Soft handover
Encyclopedia
Soft handover or soft handoff refers to a feature used by the CDMA
and WCDMA standards, where a cell phone
is simultaneously connected to two or more cells (or cell sectors) during a call. If the sectors are from the same physical cell site
(a sectorised site), it is referred to as softer handoff. This technique is a form of mobile-assisted handover, for IS-95
/CDMA2000
CDMA cell phones continuously make power measurements of a list of neighboring cell sites, and determine whether or not to request or end soft handover with the cell sectors on the list.
Due to the properties of the CDMA signaling scheme, it is possible for a CDMA phone to simultaneously receive signals from two or more radio base stations that are transmitting the same bit stream (using different transmission codes) on the different physical channels in the same frequency bandwidth. If the signal power from two or more radio base stations is nearly the same, the phone receiver can combine the received signals in such a way that the bit stream is decoded much more reliably than if only one base station were transmitting to the subscriber station. If any one of these signals fades significantly, there will be a relatively high probability of having adequate signal strength from one of the other radio base stations.
On the uplink (phone-to-cell-site), all the cell site sectors that are actively supporting a call in soft handover send the bit stream that they receive back to the Radio Network Controller (RNC), along with information about the quality of the received bit
s. The RNC examines the quality of all these bit streams and dynamically chooses the bit stream with the highest quality. Again, if the signal degrades rapidly, the chance is still good that a strong signal will be available at one of the other cell sectors that is supporting the call in soft handover.
Soft handover results in a diversity gain
called soft handover gain.
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
and WCDMA standards, where a cell phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
is simultaneously connected to two or more cells (or cell sectors) during a call. If the sectors are from the same physical cell site
Cell site
A cell site is a term used to describe a site where antennas and electronic communications equipment are placed, usually on a radio mast, tower or other high place, to create a cell in a cellular network...
(a sectorised site), it is referred to as softer handoff. This technique is a form of mobile-assisted handover, for IS-95
IS-95
Interim Standard 95 is the first CDMA-based digital cellular standard by Qualcomm. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95....
/CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
CDMA cell phones continuously make power measurements of a list of neighboring cell sites, and determine whether or not to request or end soft handover with the cell sectors on the list.
Due to the properties of the CDMA signaling scheme, it is possible for a CDMA phone to simultaneously receive signals from two or more radio base stations that are transmitting the same bit stream (using different transmission codes) on the different physical channels in the same frequency bandwidth. If the signal power from two or more radio base stations is nearly the same, the phone receiver can combine the received signals in such a way that the bit stream is decoded much more reliably than if only one base station were transmitting to the subscriber station. If any one of these signals fades significantly, there will be a relatively high probability of having adequate signal strength from one of the other radio base stations.
On the uplink (phone-to-cell-site), all the cell site sectors that are actively supporting a call in soft handover send the bit stream that they receive back to the Radio Network Controller (RNC), along with information about the quality of the received bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s. The RNC examines the quality of all these bit streams and dynamically chooses the bit stream with the highest quality. Again, if the signal degrades rapidly, the chance is still good that a strong signal will be available at one of the other cell sectors that is supporting the call in soft handover.
Soft handover results in a diversity gain
Diversity gain
In wireless communications, diversity gain is the increase in signal-to-interference ratio due to some diversity scheme, or how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss. Diversity gain is usually expressed in decibel, and sometimes...
called soft handover gain.