This paper analyzes the block error rate (BLER) performance provided by a virtual MIMO scheme which implements a modified Alamouti Space-Frequency Block Code (SFBC) encoding-decoding algorithm using two fixed Decode &Forward relays that perform selective relaying, and single-antenna devices. It also analyzes the performance of the classical relay-based distributed Alamouti scheme with single-antenna devices and, as baseline, by the non-cooperative Alamouti scheme which uses an antenna array at the source. We present the theoretical derivations of the BLER provided by the three schemes vs. the SNRs of the employed channels, using an approximation of the BLER ensured by the channel code used on each constituent link and validate them by computer simulation, both for the downlink and uplink transmissions of a generic cellular network, over Rayleigh block-faded channels, for various positions of the user terminal vs. the base station and relays.