Abstract:
An apparatus and method for generating a pilot signal for hard hand-off. A pilot signal generator of a base station for generating an identifying pilot signal corresponding to a target base station to perform inter-frequency hard hand-off in the code division multiple access cellular system comprises an intermediate frequency amplifier/divider, a service RF path unit and a RF path unit. The IF amplifier/divider divides the amplified IF signal transferred from a digital MODEM. The service RF path unit up-converts a first portion of the divided signal into a desired radio frequency and transmits the up-converted RF signal for actual communication and the RF path unit up-converts a second portion of the divided signal into a desired RF and transmits the up-converted RF signal as an identifying pilot signal corresponding to a target base station for a hard hand-off. When the IF amplifier/divider that is included in the conventional digital MODEM only transfers a signal to the RF path unit, the RF path unit then transmits all signals converted by an overhead channel or traffic channel. The RF path unit is portable for easy insertion or extraction.

Description:
BACKGROUND 
     1. Technical Field 
     The present application relates generally to a communication system and, more particularly, to an apparatus and method for generating a pilot signal for performing a hard hand-off in a code division multiple access (CDMA) cellular system using an External Pilot Transmitter (EPT). 
     2. Description of the Related Art 
     In a cellular mobile telephone system, the cellular service area is divided into a plurality of sub-areas (i.e., cells), and each cell has a base station associated therewith. In a cellular communication system, a single mobile switching center (MSC) controls all base stations, and allows a mobile station to continue communication when the mobile station travels between several service cells. In a code division multiple access (CDMA) cellular communication system or a personal communication service (PCS) system, various types of hand-off operations are utilized to ensure continuous communication when the mobile station travels between cells (i.e., when a mobile station travels from a “source base station” to a “target base station”). 
     One type of hand-off operation is referred to as a “hard hand-off.” During a hard hand-off operation, when a mobile station enters a target cell (or target base station), communication with the source base station is terminated, and then communication with the target base station is established. The hard hand-off process is completed within a very short time and the user of the mobile station does not even recognize the temporary termination of communication. 
     In a CDMA cellular system, frequency assignment to the various cells is typically offset due to the unbalanced distribution of subscribers. For instance, downtown city areas generally require more traffic capacity to service a large number of subscribers, whereas suburban areas require relatively less traffic capacity due to a smaller number of subscribers. Consequently, when a target base station (in which a mobile station enters) does not have a frequency assignment through which communication is currently established, or when, if any, the frequency assignment does not have enough traffic channels, a hard hand-off cannot occur. 
     A hard hand-off is generally performed as follows. The source base station continuously measures the signal strength of a mobile station within its cell region to determine if the signal strength drops below a predetermined threshold value. When the received signal strength falls below the threshold, the source base station determines that the mobile station is located at the boundary of its cell region, and then signals a base station controller (BSC). The BSC then decides which base station (i.e. target base station) receives a relatively strong signal from the mobile station. 
     When it is determined that a particular target base station (e.g., a neighboring base station) receives a strong signal, the BSC transmits a hand-off request message to the target base station, as well as a command to the mobile station to communicate with the target base station (neighboring base station). The mobile station then performs a hand-off and, accordingly, communication between the mobile station and the target base station is established. 
     The BSC determines whether to perform a hand-off based on a pilot signal strength of the source base station which is in current communication with the mobile station. In the conventional system, a call will be disconnected if the hand-off to the target base station is unsuccessful. There are various reasons for a hand-off operation to fail. For instance, a hand-off operation can fail if there are no available channels in the target cell for communicating the call or if the mobile station fails to receive a hand-off message. A hand-off can also be requested when a mobile station enters a shadow area of the cell area in which the pilot signal strength becomes weak. Furthermore, it is very difficult to determine a hand-off determination parameter and time in the varying cellular communication environment. And there is trade-off between coverage areas to reduce the possibility of success to perform a hand-off. 
     A conventional method for performing a hard hand-off using a pilot signal is disclosed in U.S. Pat. No. 5,594,718 entitled “A Method and Apparatus For Providing Mobile Unit Assisted Hard Handoff From A CDMA Communication System To An Alternative Access Communication System.” This method uses a pilot beacon for generating an identifying pilot signal corresponding to a target base station in order to overcome the above difficulties. 
     Referring now to FIG. 1, a diagram illustrates a conventional pilot signal generator for generating a pilot signal for performing a hard hand-off in accordance with the prior art. Each base station has a RF path unit, which includes a plurality of digital MODEMs  200 ,  210  and  220  for converting audio frequencies into intermediate frequencies, a plurality of transceivers  300 ,  310  and  320  for converting the intermediate frequencies into radio frequencies and a plurality of power amplifiers  400 ,  410  and  420  for amplifying the RF signals which are transmitted from an antenna. The digital MODEMs  200  and  210 , transceiver  300  and  310 , and power amplifiers  400  and  410  are utilized for communication, whereas a pilot signal generator (which generates an identifying pilot signal corresponding to a target base station) includes the digital MODEM  220  for generating a pilot signal, the transceiver  320  and the power amplifier  420 . As a mobile station moves to a target base station, the mobile station simultaneously receives a weak pilot signal from the source base station and a relatively strong pilot signal from the target base station. Accordingly, the mobile station will request a hand-off and the digital MODEM  220  of the pilot signal generator will only transmit the overhead channel such as, for example, a pilot, synchronization, and paging. 
     The transmission of the overhead channel (as described above) causes an unbalance of coverage areas of multiple frequencies, and this unbalance increases the load of the primary frequency. Specifically, when the coverage area of the pilot signal generator is greater than that of the forward link of the primary frequency, a mobile station will more frequently request a hand-off in a base station having a pilot signal generator than in the balanced base station where the coverage area of the primary frequency is almost balanced with that of the pilot signal generator. Accordingly, a call will more frequently be handed down to the primary frequency, which results in an increase of the load of the primary frequency and reduces the performance of the entire system. A pilot digital gain modulation can control the unbalance of the coverage areas, but it is very difficult to modulate a pilot digital gain taking into consideration the actual radio environment and system operation. 
     In addition, when the pilot signal generator is added to the base station, it requires space for installation; a digital hardware MODEM, and a channel card. The digital hardware MODEM is used for a hand-off (not for actual communication), and for generating some channels such as a pilot channel, synchronization channel and paging channel. This pilot signal generator is not portable because it is installed in an outdoor device. 
     There are various disadvantages associated with the conventional pilot signal generator. For example, the pilot signal generator is physically large and significantly costly due to the large space required for installation. Moreover, the conventional pilot signal generator is not portable and requires a significant amount of labor to install it, or extract it, from the system. In addition, as stated above, it causes unbalance in coverage areas of multiple frequencies. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for generating a pilot signal for a hard hand-off using an IF amplifier/divider without having to add the pilot signal device to frequency allocation generating hardware, the IF amplifier/divider being easily coupled to a digital MODEM. 
     It is another object of the present invention to provide a method and apparatus for generating a pilot signal to produce output to every channel in order to balance the system load. 
     It is a further object of the present invention to provide an apparatus for generating a pilot signal which does not require physical space and is portable. 
     In one aspect of the present invention, a pilot signal generator of a base station for generating an identifying pilot signal corresponding to a target base station in order to perform an inter-frequency hard hand-off operation in a code division multiple access cellular communication system, comprises an intermediate frequency (IF) amplifier/divider for dividing an intermediate frequency (IF) signal which is received from a digital MODEM; a service radio frequency (RF) path unit for up-converting a first portion of the divided signal into a radio frequency and transmitting the radio frequency, the service RF path unit being utilized solely for actual communication; and a radio frequency (RF) path unit for up-converting a second portion of the divided signal into a radio frequency and transmitting the radio frequency where the RF is used as an identifying pilot signal corresponding to the target base station for a hard hand-off, the RF path unit being used solely for transmission of a pilot signal. 
     In another aspect of the present invention, a method for generating a pilot signal for performing an inter-frequency hard hand-off in a CDMA cellular communication system, where a base station uses an identifying pilot signal corresponding to a target base station for performing an inter-frequency hard hand-off operation, includes the steps of: generating an intermediate frequency signal for communication by a digital MODEM; dividing the intermediate frequency signal to multiple paths; transmitting the up-converted signal to a service RF path unit for communication after up-converting a first portion of the divided signal; and transmitting the up-converted signal to at least one RF path unit in order to generate a pilot signal of a target base station after up-converting a second portion of the divided signal. 
     In yet another aspect of the present invention, a method for generating a pilot signal of a target base station for performing an inter-frequency hard hand-off operation in a CDMA cellular communication system includes the steps of: transmitting, at a RF path unit, all signals converted by an overhead channel or traffic channel, where the RF path unit transfers a target base station pilot signal for a hard hand-off; and transmitting, at a RF path unit, the up-converted signal after up-converting a signal of a path for a primary frequency. 
     These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments, which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a conventional pilot signal generator for generating a pilot signal corresponding to a target base station for performing a hard hand-off; 
     FIG. 2 is a block diagram of a base station having an amplifier/divider according to an embodiment of the present invention; 
     FIG. 3 is a block diagram of a self-switched IF amplifier/divider  100  according to an embodiment of the present invention; 
     FIG. 4 is a block diagram of a self-switched IF amplifier/divider according to another embodiment of the present invention. 
     FIG. 5 is a block diagram of an exemplary switch control circuit; 
     FIG. 6 is a block diagram of an exemplary IF amplifier/divider which illustrates a state of selecting a first path in accordance with the present invention; 
     FIG. 7 is a block diagram of an exemplary IF amplifier/divider which illustrates a state of selecting a second path in accordance with the present invention; 
     FIG. 8 illustrates a connection of a pilot signal generator according to the present invention; and 
     FIG. 9 is a block diagram of an exemplary pilot signal generator according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is directed to a method and apparatus for generating a pilot signal for performing a hand-off, whereby an IF amplifier/divider is added to a conventional digital MODEM such that the digital MODEM generates both a pilot signal for communication and a pilot signal for a hard hand-off. In addition, a radio frequency path unit transmits all signals converted by an overhead channel or traffic channel to balance the coverage areas of multiple frequencies. 
     Referring now to FIG. 2, a block diagram illustrates a base station having an IF amplifier/divider in accordance with one embodiment of the present invention. The base station includes a first digital MODEM  200  and an IF amplifier/divider  100 . The first digital MODEM  200  produces an intermediate frequency which results in frequency # 1 . The IF amplifier/divider  100  divides a signal from the digital MODEM  200  into a service RF path unit  500  (which includes a transceiver  300  and a power amplifier  400 ) and an RF path unit  530  (which includes a transceiver  330  and a power amplifier  430 . A second digital MODEM  210 , which is coupled to a service RF path unit  510  (which includes a transceiver  310  and a power amplifier  410 ), produces an intermediate frequency which results in a frequency # 2 . It is to be understood that the term “service RF path unit” used herein will refer to an RF path unit that is used solely for actual communication, whereas the term “RF path unit” will refer to an RF path unit that is used solely for transmission of a pilot signal. 
     As discussed above, the conventional digital MODEM (e.g., MODEM  200  in FIG. 1) produces a signal that results in a frequency # 1  for actual communication. However, in a preferred embodiment according to the present invention, the IF amplifier/divider  100  coupled to the digital MODEM  200  (FIG. 2) transmits a first portion of the divided IF signal through the service RF path unit  500  for generating a frequency # 1  (which is utilized for actual communication), and a second portion of the divided IF signal through the RF path unit  530  for producing frequency # 3  (which is utilized for generating a pilot signal). 
     The digital MODEM  200  coupled to the IF amplifier/divider  100  transmits all signals converted by an overhead channel or traffic channel. Accordingly, the RF path unit  530  can balance coverage areas of multiple frequencies by transmitting both signals converted by an overhead channel and traffic channel. It is preferable to provide a digital MODEM (which produces the primary frequency available to all base station) which divides a signal and a pilot signal generator which produces a pilot signal. This is because the coverage area of a signal from the pilot signal generator can be balanced in a coverage area of the primary frequency signal. 
     As explained in further detail below, a signal is transferred via an amplify path that is selected by the IF amplifier/divider  100  to a power divider which divides the signal into desired number of signals. In this manner, it is easy to increase or decrease the number of frequencies and pilots as demanded. This increase and decrease of the RF path unit is based on the number of subcells. That is, one RF path unit is allocated to one subcell. For example, when a hand-off is performed over two sectors, the system is designed such that transceivers and power amplifiers of the two sectors are connected to one path of the IF amplifier/divider  100 . 
     Conventionally, the IF amplifier/divider includes an IF amplifier for amplifying an input signal, an attenuator coupled to the IF amplifier for attenuating an unnecessary gain and a N-way power divider for dividing the signal transferred from the attenuator into N paths. The N-way power divider divides the amplified signal into a path for frequency # 1  and also divides into a path for transmission of a pilot signal. This problem with this circuit is that neither a signal for actual communication nor a pilot signal will be transferred when a fault occurs in the RF amplifier or attenuator. In the present invention, however, an amplify path is dualized and a self-switched circuit is utilized for detecting a fault in order to provide a safer method. In order to dualize an amplify path, either an amplifier can be dualized or both an amplifier and attenuator can be dualized. 
     Referring now to FIG. 3, a block diagram illustrates a self-switched IF amplifier/divider  100  according to one embodiment of the present invention. The self-switched IF amplifier/divider  100  includes; a first IF switch  110  for selecting one of an IF amplify path-A and an IF amplify path_B for transmitting an IF signal received from a digital MODEM (not shown). An IF amplifier A  120  amplifies an IF signal transmitted on path_A and an IF amplifier B  125  amplifies an IF signal transmitted on path_B. A second IF switch  130  is provided for selectively coupling the amplified IF signals on path_A or path_B to an attenuator  140 . The attenuator  140  attenuates unnecessary gain of the amplified IF signals on path_A or path_B, whereby the unnecessary gain is expected to be the overall gain generated by the entire IF amplifier/divider circuit. A N-way power divider  150  is connected to the attenuator  140  for transmitting the attenuated signal to N paths. 
     The first IF switch  110  and the second IF switch  130  switch each input signal to each output signal. The IF amplifiers A  120  and B  125  amplify the input signal by a predetermined value. The N-way power divider  150  divides the switched input signal into demanded paths. The attenuator  140  attenuates the signal to 0 db so as to decrease the signal gain of a self-switched IF amplifier/divider paths. 
     Referring now to FIG. 4, a block diagram illustrates a self-switched IF amplifier/divider according to a second embodiment of the present invention. The IF amplifier/divider includes a first IF switch  110  for selecting between an IF amplify path_A and an IF amplify path_B so as to transmit an IF signal received from a digital MODEM (not shown). An IF amplifier A  120  amplifies the IF signal coupled to path_A via the first IF switch  110  and an attenuator A  142  attenuates unnecessary gain of the amplified signal from the IF amplifier A  120 . An IF amplifier B  125  amplifies an IF signal coupled to path_B via the first IF switch  110  and an attenuator B  144  attenuates unnecessary gain of the amplified signal from the IF amplifier B  125 . A second IF switch  130  is connected to attenuators A  142  and B  144  for selectively coupling an attenuated signal from attenuator A  142  or attenuator B  144  to an N-way power divider  150 . The N-way power divider transmits the attenuated IF signal to N paths. The unnecessary gain, which is attenuated by the attenuator A and B, is the overall gain that is generated by the entire IF amplifier/divider circuit. 
     The function of the self-switched IF amplifier/divider of FIG. 4 is similar to the function of the self-switched IF amplifier/divider discussed above in connection with FIG.  3 . As illustrated both embodiments of the self-switched IF amplifier/divider include an IF amplify path_A and a IF amplify path_B. An input signal to the self-switched IF amplifier/divider is selectively coupled to one path of the paths (i.e., the IF amplify path_A or the IF amplify path_B). The gain of the signal(s) output from N-power divider  150  is preferably 0 dB. Each IF amplifier  120  and  125  has the same power gain G. The power gain G of each IF amplifier  120  and  125  compensates for the insertion loss L 2  of the N-way power divider  150 . The IF amplifiers  120  and  125  each have a power gain controller for controlling the power gain to keep it steady. Additionally, when a fault occurs in the circuit, the IF amplifier can easily be inserted or removed due to its modular design. In addition, the IF amplifier/divider  100  of the present invention has a constant voltage circuit for converting power transferred from outside into a constant voltage and uses the constant voltage as power. A first IF switch  110  and a second IF switch select  130  the same path at any given time under the control of a switch control circuit (discussed in further detail below). 
     Referring now to FIG. 5, a diagram illustrates an exemplary embodiment of a switch control circuit for checking the operating state of the IF amplify paths A and B, as well as for controlling the operation of the first and second IF switches  110  and  130  shown in FIGS. 3 and 4. Specifically, the IF amplifier/divider  100  has a switch control circuit  160  for detecting an alarm signal from the IF amplifiers A  120  and B  125 , and for controlling the switching operation of the first and second IF switches  110  and  130 . When the switch control circuit  160  detects a fault in one of the IF amplify paths, it blocks the faulty IF amplify path by signalling the IF switches  110  and  130  to connect to the other (non-fault) IF amplify path. 
     During initial operation, the switch control circuit  160  signals the first and second IF switches  110  and  130  to select the IF amplify path_A for transmitting a signal. The switch control circuit  160  uses a buzzer  162  to audibly notify a user of an alarm state and/or an alarm LED_A  164  and alarm LED_B  166  to visually notify the user of an alarm state in one of the IF amplify paths A and B. 
     Referring now to FIG. 6, a block diagram of the exemplary IF amplifier/divider illustrates a state when the first and second IF switches  110  and  130  select the IF amplify path_A according to the present invention. As illustrated, when the IF switches  110  and  130  (which are controlled by the switch control circuit  160  of FIG. 5) select an IF amplify path_A, a signal received from the digital MODEM  200  (not shown) is transmitted via the path_A to the N-way power divider  150 . On the other hand, FIG. 7 is a block diagram which illustrates a state when the first and second IF switches  110  and  130  select the IF amplify path_B according to the present invention. As illustrated, when the first and second IF switches  110  and  130  (which are controlled by the switch control circuit  160 ) select an IF amplify path_B, a signal received from the digital MODEM  200  (not shown) is transmitted via path_B to the N-way power divider  150 . 
     The N-way power divider  150  operates by dividing the amplified signal transmitted via IF amplify path_A or path_B into N output signals and transmits them via N paths. For example, when two outputs are demanded, the N-way power divider  150  divides the signal into two outputs and when four outputs are demanded, the N-way power divider  150  divides the signal into four outputs. The power divider terminates the non-used output ports. 
     The aforementioned N-way power divider  150  has an insertion loss L 2  and the amount of insertion loss L 2  depends on the number of output signals. Generally, a two-way power divider has approximately 3.3 dB of insertion loss and four-way power divider has approximately 6.3 dB of insertion loss. The attenuators  140 ,  142  and  144  (shown in FIGS. 3 and 4) have a predetermined attenuation L 1 . Therefore, in order to make the gain of each IF amplify path 0 db, the attenuation value L 1  is determined according to the equation: 
     
       
           L   1 = G−A−L   2  [dB], 
       
     
     where G is the gain of the IF amplifiers  120  and  125 , A is the overall gain of the IF amplifier/divider  100 , and L 2  is the insertion loss of the N-way power divider. 
     As stated above, when a fault is detected in one of the IF amplify paths of the self-switched IF amplifier/divider circuit  100 , a switch operation from the failed path to the normal path (for example from path_A to path_B when path_A has failed) is automatically implemented without any command. Advantageously, this self-switch function poses no problem with the operation of the circuit and significantly improves reliability and independence of the system. Furthermore, because the gain of the entire IF amplify path is 0 dB, the self-switched IF amplifier/divider  100  can advantageously be applied to the conventional base station without changing the signal level. 
     Referring now to FIG. 8, a diagram illustrates a connection of a pilot signal generator according to the present invention. It is to be appreciated that an RF path unit of the pilot signal generator of the present invention is portable and does not require installation within the system. An IF amplifier/divider  100  (denoted by TADU), which is installed in the base station, provides a signal to a transceiver  330  of a RF path unit. As illustrated, the RF path unit is installed externally and a pilot signal is generated via the RF path unit. The RF path unit is coupled to a High Speed IPC Node Board Assembly (HINA)  600  and is controlled by the base station. 
     Referring now to FIG. 9, a block diagram illustrates a pilot signal generator according to one embodiment of the present invention. The pilot signal generator includes a transceiver power supply  340 , a main controller  338 , a plurality of transceivers  332 ,  334 ,  336  and a plurality of power amplifiers  432 ,  434 , and  436  which are coupled to corresponding ones of the transceivers  332 ,  334 , and  336 . The number of transceivers and power amplifiers that are utilized is increased or decreased depending on certain conditions. For instance, when the pilot signal generator needs to generate a new pilot signal depending on a change of cell configuration, the addition of a transceiver and a power amplifier board results in a necessary RF path unit. 
     The pilot signal generator of the present invention is designed for use in both indoor and outdoor environments, and meets the following regulations: an RF output at an antenna port is 16 watts per frequency assignment, which requires the pilot signal generator to be less than 2 m from the base station for transmission of the output; an input voltage is standard at +27 VDC; the transmit frequency is approximately 1810-1870 MHz; an outdoor pilot signal generator is operated at a temperature range of −30° C. to +46° C. and at humidity level in the range of 10% to 90%. The pilot signal generator has an environment controller which turns on a heater at a cold start, provides power when the temperature exceeds a predetermined threshold, and blocks power when a high temperature alarm occurs. The environment controller improves the reliability of the system. 
     The aforementioned embodiments have many advantages over the existing technology. For instance, they enable performance of inter-frequency hard hand-off with only an addition of an IF amplifier/divider without another digital MODEM; they balance coverage areas of multiple frequencies transmitting all signals converted both by an overhead channel and by a traffic channel; they facilitate maintenance since they are controlled by a base station and their modular design allows them to be easily inserted into, or extracted from, the system (i.e., every board of the pilot signal generator is a module). 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.