Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
2000-05-25
2002-04-30
Ngo, Ricky (Department: 2731)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S342000, C370S335000, C370S345000, C370S441000, C370S442000, C370S479000
Reexamination Certificate
active
06381211
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to wireless telecommunications systems and more particularly to techniques for processing data transmitted and received over 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.
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 ‘Code Division Multiple Access’ (CDMA) technique. One way to implement CDMA is through the application of a set of orthogonal codes 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. A suitable set of orthogonal codes is a “Rademacher-Walsh” (RW) set of sixteen 16-bit codes. Orthogonal codes have the property that, when perfectly aligned, all codes cross-correlate to zero, thus making it possible to decode a signal to which one orthogonal code has been applied while cancelling interference from signals to which different orthogonal codes have been applied.
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, considering-the example of a set of 16 RW codes, 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 RW codes are applied to each communication signal.
It is known to provide a number of modem shelves within one central terminal, and for each modem shelf to employ a different frequency channel. Hence, if a central terminal has four modem shelves, and the set of 16 RW codes is employed for each frequency channel, one central terminal would be able to support wireless links with up to 60 subscriber terminals simultaneously.
However, 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.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a transmission controller for processing data items to be transmitted over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for transmitting data items pertaining to a plurality of wireless links, the transmission controller comprising: an orthogonal code generator for providing an orthogonal code from a set of ‘m’ orthogonal codes used to create ‘m’ orthogonal channels within the single frequency channel; a first encoder for combining a data item to be transmitted on the single frequency channel with said orthogonal code from the orthogonal code generator, the orthogonal code determining the orthogonal channel over which the data item is transmitted, whereby data items pertaining to different wireless links may be transmitted simultaneously within different orthogonal channels of said single frequency channel; and a TDM encoder arranged to apply time division multiplexing (TDM) techniques to the data item in order to insert the data item within a time slot of the orthogonal channel, whereby a plurality of data items relating to different wireless links may be transmitted within the same orthogonal channel during a predetermined frame period.
Viewed from a second aspect, the present invention provides a reception controller for processing data items received over a wireless link connecting a central terminal and a subscriber terminal of a wireless telecommunications system, a single frequency channel being employed for transmitting data items pertaining to a plurality of wireless links, and ‘m’ orthogonal channels being provided within the single frequency channel, the receiver controller comprising: an orthogonal code generator for providing an orthogonal code from a set of ‘m’ orthogonal codes used to create said ‘m’ orthogonal channels within the single frequency channel; a first decoder for applying, to signals received on the single frequency channel, the orthogonal code provided by the orthogonal code generator, in order to isolate data items transmitted within the corresponding orthogonal channel; and a TDM decoder arranged to extract a data item from a predetermined time slot within said orthogonal channel, a plurality of data items relating to different wireless links being transmitted within the same orthogonal channel during a predetermined frame period.
By using TDM techniques in addition to the known set of orthogonal codes, it is possible for selected orthogonal channels to be subdivided in the time dimension. For example, if TDM is used to divide one frame period in to four subframes, and each orthogonal channel is subject to the TDM technique, then up to 64 separate communication signals can be transmitted on the sixteen orthogonal channels during one frame period, albeit at a quarter of the rate that the communication signals could be transmitted if the TDM technique was not used.
Such an approach has the advantage that it preserves compatibility with current hardware and software equipment which use the set of orthogonal codes, but which do not support the use of TDM techniques. By designating certain orthogonal channels as channels for which TDM is not used, the current equipment can communicate over those channels without any changes being required to the equipment.
In preferred embodiments, the transmission controller further comprises: an overlay code generator for providing an overlay code from a first set of ‘n’ overlay codes which are orthogonal to each other; and a second encoder, selectively operable instead of the TDM encoder, to apply the overlay code from the overlay code generator to said data item, whereby ‘n’ data items pertaining to different wireless links may be transmitted simultaneously within the same orthogonal channel.
Similarly, the reception controller may further comprise: an overlay code generator for providing an overlay code from a first set of ‘n’ overlay codes which are orthogonal to each other, the set of ‘n’ overlay codes enabling ‘n’ data items pertaining to different wireless links to be transmitted simultaneously within the same orthogonal channel; and a second decoder, selectively operable instead of the TDM decoder, to apply to the data items of the orthogonal channel, the overlay code from the overlay code generator so as to isolate a particular data item transmitted using that overlay code.
By such an approach, data items transmitted within certain orthogonal channels can be encoded using TDM techniques whilst data items transmitted within other orthogonal
Lysejko Martin
Struhsaker Paul F.
Airspan Networks Inc.
Baker & Botts L.L.P.
Ngo Ricky
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