Abstract:
A communication channel is dynamically assigned to a mobile station in autonomous decentralized channel assignment in TDMA/TDD communications system. An interference detection criterion is determined depending on a signal strength of a call request signal. It is determined whether interference occurs in a transmitting time slot relative to the interference detection criterion in a selected channel, and further determined whether interference occurs in a receiving time slot relative to a predetermined interference detection criterion in the selected channel. When it is determined that no interference occurs in the selected transmitting and receiving time slots, the selected channel is assigned to the mobile station.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a mobile communications system, and in particular to a method for assigning a communication channel to a mobile terminal according to a carrier sensing scheme. 
     2. Description of the Related Art 
     Recently, to achieve effective use of available frequencies, a cellular communications system has employed such a channel assignment scheme that each cell station can assign one of all available channels to a mobile terminal even at the same frequency when it is determined that the degree of interference is sufficiently low at that frequency. Such a channel assignment scheme is called autonomous decentralized dynamic channel assignment. 
     In the case of personal communications system such as PHS (Personal Handy-phone System), TDMA/TDD (Time Division Multiple Access/Time Division Duplex) scheme has been employed. It is determined whether no interference occurs in a selected channel and, if no interference, then the selected channel is assigned to the mobile terminal which has requested a communication channel. On the other hand, at the mobile terminal, it is determined whether no interference occurs in the assigned channel and, if no interference, then communication is started at the assigned channel. Such a dynamic channel assignment method is called a carrier-sense channel assignment method. It should be noted that the cell station performs the carrier sensing operation, that is, detects interference level only at the timing of receiving. 
     An improved carrier sensing channel assignment method has been proposed in Japanese Patent Unexamined Publication No. 7-212821. According to the improved method, when searching for a communication channel, a cell station performs the carrier sensing operation not only at receiving timing but also at transmitting timing. If the interference levels at receiving and transmitting timing are lower than a predetermined threshold, the communication is started using the assigned channel. In other words, the cell station can detect interference levels in both up-link and down-link channels before starting the communication. Therefore, the probability of call loss is reduced, compared with the above method. 
     However, even the improved method cannot detect accurate CIR (carrier-interference ratio) at the mobile station because power reduction of radio propagation between the cell station and the mobile terminal is not taken into account. Therefore, to assign a communication channel to each mobile terminal located within the cell using the same channel assignment algorithm, it is necessary to set the interference threshold level of up-link channels to a relatively low level. When the interference threshold level is set low, there may be cases where the cell station erroneously determines that the interference of a selected channel occurs at the mobile terminal though the CIR of the selected channel is sufficiently high at the mobile terminal. In such cases, the number of available channels is reduced, resulting in increased probability of call loss. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a channel assignment method which can reduce the probability of call loss. 
     Another object of the present invention is to provide a channel assignment method which can improve use of frequencies by allocating communication channels with reliability. 
     According to the present invention, a communication channel is dynamically assigned to a mobile station in each base station of TDMA/TDD (Time Division Multiple Access/Time Division Duplex) communications system. When receiving a control signal from the mobile station, an interference detection criterion is determined depending on a signal strength of the control signal. After selecting a channel from a plurality of predetermined channels, it is determined whether interference occurs in a selected transmitting time slot relative to the interference detection criterion in a selected channel, and further determined whether interference occurs in a selected receiving time slot relative to a predetermined interference detection criterion in the selected channel. The selected channel is assigned as a communication channel to the mobile station when it is determined that no interference occurs in the selected transmitting and receiving time slots. 
     According to the present invention, it is determined whether interference occurs in a selected transmitting time slot using the interference detection criterion which varies depending on the control signal received from the mobile station. And when it is determined that no interference occurs in the selected transmitting and receiving time slots, the selected channel is assigned to the mobile station. Therefore, there is little probability that the mobile station detects interference when receiving, resulting in improved channel connection probability and reduced call loss probability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a cellular communications system employing an embodiment of a channel assignment method according to the present invention; 
     FIG. 2 is a flowchart showing the embodiment according to the present invention; 
     FIG. 3 is diagram showing an interference detection criteria table for explanation of interference detection criteria conversion in the embodiment; and 
     FIG. 4 is a time chart showing TDMA/TDD format at the cell station for explanation of carrier sensing timing in the embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a PHS system has a plurality of cell stations each forming a micro-cell and each cell station (CS) 10 can communicate with a personal station (PS) 30 located in the micro-cell 20 thereof. 
     The cell station 10 is comprised of a radio transceiver 101 and a channel controller 102 which performs channel control according to TDMA/TDD scheme. When receiving a radio signal from the personal station 30, the radio transceiver 101 outputs demodulated data and the corresponding signal level to the channel controller 102. The cell station 10 is further comprised of a processor 103 which controls the operations of the cell station 10 including the carrier sensing dynamic channel assignment. The processor 103 performs the carrier sensing dynamic channel assignment using a priority table 104 and an interference detection criteria table 105 as will be described later. Further, the processor 103 performs the operation control by running control programs including the carrier sensing dynamic channel assignment which are previously stored in a memory (not shown). 
     The priority table 104 stores a priority value for each channel so as to search for channels in order of decreasing priority. The priority table 104 is updated each time channel assignment is performed as will be described later. 
     As shown in FIG. 2, the interference detection criteria table 105 contains the relationship between received signal levels and interference detection criteria so as to search for an interference detection criterion L TH  corresponding to a received signal level. The higher the received signal level, the higher the interference detection criterion L TH . More specifically, when the received signal level falls into a range from 10 dB to 20 dB, the interference detection criterion L TH  is set to a lower criterion level 10 dBμV. When the received signal level increases to a range from 40 dB to 50 dB, the interference detection criterion L TH  also increases to a higher criterion level 40 dBμV. Therefore, the interference determination can be properly performed depending on the received signal level. 
     Referring to FIG. 3, the personal station 30 transmits a link channel establishment request signal to the cell station 10 using the up-link control channel when registering its location, calling, called, and performing band-over between cell stations (step S201). When receiving the link channel establishment request signal, the processor 103 of the cell station 10 converts the received signal level of the up-link control channel to the corresponding interference detection criterion L TH  by referring to the interference detection criteria table 105 (step S202). The interference detection criterion L TH  is stored onto a memory (not shown). 
     Subsequently, the processor 103 selects one of available frequency channels to perform the carrier sensing operation by referring to the priority table 104 (step S203). More specifically, the processor 103 searches the priority table 104 for an available channel having the highest priority at that time. 
     When such a channel is selected, the selected channel CH is subjected to the carrier sensing at transmitting and receiving timing. The respective received signal levels at receiving and transmitting timing are stored as interference levels IL UP  and IL DOWN  onto the memory (step S204). As will be described later, the carrier sensing is performed over consecutive four frames at the transmitting and receiving timing (see FIG. 4). 
     Thereafter, the processor 103 compares the interference level IL DOWN  of the transmitting timing to the interference detection criterion L TH  obtained in the step S202 to determine whether IL DOWN  is smaller than L TH  (step S205). If IL DOWN  is smaller than L TH  during four consecutive frames (YES in step S205), then the processor 103 further compares the interference level IL UP  of the receiving timing to interference level L STD1  to determine whether IL UP  is smaller than L STD1  (step S206). The interference level L STD1  of 26 dBμV is provided by Japanese standards RCR-STD28. 
     When IL DOWN  is not smaller than L TH  during at least one frame (NO in step S205) or when IL UP  is not smaller than L STD1  during at least one frame (NO in step S206), the processor 103 decreases the priority of the selected channel CH (step S207) and the control goes back to the step S203 where another channel is selected referring to the priority table 104. In this manner, the steps S203-S207 are repeatedly performed while searching the priority table 104 until an acceptable channel is found. 
     If IL UP  is smaller than L STD1  during four consecutive frames (YES in step S206), then the selected channel CH is allowed to be assigned to the personal station 30. More specifically, the cell station 10 transmits a link channel assignment signal to the personal station 30 using down-link control channel (step S208). 
     When receiving the link channel assignment signal, the personal station 30 is set to the assigned channel CH and performs the carrier sensing in the assigned channel CH at receiving timing (step S209). The received signal level at receiving timing is stored as interference level IL H-DOWN  onto a memory. The carrier sensing is performed over consecutive four frames at receiving timing. 
     Thereafter, the interference level IL H-DOWN  is compared to interference level L STD2  to determine whether IL H-DOWN  is smaller than L STD2  over consecutive four frames (step S210). The interference level L STD2  of 26 dBμV is provided by Japanese standards RCR-STD28. 
     When IL H-DOWN  is not smaller than L STD2  during at least one frame (NO in step S210), the control goes back to the step S201 where the personal station 30 transmits a link channel establishment request signal to the cell station 10 again. 
     If IL H-DOWN  is smaller than L STD2  during four consecutive frames (YES in step S210), then the personal station 30 starts communicating with the cell station 10 using the assigned channel CH (step S211). 
     At the cell station 10, when the communication is started, the processor 103 increases the priority of the assigned channel in the priority table 104 (step S212). 
     Referring to FIG. 4, in the case where time slots T3 and R3 are used for transmitting and receiving timing of the cell station 10, respectively, the carrier sensing is performed at the timing of time slot T3 to detect the interference level IL DOWN  for consecutive four frames and at the timing of time slot R3 to detect the interference level IL UP  for consecutive four frames. 
     The detected interference levels IL DOWN1  -IL DOWN4  and IL UP1  -IL UP4  are used to determine whether the selected channel is adequate for communication as described in steps S205 and S206 of FIG. 3. More specifically, if all the transmitting-timing interference levels IL DOWN1  -IL DOWN4  are smaller than L TH  (YES in step S205), then the processor 103 further compares each of the receiving-timing interference levels IL UP1  -IL UP4  to interference level L STD1  of 26 dBμV (step S206). If all the receiving-timing interference levels IL UP1  -IL UP4  are smaller than L STD1  (YES in step S206), then the selected channel CH is allowed to be assigned to the personal station 30. Contrarily, if at least one of IL DOWN1  -IL DOWN4  is not smaller than L TH  (NO in step S205) or when at least one of IL UP1  -IL UP4  is not smaller than L STD1  (NO in step S206), the processor 103 decreases the priority of the selected channel CH (step S207) and the control goes back to the step S203.