Objective

The objective of this project is to investigate the concept of cooperation in wireless networks. This concept, which is rapidly changing the design philosophies of wireless communication technologies, is capable of enhancing the reliability of networks for transmission of real-time information.

In particular, by providing diversity cooperative transmission can indeed increase throughput as well as transmission coverage. In this study we will specifically concentrate on the synchronization aspects of cooperative transmission, which is one of the major issues in its application to multihop transmission in wireless ad-hoc network environments.

Publications

N. Wu and H. Gharavi,"Combating Imperfect Synchronization In Cooperative MIMO Systems using Linear Dispersion Structure", To be Submitted.

Technical Approach

Physical Layer

When multiple relays are deployed, a 'virtual' antenna array can be formed. Therefore, co-located MIMO system design philosophies, such as space-time block coding, can be applied to cooperative scenarios.

Higher Layer Cooperation

By enabling cooperation in the Physical layer, the MAC layer scheduling as Routing protocol of the Network layer has to be re-designed in order to increase system reliability or throughput. We propose to modify the existing 802.11 protocols, which are directly compatible with current networks.

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Cooperative Transmission in Multi-hop Networks

The idea of cooperation fundamentally changes the way researchers address the problems of wireless transmission. More explicitly, by employing cooperation, 'cooperative' diversity could be achieved, such that the channel variance can be exploited, rather than minimized. On the other hand, cooperation deliberately creates 'interference' signals as a result of the broadcast nature. Conventional protocol designs try to avoid interference by enabling a single node to transmit at any given time or by employing equalizers. However, cooperation techniques aim to make use of these 'interference' signals.

Implementing the concept of cooperation requires a re-design of multiple layers of the existing communication protocol. For example, physical layer signal processing schemes have to handle signals from multiple sources. MAC layer protocols should allow the nodes to transmit simultaneously. Naturally, such improvements will lead to new route selection and maintenance in routing protocols of the network layer. Figure 1 illustrates the idea of physical layer cooperation, which normally involves a 'broadcast interval', where intermediate relays obtain the source information. During the 'cooperative interval', the nodes that can recover the information correctly using a Decode-and-Forward strategy function as relays can collaboratively transmit the information after re-encoding the useful information.

Figure 1. The source node broadcasts the source information and the relays forward the re-encoded source information after decoding successfully.

The potential application of this project could be multi-hop wireless networks as well as cellular networks, since cooperation would achieve at least one of the following advantages:

Increasing reliability over time-varying channels;
Increasing transmission rate;
Enlarging transmission range and extending network coverage.

The Issue of Synchronization for Cooperative Transmission

In order to maximize the gains of the cooperative networks, the relays are scheduled to transmit simultaneously. However, synchronous reception is often unrealistic owing to the propagation delays as well as imperfect scheduling. We address this issue by proposing a novel time-domain scheme, which has a flexible system structure and is capable of tolerating high delay differences.