Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1997-11-26
2001-02-20
Urban, Edward F. (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S514000, C455S452200, C455S454000
Reexamination Certificate
active
06192252
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to wireless telecommunications systems and more particularly to techniques for establishing a wireless link connecting a central terminal and a subscriber terminal 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.
A typical central terminal may have a number of modem shelves, each modem shelf operating with a different frequency channel. 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 wireless links 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 wireless links 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, for example, a set of sixteen orthogonal codes is used, sixteen 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 RW codes are applied to each communication signal. Thus, each modem shelf within the central terminal could support
16
wireless links.
Typically, each subscriber terminal would be arranged to always communicate on a particular orthogonal channel with a particular modem shelf of the central terminal, this arrangement often being referred to as a ‘fixed assignment’ arrangement.
As more subscribers subscribe to the wireless telecommunications network, it is becoming desirable to support more and more subscriber terminals from each central terminal. There are only a limited number of frequency channels that can be allocated to the wireless telecommunications system, and as it is desirable for neighbouring cells to use different. frequency channels so as to reduce interference, the demand cannot be met by merely adding more modem shelves to each central terminal.
Hence, using the above fixed assignment arrangement, there is a limit as to how many subscriber terminals can be supported from one central terminal.
SUMMARY OF THE INVENTION
Viewed from a first aspect, the present invention provides a channel selection controller for establishing a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, at least two frequency channels being provided over which said wireless link could be established, the channel controller comprising: a storage for storing data identifying the at least two frequency channels; a selector for selecting a frequency channel from those listed in said storage; link acquisition logic for establishing a wireless link on the frequency channel selected by the selector; the selector being responsive to the link acquisition logic being unable to establish said wireless link, to select an alternative frequency channel from those listed in said storage.
By this approach, it is possible to increase the number of subscriber terminals that can be supported by the wireless telecommunications system, since if one frequency channel is fully used at the time that a wireless link connecting a particular subscriber terminal with a central terminal is required, then another frequency channel can be selected for the establishment of that wireless link. Formerly this would not have been possible, since the subscriber terminal would have been arranged to only communicate with a central terminal using one predefined frequency channel.
In preferred embodiments, the data in said storage identifies one of said at least two frequency channels as a preferred frequency channel, and the selector is arranged to select that preferred frequency channel first. Hence the channel selection controller will have a ‘home’ frequency channel which is the preferred channel over which to establish wireless links, the other frequency channels listed in the storage only being used if the home frequency channel is fully used at the time a wireless link is required. It will be appreciated by those skilled in the art that it is not necessary to define a home frequency channel, and any suitable logic may be provided for enabling the selector to select a frequency channel from those listed in said storage.
In preferred embodiments, the subscriber terminal may establish wireless links with a plurality of central terminals forming a service domain, and the controller further comprises a receiver for receiving a message from a central terminal of said service domain identifying the frequency channels for each central terminal in the service domain over which wireless links connecting said central terminals and said subscriber terminal can be established, the storage being arranged to store the information received by the receiver. Hence, a plurality of central terminals which can be used by the subscriber terminal for establishing wireless links can be arranged as a service domain, and the necessary information about that service domain passed to the channel selection controller. The provision of more than one central terminal with which a wireless link to a subscriber terminal can be established provides the wireless telecommunications system with improved fault tolerance. For example, if a particular central terminal has a fault which affects all modem shelves of that central terminal, then in prior art ‘fixed assignment’ systems, all subscriber terminals associated with that central terminal would be unable to establish communications until the fault is fixed. However, in accordance with preferred embodiments of the present invention, the channel selection controller will be able to connect those subscriber terminals to another central terminal, thereby reducing the effect of the fault.
Preferably, the link acquisition logic attempts for a predetermined time to establish a wireless link on the frequency channel selected by the selector, and if the predetermined time elapses without the wireless link having been established, the link acquisition logic requests the selector to select an alternative frequency channel from those listed in said storage. It will be apparent that some other criteria, other than the elapse of a predetermined time, could be used to determine that the link acquisition controller has been unable to establish a wireless link on a particular frequency channel.
Viewed from a second aspect, the present invention provides a subscriber terminal of a wireless telecommunications system for communicating over wireless links with one or more central terminals provided by the wireless telecommunications system, the subscriber terminal including a channel selection controller in accordance with the present invention.
In one embodiment, one central terminal may be provided with which the subscriber terminal can establish a wireless link, a plurality of frequency channels being provided over which a wireless link between said subscriber terminal and said central termi
Lysejko Martin
Struhsaker Paul F.
Airspan Networks Inc.
Baker & Botts L.L.P.
Perez-Gutierrez Rafael
Urban Edward F.
LandOfFree
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