In this study, the performance, cellular coverage and average spectral efficiency are analysed for the uplink of small cells with dual-hop amplify-and-forward relaying. Specifically, a cellular network with an optimum combiner at a destination, multiple-antennas relays and multiple-antennas destination affected by multiple interferers is considered. The techniques of multiple relays transmission (MRT) and relay selection with transmit antenna selection (RS-TAS) are analysed and compared. The moment-generating function of the signal-to-interference-plus-noise ratio is derived in order to obtain the average bit-error rate for both transmission techniques. Then, an adaptive scheme modulation and different transmission power levels are used to obtain the coverage and the average cellular spectral efficiency by considering a path-loss model at 28 GHz. Results show that using a relay is so efficient compared to a non-relay system. Moreover, RS-TAS outperforms MRT in both coverage and efficiency.