CHANNEL COMMUNICATION DEVICE AND METHOD FOR MOBILE STATION IN ASYNCHRONOUS CDMA COMMUNICATION SYSTEM

Abstract: Disclosed is a communication device for a base station in an asynchronous CDMA communication system. A common short code generator generates a common short code used in common by multiple base stations, a pattern generator generates a unique modulation pattern, and a long spreading code generator generates a unique long spreading code at predetermined periods. A specific channel transmitter spreads a transmission signal on a specific common channel with the long spreading code. A common short code inserter modulates the common short code with the modulation pattern, punctures the transmission signal on the common channel at predetermined periods and inserts the modulated short common code in the punctured transmission signal.

Description: CHANNEL COMMUNICATION DEVICE AND METHOD FOR MOBILE STATION IN ASYNCHRONOUS CDMA COMMUNICATION SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to an asynchronous CDMA communication system, and in particular, to a device and method for rapidly searching a base station using a common short code. 2. Description of the Related Art In an IS-95 CDMA (Code Division Multiple Access) communication system wherein base stations operate in sync with one another, the respective base stations are time- synchronized to a global positioning system (GPS), and transmit the same spreading code with a unique offset value to enable mobile stations to distinguish the respective base stations. However, in an asynchronous CDMA communication system, the base stations transmit spreading codes which are not synchronized to a GPS system. In this case, prior to performing communication, the mobile stations should search for an optimal base station (i.e. cell), i.e., search for a spreading code whose receiving power level is highest so as to acquire code sync. Therefore, for an asynchronous mobile station, a cell search time is the sum of a spreading code search time and a code sync acquisition time. Thus, to perform a fast cell search, it is very important to reduce both the spreading code search time and the code sync acquisition time. To this end, the spreading code should be appropriately designed. FIG. 1 illustrates a forward channel transmitter for a base station in a conventional asynchronous CDMA communication system, which is disclosed in a paper entitled "Fast Cell Search Algorithm in DS-CDMA Mobile Radio Using Long Spreading Codes", by K. Higuchi et al., IEEE. Referring to FIG. 1 , the forward channel includes common channels applied (or assigned) in common to all the users belonging to a base station and dedicated channels uniquely assigned to the respective users. The common channels include a pilot channel, a sync channel, a paging channel and a common control channel (CCH); the dedicated channels include traffic channels (TCH) for transmitting voice and data. Each channel transmitter uses a unique orthogonal code to distinguish its own channel from the channels of other channel transmitters. A Walsh code is typically used for the orthogonal code. Subsequently, each base station scrambles its channel transmission signal using a unique long spreading code (LSC) assigned to distinguish the base station. Here, for a specific channel among the common channels, a common short code (CSC) is periodically inserted in a spreading code or an LSC after puncturing, so as to enable the mobile stations to rapidly search the base station. Herein, the specific common channel is assumed to be a common control channel. Therefore, the CSC is periodically inserted after puncturing corresponding period in the common control channel which has been previously scrambled with the LSC, according to a mask control signal. The CSC is shorter in length than the LSC and is commonly used for every base station. In addition, for the CSC, every base station has the same puncturing (i.e.insertion) period. A period of the LSC is multiples of a CSC period. The mobile station searches a CSC transmitted from an optimal base station, i.e., a CSC received with the highest power using a CSC matching receiver, to detect an exact period thereof. Since the LSC period is multiples of the CSC period, the mobile station searches the LSC with the knowledge of the locations where the CSC is inserted therein. Therefore, in searching for an optimal base station having the highest signal strength, the mobile station can reduce the search time by searching for the inserted CSC on a specific pre-designated common channel. FIG. 2 illustrates a common control channel transmitted from a base station (Fig. 1 ) in which the CSC is inserted after puncturing. As illustrated, the common control channel signal is not transmitted during the period where the CSC is inserted in the LSC. Therefore, the original common control channel signal should be carefully designed with considering the CSC insertion period. FIGs.3 A and 3B illustrate examples of the received common control channel signals using common CSC transmitted from multiple base stations (i.e. cells) A, B and C in an asynchronous system. Here, the received common control signals are shown over one LSC period. More specifically, FIG. 3 A illustrates a case where the received CSCs from multiple base stations are not aligned in time, and FIG. 3B illustrates a case where the received CSCs from multiple base stations are aligned in time. In FIG. 3B, we omit the received common control channel signals from the base station C to emphasize the time alignment of CSCs from base stations A and B. It is the same situation if more than two received CSC signals from multiple base stations are aligned in time. In FIG. 3A, the received CSCs from multiple base stations are not aligned in time, so that the mobile station may have no difficulty in searching the strongest CSC. Every base station inserts the same CSC in LSC scrambled common control channel signal using the same CSC insertion periods. Therefore, once the received CSCs from multiple base stations are aligned in time, the CSCs are repeatedly collide with each other, as illustrated in FIGs. 4A and 4B. When the received CSCs from multiple base stations are in phase as well as aligned in time as shown in FIG. 4A, the mobile station has no difficulty in searching the strongest CSC power and CSC period. However, when the received CSCs from multiple base stations are aligned in time but out of phase as shown in FIG. 4B, receiving power during the period of CSC decreases due to phase offset, so that the mobile station may have difficulty in searching the strongest CSC and period. In particular, when the two time aligned CSCs have opposite phases, the received CSCs from two different base stations A and B offset each other, and finally disturbing the repetitive CSC search. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a device and method for preventing a common short code (CSC) inserted in a predesignated specific common channel spread with a long spreading code (LSC) from being repeatedly phase offset with the CSCs from other base stations. The present invention being intended for use in an asynchronous CDMA communication system. It is another object of the present invention to provide a device and method for preventing repeated phase offset due to phase shift occurring in a channel by modulating a common short code CSC according to a unique modulation pattern and inserting the modulated common short code (CSC) to the specified position of the common control channel scrambled with long spreading code. To achieve the above objects, a communication device and associated method for rapid searching a base station using a common short code are provided. The present invention is intended for use in an asynchronous CDMA communication system. A pattern generator generates a modulation pattern unique to each base station for modulating a common short code, and a long spreading code generator generates a long spreading code unique to each base station at predetermined periods. The base station channel transmitter spreads a transmission signal on a specific common channel using the generated long spreading code. A common short code generator generates a common short code used in common by multiple base stations. A common short code inserter modulates the common short code with the modulation pattern unique to each base station and punctures the transmission signal on the common channel scrambled by the long spreading code at predetermined periods to insert the modulated common short code in the punctured transmission signal. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a diagram illustrating a forward channel transmitter for a base station in a conventional asynchronous CDMA communication system; FIG. 2 is a diagram illustrating a signal format of a common channel in a base station device having the channel structure of FIG. 1; FIG. 3A is a diagram illustrating a case where the received common short codes inserted in common channel signals from multiple base stations are aligned in time; FIG. 3B is a diagram illustrating a case where the received common short codes inserted in common channel signals from multiple base stations are are not aligned in time; FIG. 4 A is a diagram illustrating a case where the received common short codes inserted in common channel signals from multiple base stations are both aligned in time and in phase; FIG. 4B is a diagram illustrating a case where the received common short codes inserted in common channel signals from multiple base stations are aligned in time but out of phase; FIG. 5 is a diagram illustrating a channel transmission device for a base station in an asynchronous CDMA communication system according to an embodiment of the present invention; FIG. 6 is a diagram illustrating the operation of puncturing a common channel signal and inserting a common short code in the punctured common channel signal in a communication device for a base station of FIG. 5; FIG. 7 is a diagram illustrating common channel signals which are punctured to insert therein common short codes modulated according to unique modulation patterns in base station communication devices according to an embodiment of the present invention; and FIG. 8 is a diagram illustrating how to prevent repeated phase offset of the received common short codes when the common short codes inserted in common channels of different base stations are aligned in time with each other according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In an inter-base station asynchronous DS-CDMA communication system, multiple base stations using a common carrier frequency puncture a specific predesignated common channel scrambled by the long spreading code designated for a forward link at predetermined periods to insert therein common short codes modulated according to a specific modulation pattern unique to each base station. By virtue of the unique modulation applied to the respective common short codes, the received common short codes at the mobile station are prevented from offsetting each other caused by phase shifts which may occur in the communication channel during the base station search procedure. To summarize, the respective base stations insert the common short codes (CSCs) in the common control channel scrambled by the long spreading codes (LSCs) at the same periods after puncturing, to reduce a base station (i.e. cell) search time at the mobile station. The common short codes CSCs are modified by a specific pattern unique to each base station prior to their insertion into the punctured common control channel. Therefore, when the received CSCs from multiple base stations become completely aligned with one another, repeated phase offset of the common short codes can be prevented by virtue of the unique modifications applied to the CSCs. FIG. 5 illustrates a forward channel transmitter for a base station according to an embodiment of the present invention, wherein the forward channel transmitter includes a modulation pattern generator, a specific common channel and traffic channels. Herein, it is assumed that the specific common channel is a common control channel (CCH) and a Walsh code is used for an orthogonal code. Referring to FIG. 5, a multiplier 511 multiplies a transmission signal on a common control channel by a Walsh code assigned to the common control channel in order that the common control channel becomes orthogonal with respect to other traffic channels. An LSC generator 513 generates a long spreading code (LSC) having a period N LSC for discrimination of the base station. A multiplier 515 multiplies the output from the multiplier 511 by the LSC generator output. A CSC generator 517 generates a common short code (CSC) which will be inserted in the transmission signal on the common control channel scrambled by the long spreading code, at the periods of N csc after puncturing. A modulation pattern generator 519 stores a modulation pattern unique to each base station to prevent repeated offset of the CSCs from multiple base stations at the mobile station receiver. The modulation pattern generator 519 is enabled at predetermined periods to output the modulation pattern, under the control of a puncture controller 525. A multiplier 521 multiplies the CSC output from the CSC generator 517 by the modulation pattern output from the modulation pattern generator 519. A puncturer 523 receiving outputs of the multipliers 515 and 521, punctures the transmission signal on the common control channel to insert therein the modulated CSC output from the multiplier 521 in response to a puncturing control signal output from the puncture controller 525. The puncture controller 525 generates the puncturing control signal for puncturing the common control channel at predetermined periods to insert the CSC in the punctured common control channel. Multipliers 551-55N multiply transmission signals on the corresponding traffic channels by associated Walsh codes assigned to the traffic channels in order that the common control channel becomes orthogonal with respect to other traffic channels. Multipliers 561-56N multiply outputs of the corresponding multipliers 551 -55N by the LSC output from the LSC generator 513 to spread the transmission signals on the corresponding traffic channels. An adder 571 adds the spread transmission signals on the respective traffic channels, output from the multipliers 561-56N. An adder 573 adds the added transmission signal output from the adder 571 to the CSC-inserted transmission signal on the common control channel and transmits the output signal. FIG. 6 illustrates a procedure for puncturing the common control channel of FIG. 5 to insert the modulated CSC in the punctured common control channel. Referring to FIG. 6, the LSC having a period N LSC on the common control channel signal is punctured and the CSC having a period N csc is inserted in the punctured common control channel signal. Here, a gap between the CSCs is N p *N rsr chips. Further, in the N csc period, the puncture controller 525 simultaneously enables the modulation pattern generator 519 and the puncturer 523 , so that the common control channel is punctured and the modulated CSC output from the multiplier 521 is inserted in the punctured common control channel. Referring to FIGs. 5 and 6, the transmission signals on the common control channel CCH and the traffic channels TCH are orthogonally spread with the corresponding Walsh codes, for channel separation, and the orthogonally spread channel signals are multiplied by the LSC output from the LSC generator 513, for scrambling. The transmission signal on the common control channel CCH is provided to the puncturer 523. Every base station includes an identical CSC generator 517 for generating the CSC to be inserted in the punctured LSC, and the modulation pattern generator 519 for generating a unique modulation pattern comprised of "+1 " and "-1 ". The modulation pattern is multiplied by the CSC under the control of the puncturer controller 525. The puncturer 523 is activated at each N csc insertion period for an output duration equal to the length of the CSC code. The CSC is multiplied by a unique modulation pattern assigned to the base station prior to inserting the CSC under the control of the puncturer controller 525. The modulated C SC is inserted in the punctured transmission signal on the common control channel CCH which is input to the puncturer 523. Table 1 shows unique modulation patterns assigned to the base stations A, B and C, by way of example. FIG. 7 illustrates the common control channels in which the CSCs are inserted according to the modulation patterns of Table 1. [Table 1] The mobile station receiver should have the modulation patterns of all the base stations as well as the long spreading code. Therefore, by modulating the CSCs using the modulation patterns of Table 1 , it is possible to prevent repeated phase offset even though the CSCs output over the common control channels of the base stations are aligned in time with one another. FIG. 8 illustrates a state where the received CSCs on the common control channels of the two base stations A and B are aligned with each other. As illustrated, the dotted portions represent a state where the received CSCs of the cells A and B are in phase as well as aligned in time, whereas the plain portions represent a state where the received CSCs of the cells A and B are aligned in tiem but in opposite phase. FIG. 8 shows a case where the modulation pattern has a period of 5 chips, for simplicity. That is, it is assumed that 5 CSC chips are inserted in the punctured LSC spread to common control channel signal at every LSC period. Further, FIG. 8 does not illustrate the non-punctured common control channel signal, for simplicity. In FIG. 8, the cell A uses a modulation pattern (+1 +1 -1 -1 +1) and the cell B uses a modulation pattern (-1 +1 -1 +1 +1). Further, reference numeral 812 denotes a common control channel of the cell A, in which the CSCs are inserted after puncturing. Reference numerals 822-830 denote all the possible relative locations where the CSCs for the common control channel of the cell B can locate with respect to the CSCs for the common control channel of the cell A. The dotted portions in the common channel signal of the cell B represent the portions where the common channel signals are aligned in time and in phase with the common channel signals of the cell A, in the case where the phase shift does not occur during transmission The non-dotted portions represent the case where the CSCs of the cells A and B are in the opposite phase while they are aligned in time. In this case, the CSCs are offset at the portions having the opposite phase. However, the offset does not occur repeatedly. That is, due to the unique modulation applied to each CSC codes, consecutive comparisons of CSC codes from different base stations will result in phase offset occurring in some instances and not occurring in others. Therefore, it is possible to search code sync for the LSC based on the fact that the LSC always starts at the same position with the CSC. If the common control channel signal of one of two base stations is phase-shifted by 180 degree due to the phase shift during transmission, the dotted portions become same phase so that it is still possible to search the code sync. Therefore, it is possible to prevent a case where the CSCs are not detected repeatedly due to the phase difference no matter whether or not the phase shift occurs during transmission. As described above, in an asynchronous CDMA communication system, a base station transmission device punctures a specific common channel spread by the LSC to insert therein a common short code before transmission, and the mobile stations then search the base station using the common short code punctured in the transmission signal received over the specific common channel. The specific common channel transmission device for the base station inserts the CSCs using the unique modulation pattern, so that it is possible to prevent repeated phase offset of the CSCs even though the CSCs of the base stations are aligned in time with each other due to the phase shift during transmission. In this manner, the mobile station can rapidly search the base station to acquire code sync. While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A communication device for a base station in an asynchronous code division multiple access (CDMA) communication system, the base station comprising: a generator for generating a common short code used in common by multiple base stations; a modulator having a modulation pattern unique to the base station for modulating the common short code; and a puncturer-inserter for puncturing a transmission signal on a specific common channel spread by the long spreading code for a transmitting duration of the common short code and inserting said modulated common short code in the punctured transmission signal. 2. The communication device as claimed in claim 1, wherein the modulator comprises: a modulation pattern generator having the modulating pattern; and a multiplier for multiplying the common short code by the modulation pattern to modulate the common short code. 3. The communication device as claimed in claim 2 , where the common short code is inserted in the punctured transmission signal over a long spreading code duration at predetermined periods. 4. The communication device as claimed in claim 1 , wherein the specific common channel is a common control channel. 5. A communication method for a base station in a CDMA communication system, comprising the steps of: multiplying a transmission signal on a specific common channel by a long spreading code to spread the transmission signal; generating a common short code and modulating the generated common short code according to a unique modulation pattern; and puncturing the transmission signal on the specific common channel and inserting the modulated common short code in the punctured transmission signal. 6. A communication device for a base station in an asynchronous CDMA communication system, comprising: a common short code generator for generating a common short code used in common by multiple base stations; a pattern generator for generating a unique modulation pattern; a long spreading code generator for generating a unique long spreading code at predetermined periods; a specific channel transmitter for spreading a transmission signal on a specific common channel with the long spreading code; and a common short code inserter for modulating the common short code with the modulation pattern, puncturing the transmission signal on the common channel at predetermined periods and inserting the modulated short common code in the punctured transmission signal. 7. The communication device as claimed in claim 6, wherein the common short code inserter comprises: a multiplier for multiplying the common short code by the modulation pattern; and a puncturer-inserter for puncturing the transmission signal on the specific common channel, spread with the long spreading code, at predetermined periods, and inserting the modulated common short code in the punctured transmission signal. 8. The communication device as claimed in claim 7, wherein the puncturing period is equal to a period of the common short code. 9. The communication device as claimed in claim 7, wherein the specific common channel is a common control channel. 10. A communication method for a base station in an asynchronous CDMA communication system including a long spreading code generator for generating a unique long spreading code at predetermined periods and a channel transmitter for spreading a transmission signal on a specific common channel with the long spreading code, the method comprising the steps of: generating a common short code used in common by multiple base stations; generating a modulation pattern unique to each of said multiple base stations; modulating the common short code according to the modulation pattern; puncturing the transmission signal on the specific common channel at predetermined periods; and inserting the modulated common short code in the punctured transmission signal. 1 1. The communication method as claimed in claim 10, wherein the common short code inserting step comprises the steps of: multiplying the common short code by the modulation pattern; and puncturing the transmission signal on the specific common channel, spread with the long spreading code, at predetermined periods, and inserting the modulated common short code in the punctured transmission signal. 12. The communication method as claimed in claim 11, wherein the puncturing period is equal to a period of the common short code. 13. The communication method as claimed in claim 12, wherein the specific common channel is a common control channel.

Inventors: CHOI HO KYU, YOON SOON YOUNG, CHOI, HO, KYU, YOON, SOON, YOUNG
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: H04Q
IPC codes: H04B1, H04B7, H04J13, H04L1, H04L7, H04W52