Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-05-03
2001-05-15
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S442000
Reexamination Certificate
active
06233455
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to wireless communication systems and in particular to wireless systems utilizing Code Division Multiple Access (“CDMA”). More particularly, the present invention relates to maximizing sector capacity of a given system by improving call drop probability. Still more particularly, the present invention relates to improving soft handoff.
2. Description of the Related Art
CDMA, as specified by CDMA standard IS95, is a form of digital cellular phone service and generally offers increased capacity over other types of digital cellular phone service. Each phone call is combined with a code that is broadcast across a broad frequency spectrum and another phone, which is aware of the code, receives the signal among all the other signals that may be occupying that frequency band. By coding the signal so that only one phone may receive the signal, more transmissions on the same band are allowable. Each cell may be partitioned, by directional antennas, into a number of sectors. Sectors may have different pilot signals and users and resources are shared by the multiple sectors within a cell.
CDMA power requirements represent both an advantage and a disadvantage to the system. In CDMA, strong signals overpower weak signals because the noise level is raised at the base station demodulators to accommodate the strong signal. The noise level problem is somewhat overcome by power control. The base station samples signal strength indicators of each mobile and sends a power change command to the mobiles increasing or decreasing the power requirement as a function of the grade of service requirement. This causes a nearby, strong mobile to decrease its power output and a mobile with a weak signal to increase its power output.
CDMA also provides a feature called “soft handoff.” “Hard handoff,” as opposed to soft handoff, is the process a wireless phone (mobile or handset) goes through as it approaches the boundary of a new cell. The network automatically drops resources in the current cell and hands off the connection to the new cell the mobile may be entering. Soft handoff allows a mobile to maintain resource connections with multiple base stations while moving within a system, adding and dropping connections as necessary.
IS95 (digital CDMA standard for U.S. cellular radio systems) soft handoff allows an individual handset to maintain a connection with as many as six individual pilot signals. As a mobile demodulates received information and sends modulated information, the mobile is constantly searching for pilot signals. A pilot signal (identifier channel, designated P
1
, P
2
, P
3
, etc.) broadcast from each sector of each base station (fixed station for communication between a network and mobiles within base station cells), is unique to that sector and is identified with a unique code—a PSEUDO NOISE (“PN”) sequence. If a handset (“mobile”) detects a new pilot, not yet in communication with the mobile, whose pilot strength (carrier to total interference ratio) is above an upper signal strength threshold (T_ADD), it will send a Pilot Strength Measurement Message (“PSMM”) to the network via the sector(s) base station with which it is currently communicating. The PSMM is sent to request that this new sector be added to the mobile's “active set” (a set, on board the mobile and network containing sectors that are currently in communication with the mobile). The network will instruct the mobile to add this new pilot via a Handoff Direction Message (HDM) sent out by all the sectors in the mobile's current active set. The HDM includes parameter settings based on changes in signal strengths, number of pilots in an active set and new parameter values introduced by the system operator. The mobile, on receiving the message, will add this new sector to the active set utilizing the parameters provided in the HDM and acknowledge via a third message, the Handoff Completion Message (HCM). If the mobile detects that a current, active set pilot signal strength (carrier to total received signal ratio) has dropped below a certain lower signal strength threshold (T_DROP) and has remained below that threshold for a pre-determined period of time (T_TDROP), then a PSMM is sent to the network, requesting that such a sector be dropped from the active set. The HDM and HCM follow in order as explained above. Upon receiving the HCM, the network acknowledges the HCM by sending a BSAO (Base Station Acknowledgment Order).
If a PSMM is sent by a mobile requesting addition of a particular pilot into the mobile's active set, the network may choose not to add such a Pilot. In order to reduce excessive messaging back and forth, the network may not send an HDM because the mobile would just resend the PSMM. However, the network must acknowledge the PSMM to prevent the mobile from continuously sending the PSMM. In such cases, the network acknowledges the PSMM with a Base Station Acknowledgment Order (BSAO), but doesn't send the HDM. This action prevents the mobile (standard action) from sending a PSMM requesting addition of this particular pilot again, until the pilot exceeds the strength of the current, weakest active set pilot by a factor “T_COMP” (T_COMP is an assigned value for triggering a decision by the network to add a new channel to the active set. New pilot signals must exceed the weakest active set pilot signal plus T_COMP. The IS95 standard requires T_COMP to take on positive values.). As the new pilot exceeds T_COMP (plus weakest signal), a completely new PSMM will be triggered. If the message is again ignored (BSAO sent, not HDM), then another PSMM will be sent only if this particular pilot exceeds the weakest pilot in the active set by T_COMP. Keep in mind that should anything else change, i.e. another pilot needing entry to the active set or a current active set ready to drop a pilot, a subsequent PSMM will be sent.
In an active set, as indicated before, there may be as many as six sectors allocating resources for an individual handset. For example, a mobile may have an active set of P
1
, P
2
, P
3
, and a new pilot, i.e., P
4
may be increasing in strength so much so (mobile is in motion within cells causing pilots in an active set to change strength) that P
4
equals or exceeds P
3
. Consequently P
4
's signal strength generates interference with P
3
. T_COMP should be set so that a PSMM is triggered when the signal to noise ratio is such that the mobile would be dropped from the system because of too much interference. Another way to add a new pilot is that if a current, active set pilot, P
3
, drops below T_Drop (a system specified arbitrary value) for a specified time period (T_TDROP), usually four seconds, a PSMM is triggered. The PSMM requests the removal of the sector from the active set because the pilot C/I (carrier to total interference ratio) dropped below T_Drop for T_TDrop seconds. The HDM and HCM follow in order as explained above
Referring now to
FIG. 4A
, a graph illustrating signal strength of P
3
, as received by a mobile, is illustrated. P
3
400
is a pilot signal increasing in strength as the mobile moves within the system. An active set (not shown), consisting of P
1
and P
2
, will be considering P
3
400
as a candidate for the active set. As the mobile moves closer to P
3
, T_ADD
402
is reached (−6 db from P
2
, the weaker signal in the active set) and a PSMM is sent to a base station. In this instance, P
3
is not added because Delta
—
3 (arbitrary value for three pilots in an active set, set by the system) condition is not met, and P
3
continues to increase in strength to −8 db which is the signal strength of P
2
. If the mobile continues moving towards P
3
, the next PSMM to be sent will be PSMM
404
. The signal strength of P
3
when PSMM
404
is sent, meets or exceeds T_COMP
406
(including weakest signal strength) which is the trigger for PSMM
404
.
Referring to
FIG. 4B
, a chart illustrating constants required to add and drop pilot signals to an active set, i
Chheda Ashvin
Jalali Ahmad
Ramakrishna Deepa
Bracewell & Patterson L.L.P.
Crane John D.
Nortel Networks Limited
Vo Nguyen T.
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