Cooperative Communication

Implications Of The Half-Duplex Constraint On Relay-Aided Cooperation Using Rateless Codes

( Joint work with Prof. Neelesh B. Mehta, ECE Dept. Indian Institute of Science (IISc) Bangalore. )

In a cooperative communication system, the source transmits information to the destination with the help of one or more relays. Cooperation exploits the broadcast nature of wireless channel and harnesses the spatial diversity inherent in a network consisting of multiple geographically separated relays. Several cooperation protocols have been proposed to try to meet the twin goals of (i) being practically simple and robust in design and (ii) using only simple, single antenna, and low cost relays. The first goal of robust and simple protocol design has led to the development of selection-based techniques in systems with multiple relays. In these, only one relay among the many available relays is selected on the basis of its current channel state to the source and/or destination to forward information to the destination The second goal of using simple, low cost relays has motivated protocols that use half-duplex relays, which can transmit and receive but not simultaneously. This restriction reduces the hardware and signal processing capabilities required of the relays. For example, if the channels over which a relay transmits and receives are separated in frequency, the half-duplex constraint eliminates the need for a radio frequency duplexer component that would otherwise be required to isolate the transmit and receive signals.

The half-duplex constraint, which mandates that a cooperative relay cannot transmit and receive simultaneously, considerably simplifies the demands made on the hardware and signal processing capabilities of a relay. However, the very inability of a relay to transmit and receive simultaneously leads to a potential under-utilization of time and bandwidth resources available to the system. We analyze the impact of the half-duplex constraint on the throughput of a cooperative relay system that uses rateless codes to harness spatial diversity and efficiently transmit information from a source to a destination. We derive closed-form expressions for the throughput of the system, and show that as the number of relays increases, the throughput approaches that of a system that uses more sophisticated full-duplex nodes. Thus, half-duplex nodes are well suited for cooperation using rateless codes despite the simplicity of both the cooperation protocol and the relays.

  • R. Meshram, and N. B. Mehta “Implication Of The Half-Duplex Constraint On Relay-Aided Cooperation Using Rateless Codes,” in National Conference on Communication (NCC), 2010.
  • R. Meshram, P. Mandal and N. B. Mehta “Implication Of The Half-Duplex Constraint On Relay-Aided Cooperation Using Ideal Rateless Codes and Raptor Codes,” in preparation.

2. Joint Routing, Scheduling And Power Control in Multihop MIMO Networks

(Joint work with Harish Vangala and Prof. Vinod Sharma, ECE Dept. Indian Institute of Science, Bangalore.)

Multihop wireless networks (MHWN) are essential for ubiquitous computation and communication. Currently there are many experimental setups of multihop wireless networks around the world. Ad-hoc wireless networks and sensor networks are also examples of multihop wireless networks where it is necessary to employ multiple wireless hops for even the connectivity of the nodes deployed in a particular area. However, multiple wireless hops pose many new challenges in a network design. But recent studies have shown that some of these challenges can be converted into opportunities by careful network design. Thus new communication paradigms, e.g., opportunistic scheduling, cooperative communication, network coding and multiple antennas have been developed in recent years. Exploiting these techniques together in a multihop setup to optimize the system performance is very challenging.

Employing multiple antennas at a transmitter and/or at a receiver can provide transmit diversity, receive diversity, increase the capacity of the link and reduce BER. Thus, in wireless networks where bandwidth is scarce, it is important to employ multiple antennas wherever feasible. This increases the degrees of freedom one can exploit to improve the system performance. Thus even for multihop networks it is desirable to have multiple antennas at different nodes. However, as mentioned above, even with single antennas, jointly optimizing routing, scheduling and power (JRSP) in a multihop wireless network is very computationally intensive. With multiple antennas the problem becomes extremely challenging. At the moment there are comparatively few studies available for this system. We address this problem.

  • H. Vangala, R. Meshram, and V. Sharma “Joint Routing, Scheduling And Power Control For Multihop Wireless Networks With Multiple Antennas,” in National Conference on Communication (NCC), 2012.
  • H. Vangala, R. Meshram, and V. Sharma “Joint routing, scheduling and power control in multihop MIMO networks with MAC and Broadcast links,” in Wireless Communication and Networking Conference (WCNC), 2012.