Abstract:
A system and method for medium access control (MAC) protocol which controls transmissions in wireless devices to at least substantially comply with specific absorption rate (SAR) limits. The system and method of MAC protocol uses “source-based” time averaging measurements of transmitter “on-times” and an integral of transmission power to direct transmitter duty-cycle such that SAR limits are met. The system and method monitors and controls topology-dependent functions of a node transceiver, including transceiver duty cycle, transceiver transmit power levels and transceiver power-time products, using a automatic transmission control protocol (ATP) to vary functions to maintain specific absorption rate (SAR) limits for handheld and portable devices which are used close to human bodies. In cases in which SAR limits are approached, the ATP can direct the transceiver at the device to adjust each function, or combination of functions, to maintain an acceptable SAR limit.

Description:
This application claims benefit under 35 U.S.C. §119(e) from U.S. provisional patent application Ser. No. 60/385,563 entitled “MAC Protocol With Duty-Cycle Limitation For Portable Devices In A Wireless Ad-Hoc Communication Network And A Method For Using The Same”, filed Jun. 5, 2002, the entire contents of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a system and method for providing medium access control (MAC) protocol to direct transmissions in wireless devices such that specific absorption rate (SAR) limits are met. Specifically, the present invention uses “source-based” time averaging measurements of transmitter “on-times” and an integral of transmission power to direct transmitter duty-cycle such that SAR limits are met. 
   2. Description of the Related Art 
   Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive such data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in it&#39;s coverage area. 
   In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference. 
   More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks, such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, granted on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, granted on Mar. 29, 2005, the entire content of each being incorporated herein by reference. 
   A mobile node may include any number of devices, such as laptop computers or handheld mobile telephones. In the case of many of these devices, concerns regarding electric/magnetic fields have led to the creation of guidelines regarding optimum safe operation. The FCC is required by the National Environmental Policy Act of 1969 to evaluate the effect of emissions from FCC-regulated transmitters on the quality of the human environment. At the present time there is no federally-mandated radio frequency (RF) exposure standard. However, several non-government organizations, such as the American National Standards Institute (ANSI), the Institute of Electrical and Electronics Engineers, Inc. (IEEE), and the National Council on Radiation Protection and Measurements (NCRP) have issued recommendations for human exposure to RF electromagnetic fields. The potential hazards associated with RF electromagnetic fields are discussed in OET Bulletin 56 entitled, “Questions and Answers About the Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields”, the entire content of which being incorporated herein by reference. 
   As described in an FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, Supplement C to OET Bulletin 65, the entire content of which being incorporated herein by reference, FCC rules require routine environmental evaluation of RF exposure for certain mobile and portable devices. Unless the device is categorically excluded from routine environmental evaluation, applications to the FCC for equipment authorization must include an affirmative statement indicating that the device is in compliance with FCC-adopted limits for RF exposures. In some cases it may be necessary for the applicant to provide certain information to document the test procedures used to evaluate compliance. 
   Mobile devices that operate at 1.5 GHz or below with an effective radiated power (ERP) of 1.5 watts or more, and those devices that operate at frequencies above 1.5 GHZ with an ERP of 3 watts or more are required to perform routine environmental revaluation for RF exposure prior to equipment authorization or use. Mobile devices may be evaluated with respect to field strength, power density or specific absorption rate (SAR) limits. Devices may be evaluated with respect to SAR limits using either measurement or computational methods. 
   The Federal Communications Commission (FCC) sets SAR limits for handheld and portable devices which are used close to human bodies and do not pass electric/magnetic field limits due to the close proximity of device antennas to the human body. The FCC rules for evaluating portable devices for RF exposure compliance are described in 47 CFR §2.1093, the entire content of which being incorporated herein by reference. As noted in the FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, referenced above, for purposes of RF evaluation, a portable device is defined as a transmitting device designed to be used with any part of it&#39;s radiating structure in direct contact with the body of the user or within 20 centimeters of the body of the user under normal operating conditions. This category of devices would include hand-held cellular and PCS telephones that incorporate the radiating antennae into the handpiece and wireless transmitters that are carried next to the body. Portable devices are evaluated with respect to SAR limits for RF exposure. For most portable transmitters used, the applicable SAR limit is 1.6 watts/kg as averaged over any one gram of tissue. 
   Protection guidelines typically include operating warnings and warning labels for specific operation ranges, as described in an FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, Supplement C to OET Bulletin 65, referenced above. When operating and warning instructions are ineffective, the use of warning labels on the transmitting element may also be necessary to caution nearby persons to limit their exposure duration and/or conditions to ensure compliance. If warning labels are not desirable, specific absorption rate evaluations, even when not required, may be used to demonstrate compliance to obviate the need for any warning label that might otherwise be necessary. Additional details of specific absorption rates may be found in 47 CFR 2.1091, and 47 CFR 2.1093, the entire contents of each being incorporated herein by reference. 
   However, depending on antenna gain and transmission power levels at a device, SAR requirements can typically be difficult with which to comply. Evaluation of mobile devices may be performed with respect to compliance with SAR limits, but it is often simpler and more cost-effective to evaluate compliance with respect to field strength or power density. Accordingly, a need exists for a medium access control (MAC) protocol which directs and controls transmitter operation at a device, in an ad-hoc system with large cell radius, such that SAR requirements are satisfied and demonstrate compliance to obviate the need for any warning label that might otherwise be necessary. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a system and method of MAC protocol which allows handheld and portable device operation with a maximum specific absorption rate (SAR) in any condition. 
   Another object of the present invention is to provide a system and method to monitor topology-dependent functions of a node transceiver, including transceiver duty cycle, transceiver transmit power levels and transceiver power-time products. 
   Still another object of the present invention is to provide a system and method to control topology-dependent functions of a node transceiver using a automatic transmission control protocol (ATP) to vary transceiver duty cycle, transmit power levels and transceiver power-time products to maintain specific absorption rate (SAR) limits for handheld and portable devices which are used close to human bodies. 
   Still another object of the present invention is to provide a system and method to control topology-dependent functions of a node transceiver using an automatic transmission control protocol (ATP) to vary modem data rate and spreading factor to maintain specific absorption rate (SAR) limits for handheld and portable devices. 
   These and other objects are substantially achieved by a system and method of MAC protocol using measurements of device transmitter “on-times”, and an integral of transmission power to direct transmitter duty-cycles, such that SAR limits are met. The transceiver duty cycle, transmit power levels and transceiver power-time products are monitored and, where SAR limits are approached or exceeded, each function can be controlled individually or any combination, by an automatic transmission control protocol. In cases where SAR limits are approached or exceeded, the ATP can direct the transceiver at the device to adjust each function, or combination of functions, to maintain an acceptable SAR value. The ATP can also direct varying modem data rates and spreading factor as an additional function to reduce SAR values to within acceptable levels. Additionally, the system and method can be used prioritized data traffic based on function values detected at each node, thereby eliminating the potential of any single node approaching SAR limits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of an example ad-hoc packet switched wireless communications network including a plurality of nodes in accordance with an embodiment of the present invention; and 
       FIG. 2  is a block diagram illustrating an example of a mobile node employed in the network shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a block diagram illustrating an example of an ad-hoc packet-switched wireless communications network  100  employing an embodiment of the present invention. Specifically, the network  100  includes a plurality of mobile wireless user terminals  102 - 1  through  102 -n (referred to generally as nodes  102  or mobile nodes  102 ), and can, but is not required to, include a fixed network  104  having a plurality of access points  106 - 1 ,  106 - 2 , . . .  106 -n (referred to generally as nodes  106  or access points  106 ), for providing nodes  102  with access to the fixed network  104 . The fixed network  104  can include, for example, a core local access network (LAN), and a plurality of servers and gateway routers to provide network nodes with access to other networks, such as other ad-hoc networks, the public switched telephone network (PSTN) and the Internet. The network  100  can also include a plurality of fixed routers  107 - 1  through  107 -n (referred to generally as nodes  107  or fixed routers  107 ) for routing data packets between other nodes  102 ,  106  or  107 . It is noted that for purposes of this discussion, the nodes discussed above can be collectively referred to as “nodes  102 ,  106  and  107  ”, or simply “nodes”. 
   As can be appreciated by one skilled in the art, the nodes  102 ,  106  and  107  are capable of communicating with each other directly, or via one or more other nodes  102 ,  106  or  107  operating as routers for packets being sent between nodes as described in U.S. Pat. No. 5,943,322 to Mayor, and in U.S. Pat. Nos. 7,072,650, 6,807,165, and 6,873,839, referenced above. 
   As shown in  FIG. 2 , each node  102 ,  106  and  107  includes a transceiver  108  which is coupled to an antenna  110  and is capable of receiving and transmitting signals, such as packetized signals, to and from the node  102 ,  106  or  107 , under the control of a controller  112 . Additionally, the transceiver and controller are capable of monitoring and adjusting transmission power levels and duration as described in greater detail below. The packetized data signals of such transmissions can include, for example, voice, data or multimedia information, and packetized control signals, including node routing and update information. 
   Each node  102 ,  106  and  107  further includes a memory  114 , such as a random access memory (RAM), that is capable of storing, among other things, routing information pertaining to itself and other nodes in the network  100 . The nodes  102 ,  106  and  107  periodically exchange respective routing information, referred to as routing advertisements or routing table information, with each other via a broadcasting mechanism at various intervals, such as when a new node enters the network, or when existing nodes in the network move. 
   As further shown in  FIG. 2 , certain nodes, especially mobile nodes  102 , can include a host  116  which may consist of any number of devices, such as a notebook computer terminal, mobile telephone unit, mobile data unit, or any other suitable device. Each node  102 ,  106  and  107  also includes the appropriate hardware and software to perform Internet Protocol (IP) and Address Resolution Protocol (ARP), the purposes of which can be readily appreciated by one skilled in the art. The appropriate hardware and software to perform transmission control protocol (TCP) and user datagram protocol (UDP) may also be included. Additionally, each node includes the appropriate hardware and software to perform media access control (MAC) protocol, as set forth in greater detail below. 
   As stated earlier, a mobile node  102  may include any number of devices, such as laptop computers or handheld mobile telephones. In the case of many of these devices, concerns regarding electric/magnetic fields have led to the creation of guidelines regarding optimum safe operation, such as the specific absorption rate (SAR) limits for handheld and portable devices which are used close to human bodies. 
   In the embodiment of the invention described below, a MAC protocol for duty-cycle limitation at a device is disclosed which, when applied, ensures a device, or mobile node, meets SAR limits. As described in an FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, Supplement C to OET Bulletin 65, referenced above, SAR evaluation of low power devices can be achieved with either electric field measurements inside tissue media or computational methods using tissue models. In either case, SAR is determined according to equation (1) below. 
                 SAR   =              E        2     ⁢   σ     ρ             (   1   )               
wherein E is the magnitude of the measured or computed RMS electric field, σ is the tissue conductivity and ρ is the tissue mass density. As known to those skilled in the art, SAR is a measure of the rate of energy absorption per unit mass at a specific location in the tissue. SAR may be expressed in units such as watts/kg or milliwatts/gm.
 
   In accordance with an embodiment of the present invention, the MAC protocol uses measurements of transmitter, or transceiver  108 , “on-time”, and an integral of transmit power over “on-time” of the RF transmitter in a time window that is used to measure SAR. This on-time value and power integral is used to maintain an E value for use in equation (1) that keeps SAR at or below acceptable levels, where E can be determined according to equation (2) below.
 
 E= ( t   1   −t   0 ) ×P   (2)
 
where t 1 −t 0  represents an “on-time” for the transceiver  108  during use, P represents a transmission power level and E represents the product of transmission power over a time interval for use in SAR calculations. The maximum “on-time” is independent on any input data or network condition, and is measured as a power-time product in a sliding window which has the same length or is shorter in time, than the SAR averaging window which is used for measuring SAR.
 
   In a first embodiment of the present invention, depending on the integrated power over some predetermined window, the MAC protocol allows automatic transmission control protocol (ATP) to increase or decrease transmission power level P to maintain SAR at or below acceptable levels. The functions of the ATP include calculating the appropriate power level for unicast and broadcast transmissions. The library of functions comprising the ATP keeps track of the local neighbor nodes and required power levels, and provides this information to the logical link control when called. 
   If communication is not possible within a predetermined maximum integrated product of time and power without exceeding maximum limit, then transceiver duty-cycle is reduced. In this case, duty-cycle is defined as the fraction or percent of time per interval that the transceiver  108  is transmitting, and can be determined according to equation (3) below. 
   
     
       
         
           
             
               
                 
                   
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   In such an application, MAC protocol allows device operation with guaranteed maximum SAR in any condition. Such operation may be useful for radios that need to transmit high power in emergency, law enforcement and firemen applications. 
   In a second embodiment of the present invention, the MAC protocol allows automatic transmission control protocol (ATP) to increase or decrease duty cycle t 1 −t 0 . In accordance with a second embodiment of the present invention, “source-based” time-averaging based on an inherent property or transceiver duty cycle of a node is allowed to reduce SAR according to FCC rules. As stated in FCC OET Bulletin 64, for purposes of determining radio frequency (RF) exposure, the transmission protocols used by certain spread spectrum transmitters may qualify the device for source-based time averaging. The applicable duty factor may be determined according to the RF output power “on” and “off” time duration, either as a signal with a repeatable duty cycle, or by establishing a worst case duty factor using power off duration identified by the transmission protocol. Duty factors related to device usage, frequency hopping or other similar transmission conditions are normally not acceptable as source-based, time averaging factors for RF evaluations. 
   Additional details of source-based time averaging and specific absorption rates may be found in 47 CFR 2.1091, referenced above. Specifically, current FCC rules state time-averaging provisions may not be used in determining typical exposure levels for devices intended for use by consumers in general population/uncontrolled environments. However, “source-based” time averaging based on an inherent property or duty-cycle of a device is allowed. An example of this is the determination of exposure from a device that uses digital technology such as a time-division multiple-access (TDMA) scheme for transmission of a signal. In general, maximum average power levels must be used to determine SAR compliance. The second embodiment of the present invention presented here uses a MAC protocol which is adapted to control the duty cycle of the RF transceiver and guarantee that maximum transceiver “on-time”, that is t 1 −t 0  as shown in equation (2), is not exceeded in a predetermined period to maintain SAR at or below acceptable levels. 
   The embodiment includes an adaptive transmission protocol (ATP) as part of the MAC protocol which may be used to adjust any combination of RF transmit power, duty-cycle and other factors. The RF transceiver  108  can be turned “off” and/or transmit power can be reduced if maximum power-time product in a time window is going to be exceeded. This may cause problems in the network but will serve to guarantee SAR limits are not exceeded and can be used to guarantee conformity for type approval. 
   In a third embodiment of the present invention, the ATP may be used to adapt modem data rate and spreading factor depending on how much power-time product over integration period is below the maximum limit. The ATP can lower the data rate while maintaining the link when the power-time product in a window is otherwise going to be exceeded. To achieve this, the embodiment increases the spreading factor and lowers RF power. The information of power-time product can be used by MAC protocol in prioritizing traffic. 
   The embodiment described above may be used to “back-off” transmissions in time if duty-cycle limit is otherwise exceeded. Also, contention resolution back off can be a function of power-time product in a time window. The embodiment also uses ATP to adjust data rate and transmission power based on integrated transmit energy (SAR) in a time window. Logic may be included at each node to switch the RF transmitter  108  off if the integrated energy (SAR) limit is otherwise exceeded. In this case, integrated energy is integrated over window that contains transmitted energy in any window position over selected integrated period. 
   The embodiment may also inform higher layers of limitations in power-time product over some time window. This can be used to avoid other nodes routing through the device if in handheld mode. Also, the embodiment may have a different limit depending on operation mode. For example, the same device when connected to a vehicle cradle may have a higher power-time limit than a device used as a handheld. The MAC protocol of the present embodiment therefore allows MAC protocol to operate differently when located in a cradle and hence not in handheld mode. Finally, the embodiment allows higher power hand portable devices to be type approved as SAR limits are guaranteed. 
   Although only a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined.