The concept of diversity reception has been well known for some time. A diversity receiver uses multiple antennas at one reception site. Since these antenna are placed to be a non-integral number of wavelength apart, when one antenna is experiencing a multipath fade it is likely that the other antennas will not be in a fading condition.
This leads to receiver designs where the antenna with the best signal is selected to be processed by the receiver. AMPS analog cellular base stations use this type of diversity for improved fading resistance. CDMA also employs diversity reception for base stations.
One of the most problematic locations for a cellular phone is in between cells where handoffs occur. If the mobile experiences a deep fade during handoff, a dropped call can result.
If diversity reception is useful at a single receiver location, then can using multiple base station be used in a diversity network to help phone during handoffs? The answer is yes: CDMA seeks to overcome the handoff problem by using two or three base stations as a giant diversity system.
Using multiple base stations simultaneously talk to the mobile during a handoff is known as a “soft handoff”.