In this paper, we establish new capacity bounds for the multi-sender unicast index-coding problem. We first revisit existing outer and inner bounds proposed by Sadeghi et al. and identify their suboptimality in general. We then propose a new multi-sender maximal-acyclic-induced-subgraph outer bound that improves upon the existing bound in two aspects. For inner bound, we identify shortcomings of the state-of-the-art partitioned Distributed Composite Coding (DCC) in the strategy of sender partitioning and in the implementation of multi-sender composite coding. We then modify the existing sender partitioning by a new joint link-and-sender partitioning technique, which allows each sender to split its link capacity so as to contribute to collaborative transmissions in multiple groups if necessary. This leads to a modified DCC (mDCC) scheme that is shown to outperform partitioned DCC and suffice to achieve optimality for some index-coding instances. We also propose cooperative compression of composite messages in composite coding to exploit the potential overlapping of messages at different senders to support larger composite rates than those by point-to-point compression in the existing DCC schemes. Combining joint partitioning and cooperative compression said, we develop a new multi-sender Cooperative Composite Coding (CCC) scheme for the problem. The CCC scheme improves upon partitioned DCC and mDCC in general, and is the key to achieve optimality for a number of index-coding instances. The usefulness of each scheme is illuminated via examples, and the capacity region is established for each example.
['Min Li', 'Lawrence Ong', 'Sarah J. Johnson']
Cooperative Multi-Sender Index Coding