Contention-based Grant-free Transmission with Extremely Sparse Orthogonal Pilot Scheme

TL;DR

This paper introduces an extremely sparse orthogonal pilot scheme for uplink grant-free transmission, significantly reducing pilot collision and enhancing performance in high-overloading scenarios by exploiting spatial degrees of freedom.

Contribution

It proposes a novel sparse pilot scheme that minimizes resource usage and increases orthogonal pilot count, improving active user detection and interference suppression.

Findings

Significantly reduces pilot collision probability.

Enhances uplink grant-free transmission performance.

Supports high-overloading scenarios effectively.

Abstract

Due to the limited number of traditional orthogonal pilots, pilot collision will severely degrade the performance of contention-based grant-free transmission. To alleviate the pilot collision and exploit the spatial degree of freedom as much as possible, an extremely sparse orthogonal pilot scheme is proposed for uplink grant-free transmission. The proposed sparse pilot is used to perform active user detection and estimate the spatial channel. Then, inter-user interference suppression is performed by spatially combining the received data symbols using the estimated spatial channel. After that, the estimation and compensation of wireless channel and time/frequency offset are performed utilizing the geometric characteristics of combined data symbols. The task of pilot is much lightened, so that the extremely sparse orthogonal pilot can occupy minimized resources, and the number of orthogonal pilots can be increased significantly, which greatly reduces the probability of pilot collision. The numerical results show that the proposed extremely sparse orthogonal pilot scheme significantly improves the performance in high-overloading grant-free scenario.