GB2376606A

GB2376606A - A method of reducing interference to communications in Time Division Duplexing (TDD) Mode between a TDD mobile and a TDD base station

Info

Publication number: GB2376606A
Application number: GB0114671A
Authority: GB
Inventors: Luis Lopes
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-06-15
Filing date: 2001-06-15
Publication date: 2002-12-18
Anticipated expiration: 2021-06-15
Also published as: CN1516982A; WO2002104049A1; GB0114671D0; GB2376606B; CN1230020C

Abstract

The amount of interference to communications in Time Division Duplexing (TDD) mode between a TDD mobile 14 and a TDD base station 8 from a mobile 10 operating in Frequency Division Duplexing (FDD) mode that is located near to the TDD base station can be reduced by providing a simulated FDD base station 19 located close to the TDD base station. The simulated FDD base station can be a simple FDD antenna coupled to a remote FDD base station 6, thereby causing the FDD mobile to be controlled to reduce its transmitting power as though it was close to an FDD base station. Alternatively, the FDD antenna, 18 can be connected to the Radio Network Controller 2 and simulate some, but not all , the functions of an FDD base station, so that the FDD mobile is handed over to the simulated base station and, once again is controlled to reduce its transmitting power. Alternatively, when the FDD antenna detects that an FDD mobile is transmitting at high power, the FDD mobile can be controlled to be handed over to another FDD frequency either at the same or a different FDD base station.

Description

A Method for Reducing Interference to Communications in Time Division Duplexing (TDD) Mode between a TDD Mobile and a TDD Base Station Field of the Invention This invention relates to a method for reducing interference to communications in Time Division Duplexing (TDD) mode between a TDD mobile and a TDD base station, particularly from a mobile operating in Frequency Division Duplexing (FDD) mode that is located near to the TDD base station.

Background of the Invention In the Universal Mobile Telecommunications System (UMTS) communications between a mobile station, e. g. a cellular telephone, and a base station require both an uplink channel (from the mobile to the base station) and a downlink channel (from the base station to the mobile). It is usual for the channels to be provided either as a Time Division Duplexing (TDD) or a Frequency Division Duplexing system. Often different operators within the same coverage area will have one or both systems available for their customers.

In TDD, a particular frequency channel is divided into time frames, with each frame being subdivided into a plurality of timeslots. Some of the timeslots in each frame are designated for uplinking and some are designated for downlinking, with a particular mobile being allocated particular uplink and downlink timeslots for a particular communication session. Of course, different operators will, in general, have different frequency channels allocated to them.

In FDD, a number of frequencies are available as communication channels and, for a particular communication session with a mobile, the operator will allocate one frequency channel as an uplink channel and one frequency channel as a downlink channel. Those particular frequency channels then remain available all the time for that mobile for the duration of the communication session. Of course, different mobiles may well share the same uplink and downlink channels using different codes.

When a mobile station operating in FDD mode moves about a particular base station coverage area, it is controlled to increase or decrease its transmission power depending on how close to the base station it is at any particular time. It will thus be apparent that if the mobile is at a relatively far distance from the base station, its transmission power will be high. If it should happen that the mobile is close to a base station operating in TDD mode, then any frequency channels available for the TDD communication that are adjacent the FDD uplink channel that the mobile is communicating on will have interference from the FDD mobile due to power leaking across adjacent from the FDD uplink channel. Such leakage can desensitise the TDD base station by causing it to reduce its reception sensitivity in the TDD frequencies

adjacent the FDD uplink channel frequency so that it can no longer adequately receive TDD uplink communication at those frequencies.

This problem can occur even when the FDD mobile is close to the FDD base station if that happens to be co-located or closely located to the TDD base station and the FDD uplink channel has a broad transmission frequency spectrum. One way to overcome the problem is to use very sharp RF filters to preclude or reduce any crossover from the FDD uplink channel to adjacent channels, but such filters can be costly.

Brief Summary of the Invention The present invention therefore seeks to provide a method for reducing interference to communications in Time Division Duplexing (TDD) mode between a TDD mobile and a TDD base station, which overcomes, or at least reduces, the abovementioned problems of the prior art.

Accordingly, in a first aspect, the invention provides a method for reducing interference to communications in Time Division Duplexing (TDD) mode between a TDD mobile and a TDD base station from a mobile operating in Frequency Division Duplexing (FDD) mode, the method comprising the steps of providing an FDD receiver at a location at least close to the TDD base station, receiving an FDD mode communication signal from the FDD mobile, determining whether the FDD communication signal is on an FDD frequency close to a frequency used for TDD communication with the TDD base station, and controlling the FDD mobile to reduce the power of the FDD communication signal on the FDD frequency if the FDD frequency is determined to be close to a frequency used for TDD communication with the TDD base station.

In a preferred embodiment, the FDD receiver is a simple antenna coupled to a remote FDD base station and simulating an FDD base station at the TDD base station location.

Brief Description of the Drawings Embodiments of the invention will now be more fully described, by way of example, with reference to FIG. 1, which shows a schematic diagram of a communications system incorporating various embodiments of the present invention.

Detailed Description of the Drawings Thus, as shown in FIG. 1, a communications system 1 includes an FDD operator 2 and a TDD operator 3. Each operator has a number of base stations providing limited coverage over a particular area to mobiles operating in FDD and TDD mode, respectively. Thus, FDD operator 2 is coupled to FDD base stations 4,5 and 6, and TDD operator 3 is coupled to TDD base stations 7,8 and 9. The base stations are located to provide efficient coverage for the particular operator and may therefore be independently sited so that, in some cases the FDD and TDD base stations will be

separated from each other, and in other cases, they may be co-located, as in the case of FDD base station 5 and TDD base station 8.

A first FDD mobile 10 is shown communicating with FDD base station 5, as shown by jagged line 11 and a second FDD mobile 12 is shown communicating with FDD base station 4, as shown by jagged line 13. Similarly, a TDD mobile 14 is shown communicating with TDD base station 8, as shown by jagged line 15. In this example, the FDD base station 5 and the TDD base station 8 are located on the same site, or at least fairly close to each other, as shown. Thus, the FDD communication 11 between the first FDD mobile 10 and FDD base station 5 occurs fairly close to the TDD base station 8. Therefore, as described above, the uplink FDD signal from the FDD mobile 10 to the FDD base station 5 can cause interference on adjacent frequencies used for TDD communication, for example from the TDD mobile 14 to the TDD base station 8.

Although this interference may not be very high, since the power level of the FDD mobile 10 is controlled by the FDD base station 5 to be reduced when the FDD mobile 10 is close to the FDD base station 5, a more serious example is shown where FDD mobile 12 is communicating with FDD base station 4. In this case, the FDD mobile 12 is relatively far from the FDD base station 4, but is relatively close to the TDD base station 7, thus potentially causing interference on adjacent frequencies used by the TDD base station 7, as shown by dotted jagged line 16.

In order to try to reduce the interference caused by the FDD uplink from an FDD mobile located near to a TDD base station communicating with a TDD mobile, a simulated FDD base station can be located close to the TDD base station. The simulated FDD base station can be a simple FDD antenna 19 coupled to a remote FDD base station 6. In this case, the FDD antenna 19 acts as another antenna for the base station 6. Thus, when the FDD antenna 19 receives a communication from a mobile it sends the communication via link 20 to the base station 6. The base station 6 reacts, in the usual manner, when a high power communication is received from a mobile, by instructing the mobile to reduce its transmitting power. Thus, the FDD mobile is controlled to reduce its transmitting power as though it was close to an FDD base station, even though the expense of setting up another full base station co-located with the TDD base station 9 is avoided by using a simple antenna 19 coupled by link 20 to the remote FDD base station 6. The communication link 20 between the FDD antenna 19 and the base station 6 could be via a microwave point-to-point medium, or optical fiber, or could be carried out at the radio level, either before the data detection, or after.

Alternatively, an FDD receiver 18 can be connected directly to the FDD Operator (Radio Network Controller) 2 and simulate some, but not all, the functions of an FDD

base station. Thus, when the FDD mobile 12, which was in communication 13 with the FDD base station 4 moves into the coverage area of the FDD receiver 18, it is handed over, in any known manner, to the simulated base station 18 and, once again is instructed to reduce its transmitting power as it moves closer to the FDD receiver 18.

In an alternative embodiment, as shown in FIG. 1, where FDD base station 5 is co-located with TDD base station 8, if the FDD base station 5 that is communicating with the FDD mobile 10 determines that the frequency of the FDD uplink transmission from the mobile 10 is on a frequency that could cause interference to the TDD base station 8, it can instruct the mobile 10 to change to a different frequency, which is further from the TDD frequencies in use by TDD base station 8, thereby reducing the amount of interference to the TDD base station. Clearly, this is effectively a handover of the FDD communication with the mobile 20 to a different frequency. Of course, if no frequencies with reduced interference to the TDD base station 8 are available, it may alternatively be possible to handover the FDD communication with mobile 10 to an FDD base station, such as FDD base station 6, which, although further away from the mobile 10 thereby causing it to increase its transmission power, may be at a different frequency which would reduce the interference to the TDD base station, even though the power of the FDD transmission was increased.

It will be appreciated that although only a few particular embodiments of the invention have been described in detail, various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention.

Claims

Claims 1. A method for reducing interference to communications in Time Division Duplexing (TDD) mode between a TDD mobile and a TDD base station from a mobile operating in Frequency Division Duplexing (FDD) mode the method comprising the steps of : providing an FDD receiver at a location at least close to the TDD base station; receiving an FDD mode communication signal from the FDD mobile; determining whether the FDD communication signal is on an FDD frequency close to a frequency used for TDD communication with the TDD base station; and controlling the FDD mobile to reduce the power of the FDD communication signal on the FDD frequency if the FDD frequency is determined to be close to a frequency used for TDD communication with the TDD base station.
2. A method for reducing interference to communications according to Claim 1, wherein FDD communication with the FDD mobile is handed over from a more remote FDD base station to the FDD receiver when the FDD mobile is closer to the FDD receiver, whereby the FDD receiver simulates an FDD base station, but does not include the full functionality of an FDD base station.
3. A method for reducing interference to communications according to claim 2, wherein the FDD receiver is coupled to a Radio Network Controller, and the FDD mobile is controlled to reduce the power of the FDD communication signal as though the FDD receiver was an FDD base station.
4. A method for reducing interference to communications according to claim 1, wherein the FDD receiver is an antenna coupled to a remote FDD base station, whereby the remote FDD base station receives the FDD communication signal from the FDD mobile via the antenna and controls the FDD mobile to reduce the power of the FDD communication signal as though the FDD mobile was close to the remote FDD base station.
5. A method for reducing interference to communications according to claim 2, wherein the step of controlling the FDD mobile to reduce the power of the FDD communication signal involves controlling the FDD mobile to handover the FDD communication with the FDD mobile to another FDD frequency.

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6. A method for reducing interference to communications according to claim 2, wherein the step of controlling the FDD mobile to reduce the power of the FDD communication signal involves controlling the FDD mobile to handover the FDD communication with the FDD mobile to another FDD base station at another FDD frequency.
7. A method for reducing interference to communications in Time Division Duplexing (TDD) mode between a TDD mobile and a TDD base station from a mobile operating in Frequency Division Duplexing (FDD) mode substantially as hereinbefore described with reference to FIGs. 1 and 2 of the accompanying drawings.