Soft Handoff Gain for CDMA

Soft handoff is the term that is normally associated with the fact that a CDMA system makes a connection to a target cell prior to releasing (breaking) from the source site, commonly referred to as “make-before-break”. A hard handoff, associated with AMPS, GSM, or USDC, requires that the signal strength from the target cell be greater than the signal strength from the source cell by a hysteresis value in order to reduce the number of handoffs per call and the “ping-pong” effect.

This hysteresis requires an overlap between the cell coverage areas. The soft handoff gain corresponds to a decreased shadow fade margin required by the CDMA soft handoff over that of a hard handoff system. Some proponents of CDMA may have a separate entry in the RF link budget for soft handoff gain.

The purpose of this is to provide information as to the benefits of CDMA over other technologies. Some factions believe that the soft handoff gain should be accounted for in the reliability value (shadow fade margin).

For a fixed system, the gain offered by soft handoff may or may not be present depending upon the system design. For instance, a single isolated site supporting a WiLL system would have no neighboring sites to even allow soft handoff to occur. In this situation, the soft handoff gain would be zero. Another situation is for a fixed system utilizing external FWT antennas.

These antennas tend to be directional and would be sited to the best signal source and therefore minimal advantage from soft handoff would be recognized. Even for the situation of a fixed system using the FWT whip antennas, soft handoff gain may be lower than seen in a mobile environment. The FWT installation causes a form of building directionality which may decrease the soft handoff advantage.

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