Patent Abstract:
A wireless personal mobile terminal in a cellular data network includes a packet radio transceiver for communicating with cellular base stations and a paging receiver for receiving paging signals from a paging transmitter serving a large geographical area. The wireless personal mobile message unit has power-saving features built into the signal format. This personal mobile message unit accommodates many communication applications due to the high bandwidth and the low latency achieved in the cellular data network. Low-power operation and wide coverage is achieved using existing one-way paging infrastructure. The wireless personal mobile unit can be achieved inexpensively by modifying a conventional one-way pager.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application relates to (i) copending patent application, Ser. No. 08/542,860, filed on the same date as this application, entitled “Two-Way Wireless Data Network,” by Weijia Wang, also assigned to General Wireless Communication Corporation, and (ii) copending patent application, Ser. No. 08/542,770, filed on the same date as this application, entitled “Wireless Network Access Protocol,” by Weijia Wang et al, also assigned to General Wireless Communication. These applications are hereby incorporated by reference in their entireties to provide technological background of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wireless communication, and in particular, relates to wireless communication in a data network having a large number of mobile terminal units communicating with each other through one or more cellularized base stations, which are connected by wired or wireless links to form a back-haul data network. 
     2. Discussion of the Related Art 
     One-way messaging or paging application is a well-established economical technology for transmitting short messages to a mobile pager. In one-way paging, typically, a caller calls a paging station using a telephone number assigned to the pager for which the message is intended. The message is then provided to the service computer at the paging station which, in turn, broadcasts the message, using high power transmitters, in its service area. In the broadcast message, the user&#39;s message is packaged in a data frame which contains an address code which identifies the recipient pager. The recipient pager is then activated by the receipt of the message, causing a vibration or an audible signal, thereby informing the callee the arrival of the message. Other applications have also been developed for one-way paging. For example, the service computer may broadcast to subscribing users stock quotes, weather information, results of sports events, and other information of interests. 
     However, one-way paging is limited in that the callee must respond to the caller through an alternative network. Typically, the alternative network is the telephone network. However, because users of paging systems are by nature mobile, to respond to a page through the telephone network requires easy access to the telephone network. Access to the telephone network is sometimes inconvenient. This inconvenience can be eliminated if a two-way communication capability is provided to the pager. 
     Other applications using one-way messaging technology have also been developed. Some of these applications relate to dissemination of information, such as weather reports, stock quotes, news headlines, or results of sports events. However, because communication is one-way in these applications, the amount or type of information that can be disseminated in this manner is constrained by the recipient&#39;s inability to select in real time information he or she desires. A two-way communication capability can provide “information-on-demand” services which are more suitable to individual user needs, as well as providing a more efficient use of the broadcast spectrum. 
     Two-way communication services are, however, expensive because of complexity. For example, in a cellular digital packet data (CDPD) system, a large number of cellularized base stations are distributed all over the service area. Cellularizing the service area offers two advantages: (i) allow mobile units and base stations to transmit at higher data rate with relatively low power, since the expected distance between a mobile unit and a base station in the vicinity is short; and (ii) larger capacity is provided because base stations which are separated by large enough distances can use the same radio channels. Such a system provides thus very high capacity, low response delay and allows the mobile units to transmit at relatively high data rates. In such a system, two-way symmetrical and reliable data links can also be provided. 
     In a CDPD system, because connectivity is maintained over the entire duration of a data communication session, multiple channels must be provided to allow multiple sessions to be maintained simultaneously. To locate a recipient mobile unit of a message, the network broadcasts the address of the recipient mobile unit from all the base stations in the service area until the recipient unit responds. Thus, a large amount of network resources is dedicated to locating mobile units. Further, to maintain continuous connectivity and to allow real time performance, when the session is established, the CDPD unit is associated with a base station with which it communicates. In addition, because a mobile unit can be expected to be used in a moving vehicle, it is possible that the mobile unit moves out of the service range of the initially associated base station and moves into the service range or ranges of one or more such base stations during the duration of a session. Thus, provisions must be made to disengage an associated base station and to engage an additional base station or stations (“hand off”) during the course of the session. The control mechanisms for maintaining a CDPD session, including tasks typically termed “connectivity management” and “mobility management”, involve sophisticated algorithms which require high performance computers to handle setting up the session, maintaining the session, and tracking the communicating mobile units as they move between service areas of the cellularized base stations. The complexity of the system requires a large investment in expensive equipment. Often, these control mechanisms are centralized, i.e. a large network switching or control center is provided to handle the mobile units in a given service area, so that, at times of heavy data traffic, the network control center may become a bottle neck, introducing undesirable latency into the system. 
     Another major disadvantage of the CDPD system is the requirement that the receiver of the mobile unit must be on at all times to receive messages. As a result, such a mobile unit requires a battery that is, at the present time, too undesirably bulky for mobile use. It would also be extremely difficult for power-saving features to be provided in such a mobile unit. Unlike a pager, which monitors only one paging channel, the mobile unit of a CDPD system must monitor a different radio channel when it is located in a different cell. Further, control information (e.g. timing information and recipient addresses) are broadcast by base stations in dedicated control channels. While a pager can be assigned a periodically occurring time-slot during which it “wakes” up to check for messages, a similar scheme in a mobile unit of a CDPD system would require timing in all the cellular base stations to be synchronized. Failure to synchronize all base station can lead to a mobile unit checking for the broadcast of its address during the wrong time slot. 
     Thus, a data network which allows bidirectional communication between mobile units without sophisticated equipment to perform connectivity and mobility management tasks is highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a two-way personal mobile terminal having a first receiver to receive broadcast messages from a high-power transmitter and a transceiver to transmit and receive control and data signals between it and a local cellularized base station. In one embodiment, the high-power transmitter is a broadcast transmitter compatible with one-way paging systems. In fact, in that embodiment, the high-power transmitter broadcasts a data message intended for a two-way personal mobile terminal, as if the data message is intended for a one-way pager. In that embodiment, the personal mobile terminal receives its message through its first receiver. The receiver demodulates the broadcast signal, and passes the demodulated signal to an on-board microprocessor to be decoded and displayed on a display of the personal mobile terminal, just like a one-way pager. However, under the present invention, the user of the personal mobile terminal may respond by transmitting a reply message using the personal mobile terminal&#39;s transceiver. This reply message is received by a cellularized local base station, which then relays the message to the network control center for further handling. 
     Thus, the present invention provides two-way communication capability by leveraging the existing infrastructure of one-way paging. For example, expensive resources such as high-power paging transmitters can be used for both one-way and two-way applications. Further, the personal mobile terminal can be the basis for a data network, which can be built by providing relatively inexpensive (i.e. relative to high cost high-power transmitters) cellularized base stations. The existing one-way paging user-base can converted to become users of this data network by simply purchasing such a personal mobile terminal. This data network provides additional applications beyond mere two-way paging. 
     Because the personal mobile terminal is receive-only with respect to messages from one-way paging towers, and transmits messages only to the cellularized base stations within a very limited range of its immediate vicinity, the power consumption for such personal mobile terminal is small. Thus, the personal mobile terminal can be made as compact as a one-way pager. 
    
    
     The present invention is more fully understood after consideration of the following written description and accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a wireless data network  1300  in which the personal mobile terminal of the present invention can be deployed. 
     FIG. 2 shows a personal mobile terminal application in wireless data network  1300  of FIG. 1, in which the personal mobile terminal of the present invention is used. 
     FIG. 3 a  is a block diagram of a personal mobile terminal  100  in accordance with the invention. 
     FIG. 3 b  is a block diagram of CPU board  600  shown in FIG. 3 a.    
     FIG. 4 a  shows receiver circuit  400  incorporating an internal frequency oscillator  410  (shown in FIG. 4 a ) to demodulates incoming data signal  125 . 
     FIG. 4 b  shows receiver circuit  400  incorporating a programmable frequency synthesizer  420  to demodulate incoming data signal  125 . 
     FIG. 5 shows local antenna  350  (i.e. the antenna used for communication with a local cellularized base station) in the embodiment of personal mobile terminal  100  shown in FIG.  3 . 
     FIG. 6 a  shows one embodiment of transceiver  500 , which includes a transmit-receive switch circuit  520 , a local receiver  550  and a local transmitter  580 . 
     FIG. 6 b  shows an implementation of receiver  500  of the present invention. 
     FIG. 6 c  shows a second implementation of receiver  500  of the present invention, in which a frequency synthesizer circuit  552   b  is provided. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an architecture of a two-way wireless data network  1300 , in accordance with the invention. Two-way wireless data network  1300  includes (a) personal mobile terminal  1305 ; (b) cellularized base station  1303 ; (c) message switching center  1309 ; (d) interfaces  1311 - 1315  to information and communication applications; and (e) radio links  1301  and  1307 . 
     Base station  1303  is capable of two-way communication with both personal mobile terminal  1305  and network switching or control center  1309 . Consequently, personal mobile terminal  1305  can send a message to base station  1303  and receive an acknowledgement from base station  1303  over a radio link  1301 . Radio link  1301  can be a packet radio link. Base station  1303  can then relay the message from personal mobile terminal  1303  to network control center  1309 . Upon receiving the message from base station  1303 , network control center  1309  can send the message to the intended recipient using a conventional one-way paging infrastructure. If desire, network control center  1309  can also broadcast an acknowledgement packet to confirm to personal mobile terminal  1305  receipt of the message by network control center  1309 . 
     Network control center  1309  is in turn connected to various interfaces  1311 - 1315  to information and communication applications, which include: (a) existing one-way communications networks, including such wired networks as the telephone network or the internet; (b) information dissemination networks or data banks; (c) security or utility monitoring systems; (d) electronic map or positioning systems; or (e) any one of numerous other possible applications. 
     Personal mobile terminal  1305  is typically a hand-held unit with a set of keys for data and command input and a display for displaying control and data information. Personal mobile terminal  1305 , in addition to being a data terminal, can also be used as a receiving terminal for facsimile transmissions, when equipped with suitable amount of memory and communication bandwidth (e.g. 2400 bps or better) 
     Referring back to FIG. 1, radio links between a base station and a personal mobile terminal unit, such as radio link  1301 , are typically low-power local packet radio links. In this embodiment of the invention, local radio link  1301  can operate with a 100 milli-watt radiated power to provide an expected service range of approximately 3 km. A single base station  1303  and personal mobile terminal  1305  are shown in FIG.  1 . However, base station, 1303  and personal mobile terminal  1305  are only representative of a plurality of base stations and personal mobile terminals which can be part of two-way wireless data network  1300 . In particular, a heavily populated metropolitan area served by two-way wireless data network  300  will have thousands of personal mobile terminals that are like terminals  1305 , tens or even hundreds of cellularized base stations that are similar to base station  1303 , and one or more network control center that is like network control center  1309 . Indeed, one advantage of the method of the present invention is that the size of two-way wireless data network  1300  is scalable to the operating environment, i.e., as more and more customers utilize two-way wireless data network  1300 , more base stations  1303  and network control centers  1309  can be added to accommodate the new traffic. 
     FIG. 2 shows one example of a two-way paging system  1000  according to the invention. As shown in FIG. 2, two-way paging system  1000  includes (i) personal mobile terminals  1005  and  1006 , which can each be a personal mobile terminal, in accordance with the present invention; (ii) base stations  1003  and  1004 ; (iii) network control center  1009 ; and (iv) high power transmitters  1021  and  1025 , which can be part of one or more existing one-way paging systems. 
     Personal mobile terminals  1005  and  1006  are operated by subscribers designated by reference numerals  1005 A and  1006 A, respectively. When subscriber  1005 A wishes to send a message to subscriber  1006 A, subscriber  1005 A enters subscriber  1006 A&#39;s identification and a message into personal mobile terminal  1005  using the alpha-numeric keys thereon, and initiates sending the message to a nearby base stations  1003 . Unlike messages in one-way communication networks, where return routing is not provided, this message also contains a paging identification identifying the sender, which is personal mobile terminal  1005 . 
     In the example shown in FIG. 2, the message from personal mobile terminal  1005  is received by base station  1003  via local packet radio link  1001 . As with local radio link  1301  described above (FIG.  1 ), packet radio link  1001  is a two-way packet radio link which allows personal mobile terminal  1005  to both transmit messages to, and receive messages from, base station  1003 . In response, base station  1003  transmits an “acknowledgement” packet on packet radio link  1001  to inform subscriber  1005 A that his transmission was successful. Base station  1003  transmits the message received from personal mobile terminal  1005  to network control center  1009  via radio link  1007 . Base stations  1003  and  1004  are similar to base station  1303  (FIG.  1 ). Radio link  1007  is also two-way radio link, so that messages are exchanged between base station  1003  and network control center  1009 . Link  1007  need not be accomplished by wireless communication. In fact, where a telephone network is accessible, a wire communication link is possible. Such wire communication is indicated in FIG. 10 as telephone line  1040 . 
     At network control center  1009 , the message from base station  1003  (and personal mobile terminal  1005 ) is processed for authorization and billing purposes based on its service agreement with subscriber  1005 A. When such administrative tasks are completed, network control center  1009  transmits the received message from personal mobile terminal  1005  to subscriber  1006 A&#39;s paging service  1017 . The sending of this message from network control center  1009  to subscriber  1006 A can be accomplished in the same manner as the way messages are sent in one-way paging systems. Thus, when the message is received by paging service  1017 , the message is sent to transmitter  1025  (which is operated by subscriber  1006 A&#39;s paging service  1017 ) and transmitted to personal mobile terminal  1006 . Alternatively, network control center  1009  may keep subscriber  1005 A&#39;s message form personal mobile terminal  1005  and sends, instead, a notification message to subscriber  1006 A through the one-way paging system. Upon receiving this notification message, subscriber  1006 A may request for subscriber  1005 A&#39;s message by sending a message through a base station in his vicinity to network control center. By inspecting the identity of the base station from which subscriber&#39;s  1006 A is routed, network control center  1009  locates subscriber  1006 A, and subscriber  1005 &#39;s message can be routed to subscriber  1006 A through that base station. Using this alternative approach, subscriber  1005 A&#39;s message is not broadcast within the range of the entire service area, but only within the immediate vicinity of subscriber  1006 A, thus the same frequency channel can be used in other cells simultaneously, thereby increasing enormously the total bandwidth of the system. 
     Subscriber  1006 A can send an immediate reply using subscriber  1005 A&#39;s paging identification in the message received. The reply message can be composed in the same manner discussed above with respect to subscriber  1005 A&#39;s message, or it can be simply an automatically generated acknowledgement message dispatched by a special command to the user interface. The reply message or acknowledgement message is then transmitted from personal mobile terminal  1006  to a second base station  1004  via two-way radio link  1008 . The reply message finds it way to subscriber  1005 A in substantially the same way subscriber  1005 A&#39;s message reaches subscriber  1006 A. In this instance, the message is sent through base station  1004 , network control center  1009 , paging service  1020 , and transmitter  1021 . In this example, paging services  1017  and  1020  can be operated by the same company or they can be entirely separate, and even competing, paging services. Of course, the reply message can also be sent through two-way communication over a base station, using the notification mechanism described above. 
     FIG. 3 a  shows personal mobile terminal  100  in accordance with the invention. A transmission tower  101 , which may be a high-power broadcast transmitter compatible with a one-way paging system, broadcasts a data signal  125  on an assigned carrier frequency. Data signal  125  includes a conventional paging message  175  (not shown). In this embodiment, personal mobile terminal  100  has a “receive-only” link with transmission tower  101 . Personal mobile terminal is equipped with conventional antenna  300  to receive data signal  125  and a receiver  400  to extract message data  175 . 
     Antenna  300  is tuned to receive signals over a preselected range of carrier frequency. Antenna  300  consists of a single loop antenna removably mounted inside personal mobile terminal  100 &#39;s housing on a printed circuit board, which is shared with receiver circuit  400 . Receiver circuit  400  can be any conventional receiver circuit used in a one-way paging network. Receiver circuit  400  demodulates data signal  125  to provide the baseband data message  175 . In the present embodiment, receiver circuit  400  is a direct downconversion frequency shift-keyed (FSK) receiver, such as provided by the UAA2080 pager receiver integrated circuit available from Philips Microelectronics Inc. Data message  175  is then provided to personal mobile terminal  100 &#39;s on-board microprocessor, which resides on central processing unit (CPU) board  600 . Data message  175  can then be stored, further processed, or displayed on an on-board liquid crystal display (LCD)  700 . Alternatively, receiver circuit  400  can also incorporate an internal frequency oscillator  410  (shown in FIG. 4 a ) or a programmable frequency synthesizer  420  (shown in FIG. 4 b ) to directly downconvert data signal  125 . The extensive frequency range in which the frequency synthesizer operates allows the personal mobile terminal to operate over a wide range of carrier frequencies. The wide operating range is useful, since personal mobile terminal  100  is then operable, with minimal or no conversion, in a large number of locales where the assigned paging frequencies are different. Of course, receiver circuit  400  can include single or multiple stage superheterodyne circuits, and may operate under any keying schemes, such as PSK (phase shift-keying) or ASK (amplitude shift-keying) demodulation schemes, for extracting baseband message  175  from data signal  125 . 
     A block diagram of CPU board  600  is provided in FIG. 3 b . As shown in FIG. 3 b , a microprocessor  601  is provided on CPU board  600  to control the operations of personal mobile terminal  100 . Microprocessor  601  can be implemented by the 8-bit microprocessor 83CL781. Receiver circuit  400  is coupled into CPU board  600  by a connector  611  to a data signal decoder  612 , which can be implemented a POCSAG decoder PCD5003, available from Philips N.V. When a data signal (i.e. message  175 ) is received at receiver circuit  400 , signal decoder  612  asserts an interrupt at microprocessor  601  on interrupt line  613 . Data in message  175  are presented to microprocessor  601  on an industry standard I 2 C bus  610 . In the present embodiment, a connector  602  is provided to allow access to personal mobile terminal  100  by an external host computer. Connector  602  is coupled to microprocessor  601  over a serial link implemented by a universal asynchronous receiver/transmitter (UART)  604 . UART  604  also couples local transceiver  500  via connector  603  to microprocessor  601 . 
     Microprocessor  601  communicates with data memory  609  and program memory  608  over an 8-bit industry standard bus  605 . In this embodiment, data and programming memories are each provided to have 128 K-bytes. In addition, an application specific integrated circuit (ASIC)  607  provides a 220 K-byte font library for storing character fonts to be displayed on LCD display  700  and glue logic functions, such as a keypad control circuit, a display control circuit  606 , address decoding for 8-bit bus  605 , and interrupt registers. ASIC  605  can be implemented by a field-programmable gate array. 
     Referring to FIG. 3 a , personal mobile terminal  100  can also be used to establish a second communication link with a local base station  200 . The use of this link is discussed extensively in copending patent application “Two-way Wireless Data Network” incorporated by reference above. This second link is a half-duplexed two-way link between personal mobile terminal  100  and a cellularized local base station  200 , in which personal mobile terminal  100  and local base station  200  communicates using the same frequency. Using the same frequency to transmit and receive, the present invention allows for personal mobile terminal  100  and base station  200  to have a relatively simpler design. The network access protocol for communicating over this two-way link is discussed in the copending patent application “Wireless Network Access Protocol” incorporated by reference above. Because this two-way link is local, i.e. the limited range of the base station, transmission power required of personal mobile terminal  100  is low. Further, because of the limited range for this communication, the same frequency can be used simultaneously at different portions of the data network without the complication of collision. Thus, system bandwidth is dramatically increased. 
     In this embodiment, personal mobile terminal  100  establishes a two-way local link with base station  200  using a second antenna  350  and local transceiver  500 . Local antenna  350  may be any conventional antenna which can transmit or receive signals at the preassigned two-way carrier frequency between personal mobile terminal  100  and the base station  200 . However, because of the expected range of frequencies over which local radio link  301  operates (i.e. between 150 MHz to 1 GHz), antenna  350  may be required to have a dimension in the order of one meter. An antenna which is extended linearly to such length is both aesthetically unpleasant and inconvenient in an urban environment. Thus, the present invention provides a loop antenna conveniently enclosed in personal mobile terminal  100 . FIG. 5 shows local antenna  350 , which is a loop antenna  1130  molded into personal mobile terminal  100 &#39;s cover  1100 . In this embodiment, the housing is designed such that, for receiving functions, cover  1100  conceals and protects the portion of keypad  800  which is used for transmission functions. Cover  1100  also protects personal mobile terminal  100 &#39;s LCD display  700 , allowing LCD display  700  to be visible through a window in cover  1100 . However, to transmit, the user lifts cover  1100  to access the transmit function keys, thereby placing the loop antenna  1130  in position for transmission. Loop antenna  1130  is made from an electrically conductive, preferably metallic, material and is concealed in cover  1100  for aesthetic reasons. As shown in FIG. 5, ends  1132  and  1134  of loop antenna  1130  are mechanically and electrically coupled by conductive hinge pins  1101   a  and  1102   a , respectively, to conductive hinge assemblies  1101   b  and  1102   b.    
     Conductive hinge assemblies  1101   b  and  1102   b  are mechanically attached to the remainder of personal mobile terminal  100 &#39;s housing, and electrically coupled to the printed circuit board on which local transceiver  500  resides. Hinge pins  1101   a  and  1102   a  and hinge assemblies  1101   b  and  1102   b  form hinges  1101  and  1102  to allow cover  1100  to conceal and expose, when desire, keypad  800  and LCD display  700 . 
     Referring back to FIG. 3 a , local transceiver  500  can be any conventional transceiver having a carrier signal port for transmitting a data signal  1025  and receiving a base station signal  225  at the preassigned two-way carrier frequency. In addition, local transceiver  500  receives message  1075  from CPU board  600 , which is modulated for transmission as signal  1075  in local transceiver  500 . Messages from base station  200  to personal mobile terminal  100 , such as acknowledgement messages, are sent from base station  200  as signal  225 . Signal  225  is picked up by antenna  350  and demodulated by local transceiver  500  to extract embedded message  275 , which is then provided to CPU board  600 . 
     FIG. 6 a  shows one embodiment of local transceiver  500 , which includes a transmit-receive switch circuit  520 , a local receiver  550  and a local transmitter  580 . Transmit-receive switch circuit  520  serves to isolate transmitter  580  from receiver  550  during local transceiver  500 &#39;s transmit and receive states. Receiver  550  is implemented to extract the base station message data  275  from the received base station signal  225 . Transmitter  580  superimposes message data  1075  onto the preassigned two-way paging carrier frequency and transmits the data signal  1025 . 
     Transmit-receive switch circuit  520  shown in FIG. 6 a  is coupled between antenna  350 , transmitter  580  and receiver  550 . Transmit-receive switch circuit  520  can be provided by an antenna switch, such as the Motorola MRFIC2003, available from Motorola, Inc., or a circulator. Alternatively, as shown in FIG. 6 a , transmit-receive switch circuit  520  is provided by PIN diodes  522  and  526 , with the cathode terminal of PIN diode  522  and the anode terminal of PIN diode  526  coupled to terminals of a quarter-wave circuit  524 . The transmission path of the quarter-wave circuit  524  is approximately one-quarter of the wavelength of preassigned two-way data signal  1025 . PIN diodes  522  and  526  have relatively high “turn-on” voltages (e.g. 0.3V-0.7V) and do not become forward-biased when exposed to small signal levels such as those received from local base station  200  or high-power transmitter  100 . 
     The operations of PIN diodes  522  and  526  in transmit-receive switch circuit  520  are discussed next. Signal  225  received from local base station  200  at local antenna  350  is a low intensity data signal. Since signal  225  does not have an amplitude sufficient to turn on PIN diode  522 , data signal  225  is blocked from transmitter  580 . Hence, data signal  225  is coupled to the input terminal of the local receiver  550  with minimal loss. Minimal signal loss allows the local base station signal to retain a high carrier-to-noise ratio (C/N), which is required for receiver  550  to achieve a low bit error rate (BER). PIN diode  526  does not couple data signal  225  to ground because the amplitude of signal  225  received is typically insufficient to forward-bias PIN diode  526  to a conducting state. 
     During the transmission operation of transmitter  580 , data signal  1025  forward-biases PIN diode  522 , so that data signal  1025  can split at the cathode terminal of PIN diode  522  between antenna  350  and receiver  550 . However, PIN diode  526  is forward-biased by the amplitude of data signal  1025  (approximately +3 to +30 dBm). With PIN diode  526  forward-based, loss of power and possible damage to receiver  550  are avoided. To avoid significant power loss via PIN diode  526  to ground, quarter-wave circuit  524  transforms the substantially low impedance at the terminal  524   b  to a substantially high impedance at input terminal  524   a . The high impedance path at input terminal  524   a  effectively channels the power in data signal  1025  towards antenna  350 , where it is transmitted. 
     An antenna network  510  can be added to further enhance operation of transmit-receive switch circuit  520 . Antenna network  510  can include an impedance matching circuit coupled between local antenna  350  and transmit-receive switch circuit  520 . Such an impedance matching circuit increases both signal reception, during a receive state, and transmission efficiency, during a transmit state, by matching the output impedance of the local antenna  350   a  to substantially the impedance present at the input terminal  520   a  of the transmit-receive switch circuit  520 . An input filter can also be provided in addition to, or in lieu of the impedance matching circuit, so as to reduce external interference during receiving, or to restrict signal transmission to a specified band, during transmitting. 
     FIGS. 6 b  and  6   c  show alternative implementations of receiver  500  of the present invention. As shown in FIG. 6 b , receiver  550  includes an input amplifier  551 , a frequency shift-key (FSK) receiver  552  and a digital filter  553 . Input amplifier  551  typically has a high signal gain and a low noise figure to suppress the input noise figure of receiver  550  to a minimum value. Thus amplified, signal  225  is fed into a FSK receiver  552 , which can be provided by a UAA2080 direct-conversion FSK receiver integrated circuit disclosed above. Using the UAA2080, the FSK receiver  552  may utilize an internal frequency oscillator  552   a  to tune to the assigned two-way carrier frequency and to directly downconvert the signal from base station  200 . Alternatively, as shown in FIG. 6 c , programmable frequency synthesizer  552   b  can be used in lieu of internal frequency oscillator  552   a . Frequency synthesizer  552   b  has a higher frequency selectivity than internal oscillator  552   a . Digital filter  553  receives the demodulated output signal of FSK receiver  552  and produces baseband message data  275 . Message  275  is then provided to CPU board  600  where it may be stored or displayed on a display terminal such as an LCD display  700 . 
     FIG. 6 d  illustrates another implementation of receiver  550 , using a downconverting circuit  555  and a Gaussian Minimum Shift Keying (GMSK) receiver  557 . The downconverting circuit  555  includes a first-stage amplifier  555   a  for setting input noise figure, and a second-stage amplifier  555   b  for increasing signal strength. A bandpass filter  555   d  may be used between first- and second-stage amplifiers  555   a  and  555   b . Amplifiers  555   a  and  555   b  are both coupled to an automatic gain control (AGC) circuit  555   c  which monitors and increases the gain of one or both of amplifiers  555   a  and  555   b , when the received strength of the local base station signal  225  is low, and decrease their gains when the received signal strength is high. AGC circuit  555 C outputs a received signal strength indicator (RSSI) signal  558  to the GMSK receiver  557 . Down-converter circuit  555  further includes a mixer  555   e  and a local oscillator  555   f  for down-converting data signal  225  into an intermediate frequency (IF) signal  555   g . As shown in FIG. 6 d , local oscillator  555   f  includes a voltage controlled oscillator (VCO) coupled to a dual-frequency synthesizer  559  in a phase locked loop configuration. 
     The GMSK receiver  557  receives IF signal  555   g  to extract message  275 . GMSK receiver  557  performs the same function as FSK receiver  552  but requires only half the frequency deviation of a standard FSK scheme to detect orthogonal signals. Thus, within a specified frequency deviation bandwidth, as compared to FSK receiver  552 , the GMSK receiver  557  can process twice as much data. Message  275  is fed into a CPU board  600  where it may be stored, further processed, or displayed on a display terminal such as an LCD  700 . 
     Local transmitter  580  is coupled to the transmit-receive switch circuit  520  and generates the data signal  1025  by superimposing message data  1075  onto the local carrier frequency. Local transmitter  580  may be any conventional transmitter having a data input for receiving message data  1075  and a second carrier output for providing a data signal  1025  at the local carrier frequency. One choice of a local transmitter is an FSK modulator. Such an FSK modulator can use the internal oscillator and frequency multiplier circuits of an FSK receiver, as can be used to implement local receiver  550 . Alternatively, as shown in FIG. 6 d , modulator  580  can be a programmable frequency synthesizer  559 , which shares a phase-locked loop with down-converter  555 . In this embodiment, the frequency synthesizer  559  generates the local carrier signal at VCO  591 , which is used to modulate message  1075  to provide modulated signal  1025   a.    
     Output amplifier  584  amplifies the modulated signal  1025   a  for transmission to the base station  200 . As shown in FIG. 6 d , output amplifier  584  includes a two-stage amplifier circuit. In the first stage, driver amplifier  584   a  provides a high gain for signal  1025   a . Second stage amplifier  584   b  amplifies signal  1025   a  to provide data signal  1025  at a high output power. Of course, output amplifier circuit  584  may include two or more amplifiers in succession depending upon the gain requirements and desired transmission power of the data signal. 
     Since personal mobile terminal  100  is powered by a battery-based power source, as the remaining battery charge decreases over time and use, output amplifier  584  may produce data signal  1025  at varying output levels. Power variation in data signal  1025  may result in a low power signal at local base station  200 , which may prevent local base station  200  from receiving signal  1025  from personal mobile terminal  100  due to the FM capture effect causing base station  200  to lock on to the message of a more powerful near by personal mobile terminal. Thus, as shown in FIG. 6 d , an automatic level control circuit  584   c  is provided to maintain a minimum variation in transmitted output power over a wide range of battery charge conditions. 
     It should be noted that, although local receiver  550  and remote receiver  400  are shown as separate circuits in FIG.  3  and treated as such in FIGS. 4 a - 4   b  and  6   a - 6   d , the present invention can also be provided by an embodiment in which a single receiver is provided. In such a circuit, a tuning circuit can be provided to tune the receiver to either the remote carrier frequency or the local (i.e. 2-way communication) carrier frequency. Such a tuning circuit may, for example, selects between two internal local oscillators which are tuned, respectively, to the remote and local carrier frequencies. Alternatively, a programmable frequency synthesizer can also be used. 
     The above detailed description is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the appended claims.

Technology Classification (CPC): 8