Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-12-03
2004-01-06
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S338000, C455S439000, C455S442000
Reexamination Certificate
active
06674736
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for performing idle handoff in a multiple access communication system. In addition, the present invention relates to an improved method for assigning a traffic channel in a multiple access communication system. The invention also relates to a method for reducing the number of required handoffs which occur while a mobile station is queued and waiting for a traffic channel in a multiple access communications system.
II. Description of the Related Art
Typically, communications systems prohibit handoffs while the mobile station is in a system access state. The system access state is the state in which communications are initiated either by the mobile station by means of transmissions over an access channel or by a base station by means of transmissions over a paging channel. In the exemplary embodiment, messages are sent in accordance with a code division multiple access (CDMA) communication format, which is disclosed in detail in U.S. Pat. No. 4,901,307 entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters” and U.S. Pat. No. 5,103,459 entitled “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System, ” both of which are assigned to the assignee of the present invention and are incorporated by reference herein. The use of paging and access channels for call initiation is well known in the art and is detailed in TIA/EIA Interim Standard IS-95-A, entitled “Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System. ”
One of the characteristics of CDMA systems is that the same frequency is reused in every cell. Diversity combining is a method by which a receiver receiving signals carrying the same information combines those signals which are propagated through different paths to provide an improved estimate of the transmitted signal. A receiver design for taking advantage of the diversity signals carrying the same information but traveling through different propagation paths or transmitted by different transmitters is described in detail in U.S. Pat. No. 5,109,390 entitled “Diversity Receiver in a CDMA Cellular Telephone System, ” assigned to the assignee of the present invention and incorporated by reference herein.
Soft handoff is a method by which a mobile station moving from one cell into another receives information from the base stations serving the two or more cells of the boundary area as long as the mobile station is located near the boundary. The signals which are sent by the base stations are combined in the receiver of the mobile station by the diversity combining method mentioned above. A method and system for providing soft handoff in a CDMA communication system, where a plurality of base stations are in communication with a mobile station at or near cell boundaries is disclosed in U.S. Pat. No. 5,101,501 entitled “Method and System for Providing a Soft Handoff in a CDMA Cellular Telephone System, ” and U.S. Pat. No. 5,267,261, entitled “Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications System,” both of which are assigned to the assignee of the present invention and incorporated by reference herein. Hard handoff is, in contrast to soft handoff, where a mobile station passing from one cell to another is dropped by the cell being exited prior to being picked up by the cell being entered.
The use of the same frequency in every cell and the use of soft handoff result in high CDMA system capacity. The reuse of the same frequency in the neighboring cell causes rather rapid changes in the forward link signal-to-noise ratio near cell boundaries. This is because the cell being received by the mobile station may fade and the neighboring cell may increase in strength (anti-fade).
Generally, when the mobile station is receiving two cells, the received traffic channel energy per spreading chip to total spectral noise density for the signal transmitted by cell
1
is given by equation (1) below:
E
c
I
o
1
=
E
c
I
or
1
I
oc
I
^
or1
+
I
^
or2
I
^
or1
+
1
(
1
)
And the received traffic channel energy per spreading chip to total spectral noise density for the signal transmitted by cell
2
is given by equation (2) below:
E
c
I
o
2
=
E
c
I
or
2
I
oc
I
^
or2
+
I
^
or1
I
^
or2
+
1
(
2
)
where, in equations (1) and (2),
I
oc
is the total thermal noise,
E
c
I
or
1
,
E
c
I
or
2
are the fractions of traffic channel power transmitted by cell
1
and cell
2
respectively, and
Î
or1
, Î
or2
are the fractions of traffic channel power received at the mobile station from cell
1
and cell
2
respectively.
Consider I
oc
to be small relative to Î
or1
and Î
or2
. When cell
1
fades relative to cell
2
, Î
or1
becomes small relative to Î
or2
(the ratio
I
^
or2
I
^
or1
becomes large). Thus,
E
c
I
o
1
becomes small. If the mobile station is not in soft handoff, then this change in signal-to-noise ratio can cause problems. However, if the mobile station is in soft handoff with the neighboring cell, then the change in signal-to-noise ratio is not a problem because the mobile station is performing diversity combining of the forward traffic channels from both cells. While the first path given by
E
c
I
o
1
becomes small, the second path given by
E
c
I
o
2
becomes large. Thus, fading by one cell increases the signal-to-noise ratio from the other cell.
Paging is a method of sending information to a mobile station indicating the initiation of mobile terminated service or to receive an indication of new overhead information. A method for initiating a base station initiated call is described in detail in U.S. Pat. No. 5,392,287, entitled “APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE COMMUNICATIONS RECEIVER” and in U.S. patent application Ser. No. 08/206,701, filed Mar. 7, 1994, now U.S. Pat. No. 5,509,015, issued Apr. 16, 1996 to Geib et al., which is a continuation of U.S. Pat. No. 5,392,287, both of which are assigned to the assignee of the present invention and are incorporated by reference herein. The present invention is equally applicable to mobile station-initiated calls, the method for which is described in detail in copending U.S. patent application Ser. No. 08/219,867, filed Mar. 30, 1994, entitled “APPARATUS AND METHOD FOR REDUCING MESSAGE COLLISION BETWEEN MOBILE STATIONS SIMULTANEOUSLY ACCESSING A BASE STATION IN A CDMA CELULAR COMMUNICATIONS SYSTEM, ” now U.S. Pat. No. 5,544,196, issued Aug. 6, 1996 to Tiedemann, Jr., et al., which is assigned to the assignee of the present invention and is incorporated by reference herein.
In a slotted paging system, a mobile station monitors the paging channel for a short predetermined interval of time and then does not monitor the paging channel again until the next predetermined time interval. In IS-95-A this method of periodically monitoring the paging channel is called slotted mode and the mobile station may monitor the paging channel for 80 ms. every 1.28 seconds. The period between monitoring intervals can be made longer as desired by the user. Before each predesignated time slot in which a mobile station may be paged, the mobile station wakes up (becomes active) and resynchronizes or improves its synchronization with the base station. The mobile station then monitors for pages or other messages in the slot. After some interval, the mobile station can become inactive and not monitor the paging channel until just before the next assigned slot.
Prior to the time when a mobile station is actively communicating traffic information with the mobile communication system and after the time when the mobile station has achieved timing synchronization with the communication system, the mobile station is in a state referred to as the idle state. In the idle state, the mobile station can receive messages, receive an incoming call, initiate a call, i
Baker Kent D.
Elallam Ahmed
Kizou Hassan
Martin Roger W.
Qualcomm Incorporated
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