Abstract:
In a wireless communication system, the communicating stations reduce their transmitting power level when they detect interference exceeding a certain level. Interference is detected by down-shifting the received signal to place the desired signal in the baseband, then sampling the down-shifted signal, first at a sampling frequency high enough to catch the interference, then at a lower sampling frequency that excludes the interference. This system is useful for vehicle-to-vehicle communication in an environment in which vehicle-to-roadside communication may also be present at various nearby frequencies, because it does not require exact knowledge of the interfering frequencies and allows communication to continue even when interference is present.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to wireless communication, and in particular to vehicle-to-vehicle wireless communication in a frequency band shared with other types of wireless communication. 
         [0003]    2. Description of the Related Art 
         [0004]    Vehicle-to-vehicle communication is an emerging technology with numerous applications under study, including collision warning systems. Many of the systems being developed use a frequency band allocated for use under the Dedicated Short Range Communications (DSRC) standards. The same frequency band is also used for vehicle-to-roadside communications, including electronic toll collection. 
         [0005]    In a vehicle-to-roadside DSRC system, the allocated frequency band is divided into a plurality of frequency channels. Interference between communications in mutually adjacent or overlapping wireless service areas is avoided by the assignment of different channels to different areas. Packet collisions in communications on the same channel by multiple vehicles in a single wireless service area are avoided by a time division multiple access (TDMA) system. The roadside base station in each service area designates the channels used and controls the vehicles&#39; transmission timings. The mobile stations in the vehicles carry out a frequency selection process and a transition process specified in the DSRC standards, which enables them to communicate with the base station in each communication area on the designated channels. 
         [0006]    Vehicle-to-vehicle communication systems avoid packet collisions by using a carrier sense multiple access (CSMA) transmission control system in which, before transmitting on a desired carrier frequency, a mobile station detects the desired carrier signal, observes the signals transmitted by other mobile stations on this carrier frequency, waits for their transmissions to cease, and then transmits its own signal. 
         [0007]    Since vehicle-to-vehicle communication systems and vehicle-to-roadside communication systems use the same frequency band, when a vehicle communicating with another vehicle enters a vehicle-to-roadside communication service area, interference may occur. Disruption of vehicle-to-roadside communication by interference from vehicle-to-vehicle communication is particularly troublesome, because by the time the interference ends, the vehicle that is trying to communicate with the roadside base station may have left the base station&#39;s wireless service area. 
         [0008]    In Japanese Patent Application Publication No. 2001-237847, Mizoguchi et al. describe a method of avoiding interference between a wireless communication system and, for example, a weather radar system using the same frequency band. A station in the wireless communication system receives both the radar signal and the desired communication signal, detects the levels of both signals, and refrains from transmitting communication packets for a predetermined time if either signal level exceeds a predetermined level. Besides preventing packet collisions, this scheme also prevents transmitted packets from interfering with the radar signal. 
         [0009]    If this method were to be applied to vehicle-to-vehicle communications, however, then communication would be interrupted whenever interference from another system exceeded a predetermined level. Such interruptions could have serious consequences when the information to be transmitted by vehicle-to-vehicle communication is an emergency signal or warning signal. 
         [0010]    Another problem in the system described by Mizoguchi et al. is the need for extra circuitry to identify or isolate the interfering signal so that its level can be measured. 
         [0011]    Yet another problem is that the frequency of the interfering signals encountered in vehicle-to-vehicle communication systems may vary as the vehicles travel down the road, making it necessary to search for the interfering frequency by scanning the available channels, a time-consuming process. 
       SUMMARY OF THE INVENTION 
       [0012]    An object of the present invention is to provide a wireless communication method and apparatus that can react quickly to avoid interference without interrupting communication and without requiring extensive interference detection circuitry. 
         [0013]    The invention provides a method of communicating among a plurality of stations in a wireless environment in which a first signal with a first center frequency is present in a first frequency band. The plurality of stations communicate in a second frequency band with a second center frequency separated from the first center frequency by a certain separation frequency. 
         [0014]    In transmission, transmit data and a control signal are assembled into a transmit packet signal, which is modulated onto a carrier signal at the second center frequency and transmitted at a controlled transmitting power level. 
         [0015]    In reception, an incoming signal including the first signal and a second signal having the second center frequency is down-converted to obtain a third signal. The down-conversion process converts the second signal to the baseband. The third signal is sampled and thereby converted to a discrete received signal, and the interference level of the first signal is detected from the discrete received signal. 
         [0016]    The transmitting power level may be determined from the interference level. In particular, the transmitting power level may be reduced when the detected level of the first signal exceeds a predetermined threshold level. 
         [0017]    The sampling frequency may be determined from the separation frequency, if known, or from the minimum or maximum separation frequency in a permitted range of separation frequencies. The sampling frequency may be sequentially increased or decreased to search for a plurality of possible first signals. 
         [0018]    The control signal may include a reduced transmission flag, which is set when the transmitting power level is reduced, indicating that the transmitted packet signal is to be forwarded by the receiving station, and a hopping flag, indicating that the transmitted packet signal is a forwarded signal. The hopping flag is set when the reduced transmission flag is detected in the discrete received signal. 
         [0019]    By lowering the transmitting power when interference from the first signal in the received signal exceeds a certain level, it is possible to reduce interference from the second signal into the first signal. 
         [0020]    By setting the reduced transmission flag, it is possible to maintain communication even at the reduced power level, by having packets relayed over multiple hops. Communication is accordingly not interrupted when interference is present. 
         [0021]    The interference level can be detected by digitally resampling the discrete received signal at a sampling frequency low enough to eliminate the first signal and comparing the levels of the discrete received signal before and after resampling. This level detection method does not require extensive circuitry, and the size and cost of the communication apparatus can be reduced accordingly. 
         [0022]    This level detection method also does not require precise identification or isolation of the first signal. The level of interference caused by the first signal can therefore be detected quickly, even if the first center frequency is not precisely known. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    In the attached drawings: 
           [0024]      FIG. 1  illustrates a vehicle-to-vehicle communication system using a vehicle-to-vehicle communication apparatus and method embodying the invention; 
           [0025]      FIG. 2  is a block diagram of the vehicle-to-vehicle communication apparatus in this embodiment; 
           [0026]      FIG. 3  is a more detailed block diagram of the vehicle-to-vehicle communication apparatus in the embodiment; 
           [0027]      FIGS. 4A and 4B  illustrate received signal frequency spectra before and after down-shifting, and indicate attenuation characteristics and sampling frequencies used in the embodiment; 
           [0028]      FIG. 5  is a table of signal-to-interference power ratios and transmission levels used by the communication controller in the embodiment; and 
           [0029]      FIG. 6  is a diagram showing a packet frame structure used in the embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    A vehicle-to-vehicle communication system embodying the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
         [0031]      FIG. 1  illustrates a vehicle-to-vehicle communication system operating on a road  1  having two lanes  1   a ,  1   b . A base station  2  is installed on the shoulder of the road  1  adjacent lane  1   b . A plurality of vehicles  3  (individually designated  3 - 1  to  3 - 4 ) are proceeding to the left in the drawing in lane  1   a . The vehicles  3  have respective vehicle-to-vehicle communication apparatuses with respective antennas  11  (designated  11 - 1  to  11 - 4 ). A vehicle  4  with an antenna  5   a  is proceeding in the same direction, as indicated by the arrow, in lane  1   b.    
         [0032]    The base station  2  is used in a communication system that provides a service such as electronic toll collection (ETC). Vehicle  4  has just entered a vehicle-to-roadside communication area A 1  centered on the base station  2 . The base station  2  uses a first frequency band with a carrier frequency f 1  to transmit a first signal S 2  to vehicle  4 . This first signal will also be referred to as an interfering signal (IS) since it interferes with communication between the vehicles  3  in lane  1   a.    
         [0033]    The communication apparatuses in these vehicles  3  use a second frequency band with a carrier frequency f 2 , on which they communicate with each other by transmitting a second signal such as an emergency signal (ES). Carrier frequency f 2  is comparatively close to carrier frequency f 1 . The difference between the two carrier frequencies f 1 , f 2  is the separation frequency Δf(Δf=|f 1 −f 2 |). 
         [0034]    In area A 2 , vehicle  3 - 1  is communicating with vehicle  3 - 3  by transmitting a signal S 3 - 1   a  to vehicle  3 - 3 . When vehicle  3 - 1  enters area A 1  and senses interference from the base station  2 , however, it reduces its transmitting power and tries to continue communication with vehicle  3 - 3  by transmitting a signal S 3 - 1   b  to vehicle  3 - 2 , which is cruising between vehicles  3 - 1  and  3 - 3 . Vehicle  3 - 2  receives signal S 3 - 1   b  and forwards it to vehicle  3 - 3  as signal S 3 - 2 . 
         [0035]    Referring to  FIG. 2 , the vehicle-to-vehicle communication apparatus  10  in each vehicle in this embodiment includes an antenna  11 , an antenna duplexer  12 , a receiver  20 , a communication controller  29 , a transmitter  30 , and an upper layer processor  40 . The upper layer processor  40  includes a control unit  41 , a keyboard  42 , a display  43 , a speaker  44 , and a microphone  45 . The control unit  41  is connected to the communication controller  29 , and controls the keyboard  42 , display  43 , speaker  44 , and microphone  45 . 
         [0036]    The communication controller  29  is connected to the receiver  20  for input and to the transmitter  30  for output, and controls transmission and reception. The transmitter  30  receives transmit data from the communication controller  29  and outputs a modulated transmit signal S 33  to the antenna duplexer  12 . During transmission, the antenna duplexer  12  connects the antenna  11  to the transmitter  30  and sends the transmit signal S 33  to the antenna  11 . 
         [0037]    The antenna  11  is also used for reception. During reception, the antenna duplexer  12  connects the antenna  11  to the receiver  20  and sends the signal S 12  received by the antenna  11  to the receiver  20 . The receiver  20  demodulates the received signal S 12  and outputs demodulated packet data to the communication controller  29 . 
         [0038]    Referring to  FIG. 3 , the receiver  20  includes a radio frequency receiving circuit (Rx RF CKT)  21 , an analog to digital converter (ADC)  22 , a level detection circuit (DET) A  23 , a digital low pass filter (LPF)  24 , a level detection circuit B  25 , a signal to interference ratio (SIR) detector (DET)  26 , a received (Rx) packet demodulator (DEMOD)  27 , and a sampling rate converter  28 . The transmitter  30  includes a transmit (Tx) level controller  31 , a transmit (Tx) packet generator  32 , and a transmitting circuit (Tx CKT)  33 . The level detection circuit A  23 , low pass filter  24 , level detection circuit B  25 , and SIR detector  26  in the receiver  20  constitute an interference level detector  281 . 
         [0039]    During reception, the antenna duplexer  12  connects the antenna  11  to the receiver  20  and outputs the received signal S 12  as described above. The radio frequency receiving circuit  21  down-converts the received signal S 12  (reduces its frequency) to obtain a baseband signal S 21 , which is supplied to the analog to digital converter  22 . 
         [0040]    The analog to digital converter  22  samples the baseband signal S 21  with a sampling frequency fs 1  given by a sampling signal S 28   a  and outputs a resulting discrete received signal S 22  to the interference level detector  281 . 
         [0041]    Referring to the frequency spectrum in  FIG. 4A , the received signal S 12  includes an interference signal S 12   a  with carrier frequency f 1  and a desired signal S 12   b  with carrier frequency f 2 . The radio frequency receiving circuit  21  attenuates the signal intensity of the interference signal S 12   a  by, for example, half and down-converts both signals S 12   a , S 12   b  so that the desired signal Sl 2   b  is placed at the bottom end of the baseband. As shown in  FIG. 4B , the baseband signal S 21  includes both the down-converted interference signal S 21   a  and the down-converted desired signal S 21   b . The frequency spectrum of the down-converted interference signal S 21   a  is centered on the separation frequency Δf. 
         [0042]    Normally, a sampling frequency fs 2  would be used to sample the desired signal S 21   b , but the analog-to-digital converter  22  uses a higher sampling frequency fs 1  determined on the basis of the separation frequency df so that the down-converted interference signal S 21   a , as well as the desired signal S 21   b , can be detected in the discrete received signal S 22 . 
         [0043]    Referring again to  FIG. 3 , level detection circuit A  23  in the interference level detector  281  measures the level of the discrete received signal S 22  as a first received power level (level A), outputs a resulting level-A signal  23   a  to the SIR detector  26 , and passes the discrete received signal S 22  as an output discrete received signal S 23   b  to the low pass filter  24 . The low pass filter  24  resamples this discrete received signal S 23   b  according to a resampling signal S 28   b , thereby removing the interference signal IS (signal S 21   a  in  FIG. 4 ) and extracting the desired signal ES (signal S 21   b  in  FIG. 4 ), which it outputs as an LPF signal S 24  to level detection circuit B  25 . 
         [0044]    Level detection circuit B  25  measures a second received power level (level B) of the LPF signal S 24 , outputs a resulting level-B signal S 25   a  to the SIR detector  26 , and passes the LPF signal S 24  as an output LPF signal S 25   b  to the received packet demodulator  27 . From the level-A signal  23   a  and level-B signal S 25   a , the SIR detector  26  obtains the signal level S of the desired signal ES and the signal level I of the interference signal IS, calculates the signal-to-interference ratio level or SIR level, and outputs an SIR level signal S 26  to the communication controller  29 . 
         [0045]    The received packet demodulator  27  demodulates the LPF signal S 24  passed to it as the LPF signal S 25   b  and outputs a received packet signal S 27  to the communication controller  29 . 
         [0046]    The sampling rate converter  28  receives a bandwidth signal S 29   a  from the communication controller  29  and outputs the sampling signal S 28   a  to the analog to digital converter  22  and the resampling signal S 28   b  to the low pass filter  24 . 
         [0047]    The communication controller  29  comprises a central processing unit (CPU, not shown) and a storage unit (not shown) in which programs for executing various processes are stored. In combination, the CPU, the storage unit, and the stored programs constitute a set of facilities including at least a sampling processor (SPP)  29   a , an SIR processor (SRP)  29   b , a table (TBL)  29   c , a transmit packet processor (TPP)  29   d , and a received packet processor (RPP)  29   e.    
         [0048]    The sampling processor  29   a  outputs the bandwidth signal S 29   a , from which the sampling rate converter  28  derives the sampling signal S 28   a  and resampling signal S 28   b.    
         [0049]    The SIR processor  29   b  outputs a transmit level signal S 29   b  to the transmit level controller  31 , based on the SIR level signal S 26 . The SIR processor  29   b  obtains the transmit level signal S 29   b  from the SIR level signal S 26  and the table  29   c , which stores data describing a relationship between SIR levels and transmitting power levels. 
         [0050]    The table  29   c  is structured as shown in  FIG. 5 , with a set of SIR levels and the corresponding transmitting power levels (Tx levels). An interference threshold level SIRTH is preset for the SIR levels. SIR levels Y 0 , Y 1 , Y 2 , . . . , Y m  lower than the interference threshold level SIRTH all correspond to a normal transmitting power level Ps. SIR levels X 1 , X 2 , . . . , X k , . . . , X n  successively higher than the interference threshold level SIRTH correspond to transmitting power levels lower than Ps by successively larger amounts N 1 , N 2 , . . . , N k , . . . , N n  (k, m, and n are integers). 
         [0051]    The transmit packet processor  29   d  outputs the transmit data S 29   d  from which the transmit packet generator  32  in  FIG. 3  generates a transmit packet signal. The received packet processor  29   e  generates received data from the received packet signal S 27  and outputs the received data to the transmit packet processor  29   d  and the upper layer processor  40 . 
         [0052]    The transmit level controller  31  receives the transmit level signal S 29   b  and outputs a transmit level control signal S 31  to the transmitting circuit  33  to control the transmitting level. 
         [0053]    The transmit packet generator  32  receives the transmit data S 29   d , which includes a control flag for controlling packet transmission as described below, and generates a transmit packet signal S 32 , which is output to the transmitting circuit  33 . 
         [0054]    Referring to  FIG. 6 , the frame structure of the transmit packet signal S 32  comprises, from the first field to the last field, a preamble PK 1 , a unique word (UW) PK 2 , a MAC header PK 3 , and payload data PK 4 . The MAC header PK 3  includes a control signal or control flag PK 3 - 10 , which is actually a pair of flags, including a reduced transmission flag PK 3 - 11  and a hopping flag PK 3 - 12 . 
         [0055]    The transmitting circuit  33  modulates the transmit packet signal S 32  onto a carrier wave with frequency f 2 , amplifies the resulting modulated transmit packet signal S 32  up to the transmitting power level specified by the transmit level control signal S 31 , and outputs the amplified transmit signal S 33  to the antenna duplexer  12 . 
         [0056]    The general vehicle-to-vehicle communication method and the specific operation of the method during reduced level transmission will now be described. 
         [0057]    First, the general vehicle-to-vehicle communication method will be described. 
         [0058]    The vehicle-to-vehicle communication apparatus  10  in  FIG. 3  receives a communication signal from the antenna  11 . The received signal S 12  is input to the radio frequency receiving circuit  21 . 
         [0059]    As shown in  FIG. 4A , the received signal S 12  includes an interference signal S 12   a  with a carrier frequency f 1  and a desired signal S 12   b  with a carrier frequency f 2 . The radio frequency receiving circuit  21  operates with a filter-like attenuation characteristic, indicated by the dotted line in  FIG. 4A , that somewhat attenuates the interference signal S 12   a . The radio frequency receiving circuit  21  down-converts the received signal S 12  in such a way that the desired signal S 12   b  is converted to the baseband proper (the bottom part of the baseband, including the zero frequency) and outputs a baseband signal S 21  including both the down-shifted desired signal S 21   b  and the down-shifted interference signal S 21   a . As shown in  FIG. 4B , the down-shifted interference signal S 21   a  has a center frequency of |f 1 −f 2 |. 
         [0060]    The analog to digital converter  22  samples the baseband signal S 21  at a sampling frequency fs 1  equal to the frequency of the sampling signal S 28   a  and outputs the discrete received signal S 22 . The sampling frequency fs 1  satisfies the following condition (1). 
         [0000]        fs 1≧ n×|f 1 −f 2| where, n≧2  (1) 
         [0061]    In the above equation, the term |f 1 −f 2 | corresponds to the separation frequency Δf, and the factor n is the number of channels present in the interval from the desired signal ES to the interference signal IS, inclusive. When the desired signal ES and the interference signal IS use adjacent channels, for example, the factor n is equal to two (n=2). When the interference carrier frequency f 1  is unknown, a separation frequency Δf equal to the maximum separation allowed by the relevant DSRC system specification is preferably assumed, so that all possible interference can be detected. Alternatively, to detect adjacent-channel interference as quickly as possible, the assumed separation frequency Δf may be set equal to the minimum possible separation between the desired signal ES and the channel used by the base station  2 , e.g., to the channel spacing value in the relevant DSRC system specification. 
         [0062]    The interference level is detected in the interference level detector  281  as follows. Level detection circuit A  23  measures the total power level of the discrete received signal S 22  and outputs it as the level-A signal  23   a . The low pass filter  24  resamples the discrete received signal S 22  at a sampling frequency equal to the frequency of the resampling signal S 28   b . This sampling frequency is low enough to remove the interference signal IS; the filter characteristic produced by the resampling process is indicated by the dotted line in  FIG. 4B . The resampling process is carried out by, for example, dividing the discrete received signal S 22  into consecutive segments of n samples each and calculating the sum or average of the values in each segment. The output LPF signal S 24  includes only the desired signal ES. The level detection circuit B  25  measures the power level of the LPF signal S 24  and outputs it as the level-B signal S 25   a.    
         [0063]    The level-A signal  23   a  indicates the sum (S+I) of the signal level S of the desired signal ES and the signal level I of the interference signal IS. The level-B signal S 25   a  indicates only the signal level S of the desired signal ES. By subtracting level B (S) from level A (S+I), the SIR detector  26  obtains the power level (I) of the interference signal IS. The SIR level signal S 26  output by the SIR detector  26  can then be obtained as the ratio of the desired signal level S to the signal level I. 
         [0064]    In the description so far a single interference level has been detected, which is adequate for the scenario shown in  FIG. 1 , but in the general case more than one interference signal may be present. It may then be desirable to detect the power levels of the interference signals individually and determine the transmitting power level from, for example, the level of the strongest interference signal, or some other one of the interference signals. This can be done by increasing the first sampling frequency from, for example, twice the minimum possible separation frequency to twice the maximum possible separation frequency, in steps corresponding to the channel spacing in the relevant DSRC system specification, and comparing the successively detected interference levels. Alternatively, the first sampling frequency may be decreased sequentially from twice the maximum to twice the minimum possible separation frequency, so as first to detect the total interference level and then detect the levels of the interference signals individually, if interference is present. 
         [0065]    The received packet demodulator  27  demodulates the LPF signal S 24  to obtain the received packet signal S 27 . 
         [0066]    The SIR processor  29   b  in the communication controller  29  outputs the transmit level signal S 29   b  on the basis of the SIR level signal S 26  and the table  29   c  shown in  FIG. 5 . If the SIR level is one of the values Y 0  to Y m  below the threshold level SIRTH, meaning that negligible interference is detected, the SIR processor  29   b  sets the transmit level signal S 29   b  to indicate a transmitting level of Ps mW, which is the normal transmitting power. When the SIR level is, for example, X 1 , just above the interference threshold level SIRTH, the transmit level signal S 29   b  indicates a reduced transmitting level of (Ps−N 1 ) mW. If the SIR level increases to X k , the transmitting level is further reduced to (Ps−N k ) mW, where N k  is greater than N 1 . The transmit level controller  31  outputs the transmit level control signal S 31  on the basis of the transmit level signal S 29   b.    
         [0067]    Meanwhile, the transmit packet generator  32  receives transmit data S 29   d , including the control flag PK 3 - 10 , and generates the transmit packet signal S 32 . The transmitting circuit  33  modulates the transmit packet signal S 32  onto a carrier wave to produce the transmit signal S 33 , the power level of which is controlled according to the transmit level control signal S 31 . The antenna duplexer  12  connects the antenna  11  to the transmitter  30 , and the transmit signal S 33  is transmitted as a radio signal. 
         [0068]    Next, the operation of the vehicle-to-vehicle communication apparatus  10  during reduced transmission will be described through the example shown in  FIG. 1 , in which vehicle  3 - 1  communicates with vehicle  3 - 3  via vehicle  3 - 2 . 
         [0069]    By transmitting at the normal power level Ps, vehicle  3 - 1  can communicate with any of the vehicles in area A 2 . Originally, vehicle  3 - 1  transmits a signal S 3 - 1   a  directly to vehicle  3 - 3 , using carrier frequency f 2 , but when it enters area A 1 , it detects interference from the signal S 2  transmitted by the base station  2  on a nearby carrier frequency f 1 , which is separated from frequency f 2  by the separation frequency Δf, to the vehicle  4  traveling in the lane  1   b.    
         [0070]    More specifically, the SIR detector  26  in the vehicle-to-vehicle communication apparatus  10 - 1  in vehicle  3 - 1  detects the interference level and outputs the SIR level signal S 26 . The SIR processor  29   b  in the communication controller  29  refers to the table  29   c  shown in  FIG. 5 . If the SIR level is X k , for example, the SIR processor  29   b  outputs a transmit level signal S 29   b  specifying a transmitting level of (Ps−N k ). 
         [0071]    The transmit packet processor  29   d  of the communication controller  29  outputs transmit data S 29   d  in which the reduced transmission flag PK 3 - 11  is set. The transmit packet generator  32  packetizes the transmit data S 29   d  and outputs a transmit packet signal S 32 . The transmitting circuit  33  modulates the transmit packet signal S 32  onto the carrier wave, which has frequency f 2 , and transmits the resulting modulated signal as the signal S 3 - 1   b  in  FIG. 1  from the antenna  11 - 1  of vehicle  3 - 1 . 
         [0072]    Since this signal S 3 - 1   b  is transmitted at a reduced power level, it cannot be received by vehicle  3 - 3 , but vehicle  3 - 2  is traveling near vehicle  3 - 1  and has not yet entered area A 1 . The distance from vehicle  3 - 1  to vehicle  3 - 2  is short enough that vehicle  3 - 2  can receive signal S 3 - 1   b  despite the reduced transmitting level. 
         [0073]    In the vehicle-to-vehicle communication apparatus  10 - 2  mounted on vehicle  3 - 2 , the receiver  20  receives signal S 3 - 1   b , and the received packet demodulator  27  outputs a received packet signal S 27 . The sampling processor  29   a  of the communication controller  29  detects the reduced transmission flag PK 3 - 11  and instructs the transmit packet processor  29   d  to process and forward the received packet signal S 27 . From the received packet signal S 27 , the transmit packet processor  29   d  creates transmit data S 29   d  in which the hopping flag PK 3 - 12  is set to indicate forwarding, but the reduced transmission flag PK 3 - 11  is not set because vehicle  3 - 2  has not yet detected interference from the base station  2 . From the transmit data S 29   d , the transmit packet generator  32  creates a transmit packet signal S 32 . From the transmit packet signal S 32 , the transmitting circuit  33  creates the signal S 3 - 2  that is transmitted from the antenna  11 - 2  of vehicle  3 - 2  to vehicle  3 - 3  in  FIG. 1 . 
         [0074]    Signal S 3 - 2  is transmitted at the normal power level, so it can be received by vehicle  3 - 3 . A transmission path via vehicle  3 - 2  is thus formed, on which communication between vehicles  3 - 1  and  3 - 3  continues. 
         [0075]    One effect of the above embodiment is that only the desired signal ES has to be converted accurately to the baseband. The interference level is detected from the disappearance of interference when the baseband is narrowed by resampling at a lower sampling frequency. It is therefore unnecessary to identify or isolate the interfering signal by, for example, using a phase-locked loop to generate a local frequency matching the interfering signal frequency. As a result, interference levels can be detected quickly by comparatively small and inexpensive circuitry. 
         [0076]    A second effect of the above embodiment is that when a first vehicle that is communicating with a second vehicle enters a vehicle-to-roadside communication area, the first vehicle does not have to stop communicating to avoid interfering with vehicle-to-roadside communication; it only has to reduce its transmitting power level. Although this may put the second vehicle out of receiving range of the first vehicle, communication between the first and second vehicles can continue by being relayed through a third vehicle located nearby. Accordingly, the reduced transmitting level does not entail a reduced communication area or an interruption of communication. 
         [0077]    One effect of the reduced transmission flag and the hopping flag is that if a vehicle receives a packet with either one of these flags set, it knows that it is near a source of interference and can set the sampling frequency of its receiver to a frequency that permits rapid detection of the interference. In addition, if the reduced transmission flag is set, the receiving vehicle knows that it should forward the received packet to other nearby vehicles. 
         [0078]    A fourth effect of the above embodiment is that, by sequentially changing the sampling frequency, it is possible to identify the specific channels on which interference signals are present. It is therefore unnecessary to perform the conventional frequency selection process to search for interference channels. 
         [0079]    The invention is not limited to the embodiment described above. 
         [0080]    Many other modifications of the overall configuration and the individual blocks shown in  FIG. 3  are possible. For example, the analog-to-digital converter  22  in  FIG. 3  may be replaced by an analog sample-and-hold circuit, and some or all of the blocks in the interference level detector  281  may also be analog circuits. 
         [0081]    The table  29   c  in  FIG. 5  may be structured in various other ways to store the relationship between SIR levels and transmitting power levels, or may be replaced by an arithmetic or logic circuit that calculates the transmitting power level from the SIR level. 
         [0082]    The reduced transmission flag and hopping flag may be located at any recognizable positions in the packet frame structure in  FIG. 6 , not limited to the MAC header PK 3 . 
         [0083]    The source of interference is not limited to a vehicle-to-roadside communication system as shown in  FIG. 1 . The source of interference may be another type of DSRC system, such as, for example, a parking management system. 
         [0084]    The invention is not limited to use in vehicle-to-vehicle communication systems; it is applicable to wireless communication systems in general. 
         [0085]    Those skilled in the art will recognize that still further variations are possible within the scope of the invention, which is defined in the appended claims.