# Low-Cost Uplink Sparse Code Multiple Access for Spatial Modulation

## TL;DR

This paper introduces a low-cost uplink RGSM-SCMA system that maintains high spectral efficiency while reducing the number of transmit antennas and decoding complexity, making it practical for real-world applications.

## Contribution

It proposes a novel uplink RGSM-SCMA system with new transmitter design, decoding algorithms, and complexity analysis, improving efficiency and reducing hardware requirements.

## Key findings

- Achieves same spectral efficiency as traditional SM-SCMA
- Reduces number of transmit antennas significantly
- Maintains comparable bit error rate with minimal complexity increase

## Abstract

Spatial modulation (SM)-sparse code multiple access (SCMA) systems provide high spectral efficiency (SE) at the expense of using a high number of transmit antennas. To overcome this drawback, this letter proposes a novel SM-SCM A system operating in uplink transmission, referred to as rotational generalized SM-SCMA (RGSM-SCMA). For the proposed system, the following are introduced: a) transmitter design and its formulation, b) maximum likelihood and maximum a posteriori probability decoders, and c) practical low-complexity message passing algorithm and its complexity analysis. Simulation results and complexity analysis show that the proposed RGSM-SCMA system delivers the same SE with significant savings in the number of transmit antennas, at the expense of close bit error rate and a negligible increase in the decoding complexity, when compared with SM-SCMA.