  In this work, we propose buffer-aided distributed space-time coding (DSTC)
schemes and relay selection algorithms for cooperative direct-sequence
code-division multiple access (DS-CDMA) systems. We first devise a relay pair
selection algorithm that can form relay pairs and then select the optimum set
of relays among both the source-relay phase and the relay-destination phase
according to the signal-to-interference-plus-noise ratio (SINR) criterion.
Multiple relays equipped with dynamic buffers are then introduced in the
network, which allows the relays to store data received from the sources and
wait until the most appropriate time for transmission. { A greedy relay pair
selection algorithm is then developed to reduce the high cost of the exhaustive
search required when a large number of relays are involved.} The proposed
techniques effectively improve the quality of the transmission with an
acceptable delay as the buffer size is adjustable. An analysis of the
computational complexity of the proposed algorithms, the delay and a study of
the greedy algorithm are then carried out. Simulation results show that the
proposed dynamic buffer-aided DSTC schemes and algorithms outperform prior art.
