Multiplex communications – Crosstalk suppression
Reexamination Certificate
1997-11-26
2001-02-27
Yu, Huy D. (Department: 2739)
Multiplex communications
Crosstalk suppression
C370S203000
Reexamination Certificate
active
06195327
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to wireless telecommunications systems and more particularly to techniques for controlling interference in a cell of a wireless telecommunications system.
BACKGROUND OF THE INVENTION
A wireless telecommunications system has been proposed in which a geographical area is divided in to cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
Due to bandwidth constraints, it is not practical for each individual subscriber terminal to have its own dedicated frequency channel for communicating with the central terminal. Hence, techniques need to be applied to enable data items relating to different communications to be passed over the same frequency channel without interfering with each other. In current wireless telecommunications systems, this can be achieved through the use of a ‘Code Division Multiple Access’ (CDMA) technique, whereby a set of orthogonal codes may be applied to the data items to be transmitted on a particular frequency channel, data items relating to different communications being combined with different orthogonal codes from the set.
Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal channel within a particular frequency channel. Hence, if a set of 16 orthogonal codes are used, 16 orthogonal channels can be created within a single frequency channel, and hence up to sixteen separate communication signals (corresponding to sixteen separate wireless links) can be transmitted simultaneously over the single frequency channel if different orthogonal codes are applied to each communication signal.
This reduces interference between signals transmitted on the same frequency channel in a particular cell, but does not avoid the prospect of interference from signals generated by nearby cells that happen to be using the same frequency channel. This problem is alleviated somewhat by adopting a deployment arrangement whereby adjacent cells all use different frequency channels. However, there are only a limited number of frequency channels that can be allocated to the wireless telecommunications system. Hence it is inevitable, particularly in densely populated areas where there are many subscriber terminals and thus each cell only covers a small geographical area, that signals from nearby cells using the same frequency channel may interfere with those generated within a cell.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an interference controller for limiting in one cell the effect of interference generated by other cells of a wireless telecommunications system, each cell of the wireless telecommunications system having a central terminal and a plurality of subscriber terminals, communication between a central terminal and a subscriber terminal being arranged to occur over a wireless link, a plurality of code division multiplexed channels being provided within a single frequency channel to enable data items pertaining to a plurality of wireless links to be transmitted simultaneously within different code division multiplexed channels of said single frequency channel, the interference controller comprising: an analyser for comparing with predetermined criteria parameters within the cell indicative of whether the code division multiplexed channels are subject to interference from signals generated by said other cells; and a channel controller, responsive to the analyser, to selectively prevent code division multiplexed channels from being used in order to reduce the effect of the interference from said other cells.
By using the above approach, the effects of interference from other cells can be suppressed, since taking a code division multiplexed channel out of commission enhances the interference rejection of the remaining code division multiplexed channels. Further, it is not just the effect of interference from other cells which is reduced, but also the effect of any internal interference within the cell, such as multipath interference, will also be reduced. Taking a code division multiplexed channel out of commission may affect the number of calls that can be handled simultaneously by the cell, but will enable the cell to retain a satisfactory quality where no calls are affected to an unacceptable degree by outside interference.
In preferred embodiments, a parameter provided to the analyser is the bit error rate (BER) for signals transmitted within said code division multiplexed channels, and the predetermined criteria with which the analyser compares said BER is a threshold BER value identifying a predetermined maximum acceptable BER, the channel controller being responsive to the analyser indicating that the BER exceeds the predetermined maximum acceptable BER to remove a code division multiplexed channel from use. A BER signal can be provided to the analyzer for each uplink and downlink communication path, the BER signal giving an indication of the errors introduced by interference during the transmission of a data signal over the particular wireless link. Hence, for each wireless link, the analyser can be provided with information about the interference levels being experienced on those links.
Further, an additional parameter preferably provided to the analyser is a grade of service (GOS) signal indicative of the availability of the code division multiplexed channels, and the predetermined criteria with which the analyser compares said GOS signal is a threshold GOS value identifying a predetermined maximum grade of service, the channel controller being responsive to the analyser indicating that the GOS signal has exceeded the predetermined maximum grade of service to remove a code division multiplexed channel from use.
Preferably, each subscriber terminal and each modem on a modem shelf within the central terminal will have a call control function associated therewith. The call control function within an ST will collect information about how readily the ST is able to acquire an uplink channel for incoming or outgoing calls, whilst each call control functions within the CT will monitor how readily the modems within the CT can establish downlink channels. Hence, the analyser will in preferred embodiments receive information indicative of how readily the modems within the CT and each ST is able to acquire channels. If the GOS exceeds that which would be considered acceptable (as indicated by the predetermined maximum GOS), then the channel controller can remove a code division multiplexed channel from use with the aim of reducing the effect of interference within the remaining code division multiplexed channels.
Preferably, the channel controller, upon receipt of a signal from the analyser indicating that a code division multiplexed channel should be removed from use, is arranged to determine which code division multiplexed channel is least heavily used, and to remove that code division multiplexed channel from use. This ensures that the least number of users are affected by the removal of the code division multiplexed channel. However, it will be apparent that other criteria could be used by the channel controller to decide which channel to remove. For example, the least heavily used channel may be reserved for priority calls, and hence it may not be appropriate to remove that channel from use. In such cases, the channel controller might be arranged to remove the least heavily used channel that is not handling, or reserved for handling, a priority call. As yet another alternative, it may be acceptable for the channel controller not to react to the request from the analyser to remove a channel, until su
Cheung Yeung Joemanne Chi
Lysejko Martin
Struhsaker Paul F.
Airspan Networks Inc.
Baker & Botts L.L.P.
Boakye Alexander
Yu Huy D.
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