Active Set

The active set contains the pilots of those sectors that are actively exchanging traffic channel information with the mobile. If the active set has only one pilot in it, then the mobile is not in soft handoff. If the active set has two or more pilots in it, then the mobile is maintaining connections with those sectors specified by their pilots in the list. When the base station first assigns the forward traffic channel to the mobile, the base station specifies the pilots in the active set through the use of extended channel assignment message or channel assignment message. Subsequent updates to the con- tents of the active set are done by using the extended handoff direction message, gen- eral handoff direction message, or universal handoff direction message. The active set can have a maximum of six pilots.

8.2.1.1 Removing Pilots from the Active Set

Each pilot in the active set has a handoff drop timer associated with that pilot. In IS-95-A, the mobile starts the handoff drop timer for that pilot when its Ec/I0goes

below T_DROP. If the pilot’s Ec/I0comes back above T_DROP before the handoff

drop timer expires (i.e., reaches T_TDROP), then the pilot remains in the active set

Source cell (X) Target cell (Y) Mobile Base station controller (BSC)

and the timer is reinitialized. On the other hand, if the pilot remains below T_DROP until the handoff drop timer expires, then a pilot strength measurement message (PSMM) is sent to the base station, and that pilot is moved from the active set to the neighbor set.

As we can see, the administration of the pilots in the active set and therefore of the soft handoff process itself is dependent on the drop threshold parameter T_DROP. T_DROP is static in nature and does not change. In IS-2000, the system has a handoff algorithm that uses a pilot drop threshold that is dynamic in nature. In other words, this new drop threshold (which we shall designate as T_DROP*) is a function of time while the mobile is contemplating on removing a pilot from the active set. T_DROP* is given by the larger of T_DROP or

1 8 10 1 2   SOFT_SLOPE⋅_

∑

pj + _ DROP_IN j log TERCEPT (8.1)

where pjis the received pilot strength of pilot j, and the summation is over those

pilots in the active set that have strengths greater than the strength of the pilot under consideration. Expression (8.1) is called the handoff drop criterion for dropping a pilot [3]. Note that two new parameters are used for the calculation of the handoff drop criterion: SOFT_SLOPE and DROP_INTERCEPT. They are basically the slope and the intercept (in dB) of the handoff drop criterion. Note that given (8.1), the overall T_DROP* is given by

T_DROP* SOFT_SLOPE p_j DROP_INTERC

j = ⋅      +

∑

max log 8 10 EPT ,T_DROP 2         (8.2) What (8.2) tells us is that in assessing whether or not to drop a pilot (e.g., pilot i), the mobile would sum up the strengths of all those pilots that have strengths greater than that of pilot i. The mobile would then convert this sum of pilot strengths into a T_DROP-like variable (i.e., handoff drop criterion) by using SOFT_SLOPE and DROP_INTERCEPT. It would then compare the handoff drop criterion and the original T_DROP. If the original T_DROP is larger than the hand- off drop criterion, then the mobile still uses the old T_DROP as the drop threshold. If the handoff drop criterion is larger than T_DROP, then the mobile would use the value given by the handoff drop criterion as the new drop threshold. Note that by choosing the larger of the two (i.e., the handoff drop criterion or the old T_DROP), the mobile essentially implements an overall stricter drop threshold. This has the effect of dropping the pilot more quickly whenever possible. In other words, the mobile will request that the base station drops a pilot that is not value-added rela- tive to the other (stronger) pilots in the active set2_{. Also note that since the pilot}

8.2 Soft Handoff 125

2. This request is done implicitly through the PSMM or the EPSMM that is transmitted after the handoff drop timer expires.

strengths in the handoff drop criterion (8.1) change as a function of time, T_DROP* (8.2) itself also changes as a function of time.

8.2.1.2 Adding Pilots to the Active Set

Adding an additional pilot to the active set means that the mobile is to commence communication with the new sector represented by that pilot. The base station always specifies the contents of a mobile’s active set by using various handoff direc- tion messages (e.g., extended handoff direction message) and thereby directs soft handoff performed by the mobile. A pilot to be added to the active set always comes from the candidate set.

In IS-95-A, if the strength of a pilot in the candidate set exceeds the strength of a pilot in the active set by the threshold T_COMP×0.5 dB, then a PSMM transmis- sion is triggered. Upon receiving the PSMM, the base station would most likely command the mobile to move that pilot from the candidate set to the active set.

In IS-2000, the system uses a new pilot detection threshold that is dynamic in nature. This detection threshold (which we shall designate as T_ADD*) consists of the handoff add criterion for adding a pilot [3]. This criterion is given as

1 8 10 1 2   SOFT_SLOPE⋅ 

∑

pk+  ADD_INT k log ERCEPT (8.3)

where pkis the received pilot strength of pilot k. In this case, the summation is over

all the pilots in the active set regardless of their strengths. SOFT_SLOPE and ADD_INTERCEPT are the slope and intercept of the handoff add criterion equa- tion. Note that SOFT_SLOPE is identical to that shown in (8.1). Since the pilot strengths in (8.3) change as a function of time, the overall handoff add criterion is dynamic and also changes as a function of time.

Expression (8.3) states that the criterion depends on the current sum of strengths of all pilots in the active set. The sum is then linearly transformed into the handoff add criterion by using SOFT_SLOPE and ADD_INTERCEPT. In assessing whether or not to add pilot i, the mobile would compare the handoff add criterion with the strength of pilot i. If the strength of pilot i is less than the handoff add criterion, then the mobile does nothing and pilot i remains in the candidate set. On the other hand, if the strength of pilot i exceeds the handoff add criterion, then the mobile sends a PSMM or an EPSMM. Then the base station may command the mobile to move pilot i from the candidate set to the active set. In essence, the strength of a pilot in the candidate set is compared with (a linear transformation of) the sum of strengths of all pilots in the active set (i.e., the handoff add criterion). If the strength of the candi- date set pilot exceeds the criterion, then it may be moved from the candidate set to the active set.

One last note in this section, the system can also use both the T_COMP criterion and the handoff add criterion in deciding whether or not to move a pilot from the candidate set to the active set. In doing so, the mobile would send a PSMM or an EPSMM if both of the following conditions are met:

• _{The strength of a pilot in the candidate set exceeds the strength of a pilot in the}

active set by the threshold T_COMP×0.5 dB;

• _{The strength of a pilot in the candidate set exceeds the handoff add criterion}

shown in (8.3).

By demanding that the candidate set pilot meets both conditions, the system on balance enforces a stricter standard for elevating a pilot from the candidate set to the active set.

In document 3G CDMA2000 Wireless System Engineering (Page 144-147)