Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-09-01
2003-10-14
Vu, Huy D. (Department: 2732)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C370S342000, C370S479000, C455S438000, C455S442000
Reexamination Certificate
active
06633554
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to wireless networks and, more specifically, to a system for performing an idle handoff of a mobile unit between base stations during a system access in a CDMA wireless network.
BACKGROUND OF THE INVENTION
In 1996, more than 75 million people worldwide used cellular telephones. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as AT&T and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
Wireless subscribers use a wide variety of wireless devices, including cellular phones, personal communication services (PCS) devices, and wireless modem-equipped personal computer (PCs), among others. The large number of subscribers and the many applications for wireless communications have created a heavy subscriber demand for RF bandwidth. To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Although TDMA-based wireless networks were developed on a large scale first, CDMA-based wireless networks are now also widely used. As is well known, CDMA systems divide the RF spectrum into a number of wideband digital radio signals. Each digital radio signal carries several different coded “channels”. Each coded channel is distinguished by a unique pseudo-random noise (PN) code used by the mobile station and/or the base station. In a CDMA receiver, the coded channels are decoded by a signal correlator that matches PN sequences. Some coded channels are used as data traffic channels to transport subscriber voice and/or data signals, while other coded channels are used as control channels, including pilot, synchronization, paging and access channels. Some CDMA systems may use an analog control channel to allow a subscriber's wireless device (or mobile station) to access the CDMA system.
When a mobile station accesses a base station in a CDMA wireless network, the CDMA control channel (which has one unique PN code) assigns the mobile station to a data traffic channel (which has a different PN code) on which the mobile station exchanges data traffic with another party, a data terminal, a fax machine, or the like. Typically, the coded control channel and the data traffic channel(s)s used by the mobile station and the base station are on the same RF carrier frequency. Advantageously, in many CDMA networks, the control channels and the data traffic channels are on the same RF carrier frequency in all cells (i.e., base station coverage areas) in the networks.
Because adjoining base stations in the CDMA network are operating on the same RF carrier frequency, a mobile station can simultaneously access two (or more) base stations by using the same PN codes to communicate in data traffic channels and control channels with each base station. This enables a process known as “soft handoff” to occur when a mobile station communicating with a first base station enters a second base station's cell site. A soft handoff may occur while a mobile station is actively communicating with another device (e.g., a voice phone call). A soft handoff also may occur when the mobile station is turned “ON” and has accessed the CDMA network, but is not actively communicating with another device. This is known as an “idle” handoff.
In a CDMA wireless network, call setup failures frequently occur due to very quick propagation environment changes, such as when a mobile station moves behind a hill or a large building. This can cause problems when the mobile station is in the middle of a system access operation. For example, when a mobile station sends an Origination message to a first base station or receives a Page message from that first base station, the mobile station enters a “system access” state during which the mobile station cannot undergo a handoff to a second base station. If the RF propagation environment changes quickly enough, the mobile station may not receive a channel assignment message from the first base station and the data traffic channel will not be set up. In conventional CDMA networks, the mobile station cannot be put into a soft handoff at the time of initial channel assignment. A soft handoff setup is initiated only after a mobile station is assigned to a traffic channel and is sending a Pilot Strength Measurement message to the network. As a result, the attempted set up process with the first base station and all related messages are lost, and the mobile station must start over with another base station.
There is therefore a need in the art for improved CDMA wireless networks that allow a mobile station to perform an idle handoff while in a system access state in order to reduce the number of lost messages caused by changes in the RF propagation environment. In particular, there is a need in the art for improved CDMA systems that more rapidly initiate a soft handoff procedure for a mobile station to thereby reduce the call setup failure rate of the CDMA network. More particularly, there is a need for CDMA systems that can initiate a handoff procedure without waiting for a Pilot Strength Measurement message from the mobile station.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a CDMA wireless network, a base station capable of wirelessly communicating with a mobile station. According to one embodiment of the base station comprises a receiver capable of receiving a system access message transmitted by the mobile station, wherein the system access message initiates in the wireless network a system access procedure operable to establish a communications channel between the mobile station and the CDMA-based wireless network. The base station also comprises a handoff controller, coupled to the receiver and receiving the system access message therefrom, the handoff controller capable of transferring control over the system access procedure from the base station to a first selected base station in the CDMA-based wireless network, wherein the handoff controller sends to the first selected base station a handoff control message operable to initiate in the first selected base station a handoff procedure capable of assuming control over the system access procedure.
The present invention therefore introduces the novel concept of speculatively initiating a handoff procedure to another base station without waiting for a message from the mobile station that initiates the handoff procedure. This allows a handoff procedure to be performed more rapidly and allows the handoff procedure to be initiated while the mobile station is still in a system access state, rather than in a conversation state.
In one embodiment of the present invention, the system access message comprises signal strength indicia indicating a strength of at least one signal transmitted by at least one base station in the wireless network.
In another one embodiment of the present invention, the handoff controller sends the handoff control message to the first selected base station in response to a signal strength indicia in the system access message corresponding to a signal transmitted b
Phan Tri H.
Samsung Electronics Co,. Ltd.
Vu Huy D.
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