Abstract:
A base station simulator ( 400 ) provides testing of soft handoff functionality for a mobile station ( 400 ), such as a mobile telephone, between a first base station (BS 1 ) and a second base station (BS 2 ). The base station simulator ( 400 ) includes two digital baseband processors ( 402 ) and ( 404 ) for generating forward transmission signals to simulate the signals from the two base stations (BS 1 ) and (BS 2 ). The two digital base station signals have gain digitally controlled using multipliers ( 407-409 ) and the signals are added together digitally using a digital adder ( 410 ) before conversion to an analog signal using an A/D converter ( 412 ). The base station simulator ( 400 ) avoids the additional cost and space required for the redundant parts from two separate base station simulators, as conventionally used for testing soft handoff functionality. Adding of the digital signals before converting to analog in A/D converter ( 412 ) increases measurement accuracy, and control of the combined baseband processors ( 402 ) and ( 404 ) as opposed to separate controllers for separate base station simulators simplifies operation from conventional systems for testing soft handoff.

Description:
CROSS-REFERENCE TO PROVISIONAL APPLICATION 
     This Patent Application claims the benefit of Provisional Application No. 60/155,284 filed Sep. 21, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for testing the soft handoff functionality of a mobile station, such as cellular telephone acting as the mobile station being handed off between two base stations. More particularly, the present invention relates to an apparatus for testing soft handoff functionality of a Code Division Multi-Access (CDMA) communication system. 
     2. Description of the Related Art 
     An explanation of soft handoff and a description of a conventional apparatus to test soft handoff are provided below. 
     A. Soft Handoff 
     Soft handoff is a handoff between CDMA channels that have identical frequency assignments. Soft handoff provides diversity of Forward Traffic Channel and Reverse Traffic Channel paths on the boundaries between base stations. A traffic channel is defined as a communication path between a mobile station and a base station used by both a user and signaling traffic, the signaling traffic controlling communication between the mobile station and base station. The term “traffic” implies a forward traffic channel (base station to a mobile station) and a reverse traffic channel (mobile station to base station) functioning as a pair. 
     Before and during soft handoff, a base station and its neighboring base stations send pilot channel signals on a carrier and a MS (Mobile Station) monitors the signal strength of the pilot channels. The pilot channel is an unmodulated direct-sequence spread spectrum signal transmitted continuously to each CDMA base station. A pilot channel allows a MS to acquire the timing of a forward CDMA channel, provide a phase reference for coherent demodulation, and provide a means for signal strength comparisons between base stations for determining when to handoff. The MS measures the pilot strengths and provides a signal back to the base stations reporting the pilot signal strengths. 
     FIG. 1A illustrates soft handoff between two base stations. FIG. 1A shows a MS moving away from a Base Station (BS 1 ) and approaching a Base Station  2  (BS 2 ), while the MS is receiving a traffic channel from BS 1  on the boundary between BS 1  and BS 2 . When the MS detects a pilot signal from BS 2  of sufficient strength, the MS reports the pilot strength to BS 1 . Then BS 1  sends the MS a message telling the MS to receive the traffic channel of BS 1  and the traffic channel of BS 2 . The MS then begins to receive both BS 1  and BS 2  traffic channels simultaneously (providing a diversity of forward link). At this point, both BS 1  and BS 2  are receiving traffic channels from the MS, which allows the transmission of higher quality traffic data to the network (providing diversity of reverse link). As the MS moves farther away from BS 1 , the MS will detect a weak pilot strength and report the weak pilot strength from BS 1 . The MS then begins to receive only the traffic channel from BS 2 . During the entire process, the MS can make handoffs between BS 1  and BS 2  without interrupting communication. 
     FIG. 1B illustrates sector-to-sector handoff for a single base station  100  transmitting over different antennas to separate sectors using different sector units  102  and  104  of the single base station  100 . Such sector-to-sector handoff is sometimes referred to as softer handoff. FIG. 1B shows the two sectors  102  and  104  located in the single base station  100  for covering adjacent sectors. Softer handoff occurs when the MS is moving on the boundary between a sector  1  controlled by a sector unit  102  and a sector  2  controlled by sector unit  104 . 
     B. Conventional System To Test Soft Handoff Functionality 
     Soft handoff is a procedure enabling an interruption-free handoff. Manufacturers of CDMA phones assure communication quality by verifying the soft handoff functionality. The following are requirement specifications for a conventional test apparatus for testing soft handoff functionality of a MS: 
     Be able to simulate two base stations. 
     Be able to add Additive White Gaussian Noise (AWGN) to the forward link signal. 
     Be able to control the level of the forward link signal (including the forward link signal of BS 1 , the forward link signal of BS 2  and (AWGN) correctly and with sufficient resolution. 
     Be able to exchange messages needed during handoff with the MS. 
     Be able to display the contents of the pilot strength measurement report sent from the MS. 
     Be able to define the contents of the system parameter message from the base station (some parameters are used to inform the MS of a condition to monitor pilot strength). 
     FIG. 2 shows one example of a conventional test system for testing soft handoff. The test system includes two base station simulators (BS 1 )  202  and (BS 2 )  204 , an AWGN generator  206 , a test system controller  200 , couplers  207 ,  208 , a duplexer  210  and a power divider  212 . Generally the test system shown in FIG. 2 and a (MS)  214  are connected using RF cables. An antenna or aerial interface may be used between the duplexer  210  and MS  214  instead of the RF cables depending on the condition of the test. 
     1. Base Station Simulators  202 ,  204   
     Two base station simulators  202  and  204  are needed to simulate two base stations. The MS  214  will make a handoff from one base station simulator to another during testing. FIG. 3 shows a block diagram of a conventional base station simulator as described in more detail below. 
     2. AWGN Generator  206   
     The AWGN generator  206  is used to simulate noise and signals sent from other base stations in the field. Some base station simulators have a built-in AWGN generator. The base station simulators in a conventional test system can have built in AWGN generators, so a separate AWGN generator as shown in FIG. 2 is not needed. 
     3. Test System Controller  200   
     In order to execute soft handoff between one base station simulator to the other, the two base stations  202  and  204  need to synchronize with each other. The test system controller  200  is used to control both base station simulators, allowing them to handle call processing or message transmission/receptions to and from the MS  214  being tested. The test system controller  200  also controls the AWGN. The test system controller  200  controls the forward link signal for both of the base station simulators  202  and  204  and the AWGN  206 . The test system controller  200  further provides for displaying of the contents of the pilot strength measurement report message sent from the MS  214 . 
     4. Couplers  207 ,  208  &amp; Duplexer  214   
     The couplers  207  and  208  are used to combine the forward link signals of the base station simulators, and the AWGN and provides those signals to a duplexer  210 . The duplexer directs the signal from the coupler  208  through either a cable or antenna to a mobile station  214 . 
     5. Power Divider  212   
     The reverse link signal from the MS  214  is directed by the duplexer which then feeds the signal received to both base station simulators using the power divider  212 . 
     6. Conventional Base Station Simulator Circuitry (FIG. 3) 
     FIG. 3 shows the components of a conventional base station simulator, such as either  202  or  204 . The conventional components of the base station simulator includes a transmit (TX) baseband processor  302  which generates a digital forward link signal with a desired indication for amplitude. The digital forward link signal is converted to an analog signal in a digital to analog (D/A) converter  304 . The output of the D/A converter  304  is then provided through a quadurature modulator  306  and upconverter  308  for transmission from the base station simulator. A signal received by the base station simulator shown in FIG. 3 is downconverted in downconverter  310 , demodulated in quadrature demodulator  312  and converted from an analog to a digital signal in the A/D converter  314 . The output of the A/D converter is provided to a receive (RX) baseband processor  316  which functions to further demodulate the received IF digital, decode the demodulated data and provide the demodulated data to the control part  320 . The output of the A/D converter further provides its output to a MS performance measurement unit  318  which measures the accuracy of the signal level and determines modulation accuracy of a MS. The control part  320  controls the call processing status and hardware settings that simulate a base station according to the condition that a test operator sets via the user interface  322 . The user interface  322  can include a display screen and keypad, or a remote controlling computer. 
     C. Drawbacks of Conventional System 
     The conventional system for testing the soft handoff function of a MS has the following drawbacks: 
     The test system needs two base station simulators and an external controller (including hardware and software). The cost of a conventional test system is, thus, more than double the cost of one base station simulator. 
     The individual forward link signal levels of BS 1   202 , BS 2   204  and AWGN  206  are specified relative to the total forward link signal power, i.e. BS 1 +BS 2 +AWGN. Maintaining accurate forward link signal levels is then critical to testing the soft handoff functionality of the MS  214 , and maintaining the accuracy is very difficult since it typically requires rigorous calibration for each signal source. 
     The test system operator needs to operate the two base station simulators  202  and  204  and a test system controller  200 . The different connections for these components during a test procedure, and steps for operating each of the components during the test means an operator must perform a number of complicated tasks. A test system controller might be configured which could automatically make the connections and automatically preform the steps required for testing to minimize the operator tasks, but the cost for development of such a system could be high. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a system is provided for testing soft handoff functionality which avoids the cost and space required for the redundant components of two separate base station simulators. The system in accordance with the present invention further provides improved accuracy in generating signal strength levels for the forward link signals. The system further provides a reduction in complexity for the test procedure relative to a conventional system for testing soft handoff functionality. The system further provides testing for soft handoff between two base stations as illustrated in FIG. 1A, or softer handoff between two sectors of a single base station as shown in FIG.  1 B. 
     Referring to FIG. 4, the base station simulator  400  in accordance with the present invention includes two digital baseband processors  402  and  404  for generating forward transmission signals to simulate the signals from the two base stations (BS 1 ) and (BS 2 ). The two digital base station signals have gain digitally controlled using multipliers  407 - 409  and the signals are added together digitally using a digital adder  410  before conversion to an analog signal using an A/D converter  412 . Adding of the digital signals before converting to analog in A/D converter  412  increases measurement accuracy, as opposed to coupling the base station analog signals together. Further, adding of a digital noise signal from an AWGN generator  406  before conversion to analog can further improve accuracy. 
     The system further includes components for upconverting the signal from the D/A converter  412  and transmitting the upconverted signal to a mobile station  430 . Additionally, the system includes components for downconverting the signal transmitted from the mobile station  430  and measuring the performance of the MS  430 , similar to the components of a single base station simulator shown in FIG.  3 . But, by combining the signals from two baseband processors  402  and  404  before upconversion using a single upconversion system, and by using a single system for downconversion, the additional cost and space required for the redundant parts from two separate base station simulators is avoided. Further, control of the combined baseband processors  402  and  404 , as opposed to separate controls for separate base station simulators, simplifies operation of the system in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the present invention are explained with the help of the attached drawings in which: 
     FIG. 1A illustrates soft handoff between two base stations; 
     FIG. 1B illustrates softer handoff between two sector units of a single base station; 
     FIG. 2 shows components of a conventional test system for testing soft handoff; 
     FIG. 3 shows components of a conventional base station simulator; and 
     FIG. 4 shows components of a base station simulator in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 shows a block diagram of components used to test soft handoff in accordance with the present invention. A central aspect of the apparatus of FIG. 4 is that it contains two independent baseband processors  402  and  404  within one base station housing  400  to simulate the individual forward link signals of two base station simulators. The two base station processors  402  and  404  allow one base station simulator  400  to generate the forward link signals of two base stations (BS 1  and BS 2 ) by itself. 
     In the system of FIG. 4, signals from two base station processors  402  and  404  and an AWGN noise generator  406 , also included in the simulator housing  400 , are summed digitally in baseband using a digital adder  410  prior to conversion to an analog signal using a single D/A converter  412 . The D/A converter  412  signal is provided through quadrature modulator  414  and upconverted to an RF frequency using upconverter  416 . The upconverted signal is finally provided through an amplifier  418  and duplexer  420  for provision to a mobile station (MS) or device under test (DUT)  430 . The use of digital technology allows for accurate signal level settings for BS 1  and BS 2  and AWGN because their gains are set through simple digital multiplication. Summation of the Forward Link signals in baseband virtually eliminates the effects that typical gain variations associated with the use of independent signal sources have on the relative power levels for BS 1 , BS 2 , or AWGN with the total forward link power (BS 1 +BS 2 +AWGN). 
     Since the system shown in FIG. 4 requires only one base station simulator  400 , it provides an inexpensive and attractive solution for testing soft handoff compared with a conventional system which requires two base station simulators, such as  202  and  204  of FIG.  2 . For transmission to the MS  430 , the system of FIG. 4, thus, does not require an additional D/A converter  412 , quadrature modulator  414  or upconverter  416 , as would be required when two separate systems with components shown in FIG. 3 are used. 
     To process signals received from the MS  430 , the duplexer  420  of FIG. 4 directs signals received from the MS  430  through an amplifier  432 , and through a single downconverter  434  and quadrature demodulator  436  to an A/D converter  438 . The A/D converter  438  provides its output to a receive (RX) baseband processor  440  which functions to further demodulate the received IF digital, decode the demodulated data and provide the demodulated data to the control part  444 . The output of the A/D converter  438  further provides its output to a MS performance measurement unit  442  which measures the accuracy of the signal level and determines modulation accuracy of the MS  430 . The control part  444  controls the call processing status and hardware settings that simulate a base station according to the condition that a test operator sets via the user interface  446 . As with the transmitting portion of the base station simulator  400 , the receiving portion only requires one set of receiving components  432 ,  434 ,  436 ,  438 ,  440 ,  442 ,  444  and  446  as opposed to two sets of the components  310 ,  312 ,  314 ,  316 ,  318 ,  300  and  320  shown in FIG.  3 . 
     A. Base Station Simulator Details 
     Further information relating to the components of the base station simulator  400  shown in FIG. 4 is provided in the sections to follow. 
     1. BS 1  TX Baseband Processor  402   
     The BS 1  TX Baseband Processor  402  does coding and modulation to generate a baseband signal that simulates the forward link signal of a first base station-BS 1 . 
     2. BS 2  TX Baseband Processor  404   
     The BS 2  TX Baseband Processor  404  does coding and modulation to generate a baseband signal that simulates the forward link signal of a second base station-BS 2 . 
     3. AWGN Generator  406   
     The AWGN generator  406  generates additive white Gaussian noise to simulate noise in the forward link signal going to a MS  430  being tested. 
     4. Gain Controller Multipliers  407 - 409  And Adder  410   
     Digital gain controller multipliers  407 - 409  are used to set the signal gain of the forward link signals from the base stations  402  and  404  and the AWGN generator  406  in baseband. The gain control is set by multiplying the signal is set by multiplying the signal from the base stations  402  and  404  and the signal from the AWGN generator  406  by signals provided from the control part  404  in the respective gain control multipliers  407  and  409  signal according to conditions set by a user using the user interface  446  to set the control part. The BS 1 , BS 2  and AWGN signals are added by an adder  410  after gain control. 
     5. D/A Converter  412 , Quad Modulator  414 , Up Converter  416  And Amplifier  418   
     The D/A converter  412  converts the digital baseband signal to an analog signal. A carrier signal is modulated by the analog baseband signal from the D/A converter in the quadrature modulator  414 . The up converter  416  converts the modulated carrier to an RF signal, and the amplifier  418  sets the carrier signal level to a proper level for testing and applies the signal to the MS  430  through duplexer  418 . 
     6. Amplifier  432 , Down Converter  434 , Quad Demodulator  436  And A/D Converter  438 , RX Baseband Processor  440 , MS Performance Measurement Unit  442  And Control Part  444   
     The amplifier/attenuator  432  receiving a signal from the MS  430  through the duplexer  418  adjusts the received signal to a proper level that the test system can process. The down converter  434  converts the carrier frequency to an intermediate frequency (IF). The IF is then demodulated using the quadrature demodulator  436 . The A/D converter  438  the converts the analog demodulated signal to a digital signal. The RX baseband processor  440  further demodulates the received IF signal and decodes the demodulated data. The MS performance measurement unit  442  measures the performance, including signal level or modulation accuracy of the signal transmitted from the MS  430  during the soft handoff test. The control part  444  controls the call processing status and hardware settings that simulate a base station according to the condition that a test operator sets via the user interface  446 . 
     7. User Interface  446   
     The test system operator can set the conditions for test and perform the call processing and soft handoff test via the user interface  446 . The user interface  446  can include a display screen and keypad, or a remote controlling computer. The major purposes of the user interface  446  are: 
     Setting parameters to define call processing and signal condition, i.e. frequency, signal level, etc. 
     Ability to change the call processing state, call process parameters, or signal level, etc. 
     The ability to display the call processing status, the soft handoff test status, the contents of the mobile station messages, or measurement results of the mobile station&#39;s transmission signal performance. 
     B. Test Procedure 
     A steps of a test procedure using the test system of FIG. 4 to test a MS  430  is provided below. In the procedure steps, the test system  400  is referred to as a Base Station Simulator (BSS) and MS being tested  430  is referred to as the MS. The steps are as follows: 
     (1) The operator connects the BSS and MS. 
     (2) The operator sets the condition for call processing and soft handoff on the BSS and begins testing. 
     (3) The BSS begins transmitting configuration and access information (for BS 1 ) to the MS. The channels used for configuration and access information include the pilot, synchronization and paging channels. 
     (4) The operator turns on the MS. 
     (5) The MS receives the configuration and access information and executes the Registration process with the BSS. 
     (6) The BSS establishes a call with the MS. The BSS and the MS change to the “conversation” state. 
     (7) The BSS generates a forward link signal from the secondary base station (BS 2 ). 
     (8) The operator changes the levels of BS 1 , BS 2  or AWGN and checks the contents of messages then sent from the MS that report the receiving level at the MS. The operator then verifies whether the MS is measuring the signal levels correctly. 
     (9) The operator then orders the BSS to send a message to the MS to enter soft handoff, combining the signals from BS 1  and BS 2  and repeats step (8). 
     C. System Advantages 
     The system in accordance with the present invention offers the following advantages over a conventional system: 
     1. Cost And Space Savings 
     A conventional system needs two base station simulators, an external controller, a coupler and a power divider to test the soft handoff functionality of a mobile station as illustrated in FIGS. 2 and 3. The system in accordance with the present invention as shown in FIG. 4 only needs additional baseband process circuitry in a conventional base station simulator and will not require a large number of other duplicated components. The cost for the apparatus in accordance with the present invention is then roughly half of a conventional test apparatus. 
     2. Operation Simplification 
     The operator needs to control and set conditions for two base station simulators  202  and  204  and a separate controller  200  when performing tests using a conventional test apparatus. The test steps the operator performs are time consuming and difficult. Although it may be possible to program the controller  200  to perform a majority of the operator steps, it may require lengthy and costly development of the software as well as the hardware to interface between the controller  200 , base stations  202  and  204  and MS  214  for all the test conditions. When an operator tests the soft handoff functionality with the system as shown in FIG. 4 in accordance with the present invention, the operator can connect the MS being tested  430  and test system directly without changing the test system hardware configuration during testing and without manually controlling two separate base station simulators. Testing is, thus, simpler and less time consuming. 
     3. High Accuracy Of Relative Level Between BS 1 , BS 2 , AWGN And Total Level 
     As indicated previously, the baseband processing of the signal levels from the baseband processors  404  and  404  and AWGN generator  406  is performed with the test system of FIG. 4 in accordance with the present invention using digital technology (including digital signal processing and/or digital circuits) allowing the relative signal levels to be set very accurately. The summation of the digital signals in baseband using a digital adder or summer  410  eliminates the effect that RF amplifier variations have on the overall relative signal level accuracy and eliminates the need for rigorous calibration that individual signal sources of a conventional test apparatus require. 
     D. Conclusion 
     Although the system of FIG. 4 is shown and described for testing the soft handoff functionality between two base stations, as illustrated in FIG. 1A, the system can be used to test softer handoff between two sector units of a base station, as illustrated in FIG.  1 B. For testing sector-to-sector handoff, the baseband processor  402  can be used alone to transmit the sector units&#39; signals for handoff, or both baseband processors  402  and  404  can be controlled to act as the separate sector units of a single base station. 
     Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many additional modifications will fall within the scope of the invention, as that scope is defined by the claims which follow.