Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-06-30
2001-12-25
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S423000, C370S329000, C370S337000, C370S347000, C370S442000, C370S458000
Reexamination Certificate
active
06334057
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to wireless telecommunications systems, such as cellular radio telecommunications systems. More particularly, the present invention relates to allocating channels in a cellular radio telecommunications system.
BACKGROUND
In a cellular radio telecommunications system, any number of multiple access strategies may be employed, such as, frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In a system that employs a FDMA strategy, the frequency spectrum is divided into a number of disjunctive frequency bands, wherein each frequency band serves as a separate radio channel. In a system that employs CDMA, different modulation codes, known as spreading codes, are used to distinguish the various radio channels. In a TDMA system, channel separation is accomplished by dividing the time domain into time frames and time slots as described in more detail below.
FIG. 1
illustrates a typical time division duplex (TDD), TDMA time frame
100
. Generally, in a TDD-TDMA based system, the uplink channels and the downlink channels are separated in time over a common frequency. More specifically,
FIG. 1
illustrates that the TDD-TDMA time frame
100
is divided into a number of fields including a downlink control channel (DL CC) field
105
, a downlink traffic channel field
110
, an uplink traffic channel field
115
, and a random access channel (RACH) field
120
. The downlink traffic channel field
110
and the uplink traffic channel field
115
are still further divided into a number of time slots (not shown). In the case of the downlink traffic channel field
110
, a time slot is utilized for carrying information from the cell base station to an assigned mobile unit (e.g., cellular telephone) located in the cell. In the case of the uplink traffic channel field
115
, a time slot is utilized for carrying information from an assigned mobile unit in the cell to the cell base station. Typically, the DL CC field
105
contains, among other things, an announcement list which identifies the mobile units that are to receive information from the base station in the current downlink period as well as the time slots during which the identified mobile units are to receive that information. The DL CC field
105
may also contain an assignment list which identifies those mobile units that have been allocated a time slot during the current uplink period. The RACH field
120
is typically utilized for transporting such information as scheduling information and control information, including retransmission requests.
FIG. 1
also illustrates that the TDMA time frame
100
includes an asymmetric uplink/downlink format. This means that the size (i.e., length) of the downlink traffic channel field
110
may be different than the size of the uplink traffic channel field
115
. Moreover, this means that the relative size of the downlink traffic channel field
110
compared to the size of the uplink traffic channel field
115
may vary from cell to cell. Consequently, the time period associated with a downlink traffic channel field
110
in a first cell may, to some extent, overlap the time period associated with the uplink traffic channel field
115
in an adjacent or nearby cell, and vice versa.
When the downlink traffic channel field
110
in one cell overlaps the uplink traffic channel field
115
in an adjacent or nearby cell, the transmission of information in one cell is more likely to disturb the transmission of information in the other cell. That is, the overlap between the downlink traffic channel field
110
in one cell and the uplink traffic channel field
115
in the adjacent or nearby cell is more likely to result in cross-channel and/or co-channel interference. The situation is especially problematic wherein the two cells share (i.e., reuse) a portion of the frequency band, thereby increasing the likelihood of co-channel interference. As illustrated in
FIG. 2
, for example, if a base station
205
in a first cell is presently receiving uplink transmissions from mobile units located in the first cell, such as mobile unit
210
, while a base station
215
in a second cell is presently transmitting downlink messages to various mobile units located in the second cell, such as mobile unit
220
, the base station
205
in the first cell may inadvertently receive a transmission from the base station
215
in the second cell. In a second example, as illustrated in
FIG. 3
, the mobile unit
310
in the first cell may pick up transmissions from the mobile unit
320
located in the second cell.
The problem of intercell interference in a time division system is described in G. Povey et al., “TDD-CDMA Extension to FDD-CDMA Based Third Generation Cellular System”, Proceedings of the ICUPC, IEEE (1997). Povey suggests synchronizing all of the cells in the network. In a large cellular network, however, this is an unrealistic and, most likely, a very complex proposal. Accordingly, there is a need to reduce and/or eliminate intercell interference due to asymmetric uplink/downlink traffic channel fields in a TDMA time frame.
SUMMARY OF THE INVENTION
The present invention provides a more effective technique for assigning (i.e., allocating) uplink and downlink radio channels in a TDD-TDMA based network. In general, the present invention accomplishes this by determining the level of susceptibility associated with each mobile unit during both the uplink and downlink period. The mobile unit is then assigned a radio channel in either or both the uplink and downlink traffic channel fields as a function of these determinations.
Accordingly, it is an object of the present invention to provide an effective channel allocation technique in a TDD-TDMA based network employing asymmetric TDMA time frames.
It is another object of the present invention to assign time slots for the uplink and/or downlink traffic channel fields in such a way that overall system performance increases.
It is still another object of the present invention to assign time slots for the uplink and/or downlink traffic channel fields in such a way that overall intercell interference decreases.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method and/or apparatus for allocating radio channels in a time division multiple access time frame which contains asymmetric downlink and uplink traffic channel fields. The method and/or apparatus involves, among other things, dividing the asymmetric downlink and uplink traffic channel fields into a number of regions, wherein each of the regions has associated with it an expected level of intercell interference. In addition, the level of susceptibility to intercell interference for each of a plurality of mobile units is continuously determined. Then, one region is selected and a radio channel from that region is allocated to one of the plurality of mobile units, based on the expected level of intercell interference associated with the selected region and the level of susceptibility to intercell interference for the mobile unit.
In accordance with another aspect of the present invention, the foregoing and other objects are achieved by a method and/or apparatus for allocating downlink and uplink channels, in a synchronous cellular radio telecommunications network that employs a time division multiple access (TDMA) channel allocation scheme, associated with a TDMA time frame which contains asymmetric downlink and uplink traffic channel fields. The method and/or apparatus involves dividing the downlink and uplink traffic channel fields into a total of at least three regions. The three regions include a first region exclusively coinciding with a portion of the downlink traffic channel field; a second region exclusively coinciding with a portion of the uplink traffic channel field; and a third region coinciding with a portion of the downlink traffic channel field and a portion of the uplink traffic channel field. The third region has associated wit
Kronestedt Fredric
Malmgren Göran
Bost Dwayne
Burns Doane Swecker & Mathis L.L.P.
Craver Charles
Telefonaktiebolaget LM Ericsson (publ)
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