Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1998-07-13
2001-08-14
Vu, Huy D. (Department: 2664)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C455S515000, C455S434000
Reexamination Certificate
active
06275487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is chat of digital cellular mobile radio systems such as those conforming to the GSM public land mobile radio network standard.
The expression “GSM standard” here means not only GSM standard 900 covering GSX mobile radio systems operating in the 900 MHz band but also the DCS 1800 standard covering systems operating in the 1800 MHz band.
To be more precise, the invention concerns a particular BCCH carrier structure and a method of measuring the signal level received by a mobile station on a BCCH carrier of this kind.
2. Description of the Prior Art
A digital cellular mobile radio system is generally implemented within a network of geographical cells through which mobile stations travel. A base station is associated with each cell and a mobile station communicates via the base station associated with the cell in which it is located.
Each base station (and therefore each cell) uses one or more pairs of radio carriers, necessarily including a pair of BCCH carriers specific to it The carriers of the same pair are respectively used for the uplink (mobile station to base station) and downlink (base station to mobile station) directions.
In the conventional way, each carrier is segmented temporally using a fixed time-division multiple access (TDMA) scheme. The time axis is divided into successive frames of fixed duration each divided into a particular number of time slots, the recurrence of a particular time slot in each frame constituting a physical channel onto which a plurality of logical channels can be multiplexed.
The uplink BCCH carrier (transmitted by the mobile station) generally supports a logical traffic channel (TCH) which is used to transmit user data or speech and a logical random access channel (RACH) which a mobile station uses to access the network in order to log on in a cell or to make a call.
The downlink BCCH carrier (transmitted by the base station) supports one or more traffic channels multiplexed onto one or more physical channels and the following signaling channels, which are generally multiplexed onto a physical channel consisting of the recurring first time slot of each frame:
a logical broadcast control channel (BCCH) which supplies to all mobile stations general information on the network, the cell in which the mobile station is located and the adjacent cells,
a logical synchronization channel (SCH) which carries information for synchronizing frames and identifying the base station transmitter,
a logical frequency channel (FCH) which provides information on the carrier used,
a logical access grant channel (AGCH) which is used to assign dedicated resources (signaling channel (SDCCH) or traffic channel) to the mobile station requesting them via the random access channel, and
a logical paging channel (PCH) which is used to locate a mobile station and to advise it of a call from the network.
The present invention is specifically concerned with the downlink BCCH carrier which in the remainder of the description is simply referred to as the BCCH carrier.
An essential role of the BCCH carriers is to enable the system to determine the cell in which each mobile station is located so that it can detect when a mobile station moves from one cell to another. A mobile station in a given cell, known as the current cell, monitors the BCCH carriers of adjoining cells continuously and sends the corresponding measurements to the system so that it can determine if the mobile station is moving from one cell to another.
During a call, monitoring of adjoining cells by the mobile station enables the system to decide when handover must take place, i.e. when an adjoining cell must become the new current cell.
Similarly, on standby, monitoring of adjoining cells by the mobile station enables the system to detect a change of location area identity (LAI).
In the conventional way a mobile station monitors the BCCH carriers of adjoining cells by measuring the signal level (i.e. the power) received on each BCCH carrier. The adjoining cell whose identifier has been decoded and whose BCCH carrier is received with the maximal power is then chosen as the new current cell.
At present each base station transmits its BCCH carrier at constant power. It is generally accepted that all time slots of all frames of the same BCCH carrier must be transmitted with the same power. In other words, all physical channels (themselves carrying multiplexed logical channels, i.e. logical signaling channels and traffic channels) are transmitted with a maximum power.
Transmitting a BCCH carrier with a constant power has the major drawback of implying a high level of interference on the BCCH carrier. Consequently, re-use of BCCH carriers in the cells is low. In other words, it is difficult to use the same BCCH carrier again in other cells and the frequency band needed to assure a given traffic is therefore greater, which constitutes a non-negligible economic drawback.
Also, it is not possible to apply to a constant power BCCH carrier interference reduction techniques such as power control or discontinuous transmission (DTX).
An objective of the invention is to alleviate these various drawbacks of the prior art.
To be more precise, one objective of the present invention is to provide a BCCH carrier that can be transmitted at non-constant power, in particular to reduce the level of interference on the BCCH carrier and to increase re-use of BCCH carriers in the various cells.
Another objective of the invention is to provide a monitoring strategy suitable for a BCCH carrier of the above kind transmitted at non-constant power In other words, another objective of the invention is to provide a method of measuring the signal level received by a mobile station on a BCCH carrier of the above kind.
SUMMARY OF THE INVENTION
The above objectives, and others that will become apparent hereinafter, are achieved in accordance with the invention by means of a BCCH carrier transmitted by a base station of a digital cellular mobile radio system to a plurality of mobile stations, the BCCH carrier being segmented temporally into successive frames of fixed duration, each of the frames being itself divided into a particular number N
IT
of time slots; the BCCH carrier supporting a physical BCCH itself carrying at least one logical BCCH, the logical BCCH transmitting general signaling information known as BCCH information, each mobile station, for which the base station transmitting the BCCH carrier is an adjoining base station. having an least one monitoring window in each of at least N
T
successive frames, where N
T
≧1, i.e. at leas. N
F
successive monitoring windows for receiving the BCCH carrier in particular, each monitoring window enabling the mobile station concerned to receive at least N
E
time slots of the BCCH carrier, where N
E
≧1, wherein the BCCH carrier supports at least N
R
physical BCCH transmitted at constant power and consisting in the recurrence in each frame of N
R
time slots known as BCCH time slots, the physical BCCH or each of the N
R
physical BCCH carries at least the logical BCCH, the BCCH time slot or each of the N
R
BCCH time slots containing the BCCH information at least once in N
T
successive frames, the number and the spacing of the BCCH time slot or time slots are such that the BCCH information is entirely received in the monitoring window or one of the at least N
p
successive monitoring windows of each mobile station, and the time slots other than the N
R
BCCH time slots are transmitted with a power that can be controlled.
Thus the BCCH carrier of the invention is not transmitted at constant power, which reduces the level of interference on the BCCH carrier and increases re-use of the BCCH carrier in the various cells.
The general principle of the invention is to transmit at constant power only one or more time slots (called “BCCH time slots” in the remainder of the description), the recurrence of each of these BCCH time slots constituting a physical BCCH carrying the logical BCCH (possibly multiplexed with ot
Le Strat Evelyne
Szalajski David
Alcatel
Harper Kevin C.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Vu Huy D.
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