Background of the Invention Wireless telecommunication systems provide voice and data transmission from places where communication based on Public Switched Telephone Network (PSTN) or other wired networks is not possible. Quality of communication link in radio transmission can be affected by interference and jamming. It is important to highlight that the term"jamming"is used in situations where an interfering signal is generated intentionally and for unintentional signals the same phenomenon is called interference. However in both situations the physical phenomenon is the same and is known as interference, thus each time herein below when the term "interference"is used it covers both intentional and unintentional signals that cause interference unless otherwise stated. For systems operating in trunking mode, with a base station serving in one cell the interference on the main control channel might make the cell unusable to legitimate subscribers'wireless communication units.

The impact of the interference on a control channel frequency can be reduced in such a way that the cell shall still remain operational and continue the expected

service, with a possible reduction in capacity efficiency of the cell.

The term a"jamming device"herein below refers to a transmitter that intentionally transmits radio waves (interferences) at the same frequency which system transmitters (mobile and base station) work on. That signal corrupts some of data exchanged between the wireless communication unit and the base station (BS), and makes it impossible for these terminals to decode the information.

A jamming device might make a whole cell useless by generating an interfering signal on that control channel. Once it happens, all the wireless communication units that are camped on that control channel and listen to the broadcast on that channel, lose the connection to the BS, and leave the cell.

The jamming signal is considered to be generated on either downlink or uplink frequencies of a main carrier to corrupt the received information to the wireless communication units or BSs. If the interference source succeeds in this, the entire cell would be useless to the subscribers using the wireless communication units.

Jamming of downlink signalling is easier due to the fact that the downlink signal is always present and therefore is more vulnerable to analysis and selective attack.

There is known in the prior art a solution to the problem of jamming and interference defined in a standard for TErrestrial Trunked RAdio (TETRA) system, as defined by the European Telecommunications Standards Institute (ETSI). A primary focus for TETRA equipment is the emergency services, as TETRA provides a dispatch and control operation. Each wireless communication unit is able to transmit and receive control, voice and data

information. The system infrastructure in a TETRA system is generally referred to as a switching and management infrastructure (SWMI), which substantially contains all of the system elements apart from the wireless communication units.

A backup control channel as defined in the TETRA standard, replaces the main control channel, once the main control channel goes off. A jamming device which is capable of jamming the control channel, may switch to the backup channel and to jam it as well once it detects that the SWMI has switched to the backup. There is no need to a more powerful jamming device for that.

Besides, the wireless communication units are always on the same channel, so all of them are affected, by jamming a single channel.

Summary of the Invention There is a need for a method of reducing the impact of an interference of a control channel and for a wireless communication unit and base transceiver station which are able to operate according to the method and which alleviate or overcome the disadvantages of the prior art.

According to a first aspect of the present invention there is provided a method of reducing the impact of interference on a control channel as claimed in claim 1.

According to a second aspect of the present invention there is provided a wireless communication unit as claimed in claim 12.

According to a third aspect of the present invention there is provided a base transceiver station as claimed in claim 19.

The present invention beneficially allows for minimizing the effect of interference by the fact that the communication devices are distributed over different control channels and jamming one of the channel affects only part of the wireless communication units operating in the cell. As there are other control channels the wireless communication units from the jammed or affected by interference control channel can switch to another one. Jamming of all control channels is difficult as it requires more powerful jamming device. To jam all the control channels the jamming device needs N times power, assuming that there are N control channels.

Brief description of the drawing The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: FIG. 1 is a flow chart illustrating a method of reducing impact of jamming of a control channel in one embodiment of the present invention, FIG. 2 is a block diagram of a mobile station in one embodiment of the present invention.

Detailed description of an embodiment of the invention Referring to FIG. 1 one embodiment of a method of reducing impact of interference on a control channel according to the present invention is shown.

The impact of interference on a control channel is reduced by replication 104 of the first control channel into plurality of control channels and switching communication devices to the replicated control channels. Each of these replicated control channels comprises exactly the same downlink information as the first control channel. The first control channel is replicated after detection 102 of interference on the first control channel. Once the base transceiver station detects the interference the replication is performed by the base transceiver station. When the first control channel is replicated a base transceiver station transmits 106 over a broadcast control channel a list of available, replicated, control channels.

When the information transmitted 106 over the broadcast control channel is received by the communication device said information is further decoded and stored 108 in a form of a list of the available control channels in a memory of said wireless communication unit. The list is dynamically updated as the base transceiver station transmits the new lists with available control channels. Some of the control channels can be closed 124 due to necessity of allocation of bandwidth to other services.

To provide good quality of service, in one embodiment, if said list of available control channels contains more than one control channel the communication device measures quality 110 of the available control channels. In one embodiment the measure of quality can be a ratio of good packets to bad packets received on the control channel by the communication device. However for those skilled in the art it is clear that other methods of measuring quality could also be applied.

As the result of measuring of the quality of the control channels they are being prioritized according to their quality 112 and the ones with the quality below a predefined value are not selected by the wireless communication unit 114 for switching. The communication device switches 118 to one of the control channels with quality equal or above the predefined control level.

In one embodiment the wireless communication unit switches 118 to one of the replicated control channels after predefined period of time of operation on a current control channel 126,128. Selection of the control channel for switching 118 is done randomly.

Random selection has the advantage that the communication devices are equally distributed over the control channels and additionally makes jamming more difficult as it is not possible to predict on which control channel operates and will operate particular communication device. However switching to other control channels according to a predefined pattern is also possible.

If the jamming device continues its operation also on the replicated control channels or interference appears on any of the replicated control channels the wireless communication units operating on the control channel affected by the interference switch to other of the replicated control channels when the interference is detected on their current control channel. The wireless communication unit always switches to other control channel once it detects interference on its current control channel.

In one embodiment the replicated control channels are closed and their bandwidth is freed for other

services if the interference or jamming signal disappeared.

In one embodiment the base transceiver station transmits the information on the available control channels in repetitions and for the repetitions different broadcast control channels are used.

With reference to Fig. 2 an embodiment of a wireless communication unit 200, which is capable to operate according to the above described method is shown. The wireless communication unit 200 comprises an antenna 206, a Radio Frequency (RF) switch 204 and a receiver section 208, which together form a receiver circuitry. A transmitter section 202, the RF switch 204 and the antenna 206 form together a transmitter circuitry. A signal processor 210 is operably coupled to the receiver and/or transmitter circuitries as well as to a memory 212. The communication device comprises also an audio output section 224 and 226 as well as a microphone 220. A keypad 216 and a display 218 form a user interface that allows controlling the communication device 200.

In operation, when the base transceiver station transmits over the broadcast control channel information on available, replicated, control channels, the receiving circuitry and the signal processor 210 receives and decodes that information and stores in a form of a table in the memory 212. The communication device 200 switches the current control channel to one of the replicated control channels.

The wireless communication unit 200 switches to one of the replicated channels after predefined period of

time of operation on a current control channel.

Selection of the control channel to switch is performed in a random manner. However in alternative embodiment the control channel to switch is selected in accordance with a predefined pattern.

In one embodiment the signal processor 210 of the communication device 200 calculates a ratio of good packets to bad packets received on each of the control channels. The calculated ratio is a measure of quality of each of the control channels and is used for prioritization of the control channels. The control channels with the quality measure below a predefined value are not selected for switching.

It is obvious for those skilled in the art that the embodiments described above may be equally used for reducing impact of both unintentional and intentional (jamming) interference.

It is worth to emphasize that the information on all control channels is identical, and a wireless communication unit may switch from one control channel to another without missing any broadcast information. In one embodiment of the present invention, the wireless communication unit switches to a control channel for a predefined period of time, and then picks up another control channel randomly from the pool of the available control channels in the cell, and switches over. This mechanism also overcomes a smart jamming device that tries to chase after a specific wireless communication unit.

What is also worth to emphasize is that the present invention allows for dynamic change of number of control

channels depending on the available bandwidth. If there is a free bandwidth in the cell it can be used for the replicated control channel. If there is a plurality of control channels in the cell and there is a demand for a bandwidth, some of the control channels can be closed and the bandwidth can be made available for other services.

In case of a cell with multiple control channel the wireless communication units that operate on the jammed control channel are affected, but the ones that listening to the other control channel are getting full service from the base transceiver station.