Achieving Full DoF in Heterogeneous Parallel Broadcast Channels with Outdated CSIT

TL;DR

This paper characterizes the optimal degrees of freedom in heterogeneous parallel channels with outdated CSIT, showing that joint coding strategies can outperform simple aggregation and achieve instantaneous CSIT performance under certain conditions.

Contribution

It introduces a two-phase overload-multicast strategy that achieves sum DoF optimality with outdated CSIT in heterogeneous channels, extending to K-user MIMO BCs.

Findings

Joint coding over channels outperforms simple aggregation.

The overload-multicast scheme achieves instantaneous CSIT performance with outdated CSIT.

The strategy is sum DoF optimal for high multicast rates and small multicast capacity regimes.

Abstract

We consider communication over heterogeneous parallel channels, where a transmitter is connected to two users via two parallel channels: a MIMO broadcast channel (BC) and a noiseless rate-limited multicast channel. We characterize the optimal degrees of freedom (DoF) region of this setting when the transmitter has delayed channel state information (CSIT) regarding the MIMO BC. Our results show that jointly coding over the two channels strictly outperforms simple channel aggregation and can even achieve the instantaneous CSIT performance with completely outdated CSIT on the MIMO BC in the sum DoF sense; this happens when the multicast rate of the second channel is larger than a certain threshold. The main idea is to send information over the MIMO BC at a rate above its capacity and then use the second channel to send additional side information to allow for reliable decoding at both receivers. We call this scheme a two-phase overload-multicast strategy. We show that such a strategy is also sum DoF optimal for the K-user MIMO BC with a parallel multicast channel when the rate of the multicast channel is high enough and can again achieve the instantaneous CSIT performance (optimal sum DoF) with completely outdated CSIT. For the regime where the capacity of the multicast channel is small, we propose another joint coding strategy which is sum DoF optimal.