Abstract:
The distance between a first Multi Band Orthogonal Frequency Division Multiplex (MB-OFDM) data transceiver and a second or more such transceiver is determined using known techniques. The radio frequency path loss between transceivers is estimated given said distance, using a known relationship between distance and path loss, and further accounting for line-of-sight or non-line-of-sight conditions if desired. This path loss value is added to the typically minimum transmit power level, absent path loss, needed for reliable data communication. This modified initial transmit power level is then used by the first transceiver to begin the known iterative feedback process of transmit power control (TPC). Because this modified initial transmit power level, based on distance, is closer to the final optimum level, convergence in the TPC process occurs in fewer steps and less time than had the initial transmit power been maximum power as is typical in known TPC systems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to wireless data communication, and, in particular, to transmit power control by which an optimal transmitter power is determined that is high enough to enable reliable communication while low enough to minimize interference to other users or devices sharing the same spectrum.  
         [0003]     2. Description of the Related Art  
         [0004]     In wireless data communication systems, it is often beneficial to employ transmit power control (TPC), limiting the transmit power to a level high enough for reliable communication but typically less than the maximum available power. Benefits include but are not limited to reduced transmitter power drain—especially important in battery-powered applications—and reduced interference to other users of the same spectrum. In some systems such as ultra wideband (UWB), concurrent users share all or a portion of the spectrum used by other users&#39; transmissions.  
         [0005]     TPC is widely used in cellular telephone systems and wireless data communication systems utilizing unlicensed spectrum, such as that system commonly referred to as Wi-Fi. In communication systems utilizing spread-spectrum modulation, minimizing the transmit power is especially important, as multiple transceivers in an area share common spectrum. The effectiveness of communication between devices may be reduced considerably if one or more transmitters in the area are transmitting at significantly higher power than the other transmitters. TPC is typically implemented as an iterative process converging on an optimal transmit power, wherein a first transceiver transmits a first data packet at a high level, typically maximum power. If a second transceiver is within range, it receives this transmission and computes a figure of merit, such as frame error rate (FER), which is related to received signal power. This figure of merit is compared in the second data transceiver to desired limits, and a command to increase power or decrease power is transmitted back to the first data transceiver. The first data transceiver then typically raises or lowers power in a stepwise manner, or according to another power level progression. Another data packet is then sent to the second transceiver, using this modified transmit power, and a new figure of merit is computed and compared to desired limits, causing another increase or decrease power command to be sent to the first transceiver. In this iterative manner, a transmit power level for the first transceiver is found which generates the desired figure of merit in the second transceiver. Measurement of the figure of merit may continue as the payload data transfer occurs, so that the iterative process of adjusting transmit power may be repeated if the figure of merit deviates from the prescribed range. In typical systems, the transmit power levels in both transceivers are adjusted in this manner, either concurrently or sequentially.  
         [0006]     The iterative process described above typically requires multiple bidirectional data exchanges to arrive at optimal transmit power levels for each transceiver. These data exchanges add undesired overhead to the communication link, putting additional drain on the power source in each transceiver, and lengthening the time each transmitter is on the air and thus potentially interfering with other transceivers in the area. A method and apparatus for optimizing this process so as to more rapidly determine an optimal transmit power is therefore desirable.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention provides an apparatus and method for estimating, based on distance between transmitter and receiver, an initial transmit power close to an optimal level, thereby minimizing the number of additional iterative steps required to reach an optimal transmit power level.  
         [0008]     In a preferred embodiment of the invention described in greater detail below, the initial transmit power of a first transceiver is determined in part by the distance between the first and a second or more data transceiver. This distance or range is computed in some data communication systems by measuring the propagation time of a data packet from the first transceiver to the second or more transceiver and back. Because radio waves travel at a nearly constant velocity in air, this round-trip propagation time correlates well to the distance between the transceivers. An example of ranging capability in an Ultra Wideband (UWB) wireless system is described in standard ECMA-368 and in Texas Instruments patent T37034 on UWB ranging for MB-OFDM systems. In some UWB systems including Multi-Band Orthogonal Frequency Division Multiplex (MB-OFDM) systems, the data packets used for ranging support desired functions such as limiting association, whereby data communication is limited to transceivers within a certain distance of each other. Because ranging is typically already occurring for such desired functions, its use for the disclosed invention typically adds no additional overhead.  
         [0009]     The reduction in received signal strength as the distance between transceivers increases is well understood theoretically and is readily determined empirically. This loss in received signal strength attributable to the distance between transceivers is typically referred to as path loss.  
         [0010]     By measuring the distance between transceivers, and estimating the path loss based on this distance, an initial transmit power level may be determined which is closer to the optimum value. This is done by adding the estimated path loss to the known typically lowest transmit power level which yields reliable data transfer with minimal or no path loss. Further refinement of transmit power level may then occur using the known iterative closed-loop method described above, wherein the receiver in the second or more transceiver commands the transmitter in the first transceiver to increase or decrease power based on a receiver figure of merit such as FER. Convergence to the optimal transmit power thus occurs more rapidly when the initial transmit power level is based on distance and estimated path loss, rather than if the initial transmit power had been maximum power or some other arbitrary value. In some cases, the initial transmit power is close enough to the optimum that no further iteration is required.  
         [0011]     Further benefits and advantages will become apparent to those skilled in the art to which the invention relates.  
     
    
     DESCRIPTION OF THE VIEWS OF THE DRAWINGS  
       [0012]      FIG. 1  is a diagram of a wireless data communication system having wide variation in the distance (range) between multiple transceivers, in which transceivers have distance measurement (ranging) and transmit power adjustment capabilities.  
         [0013]      FIG. 2  is a block diagram of a data transceiver having distance measurement (ranging) and transmit power adjustment capabilities.  
         [0014]      FIG. 3  is a known graph of path loss as a function of distance between two transceivers.  
         [0015]      FIG. 4  is a flow chart showing the iterative process of transmit power control in a known system.  
         [0016]      FIG. 5  is a flow chart showing the steps used in the preferred embodiment to determine an initial transmit power level nearer the optimal transmit power level.  
         [0017]      FIG. 6  is a flow chart showing the steps used in another embodiment to determine an initial transmit power level nearer the optimal transmit power level. 
     
    
       [0018]     Throughout the drawings, like elements are referred to by like numerals.  
       DETAILED DESCRIPTION  
       [0019]     As shown in  FIG. 1 , a digital camera  100  has within it a digital data transceiver  102  which enables wireless transfer of image files from camera  100  to computer  104  having data transceiver  106 . In this example the distance  116  between camera  100  and computer  104  is small compared to distance  118  between camera  100  and television  112 . Television  112  has within or coupled to it data transceiver  114  which enables camera  100  to display images on television  112  without physically connecting the two. DVD player  108  having transceiver  110  is next to television  112 , and transmits video and audio from transceiver  110  in DVD player  108  to transceiver  114  in television  112 . As the DVD player and television are in close proximity in this example, distance  120  is small compared to distance  118 .  
         [0020]     Within each transceiver  102 ,  106 ,  110 ,  114  are circuits which determine distances between transceivers, and transmit power control circuitry responsive both to this path distance and to an increase or decrease power command sent to the transmitting transceiver by the receiving transceiver. The increase or decrease power decision is made in the receiving transceiver based on a figure of merit representative of data communication quality, such as frame error rate (FER). These circuits are detailed in  FIG. 2 .  
         [0021]     In a scenario such as one in which DVD player  108  is transmitting a movie to television  112  over a short distance  120  concurrent with camera  100  transmitting images to television  112  over a much longer distance  118 , it is important that the transmit power utilized by transceiver  110  is less than the transmit power utilized by transceiver  102 , so as to make approximately equal the signal levels from both the camera and the DVD player at television  112 . If the transmit power of DVD player  108  were significantly higher than that of camera  100 , the higher level signal would likely cause interference with the lower level signal, resulting in data errors or failure to establish communication between camera  100  and television  112 . However, when each transceiver has knowledge of distance between itself and others, and is able to adjust it&#39;s transmit power accordingly, such interference is significantly reduced.  
         [0022]     In  FIG. 2 , further detail of the transceiver  102  and TPC subsystem of the preferred embodiment is shown. Functional elements of  FIG. 2  may be realized in hardware, software, or some combination, as will be obvious to those skilled in the art.  
         [0023]     Transmitter  200  facilitates modulation of a data signal onto a carrier of appropriate frequency using an appropriate modulation scheme. The output of transmitter  200  is coupled through variable gain  202  to power amplifier  204 , which in turn is coupled to transmit antenna  206 . Receive antenna  208  is coupled to receiver  210 , which demodulates and decodes received data. Ranging and power control  212  measures the time delay from transmission by transmitter  200  to receipt of an acknowledgement from a second or more transceiver, and facilitates computation of the distance (range) between transceiver  102  and this second or more transceiver. Ranging and power control  212  also facilitates estimation of path loss based on this distance, and couples a control signal responsive to this path loss to variable gain  202 , thereby modifying the transmitted power to approximate the optimal power for the measured distance.  
         [0024]     Given a first and a second transceiver, wherein T 1  is the time of transmission by the first transceiver of a first data packet, R 2  is the time of receipt by the second transceiver of this first data packet, T 2  is the time of subsequent transmission of an acknowledging second data packet by the second transceiver, R 1  is the time of receipt by the first transceiver of this second data packet, and C is the speed of light in meters per second, the distance D between the first and second transceivers, compensating for known processing delays in the transceivers, is given by: 
 
 D=C * [( R 2 −T 1)+( R 1 −T 2)]/2 
 
         [0025]     Ranging and power control  212  is also coupled to data from receiver  210 , such that it may receive commands from the second transceiver to increase or decrease power as needed. The transmitted power from transceiver  102  is thus initially at a substantially optimal level based on measured range between the transceivers, and is then iteratively refined if necessary, responsive to power increase or decrease commands from the second or more transceiver.  
         [0026]      FIG. 3  graphically shows the known relationship between distance between transceivers and path loss. Horizontal axis  302  represents distance in meters between the two transceivers. Vertical axis  304  represents path loss. Line segments  306  and  308 , taken together, show the functional relationship between distance and path loss. This relationship is contained within ranging and power control  212 , typically as an algorithm or a lookup table, and is used by ranging and power control  212  to determine approximate path loss between transceivers once range between transceivers is determined. In an embodiment using an algorithm to calculate path loss, different path loss functions or parameters may be used for different distance ranges. For example, the function may have path loss increasing as the square of distance for distance from 0 to 4 m, as shown by segment  306 , and path loss increasing by the cube of distance for distance from 4 to 10 m, as shown by segment  308 . Further refinement of the relationship between distance and path loss may be made if additional information is available, such as whether the path is line-of-sight (LOS) or non-line-of-sight (NLOS). Some transceivers, such as ultra wideband systems using multi-band OFDM or spread spectrum modulation, are able to determine in a known manner the LOS or NLOS nature of the path they are using. Line segments  306  and  308  represent a typical LOS path loss relationship, while line segment  310  represents a typical NLOS path having higher path loss than the LOS path at a given distance.  
         [0027]     In  FIG. 4 , a flow chart shows the steps used by a typical known iterative closed loop transmit power control system.  
         [0028]     At step  402 , the transmitter in the first data transceiver is set to maximum power in preparation for its initial communication with a second or more transceiver an unknown distance away.  
         [0029]     At step  404 , the initial data transmission from the first transceiver is made at maximum power. Subsequent data transmissions are made at a modified transmit power level.  
         [0030]     At step  406 , data from the first transceiver is received at the second or more transceiver, which computes a figure of merit for data quality, such as frame error rate (FER), and based on this figure of merit, determines whether the received power level needs to be increased or decreased.  
         [0031]     At step  408 , an “increase power” or “decrease power” command is transmitted back to the first data transceiver.  
         [0032]     At step  410 , the first transceiver receives the command to increase or decrease transmit power level.  
         [0033]     At step  412 , if the received command was to increase power, in step  414  the first transceiver power level is increased by one step, and process flows to start of step  404 . If there was no command to increase power, flow continues to step  416 .  
         [0034]     At step  416 , if the received command was to decrease power, in step  418  the first transceiver power level is decreased by one step, and process flows to start of step  404 . If there was no command to decrease power, flow continues to step  420 .  
         [0035]     At step  420 , TPC is complete, and data communication begins using the current transmit power level.  
         [0036]     In  FIG. 5 , a flow chart describes the steps of the method of the preferred embodiment, wherein the initial transmit power level is based on distance between transceivers.  
         [0037]     At step  502 , the distance between a first MB-OFDM transceiver and a second transceiver is measured using known techniques such as specified in ECMA-368 and described in Texas Instruments patent T37034 on UWB ranging for MB-OFDM systems.  
         [0038]     At step  504 , this measured distance is input to an algorithm to determine approximate path loss (PL) for the distance. The algorithm may also account for path characteristics in addition to distance, such as LOS or NLOS.  
         [0039]     At step  506 , the signal to noise ratio (SNR) desired at the receiver is estimated, based on the known packet data rate. This SNR, when added to the noise power N, approximates the minimum received signal level needed to receive data with a given frame error rate in the absence of fading or other path impairments.  
         [0040]     At step  508 , an additional margin M is determined, dependent on characteristics of the system and the desired level of certainty of communication, where M is the sum of such parameters as fading margin (typically on the order of 3 dB), receiver implementation loss (typically on the order of 2.5 dB), and any other margins (typically on the order of 3 dB).  
         [0041]     At step  510 , the initial transmit power level Pmod is determined, where: 
 
 P mod= N+SNR+PL+M.  
 
         [0042]     At step  512 , this initial transmit power level is rounded to the nearest TPC step.  
         [0043]     At step  514 , data communication begins at this rounded initial transmit power level.  
         [0044]     In  FIG. 6 , a flow chart describes the steps of the method of yet another embodiment, wherein a nominal transmit power Pnom is modified by a combination of transmit data rate and distance between transceivers. Pnom is that transmit power level which, for a given data rate (for example, the lowest data rate of a plurality of possible data rates to be supported) and distance (for example, the shortest distance of a range of distances to be supported), results in reliable data communication accounting for desired margins for fading, receiver implementation loss, receiver SNR requirements, and other margins. In this embodiment, since transmit power needs to increase as data rate increases, and also needs to increase as distance increases, Pnom represents the typically lowest transmit power level to be used. By knowing the actual data rate and distance, a gain value may be determined which modifies Pnom such that margins are retained and reliable data communication is enabled.  
         [0045]     At step  602 , the distance D between a first MB-OFDM transceiver and a second or more transceiver is measured using known techniques such as specified in ECMA-368.  
         [0046]     At step  604 , a test of data rate is made, to determine if the transmitted data rate is at rate R 1 , the first of M possible data rates to be used by the system. If yes, flow proceeds to step  606 . If no, flow proceeds to step  608 .  
         [0047]     At step  606 , path loss as a function of distance D and data rate R 1  is determined using a first lookup table. The resulting path loss PL 1  is passed to step  616 .  
         [0048]     At step  608 , a test of data rate is made, to determine if the transmitted data rate is at rate R 2 . If yes, flow proceeds to step  610 . If no, flow proceeds to step  612 .  
         [0049]     At step  610 , path loss as a function of distance D and data rate R 2  is determined using a second lookup table. The resulting path loss PL 2  is passed to step  616 .  
         [0050]     At step  612 , a test of data rate is made, to determine if the transmitted data rate is at rate RM. If yes, flow proceeds to step  614 . If no, flow proceeds to step  602  or alternatively to an error handling process.  
         [0051]     At step  614 , path loss as a function of distance D and data rate RM is determined using the Mth lookup table. The resulting path loss PLM is passed to step  616 .  
         [0052]     At step  616 , the nominal transmit power level Pnom is increased by PL(m), such that Pmod=Pnom+PL(m), where m is one of 1,2, . . . M.  
         [0053]     At step  618 , data communication occurs at this modified transmit power level Pmod.  
         [0054]     It is apparent to those skilled in the art that additional lookup tables accounting for other variables such as LOS/NLOS can be employed, and/or that lookup tables may be replaced or augmented by appropriate algorithms for generating the PL(m), without deviating from the spirit of the invention.  
         [0055]     Those skilled in the art to which the invention relates will appreciate that yet other substitutions and modifications can be made to the described embodiments, without departing from the spirit and scope of the invention as described by the claims below.