Abstract:
In a communication network, a wireless channel is selected from multiple channels. The interference level of the selected channel is determined and a first quality value is derived and compared to a first decision threshold. If the first quality value is smaller than the first decision threshold, the selected channel is abandoned and a new channel is searched. If the first quality value is greater than the first decision threshold, the selected channel is maintained for transmission of a packet. A second quality value of the maintained channel is continuously determined and compared to a second decision threshold which is lower than the first decision threshold. Only if the second quality value is smaller than the second decision threshold, the channel is abandoned and a channel search is initiated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to wireless communication networks, and more specifically to a technique for selecting a wireless channel from multiple channels according to the interference level of the channel and maintaining the selected channel for transmission of data packets as long as the channel is of satisfactory quality. The present invention particularly relates to a method of determining the interference level of a Wireless channel of a communication network. 
     2. Description of the Related Art 
     Japanese Laid-Open Patent Application 10-66140 discloses a wireless communication network in which multiple wireless channels are shared by a plurality of subnetworks each comprising a parent host and multiple child hosts. The parent host provides management of the subnetwork and establishes packet communication with its own child hosts by sharing a single wireless channel. When the parent host of each subnetwork is powered on, it selects an idle channel from a plurality of channels allocated to the network and establishes the selected channel if there is no interference. If the upper layer of the parent host has a packet to send, it is transmitted on the established channel. On the other hand, each child host of the subnetwork, when powered on, makes a search for a channel of highest strength and selects it as the channel established by the parent host of its own subnetwork, based on the result of control packets exchanged with the parent host. If the child host detects even a single control packet on the selected channel that is transmitted from other parent host, it recognizes that there is interference and abandons the selected channel in favor of a new channel used by another parent host. Once the parent host is determined, the child host is ready to send packets on the channel established by the parent host. 
     A similar technique is disclosed in Japanese Laid-Open Patent Application 10-229579. According to this prior art, control packets are exchanged on a selected channel between hosts to determine its interference level. If the channel is found to be of acceptable quality, it is maintained and used for transmission of data packets. When the channel is being used for packet transmission, the channel is monitored for a channel ownership packet broadcast from other subnetwork. If such a packet is detected, it is determined that there is interference and the current channel is abandoned and a search for a new channel is initiated. 
     However, since the presence or absence of an interfering packet is the only factor for determining channel quality, precision measurement of interference is desired for efficient utilization of available channels. In particular, channel ownership packets broadcast from one subnetwork may be received by hosts of another subnetwork at a rate that varies with the severity of interference. Another shortcoming is that, since channel quality detection is performed independently on channel selection phase and data transmission phase based on comparison between channel quality and decision threshold, the communication between hosts may suffer from channel instability. If the decision threshold of the channel selection phase is lower than that of the data transmission phase, needs may often occur during transmission to initiate a channel search for a better channel. However, the use of lower decision threshold tends to increase the probability of lower quality channels being selected. Thus, the reselection of a channel may cause a further channel reselection. 
     The present invention is intended to solve these shortcomings. Prior art references which are of interest to the present invention are Japanese Laid-Open Patent Applications 8-33020 and 8-336177. In JP 8-33020, a base station collects traffic data from the network and establishes a channel to a mobile unit according to the start and end timing determined by the traffic data. Another interference detection technique disclosed in JP 8-336177 is based on the total length of time in which signals are received without error at a given rate, compared to the total length of time in which signals are received in error at the same rate. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and a network for establishing a wireless communication channel that can avoid frequent switching of channels during packet transmission. 
     A further object of the present invention is to provide a method of precisely determining the interference level of a wireless channel to prevent frequent channel switching. 
     Briefly, the method of the present invention comprises the steps of (a) selecting a wireless channel from a plurality of wireless channels, (b) detecting interference level of the selected channel and determining therefrom a first quality value and comparing the first quality value to a first decision threshold, (c) if the first quality value is smaller than the first decision threshold, returning to the step (a) to select another channel, (d) if the first quality value is greater than the first decision threshold, using the selected channel for transmission of a packet, (e) determining a second quality value of the maintained channel and comparing the second quality value to a second decision threshold which is lower than the first decision threshold, (f) if the second quality value is smaller than the second decision threshold, returning to the step (a) to select another channel, and (g) if the second quality value is greater than the second decision threshold, maintaining the used channel and returning to the step (e). 
     In one aspect of the invention, the interference determination step comprises the steps of (a) broadcasting a polling packet to the network and starting a timing operation, (b) receiving a response packet from the network and incrementing a count value, (c) repeating the step (b) until the timing operation expires, (d) repeating the steps (c) to (d) a predetermined number of times each time the timing operation expires, and (e) determining the interference level of the selected channel from a ratio of the count value to the predetermined number. 
     In a second aspect of the invention, the interference determination step comprises the steps of (a) identifying a parent host that can be accessed from a child host via the wireless channel, (b) broadcasting a polling packet from the child host to the network and starting a timing operation, (c) receiving at the child host, a response packet from the network and incrementing a count value if the packet is received from the identified parent host or from another child host which is communicating with the identified parent host, (d) repeating the step (c) until the timing operation expires, (e) repeating the steps (b) to (d) a predetermined number of times each time the timing operation expires, and (f) deriving the interference level of the selected channel from the count value and the predetermined number. 
     In a third aspect of the invention in which a channel ownership packet is broadcast to the network at intervals determined by a first timer, the interference determination step comprises the steps (a) starting a second timer, (b) receiving the channel ownership packet from the network and incrementing a count value in response to the receipt of the packet and identifying a source parent host of the received packet, (c) repeating the steps (b) until the timer expires, (d) repeating the steps (a) to (c) a predetermined number of times each time the second timer expires, and (e) deriving the interference level of the channel from the count value, timeout periods of the first and second timers, a number of different source parent hosts identified by the step (b), and the predetermined number. 
     In a fourth aspect of the invention, the interference determining step comprises the steps of (a) starting a timer, (b) repeatedly determining the interference power level of the wireless channel until the timer expires, (c) repeating the steps (a) and (b) a predetermined number of times each time the timer expires, (d) producing a sum of the interference power levels repeatedly determined by the step (b), and (e) deriving the interference level of the selected channel from the sum, the predetermined number and a timeout period of the timer. 
     In a fifth aspect of the invention, the interference determination step comprises the steps of (a) starting a timer, (b) detecting interference power level of the wireless channel higher than a predetermined level and incrementing a variable by a predetermined amount in response to the detection of the higher interference power level, (c) repeating the step (b) until the timer expires, (d) repeating the steps (a) and (b) a predetermined number of times each time the timer expires, and (e) deriving the interference level of the selected channel from the incremented variable, the predetermined number and a timeout period of the timer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIGS. 1A and 1B are block diagrams of a wireless communication network of the present invention; 
     FIG. 2 is a block diagram of each host apparatus of the wireless communication network of the present invention; 
     FIG. 3 is a flowchart of the operation of each host of the network according to a first embodiment of the present invention; 
     FIG. 4 is a flowchart of the operation of each host of the network according to a modification of the previous embodiment; 
     FIG. 5 is a flowchart of the operation of each host of the network according to a second embodiment of the present invention; 
     FIG. 6 is a flowchart of the operation of the host associated with FIGS. 3 and 5 during the process of interference determination; 
     FIGS. 7A and 7B are flowcharts of the operation of a child host of the network according to a third embodiment of the present invention; 
     FIG. 8 is a flowchart of the operation of the child host associated with FIGS. 7A and 7B during the process of interference determination; 
     FIGS. 9A,  9 B and  9 C are flowcharts of the operation of a parent host of the network according to a fourth embodiment of the present invention; 
     FIG. 10 is a flowchart of the operation of each host of the network associated with FIG. 5 according to one form of the present invention during the process of interference detection; and 
     FIG. 11 is a flowchart of the operation of each host of the network associated with FIG. 5 according to a modified form of the present invention during the process of interference detection. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1A, a wireless network of the present invention is shown as comprising base hosts  1  and mobile hosts  2 . The network may be used as a wireless LAN (local area network). Thus, the base hosts are desktop or notebook computers and mobile hosts are notebook computers. Base hosts  1   a  and  1   b  are connected to the common medium  3  of the subnetwork to operate as parent hosts and mobile hosts  2   a,    2   b  constitute child hosts of the parent host  1   a  and mobile hosts  2   c,    2   d  form child hosts of the parent host  1   b.  Hosts  1   a,    2   a  and  2   b  comprise a first wireless subnetwork and hosts  1   b,    2   c,    2   d  comprise a second wireless subnetwork. The local area network is allocated a frequency spectrum which is divided into a number of communication channels. In each subnetwork, each parent host selects one of the communication channels and the child hosts of the same subnetwork use the same channel for mutual communication. All channels of the network are shared by all hosts, so that when a packet is sent from a host of a given subnetwork it may also be received by the hosts of other subnetworks using the same channel. 
     The parent host of each subnetwork has the responsibility to select and establish a wireless communication channel for communication within that subnetwork and each of the child hosts of the same subnetwork selects the channel if one is already established by the parent host. If no channels are established by the parent host, the child host is responsible to select a new channel and informs the parent host of the identity of the selected channel. 
     As shown in FIG. 1B, mobile hosts  2   e  to  2   j  may be additionally provided to form third and fourth wireless subnetworks. In this case, mobile host  2   e  operates as a parent of the mobile hosts  2   f  and  2   g  and mobile host  2   h  operates as a parent of the mobile hosts  2   i  and  2   j.  Further, the wireless communication network may be comprised of all mobile hosts such as mobile host terminals  2   e  to  2   j.    
     As shown in FIG. 2, each host of the network includes a wireless transceiver  201  connected to the upper layer of the network protocol, an air interface  202  for interfacing the transceiver  201  to the network through a wireless link, and an interference detection and channel control unit  203  connected to the transceiver  201 . As will be described in detail below, the interference detection and channel control unit  203  controls the transceiver  201  to select a wireless communication channel during a channel search phase and maintain the selected channel for transmission of packets. Channel control unit  203  detects the interference level of the selected channel during the channel search phase using a high channel quality decision threshold and continuously detects the interference level of the maintained channel during the packet transmission phase using a low channel quality decision threshold. 
     The operating sequence of the interference detection and channel control unit  203  of each host (either parent or child) according to a first embodiment of the present invention is illustrated in FIG.  3 . 
     When each host of a subnetwork is powered on, it proceeds to step  302 . If the host is a parent host, it selects an idle channel and if the host is a child host, it selects a highest strength channel as one established by a parent host. At step  303 , the host determines the channel quality Q 1  of the selected channel. The channel quality Q 1  is then compared to a threshold K 1  (step  304 ). If Q 1  is smaller than K 1 , it is determined that the selected channel is unacceptable quality and flow returns to step  302  to reselect another channel if there are still channels not tested (step  306 ). If it is determined that all channels are of unacceptable quality, flow proceeds from step  306  to step  307  to send an indication to the upper layer no communication channels are currently available. 
     If 1/T 1  is greater than K 1 , it is determined that the selected channel is of acceptable quality and the routine proceeds from step  305  to step  308  to forward data packets received from the upper layer onto the selected channel. 
     At step  309 , the quality of the selected channel is continuously determined as a channel quality value Q 2  by determining the bit error rate of packets received from destination host or by determining the interference level of the channel in a manner as will be described later. The channel quality value Q 2  is compared to a threshold K 2  that is smaller than K 1  (step  310 ). If Q 2  is greater than K 2 , it is determined that the current channel is of acceptable quality and flow returns from step  311  to step  308  to transmit packets and repeatedly perform the channel test. 
     If Q 2  is smaller than K 2 , it is determined that the current channel is of unacceptable quality and the routine returns from step  311  to step  302  to abandon the current channel and restart a search for a new channel. 
     Since the threshold K 1  for channel selection is greater than the threshold K 2  for data transmission, channel switchover events are less likely to occur and hence high system stability is achieved. 
     FIG. 4 is a modified form of the flowchart of FIG.  3 . In this modification, steps  401  and  402  are additionally provided following step  306  of the flowchart of FIG.  3 . When the decision at step  306  is affirmative, flow proceeds to step  401  to decrement the threshold K 1  by a predetermined amount and the decremented K 1  is compared to a predetermined minimum value of K 1 , which minimum value is greater than threshold K 2 . If the decremented K 1  is not equal to the minimum K 1  (step  402 ), control returns to step  302  to repeat the channel selection process. Otherwise, it is determined that no channels are available and flow proceeds to step  307 . As long as K 1  is greater than the predetermined minimum value, channel selection is repeated. 
     Note that the threshold values K 1  and K 2  of a parent host may not necessarily be the same as those of its child hosts. Use of different thresholds K 1 , K 2  in parent hosts from those of child hosts ensures that the operational stability of parent hosts is independent of the operational stability of child hosts. 
     A call establishment method for each host is shown in FIG. 5 according to a second embodiment of the present invention. 
     When the host (either parent or child) is powered on, it selects a channel at step  502  and determines its interference (noise) level T 1  at step  503 . If no interference (T 1 =0) exists (step  504 ), flow proceeds from step  504  to step  509 . If T 1  is not equal to zero, flow proceeds from step  504  to step  505  to compare the reciprocal (1/T 1 ), which represents the quality of the selected channel, to the threshold K 1 . If the quality value 1/T 1  is smaller than K 1 , it is determined that the selected channel cannot be used and flow returns to step  502  to select another channel if all channels are not tested (step  507 ). If all channels are of low quality, flow proceeds from step  507  to step  508  to send a no-channel indication to the upper layer. 
     If the quality value 1/T 1  is greater than K 1 , it is determined that the selected channel can be used and the routine proceeds from step  506  to step  509  to forward data packets received from the upper layer onto the selected channel. 
     The interference level T 2  of the selected channel is continuously determined at step  510 . If T 2 =0 (step  511 ), steps  509  and  510  are repeated. Otherwise, flow proceeds from step  511  to step  512  to compare the reciprocal 1/T 2  to the threshold K 2 . If the channel quality 1/T 2  is greater than K 2  (step  513 ), steps  509  to  512  are repeated. If 1/T 2  is smaller than K 2 , it is determined that the current channel is of poor quality and the routine returns to step  502  to reselect another channel. 
     Interference level can be precisely determined by the flowchart of FIG.  6 . This flowchart can be used for interference determination steps  303 ,  503  and  510 . 
     At step  601 , variables n 1  and R 1  are set equal to zero. A polling packet is broadcast from the source host to every other hosts of the same subnetwork (step  602 ) and the variable n 1  is incremented by one (step  603 ) and a timer is started (step  604 ). The polling packets urge responders to return a response packet containing the identity of the responding host. Hosts of other subnetworks as well as the hosts of the same subnetwork may receive the polling packet if they are currently using the same channel and return a response packet. 
     When a response packet is received (step  605 ), the variable R 1  is incremented by one at step  606  and elapsed time of the timer is checked (step  607 ). If the timer is still running, flow returns from step  607  to step  605  to wait for the next response packet. If the timer has expired, flow proceeds to step  608  to check to see if the variable n 1  is equal to or greater than a predetermined value N 1 . If n 1  is smaller than N 1 , flow returns to step  602  to transmit the broadcast packet again to receive a response packet. If n 1 ≧N 1 , flow proceeds from step  608  to step  609  to calculate the square root of (R 1 /N 1 ) as the level of interference T 1  (or T 2 ). Since N 1  equals the number of broadcast packets transmitted, the ratio R 1 /N 1  represents the ratio of the number of received packets to the number of transmitted packets. If the interference level of a channel is high, it is likely that undesired signals are arriving from many sources. Thus, the interference level can be represented by the ratio R 1 /N 1 . 
     Flowcharts shown in FIGS. 7A and 7B concerns a communication method performed by a child host according to a third embodiment of the present invention. This embodiment is useful for child hosts to precisely determine the level of interference. 
     When a child host is powered on, it makes a search for a parent host transmitting a high strength signal (step  702 ). If such a parent host is not found (step  703 ), the child host proceeds to step  704  to inform the upper layer that no channels are available. If a parent host is found (step  703 ), the child host proceeds to step  705  to store the identifier PHID of the parent host and enters an interference determination subroutine  750  which is identical to the flowchart of FIG.  6 . 
     Specifically, at step  706 , variables n 1  and R 1  are set equal to zero. A polling packet is broadcast from the child host to every other hosts of the same subnetwork (step  707 ). Variable n 1  is incremented by one at step  708 , and a timing operation is started at step  709 , and the routine checks to see if a response packet is received (step  710 ). Variable R 1  is incremented by one at step  711  if a response packet is received, and steps  710  and  711  are repeated if the timing action is still in progress. When the timing action expires, the child host returns from step  713  to step  707  if n 1  is smaller than N 1 . When n 1  is equal to or greater than N 1 , the child host determines the interference level T 1  at step  714 , and exits subroutine  750 . 
     If an accessible parent host is not found (step  704 ), it is determined that the selected channel cannot be used and the routine returns to channel selection step  702  if all channels are not tested (step  718 ). 
     Next, the interference level T 1  is tested. If T 1 =0 (step  715 ), the child host proceeds to step  721  to transmit data packets (FIG.  7 B). Otherwise, it proceeds to step  716  to compare the quality value 1/T 1  to the threshold K 1 . If 1/T 1  is greater than K 1  (step  717 ), flow proceeds to step  721 . Otherwise, it returns to channel selection step  702 . 
     Following step  721 , the interference level T 2  of the selected channel is determined by subroutine  760  which includes steps  722  to  730  respectively corresponding in significance to steps  706  to  714  of FIG.  7 A. 
     If the interference level T 2  that is determined by step  730  is equal to  0  (step  731 ), steps  721  to  730  are repeated. Otherwise, flow proceeds from step  731  to step  732  to compare the quality value 1/T 2  to the threshold K 2 . If 1/T 2  is greater than K 2  (step  733 ), steps  721  to  732  are repeated. If 1/T 2  is smaller than K 2 , the child host determines that the current channel is of poor quality and returns to step  702 . 
     By using the parent host identifier PHID stored at step  705  (FIG.  7 A), the number of received response packets (i.e., represented by variable R 1 ) is precisely determined by a subroutine shown in FIG.  8 . This subroutine corresponds to steps  711  and  727  of FIGS. 7A and 7B. 
     In FIG. 8, address data contained in a received response packet is stored in memory (step  801 ) following the execution of step  710  (FIG.  7 A). The address data includes a source address if the source of the response packet is a parent host. If the source is a child host, the response packet includes its identity and the identity of a parent host with which it is communicating. 
     At step  802 , the child host examines the address data of the response packet to determine if the source of the packet is a parent host or a child host. If the packet source is a parent host, control proceeds from step  802  to step  803  to detect a match between the stored parent host identifier PHID and the identifier of the source parent host. If they match, flow proceeds from step  803  to step  711 . Otherwise, the variable R 1  is incremented by one at step  805 . If the packet source is a child host, control proceeds from step  802  to step  804  to detect a match between the stored parent host identifier PHID and the identifier of the parent host with which the source child host is communicating. If they match, flow proceeds from step  804  to step  711 . Otherwise, the variable R 1  is incremented by one at step  805 . 
     Flowcharts shown in FIGS. 9A,  9 B and  9 C concerns a communication method performed by a parent host according to a fourth embodiment of the present invention. This method is useful for parent hosts to precisely determine the level of interference. 
     The parent host executes a subroutine  950  which includes steps  902  to  907  that respectively correspond to steps  502  to  508  of FIG.  5 . 
     When a parent host is powered on, it selects an idle channel (step  902 ) and determines its interference (noise) level T 1  at step  903 . If TI is not equal to zero (step  904 ), the parent hold compares the reciprocal (1/T 1 ) to the threshold K 1 . If 1/T 1  is smaller than K 1 , the parent host returns to channel selection step  902  if all channels are not tested (step  907 ) to reselect another idle channel. If all channels are of low quality, the parent host proceeds to step  908  to send a no-channel indication to the upper layer. If 1/T 1  is greater than K 1  (step  906 ) or T 1 =0 (step  904 ), the parent host broadcasts a channel ownership packet to the network (step  909 ) and starts a timer (with a timeout period C 1 ) at step  910 . Data packets are then sent to the selected channel (step  911 ). Following the execution of step  910 , the parent host proceeds to the flowchart of FIG.  9 B. 
     When the first timer expires, the parent host exist the main routine and initiates a timer (C 1 ) interrupt routine as shown in FIG.  9 C. In response, the parent host broadcasts a channel ownership packet containing its identity and the identity of the channel selected by the parent host (step  930 ), restarts the first timer (step  931 ) and returns to the main routine. 
     In FIG. 9B, the parent host sets variables n 2  and R 2  to zero (step  912 ) and proceeds to step  913  to start a second timer (with a timeout period C 2 ) and increments the variable n 2  by one (step  914 ). At decision step  915 , the parent host checks to see if a channel ownership packet is received from another parent host, announcing that it is using the same channel. If so, flow proceeds to step  916  to increment the variable R 2  by one and store the identity of that parent host. If no channel ownership packet is received, steps  915  and  916  are repeated as long as the second timer is running (step  917 ). When the second timer (C 2 ) expires, the parent host proceeds from step  917  to step  918  to determine if n 2  is equal to or greater than a predetermined value N 2 . If not, flow returns from step  918  to step  913  to repeat the counting of ownership packets from other parent hosts. 
     If n 2  is equal to or greater than N 2 , the parent host proceeds from the packet counting routine to step  919  where it determines the interference level T 2  by calculating (R 2 ·C 1 )/(N 2 ·NP·C 2 ), where NP is the number of parent hosts from which the channel ownership packets are received. 
     In more detail, the timeout period C 1  is the interval at which channel ownership packets are broadcast from a parent host and the timeout period C 2  represents the observation time for receiving channel ownership packets. The ratio C 2 /C 1  represents the number of channel ownership packets broadcast from a single parent host within the observation time C 2 . By multiplying this ratio by NP (i.e., NP·C 2 /C 1 ), the number of channel ownership packets received by a parent host within the observation time C 2  is obtained. Since N 2  is the maximum number of times the interference measurements are repeated, multiplying NP·C 2 /C 1  by N 2  results. in the total number of channel ownership packets broadcast during repeated measurement times. Since R 2  indicates the total number of broadcast packets received during the repeated measurement times, the interference level T 2  is obtained by dividing R 2  by N 2 ·NP·C 2 /C 1 . 
     If the interference level T 2  is equal to zero (step  920 ), flow returns to step  911  to transmit data packets. If T 2  is not equal to zero, flow proceeds from step  920  to step  921  to compare the reciprocal 1/T 2  to the threshold K 2 . If 1/T 2  is smaller than K 2  (step  922 ), the packet transmission and interference determination are repeated. Otherwise, it is determined that the current channel is of poor quality and the parent host changes the channel status from “busy” to “idle” (step  923 ). 
     With the channel,status being changed to “idle”, the patent host stops the first timer at step  924 , and returns to channel reselection step  902 . 
     It is seen therefore that channel ownership packets are repeatedly broadcast from a parent host at intervals determined by the timeout period C 1  during the time a channel is used for transmission of data packets within a subnetwork. 
     Another method for interference determination step  503  of channel search phase and step  510  of communication phase is shown in FIG. 10. A variable n 3  is set to zero (step  1001 ) and a timer (with a timeout period C 3 ) is started (step  1002 ) and the variable n 3  is incremented by one (step  1003 ). At step  1004 , interference power level is detected and stored in memory. Step  1004  is repeated until the timer expires (step  1005 ). When the timer expires and the variable n 3  is smaller than a predetermined value N 3  (step  1006 ), the timer is restarted at step  1002 , n 3  is incremented by one (step  1003 ) and interference power level is detected and stored again at step  1004 . When n 3  becomes equal to or greater than N 3  at step  1006 , all stored interference power levels are added up to produce a total power value (step  1007 ). At step  1008 , the interference level T 2  is obtained by dividing the total power value by the total observation time C 3 ·N 3 . 
     FIG. 11 shows a modification of FIG.  10 . Variable n 3  is set to zero (step  1101 ) and timer (C 3 ) is started (step  1102 ) and the variable n 3  is incremented by one (step  1103 ). Variable D is set to zero (step  1104 ) and the interference power level (IPL) is determined (step  1105 ). At step  1106 , the interference power level is compared to a threshold value K 3 . If IPL is equal to or greater than K 3  (step  1107 ), control proceeds to step  1108  to increment D by a constant DT which represents the length of time taken to determine the interference power level IPL. If IPL is smaller than K 3  (step  1107 ), step  1108  is skipped. While the timer is still running, steps  1105  to  1108  are repeated so that the time-factor variable D is integrated. The integrated value D represents the amount of time in which interference of unacceptable power level is present. Steps  1102  to  1109  are repeated until n 3  equals N 3  at step  1110 . At step  1111 , the interference level T 2  is obtained by the ratio of the interference presence time D to the total observation time C 3 ·N 3 .