Downlink Noncoherent Cooperation without Transmitter Phase Alignment

TL;DR

This paper proposes a noncoherent downlink cooperation scheme for cellular systems that enhances spectral efficiency without requiring phase alignment, using joint transmission and power optimization across sub-carriers.

Contribution

It introduces a novel noncoherent cooperative transmission method that does not need phase alignment, with optimized power allocation for improved sum rates in cellular networks.

Findings

Significant sum rate improvements at low to moderate SNR.

Efficient numerical solutions for power allocation in wideband systems.

Effective joint transmission without phase synchronization.

Abstract

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base transceiver stations (BTSs), which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission scheme for the downlink is studied, which does not require phase alignment. The focus is on jointly serving two users in adjacent cells sharing the same resource block. The two BTSs partially share their messages through a backhaul link, and each BTS transmits a superposition of two codewords, one for each receiver. Each receiver decodes its own message, and treats the signals for the other receiver as background noise. With narrowband transmissions the achievable rate region and maximum achievable weighted sum rate are characterized by optimizing the power allocation (and the beamforming vectors in the case of multiple transmit antennas) at each BTS between its two codewords. For a wideband (multicarrier) system, a dual formulation of the optimal power allocation problem across sub-carriers is presented, which can be efficiently solved by numerical methods. Results show that the proposed cooperation scheme can improve the sum rate substantially in the low to moderate signal-to-noise ratio (SNR) range.