Abstract:
A system and method for maximizing signal strength while limiting specific absorption rate in diversity transmission network is achieved by coupling a first input signal to a second input signal. The system includes a sampler, a coupling network, and combiner.

Description:
PRIORITY 
     This application claims priority of U.S. Provisional Patent Application No. 61/505,872, filed Jul. 8, 2011, which is incorporated herein by reference. Additionally, this application is related to PCT/US2012/043632, filed Jun. 21, 2012, which is also incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to mobile transmission diversity systems having control over Specific Absorption Rate and Total Radiated Power. 
     BACKGROUND 
     Signal transmission between a mobile device and a base station is enhanced through mobile transmission diversity (MTD), sometimes referred to as beam forming. One form of MTD employs multiple antennas in the mobile unit, each antenna transmitting identical data. The phase difference between the signals from the two or more antennas is controlled so that constructive interference of the signals received at a base station provides power received at the base station greater than the arithmetic sum of the received power that would be radiated from each of the antennas in the absence of the other(s). By varying the phase difference between the signals from the antennas, the peak radiated power may be steered to focus upon the antenna of the base station. Typically two antennas are used for MTD. 
     Since the phase of a signal reaching a base antenna is affected by changes of path length such as due to reflections, closed loop MTD has been developed where the phase difference at the multiple mobile unit antennas is adjusted in response to feedback from the base station. In particular a quality factor such as bit error rate, or a Power Control Bit (PCB) or Transmitter Power Control (TPC) parameter determined at the base station may be communicated back to the mobile device, or the base station may return a signal indicating whether the received power should be lowered or raised. This communication from the base signals the need to adjust the phase difference between the antenna outputs. 
     Since the mobile device is located close to a user, the amount of radiation impinging on the user is also monitored. The Specific Absorption Rate (SAR) is a measure of the amount of power absorbed by biologic tissue and a goal is to keep the SAR below a predetermined value. Absorption levels may be typically defined and measured by placing a liquid-filled phantom head, hand, or other emulated body part close to the edge(s) of a mobile device while transmitting. Measurements of a rise in the liquid&#39;s temperature provide an indication of the radiation exposure overall or at particular points 
     To meet SAR requirements the conventional approach may be to set the antenna radiation limits based on peak radiation points rather than peak radiated averages (i.e. total radiated power (TRP)). This conventional approach may cause vendors to limit the maximum power and or the maximum data rate. 
     BRIEF DESCRIPTION 
     An apparatus and method has been developed to reduce SAR at near field locations for a mobile device where SAR would otherwise attain its highest value. It relies on what hitherto has been regarded as a problem in mobile diversity systems. In particular, because the antennas in a MTD system are close to each other, the broadcasting antennas receive strong signals from each other inducing currents in each other&#39;s antenna circuits. These currents introduce, by means of standing waves resulting from antenna interaction, a complexity in assigning phases to the antenna transmission. In the present disclosure instead of treating the interaction between antenna currents as a detriment the antenna circuits are coupled in such a manner as to convert the interaction between currents in the antenna circuits into a feature utilized to reduce SAR by designing appropriate coupling circuits between antennas and by providing a system so designed. VSWR (Voltage Standing Wave Ratio) is a measure of the impedance mismatch between an antenna and its power amplifier. In the presence of such coupling circuits the phase adjustments may be carried out and the presence of the standing waves, as determined by the coupling circuits (and quantified as VSWR), used to provide a preferred safe SAR level that can be maintained during phase adjustment. The signal being coupled from one antenna to another may be used to constructively or destructively interfere with the existing signal to manipulate the output signal to provide a preferred SAR level. 
     This disclosure enables a mobile transmit diversity device comprising a plurality of antennas, each antenna receiving a signal from a respective power amplifier. The power amplifiers and antennas are each connected by a circuit, where the circuits include coupling elements located between separate circuits. The coupling circuits are chosen to produce reflection coefficients and VSWR values in an antenna circuit that reduce TRP at a phase setting that produces the worst case SAR by the coupled signals among the antennas. That example may produce maximum TRP without exceeding a preset SAR limit or threshold. 
    
    
     
       DESCRIPTION OF FIGURES 
         FIG. 1  depicts a basic method of adjusting diversity parameters for beamforming transmit diversity; 
         FIG. 2  depicts sampling the signal on one branch of a two branch diversity transmit system, modifying the parameters of that signal and injecting the signal into the second branch in one example of this disclosure; 
         FIG. 3  depicts another example of a depiction of a diversity transmit system of the present disclosure; 
         FIG. 4  is a chart showing the far field signal strength at varying azimuth points of a signal transmitted by an example of the present disclosure; 
         FIG. 5  is a chart showing the data of  FIG. 4 , modified to show only maximum and minimum values; 
         FIG. 6  is a chart showing the near field signal strength at varying azimuth points of a signal transmitted by an example of the present disclosure; and 
         FIG. 7  is a chart showing the data of  FIG. 6 , modified to show only maximum and minimum values. 
     
    
    
     DETAILED DESCRIPTION 
     Two issues affect MTD performance: (1) In a situation where there are two antennas termed primary and secondary, there is a “worst” phase in which antennas are set where the signals from each antenna add to create the highest Specific Absorption Rate (SAR). In general, this effect is caused by a near-field addition of the signal fluxes from each antenna at a phase relationship where the signals at the antennas are in phase to that point. (2) The second issue is related to the interaction between antenna Voltage Standing Wave Ratio (VSWR) and the coupling between antennas. As the phase between the two antennas is changed (such as is done in “pointing a beam”) there is a change in the Total Radiated Power (TRP). This change in TRP is likely due to the fact that the power coupled from one antenna to another combines with the voltage reflected from that antenna, increasing and decreasing the apparent VSWR seen by the power amplifier (PA). Since this effect is due to the two voltages adding or subtracting, it is dependent on the relative phase of the two signals. 
     The present disclosure relates to a design methodology such that the electrical phase difference that creates the maximum value of SAR has a somewhat lower TRP, but with such an implementation, the mobile device will operate with higher average TRP without exceeding the SAR limit. 
       FIG. 1  illustrates a basic two antenna MTD system  100 , having a coupling system  110 . The signal  101  is divided into two copies, where the lower copy is modified by the Parameter Modifier  112 . The Parameter modifier  112  modifies the phase of the signal  101  in order to maximize TRP at the receiver. The two copies of signal  101  are further amplified by power amplifiers  114  and  116  to form respective signals  118  and  120 . The two signals  118  and  120  are radiated by antennas  122  and  124 , respectively. 
     As shown in the example of  FIG. 2 , the coupling system  200  receives two input signals, signal  202  and signal  204 . The second input signal  204  has been modified in accordance with known diversity transmission techniques. Signal  202  is sampled by sampler  206 , outputting signal  208  and first output signal  218 . Signal  218  is relayed to a first antenna  222 . Signal  208  is relayed to a coupling network  210  where it is modified to form signal  212 . Signal  212  is combined with the second input signal  204  at combiner  214  to form a second output signal  216 . The second output signal  216  is relayed to a second antenna  220 . The second output signal  216  will be increased or decreased in amplitude by the coupling system  200  depending on the relative phase of signal  202  with respect to signal  204 . Further, the coupling network  210  may shift the phase of signal  212  in order to establish constructive or destructive interference between the signals. 
     The voltage amplitude of the second output signal  216  is proportional to the sum of the voltage of signal  204  plus the voltage of signal  212  when the two signals are substantially coherent and in-phase. Further, the voltage amplitude of the second output signal  216  is proportional to the difference of the voltages of signal  204  and the voltage of signal  212  when the two signals are coherent and opposite (+/−180 degrees) in phase. Since the phase of signal  212  is manipulated by the coupling network  210 , the coupling network  210  may be used to set the relative phase difference of signal  204  and signal  212 , thereby establishing the voltage amplitude of the second output signal  216 . Accordingly, the coupling network  210  may be adjusted to establish a maximum TRP at a receiver without exceeding a predetermined SAR. 
     EXAMPLES 
     In one example, the second output signal  216  is 10 dB weaker than the second input signal  204 . This variation caused by the coupling network  210  results in a 3 dB decrease in SAR caused by the system  200 . 
       FIG. 3  illustrates another example of an MTD system using two coupled antennas. In this example, system  300  includes two input signals—signal  302  and signal  304 . The first input signal  302  is divided by sampler  306  into a first output signal  322  and a coupling signal  308 . The coupling signal  308  passes through amplifier  310 , attenuator  312 , and phase shifter  314  to form signal  316 . Signal  316  and the second input signal  304  are combined by combiner  318  to produce a second output signal  320 . The first output signal  322  and the second output signal  320  are radiated by antenna  324  and antenna  326 , respectively. 
       FIG. 4  shows sample data illustrating the Far Field Signal of the diversity transmission system  200 ,  300  at 30 degree Azimuth intervals. The various lines in the graph represent varying phase shifts of the coupling network. The Far Field Signal corresponds to the TRP of the system. In  FIG. 5 , the data is simplified to show only the peak values relating to constructive interference phase values. The lighter line represents the Far Field Signal with no coupling and the darker line represents the Far Field Signal when coupling is applied. Based on these results, it is shown that the Far Field Signal may be increased at certain Azimuth positions using the coupling system  200 . 
     Similarly,  FIGS. 6 and 7  show sample data illustrating the Near Field Output of the diversity transmission system  200 ,  300  at 45 degree Azimuth intervals. The varying lines in  FIG. 6  represent varying phase shifted signals of the coupling network. In  FIG. 7 , only the peak values are shown. The Near Field Output corresponds to the SAR caused by the system. The data illustrate that the coupling system  200 ,  300  will lower maximum Near Field Signal levels at certain Azimuths, e.g., 90 degrees and 270 degrees. 
     Accordingly, the system  200 ,  300  may be used to maximize TRP while maintaining predetermined SAR levels. 
     Although this example has been described with particular parameter values, it should be understood that the example is representative of a system/method that is not tied to those particular values or to the circuitry under which the example is assumed to function. Persons of skill in this art will know how to adapt this example to different parameter values and different specific hardware. While certain features of the disclosure have been illustrated and described herein, many modification, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure.