Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Patent
1996-06-14
1998-02-24
Urban, Edward F.
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
455422, 455436, 455502, 375206, 370331, 370335, 370337, H04Q 700
Patent
active
057220743
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/FI93/00385, filed Sep. 24, 1993.
FIELD OF THE INVENTION
The present invention relates to cellular telecommunications systems. More specifically, the present invention relates to a novel and improved handoff between a mobile station and base stations within a cellular telecommunications system.
BACKGROUND OF THE INVENTION
The use of code division multiple access (CDMA) modulation is but one of several techniques enabling digital communications among a number of mobile users utilizing a common part of the radio spectrum, as is the case for cellular telecommunications systems.
Other well-known radio access techniques are time division multiple access (TDMA) and frequency division access (FDMA). The concept of soft handoff to which the present invention is closely related, is indeed applicable to all three of the mentioned multiple access techniques and will result, if applied instead of the conventional hard handoff schemes, in increased system capacity and fewer dropped calls. However, soft handoff is mandatory for CDMA, as the use of conventional hard handoff would result in a very poor system performance. For these reasons, the embodiment of the present invention is presented for a CDMA cellular telecommunications system, however, it should be understood that the present invention is not limited to CDMA.
An exemplary application of CDMA to cellular telecommunications systems has been substantially described in "On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal Communications Networks", Allen Salmasi and Klein S. Gilhousen, presented at the 41st IEEE Vehicular Technology Conference on May 19-22, 1991 in St. Louis, Mo.
In the just mentioned publication a direct-sequence CDMA (DS-CDMA, or shortly, CDMA in the following) technique is described in which a number of user mobile stations (MSs) communicate via CDMA radio spread spectrum signals with base stations (BSs, also referred to as cell sites) in the uplink (mobile station to base station) and downlink (base station to mobile station) directions. The base stations convert these CDMA radio signals originating from, respectively terminating at, the user's MS into a form appropriate for the use in conjunction with terrestrial telecommunications transmission equipment such as the commonly deployed Pulse Code Modulation (PCM) circuit facilities. The base stations further relay these user signals in the uplink and downlink directions to the mobile switching center (MSC, also referred to as mobile exchanges or mobile telephone switching office (MTSO)) for further processing. Usually the user information signals are transferred on these terrestrial links in a digital, framed format. This frame structure may be derived and adapted from the frame structure used for the radio signals which is typically defined by the voice coding process.
The just mentioned framed user communication signals comprise digitized voice signals and control information (also referred to as signaling). The MSC performs multiplexing and conversion operations on the just mentioned tributaries and relays the voice signal to the other communication user within the Public Switched Telephone Network (PSTN). The MSC also interprets, reacts upon, and generates signaling information, thus controlling the overall communication link between the system users. These communications link control functions comprise the management of general call related events such as call setup or tear down as well as CDMA radio link related events such as the deterioration of the CDMA radio link quality and subsequent handoff initiation.
If CDMA is deployed within the typical medium to large sized cells of land mobile telecommunications systems then the average time delay spread of the multipath radio propagation environment is usually larger than the chip duration of the DS-CDMA signal. This forces CDMA to operate in an asynchronous mode with the consequence that the orthogonality of the spre
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patent: 4696052 (1987-09-01), Breeden
patent: 5267261 (1993-11-01), Blakeney, II et al.
patent: 5345448 (1994-09-01), Keskitalo
patent: 5408517 (1995-04-01), Nyhart et al.
"An Overview of the Application of Code Division Multiple Access (CDMA) to Digital Cellular Systems and Personal Cellular Networks", May 21, 1992, pp. 1-58.
Nokia Telecommunications Oy
To Doris
Urban Edward F.
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