Abstract:
A duty cycle management system and method for use in a wireless device having a transmitter which transmits packets in the form of individual packets and/or packet bursts comprising a predetermined maximum number of contiguous packets. The transmitter has an associated predetermined duty cycle limit for transmission of packets and is operable over a duration measurable as a series of time windows each of which comprises a predetermined number of timeslots with each timeslot having a predetermined nominal duration. Transmission of one packet occupies one timeslot. A controller in combination with an instruction set limits the number of packets transmitted during each time period defined by one time window or adjacent time windows, on a sliding basis, so as to limit the duty cycle of transmissions during successive adjacent time windows to the predetermined duty cycle limit. To do so transmission of packets is delayed as needed to establish sufficient idle period(s) during the one or adjacent time window(s) to maintain the duty cycle within the duty cycle limit. An identification is made prior to the transmission of a packet whether an individual packet or a packet burst transmission mode is to be applied. A counter is used in the preferred embodiment for keeping a running count which correlates to the duty cycle over the period of a time window. The counter is incremented by an amount IC for each timeslot in which a packet is transmitted and is decremented by an amount DC for each idle timeslot, whereby the ratio of IC to DC is the duty cycle limit. Where a maximum count amount, equal to the predetermined number of timeslots per time window, would be exceeded packet transmission is inhibited.

Description:
FIELD OF INVENTION  
         [0001]    The present invention is directed to management of the duty cycle of wireless data transmissions and, in particular, to an adaptive duty cycle management method and system for digital radio frequency (RF) transmitters which limit the duty cycle on a sliding window basis to a preselected maximum amount.  
         BACKGROUND OF THE INVENTION  
         [0002]    There exists a concern regarding the safety to users of wireless communications devices, such as mobile radios and cellular telephones, when these devices are operated in their transmit mode by reason that it is perceived the electromagnetic radiation associated with the transmission of the signals from such devices may pose a potential health hazard to the user. When using a hand-held wireless device the user holds the device near the ear and this positions the devices transmission antenna in close proximity to the user&#39;s hand and head as it transmits electromagnetic radiation and it is postulated that a portion of this radiation may be absorbed by the head and hand and that such absorption may cause health risks to the user. The perceived potential health hazard associated with wireless transmitting devices increases as the power of the signals transmitted from the device increases.  
           [0003]    To address this perceived health risk many countries have adopted legislation requiring that the effective radiated power (ERP) of wireless devices be limited. For instance, the American National Standards Institute (ANSI) in association with the Institute of Electrical and Electronic Engineers, Inc. (IEEE) has adopted new standards for RF exposure viz. ANSI/IEEE C95.1-1992. The Federal Communications Commission (FCC) has adopted limits for maximum permissible exposure (MPE) time which, generally, are more restrictive than the previously adopted limits and guidelines and apply to land-mobile systems, such as cellular radio, pocket and hand-held radio telephones. These limits are based on recommended exposure guidelines published by the National Council on Radiation Protection and Measurements (NCRP) in “Biological Effects and Exposure Criteria for Radio frequency Electromagnetic Fields, “NCRP Report No. 86, Sections 17.4.1, 17.4.1.1, 17.4.2 and 17.4.3. Copyright NCRP, 1986, Bethesda, Md. 20814. The guidelines provide for exclusions to the regulations if it can be shown through laboratory procedures that exposure conditions do not exceed a certain specific absorption rate (SAR) or, alternatively, if the radiated power is below a certain level.  
           [0004]    A quantification of MPE for uncontrolled environments is based on, inter alia, the electric and magnetic field strengths, the power density, the frequency range of the radio signal, the distance between the user&#39;s head and the transmitter radiating point. Thus, the MPE can be correlated to a maximum permitted transmission power, which may vary with the frequency range of a wireless device.  
           [0005]    One method of meeting the applicable MPE limitations is to reduce the duty cycle of the transmitter. While techniques such as TDM (time division multiplexing) inherently activate the transmitter for less than 100% percentage of the total communication time, it is not an easy task to reduce the duty cycle of the transmitter on a source basis (i.e. without regard to network traffic) in such a way as to effectively control the duty cycle without negatively degrading the performance of the network and without averaging duty cycles over unreasonably long periods (tens of minutes or hours). Instead, since data is randomly transmitted over various time intervals, it is desired that the wireless device itself limit the effective transmit (active) time.  
           [0006]    In order to address the foregoing need for an effective method and system for managing the duty cycle of a transmitter the inventor developed the subject matter herein which makes effective use the quiet time inherent during a transmission.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method and system for adaptively limiting the duty cycle of a transmitter of a wireless device in order to comply with MPE requirements while preserving the ability to broadcast RF signals at a level sufficient to establish and/or maintain a quality radio link. This is achieved by sliding (moving) a reference window of time over which a preselected duty cycle is calculated (e.g. one minute), thereby taking advantage of any quiet time preceding a burst, and afterwards enforcing only as much quiet time as required to meet the desired duty cycle limitation. The specific duty cycle to be imposed by the method and system is predetermined and selectable (i.e. a duty cycle of, say, 2% can be set as can a duty cycle of, say, 25%). The resulting average power output is therefore also controllable by selecting the appropriate duty cycle.  
           [0008]    In accordance with the present invention there is provided a duty cycle management system and method for use in a wireless device comprising a transmitter for transmitting packets in the form of individual packets and/or packet bursts comprising a predetermined maximum number of contiguous packets. The transmitter is operable over a duration measurable as a series of contiguous time windows with each time window comprising a predetermined number of timeslots and each timeslot having a predetermined nominal duration. The transmitter is configured for transmitting one packet during one timeslot and has an associated predetermined duty cycle limit for the transmitted packets. A controller in combination with a storage medium containing instructions executable by the controller limits the number of packets transmitted during each time period defined by one time window or adjacent time windows, on a sliding basis, so as to limit the duty cycle of transmissions during successive adjacent time window(s) to the predetermined duty cycle limit. In doing so the transmission of packets is delayed as needed to establish sufficient idle period(s) during adjacent time windows to maintain the duty cycle within the duty cycle limit.  
           [0009]    The controller identifies prior to the transmission of a packet whether such packet is an individual packet or a burst packet. A counter is used in the preferred embodiment for keeping a running count which may be correlated to the duty cycle over one or more time windows. The counter is incremented by an amount of either IC or two times IC for each timeslot in which a packet is transmitted and is decremented by an amount DC for each idle timeslot, whereby the ratio of IC to DC is the duty cycle limit. For an individual packet, transmission of the packet is permitted when the counter amount does not exceed a maximum counter amount equal to the predetermined number of timeslots per time window. However, transmission of the packet is delayed when the counter amount has reached that maximum counter amount (viz. the predetermined number of timeslots per time window) until the counter amount falls below the maximum counter amount. For a burst packet, the controller tests, by determining whether the counter amount plus the number of packets in the burst times IC is equal to or greater than the maximum counter amount, whether transmission of the number of packets in the burst would cause the counter amount to reach the maximum counter amount. If it would only an individual packet is transmitted (not the burst) and if it wouldn&#39;t the burst is transmitted. In a special case where the duty cycle is so low that transmission of a single packet burst within one time window would increase the counter amount by an amount near or equal to the maximum counter amount, the controller permits transmission of the burst packets only if the counter amount is zero and, if the counter amount has been zero for at least a number of timeslots equal to the predetermined number of timeslots per time window, the counter is incremented by IC or, if the counter amount has not been zero for at least a number of timeslots equal to the predetermined number of timeslots per time window, the counter is incremented by two times IC.  
           [0010]    Preferably, the controller in combination with the instructions is configured for providing priority, for transmission, to a predetermined maximum number of acknowledgement packets by permitting transmission of up to maximum number of acknowledgement packets when the counter amount is equal to or greater than IC/DC times the maximum number of packets per packet burst but less than a predetermined maximum count.  
           [0011]    Advantageously, the invention provides for a wireless transmission not exceeding an established duty cycle limit based on short fixed-length windows of time, while permitting longer transmission times in the form of bursts in order to improve response times. This results in a greater range of power output, which maximizes transmitter&#39;s ability to maintain quality radio links while avoiding possible detrimental effects associated with uncontrolled radiation environments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A preferred embodiment of the invention is described below, by way of example only and without intending to limit the scope of the invention claimed herein, with reference to the following drawings.  
         [0013]    [0013]FIG. 1 is a block diagram of the adaptive duty cycle management system components of a digital wireless device (such as a private network mobile radio unit or public network wireless phone) in accordance with the invention;  
         [0014]    [0014]FIG. 2( a ) is a flow chart of the steps performed by the adaptive duty cycle management system shown in FIG. 1 whereby a relatively high duty cycle, e.g. 25%, is applied and  
         [0015]    [0015]FIG. 2( b ) is a flow chart showing the special case steps performed by the adaptive duty cycle management system whereby a relatively low duty cycle, e.g. 2%, is applied such that transmission of a single packet burst within a number of timeslots equal to the predetermined number of timeslots per time window would increase the counter amount by an amount near or equal to the maximum counter amount;  
         [0016]    FIGS.  3 ( a ) and  3 ( b ) are graphs showing the count pattern established by the controller for the special case example of FIG. 2( b ) by which a low duty cycle of 2% is applied, with three full one-minute time windows being shown, wherein the graph of  
         [0017]    [0017]FIG. 3( a ) shows the situation in which a burst occurs during the second time window following an idle period during the first time window and the graph of  
         [0018]    [0018]FIG. 3( b ) shows the special case situation in which a burst occurs during the second time window following the transmission of some packets during the first time window. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    The following description is of an example of a preferred embodiment of the invention which is provided only for purposes of illustration and without any intention to limit the invention to this particular example or to any particular duty cycle limit.  
         [0020]    In digital wireless transmission, the transmitter circuitry modulates a carrier signal with a digital signal comprised of data packets. The transmission of the modulated data packets is typically performed according one of two modes being that of individual packets or bursts of packets. Each packet comprises address, data, sender identification and control portions and instead of being transmitted continuously they are stored until an individual packet, or a group of packets is ready for transmission. Therefore, the output power generated by the transmitter follows a pattern of short intervals in order to transmit bursts (being groups of packets) coupled with relatively long periods of quiet time (also referred to as idle time) between such short burst intervals. In the example of the preferred embodiment a packet burst is defined to be from 3 to 23 packets.  
         [0021]    Referring to FIG. 1, there is shown a block diagram of adaptive duty cycle management system components of a digital wireless device (such as a private network mobile radio unit or public network wireless phone) in accordance with the present invention. It is to be understood that only those components of the device which relate to this invention are shown and that many other components of the wireless device are not shown.  
         [0022]    The wireless device  10  includes a transmitter  4 , a controller  6  which is a suitable microprocessor in the preferred embodiment, a receiver  8 , and a counter  2  which is provided by a memory component and the microprocessor in the preferred embodiment. Device  10  may be a mobile unit, or any transceiver used in the industry. The transmitter  4  and receiver  8  establish connection with the wireless network (e.g. cellular network) in the manner well-known to persons skilled in the art, under control of the controller  6 . Machine readable storage  9 , comprising non-volatile memory, stores the duty cycle management system algorithm and information and parameters utilized thereby including the parameters IC and DC representing the value of an increment count and the value of a decrement count, respectively, whereby the parameters IC and DC are fixed numbers for any given embodiment and used by the system in applying the duty cycle management method. In this embodiment a counter  2  is provided for use in controlling the active time of a transmitter.  
         [0023]    The counter  2  functions in co-operation with other components of the system shown in FIG. 1 to limit the transmitter duty cycle to a selected duty cycle (DS) which, for one exemplary embodiment presented in the following, is selected to be 2% in order to achieve 1.5W ERP using a 40W transmitter with a 3 dB (approximately) gain antenna. In this embodiment the counter maintains a running count which may be correlated to the duty cycle over one or more time windows and it does so by incrementing an amount of either IC or two times IC for each timeslot in which a packet is transmitted and decrementing by an amount DC for each idle timeslot, the ratio of IC to DC being the duty cycle limit. Thus, the duty cycle limit and the resulting average power output can be varied simply by changing the value of IC and/or DC so as to change the ratio IC/DC (which equals the duty cycle limit).  
         [0024]    For the example of a 2% duty cycle, when the transmitter transmits in the individual packet mode the packets are sent in timeslots (alternatively referred to as “slots” herein) of 52 ms nominal duration (which corresponds to about 1153 slots/minute), at a maximum rate of 23 packets per minute, and the time window (T) in this example is considered to be 1 minute (i.e. 2%=X packets/60 sec.×0.052 sec/timeslot, so X=23). When operating in burst mode the transmitter transmits a contiguous series of 3 to 23 packets (maximum) over a time period totalling about 1.2 seconds maximum (i.e. 23×0.052 seconds) at a transmission rate of 19,200 bits/sec. Bursts are considered to represent a special case in this example, due to the relatively low duty cycle limit of 2%, and can be transmitted no more than once per minute (this one minute window corresponding to about 1153 slots). A further example, applying a relatively high duty cycle limit of 25%, for timeslots of the same 52 ms nominal duration, allows for a maximum of 288 packets per minute (i.e. 25%=X packets/60 sec×0.052 sec/timeslot, so X=288) so many bursts of up to 23 packets may be transmitted within one time window of 1 minute without nearing the maximum duty cycle of 25%.  
         [0025]    As is well-known in the art, the determination of when to transmit a burst is made when the wireless device makes a reservation request to the base station to request permission to send a burst and the base station grants such reservation request.  
         [0026]    FIGS.  2 ( a ) and  2 ( b ) are flowcharts showing the steps performed by the duty cycle management system which utilizes the controller  6 , the counter  2  and memory  9 . These flowcharts each represent a repetitive algorithm which is repeated for each time slot and this algorithm is implemented in one or more computer programs which are executable by the controller  6  (microprocessor). FIG. 2( a ) shows the normal steps of the algorithm to be used for individual packet and burst packet transmission when a relatively high duty cycle limit is applied (e.g. 25%) and FIG. 2( b ) shows the special case steps of the algorithm to be used for individual packet and burst packet transmission when a relatively small duty cycle limit is applied (e.g. 2%). For a selected duty cycle of 25% in this illustrated example a firmware counter  2  increments by the increment count (IC) amount of 4 upon each individual packet transmission and decrements by the decrement count (DC) amount of 1 upon each idle (receive) slot (i.e. ¼ equals 25%) whereas for a duty cycle of 2% the counter  2  is incremented by 50 (i.e. IC=50) upon each individual packet transmission and decremented by 1 (i.e. DC=1) upon each idle (receive) slot (i.e. {fraction (1/50)} equals 2%). When the counter  2  exceeds a maximum counter amount equal to the predetermined number of timeslots per time window, being 1153 for these examples, the system halts any further transmissions because this means that, at this point in time, a ratio of 4 to 1 packets (representing a duty cycle of 25%) or 50 to 1 packets (representing a duty cycle of 2%) have been transmitted over the preceding window time period T.  
         [0027]    For the example of a 25% duty cycle limit a single packet burst of the maximum 23 packets will only increment the counter  2  by 92 counts (i.e. 4×23), which is far below the maximum count amount of 1153, so several bursts may be transmitted within one time window without pushing the counter amount close to the maximum count threshold. However, for the low duty cycle limit example of 2% a single packet burst can, if it consists of 23 packets, use up essentially all of the permissible time slots for a given time window. Consequently, for this special case example a packet burst may only be sent when the value of the counter  2  is zero. From a time line perspective this means that a 1.2 sec maximum-length burst transmission (i.e. 52 ms×23=1.2 sec) leaves the counter  2  at its maximum value of 1150 for that increment level (IC) and this inhibits any further burst transmissions for the next period of 59.8 seconds calculated on the basis of 1150×52 ms. This represents the worst-case duty cycle limit of 2% and results in a calculated duty cycle value of 1.96% over the period of those 1150 timeslots which satisfies this duty cycle limitation of 2% (i.e. 1.2 sec divided by (59.8 sec+1.2 sec=1.2/61=1.96%).  
         [0028]    As shown by FIG. 2, the special case algorithm of the duty cycle management system (i.e. the situation in which a single packet burst would render the counter close to the maximum counter amount within a single time window) is performed on an adaptive basis whereby one of two methods (algorithms), A or B, is selected and performed for the transmission of bursts and the selection of which algorithm is performed impacts upon the performance of the system in relation to the communications network. FIGS.  3 ( a ) and  3 ( b ) illustrate, graphically, the results of performing methods A and B, respectively, for a duty cycle limit of 2%. The steps of method A are performed when there have been no transmissions at all (i.e. no burst and no individual packet) for a number of timeslots equal to the predetermined number of timeslots per time window prior to the pending burst (i.e. the counter value is zero and there must have been no increment to the counter during the immediately preceding time window). This limits the duty cycle of bursts plus individual packets to 2% over any double window period (i.e. any period of 2 minutes in this example). Application of this method will occur in networks where a burst is typically the first of a series of transmissions, and may be followed by a small number of individual packets such as an application-level acknowledgement packet (alternatively referred to herein as an ‘ACK’ packet, being a type of packet used to acknowledge receipt of an incoming message). For purposes of network efficiency it is desirable to avoid delays in transmitting “ACK” packets as this may cause an unnecessary retransmission of the message and, thus, an “ACK” priority feature, as detailed more fully below, may be used in combination with duty cycle algorithm of the invention in order to enhance the overall network performance.  
         [0029]    The steps of method B are performed when the value of the counter is zero but the counter has been incremented during the immediately preceding window i.e where some individual packets have been sent during that window. For this method the counter is incremented by 2 times the value of IC (i.e. by 100 in the foregoing example applying a 2% duty cycle limit) for each packet in a burst. Following the transmission of the burst in this example the value of the counter is 2300 (calculated on the basis of 100 times 23 packets) and this means that following a maximum-length burst all transmissions will be halted for one time window (i.e. 1 minute in this example) and any further burst will be halted for two time windows (i.e. 2 minutes in this example). Application of this method will occur in networks where a burst typically ends a series of exchanges.  
         [0030]    These two alternate burst-mode methods (A and B) are selected adaptively to yield best performance by using the following algorithm (this is also illustrated by FIG. 2):  
         [0031]    IF  
         [0032]    there have no transmissions during the preceding window  
         [0033]    THEN  
         [0034]    debit burst packets at 1× IC each (IC=50 in the example) (It is necessary to exclude the reservation-request packet, or make the debit-rate decision at reservation-request time and not at burst transmit time or the above case will never be true.)  
         [0035]    ELSE  
         [0036]    debit burst packets at 2× IC each (IC=50 in the example)  
         [0037]    By performing the steps of this algorithm the system automatically allows individual packets to follow a burst at 1 packet per 2.6 seconds if the transmitter was idle for the minute before the burst and if the transmitter was not idle for the minute before the burst it defers all post-burst transmissions for 1 minute and prevents additional bursts for 2 minutes.  
         [0038]    When the counter  2  is non-zero but still below the maximum counter amount only individual packet transmissions are allowed. Each packet transmission increments the counter by 50, limiting the sum of individual packets to the same 2% duty cycle as burst transmissions.  
         [0039]    The pre-burst zero-count limitation prevents a burst from following individual packets and exceeding the 2% duty cycle. Following a burst transmission, individual packets can be sent only if the original burst was shorter than the full 23-packet maximum, or if sufficient time has elapsed to decrement the counter sufficiently to enable an amount IC to be added without going over the maximum counter amount. These limitations maintain the duty cycle, defined by the ratio of (burst+packet) to (burst+idle+packet), to 2% or less. In other terms, these limitations impose a maximum transmission rate of one individual packet per 2.6 seconds (52 ms/2600 ms=2%) or groups (not bursts) of 2-3 individual packets at shorter intervals only if surrounded by longer periods of idle.  
         [0040]    At data rates below 19,200 bits/sec, the slot duration may differ from the averaged value of 52 msec used for this example but, because the duty-cycle management method counts slots at the rate of IC for each transmitted packet (i.e 50× tx) minus DC for each idle slot (i.e. 1× rx), it automatically scales for other slot durations while still enforcing the selected duty cycle (being 2% in this example). For example, if the averaged slot duration were to be 112 ms the maximum burst duration would be 2.576 seconds (i.e. 23 packets times 112 ms) and this would be followed by 128.8 seconds of idle (i.e. if 1150 idle slots are used for a calculation of the remainder of the time window whereby 1150×112 ms=128.8). Therefore, the resulting average duty cycle over the calculated timeslots would be the same as that shown above for the situation in which the averaged slot duration was 52 ms: 2.576 s/(2.576 s+128.8 s)=1.96%.  
         [0041]    Optionally, a provision for acknowledgement (ACK) priority may be implemented in combination with the duty cycle management method of the invention, it being recognized that ACK packets are always short individual transmissions and never bursts. This option may be used to address a need to avoid the situation in which the wireless communication device, having reached its duty-cycle limit, will be unable to acknowledge receipt of an incoming message, one overall network performance objective being to allow for the transmission of acknowledgments in a timely fashion so as to prevent unnecessary re-transmissions from the base station and maintain network throughput. Accordingly, in one preferred embodiment of the invention an ACK priority exception is implemented in combination with the foregoing duty cycle management algorithm. This exception algorithm is configured to permit the transmission of an ACK packet (if required) following the transmission of a packet. This anticipates that an inbound (from mobile to base) transmission may contain a query which will generate an outbound (from host to mobile) response. Unlike the duty-cycle management method, this permitted priority-ACK limit is not cumulative but is instead a fixed (configurable) maximum number of ACKs which can follow a data transmission.  
         [0042]    The acknowledgment transmissions are counted (debited) as normal transmissions by the duty-cycle calculation and will further delay other data transmissions as required to respect the duty cycle limit.  
         [0043]    To respect the duty-cycle limit, only a small number (typically one) of ACKs can follow a data transmission. This provides a two-stage limit (or a ‘soft threshold’) for the duty-cycle; a first level after which a unit will not transmit further data but may send an ACK, and a second limit (i.e. a predetermined priority maximum count), shortly beyond, after which the unit will not make any further transmissions. Alternatively, the system could apply a lower maximum threshold (e.g. slightly less than 1150) for the transmission of data packets and a higher one (i.e. greater than 1150) for ACKs.  
         [0044]    For the foregoing example, providing for a 2% duty cycle limit, the ACK priority feature utilizes the fact that the maximum counter amount of 1150 is less, by about 3 timeslots (actually 3.8 timeslots in that there are actually 60 sec/52 msec=1153.8 slots in one minute), than the actual number of timeslots in one time window of one minute. The ‘priority’ mechanism operates so as to assure that the full maximum number of timeslots is not consumed by data such that there remains some ‘headroom’ for the priority-acknowledgement packets. In other words, data is throttled at 1150 timeslots, but an acknowledgement packet would be permitted to be transmitted up to a maximum point of the 1152 or 1153 timeslot so as to allow an ACK packet to be sent even if data cannot be sent pursuant to the governing duty cycle management algorithm.  
         [0045]    The individual circuit and processing functions utilised in the foregoing described preferred embodiment are, individually, well understood by those skilled in the art, and it is to be understood by the reader that a variety of other implementations may be devised by skilled persons for substitution. Further, it should be noted that although the some of the system components described herein are stated to be implemented by firmware in the preferred embodiment a person skilled in the art will recognize that it may be preferred for another application to implement such components in hardware. Persons skilled in the field of communication design will be readily able to apply the present invention to an appropriate implementation method for a given application.  
         [0046]    Consequently, it is to be understood that the particular embodiment shown and described herein by way of illustration is not intended to limit the scope of the invention claimed by the inventor which is defined by the appended claims.