Abstract:
A radar, or other repetitive interference, detection and data flow control system and method for RF data transmissions uses an RF detector and an omni directional antenna to detect radar or similar signals. The information from the RF detector is communicated to a hub data transmission unit which processes the data about the signal provided by the detector. This information is used by the hub to schedule communications between the hub and subscriber units so that none of the communications between the hub and subscriber fall within the time period of the interfering pulses. Although there are no communications during the time periods of the pulses, the efficiency of communications is improved because collisions and retransmission of data are avoided.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to communications interference detection and mitigation systems and more particularly to a system and method for detecting certain types of RF interference and blanking RF data flow control in response thereto. 
     BACKGROUND OF THE INVENTION 
     Currently, there are several so-called “last mile” and “last foot” transmission systems which are designed to deliver high speed and/or high data capacity from one location to another. Several such systems use RF transmission to replace copper wire. Some of these systems are called point to point or point to multipoint systems and operate in various RF bands. A fundamental characteristic of such existing systems is that their RF transmissions occur in a frequency spectrum protected and regulated by a government body. These protected frequency spectrums, or bands, are licensed to certain license holders and only one operator (or a selected few) may operate in any given physical area. In such situations, rigorous rules apply to anyone holding permits for the usage of those protected bands. Another fundamental characteristic of such protected bands is that all users are licensed to perform the same type of RF transmission. 
     When operating in a licensed band the interference between transmissions is not only homogeneous, i.e., wideband, it originates from the same type of antenna to accomplish the same type of transmission and is thus controllable. Accordingly, noise (interference from another transmitter on the same frequency or on an interfering frequency) typically will be evenly spread. 
     In a typical licensed application, the frequency coordination would mathematically predict a certain low level of interference, and if you could not achieve a low level of interference, the license would not be granted. Once a governing body grants a license, then the user is afforded protection. Thus, in a protected band, if a narrow band interferer is detected, the licensed user could call the FCC (or other regulating agency) and ask that the agency investigate and rectify the problem. In an unlicensed band, the user is essentially on his/her own and usually no such official remedy is available. 
     Because of the licensed nature of some RF bands, only a limited number of companies may provide service within those bands. Thus, in order to widen the choices consumers have, it is desirable for service providers to be able to use unlicensed RF bands to provide high data rate capability to deliver high speed, high capacity data services. 
     In 1997 the FCC created a wireless arena called Unlicensed National Information Infrastructure (U-NII). System operators are free to operate wireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35 GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequency spectrum. Part 15 of the FCC document specifies the conditions for operating wireless equipment in the U-NII frequency band. However, operators are not protected from possible interference from other U-NII operators transmitting in the vicinity or even other systems which utilize the same frequencies. 
     The IEEE, a standards group, is defining a wireless LAN standard, referred to as IEEE 802.11a for operation in the U-NII band. Equipment that conforms to this standard will operate indoors at the lower frequency sub-band i.e. 5.15 to 5.25 GHz. The ETSI BRAN group in Europe has defined an air interface standard for high-speed wireless LAN equipment that may operate in the U-NII frequency band. Equipment that is compatible with this standard may cause interference with use of these unlicensed bands. 
     One major problem with the use of such unlicensed bands is that it is very difficult, if not impossible, to control RF interference from other users of the unlicensed band. These other users may be using the selected unlicensed band for uses which are essentially different from that employed to deliver communication services. For example, the 5.25 to 5.35 GHz and 5.725 to 5.825 GHz bands are available for use for outdoor data communication between two points. This is typically a wideband use. The same bands are also available for other applications including radar. When the same band is used for wideband communication and also used by others for uses such as radar, data communications between sending and receiving antennas will experience significant interference from radar pulses, which are broadcast over a wide area in repetitive bursts. 
     In the current state of the art, there is no discrimination between narrow band or wideband interference. When interference is detected, it is usually based on a signal to noise ratio for any given channel, then the radio switches to a lower order modulation, from either 64QAM to 16QAM, or 16QAM to QPSK, or QPSK to BPSK. Such a lower modulation shift allows more tolerance for noise and interference. Similarly, for Orthogonal Frequency Division Multiplexing (OFDM), the modulation order of the subcarriers is optimized for any given signal to noise ratio. 
     The prior art of radar interference mitigation is intended for use in currently licensed RF bands. However, radar interference is not an issue of great concern in licensed bands because there is little or no such interference. Most licensed bands are free and clear of other harmful interferers. As a result licensees do not have to deal with the issue of narrow band interference such as radar. Additionally, most unlicensed bands do not have strong radar interferers. However, there is other low level interference in the unlicensed RF bands. This interference is at a much lower level and has a different signature than high powered radar. Therefore, generally speaking, prior art interference mitigation systems do not detect radar interference nor do they attempt to avoid it. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method which detects repetitive RF signals, including radar, and mitigates the effects of these signals on data transmissions in RF bands by blanking the data transmissions for a period of time coinciding with the period of the repetitive interference signal and rescheduling transmissions. 
     The system operates with an RF detector and an omni directional antenna to detect RF or other periodic signals. The information from the RF detector is communicated to a hub data transmission unit which processes the data pertaining to the signal. This information is used by the hub to schedule communications between the hub and subscriber stations so that none of the communications between hub and subscriber stations fall within the time period that a radar or other interfering pulse is present. Although there are no communications during the time period of the radar pulses, which increases overhead, the efficiency of the communications is improved. Efficiency is improved because collisions and retransmission of data that would otherwise occur, if transmissions were carried on during the radar pulses, are avoided. As a result, the overall throughput is increased because of this intelligent scheduling. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  is a diagram of an RF data transmission system using the system and method of radar detection and data flow blanking control of the present invention; 
         FIG. 2  is a flow diagram showing typical operation; and 
         FIGS. 3 and 4  show how time slots can be skipped, shifted or rescheduled to avoid radar interference. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a system and method which detects repetitive, RF signals, such as radar, and mitigates their effect on data transmission in various RF bands by blocking data transmissions for a period of time coinciding with the period of the radar signal. Turning to  FIG. 1 , data transmission system  10  has hub  11  (which could be one of many) and subscriber station  12 , again one of many. Hub  11  is connected in a typical installation to other remotely located users (not shown) via one or more networks, such as MAN/WAN  111 , Internet  112 , or any other network, such as network  113 , all via switch, router and/or add drop multiplexer (ADM)  110  and interface  104 . These networks could be internal to an enterprise or could be connected to public or private networks either directly or via an intermediary network. Power for the hub  11  is provided via power supply  103 . 
     Hub  11  serves to direct communications between subscriber  12  and other users over RF link RF 2  between one or more hub antennas  106  and subscriber antenna  107 . Transmission between these antennas can use one or more modulations, such as but not limited to, OFDM, 64QAM, 16QAM, QPSK or BPSK. At subscriber station  12 , transmission to/from customer premises equipment (CPE)  109  flows, by way of example, via wall jack  108 . A repetitive interferer, such as radar antenna  13  sending out RF signals RF 1  impinging on antenna  106 - 2  causes interference with transmission between hub  11  and subscriber  12 . 
     An omnidirectional antenna  1301  is connected to an RF detector  14 . The RF detector  14  provides strength of signal, duration and timing information associated with a radar or other interfering pulse to the processor  101  for an interference source  13 . Preferably, the omnidirectional antenna and radar detector data can be supplemented by directional information from the hub antennas  106 - 1 ,  106 - 2  and  106 - 3 , which are sectorized. 
     The subscriber station antenna  107  can also detect radar interferers. The subscriber antenna is a much more directional antenna and it is not generally set up as high above the ground as the hub antennas, so there is much less chance for interference on the subscriber station. However, the subscriber station can perform the same type of radar detection and rescheduling for data transmissions as discussed for the hub to avoid interference. However, The subscriber station preferably does not have dedicated RF detector hardware. Preferably it uses its primary receiver to detect interference. 
     The timing of data transmission to avoid radar or other interfering pulses is handled by the hub&#39;s processor  101  and the media access control layer (MAC)  102  for each modem  105 . Timing is coordinated so that interference will be mitigated by scheduling data transmissions via modems  105  around interfering pulses. Similarly, for the subscriber stations, additional functionality is built into the subscriber station processor and MAC for interference mitigation. 
     System  10  preferably synchronizes to the radar signal RF 1 . The processor  101  predicts the next pulses staying locked to the frequency of the pulses, and reschedules data transmissions for the period that the interference from the pulses of the radar occurs. This method is much more efficient in terms of data throughput than not rescheduling the data. In other words, if a data/radar collision occurs, the data must be resent resulting in a loss of time. It is much more efficient to just avoid transmission for the period of the radar pulse. The software acting as the MAC makes a decision whether it is more efficient in terms of payload, customer payload or data payload to reschedule the data, or to allow the forward error correction (FEC) to attempt to correct errors in the data transmission. If the radar pulses are short enough in duration, it may be more efficient to let the FEC circuit associated with the subscriber CPE  109  handle the errors, or allow the errors to occur and resend the data. Hence, the intelligence and the algorithm of the MAC software will need to determine the most efficient action, be it rescheduling or to do nothing. 
     Preferably, the period of the radar pulse and the repetition rate of the radar pulses are tracked. If the repetition rate of the radar pulses is too short or too long to be handled by an interleaver, then the MAC software will also make the decision on whether to reschedule or to allow the FEC to fix the errors. Interleaving coded data transforms bursty channel errors into discrete independent errors. An interleaver formats encoded data in a rectangular array of rows and columns. The bits are stored row-wise and read out column-wise at a transmitter. At a receiver, the data is read out row-wise, restoring the original data sequence. As a result of this reordering of the data during transmission, a large burst or block of errors is broken up into a stream of smaller independent errors, which can be corrected. 
     The logical branch diagram of  FIG. 2  illustrates the present method for radar and other interference detection and rescheduling of data flow control  200 . In box  201  extraneous RF signals are monitored in accordance with the methods described herein, using an omnidirectional antenna and an RF detector. The hub antennas can be used to provide directional information about the interference. In box  202  the periods and duration of the interference is determined by processor  101 . Processor  101  uses this information to develop a profile of interference so that it can schedule data transmissions around the interference, as indicated in box  203 . This scheduling will avoid data transmission during predictable times of interference from a regular interferer such as a radar installation  13 . The data transmissions will be rescheduled to avoid the profiled radar or offending pulse. 
     For example, in  FIG. 3  scheme  300  is shown for minimizing the effects of interference  301  in accordance with the mitigation technique of  FIG. 2 . For a number (n) of various length transmission time slots,  302 , broadcasting at a given frequency and polarity, one time slot, B,  303  is disrupted by interference pulse  301 . As shown in the lower left portion of  FIG. 3 , time slot B&#39;s transmission is rescheduled, shifting transmission to avoid interference  301  leaving an unused time slot or blank  304 . Since interference pulse  301  can be of varying length and is not necessarily synchronized to the transmission time slots, interference  301  can later impinge on a different time slot, as illustrated in the right side of  FIG. 3 . Here interference pulse  301  has disrupted time slot A,  306 , of a number of varying length time slots,  305 . Here the time slots of the transmissions are shifted or rescheduled to allow time slot A,  306 , to transmit in the clear. Thus, creating blank  307 . If the interference is permanent or continues for a long period of time, a higher order modulation or different code rate may be used to accommodate the data within the remaining available time. Alternatively, the overall code rate can be reduced to accommodate the data within fewer time slots, or the overall data throughput reduced. 
     Turning to  FIG. 4  another interference mitigation scheme  400  in accordance with the mitigation technique of  FIG. 2  is shown for interference  401  affecting two time slots  403  and  404 , or  407  and  408  of anumber of various length time slots,  402  and  406 , respectively. Shifting the transmissions in time allows transmission of time slots  403 ,  404 ,  407  and  408  in the clear. Here the interference originally impinges upon two separate time slots  403  and  404 , or  407  and  408 . In the lower portion of  FIG. 4  it is shown that by shifting or rescheduling the time slots of data transmissions blanks  405  and  409  is created as opposed to corrupting the pairs of data time  403  and  404 , or  407  and  408 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.