  In this paper, we consider the ChannelComp framework, which facilitates the
computation of desired functions by multiple transmitters over a common
receiver using digital modulations across a multiple access channel. While
ChannelComp currently offers a broad framework for computation by designing
digital constellations for over-the-air computation and employing symbol-level
encoding, encoding the repeated transmissions of the same symbol and using the
corresponding received sequence may significantly improve the computation
performance and reduce the encoding complexity. In this paper, we propose an
enhancement involving the encoding of the repetitive transmission of the same
symbol at each transmitter over multiple time slots and the design of
constellation diagrams, with the aim of minimizing computational errors. We
frame this enhancement as an optimization problem, which jointly identifies the
constellation diagram and the channel code for repetition, which we call
ReChCompCode. To manage the computational complexity of the optimization, we
divide it into two tractable subproblems. Through numerical experiments, we
evaluate the performance of ReChCompCode. The simulation results reveal that
ReChCompCode can reduce the computation error by approximately up to 30 dB
compared to standard ChannelComp, particularly for product functions.
