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1. WO1998052368 - TRANSMISSION OF DATA WITHIN A DIGITAL WIRELESS COMMUNICATION SYSTEM

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TRANSMISSION OF DATA WITHIN A DIGITAL WIRELESS
COMMUNICATION SYSTEM

Field of the Invention

The present invention relates generally to wireless communication systems and, in particular, to a method and apparatus for transmission of data within such wireless communication systems.

Background of the Invention

Wireless communication systems are well known and consist of many types including land mobile radio, cellular radiotelephone, fixed wireless, personal communication systems, and other communication system types. Within the communication system, transmissions are conducted between a transmitting device and a receiving device over an air interface, commonly referred to as a communication channel. To date, the transmissions have typically consisted of analog and digital voice signals. More recently however, it has been proposed to carry other forms of signals, including data signals such as facsimile transmissions over the communication resource. Current facsimile transmissions over a digital cellular interface utilize a digital facsimile standard such as the Class 2.0 standard described in the Electronic Industry Association/Telecommunications Industry Association (EIA/TIA) ANSI-592 "Asynchronous Facsimile DCE Control Standard." EIA TIA can be contacted at 2001 Pennsylvania Ave. NW Washington DC 20006. In such digital facsimile communication, data that is to be transmitted over the communication system is first digitized by an intelligent peripheral (such as a laptop computer) before being sent to a transmitter. After digitized facsimile data has been received by the transmitter, the transmitter bypasses any vocoder and transmits the data stream over the air interface.

ln a landline environment, paper facsimile machines utilize a standard 2-wire analog telephone circuit which typically uses a Registered- Jack type 11 (RJ-11) as a physical connection to the public switched telephone network (PSTN) to transmit specific tones (representative of data) that communicate information over the PSTN. Because the transmissions from standard facsimile machines consist of transmitting analog tones (as opposed to digital data), many types of digital wireless communication systems currently do not allow for transmission of such analog signals over a communication channel. In particular, current transmission of data over many of the digital air interfaces routes analog data through voice coding operations. Thus analog facsimile transmissions entering a transmitter will be passed through a vocoder, corrupting the analog data stream. Therefore, a need exists for a method and apparatus for transmission of data within a digital wireless communication system that can effectively transmit analog data over a communication channel.

Brief Description of the Drawings

FIG. 1 shows a wireless communication system in accordance with the preferred embodiment of the present invention.
FIG. 2 is a block diagram of a remote unit transmitter in accordance with the preferred embodiment of the present invention.
FIG. 3 is a block diagram of a base station receiver and a centralized base station controller of FIG. 1 for receiving analog data in accordance with the preferred embodiment of the present invention.
FIG. 4 is a block diagram of a centralized base station controller and a base station transmitter of FIG. 1 for transmitting analog data in accordance with the preferred embodiment of the present invention.
FIG. 5 is a flow chart showing the steps necessary for analog data transmission within a digital wireless communication system in accordance with the preferred embodiment of the present invention.
FIG. 6 is a flow chart showing the steps necessary for proper error control of analog data transmission within a digital wireless communication system in accordance with the preferred embodiment of the present invention.
FIG. 7 is a traffic channel frame for transmission over an air interface in accordance with the preferred embodiment of the present invention.
FIG. 8 is a flow chart illustrating the steps necessary for obtaining acceptable transmission rates in accordance with the preferred embodiment of the present invention.

Detailed Description of the Drawings

Stated generally, the transmission of analog data within a digital communication system takes place by transmitting analog tones (representative of data) to a remote unit. Analog tones enter the remote unit and are appropriately digitized via a modem for transmission over an air interface to a base station. Once the digitized data has been received by the base station, it is appropriately demodulated and transmitted to a Centralized Base Station Controller (CBSC). The CBSC performs a digital to analog conversion via a modem, and the digitized analog signal is converted back to its original analog tone form and transmitted to a Public Switched Telephone Network. Bypassing any vocoder, and passing the analog tones through a modem allows for transmission of data within a digital wireless communication system that can effectively transmit analog data (in the form of analog tones) over a communication channel.
The present invention encompasses a method for transmission of data within a wireless communication system. The method comprises the steps of receiving analog tones transmitted by a first peripheral and extracting baseband information from the analog tones. In the preferred embodiment the analog tones are representative of a data pattern to be transmitted over an air interface. Next, transmission of the baseband digital signal over an air interface occurs, and the baseband digital signal is received at a base site and converted back to analog tone form. The analog tones are transmitted to a second peripheral where they are converted to the original data pattern. In the preferred embodiment of the present invention, the first and the second peripherals are facsimile machines that conform to the CCITT Blue Book Recommendations T.O through T.63. The description of the invention below focuses on facsimile applications, however, the invention is not limited to facsimile. The methods of the invention can be easily extended in order to support the scenario where the first and second peripherals are data modems, such as those that conform to V.32, V.32 bis and other data modem standards. Additionally the present invention encompasses a method for transmission of data within a wireless communication system. The method comprises the steps of transmitting analog tones from a first peripheral to a first modem at a first data rate and converting the transmitted data from an analog format to a digital format at the first data rate. The first data rate and the digital data are encoded into a frame for transmission over an air interface and the frame is transmitted over the air interface. In the preferred embodiment of the present invention the frame is received and decoded and the first data rate is determined from the decoded frame. After decoding, a determination is made whether the first data rate is greater than a second data rate. In particular, a determination is made if the first data rate is greater than a maximum sustainable data rate that can be transmitted over the air interface. If it is determined that the first data rate is greater than the maximum sustainable data rate, then the transmitted data is replaced with a predetermined numerical sequence to produce corrupted data and the corrupted data is sent to a second peripheral at the first data rate. If the transmitted data consists of a training pattern utilized to synchronize the two peripherals, then the corrupted data forces the two peripherals to step down the first data rate to a lower data rate.
Additionally, the present invention encompasses a method comprising the steps of transmitting by a first facsimile machine, an analog data signal to a first cellular infrastructure device, determining, by the first infrastructure device, if the analog data signai is voice information or analog data, routing the analog data signal to a first modem for digital coding of the analog data signal based on the determination to produce a digital signal, transmitting the digital signal over an air interface for reception by a second cellular infrastructure device, receiving the digital signal by the second cellular infrastructure device, routing the digital signal to a second modem for converting the digital signal back to the analog data signal, and transmitting the analog data signal to a second facsimile machine. Similar steps are used in the reverse direction when transmitting from the second facsimile machine to the first facsimile machine.
Finally, the present invention encompasses a remote unit (which is one of the two infrastructure devices mentioned above) for transmitting analog data within a digital wireless communication system. The remote unit comprises a "switch" having a first analog signal as an input and an output for routing the first analog signal based on the switch's determination if the first analog signal comprises voice data or analog data. In the preferred embodiment of the present invention the switch a device specifically utilized to detect the type of signal input and direct the signal to a particular output. The remote unit also includes a first modem having an input coupled to the output of the switch that converts the first analog signal to baseband information and a transmitter coupled to the first modem for transmitting the baseband information via an uplink communication signal. Finally, the remote unit comprises a receiver for receiving a digital signal transmitted via a downlink communication signal in response to the uplink communication signal and a second modem having the digital signal as an input for converting the digital signal to a second analog signal.
FIG. 1 shows a wireless communication system 100 in accordance with the preferred embodiment of the present invention. Wireless communication system 100 is preferably a cellular communication system that utilizes a Code Division Multiple Access (CDMA) system protocol, however, in alternate embodiments of the present invention communication system 100 may utilize any analog or digital system protocol such as, but not limited to, the Global System for Mobile Communications (GSM) protocol, the Personal Digital Cellular (PDC) protocol, or the United States Digital Cellular (USDC) protocol.
Communication system 100 comprises base transceiver station (BTS) 101 , remote unit (RU) 113, Centralized Base Station Controller (CBSC) 103, Mobile Switching Center (MSC) 104, and peripheral 115. In the preferred embodiment of the present invention, peripheral 115 is a standard facsimile machine utilizing analog tones (as opposed to digital data) to communicate data to remote unit 113, although in alternate embodiments of the present invention, peripheral 115 may be other analog devices (such as computer modems or Telephone for Deaf and Disabled/Teletype (TDD/TTY). As shown, remote unit 113 is communicating with base station 101 via uplink communication signal 119 and base station 101 is communicating with remote unit 113 via downlink communication signal 116. Additionally, base station 101 is coupled to CBSC 103, which is in turn coupled to MSC 104 and finally to PSTN 105. A communication system utilizing the CDMA system protocol is described in detail in EIA/TIA Interim Standard 95 (IS-95A) which is incorporated by reference herein.
The description of transmission of analog data in accordance with the invention will be described below in two preferred embodiments. The first embodiment of the invention, which is referred to as remote-to-PSTN transmission, describes facsimile transmission from peripheral 115, through PSTN 105 and ultimately to peripheral 120. The second embodiment of the present invention, referred to as land-to-PSTN transmission, describes facsimile transmission originating from peripheral 120 and transmitted through PSTN 105, ultimately to peripheral 115.

Remote-to-PSTN Transmission of Analog Data

The following description of remote-to-PSTN transmission in accordance with the preferred embodiment details operation of communication system 100 when facsimile data is transmitted by remote unit 113 to BTS 101 and eventually to peripheral 120. In other words, the following description of the preferred embodiment establishes transmission of data from peripheral 115 to peripheral 120.
Peripheral 115, which in the preferred embodiment is a standard facsimile machine utilizing analog tones to convey information (as opposed to digital data), communicates facsimile data over a standard 2-wire loop analog telephone circuit. In the preferred embodiment, the analog tones represent a data pattern that is to be transmitted over an air interface by remote unit 113. Typically the standard 2-wire loop analog telephone circuit uses a four conductor modular jack such as a Registered Jack type 11 (RJ-11) connection shown in FIG. 1 as connection 114. As shown, remote unit 113 and facsimile machine 1 15 are physically separated from each other with RJ-11 connection 1 14 serving to link remote unit 113 to facsimile machine 115, however, in an alternate embodiment of the present invention, facsimile machine 1 15 can be physically located within remote unit 113.
Continuing, analog data enters remote unit 113 and remote unit 113 appropriately converts the analog data to baseband information for transmission via uplink communication signal 119 to base station 101. Further details regarding digital transmission of analog data by remote unit 113 will be described later. Once the digital data has been received by base station 101 , it is appropriately demodulated and transmitted to CBSC 103. CBSC 103 performs a digital to analog conversion where the digitized analog signal is converted back to its analog tone form, and transmitted to PSTN 105 and eventually to peripheral 120.
Referring now to FIG. 2 and FIG. 4. In FIG. 2, a block diagram of a remote unit transmitter, generally designated 200, is illustrated which utilizes the present invention. Transmitter 200 includes convolutional encoder 217, interleaver 219, orthogonal encoder 221 , modulator (not shown), and upconverter 256. During operation, signal 210 (which for the purposes of this description comprises analog facsimile data) is output by facsimile machine 115 and enters remote unit 113. In the preferred embodiment of the present invention signal 210 comprises analog tones that conform to the facsimile protocol CCITT Blue Book Recommendations T.O through T.63 Volume VII fascicle 7.3 "Terminal Equipment and Protocols for Telematic Services." A facsimile machine utilizing the T.4 and T.30 system protocol is described in detail in the above referenced CCITT Specifications which is incorporated by reference herein (CCITT Recommendations can be obtained through the International Telecommunication Union which can be reached at International Telecommunication Union (ITU) Sales and Marketing Service, Place des Nations, CH-1211 Geneva 20, Switzerland).
Continuing, in the preferred embodiment, facsimile machine 1 15 dials the phone number of destination facsimile machine 120 and the call is established in a similar manner to that of voice telephone calls. It should be noted that the T.30 system protocol does not always require the sending facsimile machine to generate any signal indicating that a facsimile is ready to be transmitted. T.30 does require that a facsimile machine that is receiving a call (in this case facsimile machine 120), generate an indication it is ready to begin a T.30 facsimile session. Because of this, if facsimile machine 115 is originating a facsimile, switch 211 will need an indication from the receiving facsimile machine (machine 120) that signal 210 should be routed through modem 213 instead of vocoder 212. Therefore, in the preferred embodiment of the present invention, switch 211 will route signal 210 through modem 213 when an analog facsimile service option message is received at remote unit 113. Additionally, CBSC 103 will generate an analog facsimile service option message when switch 411 detects a T.30 signal via signal 401 that is generated by facsimile machine 120.
Continuing, switch 211 is instructed via the service option message to route signal 210 accordingly. In particular, signal 210 containing analog facsimile data is routed to modem 213 where modulator/demodulator operations transform analog signal 210 to digital signal 214. In the preferred embodiment of the present invention, modem 213 is a device that converts analog modem tones to the corresponding baseband representation and vice versa as specified in CCITT Fascicle VIII.1 "Data Communication over the Telephone Network Series V Recommendations," which is incorporated by reference herein. In particular, analog signal 210 is converted to baseband signal 214, where baseband signal 214 is a baseband representation of the analog wave form. Signal 214 is output to Radio Link Protocol (RLP) processor 215 which provides a Radio Link Protocol (RLP) layer to the data as a bit stream service over IS-95A forward and reverse traffic channels, substantially reducing the error rate typically exhibited by these channels. In particular, as described in Interim Standard 99 (IS-99) section 3.7.1 , RLP processor 215 divides the digital signal into IS-95A traffic channel frames for transmission, and outputs the traffic channel frames to convolutional encoder 217.
The traffic channel frames received by convolutional encoder 217 are at a particular bit rate (e.g., 9.6 kb/second). Convolutional encoder

217 encodes input data bits 216 into data symbols at a fixed encoding rate with an encoding algorithm which facilitates subsequent maximum likelihood decoding of the data symbols into data bits (e.g. convolutional or block coding algorithms). For example, convolutional encoder 217 encodes input data bits 216 (received at a rate of 9.6 kbit/second) at a fixed encoding rate of one data bit to three data symbols (i.e., rate 1/3) such that convolutional encoder 217 outputs data symbols 218 at a 28.8 ksymbol/second rate.
Data symbols 218 are then input into interleaver 219. Interleaver 219 interleaves the data symbols 218 at the symbol level. In interleaver 219, data symbols 218 are individually input into locations within a matrix so that the matrix is filled in a column by column manner. Data symbols

218 are individually output from locations within the matrix so that the matrix is emptied in a row by row manner. Typically, the matrix is a square matrix having a number of rows equal to the number of columns; however, other matrix forms can be chosen to increase the output interleaving distance between the consecutively input non-interleaved data symbols. Interleaved data symbols 220 are output by interleaver

219 at the same data symbol rate that they were input (e.g., 28.8 ksymbol/second). The predetermined size of the block of data symbols defined by the matrix is derived from the maximum number of data symbols which can be transmitted at a predetermined symbol rate within a predetermined length transmission block. For example, in a full rate transmission if the predetermined length of the transmission block is 20 milliseconds, then the predetermined size of the block of data symbols is 9.6 ksymbol/second times 20 milliseconds times three which equals 576 data symbols which defines a 24 by 24 matrix.
Interleaved data symbols 220 are input to orthogonal encoder 221. For IS-95-type transmission orthogonal encoder 221 m-ary modulates the interleaved data symbols 220. For example, in 64-ary orthogonal encoding, each sequence of six interleaved data symbols 220 are replaced by a 64 symbol orthogonal code. These 64 orthogonal codes preferably correspond to Walsh codes from a 64 by 64 Hadamard matrix wherein a Walsh code is a single row or column of the matrix.
A sequence of Walsh codes 222 is burst randomized by burst randomizer 224 such that the transmission is pseudo randomized by dividing each 20 ms frame into 16 power control groups of 1.25 ms each and randomly assigning the transmit bursts among the 16 groups while satisfying a required "duty cycle" for the data rate used in that frame. Long Code Generator 227 provides a spreading code which is combined with the output from randomizer 224. Next, the long code spreaded signal is further spread by a specific sequence of symbols which is output at a fixed chip rate (e.g., 1.228 Mchip/second). In practice, the code spread encoded chips are a pair of pseudorandom (PN) codes used to generate an l-channel and Q-channel code spread sequence 226. The I-channel and Q-channel code spread sequences 226 are used to biphase modulate a quadrature pair of sinusoids by driving the power level controls of the pair of sinusoids. The sinusoids output signals are summed, bandpass filtered, translated to an RF frequency, amplified, and QPSK modulated via modulator/upconverter 256 and radiated by antenna 258 to complete transmission of signal 210.
FIG. 3 is a block diagram of a base station receiver and a centralized base station controller of FIG. 1 for receiving facsimile data in accordance with the preferred embodiment of the present invention. Orthogonally encoded spread-spectrum digital signal 119 is received at receive antenna 331 and amplified by receiver 332 before being despread by despreader 336 into in-phase 340 and quadrature 338 components. Components 338, 340 of despread digital samples are then grouped into predetermined length groups (e.g., 64 sample length groups) of sampled signals that are independently input to orthogonal decoders in the form of fast Hadamard transformers 342, 344, which despread the orthogonally encoded signal components producing a plurality of despread signal components 346 and 360, respectively (e.g. when 64 sample length groups are input, then 64 despread signals are generated). In addition, each transformer output signal 346, 360 has an associated Walsh index symbol which identifies each particular orthogonal code from within a set of mutually orthogonal codes (e.g. when 64 sample length groups are input, then a 6 bit length index data symbol can be associated with the transformer output signal to indicate the particular 64 bit length orthogonal code to which the transformer output signal corresponds). Outputs 346 and 360 are then demodulated by demodulator 368. Demodulated data 370 is then deinterleaved by deinterleaver 372 prior to final maximum likelihood decoding by decoder 376 and passed as signal 378 to CBSC 103.
As shown in FIG. 3, CBSC 103 comprises signal quality indicator

(not shown), controller 303, transcoder (XCDR) 306, facsimile transcoder (FXCDR) 305, and Pulse Code Modulator (PCM) 311. Within CBSC 103, signal 378 is routed to controller 303. Controller 303 also performs the radio link protocol (RLP) processor function for the CBSC 103. Based on the determination made by switch 411 of whether signal 401 is voice or facsimile data, controller 303 routes signal 378 to either XCDR 306 or FXCDR 305. In particular, switch 411 causes a standard service option message to be generated that indicates the call is an analog facsimile session. Specifically, CBSC 103 uses its tone detector in switch 411 to determine the disposition of the call as either voice, facsimile, or modem type data. If signal 378 contains facsimile data, then signal 378 is routed to Facsimile Transcoder (FXCDR) 305. In the preferred embodiment of the present invention, FXCDR 305 performs the function of converting digital data contained within signal 378 to analog form.
Continuing, signal 378 enters modem 307 and is converted back to its original analog form and is output from modem 307 as signal 381. Additionally, as will be discussed below in reference to FIG. 6, signal 378 is initially buffered by buffer 309 for up to 1 second prior to entering modem 307 in order to compensate for errors that can occur in air interface signal 119 that prevent timely delivery of facsimile line information contained in signal 378. The exact amount of buffering is optimized to be slightly longer in time than the maximum time that the communication signal 119 can become unavailable or unreliable. Buffering ensures continuous transmission of facsimile data to MSC 104 (and eventually to peripheral 120). In particular, since most facsimile machines require a continuous transmission of data, continuous transmission of data to peripheral 120 is required, even when corrupted data is received by CBSC 103 due to brief outages or unreliability of communication link 119. In the preferred embodiment, a signal quality indicator (not shown) determines if signal 378 contains corrupted data, and if so instructs buffer 309 to stop buffering corrupted data 378. In the preferred embodiment of the present invention a signal quality indicator analyzes signal 378 to determine a Bit Error Rate (BER) for signal 378 to determine a signal quality indication, although in alternate embodiments of the present invention other signal quality indications may be utilized. During normal operation buffer 309 acts as a "pipeline" whereas data comes into the buffer and is later removed from the buffer. When communication link 119 contains errors, however, buffer 309 can become depleted. When this happens, buffer 309 is instructed by controller 303 to begin transmitting a "FILL" character. As defined in the T.4 standard, "FILL" is a variable length of bit zeroes which can be sent for up to 5-seconds. The Error Correction Mode (ECM) which is used with the T.6 standard will allow HDLC flags to be used for "FILL". This transmission of "FILL" characters will continue until which time the amount of data in the buffer 309 becomes sufficient (such as a complete T.4 line or a complete frame of T.6 data). Once the amount of data in the buffer 309 becomes sufficient, controller 303 instructs buffer 309 to once again start transmitting data from buffer 309.
Continuing, when a signal quality indicator determines that signal 378 is corrupt, controller 303 instructs buffer 309 to continue transmission of "good data" that has been stored in buffer 309, but does not store data transmitted with the corrupt signal. As described earlier, buffer 309 will either continue to provide good data to be transmitted during a brief period of reception of corrupt data from signal 378 or if the duration of time that the corrupt data persists and causes buffer 309 to empty, buffer 309 will as described earlier, transmit FILL characters instead until a sufficient amount of non corrupted data is again received by CBSC 103.

By continuously transmitting a "FILL" character to modulator 31 1 when corrupted data is received by CBSC 103, the T.4 facsimile protocol is not violated (i.e., continuous transmission occurs). Therefore the facsimile session is not dropped while communication signal 1 19 undergoes occasional periods of unreliability. Also, use of the "FILL" in this manner allows time to replenish buffer 309 once reliable communication link 119 is restored. After buffering, signal 380 is output to the modem 307 for conversion from digital to analog and analog facsimile modem signal 381 is sent to the Pulse Code Modulator 311 to be modulated and sent as a PCM stream to MSC 104 and eventually through PSTN 105 to peripheral 120.

PSTN-to-remote Transmission of Analog Data

The above description of the preferred embodiment details operation of communication system 100 when facsimile data is transmitted by remote unit 113 to BTS 301 and eventually to peripheral 120. In other words, the above description of the preferred embodiment establishes transmission of data from peripheral 115 to peripheral 120. However, in order to establish real-time communication between peripheral 115 and peripheral 120 (as required by T.4 and T.30 system protocols), transmission of data from peripheral 120 to peripheral 1 15 needs to be established. The discussion below details operation of communication system 100 when facsimile data is sent from peripheral 120 through PSTN 105 to peripheral 115. The facsimile data sent by peripheral 120 may be in response to facsimile data sent from peripheral 115 (e.g., in response to an error in transmission by peripheral 115), or peripheral 120 may be initiating an original facsimile to peripheral 115.
FIG. 4 is a block diagram of CBSC 103, base station 101 , and remote unit 113 for transmitting facsimile data in accordance with the preferred embodiment of the present invention. PCM stream 401 , originating from PSTN 105 enters CBSC 103. As discussed above, the T.30 system protocol does not always require the sending facsimile machine to generate any signal indicating that a facsimile is ready to be transmitted. T.30 does require that a facsimile machine that is receiving a call (in this case facsimile machine 115), generate an indication it is ready to begin a T.30 facsimile session. Because of this, if facsimile machine 120 is originating a facsimile, switch 411 will need an indication from the receiving facsimile machine (machine 115) that signal 401 should be routed through modem 413 instead of vocoder 412. Therefore, in the preferred embodiment of the present invention, switch 411 will route signal 401 through modem 413 when an analog facsimile service option message is received at CBSC 103. Additionally, remote unit 1 13 will generate an analog facsimile service option message when switch 211 detects a T.30 signal via signal 210 that is generated by facsimile machine 115.
Continuing, signal 401 containing analog facsimile data is routed to modem 413 where standard demodulator operations transform analog signal 401 to baseband signal 414. RLP processor 415 which provides a Radio Link Protocol (RLP) layer to the data as an octet stream service over IS-95A forward and reverse traffic channels, substantially reducing the error rate typically exhibited by these channels. In particular, as described in Interim Standard 99 (IS-99) section 3.7.1 , RLP processor 415 divides the digital signal into IS-95A traffic channel frames for transmission, and outputs the traffic channel frames as signal 417 for transmission to base station 101.
Base station 101 receives signal 417 for transmission and performs necessary processing for IS-95A CDMA transmission via downlink signal 116. In particular, base station 101 convolutionally encodes and interleaves signal 417. Interleaved data symbols are orthogonally encoded, spread by a sequence of weighted Walsh codes and modulated for transmission over a communication channel by antenna 419 to complete transmission of channel data bits to remote unit 113.
Remote unit 113 receives downlink communication signal 116 and performs standard IS-95A decoding and demodulating of signal 116. In particular, signal 116 is properly demodulated, despread, and decoded by IS-95A front end 421. The resulting signal 423 contains IS-95A traffic channel frames. Controller 427 determines if the call is a voice or facsimile type call from the service option message generated earlier. If signal 423 contains facsimile data, then signal 423 is routed to the remote unit's buffer 441 and modulator 439. In the preferred embodiment of the present invention, modulator 439 performs the function of converting digital data contained within signal 423 to analog form.
In the preferred embodiment of the present invention, buffer 441 receives signal 480 from controller 427. Signal 480 is then buffered by buffer 441 until sufficient data has been collected (such as a line of T.4 data or a frame of T.6 data). Buffer 441 then sends the signal to modem 439 to be converted to an analog facsimile signal to be sent to peripheral 115. As discussed above, buffering ensures continuous transmission of facsimile data to peripheral 115. Buffer 441 operates in the same manner as described for buffer 309. In particular, buffer 441 uses T.4 lines, T.6 frames and FILL characters as previously described for buffer 309 to ensure continuous transmission of data from buffer 441. In the preferred embodiment, signal quality indicator 425 determines if signal 423 contains corrupted data, and if so instructs buffer 441 to stop buffering the corrupted data. Signal quality indicator 425 analyzes signal 423 to determine a Bit Error Rate (BER) for signal 423 to determine a signal quality indication, although in alternate embodiments of the present invention other signal quality indications may be utilized.
Continuing, when signal quality indicator 425 determines that signal 423 is corrupt, buffer 441 will either continue to provide good data to be transmitted during a brief period of reception of corrupt data from signal 423 or if the duration of time that the corrupt data persists and causes buffer 441 to become empty, buffer 441 will as described earlier, transmit FILL characters until non corrupted data is again received by remote unit 113.
The above description of the present invention describes transmission of communication system for remote-to-PSTN, and land-to-PSTN operation. FIGs. 5-8 illustrate operation of communication system 100 for both remote-to-PSTN and land-to-PSTN scenarios. FIG. 5 is a flow chart showing the steps necessary for analog transmission of data within a digital wireless communication system in accordance with the preferred embodiment of the present invention. The logic flow begins at step 501 where a first peripheral transmits a data signal to a first cellular infrastructure device. As discussed above, the first cellular infrastructure device may comprise remote unit 113 or may comprise CBSC 103 depending on whether transmission of data is to take place on an uplink (remote-to-PSTN) or downlink (PSTN-to-remote) transmission. Next, at step 503 the first infrastructure device determines if the data signal is voice information or analog data. If at step 503 it is determined that the data signal is voice data, then the data signal is passed to a voice coder for subsequent coding and transmission of the voice data (step 505), otherwise the data signal is routed to a first modem for digital coding of the analog signal (step 507). As discussed above, the first modem appropriately converts the analog signal modem tones to the corresponding digital representation and vice versa as specified in CCITT Fascicle VIII.1. Next, at step 509, the digitized signal is transmitted over an air interface for reception by a second cellular infrastructure device. As discussed above, the second cellular infrastructure device may comprise either base station 101 or remote unit 113 depending on whether downlink or uplink transmission occurs. Additionally, in the preferred embodiment of the present invention transmission over the air interface takes place by utilizing a CDMA system protocol as described in IS-95A.
Continuing, at step 511 the received signal is appropriately demodulated and decoded and the resulting digital signal is analyzed to determine if error-free transmission of the signal occurred (step 515). If at step 515 it is determined that the signal was transmitted with an unacceptably high error level, then at step 520 appropriate error control is performed. As discussed above, error control comprises a signal quality indicator determining that the received signal is corrupt and controller 303 instructing buffer 309 to empty buffer 309 until an "end of line" is reached, and then continuously transmit a "FILL" character before the "end of line code." (as defined in the T.4 standard, "FILL" is a variable length of bit zeroes which can be sent for up to 5-seconds). If at step 515 it is determined that error free transmission of the signal occurred, the signal is routed to a buffer where the digital signal is buffered (step 525). Next, at step 530 digital data is output from the buffer and routed to a second modem where conversion of the digital signal to analog signal modem tones takes place. Finally, at step 535, the analog signal modem tones are routed to a second peripheral and the logic flow ends at step 540.
FIG. 6 is a flow chart showing the steps necessary for proper error control of analog transmission of data within a digital wireless communication system in accordance with the preferred embodiment of the present invention. Although the steps necessary for proper error control are illustrated below with respect to error control within CBSC 103, error control within remote unit 113 occurs similarly.. The logic flow begins at step 601 where controller 303 receives data. Next at step 603 a test for corrupted data is performed. If the data is corrupt, the logic flow continues to step 605 where the data is discarded and a re-transmission is requested and the logic flow continues to step 610. If at step 603 it is determined that the data is not corrupt, then the logic flow continues to step 607 where the data is stored in buffer 309. Next, at step 610 it is determined if the buffer contains enough information (such as a line of a

T.4 facsimile image, T.6 data in an Error Correction Mode frame, training bit pattern, etc.) and if not, a "FILL" is output (step 612), otherwise the buffered data is output (step 614). The logic flow then ends at step 616.
As described in the CCITT VIII.1 standard for facsimile transmission, facsimile messages comprise tones, control messages, and data messages. In particular, tone messages comprise origination messages that establish that a facsimile is being sent, and are utilized to establish initial contact between facsimile machines; control messages are sent out utilizing 300 BPS Frequency Shift Keying (FSK) modulation and indicate control information (such as data rate); and data messages comprise actual transmission of facsimile data utilizing CCITT VIII.1 QAM modulation based on a data rate agreed upon. Because a standard IS-95A traffic channel frame can carry 171 bits, and given an RLP overhead of 19 bits, allocating 8 bits for modulation type (data rate) leaves only 144 bits for actual data to be carried within a given frame. Though 8 bits have been allocated for the modulation type in the preferred embodiment of the present invention, variable length can be employed and thus more bits would be available for data. Thus, current IS-95A traffic channel protocol allows for a maximum transmission rate of 7200 BPS. Because it is possible that facsimile machines may come to a mutual understanding between themselves to transmit at a higher data rates than 7200 BPS, in the preferred embodiment of the present invention, means for ensuring an acceptable data rate are necessary. In the preferred embodiment of the present invention, a 9600 BPS air interface is utilized, however, in alternate embodiments of the present invention, higher bit rates can be utilized. (14,400 BPS, etc.).
Referring to FIG. 4 and FIG. 7, acceptable data transmission rates are ensured by controller 427 determining if data messages are arriving at a higher than acceptable rate (i.e., higher than 7200 BPS) and if so, sending an invalid data pattern to the destination as a training pattern. Operation in accordance with the preferred embodiment ensures that the only time data arrives at a higher than acceptable rate is during the Group 3 facsimile training portion of the facsimile session. In particular, during negotiation between peripheral 120 and peripheral 115, both machines may agree to operate at data rates higher than 7200 BPS. Once an agreed upon rate has been established, the transmitting facsimile (in this case peripheral 120) will send a training pattern of zeros, as described in section T.4 in the above referenced CCITT Specifications. In the preferred embodiment of the present invention, modem 413 receives the training pattern at the higher data rate (e.g., 9.6 kbps, 12 kbps, or 14.4 kbps), converts them to digital format and transmits them at the higher data rate to RLP processor 415. Because RLP processor 415 can only operate at 7200 BPS, RLP processor 415 will drop data. In other words, because modem 413 is transmitting faster than RLP processor 415 is capable of transmitting, an overflow situation will occur in which data will be lost. In this case, some of the zeroes of the all zeroes training pattern will be discarded.
RLP processor 415, receiving the data at the higher data rate will then format the data (some of which has been lost) into IS-95A traffic channel frames for transmission. In the preferred embodiment of the present invention each frame contains a variable length header 701 containing modem's 413 transmission rate and modulation type. In other words, if modem 413 is transmitting a training pattern at 14.4 kbps, each frame 700 output from RLP processor 415 will contain header 701 indicating that modem 413 is operating at 14.4 kbps and is using Quadrature Amplitude Modulation.
After transmission by base station 101 and subsequent IS-95A processing by receiver 421 in remote unit 113, controller 427 receives the frames transmitted from RLP processor 415, each of which is encoded with the particular data rate of modem 413. Controller 427 determines (from header 701) the data rate of modem 413, and if the data rate is higher than 7200 BPS, eliminates all data bits transmitted within the higher data rate frame (i.e., data bits 703), and replaces data bits 703 with a predetermined numerical sequence. In the preferred embodiment of the present invention the predetermined sequence is a number sequence that does not correspond to a training pattern (such as all ones). Additionally, controller 427 adds additional data to each data frame so that buffer 441 is filled at the original higher data rate. Finally, modem 439 transmits the data to peripheral 115 for decoding.
Since all data transmitted above the acceptable data rate is corrupted by controller 427, peripheral 115, which expects to see a training pattern of zeros, will instead see corrupted data (all ones), and will automatically request peripheral 120 to step down its transmission rate. In particular, once received at the second peripheral, the data is perceived as being corrupt since the training pattern is determined to have an error rate above a predetermined threshold, and the second peripheral will communicate this fact to the first peripheral via PSTN-to-remote communication of analog data (described above). This will automatically cause the first peripheral to step down the rate of transmission and repeat sending the training pattern at the lower data rate.
FIG. 8 is a flow chart illustrating the steps necessary for obtaining acceptable transmission rates in accordance with the preferred embodiment of the present invention. The logic flow begins at step 801 where a first peripheral (peripheral 120) transmits to a first modem (modem 413) at a first data rate. Next, at step 805, modem 413 converts the transmission from peripheral 120 from analog to digital format at the first data rate. At step 810, a processor (RLP processor 415) determines the first data rate and encodes the first data into frames for transmission over an air interface. As discussed above, each frame contains a header indicative of the first data rate. Additionally, in the preferred embodiment of the present invention, processor 415 operates at a second data rate, such that when the first data rate is higher than the second data rate, some of the digital data converted from the analog data at the first data rate will be lost.
Continuing, at step 815, the frames are transmitted over the air interface, and received by a receiver (step 820). The received frames are then decoded (step 825) and at step 830, sent to a controller (controller 427) which determines the first data rate by analyzing the data rate encoded onto the frames. At step 835, controller 427 determines if the first data rate is greater than the second data rate. In other words, controller 427 determines if the data rate of modem 413 is higher than the maximum throughput supported by the system (7200 BPS in this example), and if not the logic flow continues to step 845 where the data existing within the transmitted frames are passed on to a buffer (buffer 441). If at step 835 it is determined that the first data rate is higher than 7200 BPS then controller 427 eliminates all data bits transmitted within the higher data rate frame and replaces data bits with an invalid training number sequence (step 840) and the logic flow continues to step 845. Additionally, at step 840 controller 427 adds additional invalid training data to each data frame so that buffer 441 is filled at the higher data rate. Finally, at step 850 a second modem (modem 439) transmits the buffered data to a second peripheral (peripheral 115) for decoding (step 855).
Once received at the second peripheral, the data is perceived as being corrupted, and the second peripheral will communicate this fact to the first peripheral via PSTN-to-remote communication of analog data. This will automatically cause the first peripheral to step down the rate of transmission and repeat sending the training pattern at the lower data rate.
The above description of downlink (land-to-PSTN) and uplink (remote-to-PSTN) transmission of facsimile data allows for a seamless communication between two facsimile machines utilizing one T.30 session to transmit analog data. In particular, utilizing the present invention for downlink and uplink transmission allows for the communication between analog peripherals over a digital communication network in a real-time fashion. For example, once communication is established from a first to a second peripheral via remote-to-PSTN communication, the first peripheral will receive real-time communication from the second peripheral via PSTN-to-remote communication.
The descriptions of the invention, the specific details, and the drawings mentioned above, are not meant to limit the scope of the present invention. For example, in many cellular communication systems, voice coding and decoding functions take place within mobile switching centers instead of centralized base station controllers, and it is the intent of the inventors that the necessary modifications can be made to the mobile switching centers to facilitate transmission of data in accordance with the present invention without varying from the spirit and scope of the invention. It is intended that all such modifications come within the scope of the following claims.

What is claimed is: