Hi,
regarding the different parameters. 'tx_diversity' should always be left as it is. In case of a 1x1 MIMO configuration it will just have no effect. This parameter reflects the LTE standard with its two options 'tx_diversity' and 'spatial_multiplexing'. The latter isn't used for control channels.
In order to specify a certain bandwidth, use the parameter 'N_rb_dl', which specifies the number of resource blocks used. This will also set the bandwidth as this is the unit given by the system to determine the bandwidth. e.g. have a look at the paper which is included in the project there is a table which shows the dependencies.
25 RBs --> 25*12 = 300 subcarrier --> 15kHz * 300 + spacing = 5MHz.