Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-10-22
2002-12-10
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S439000
Reexamination Certificate
active
06493554
ABSTRACT:
This application is the national phase of international application PCT/FI98/00123 filed Feb. 11, 1998 which designated the U.S.
FIELD OF THE INVENTION
The invention relates to a handover method in a mobile communication system in which error correction of a radio signal can be arranged with different protection levels. The method comprises measuring the signal level and/or quality of base stations in a mobile station, measuring the signal quality and/or level of the mobile station at the base station, comparing the measurement results thus obtained and other variables of the connection with handover criteria, and performing a handover from the source cell to the target cell when the handover criteria are met.
The invention also relates to a mobile communication system for performing handover.
BACKGROUND OF THE INVENTION
In cellular mobile communication systems, a radio coverage area is implemented with a plurality of slightly overlapping radio cells. When a mobile station moves from one cell to another, handover to a new radio cell is performed on the basis of predetermined handover criteria. The aim is to perform handover in a manner that disturbs an ongoing call as little as possible. Handover is normally performed on the basis of radio path criteria, but it may also be performed for other reasons, e.g. to divide the load, or to reduce transmission powers. Handover may also be performed inside a cell from one traffic channel to another.
FIG. 1
of the appended drawings shows a simplified block diagram of the pan-European GSM mobile communication system. A mobile station MS is connected over a radio path to a base transceiver station BTS, in the case of
FIG. 1
to BTS
1
. A base station system BSS consists of a base station controller BSC and base stations BTS controlled by the BSC. A plurality of base station controllers BSC usually operate under the control of a mobile services switching centre MSC. An MSC communicates with other MSCs and, through a gateway mobile services switching centre GMSC, with a public switched telephone network. The operation of the entire system is controlled by an operation and maintenance centre OMC. The subscriber data of a mobile station MS are stored permanently in a home location register HLR of the system and temporarily in the visitor location register VLR in the area of which the MS is located at a given moment.
A mobile station MS and the serving base station BTS
1
continuously measure the signal level and quality of the radio connection for example to determine the need for handover. The MS measures the signals of the serving base station BTS
1
and the base stations BTS that are closest to its location area for instance to select a suitable target cell for handover. In the GSM mobile communication network, for example, an MS may simultaneously measure the signal level of both the serving base station and up to 32 other base stations. Via the serving base station BTS
1
, the MS is informed of the neighbouring cells it should measure. The measurement results of each cell are identified on the basis of the combination of a base station identity code BSIC and the frequency of the broadcast control channel BCCH. The base station BTS measures the signal level and quality of ongoing radio connections at the base station.
The mobile station MS sends the measuring results regularly as a report message through the serving base station BTS
1
to the base station controller BSC. A report message contains the measurement results of the serving base station and up to six best neighbouring base stations. Handover from a serving cell to a neighbouring cell or to another channel of the serving cell may take place, for example, when the measurement results of the mobile station/base station indicate a low signal level and/or quality of the traffic channel of the current serving cell and a higher signal level is achieved in the neighbouring cell or a better signal quality can be achieved with another channel, or when a neighbouring cell/another channel allows communication at lower transmission powers. The signal level and/or load of the target cell, for example, affect the choice of the handover target cell. Generally used handover criteria include the signal level and quality of the radio connection, the signal levels of the source cell and the target cell, the signal quality of the source cell and the transmission power required of and allowed for a mobile station in the target cell. Handover from the source cell to the target cell is performed when the handover criteria set by an operator are met. In order for the stability of the mobile communication network to be ensured, the measurement results and parameters used in handover are averaged over a given period of time. The averaging process makes handover less sensitive to measurement results distorted by instantaneous interference or fading.
The base station controller BSC makes the decisions relating to handover. If the target cell is controlled by another BSC, the handover may be performed under the control of the MSC. Another possibility is that handover decisions are always made in a centralized manner in the MSC. If necessary, the BSC gives a handover command to the MS through the BTS.
In a mobile communication system implemented by code division multiple access (CDMA) technology, handover performed in the manner described above is called hard handover. In addition, CDMA systems can use so-called soft handover, in which a mobile station, during a handover, may be simultaneously connected to the network through several base stations. When one of these base stations proves to be better than the others on the basis of its signal, the connections of the mobile station with the other base stations are released, and the call is continued only through the best base station. Soft handover prevents recurring handovers between base stations when a mobile station is located on the periphery of cells.
Transmission errors which deteriorate the quality of a transmitted signal occur on the transmission path when speech or data is transmitted in a digital telecommunication system. Transmission errors occur on the transmission path when a signal is disturbed, for example, on account of multipath propagation, an interfering signal or high background noise level. Error correction of a digital signal to be transmitted, e.g. channel coding and/or retransmission, is used for improving the quality of the transmission and the tolerance of transmission errors. In channel coding, repetition is added to the original bit string of encoded speech or data by error correcting bits calculated from the original signal. In the receiver, the channel coding is decoded in a channel decoder, whereby the signal errors that have occurred during the transmission can be detected or even corrected by means of the correcting bits. Retransmission is used for correcting transmission errors either independently or, for example, in addition to channel coding: the errors in a channel coded transmission are corrected by retransmission of distorted frames. When the quality of the connection deteriorates, the number of erroneous and lost frames grows, and thus also the number of retransmissions grows.
Channel coding increases the number of bits to be transmitted. In the GSM mobile communication system, for example, error correcting bits with a transmission rate of 9.8 kbit/s are added to a full-rate 13 kbit/s speech signal, whereby the total transmission rate is 22.8 kbit/s. The level of the protection provided by channel coding is arranged according to the need. If a large number of data are to be transmitted fast, the amount of channel coding is reduced to allow more payload to be transmitted on the transmission channel. Channel coding may be arranged either to both detect errors occurred during the transmission and correct them or to merely detect them. In the GSM system, bits to be transmitted are divided according to their importance into different classes, in which channel coding is provided at a predetermined level. The different elem
Aalto Risto
Kanerva Mikko
Räsänen Juha
Vainikka Jari
Nokia Telecommunications Oy
Pillsbury & Winthrop LLP
Vo Nguyen T.
LandOfFree
Handover in a mobile communication system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Handover in a mobile communication system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Handover in a mobile communication system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2929564