Mobile station capable of determining base station suitability

Details Mobile station capable of determining base station suitability Mobile station capable of determining base station suitability

1999-08-05

2003-03-25

Moore, James (Department: 2682)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S331000, C370S342000, C370S479000, C375S144000, C375S148000

Reexamination Certificate

active

06539006

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed toward a mobile station for use in a wireless communications system and, more particularly, toward a mobile station capable of determining base station suitability.
BACKGROUND OF THE INVENTION
Cellular systems designed according to the TIA (Telecommunications Industry of America) IS-95 standards typically utilize CDMA (Code Division Multiple Access) as the system platform. A CDMA system is a spread-spectrum system permitting multiple transceiver pairs (mobile phones and base stations) to use a wideband frequency channel, e.g., a 1.23 MHz wide channel, for receiving and transmitting messages. Within each wideband frequency channel, Walsh-Hadamard codes, commonly called Walsh codes, are typically utilized to uniquely identify each mobile station. A Walsh code generator in each mobile station transceiver encodes the message data with one of 64 unique orthogonal codes, thus permitting simultaneous communication by 64 mobile stations in each wideband frequency channel.
Base stations within a CDMA system generally use the same Pseudorandom Noise (PN) sequence to spread the signal across the wideband frequency channel. Each base station is uniquely identified in the system by using a unique starting phase, also known as a PN offset, with each base station having the capability to use all 64 Walsh codes. PN offsets thus provide an additional layer of coding in the time domain, permitting reuse of each of the 64 Walsh codes 512 times (currently there are 512 possible PN offsets available for use).
A mobile phone, or mobile station, configured for use in a CDMA system, typically utilizes a RAKE receiver generally having three or four input ports called fingers. The RAKE fingers are tuned to receive specific rays of energy that are transmitted from a base station. These rays can include the primary ray of energy transmitted from a base station, as well as reflections of that ray as the ray travels from the base station to the mobile station. The RAKE receiver receives the three or four rays of energy from the base station, which rays are then demodulated and combined to create a more error free signal.
In addition to traffic signals, each base station also broadcasts a pilot signal and a synchronization (“sync”) signal. The pilot signal broadcast by each base station is formed by a constant level signal that is covered by Walsh code 0, which consists of all zeros. The pilot signal is commonly received by all mobile stations within range of the base station, and is used by the mobile station for identifying the presence of a CDMA system. The sync signal, which is covered by Walsh code 32, includes information concerning the respective base station. By tuning to a particular base station's sync channel, the mobile station can evaluate the suitability of a base station for use.
To detect pilot signals, CDMA mobile stations utilize a receiver-like device called a searcher. The searcher is configured to detect pilot signals broadcast by various base stations and measure their signal strength. The searcher can measure the received signal strength of the base station by tuning to its PN offset and measuring the strength of its pilot signal. The searcher operates independent of the RAKE receiver.
When a mobile station is in an idle mode, that is, when it is not on a call or is not trying to receive or originate a call, the mobile station simply monitors the base station with which it has last communicated. The base station that the mobile station is monitoring is said to be in the “Active Set” of the mobile station. In the idle mode, there is only one base station in the mobile station's Active Set.
Base stations also transmit what is called a Neighbor Set, which is a list of base stations in the general area that may be considered for use if the particular base station's signal becomes too weak. While in the idle mode, the mobile station receives this Neighbor Set of base stations from the currently monitored base station in its Active Set. While the mobile station is monitoring its current base station in its Active Set via its RAKE receiver, the searcher may be configured to measure the signal strengths of the pilot signals from the base stations in the Neighbor Set. Since only one base station is permitted in the mobile station's Active Set, i.e., the base station currently being monitored, all RAKE receiver fingers are required to be tuned to the same PN offset in the idle mode so that their demodulated signals can be additive, implying that they are energy rays or reflected rays of the same base station signal.
Generally, when a mobile station is in the idle mode, it will move to the base station in the Neighbor Set with the strongest pilot signal when the pilot signal of its Active Set base station becomes too weak. This is accomplished by directing the mobile station searcher to measure the pilot signal of the Active Set base station and the pilot signal of each prospective base station in the Neighbor Set. After evaluation of this search, the mobile station may choose to begin monitoring a different base station by tuning the RAKE receiver fingers to the PN offset associated with the different base station. However, only after switching over to monitor the new base station can the mobile station make a complete determination of that particular base station's suitability with the mobile station. If the mobile station determines that the new base station is not suitable for whatever reason, it must re-establish communication with the old base station, or the mobile station may enter a system determination sub-state which may require a substantial amount of time before contact with another base station is established. This is due to the restrictive nature of the RAKE finger assignments, demodulation and signal combination in the idle mode.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
SUMMARY OF THE INVENTION
In a wireless communications system, a method of determining the suitability of a base station for use with a mobile station with the mobile station in an idle mode is provided. The method generally includes the steps of measuring the strength of pilot signals transmitted by a plurality of base stations, assigning a plurality of RAKE receiver fingers to a base station currently monitored by the mobile station for communication purposes, and selectively assigning at least one of the RAKE receiver fingers to a first potential base station in the plurality of base stations, with the first potential base station being different from the currently monitored base station. The data received at the at least one RAKE receiver finger from the first potential base station is demodulated independent of data received at the other RAKE receiver fingers from the currently monitored base station. From the independently demodulated data, it is determined whether the first potential base station is suitable for use with the mobile station. The determined result is stored in a memory, and the above-identified steps are repeated for all potential base stations in the plurality of base stations, the potential base stations being those whose measured pilot signal strength exceeds a threshold value.
In one form, the pilot signal strengths of the potential base stations determined to be suitable for use with the mobile station are monitored and, if the pilot signal strength of the currently monitored base station falls below a threshold value, the currently monitored base station is replaced with one of the potential base stations determined to be suitable for use with the mobile station. In a preferred form, the replaced potential base station includes the potential base station with the strongest monitored pilot signal strength.
The plurality of base stations generally includes a Neighbor Set of base stations transmitted by the currently monitored base station to the mobile station, with the Neighbor Set of base stations including those base stations potentially usable by th

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