  This paper investigates the massive random access Gaussian channel with a
focus on small payloads. For this problem, grant-based schemes have been
regarded as inefficient due to the necessity of large feedbacks and the use of
inefficient scheduling request methods. This papers attempts to answer whether
grant-based schemes can be competitive against state-ot-art grantless schemes
and worthy of further investigation. In order to compare these schemes fairly,
a novel model is proposed, and, under this model, a novel grant-based scheme is
proposed. The scheme uses Ordentlich and Polyanskiy's grantless method to
transmit small coordination indices in order to perform the scheduling request,
which allows both the request from the users to be efficient and the feedback
to be small. We also present improvements to the Ordentlich and Polyanskiy's
scheme, allowing it to transmit information through the choice of sub-block, as
well as to handle collisions of the same message, significantly improving the
method for very small messages. Simulation results show that, if a small
feedback is allowed, the proposed scheme performs closely to the state-of-art
while using simpler coding schemes, suggesting that novel grant-based schemes
should not be dismissed as a potential solution to the massive random access
problem.
