Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-09-21
2001-09-18
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000, C455S438000, C455S439000, C455S442000, C455S452200, C455S525000
Reexamination Certificate
active
06292661
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of implementing a handover in a cellular radio system, the system comprising: in each cell, at least one base station which communicates with subscriber terminals located within its area, which transmits at least at one carrier frequency and which broadcasts information about itself on control channels to subscriber terminals; in at least one cell, at least two subcells which comprise an antenna unit and in which all available frequencies are dynamically used; and, at each frequency, a signal to be transmitted, the signal being divided on a time-division basis into frames comprising a plural number of time slots; the base station measuring through all antenna units the power level of a traffic channel used by a subscriber terminal, on the basis of which an antenna unit providing the strongest signal is selected for communication.
The invention also relates to a method of implementing a handover in a cellular radio system, the system comprising: in each cell, at least one base station which communicates with subscriber terminals located within its area, which transmits at least at one carrier frequency, and which broadcasts information about itself on control channels to subscriber terminals; in at least one cell, at least two subcells which comprise an antenna unit and in which all available frequencies are dynamically used; and, at each frequency, a signal to be transmitted, the signal being divided on a time-division basis into frames comprising a plural number of time slots; the base station measuring through all antenna units the strength of a signal of a subscriber terminal on a traffic channel of an old cell, an antenna unit providing the strongest signal being selected on the basis of the measurements for a handover to a new cell.
DESCRIPTION OF RELATED ART
In cellular radio systems, a user's speech and data between a base station and a subscriber terminal are transmitted on a traffic channel. Between a base station and a subscriber terminal are also needed various control messages and system information, which are transmitted on control channels. An example of a control channel that can be mentioned is a BCCH channel, which is used in the GSM system for transmitting connection set-up information from a base station to subscriber terminals.
In the current GSM system, a carrier frequency that comprises a BCCH channel, i.e. a BCCH carrier frequency, is transmitted uninterruptedly at a constant power level. A subscriber terminal continuously measures the power level of the BCCH carrier frequencies transmitted by adjacent base stations and reports the measurement results to the base station serving the subscriber terminal. On the basis of the measurement results, the system decides an appropriate moment for a handover to another base station.
Effective utilization of a frequency spectrum is one of the main objectives in cellular radio systems. Most channel allocation methods are based on the reuse of frequencies beyond a given interference distance. The conventional GSM system is implemented by using Fixed Channel Allocation (FCA). In this method the frequencies available for use in the system are divided into groups and fixedly allocated to different cells. The frequency groups can be reused in cells which are at a sufficient distance from each other. The reuse distance is determined by the level of co-channel interference that the system tolerates.
If the traffic load estimate concerning each cell is correct and the traffic loads do not vary greatly, fixed channel allocation functions reasonably well in macrocells, i.e. in cells the size of which is between one kilometer and several dozens of kilometers. A disadvantage in fixed channel allocation is that precise and laborious frequency planning is required to allow co-channel interference to be minimised. In addition, fixed channel allocation adjusts poorly to variations in traffic loads.
The above disadvantages in fixed channel allocation are emphasised in cellular radio systems with a small cell size, such as office systems, where variations in radio traffic load are usually larger than in macrocell systems, which complicates radio network planning. The load of radio traffic grows strongly in environments using small cells, and the network should be able to easily adapt to the increase in traffic. The adding of new cells to a network applying fixed channel allocation requires, however, that frequency planning as a whole is renewed.
BRIEF SUMMARY OF THE INVENTION
To obtain a solution to the above-mentioned problems, a more flexible and more adjustable channel allocation method than the one provided by fixed channel allocation is required. In the purest form of Dynamic Channel Allocation (DCA), all frequencies of the system are available in every cell. No frequency group is allocated to a particular base station; instead, all channels can be taken in use at any base station. Channel selection can be made at the moment of call set-up on the basis of the interference situation at the time concerned. The major advantages of dynamic channel allocation are its flexibility in connection with different traffic loads and its greater efficiency in the utilization of the frequency spectrum. In addition, frequency planning is no longer needed. As cell size diminishes, the advantages of and the need for dynamic channel allocation become further emphasised.
An intermediate form of fixed and dynamic channel allocation is a channel allocation method in which some of the channels are fixedly allocated and others are dynamically available. Also in such a method the advantages offered by dynamic channel allocation can be utilized.
In cellular radio systems that comprise not only conventional cells but also cells applying dynamic channel allocation, handover situations involve technical solutions that require particular attention. A cell applying dynamic channel allocation comprises at least two subcells which, in turn, comprise an antenna unit. When a handover to a cell applying dynamic channel allocation is performed in the system, the system does not know which antenna unit in the new cell is closest to the subscriber terminal involved in the handover. In a cell applying dynamic channel allocation a BCCH carrier frequency is therefore usually transmitted through all antenna units. But because of this, the traffic channels of the BCCH carrier frequency cannot be reused within the area of the cell, so the system capacity is not the best possible. In addition to the above mentioned problem, the limited number of traffic channels in a BCCH carrier frequency also sets a permanent upper limit to the handover capacity of a dynamic cell.
An object of the present invention is thus to implement a handover in a cellular radio system so as to provide improved system capacity.
This is achieved with a method described in the preamble, characterized in that only BCCH carrier frequency time slots comprising control channels are transmitted in a cell all the time through all antenna units; and characterized in that in a handover situation, a base station controller controlling the cell selects in the cell for the handover a traffic channel which is not in use within the area of the cell, and the base station of the cell starts to transmit the traffic channel through all antenna units, the handover to the cell being performed thereafter to the traffic channel concerned; and characterized in that through an antenna unit selected for communication the base station measures the quality of all traffic channels available in the cell, on the basis of which a traffic channel is selected for communication and a handover to the traffic channel concerned is performed.
Alternatively, a method of the invention is characterized in that only those BCCH carrier frequency time slots which comprise control channels are transmitted through all antenna units; and characterized in that in a handover situation, a base station controller controlling a cell informs the base station about the traffic channel used by
Patronen Petri
Posti Harri
Ryynänen Jari
Silventoinen Marko
Hunter Daniel
Nokia Telecommunications Oy
Pillsbury & Winthrop LLP
Zewdu Meless
LandOfFree
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