Hi Nyembezi,
I am not sure what you mean by slowing the rate down. I will
suppose by "more bits at a go" you are referring to the frames that
the system sets up prior to sending them through the channel ( in
some books they mention about "sending the bits in parallel" ).
I think it will be clearer if we consider the OFDM system in terms
of the transmitter, channel and receiver. In the transmitter and
receiver, the frames or symbols are used for IFFT/FFT and other
operations. However, when transmitting through the channel, you have
to send them in bits/samples.
The closest correlation to slowing the rate down will be the
extension of the symbol period. Because the data bits of a single
data symbol are spread over the data carriers and interleaved with
other data symbols, the data symbol will now take a longer time to
transmit over the channel.
Not sure if this answers your question, but I hope it helps abit.
Cheers
Alvin