Zero-Outage Cellular Downlink with Fixed-Rate D2D Underlay

TL;DR

This paper demonstrates that fixed-rate D2D communications can enable zero-outage cellular downlink transmission through joint decoding, even with multiple interfering devices, highlighting a novel approach to spectrum reuse in wireless networks.

Contribution

It proves that fixed-rate D2D underlay can always support a positive downlink rate with joint decoding, a significant advancement over traditional single-user decoding methods.

Findings

Joint decoding enables zero-outage downlink with multiple D2D devices.

Single-user decoding fails to support multiple interfering devices.

Fixed-rate D2D underlay improves spectrum efficiency.

Abstract

Two of the emerging trends in wireless cellular systems are Device-to-Device (D2D) and Machine-to-Machine (M2M) communications. D2D enables efficient reuse of the licensed spectrum to support localized transmissions, while M2M connections are often characterized by fixed and low transmission rates. D2D connections can be instrumental in localized aggregation of uplink M2M traffic to a more capable cellular device, before being finally delivered to the Base Station (BS). In this paper we show that a fixed M2M rate is an enabler of efficient Machine-Type D2D underlay operation taking place simultaneously with another \emph{downlink} cellular transmission. In the considered scenario, a BS $B$ transmits to a user $U$, while there are $N_M$ Machine-Type Devices (MTDs) attached to $U$, all sending simultaneously to $U$ and each using the same rate $R_M$. While assuming that $B$ knows the channel $B-U$, but not the interfering channels from the MTDs to $U$, we prove that there is a positive downlink rate that can always be decoded by $U$, leading to zero-outage of the downlink signal. This is a rather surprising consequence of the features of the multiple access channel and the fixed rate $R_M$. We also consider the case of a simpler, single-user decoder at $U$ with successive interference cancellation. However, with single-user decoder, a positive zero-outage rate exists only when $N_M=1$ and is zero when $N_M>1$. This implies that joint decoding is instrumental in enabling fixed-rate underlay operation.