Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1993-05-17
2001-10-09
Le, Thanh Cong (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S063300
Reexamination Certificate
active
06301478
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a method for intra cell handover and channel allocation in cellular phone systems, and specifically to a method for reducing interference with other users by handoff of a call causing interference.
2) Discussion of Related Art
In cellular telephone systems, a radio link, i.e., a call, between a mobile station and a base station may be switched to a different channel when prevailing reception conditions make this handoff procedure advantageous. The main handover strategies existing in cellular networks are handover between two different cells, i.e., inter cell handover, and handover within a cell, i.e., intra cell handover. Inter cell handover is made when the path loss between a mobile station (MS) and a neighboring base station (BS) is less than the path loss to the base station it is presently in communication with. Intra cell handover is made when a carrier to interference ratio (C/I) is below a certain limit at the same time the path loss in the cell that the mobile is in communication with is less than another limit. These rules can be expressed as:
Inter cell handover:
Path loss between MS and a neighboring BS <
Path loss between MS and own MS - hysteresis.
Intra cell handover:
C/I in uplink or C/I in downlink < C/I limit
AND
Path loss < path loss limit.
The hysteresis in inter cell handover is needed to avoid repeated handovers at cell borders.
In the TDMA system based on the standard EIA/TIA—IS-54, power is transmitted on all time slots on a carrier in the downlink if at least one time slot is in use. However, the C/I ratio is only checked in the time slot that is currently in use.
In this system, a problem has arisen as follows. The quality, or C/I ratio, in a link depends on both the signal strength C and the interference level I. Both are equally important. When the call suffers from poor quality, a handover should be made in order to find a better channel. A handover order is sent from the base station BS to the mobile station MS and the mobile station MS has to receive the order before it can tune to a new channel. The problem is that the reason for the handover attempt may have been poor quality in the downlink, and thereby the handover order may not be received by the mobile station MS. The call will be lost if the interference in the downlink does not decrease.
The goal for intra cell handover is to find a channel which suffers from less interference in order to improve the C/I ratio. The reason for high interference in the uplink is that some other call is currently using the same channel. The other call may be on the same channel causing interference because it is in communication with a neighboring cell on the same channel or because of misalignment of the time slot due to propagation delay, for example. In the downlink, interference is caused by at least one other call linked to a different cell currently using the same carrier. Even though the quality may be high in both links for a call, the call may severely interfere with another call in the downlink. It is equally important for a user not to interfere with another user as it is for the user not to be subject to interference, but the former cannot be avoided in current intra cell handover criteria.
SUMMARY OF THE INVENTION
The above problems are eliminated or ameliorated by the present invention. When a call is interfering with another call, the present invention has the call causing the interference change channel, rather than requiring the call being subjected to the interference to change channel. This is because the risk of not receiving a handover command is less if the link is not subject to interference. The call being interfered with can be either in communication with a neighboring base station (with the interference being caused by cellular coverage overlap), or in communication with the same base station as the interfering call (with interference being caused by, e.g., propagation delay).
In the existing intra cell handover procedure, only the C/I ratio is checked and then only in the time slot it is allotted. The C/I ratio is checked in the mobile station MS and in the base station BS. However, a call may cause interference on one time slot in the uplink and three time slots in the downlink because the MS only transmits the carrier in its time slot, whereas the BS transmits the carrier in all three when one or more time slots are being used, when the system operates in accordance with IS-54. The latter three time slots are to be checked by the base station BS as well.
The present invention modifies the intra cell handover procedure by introducing a courtesy handover (CHO). If a call in any one time slot in the uplink is interfered with, then it is determined that another call is being interfered with, and the frequency is to be cleared by means of an intra cell handover. The total intra cell handover strategy is outlined below where the last criteria is the courtesy handover.
Intra cell handover:
(C/I in uplink or C/I in downlink < C/I limit
AND
Path loss < path loss limit)
OR
(Interference at most interfered time slot >
Interferenee limit).
The interference limit is a threshold parameter. When a new channel is allocated during call set up, inter cell handover or intra cell handover, the same check is performed. This is especially important when a new carrier is allocated because introduction of a new carrier has the potential of creating a lot of interference.
As meant herein, interference of a call in any time slot includes interference in its own time slot. It is important to clear the frequency when a call is interfering with another call which is also in the same time slot and linked to a different BS.
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Almgren Knut Magnus
Andersson Claes Håkan
Wallstedt Yngve Kenneth
Burns Doane Swecker & Mathis L.L.P.
Cong Le Thanh
Telefonaktiebolaget LM Ericsson (publ)
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