Pattern mapper, (or in full name - Manchester Pattern Mapper), is a physical layer channel encoding/decoding technique to acquire extra channel gain.

Manchester encoding replace every binary bit for transmit into N symbols following set of pre-defined patterns.
For example,
1 -> 10
0 -> 01

With this pattern, a binary bit sequence :
0011 1001
will be encoded into:
0101 1010 1001 0110

And during receiving, the same pattern will be applied onto received bit stream and the original bit sequence will be decoded. In this way if one or more bits get corrupted Manchester mapper can recover these bits.
Example (still the above bit sequence):
Data_corrupted: 0101 1011 1001 0110
Decoded pattern: 0011 1001 (Note, that the decoded sequence is still correct)

It is then obvious that patterns with more symbols will be more robust to bit corruption. The number of symbol in each pattern is represented as S. in above example S=2, and we can also define patterns with S=8, which encode each bit into a 8-symbol pattern.
Usually with S=2 or S=8 encoding we can get a 5 ~ 8 dB gain in receiver sensitivity. However, the larger S, the more symbols one needs to be transferred over the link, which brings the down effect of lower overall data throughput.