Soft handover or soft handoff refers to a feature used by the
CDMA and
W-CDMA standards, where a
cell phone
A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...
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 cell site, cell tower, or cellular base station is a cellular-enabled mobile device site where antennas and electronic communications equipment are placed (typically on a radio mast, tower, or other raised structure) to create a cell, or adj ...
(a sectorised site), it is referred to as softer handoff. This technique is a form of mobile-assisted handover, for
IS-95/
CDMA2000
CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and Signaling (telecommunication), signaling data between mobile phones and cell sites. It is developed by 3GP ...
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
Base station (or base radio station) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a " land station in the land mobile service."
The term is used in the context of mobile telephony, wireless co ...
that are transmitting the same
bit stream
A bitstream (or bit stream), also known as binary sequence, is a sequence of bits.
A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet ma ...
(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
bits. 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[Vanghi, Damnjanovic, Vojcic, The Cdma2000 System for Mobile Communications, Prentice Hall, 2004, Ch. 1] called soft handover gain.
See also
*
Handoff
*
Macrodiversity
*
Dynamic Single-Frequency Networks {{Unreferenced, date=January 2008
Dynamic Single Frequency Networks (DSFN) is a transmitter macrodiversity technique for OFDM based cellular networks.
DSFN is based on the idea of single frequency networks (SFN), which is a group of radio transmit ...
External links
Mobile and Broadband Access Networks
References
{{Reflist
Radio resource management
Mobile technology