# A New View of Multi-User Hybrid Massive MIMO: Non-Orthogonal Angle Division Multiple Access

## TL;DR

This paper introduces a new perspective on multi-user hybrid massive MIMO systems using angle domain analysis, proposing a novel channel estimation method and viewing the system as non-orthogonal ADMA, validated by simulations.

## Contribution

It presents an angle domain decomposition approach for hybrid MIMO, enabling efficient channel estimation and a new non-orthogonal ADMA framework.

## Key findings

- Asymptotic orthogonality of channel vectors with different AOAs
- Significant reduction in training overhead for channel estimation
- Validation of the proposed scheme through computer simulations

## Abstract

This paper presents a new view of multi-user (MU) hybrid massive multiple-input and multiple-output (MIMO) systems from array signal processing perspective. We first show that the instantaneous channel vectors corresponding to different users are asymptotically orthogonal if the angles of arrival (AOAs) of users are different. We then decompose the channel matrix into an angle domain basis matrix and a gain matrix. The former can be formulated by steering vectors and the latter has the same size as the number of RF chains, which perfectly matches the structure of hybrid precoding. A novel hybrid channel estimation is proposed by separately estimating the angle information and the gain matrix, which could significantly save the training overhead and substantially improve the channel estimation accuracy compared to the conventional beamspace approach. Moreover, with the aid of the angle domain matrix, the MU massive MIMO system can be viewed as a type of non-orthogonal angle division multiple access (ADMA) to simultaneously serve multiple users at the same frequency band. Finally, the performance of the proposed scheme is validated by computer simulation results.