T_ADD, T_COMP, T_DROP, and T_TDROP
Definition: These four parameters are used to control the dynamic response or behavior of the handoff algorithm. T_ADD and T_DROP are threshold values against which the power spectral density of a pilot, equal to its energy per chip, Ec, is measured relative to the total power spectral density NoT. If more than one RAKE-receiver finger is available, the sum of the Ec/NoT ratios on the multipath components to be processed by the RAKE-receiver fingers is used as a measure of the pilot strength. T_COMP is a ratio of the value of Ec/NoT for a pilot in the candidate set to the value of Ec/NoT for a pilot in the active set. T_TDROP is a time interval used to help avoid deleting a weak pilot from the active set simply because it is in a momentary deep fade. The four parameters are defined as follows:
- T_ADD: The threshold value of Ec/NoT at which a pilot is moved into the candidate set, usually from the neighbor set.
- T_COMP: The threshold value of the ratios of Ec/NoT at which a pilot in the candidate set appears to be worthy of being promoted to the active set.
- T_DROP: The threshold value of Ec/NoT at which a counter is started to determine if that pilot should be removed from the active set.
- T_TDROP: The drop-timer threshold interval which is the threshold of time beyond which a pilot with Ec/NoT below T_DROP is removed from the active set.
In the traffic mode, the thresholds T_ADD, T_COMP, T_DROP, and T_TDROP are used as follows:
- If the Ec/NoT measured at the mobile for a particular pilot exceeds T_ADD, that pilot is moved to the candidate set and a Pilot Strength Measurement Message (PSMM) is transmitted to the current base stations.
- If the Ec/NoT measured for a pilot in the candidate set exceeds the Ec/NoT measured for a pilot in the active set by more than a factor of T_COMP times .5 dB, then the mobile sends a PSMM, but it does not unilaterally move the pilot from the candidate set into the active set.
Movement of the pilot into the active set occurs by direction contained in the Handoff Direction Message. Some suppliers do not favor the use of T_COMP at least in part because it slows down the movement of the pilots into the active set.
- If the Ec/NoT measured at the mobile for a pilot in the active set is below T_DROP, the pilot is not immediately removed from the active set. Instead a timer, called the drop timer, is started. If a later measurement of Ec/NoT on this pilot yields a value above T_DROP before the timer "times out" by reaching T_TDROP, the drop timer is stopped and reset to zero.
- As described above, if the drop timer counts up to T_TDROP seconds, indicating that the pilot has been below T_TDROP for an interval of time equal to T_TDROP, the mobile sends a PSMM to the current base stations. The mobile only removes this pilot from the active set by direction contained in the Handoff Direction Message (HDM).
T_ADD, T_COMP, T_DROP, and T_TDROP
Example: Changing the values of T_ADD, T_COMP, T_DROP, and T_TDROP generally have the following effects:
- Increasing T_ADD and T_COMP makes it more difficult for a pilot to qualify for promotion to the active set. Hence, movement into the active set is slowed down. Decreasing these parameters generally has the opposite effect.
- Decreasing T_ADD but increasing T_COMP makes it easier for a pilot to get into the candidate set, but harder for it to be promoted to the active set, particularly for relatively large values of T_COMP.
- Decreasing T_DROP and increasing T_TDROP generally tend to retain pilots in the active set and, hence, movement out of the active set is slowed down. Increasing T_DROP and decreasing/shortening T_TDROP tend to more rapidly discharge pilots from the active set.
- To take advantage of the increased performance typically achieved when a mobile is in two-way handoff, with one pilot in the active set, the handoff algorithm may well be set to facilitate the movement of a second pilot into the active set and to retain a pilot in the active set. Hence, at this time, T_ADD and T_COMP might be relatively low and T_DROP might be relatively low and T_TDROP relatively long to keep two pilots in the active.
- To avoid possible problems with handoff dynamics when three pilots are in the active set (assuming the mobile has three RAKE-receiver fingers available), once two pilots are in the active set, the new handoff parameters conveyed with the HDM might well be set to allow only a very strong third pilot into the active set. At this time, T-ADD might set high perhaps to a value of - 6 dB or so.
- If there are three pilots in the active set, the handoff parameters should typically be set so it is very easy to promote a strong pilot to the active set. This, however, requires that it is easy to demote at least one pilot from the active set. If replacement of a weak pilot in an active set containing three pilots with a strong new pilot is too time consuming regardless of the manner in which the handoff parameters are set, then direct intervention by using the HDM may be needed. Alternately, recognition of the likelihood of this problem could be used to speed-up the response of the computer-generated HDM.
- In general, making it difficult to have three (but easy to have two) pilots in the active set, unless one is very strong relative to the others, seems to be a reasonable way to set the handoff parameters.
Typical values for T_ADD are expected to be in the - 9 to -12 dB range, although, special cases may cause different values. The minimum T_ADD is expected to be about -17 dB. T_COMP is typically set to provide a ratio of between 1 and 2, or zero and 3 dB. Hence, T_COMP is between zero and 6. The System Parameter Message and the HDM both allow 4 bits for T_COMP. It is not unusual for T_COMP to equal zero. T_DROP is often set to be about the same as T_ADD. It may be a little lower, and T_TDROP a little longer, to retain pilots in the active set. T_TDROP is expected to be only a few (4 to 9) seconds, but Table 6.6.6.2.3-1 in IS-95 allows much longer values. Lower speed mobiles may use a longer T_TDROP since deep nulls may last longer.
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