Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-12-30
2001-02-13
Eisenzopf, Reinhard J. (Department: 2745)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S450000, C455S522000, C370S331000
Reexamination Certificate
active
06188906
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless telecommunications and more particularly to a method for coverage optimization of a multi-frequency assignment (multi-FA) system.
DESCRIPTION OF THE RELATED ART
In a code division multiple access (CDMA) wireless communications system, a spread spectrum technique is employed which enables the same frequency spectrum to support multiple communication signals. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques, thus permitting an increase in system user capacity.
Often, a CDMA base station uses only a single CDMA frequency band to support communications to subscriber stations within the cell. However, in a geographical area where a large volume of telephone traffic exists, such as in the center of a city, a multi-frequency assignment (multi-FA) method for accommodating a large number of subscribers is employed. With the multi-FA approach, two or more radio frequency (RF) channels are operated simultaneously, with each RF channel supporting multiple communication signals via CDMA. In practice, it is important to maintain approximately equal geographical coverage for the frequency assignments. In this manner, system resources per frequency assignment are used uniformly; hence, the subscriber load is distributed equally among the allocated frequency bands.
In the typical CDMA system, each base station transmits a unique pilot signal on a pilot channel (using the same frequency band as the associated traffic channels). The pilot signal is an unmodulated, direct sequence, spread spectrum signal transmitted continuously by each base station using a common pseudorandom noise (PN) spreading code. Each base station or base station sector transmits the common pilot sequence offset in time from the other base stations or sectors. The subscriber stations can identify a base station based on the code phase offset of the pilot signal that it receives from the base station. The pilot signal also provides a phase reference for coherent demodulation and the basis of the signal strength measurements used in handoff determination. In a multi-FA system, each frequency band has its own pilot channel. In practice, it is necessary to determine the power allocation for the pilot channel relative to the traffic channels.
Generally, the following tests are performed to maintain coverage for each frequency assignment in a multi-FA system:
1. Forward Link (base station to subscriber station):
traffic channel calibration;
determine output characteristics of transmit intermediate frequency (Tx IF) level;
frequency accuracy test;
pilot time error;
time error between pilot channel and code channel;
spurious measurement;
total power measurement;
pilot power measurement.
2. Reverse Link (subscriber station to base station):
output characteristics of Receive (Rx) IF level;
Automatic Gain Control (AGC) measurement;
Rx IF cable test.
If the above tests satisfy a predetermined specification, coverage for the associated frequency assignment is maintained.
One method for maintaining equal coverage for each frequency assignment is as follows:
(1) At the base station, a transmit attenuator value, “Tx_Atten”, is confirmed for a first frequency assignment FA#
1
of each sector in current operation. (The value of Tx_Atten controls adjustment of the transmit power level.);
(2) The output RF transmit power for FA#
1
per each currently operated sector is measured;
(3) A value for Tx_Atten of a second frequency assignment, FA#
2
, is set equal to that for FA#
1
;
(4) The output RF transmit power for FA#
2
per each sector is measured and equalized with the RF output of FA#
1
by adjusting the value of Tx_Atten for FA#
2
;
(5) The final value for Tx_Atten of FA#
2
is optimized; and
(6) The optimized value for Tx_Atten of FA#
2
is output, e.g., to an operator who manages parameters of a base station manager (BSM).
For CDMA systems, the conventional optimization method outlined above is limited to the case in which there is a constant number of RF channels in all base stations.
For base stations that handle low traffic volume, such as those located in outer regions of a city, it is unnecessary to provide a large number of frequency assignments. However, a handoff problem may occur between adjacent base stations that have a different number of frequency assignments. For example, assume that two base stations A and B are adjacent, FA#
1
and FA#
2
are being used in base station A and only FA#
1
is being used in base station B. A mobile station can not search the pilot of base station B when the mobile station is tuned to FA#
2
of base station A. As a result, when this mobile station is traveling towards base station B, a handoff to base station B can not be performed (absent methodology to handle this situation) and it is highly probable that the call will be dropped due to the link deterioration. To address this problem, a base station handling low traffic volume as in the above case may adopt a method that utilizes a pilot channel for handoff, rather than a traffic channel. Such a frequency assignment is sometimes referred to as a dummy frequency assignment. The “dummy pilot channel” used for this handoff is within the dummy frequency assignmnt, which is an unused frequency band of the low traffic volume base station (e.g., within FA#
2
in the above example).
A similar handoff problem is addressed in U.S. Pat. No. 5,649,000, which discloses a method for providing a different frequency handoff in a CDMA cellular telephone system. A mobile unit measures the strength of all pilot signals emanating from surrounding base stations. A different frequency handoff is initiated by the mobile unit when all pilot signals are lower than a threshold, or is initiated by a system controller with consideration given to the frequency band occupation state of surrounding base stations and the strength information reported from the mobile unit.
SUMMARY OF THE INVENTION
The present invention is intended to provide a method for optimizing the geographical coverage for each frequency assignment of a multi-frequency assignment wireless communications system employing a dummy pilot. The coverage is optimized by equalizing handover boundaries for each frequency assignment between adjacent base stations, even when the base stations have a different number of frequency assignments. In this manner, the subscriber load among the frequency bands is equally distributed, so that system resources per FA are used in a more uniform manner. The method is particularly advantageous for a multi-FA CDMA system.
In an illustrative embodiment of the present invention, there is provided a method for uniformly using the system resources of each frequency assignment of a multi-FA wireless communications system employing a dummy pilot channel. The communications system includes a first base station configured to transmit communication signals to subscriber terminals using at least a first frequency allocation (FA#
1
) and a second frequency allocation (FA#
2
). A second base station transmits communication signals to subscriber terminals using FA#
1
substantially more frequently than FA#
2
. The second base station has a dummy pilot at FA#
2
which is used for handoff at FA#
2
. The method equalizes coverage in the first base station at FA#
1
and FA#
2
by transmitting the dummy pilot with less RF power than that of a pilot channel at FA#
1
of the second base station. The dummy pilot RF power is sufficient to substantially equalize a handoff boundary between first and second cells at both FA#
1
and FA#
2
in an environment of more communication traffic and associated interference at FA#
1
than at FA#
2
.
REFERENCES:
patent: 4639914 (1987-01-01), Winters
patent: 5551057 (1996-08-01), Mitra
patent: 5594718 (1997-01-01), Weavers, Jr. et al.
patent: 5649000 (1997-07-01), Lee et al.
patent: 5771451 (1998-06-01), Takai
Lim Young-sik
Wee Pyeong-hwan
Appiah Charles N.
Dilworth & Barrese LLP
Eisenzopf Reinhard J.
Samsung Electronics Co,. Ltd.
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