Abstract:
A method of reducing interference between wireless networks operating on channels with different bandwidths is proposed. One wireless network operates on a narrow band channel while the other network operates on a wide band channel that overlaps the narrow band channel. The method involves communicating information regarding the wide band channel to the devices of the network operating on the narrow band channel. A device in the network operating on the wide band channel constructs a special frame that carries its network ID as well as information regarding the wide band channel that it is operating on, namely the center frequency and the channel bandwidth. The device then broadcasts this frame on the narrow band channel. In addition, this frame is protected from possible collision by preceding the transmission of the frame by protection frames on both the channels.

Description:
TECHNICAL FIELD 
     The present invention generally pertains to wireless communications and, more particularly, to a method for reducing the interference caused by overlapping channels in a wireless communication system. 
     BACKGROUND ART 
     In a Wireless Communication system in which the wireless networks have a choice of operating on channels with different channel bandwidth, the likelihood of one wireless network operating on a wide bandwidth overlapping another wireless network operating on a narrow bandwidth is high. Here, the spectrum of the narrow band channel forms a portion of the wide band channel. 
     In situations where the interference between the two networks is high, one possible solution to reduce the interference could be to move one or both networks to non-overlapping channels, provided such channels exist. Both networks will become aware of the interference, possibly due to deteriorating throughput or by other means. It is relatively easier for a device operating on the wide channel to determine the exact characteristics of the narrow band channel by using techniques including but not limited to FFT analysis. On the other hand, due to limitations such as channel-filter etc., placed on the receiver of the devices operating on the narrow band channel, it is much harder for these devices to correctly detect the exact characteristics of the wide band channel. In all wireless communication systems, the number of available wide band channels is limited and in many cases might even be limited to one. In such cases, where there does not exist an alternative wide band channel, it makes more sense for the network operating on the narrow band channel to shift to another non-overlapping channel. 
     Traditionally, if the controller device of a network (an AP of a WLAN network, or a base station of a cell) wants to switch channels, it needs to perform various forms of scans (energy detection, active/passive beacon detection etc.) in order to determine which of the available channels is best. The controller device can spend substantial amount of time and energy in this process. If the controller device had a prior knowledge of the exact characteristics of the interfering wide band channel, it can save substantial time and energy in the scan process since it can safely skip scanning the channels overlapped by the wide band channel. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL1: U.S. Pat. No. 7,640,022(B2) Techniques for interference reduction in wireless communication networks, Juha Salokannel et al. 
         PTL2: WO2010/124729(A1) Spectrum arrangement for co-channel interference reduction 
         PTL3: US2012/0184206(A1) Method and apparatus for reducing inter-cell interference in a wireless communication system 
         PTL4: US2012/0257574(A1) Method of reducing interference between stations in Wireless LAN system, and apparatus supporting the same 
       
    
     Non Patent Literature 
     
         
         NPL1: IEEE STD 802.11-2012 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Summary of Patent document 1: The interference between two networks occur due to overlapping of scheduled transmission time slots. The proposed solution involves communicating the interference situation by one network to another network by means of regular beacon frames and requesting the other network to change its transmission time slot so as to avoid the interference. The proposed solution does not solve our problem since a network operating on a channel with different bandwidth is not able to receive beacon frames of another network. 
     Summary of Patent document 2: Interference occur between two frequency channels, one of them being wider than the other and the narrow channel overlapping one end of the frequency spectrum of the wider channel. Proposed solution does not involve any communication between the two networks. Solution is based on limiting the transmission on the wider channel to the non-overlapping portion of the frequency spectrum. This approach is different from our proposed solution. 
     Summary of Patent document 3: Interference occurs between a Macro cell and a Micro cell. Solution involves negotiating a time schedule during which communication may occur between the two cells. One cell may send a request to the other cell to reduce its transmit power in order to reduce the effects of the interference. Means to negotiate such inter-cell communication schedule does not exist in our case. 
     Summary of Patent document 4: The proposed solution involves the use of sounding PPDUs to gather information regarding interfering channel and using beam-forming techniques to reduce interference. Proposed solution technique is very different from ours. 
     Summary of Non-patent document 1: Under section 9.3.2.7 Dual CTS protection, mechanism of protecting a STBC data frame in the presence of non-STBC STAs is described. The reverse case of protecting a non-STBC data frame in the presence of STBC STAs is also described. This protection mechanism is not suitable for the scenario described in this text where the two networks are operating on channels with different channel bandwidths. 
     An object of the present invention is to provide a method of avoiding/reducing interference between two overlapping wireless communication systems operating on overlapping channels of different channel bandwidths and an apparatus to implement the method. 
     Objects of the present invention are not limited to that described above and other objects will be clear to people skilled in the art from the description of the invention in the following sections. 
     Solution to Problem 
     As an example, a method of avoiding/reducing the interference between two overlapping wireless communication systems operating on overlapping channels of different channels is provided. The method is summarized in the flowchart  600  presented in  FIG. 7 . The process starts at  602  when a first wireless network operating on wide band channel is interfered by a second wireless network operating a narrow band channel that is completely overlapped by the wide band channel. In step  604  the controller/coordinator device of the first network detects the presence of the interference and attempts, with finite number of retries, to gather information regarding the interfering channel in step  606 . In step  608 , if the information gathering in step  606  was successful and that the controller device was able to determine that the interfering channel is of a narrower bandwidth, the controller device proceed to step  610 . Otherwise, if the controller device was not able to gather interfering channel information in step  606 , or that the interfering channel was determined to be not of a narrower bandwidth, the controller device may proceed to fall back on alternative interference mitigation mechanisms (step  609 ) that is beyond the scope of the patent. In step  610  the controller device constructs a special frame which may be called “Interference Notification” frame. The frame comprises of information regarding the first network like a unique network ID, center frequency, channel bandwidth etc. In step  612 , the controller device confirms that both the wide band as well as the narrow band channels is idle and proceeds to transmit a protection frame with a proper duration setting on its own channel. The purpose of this protection frame is to prevent transmission of any other frames in the wide band channel within the stipulated duration. In step  614 , after waiting for a short interval, the controller device proceeds to transmit another protection frame with a proper duration setting on the narrow band channel. In step  616 , after waiting another short interval, the controller device proceeds to transmit the “Interference Notification” frame on the narrow band channel. In step  618 , a device in the interfering network receives the “Interference Notification” frame and becomes aware of the existence of the first network. It then proceeds to report this information to the controller device of its own network. In step  620 , based on this information, the controller device of the interfering network selects an alternative non-overlapping channel and moves the network to this channel, thereby removing the interference condition. 
     In addition, an apparatus to implement the method described above is also presented. 
     Advantageous Effects of Invention 
     An advantage of the present invention is the substantial saving in energy and time for a network to select an alternative channel in the event of heavy interference from a neighboring network. 
     Advantages of the present invention are not limited to that described above and other advantages will be clear to the people skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example environment where the invention presented might be applied. Two overlapping wireless communication networks, one running on a wide band channel, and one running on a narrow band are shown; 
         FIG. 2  is a diagram of available channels of two frequency spectrums, one narrow band and one wide band; 
         FIG. 3  shows the frame exchange sequence proposed; 
         FIG. 4  shows an alternative frame exchange sequence that may be used instead of the frame exchange sequence depicted in  FIG. 3 ; 
         FIG. 5  is the structure of the proposed “Interference Notification” frame; 
         FIG. 6  is the block diagram of a device that implements the proposed invention; and 
         FIG. 7  is a flowchart describing the sequence of action involved in the proposed invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following description is based on embodiments of the invention and should not be taken as limiting the invention with regard to alternative embodiments that are not explicitly described herein. 
     First Embodiment 
       FIG. 1  shows an example environment where the present invention may be applied. The environment includes two Wireless LAN (WLAN) networks also called Basic Service Set (BSS), an extended range BSS  100 , and a short range BSS  110 . Each BSS contain at least one Access Point (AP) and one or more stations (STA). The extended range BSS  100  comprises one AP  102  and two STAs  104   a  and  104   b.  The short range BSS  110  comprises one AP  112  and one STA  114 . As seen in  FIG. 1 , the extended range BSS  100  completely overlaps the short range BSS  110  and forms an Overlapping Basic Service Set (OBSS). Furthermore, one of the STA  104   a  belonging to the extended range BSS  100 , is also within the transmission range of the AP  112  of the short range BSS  110 . As an example, the extended range BSS  100  can be an extended range WiFi hotspot, with range up to 1 km. The short range BSS  110  on the other hand can be a home WiFi network with range less than 100 meters. The channel used for the wireless communication in the extended range BSS  100  is represented by the dotted line  106  while the channel used for the wireless communication in the short range BSS  110  is represented by the solid line  116 . 
       FIG. 2  shows the frequency spectrum of the two BSSs. The extended range BSS  100  is operating on the narrow band channels  208 , which is one of the available narrow band channels in the frequency spectrum  200 . Similarly, the short range BSS  110  is operating on a wide band channel  251  in the frequency spectrum  250 . Here, the narrow band channel  208  is completely overlapped by the wide band channel  251  i.e. both the starting frequency  221  (f 1s ) and the ending frequency  222  (f 1e ) of the narrow band channel  208  lies between the starting frequency  225  (f ws )and the ending frequency  256  (f we ) of the wide band channel  251 . In addition, all the narrow band channels  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  209 ,  210 ,  211 ,  212 ,  213 ,  214 ,  215  and  216  are also overlapped by the wide band channel  251 . 
     Due to this overlapping of the operating frequencies, any wireless communications on either channels will interfere with the communication on the other channel. For example, data transfer between AP  102  and STA  104   a  on link  106  will affect data transfer between AP  112  and STA  114  on link  116  and vice versa. Since the two links are operating on two different channel band widths, the devices partaking in the communication might not become immediately aware of this interference, but this interference will be manifested as a reduction in the throughput, increased retransmissions etc. on one or both links. If this interference is very heavy, communication on both links will deteriorate heavily. The situation is more critical for the short range BSS  110  since all the devices in it will be affected, where as for the extended range BSS  100 , only the devices that are within the transmission range of AP  112  are affected. Since wider band channels are more scarce in numbers, in many instances there might be only one wide band channel in a given frequency spectrum where as there can be many narrow band channels as shown in  FIG. 2 . In such a circumstance, even upon becoming aware of the interference, the short range BSS  110  might have only two options to mitigate the interference: One, it can move itself to a narrower band channel that does not overlap the narrow band channel  208 ; or Two, it can request the interfering BSS  100  to move to another narrow band channel that does not overlap with the wide band channel  251  for e.g. any of the narrow band channels shown in  FIG. 2 . aside from channels  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  209 ,  210 ,  211 ,  212 ,  213 ,  214 ,  215  and  216 . To accomplish this, the short range BSS  110  has to be able to somehow communicate this request to the extended range BSS  100 . As of the present, there does not exist any explicit mechanism to accomplish this kind of inter-BSS communication. A new control frame called “Interference Notification” frame  303  is proposed for this purpose and the structure of the frame is described in  FIG. 5 . The fields of the frame comprise as one example: Frame Control field  401 , Duration field  402 , Receiver Address (RA) field  403 , BSSID field  404 , Channel Information field  405  and the FCS field  406 . The Frame Control field  401 , Duration field  402  and the FCS field  406  are construction following the same rules used in construction other control fields. The Receiver Address (RA) field  403  shall be set as the broadcast MAC address and the BSSID field  404  is the network ID of the BSS. The most unique element of the “Interference Notification” frame  303  is the Channel Information field  405 , which explicitly identifies the channel that the BSS is operating on. The Channel Information field  405  is made up of the sub-fields Center Frequency  410 , which specifies the center frequency of the BSS channel and the sub-field Bandwidth  411 , which specifies the bandwidth of the BSS channel. 
       FIG. 3  describes the frame sequence involved in transmitting the “Interference Notification” frame  303 . Due to presence of OBSS, the probability of the transmission of the “Interference Notification” frame  303  failing is reasonably high. In order to increase the success rate of the transmission, the “Interference Notification” frame  303  needs to be protected in both the wide band channel  251  as well as the narrow band channel  208 . This is achieved by the AP  112  gaining access to the medium on both the wide band channel as well as the narrow band channel by using appropriate channel access procedures and then transmitting a CTS-to-self frame  301 , addressed to itself in the wide band channel  251 . In this embodiment, the CTS-to-self frame  301  is transmitted using the default Modulation and Coding Scheme (MCS) used for transmitting control frames in the short range BSS  110 . A CTS-to-self frame is a standard CTS frame with its Receiver Address (RA) field set to the transmitting device&#39;s own MAC address. The protection duration  320  of the CTS-to-self frame  301  is set as the sum of the time intervals  310   a  and  310   b  and the transmission times of the CTS-to-self frame  302  and the “Interference Notification” frame  303 . The time intervals  310   a  and  310   b  are set as one Short Interframe Space (SIFS). The time intervals  310   a  and  310   b  may have the same duration, or each the time intervals  310   a  and  310   b  may have different duration. All the devices in the short range BSS  110  can receive the CTS-to-self frame  301 . Since the Receiver Address (RA) field of the CTS-to-self frame  301  does not match the MAC address of the device receiving the frame, the device will set its Network Allocation Vector (NAV) to the duration  320  set in the CTS-to-self frame  301 . In this manner, all the devices in the short range BSS  110 , except the AP  112 , are restricted from transmitting any frame for the entire duration required to complete the frame sequence shown in  FIG. 3 . After a time interval  310   a  equal to one SIFS, the AP  112  transmits another CTS-to-self frame  302 , addressed to itself in the narrow band channel  208 . In this embodiment, this CTS-to-self frame  302  is transmitted using the default MCS used for transmitting control frames in the extended range BSS  100 . All the devices that belong to the extended range BSS  100  within the transmission range of AP  112  can receive the CTS-to-self frame  302 . Since the Receiver Address (RA) field of the CTS-to-self frame  302  does not match the MAC address of the device receiving the frame, the device will set its Network Allocation Vector (NAV) to the duration  321  set in the CTS-to-self frame  302 . In this manner, all the devices belonging to the extended range BSS  100  which are within the transmission range of AP  112 , are restricted from transmitting any frame for the duration  321  set in the CTS-to-self frame  302 . The protection duration  321  of the CTS-to-self frame  302  is set as the sum of the time interval  310   b  and the transmission time of the “Interference Notification” frame  303 . After a time interval  310   b  equal to one SIFS, the AP  112  finally transmits the “Interference Notification” frame  303  in the narrow band channel  208 . In this embodiment, this “Interference Notification” frame  303  is transmitted using the default MCS used for transmitting control frames in the extended range BSS  100 . The “Interference Notification” frame  303  is received by the STA  104   a  belonging to the extended range BSS  100 , which is also within the transmission range of the AP  112  of the short range BSS  110 . Upon inspecting the BSSID field  404  of the “Interference Notification” frame  303 , the STA  104   a  will become aware of the existence of the OBSS caused by the short range BSS  110 . The STA  104   a  can report this to its AP  102  by using some reporting mechanism and it will include the information regarding the interfering channel in this report. AP  102 , upon receiving this report from STA  104   a  can start the process to find an alternative channel that is not overlapped by the wide band channel  251 . This process may involve various forms of scans (energy detection, active/passive beacon detection etc.) in order to determine which of the available channels is best. Due to the explicit information regarding the wide band channel that was communicated by means of the “Interference Notification” frame  303 , namely the center frequency and the channel bandwidth, the AP  102  can skip scanning channels  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  209 ,  210 ,  211 ,  212 ,  213 ,  214 ,  215  and  216  since they are all overlapped by the wide band channel  251  and concentrate its scanning efforts on the remaining channels. This a priori knowledge of the exact characteristics of the interfering wide band channel can save substantial time and energy for AP  102  in the scan process since it can safely skip scanning the channels overlapped by the wide band channel. 
     The method described above may be implemented by an wireless apparatus  500  as shown in  FIG. 6 . The wireless apparatus  500  may be an AP or a non-AP STA. The wireless apparatus  500  includes an RF antenna  502 , a Transmit/Receive unit  504 , a TX processor  506 , a RX processor  510 , a central controller  508 , a Data Source  512 , a Data Sink  514  and also memory  514 . The RF antenna  502  is responsible for transmitting and receiving radio signals, the Transmit/Receive unit  504 , is responsible for putting the bitstream to be transmitted onto the air as well as receive the incoming radio signals into bitstream. The TX processor  506  takes the responsibility of converting the various frames passed by the MAC layer into a bitstream to be passed to the Transmit/Receive unit  504 , while the RX processor  510  assumes the responsibility of the incoming bitstream from the Transmit/Receive unit  504  and passing the relevant information to the MAC layer. The MAC layer is implemented by the central controller  508 . Depending on the type of MAC program loaded on central controller  508 , the wireless apparatus acts either as an AP or as a non-AP STA. 
     Second Embodiment 
     According to the second embodiment of the present invention, while all the procedures described in the first embodiment remain the same, the CTS-to-self frame  301 , the CTS-to-self frame  302  and the “Interference Notification” frame  303  are all transmitted at the lowest possible MCS value. 
     Third Embodiment 
     According to the third embodiment, while all other procedures described in the first embodiment remain the same, the procedures of the frame exchange sequence are replaced with the following explanation referring to  FIG. 4 . 
       FIG. 4  describes an alternative frame sequence involved in transmitting the “Interference Notification” frame  303 . Due to presence of OBSS, the probability of the transmission of the “Interference Notification” frame  303  failing is reasonably high. In order to increase the success rate of the transmission, the “Interference Notification” frame  303  needs to be protected in both the wide band channel  251  as well as the narrow band channel  208 . This is achieved by the AP  112  gaining access to the medium on each of the narrow band channels that are overlapped by the wide band channel  251  by using appropriate channel access procedures and then simultaneously transmitting the CTS-to-self frames  304  addressed to itself in each of the narrow band channels that are overlapped by the wide band channel  251 . In this embodiment the CTS-to-self frame  304  is transmitted using the default Modulation and Coding Scheme (MCS) used for transmitting control frames in the short range BSS  110 . The protection duration  330  of the CTS-to-self frame  304  is set as the sum of the time interval  310   c  and the time required to transmit the “Interference Notification” frame  303  in the narrow band channel  208 . The time interval  310   c  is set as one Short Interframe Space (SIFS). Furthermore, the CTS-to-self frame  304  is transmitted by AP  112  at maximum TX power to ensure that all the devices within the maximum transmission range of AP  112  can receive the CTS-to-self frame  304 . Since the Receiver Address (RA) field of the CTS-to-self frame  304  does not match the MAC address of the device receiving the frame, the device will set its Network Allocation Vector (NAV) to the duration  330  set in the CTS-to-self frame  304 . In this manner, all the devices within the maximum transmission range of AP  112 , except the AP  112  itself, are restricted from transmitting any frame for the entire duration  330  required to complete the frame sequence shown in  FIG. 4 . After a time interval  310   c  equal to one SIFS, the AP  112  finally transmits the “Interference Notification” frame  303  in the narrow band channel  208  at maximum TX Power. In this embodiment, this “Interference Notification” frame  303  is transmitted using the default MCS used for transmitting control frames in the short range BSS  110 . AP  102 , upon receiving the “Interference Notification” frame  303 , becomes aware of the existence of the OBSS caused by the short range BSS  110 . AP  102  can start the process to find an alternative channel that is not overlapped by the wide band channel  251 . This process may involve various forms of scans (energy detection, active/passive beacon detection etc.) in order to determine which of the available channels is best. Due to the explicit information regarding the wide band channel that was communicated by means of the “Interference Notification” frame  303 , namely the center frequency and the channel bandwidth, the AP  102  can skip scanning channels  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  209 ,  210 ,  211 ,  212 ,  213 ,  214 ,  215  and  216  since they are all overlapped by the wide band channel  251  and concentrate its scanning efforts on the remaining channels. This a priori knowledge of the exact characteristics of the interfering wide band channel can save substantial time and energy for AP  102  in the scan process since it can safely skip scanning the channels overlapped by the wide band channel. 
     Fourth Embodiment 
     According to the fourth embodiment of the present invention, while all the procedures described in the third embodiment remain the same, the CTS-to-self frame  304  and the “Interference Notification” frame  303  are all transmitted at the lowest possible MCS value. 
     Fifth Embodiment 
     According to the fifth embodiment of the present invention, while all the procedures described in the third embodiment remain the same, the CTS-to-self frame  304  and the “Interference Notification” frame  303  are all transmitted at default MCS used for transmitting control frames in the extended range BSS  100 . 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2012-270380 which was filed on Dec. 11, 2012 and No. 2013-048334 which was filed on Mar. 11, 2013, the entire contents of which are incorporated herein by reference.