I am trying to understand STBC and Alamouti scheme.
However, i am very confused about the BER of such systems.

Referring to the performance results in Alamouti and Tarokh's papers, for QPSK (for example), the BER/SER shown is against SNR for 2 bit/sec/hz.

1- what does the "per hz" mean here?
2- why do we use SNR here instead of Eb/No?
3- how can i calculate the BER/SER for a 20Khz bandwidth channel for example?