The rate memory is a part of the SHM, it is probably used to recognize PLCP headers.
Initializing the Rate Memory
To initialize the Rate Memory, first, find the offset of each rate that you'd like to initialize. If the rate is an OFDM rate, the base offset is 0x480. If the rate is not an OFDM rate, the base offset is 0x4C0. Next, add twice the lower 4 bits of the PLCP rate (see PLCP header) to the base offset to find the rate offset in the SHM. Finally, copy a 16 bit word from this offset to the offset 0x20 more than the offset copied from.
For a better explanation see 802.11/ObjectMemory but use the offsets from here.