12, no. 11, pp. 5506-5519, November 2013.
    • V. Jamali, N. Zlatanov, and R. Schober
      Buffer-Aided Bidirectional Relay Networks with Fixed Rate Transmission – Part II: Delay-Constrained Case.
      IEEE Trans. Wireless Commun., 2014.
    • Y. Cui, V. K. N. Lau and E. Yeh
      Delay Optimal Buffered Decode-and-Forward for Two-Hop Networks With Random Link Connectivity.
      IEEE Transactions on Information Theory, vol. 61, no. 1, pp. 404-425, Jan. 2015.
    • J. Hajipour, A. Mohamed and V. C. M. Leung
      Channel-, Queue-, and Delay-Aware Resource Allocation in Buffer-Aided Relay-Enhanced OFDMA Networks.
      IEEE Transactions on Vehicular Technology, vol. 65, no. 4, pp. 2397-2412, April 2016.
  • Applications of Buffer-Aided Relaying
    • Cognitive Radio
      • H. Tran, H. J. Zepernick, H. Phan and L. Sibomana
        Performance Analysis of a Cognitive Radio Network With a Buffered Relay.
        IEEE Transactions on Vehicular Technology, vol. 64, no. 2, pp. 566-579, Feb. 2015.
      • M. Darabi, V. Jamali, B. Maham and R. Schober
        Adaptive Link Selection for Cognitive Buffer-Aided Relay Networks.
        IEEE Communications Letters, vol. 19, no. 4, pp. 693-696, April 2015.
      • M. Shaqfeh, A. Zafar, H. Alnuweiri and M. S. Alouini
        Overlay Cognitive Radios With Channel-Aware Adaptive Link Selection and Buffer-Aided Relaying.
        IEEE Transactions on Communications, vol. 63, no. 8, pp. 2810-2822, Aug. 2015.
    • Security:
      • G. Chen, Z. Tian, Y. Gong, Z. Chen and J. A. Chambers
        Max-Ratio Relay Selection in Secure Buffer-Aided Cooperative Wireless Networks.
        IEEE Transactions on Information Forensics and Security, vol. 9, no. 4, pp. 719-729, April 2014.
      • J. Huang and A. L. Swindlehurst
        Buffer-Aided Relaying for Two-Hop Secure Communication.
        IEEE Transactions on Wireless Communications, vol. 14, no. 1, pp. 152-164, Jan. 2015.
    • Free-Space Optical Communications
      • V. Jamali, D. S. Michalopoulos, M. Uysal and R. Schober
        Link Allocation for Multiuser Systems With Hybrid RF/FSO Backhaul: Delay-Limited and Delay-Tolerant Designs.
        IEEE Transactions on Wireless Communications, vol. 15, no. 5, pp. 3281-3295, May 2016.
      • M. Najafi, V. Jamali and R. Schober
        Adaptive Relay Selection Protocol for the Parallel Hybrid RF/FSO Relay Channel: Buffer-Aided and Non-Buffer-Aided Designs.
        IEEE Transactions on Communications, 2016.