Patent Publication Number: US-6985757-B2

Title: Smart host power supply detection for PC card wireless modem

Description:
FIELD OF THE INVENTION 
   The present invention relates to wireless communications. More specifically, the present invention relates to methods of and apparatuses for determining the power supplying capabilities of host power supplies of host computers providing power to PC card wireless modems. 
   BACKGROUND OF THE INVENTION 
   Many modern portable computers, including laptops and personal digital assistants (PDAs), have a built-in PCMCIA slot (i.e. socket) for accepting another electronic device packaged in a PC (personal computer) card conforming to the PCMCIA standard. PCMCIA (Personal Computer Memory Card International Association) is an organization that was formed in 1989 with the original purpose of developing and promoting standards for PC cards that could provide additional memory resources for the host computer. This purpose has since expanded to encompass other types of electronic devices such as, for example, PC card wireless modems that are capable of communicating with a remote device over a wireless link. 
     FIG. 1  shows a block diagram of a wireless-communication-enabled computer system  10  comprising a host computer  100  and a PC card wireless modem  102 . PC card wireless modem  102  includes an antenna  104  for transmitting/receiving radio frequency (RF) signals to/from a remote device over a wireless network. PC card wireless modem  102  also includes various input/output (I/O), power and ground terminals  106 , which are arranged according to the PCMCIA standard. Host computer  100  communicates with PC card wireless modem  102  via a PCMCIA interface  108 , when terminals  106  are plugged into a PCMCIA slot  110  of host computer  100 . PCMCIA interface  108  not only provides a communication means, it also includes power and ground terminals that couple a power supply of host computer  100  to the power and ground terminals of PC card wireless modem  102 . 
     FIG. 2  shows a wireless communications system  20  that includes a wireless-communication-enabled host computer, such as that shown in  FIG. 1 . A host computer  200 , shown as a laptop, has a PCMCIA slot  202  with a PC card wireless modem  204  plugged into the slot  202 . PC card wireless modem  204  has an antenna  206  that transmits/receives RF signals modulated by data and voice information to/from a base station  208  over a wireless link  210 . Base station  208  transmits/receives voice modulated signals to/from a mobile switching center  212 , which communicates with a remote device (e.g. a telephone) over the PSTN (Public Switched Telephone Network)  214 . Base station  208  also transmits/receives data modulated signals to an ISP (Internet Service Provider) Server  216 . ISP server  216  transmits/receives data to/from a gateway/router  218 , which sends/receives the data to/from a remote over the Internet  220 . 
   Base station  208  in  FIG. 2  may be associated with any number of networks. For example, it may be associated with a pager network or a wireless communications network used by cellular telephones. One particular cellular telephone network that is in common use in Europe and of increasing use throughout the rest of the world is GSM (Global System for Mobile communications). Besides functioning as a voice network, GSM is becoming particularly attractive to users and developers of wireless-communication-enabled computers, such as the ones described above in connection with  FIGS. 1 and 2 . A large reason for this is that GSM supports packet-switched data protocols like GPRS (General Packet Radio Service). Packet-switched data makes more efficient use of available bandwidth and is typically faster than traditional circuit-switched data protocols. GPRS also supports the Internet Protocol (IP), thereby allowing users of a computing device with a GPRS-compatible PC card wireless modem to gain access to the Internet. 
   GPRS operates by allocating timeslots for packet data transmissions upon a request by a user and freeing up timeslots when not required by the user. The wireless-communication-enabled computer system breaks down an Internet TCP/IP (Transmission Control Protocol/Internet Protcol) data message into data packets. When the data is ready to be sent, the network assigns timeslots on a channel for the transmission. The GPRS-compatible modem transmits the data packets in the assigned timeslots to the cellular base-station where the packets are reassembled into the original TCP/IP data message and finally passed to the Internet for transport to the destination. 
   Timeslots in a GSM/GPRS network are delineated similar to that in TDMA (Time Division Multiple Access) technology. Each channel is divided into eight timeslots, which are then allocated to different requesting users. More than one timeslot may be requested and allocated to increases the rate at which the modem is permitted to transmit data. However, when this is done more power is demanded from the host power supply, which explained above, functions as the power source of the modem. When the current increases, the voltage supplied by the host power supply tends to drop, due to the internal resistance of the power supply. If the current demanded by the modem exceeds that capable of being delivered by the host supply, the host power supply may be damaged and/or the modem may shut down or reset. Unfortunately, the PC card wireless modem does not know what the supplying capability of the host power supply is. 
   One solution proposed to avoid the current overdraw problem is to include a supplemental battery pack on the PC card. This approach is undesirable, however, as it increases the size of the PC card, making it more bulky and less popular with users. Another solution would be to simply reduce or limit the RF power of the modem. This approach is also undesirable, however, as it reduces the range of operation of the modem and also unnecessarily sacrifices performance of systems in which the power supply is not the limiting factor. 
   SUMMARY OF THE INVENTION 
   Generally, methods of and apparatuses for characterizing the power supplying capabilities of host power supplies of host computers providing power to a PC card wireless modem are disclosed. The host computer may comprise any computer device, such as a laptop or personal digital assistant (PDA), which is configured to provide power to the PC card wireless modem. 
   According to one aspect of the invention, a method of determining permissible transmission power classes of a wireless modem, comprises the steps of determining an equivalent series resistance of the power supply, comparing the equivalent series resistance of the power supply to a plurality of maximum equivalent series resistance values associated with a corresponding plurality of transmission power classes, and designating each power class as a permisssible power class at which the wireless modem may be configured to transmit for each maximum equivalent series resistance value that is greater than the equivalent series resistance of the power supply. 
   According to another aspect of the invention, a method of determining whether a power supply of a host computer is capable of providing sufficient power to a PC card wireless modem, comprises the steps of determining a maximum allowable equivalent series resistance of a power supply configured to supply power to the PC card wireless modem when the modem is configured to transmit at a particular transmission power class and slot configuration, inserting the PC card wireless modem into a PCMCIA slot of a host computer having a host power supply, determining an equivalent series resistance of the host power supply, comparing the maximum allowable equivalent series resistance to the equivalent series resistance of the host power supply, and determining that the host power supply is capable of providing sufficient power to the PC card wireless modem if the equivalent series resistance of the host power supply is less than the maximum equivalent series resistance. 
   According to another aspect of the present invention, a wireless modem capable of configuring itself to one of a plurality of particular transmission power classes and slot configurations, comprises a DC/DC converter having a power input coupled to an output of a power supply, an analog-to-digital converter (ADC) operable to convert an input voltage applied to the power input of the DC/DC converter when the wireless modem is configured to draw a known current from the power supply, and a baseband processor operable to calculate an equivalent series resistance of the power supply, based on the input voltage applied to the power input of the DC/DC converter and the current drawn from the power supply. The baseband processor is also operable to compare the calculated equivalent series resistance to a plurality of maximum equivalent resistance values associated with a corresponding plurality of transmission power classes and slot configurations. 
   According to yet another aspect of the present invention, a wireless modem capable of configuring itself to one of a plurality of particular transmission power classes and slot configurations comprises a DC/DC converter means for converting an input voltage from a power supply to an output voltage, analog-to-digital converter means for converting said input voltage to a digital signal when the wireless modem is configured to draw a known current from the power supply, and baseband processor means for calculating an equivalent series resistance of the power supply, based on the input voltage applied to the power input of the DC/DC converter and the current drawn from the power supply. The baseband processor means also functions to compare the calculated equivalent series resistance to a plurality of maximum equivalent resistance values associated with a corresponding plurality of transmission power classes and slot configurations. 
   Other aspects of the invention are described and claimed below, and a further understanding of the nature and advantages of the inventions may be realized by reference to the remaining portions of the specification and the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of a wireless-communication-enabled computer system comprising a host computer and a PC card wireless modem; 
       FIG. 2  shows a wireless communications system that includes a wireless-communication-enabled host computer, such as that shown in  FIG. 1 ; 
       FIG. 3  shows a block diagram of a system for characterizing the current supplying capability of a host power supply of a host computer supplying power to a PC card wireless modem, according to an embodiment of the present invention; 
       FIG. 4  shows a test set-up, which may be used to determine a plurality of R esr (max) values of a PC card wireless modem, the R esr (max) values associated with a corresponding plurality of transmission power classes and slot configurations; 
       FIG. 5  shows an exemplary classification table for a GSM/GPRS PC card wireless modem, including R esr (max) values associated with various power transmission power classes and slot configurations, according to a specific exemplary embodiment of the present invention; 
       FIG. 6  shows a simplified diagram of the system in  FIG. 3 , which may be used to characterize the current supplying capability of host power supply of a host computer, while supplying power to a PC card wireless modem, according to an embodiment of the present invention; and 
       FIG. 7  shows a method of characterizing the equivalent series resistance R esr  of a host power supply, according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Those of ordinary skill in the art will understand that the following detailed description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to exemplary implementations of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like elements. 
   Referring to  FIG. 3 , there is shown a block diagram of a system  30  for characterizing the current supplying capability of a host power supply of a host computer supplying power to a PC card wireless modem, according to an embodiment of the present invention. A PC card wireless modem  300  communicates with and is powered by a host computer  302 , via a PCMCIA interface  304  comprising a power bus  306  and communications bus  308 . Host computer  302  includes a host processor  310 , which addresses and receives processing instructions from a read only memory (ROM)  312  via host computer system bus  314 . A random access memory (RAM)  316  is also coupled to host computer system bus  314  and provides temporary storage of data, address, and control information processed by, or to be processed by, host processor  310 . Host processor  310 , ROM  312 , RAM  316  and other electrical components of host computer  302  not shown in  FIG. 3  are powered by a host power supply  318 . The illustrations of both PC card wireless modem  300  and host computer  302  are simplified and do not include components that are not necessary to understand the inventions described herein. For example, whereas only the salient portions of the transmitting portion of the modem PC card wireless modem are illustrated in block  300 , those skilled in the art will readily understand that modem  300  also includes a receiving portion. 
   PC card wireless modem  300  is powered by host power supply  318 . Specifically, a DC/DC converter  320  of PC card wireless modem  300  is coupled to power bus  306 , such that host power supply  318  may provide power to PC card wireless modem  300 . DC/DC converter  320  operates to translate the voltage supplied by the host power supply (e.g. 5 volts) to a voltage that is appropriate to power up the modem (e.g. 3.3 volts). DC/DC converter  320  supplies power to, among other components on PC card wireless modem  300 , a power amplifier (PA)  322 . PA  322  amplifies radio frequency RF signals modulated by data messages originating from host computer  302 . The data messages, which are originally digitally formatted, are delivered over communications bus  308  to a baseband processor  324 . Baseband processor  324  processes and formats the digital data messages according to information stored in the look-up-table (LUT)  326  and then sends the processed messages to a digital-to-analog converter DAC in CODEC  328 . Baseband processor  324  also provides a PA control signal to PA  322 , on PA control line  330 , to control the amount by which PA  322  amplifies the RF signals. After the DAC in CODEC  328  converts the digital data messages into analog signals, the analog signals are modulated and converted to intermediate frequency (IF) signals by modulator  332  and then ultimately up-converted to radio frequency (RF) signals by RF up-converter  334 . The RF signals are then amplified by PA  322  and transmitted by an antenna  336  over a wireless link to a remote destination, so long as switch  338  is closed. If switch  338  is open, antenna  336  does not radiate the RF signals. Accordingly, the function of switch  338  is to connect/disconnect antenna  336  to/from the output of PA  322 . By disconnecting antenna  336 , the PC card wireless modem  300  and host power supply  318  may be tested and characterized without having to transmit the RF signals, which under test conditions may not be in compliance with wireless communications regulations, e.g., regulations set forth by the Federal Communications Commission in the United States of America. 
   As described in more detail below, baseband processor  324  configures PC card wireless modem  300  to transmit in a power class and/or slot configuration depending on the current supplying capabilities of host power supply  318  and information stored in LUT  326 . LUT  326  comprises a memory device, for example a FLASH memory chip, which contains a list of maximum equivalent series resistance R esr (max) values that the equivalent series resistance R esr  of a host power supply must not exceed when the PC card wireless modem  300  is operating according to a corresponding power class and slot configuration. As explained in the next paragraph, the R esr (max) values are determined and stored in LUT  326  prior to characterizing the equivalent series resistance R esr  of host power supply  318 . 
     FIG. 4  shows a test set-up  40 , which may be used to determine the R esr (max) values for storing in LUT  326 . Test set-up  40  includes a test power supply  400  capable of delivering any amount of current demanded by PC card wireless modem. Hence, power supply  400  may be treated as an ideal power supply under all test conditions. From the perspective of test power supply  400 , PC card wireless modem  300  presents an input resistance, represented by resistor R in    402  in  FIG. 4 . During testing, baseband processor  324  (see  FIG. 3 ) of PC card wireless modem  300  and PA control line  330  are set so that PC card wireless modem  300  transmits in accordance with a variety of power classes and slot configurations. Switch  338  remains open during testing so that antenna  336  does not radiate while the testing is being performed. This ensures that any wireless communications RF regulations are not violated. 
   For each power class and slot configuration, the peak current I peak  drawn by PC card wireless modem  300  is measured and recorded. Then, for each of these measured peak currents an R esr (max) is determined according to the following formula:
 
 R   esr (max)=( V   supp   −V   reg (min))/ I   peak   (1)
 
where V reg (min) is the minimum voltage required at the input of DC/DC converter  320  of PC card wireless modem  300  to maintain proper voltage regulation.
 
   Each R esr (max) value corresponds to a specific transmission power class and slot transmission configuration that is unique to PC card wireless modem  300 . These R esr (max) values are stored in LUT  326 . As explained in more detail below, the equivalent series resistance R esr  of host power supply  318  is measured and compared to the R esr (max) values stored in LUT  326  to determine which power classes and slot configurations host power supply  318  is capable of supporting.  FIG. 5  shows an exemplary classification table for a GSM/GPRS PC card wireless modem for various transmission (TX) power classes and slot configurations. Similar classification tables can be created for other types of cellular networks, e.g., DCS (Digital Communication System), PCS (Personal Communications Service), etc. The peak currents I peak  recorded in the table in  FIG. 5  were measured according to the method described above and the corresponding R esr (max) values were determined using formula (1) with a V reg (min) of 4.2 volts. 
   Referring now to  FIG. 6 , there is shown a simplified diagram of system  30  in  FIG. 3 , for characterizing the current supplying capability of host power supply  318  while supplying power to PC card wireless modem  300 , according to an embodiment of the present invention. Host computer  302  includes a power supply  318  that provides power to PC card wireless modem  300  via power bus  306 , which explained above comprises part of PCMCIA Interface  304 . Host power supply  318  comprises an ideal voltage supply component  600  and an equivalent series resistor  602  having an equivalent series resistance R esr . From the perspective of host power supply  318 , PC card wireless modem  300  presents an input resistance, which is represented by resistor R in    604  in  FIG. 6 . 
   Referring now to  FIG. 7 , there is shown a method  70  of characterizing the equivalent series resistance R esr  of host power supply  318 , according to an embodiment of the present invention. First, at step  700  PC card wireless modem  300  is inserted into the PCMCIA slot of host computer  302 , thereby interfacing PC card wireless modem in accordance with PCMCIA interface  304 . Next, at step  702  baseband processor  324  sends a PA control signal on PA control line  330  to PA  322 , to adjust the modem so that the modem test input current I t  is set to a constant value (e.g. 1 A in one exemplary embodiment) within the normal supply range of host power supply  318 . With this known test current being drawn, at step  704  the test input voltage V t  applied to PC card wireless modem  300  is measured and recorded. Test input voltage V t  may be measured by an external voltage-measuring device (e.g. a voltmeter) or by an analog-to-digital converter ADC in CODEC  328 . With knowledge of V t  and I t  and the input voltage and current V i  and I i  of the modem in its idle state, the equivalent series resistance R esr  of power supply  318  is calculated in step  706  using the following formula:
 
 R   esr =( V   i   −V   t )/( I   t   −I   i )  (2)
 
Note that the idle voltage and current, V i  and I i , can be measured during manufacturing testing or in a test set-up similar to that shown and described in connection with  FIG. 4  or  6 . Formula (2) is based on the observation that the R esr  of most power supplies remains essentially constant, at least over the range of load currents applied in the formula.
 
   After the equivalent series resistance R esr  of host power supply  318  is calculated, at step  708  baseband processor  324  compares R esr  of host power supply  318  to the R esr (max) values stored in LUT  326 . Based on the comparison in step  708 , at step  710  baseband processor  324  determines the permissible transmission power classes and slot configurations that PC card wireless modem  300  may operate at when powered by host power supply  318 . As an example, if the R esr  of host power supply is calculated to be 900 mΩ and the R esr (max) values stored in LUT  326  are those shown in the classification table in  FIG. 5 , baseband processor  324  would determine that, when powered by a power supply like host power supply  318 , PC card wireless modem  300  could be configured for single or dual slot transmission at 1 W or single slot transmission at 2 W. Because the R esr (max) values for the other slot configurations and power classes are less than R esr =900 mΩ, they would not be permissible when being powered by a power supply like host power supply  318 . Finally, at step  712  baseband processor  320  configures PC card wireless modem  300  so that it transmits according to one of the permissible power class and slot configurations determined in step  710 . 
   Whereas the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. For example, whereas a PC card socketed in a PCMCIA slot of a host computer has been described in the exemplary embodiments above, the methods described above also apply to other types of card/slot types where a host device includes a host supply that provides power to the wireless modem. Therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.