Multipath equalization
At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.
Here are two old Western Union images. The top graphic shows transmission without a delay equalizer. The image below it shows the same transmission corrected by a delay equalizer.
Above. No equalizer.
Above. Delay equalizer introduced. Pretty dramatic difference, eh?