Code Division Multiple Access (CDMA) phần 7 pps

P1: IML/FFX P2: IML

MOBK023-03 MOBK023-Buehrer.cls September 28, 2006 15:55

106 CODE DIVISION MULTIPLE ACCESS (CDMA)

T_ADD

T_DROP

(1)(2)(3) (4) (5)(6) (7)

Measured pilot strength

Neighbor set Active set

Candidate set

Neighbor set

FIGURE 3.19: Illustration of soft hand-off.

set and into its candidate set. The mobile then requests a hand-off to that cell. (2) If the cell

has sufficient resources, the mobile switching center will send a message to the base station and

the mobile to begin a hand-off. (3) The mobile moves the pilot to its active set and completes

hand-off. As long as the signal strength remains above a drop threshold (T DROP), the signal

will remain in its active set. The mobile then communicates simultaneously with all base stations

in its active set. Most CDMA systems support at least three-way soft hand-off, with some

supporting up to six-way soft hand-off. (4) When the pilot strength drops below the drop

threshold, the mobile begins a hand-off drop timer. (5) When the hand-off drop timer expires,

the mobile sends a hand-off message to the base station. (6) The base station then acknowledges

receipt of the hand-off request by sending its own hand-off message. (7) Finally, the mobile

terminates its connection and moves the pilot to its neighbor set.

Besides macro-diversity, soft hand-off ensures that a mobile is always communicating

with the strongest base station in its view. In classic hard hand-off techniques, the hysteresis

effect ensures that a mobile does not ping-pong between base stations. However, in doing so, the

mobile is not always communicating with the strongest base station. This is tolerable, although

not optimal, in FDMA/TDMA systems but is a problem in CDMA systems since it means that

the strongest base station is actually causing substantial interference. Soft hand-off avoids this.

Finally, a distinction between soft hand-off between two base stations and soft hand-off

between two sectors of the same base station must be explained. The latter is usually termed softer

hand-off. Soft and softer hand-off look identical to the mobile station since it cannot distinguish

between two cells and two sectors from the same cell. However, it makes a difference on uplink

performance; in softer hand-off, uplink signals can be combined before decisions are made. In

soft hand-off, separate decisions must be made on the uplink signals at the two base stations

and decoded frames sent to the mobile switching center. However, softer hand-off typically

fails to provide the same diversity advantage as soft hand-off.