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Follow Us. Soft handoff refers to the overlapping of repeater coverage zones, so that every cell phone set is always well within range of at least one repeater also called a base station.
In some cases, mobile sets transmit signals to, and receive signals from, more than one repeater at a time. Soft handoff technology is used by code-division multiple access CDMA systems. This occurs when several sectors may be served from the same NodeB, thereby simplifying the combining as it can be achieved within the NodeB and not require linking further back into the network.
This may occur as a result of the sectors overlapping, or more commonly as a result of multipath propagation resulting from reflections from buildings, etc. In the uplink, the signals received by the NodeB, the signals from the two sectors can be routed to the same RAKE receiver and then combined to provide an enhanced signal. In the downlink, it is a little more complicated because the different sectors of the NodeB use different scrambling codes. To overcome this, different fingers of the RAKE receiver apply the appropriate de-spreading or de-scrambling codes to the received signals.
Once this has been done, they can be combined as before. In view of the fact that a single transmitter is used within the UE, only one power control loop is active.
This may not be optimal for all instances but it simplifies the hardware and general operation. These handovers may be required for one of a variety of reasons including:. One of the benefits of using a scheme like CDMA is that handover can be effected more easily than with systems like GSM where a hard handover is always needed. Using a soft or softer form of handover it is easier to effect a successful handover. Obviously the Inter-RAT handover is the most difficult to achieve, but nevertheless it is done successfully.
For purely inter W-CDMA 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. UMTS hard handover The name hard handover indicates that there is a "hard" change during the handover process.
In soft handover the mobile station is in the overlapping cell coverage area of two sectors that belongs to different base stations. In the uplink, for the particular UE, the scrambled signals from different base station are compared frame by frame at RNC and the best candidate is selected after ach interleaving period i.
In downlink direction signals that are received from different base stations are combined at the rake receiver of the UE. Now, Basic reasons why LTE does not have soft handover are:. Also, there is no need for power control in LTE because of the presence of orthogonal modulation scheme; there is no self interference like CDMA. So no worries about Rx diversity gain and so soft handover can be skipped. Cell edge reception which was the heart of the soft handover design in WCDMA is not applicable in LTE networks due to orthogonality in both downlink and uplink.
Thus soft handover is dropped from LTE system. Post a Comment. This standardization was concluded in 3GPP Rel
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