• When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This procedure is called handoff.
  • The handoffs are of following types:

1. Hard Handoff
2. Soft Handoff
3. Queued Handoff
4. Delayed Handoff
5. Intersystem Handoff
6. Intrasystem Handoff
7. Network controlled Handoff (NCHO)
8. Mobile Assisted Handoff (MAHO)

Hard Handoff:

• The definition of a hard handover or handoff is one where an existing connection must be broken before the new one is established.
• Hard handoff allocate different frequency of user.
• In hard hand off a handset always communicates with one BS at any given time
• Hard handoff is typically used in TDMA and FDMA systems.
• Hard handoff is not very complicated.
• Since the radio link between the BS and the handset is broken before it is connected in hard handoff, the link transfer may fail due to long network response time even if radio channels are available in the new BS.

Soft handoff:

• Soft handoff is defined as a handover where a new connection is established before the old one is released.
• Soft hand off allocate same frequency.
• In soft handoff a handset may connect up to three or four radio links at the same time.
• Soft handoff used in CDMA and some TDMA systems.
• Soft handoff is more complicated than hard handoff.
• On the other hand, soft handoff degrades channel availability because a handset may consume multiple radio channels.

Delayed handoff

• A Delayed handoff is a two hand off level algorithm. It provides more opportunity for a successful hand off.
• The MTSO always handles the handoff first and the originating calls second. If no neighboring cells are available after the second handoff level is reached, the call continues until the signal strength drops below the threshold level then the call is dropped.
• Lower handoffs help in handling call processing more adequately.
• It makes the hand off occur at the proper location and eliminates possible interference in the system.

Queued hand off:

• Queued hand off is more effective than two threshold level handoffs.
• The MTSO will queue the requests of handoff calls instead of rejecting them if the new cell sites are busy..
• With Queuing of originating calls only, the probability of blocking is reduced.
• It is effective when implementing a simple queue for hand off calls which reduces call drops.

Intersystem Handoff:

• In Intersystem Handoff, If during an ongoing call a mobile unit moves from one cellular system to a different cellular system which is controlled by different MTSO, a handoff procedure which is used to avoid dropping of call referred as Intersystem Handoff takes place.

• An MTSO engages in this handoff system when a mobile signal becomes weak in a given cell and MTSO cannot find another cell within its system to which it can transfer the call then in progress.

• Before implementation of Intersystem Handoff, the MTSO compatibility must be checked and in an Intersystem Handoff a local call may become a long distance call as the mobile moves out of its home system and becomes a roamer in a neighboring system..

Intrasystem Handoff:

• In Intrasystem Handoff, if during an ongoing call a mobile unit moves from one cellular system to an adjacent cellular system which is controlled by the same MTSO, a handoff procedure which is used to avoid dropping of call referred to as Intra System Handoff takes place.
• An MTSO engages in this handoff system when a mobile signal becomes weak in a given cell and the MTSO finds another cell within its system to which it can transfer the call in progress.
• In Intra System Handoff local calls always remain local calls only since after handoff also the call is handled by the same MTSO. In first generation analog cellular systems, signal strength measurements are made by the base stations and supervised by the MSC.
• Each base station constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the base station tower.
• In addition, to measuring the RSSI of calls in progress within the cell, a spare receiver in each base station, called the locator receiver, is used to scan and determine signal strengths of mobile users which are in the neighboring cells which appear to be in need of handoff.
• The locator receiver, which is controlled by the MSC, reports all RSSI values to the MSC. Based on the locator receiver signal strength information from each base station, the MSC decides if a handoff is necessary or not. This handoff is called as NETWORK CONTROLLED HANDOFF (NCHO).

Mobile Assisted Handoff (MAHO)

• In Mobile Assisted Handoff (MAHO) every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station.

• A handoff is initiated, when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time.

• In MAHO method call handed over between base stations is much faster than first generation analog systems .As handoff measurements are made by each mobile . MSC no longer constantly monitors signal strengths. MAHO is particularly suited for microcellular environments where handoffs are more frequent. During the course of a call, if a mobile moves from one cellular system to a different cellular system controlled by a different MSC, an intersystem handoff becomes necessary.
• An MSC engages in an intersystem handoff when a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress.