Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Patent
1996-12-13
1998-08-04
Marcelo, Melvin
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
455442, 455443, H04Q 738
Patent
active
057905282
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to cellular telecommunications systems. More specifically, the present invention relates to a novel and improved system for handoff between a mobile station and base stations which are connected to different mobile switching centers 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. The background of the present invention will be 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 above 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) direction. A typical network configuration of a prior art CDMA telecommunications system is shown in FIG. 1. The base stations BS 10, 12, 14 and 16 convert these CDMA radio signals originating from, respectively terminating at, the user's MS 20 into a form appropriate for 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 direction to the mobile switching center 1 or 2 (MSC, also referred to as mobile exchange or mobile telephone switching office (MTSO)) for further processing.
The above mentioned user communication signals comprise digitized voice signals and control information (also referred to as signalling). The MSC performs multiplexing and conversion operations on the above mentioned tributaries and relays the voice signal to another user, e.g. within the Public Switched Telephone Network (PSTN). The MSC also interprets, reacts upon, and generates signalling information, thus controlling the overall communication link between the 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 spread spectrum multiple access user signals cannot be achieved by means of orthogonal spreading codes alone. Therefore, the communications suffer from system self-induced interference not only among signals ori
REFERENCES:
patent: 5504935 (1996-04-01), Vercauteren
patent: 5697055 (1997-12-01), Gilhousen et al.
A. Salmasi et al., "On The System Design Aspects Of Code Division Multiple Access (CDMA) Applied To Digital Cellular And Personal Communications Networks," 41st IEEE Vehicular Technology Conference. Gateway to the Future Technology in Motion (Cat. No. 91C) Jul. 1991.
Marcelo Melvin
Nokia Telecommunications Oy
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