Soft/Softer Handoff Concepts

Soft Handoff vs. Softer Handoff

Soft Handoff

Soft handoff is a "Make before break" handoff. That is, the mobile station (MS) is up on a call and moves from one base station (BS) to another, but the MS starts communicating with a new BS before terminating communications with the old BS.

• Soft handoffs can only be used between BSs on the same frequency. The technique improves reception as MSs move between cells (on cell boundaries).
• During soft handoff the MS actually communicates with more than one BS at a time, so that when it's time to move from the weaker BS to the stronger one, the MS is already in communication with the stronger one.
• During a soft handoff, the mobile station receives independent closed loop power control bits from the two BSs and perform "Or of Downs" logic to determine how to adjust its power. That means the mobile station will increase its power level if and only if both power control bits from the two BSs are 0 (indicating up). If the power control bit from any base station equals to '1' (indicating down), the mobile station shall decrease its power.
  To learn more about how to use the test set to verify the MS behavior on independent closed loop power control from the two BSs, see Verifying Mobile Station (MS) Closed Loop Power Control (CLPC) Behavior in Soft/Softer Handoffs .

Softer Handoffs

A softer handoff occurs when the MS is communicating with two sectors of a cell. Softer handoff is identical to the soft handoff with the following exceptions.

• The mobile station receives identical power control from both sectors and provides diversity combining of the power control bits to determine whether the BSs are sending an up bit or a down bit (ignore the weaker bits). It's NOT "or of downs" logic.
  To learn more about how to use the test set to verify the MS behavior on identical closed loop power control from two sectors, see Verifying Mobile Station (MS) Closed Loop Power Control (CLPC) Behavior in Soft/Softer Handoffs .

Key Parameters Involved in the Soft/Softer Handoff

The mobile station assists the base station in the handoff process by measuring and reporting the strengths of received pilots. It searches for pilots on the current CDMA Frequency Assignment to detect the presence of CDMA Channels and to measure their strengths. When the mobile station detects a pilot of sufficient strength that is not associated with any of the Forward Traffic Channels assigned to it, it sends a Pilot Strength Measurement Message or an Extended Pilot Strength Measurement Message to the base station. The base station can then assign a Forward Traffic Channel associated with that pilot to the mobile station and direct the mobile station to perform a handoff.

The following parameters effect the MS pilot strength reporting behavior. To learn more about how to use the test set to verify the MS behavior regarding these parameters, see Verifying MS Pilot Reporting Behavior in Soft/Softer Handoff .

• T_ADD: As shown in T_Add, T_Comp and T_Drop , if a Pilot Channel E _c /I ₀ exceeds T_ADD, the MS notifies the BS. The new pilot is added to the Candidate Set. It is now a candidate to be considered for the Active Set. Default T_ADD value is 28 (in incremental units of -0.5 dB, so 28 corresponds to -14 dB).
• T_COMP: As shown in T_Add, T_Comp and T_Drop , if Candidate Set Pilot Channel E _c /I ₀ exceeds lowest Active Set Pilot Channel E _c /I ₀ by 0.5xT_COMP, the MS notifies the BS. The BS adds the pilot to the Active Set (MS commences soft handoff with it). Default value for 0.5xT_COMP is 2.5 dB.
• T_DROP and T_TDROP: As shown in T_Add, T_Comp and T_Drop , if a Pilot Channel E _c /I ₀ in the Active Set falls below T_DROP for a time T_TDROP, then the MS notifies the BS. The BS removes it from the Active Set. Default value for T_DROP is 32 (in incremental units of -0.5 dB, so 32 corresponds to -16 dB) and for T_TDROP is 3 (corresponds to 4 seconds).

T_Add, T_Comp and T_Drop

Pilot Sets

The pilot search parameters and the rules for Pilot Strength Measurement Message or Extended Pilot Strength Measurement Message transmission are expressed in terms of the following sets of pilots:

• Active Set: The set of pilots associated with the Forward Traffic Channels assigned to the MS. Any BS that the MS is up on a call with (or in soft handoff with) is in the MS's "Active Set".
• Candidate Set: The set of pilots that have been received with sufficient strength by the MS to be successfully demodulated, but have not been placed in the "Active Set" by the BS. MS has notified the BS that these strong pilots are present.
• Neighbor Set: The pilots that are not currently in the Active Set or the Candidate Set and are likely candidates for handoff.

Related Topics

Performing Soft/Softer Handoffs with the Test Set