Mumbai University > Electronics and telecommunication engineering> Sem 7 > mobile communication

Marks: 10M

Year: Dec 2015

• Handover or handoff is as important for UMTS as any other form of cellular telecommunications system and is performed seamlessly so that the user is not aware of any change.
• Any failures within the UMTS handover (or UMTS handoff) procedure will lead to dropped calls which will in turn result in user dissatisfaction.
• For purely inter UMTS technology, there are three basic types of handover:
• Hard handover: This form of handover is essentially the same as that used for 2G networks where one link is broken and another established.
• Soft handover: This form of handover is a more gradual and the UE communicates simultaneously with more than one Node B or base station during the handover process.
• Softer handover: Not a full form of UMTS handover, but the UE communicates with more than one sector managed by the same NodeB.

HARD HANDOVER:

• Break Before Make.
• The network decides a handover is required dependent upon the signal strengths of the existing link, and the strengths of broadcast channels of adjacent cells.
• The link between the existing Node B and the UE is broken.
• A new link is established between the new Node B and the UE.

UMTS hard handovers may be used in a number of instances:

• When moving from one cell to an adjacent cell that may be on a different frequency.
• When implementing a mode change, e.g. from FDD to TDD mode, for example.
• When moving from one cell to another where there is no capacity on the existing channel and a change to a new frequency is required.

One of the issues facing UMTS hard handovers was also experienced in GSM. When usage levels are high, the capacity of a particular cell that a UE is trying to enter may be insufficient to support a new user. To overcome this, it may be necessary to reserve some capacity for new users.

SOFT HANDOVER:

Soft handover is a form of handover that was enabled by the introduction of CDMA. Soft handover occurs when a UE is in the overlapping coverage area of two cells. It is make before break.

• The UE communicate and receive the signals from the NodeB’s which their signals are overlapping.
• The rake receiver is needed in the UE (ME) to combine the two signals
• In the uplink, the best quality frame of the two signals is selected. The selection is made by the outer loop power control algorithm measurement.
• Negatives: It needs to indicate resources (capacity) on both Node Bs.

SOFTER HANDOVER:

• Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B.
• In softer Handover, the NodeB can receive the signal in macro diversity with maximum ratio combining.
• In soft handover macro diversity with selection combining is selected.