CDMA soft handoff control method

Details CDMA soft handoff control method CDMA soft handoff control method

1998-06-16

2003-07-22

Ton, Dang (Department: 2666)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S335000, C455S442000

Reexamination Certificate

active

06597673

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a wireless link changeover control method (handoff control method) carried out when a mobile station (a car telephone, a portable telephone, etc.) moves from one cell to another cell. More particularly, the invention relates to a CDMA (Code Division Multiple Access) soft handoff control method.
In a cellular mobile telecommunications system, control for changing over a wireless link when a mobile station (a car telephone, a portable telephone, etc.) moves from one cell to another cell is referred to as “handoff”. One method of such handoff control is soft handoff.
According to ordinary handoff (referred to as “hard” handoff as opposed to “soft” handoff), the wireless link between the mobile station and a base station in a first cell is severed at the time of handoff, after which a wireless link is established between the mobile station and a base station in a second cell. By contrast, soft handoff establishes a wireless link between the mobile station and the base station in the second cell while the wireless link between the mobile station and the base station in the first cell is maintained. In other words, soft handoff can be thought of as a handoff technique characterized by the fact that wireless links are established with two (or more than two) base stations at the same time. This technique is often used in CDMA schemes (see the specification of International Application Laid-Open No. WO91/07020).
FIG. 11
is a diagram showing the configuration of a mobile telecommunications system, and
FIG. 12
is a diagram useful in describing the soft handoff procedure. The system shown in
FIG. 11
includes a mobile station (MS)
1
, base stations (A, B)
2
,
3
which communicate wirelessly with the mobile station in cells
4
,
5
, respectively, and a base station controller (BSC)
6
. When the mobile station
1
in cell
4
moves and approaches the side of base station B while communicating with the base station A, the strength of radio waves received from the base station B gradually increases. When the mobile station
1
arrives at the region where the cells
4
and
5
intersect and the electric field strength of the radio waves received from the base station B surpasses a set level, the mobile station
1
notifies the base station controller
6
of this fact via the base station
2
. As a result, the base station controller
6
establishes a wireless link between the mobile station
1
and the base station B without severing the wireless link between the mobile station
1
and the base station A, and the mobile station
1
therefore communicates with the plurality of base stations A, B simultaneously. When the reception field strength between the mobile station
1
and the base station A falls below the set level and remains there for a predetermined period of time, the wireless link between them is severed and the mobile station
1
communicates with the base station B from this point onward.
When the strength of a pilot signal from the base station B exceeds a set level T-ADD in a CDMA scheme, the mobile station
1
sends a pilot strength measurement message to the base station controller BSC via the base station A (
1
a
,
2
a
in FIG.
12
). Upon receiving this message, the base station controller BSC judges whether handoff between the mobile station
1
and the base station B should be executed. If the base station controller BSC decides to execute handoff, then it instructs the base station B of a communication channel CH, etc., allocated to the mobile station
1
. In response, the base station B starts the transmission of a channel designating message on a channel Forward T-CH (
2
b
).
Next, the base station controller BSC transmits a handoff direction message to the mobile station
1
via the base stations A and B (
2
c
). Upon receiving the handoff direction message, the mobile station
1
acquires the communication channel CH, on which transmission is taking place, from the base station B and thenceforth is capable of communicating with the base station B over this channel and sends a handoff completion message to the base station controller BSC via the base station A and B (
1
b
,
2
d
).
The mobile station
1
thenceforth communicates with the base station A, B simultaneously. If the strength of the pilot signal from the base station A falls below the set level and remains there for a set period of time, the mobile station
1
transmits the pilot strength measurement message to the base station controller BSC via the base station A. In response to this notification, the base station controller BSC decides to terminate communication between the mobile station
1
and the base station A and transmits the handoff direction message to the mobile station
1
via the base stations A, B (
1
c
,
2
e
).
Upon receiving the handoff direction message, the mobile station
1
transmits the handoff completion message to the base station controller BSC via the base stations A, B and severs the wireless link with the base station A (
1
d
).
Upon receiving the handoff completion message via the base stations A, B, the base station controller BSC instructs the base station A to stop using Forward T-CH. The base station A responds by stopping transmission/reception on Forward T-CH (
2
f
). From this point onward the mobile station
1
communicates with the base station B.
Soft handoff has the following two characterizing features:
(1) Hitless handoff is possible.
Unlike hard handoff, wireless links can be changed over without conversation being interrupted at the time of handoff.
(2) Inter-cell diversity is possible.
Wireless link quality within a cell usually is poor near the perimeter of the cell. However, enabling communication with a plurality of base stations simultaneously by soft handoff makes possible inter-cell diversity (namely communication in which the base station that is capable of communicating at the lowest power is selected for communication). This reduces susceptibility to the effects of shadowing and fading.
Feature (1) mentioned above generally is noted as being the advantage of soft handoff. In actuality, however, feature (2) is more important because it increases the number of channels that can be accommodated by a base station (i.e., the channel capacity of the base station), as evident from the analysis set forth in the literature [A. J. Viterbi and A. M. Viterbi, “Other-Cell Interference in Cellular Power-Controlled CDMA”, IEEE Trans. on. Commun., Vol. 42, No. 2/3/4/(1994)]. Specifically, if soft handoff is not carried out in CDMA uplink (the wireless link from the mobile station to the base station), interference on other cells increases greatly and the number of wireless channels that can be used decreases significantly (i.e., subscriber capacity declines greatly). In order to assure CDMA uplink channel capacity, therefore, soft handoff is an essential technique. Since communication with soft handoff is performed upon selecting the base station that is capable of communicating at the lowest power, interference with other cells is reduced and the number of wireless channels (channel capacity) that can be used is increased.
FIGS. 13A
,
13
B are diagrams useful in describing the effect of raising uplink channel capacity by soft handoff, in which
FIG. 13A
shows a case where soft handoff is not carried out and
FIG. 13B
a case where soft handoff is carried out.
Consider the situation where soft handoff is not performed (FIG.
13
A). If a great deal of shadowing is caused by a building BLD or some other structure between a mobile station MS
1
and a base station BS
1
in a cell CL
1
, the mobile station MS
1
controls transmission power in such a manner that the incoming call level at the base station BS
1
will take on a stipulated value. Transmission power of the mobile station MS
1
rises as a result. When this takes place, interference (see the arrow indicated by the dashed line) with respect to a cell of interest (cell CL
0
) increases and a limitation is imposed upon the number of mobile

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