METHOD AND APPARATUS FOR POINT-TO-MULTIPOINT DATA COMMUNICATION WITH ARQ

Abstract: A novel method and apparatus provides an efficient means for delivering data to multiple wireless destinations by the issuing of a negative acknowledge message from a receiving terminal upon detecting a second outstanding block error. The transmitting terminal sends an error correction block for all the blocks up to but not including the block with the second detected error, which increases the throughput. Each receiver is independent of the other receivers and decodes the incoming blocks. Every receiver performs the error detection function on every received block and detects whether or not the block has been received correctly.

Description: METHOD AND APPARATUS FOR POINT-TO-MULTTPOTNT DATA COMMUNICATION WITH ARQ Field of Invention The present invention is generally related to communication devices and systems, and more particularly, to a method and apparatus for transmitting data in a communication system. Background of the Invention There is great demand for reliable point-to-multipoint data services, as provided by a point-to-multipoint communication network shown in FIG. 1. In particular, a transmitting terminal (BS) receives data signals from a transmitting terminal MSI, and transmits the data signals to multiple receiving terminals (MS2, MS3 and MS4). Protocols for point-to-point data transfer typically rely on windowing protocols, which protect terminals from overflow conditions. When transferring data over a radio interface, it is likely to be the radio interface rather than the buffering capability of the terminal which will be the rate-determining factor. This is a factor which, when combined with a requirement to provide a point-to-multipoint data transfer, dramatically changes the degrees of freedom available when designing an optimum data transfer protocol. For packet switched communications, ARQ (Automatic Repeat Request) protocols have been designed for point-to-multipoint applications which are able to cope with stringent memory requirements, but they either lack integrity at high block error rates, or they require a large bandwidth due to low throughput. The various ARQ methods differ in the transmissions of erroneous blocks. With continuous ARQ strategies, the sender continuously transmits new blocks, and the receiver accepts each error-free block and acknowledges it by sending an ACK message. On receipt of an erroneous block, the receiver negatively acknowledges the block by sending a "NACK". When the NACK arrives at the sender, the transmission of the blocks is interrupted. In case of the "Go-Back-N" strategy, the sender does not only retransmit the negative acknowledge block, but also all the following blocks. These are the blocks which have been transmitted over one round trip delay after the erroneous block. In contrast, the selective repeat strategy (SR), the sender retransmits only the negatively acknowledged block. These strategies are also used for point-to- multipoint communications. In this case, blocks are considered to be correct if the blocks are correctly received by all receivers. Accordingly, the sender decides about retransmissions of a block after receiving acknowledgements from all K receivers. Because data packet services require the transmission of many data packets at a very low bit error rate, there is a need to maximise the throughput for wireless point-to-multipoint protocols. In a paper entitled "Improved Selective Repeat ARQ Schemes for Data Communication" by M Aghadavoodi et al. presented at the VTC '94 Conference, an improved strategy for multicast communication is disclosed. As set forth in the paper, with the exclusive or, selective repeat (XOR-SR) technique, the sender does not retransmit NACKed blocks immediately upon reception of a NACK, but it gathers window numbers of ACK/NACK acknowledgements and then decides on retransmitting not acknowledged blocks. Several NACKed blocks are combined by XORing (i.e. modulo-2-adding) them to minimise the number of retransmissions and to increase throughput. The basic XOR retry mechanism allowing the efficient calculation by multiple mobile terminals of the correct value of one bad block per terminal is well known in the prior art. The resulting block is called an XOR block. A receiver expecting a particular block must be able to reconstruct it from the XOR block (i.e. it must know all other blocks comprising the XOR block). Applying the XOR operation again to the XOR block with all the participating and correctly received blocks the receiver obtains the expected block. Thus, the basic strategy is to combine as many NACKed blocks in an XOR block as possible, while ensuring that each intended receiver will know all but one of the participating blocks. FIG. 2 shows a simulation of the improvement of throughput as a function of the number of receivers for the XOR-SR compared to the SR scheme. Although the XOR-SR scheme provides improvement, it is nevertheless suboptimal for a point-to-multipoint data transfer. Accordingly, there is also a need for a method and apparatus for efficiently transmitting data in a point-to-multipoint system. There is also a need for a method for dynamically defining a data recovery window in a communication network. Brief Description of the Drawings FIG. 1 is a block diagram of a communication system having point- to-multipoint data transmission capability according to the present invention; FIG. 2 is a chart showing the throughput of data according to conventional SR and XOR-SR schemes FIG. 3 is a diagram showing frames of receiving terminals according to the present invention; FIGs. 4 A and 4B are a flow diagram showing the preferred method of transmitting data according to the present invention. FIG. 5 is a chart showing the throughput of data according to the present invention, compared to conventional XOR-SR techniques; FIG. 6 is a block diagram of a terminal incorporating the present invention; FIG. 7 is a block diagram of a transmitting portion of control circuit of FIG. 6 according to the present invention; and FIG. 8 is a block diagram of a receiving portion of control circuit of FIG. 6 according to the present invention. Detailed Description of the Invention The present invention provides an efficient means for delivering data to multiple wireless destinations by the issuing of a negative acknowledge message from a receiving terminal upon detecting a second outstanding block error, followed by the sending by the transmission source of an error detection block for all the blocks up to but not including the block with the second detected error, which gives rise to the benefit of greater throughput, especially for point-to-multipoint transfers. The data to be transmitted is segmented into blocks of a known fixed size. Each block is given a sequence number and an error detection field is added. The blocks are transmitted in sequence. Each receiver is independent of the other receivers and decodes the incoming blocks. Every receiver performs the error detection function on every received block and detects whether or not the block has been received correctly. Sequencing and error detection mechanisms are both very well known to those skilled in the art. Generally, each transmitted block includes a sequence number, a verification field, and an error detection field. The receiving terminal generates an error detection value, and compares it to the error detection field of the received block. The method for transmitting data according to the present invention will be described in more detail in reference to the frame diagrams of FIG. 3 and the flow chart of FIGs. 4A and 4B. According to one novel feature of the present invention, the first incorrect block which is detected in a mobile terminal does not cause a negative acknowledgement to be transmitted back to the transmitting terminal. Instead, a pointer identified here as "First Error Pointer" in each receiving terminal is configured to point at the sequence number of the bad block received by that terminal, as shown in FIG. 3. This is possible because the blocks have been transmitted in sequence and are associated with a sequence number. The data transfer continues, with each receiving terminal effectively remembering which block has been incorrectly received based upon the First Error Pointer. A counter of all the outstanding incorrectly received data blocks is also maintained at all times by each receiving terminal. If a receiving terminal subsequently detects a second block which has been incorrectly received, a second pointer identified here as "Second Error Pointer" is configured to point at the sequence number of the second bad block. At this point, a negative acknowledgement ("NACK") is transmitted by the receiving terminal receiving the second bad block to the transmitting terminal, which is normally a transmitting terminal shown in Figure 1. Given that multiple mobile terminals may be attempting to transmit a NACK at the same time (e.g. MS2 and MS4 of FIG. 3), a simple energy burst would be a suitable technique for this NACK message. Such an energy burst technique is described in detail in European Patent Publication No. 9422449.0, filed on 7 November 1994 by applicants of the present invention. When the transmitting terminal receives the NACK message, it transmits an error correction block which is sufficient for all of the terminals which have an outstanding block error to calculate the correct value for the incorrect block regardless of the position of the incorrect block in sequence of blocks. Preferably, an XOR scheme, which is well known in the art, is used for generating an error correction block. The remaining description will include references to the XOR scheme. However, other error correction schemes could be used within the spirit and scope of the present invention. Upon receiving an XOR block, the terminal calculates the correct value for the bad block and then moves the value stored in Second Error Pointer into the First Error Pointer and deletes the contents of the Second Error Pointer. A mobile terminal may have only detected one block error, when the transmitting terminal transmits an XOR block. This may occur because one of the other terminals had transmitted a NACK to the transmitting terminal. As shown for example in FIG. 3, MS2 and MS3 had only received one error block when MS4 received its second error block. In this case, the mobile terminals with only one block error is able to calculate the correct value of the bad block and delete the contents of the First Error Pointer. It may sometimes be the case that an XOR block itself is incorrectly received, by a terminal which needs the XOR block to re-calculate the first of its two bad blocks. If this occurs, the terminal transmits a NACK message to the transmitting terminal in response to the XOR bad block. However, if a terminal had received only one error when the XOR block is incorrectly received, it is not necessary to transmit the NACK message. Rather, the receiver with only one outstanding block error can wait for the next transmission of the XOR block when it or another terminal detects a second block error. The last block to be transmitted is a special XOR block which covers the complete data set and is used by all of the mobile terminals which have an outstanding block error to re-calculate the correct value of the bad block. This XOR block is always negatively acknowledged if incorrectly received by a terminal which has a bad block to recover. Referring specifically to FIGs. 4A and 4B, the preferred steps for transmitting data according to the present invention are shown. At a step 402, the receiving terminal sets an error counter equal to zero and resets the first and second error pointers. At a step 404, the receiving terminal receives the next block of data and determines whether the block is a data block or an XOR block at a step 406. If the block is a data block, the receiving terminal determines whether the block is a good or bad data block at a step 408. If it is a good data block, the terminal accepts the block at a Step 410. However, if it is a bad data block, the receiving terminal determines whether the error counter is greater than zero at a step 412. If the error counter is not greater than zero, the receiving terminal sets the first error pointer to the data block and increments the error counter at a step 414. However, if the error counter is greater than zero, the receiving terminal sets the second error pointer and increments the error counter at a step 416. In this case, the receiving terminal also transmits a NACK at a step 418. If the block is an XOR block, the receiving terminal determines whether the XOR block is a normal block or the last XOR block which is sent at a step 420. If it is a normal XOR block, the receiving terminal determines whether it is a good or bad XOR block at a step 422. If it is a good XOR block, the receiving terminal determines whether the error counter is greater than zero at a step 424. If the error counter is greater than zero, the receiving terminal calculates the value of the bad block from the XOR block, and decrements the error counter at a step 426. The terminal also sets a first error pointer to a value held in the second error pointer at a step 428. Finally, the receiving terminal resets the second error pointer at a step 430. However, if it is a bad XOR block, the receiving terminal determines whether the error counter is greater than one at a step 432. If the error counter is greater than one, the receiving terminal transmits a NACK at a step 434. However, if it is the last XOR block, the receiving terminal determines whether it is a good or bad XOR block at a step 436. If it is a good XOR block, the receiving terminal determines whether the error counter is greater than zero at a step 438. If the error counter is greater than zero, the terminal calculates the value of the bad error block at a step 440. The receiving terminal terminates receiving data at that point. However, if the error counter is not greater than zero, the receiving terminal terminates receiving data at this point. If the last XOR block is a bad block, the receiving terminal determines whether the error counter is greater than zero at a step 442. If it is not greater than zero, the receiving terminal terminates the receiving data at that time. Otherwise, the receiving terminal transmits a NACK at a step 444 to receive another XOR block. Figure 5 shows the throughput of data of the present invention compared with the conventional XOR-SR protocol. As compared with FIG. 2, it can be clearly seen that the method of the present invention has a significantly better throughput than the conventional SR or XOR-SR protocols. This advantage is particularly noticeable for higher Block Error Rate (BLER) values, for example 0.1, where the XOR-SR protocol tends to approach the performance of simple selective retry for large numbers of receiving terminals, whereas the invention described here continues to be significantly better than the simple SR protocol, and in fact stays well above 0.6 even when there are 100 receiving terminals. Turning now to FIG. 6, a block diagram of a terminal for implementing the present invention is shown. In the preferred embodiment, an ASIC (Application Specific Integrated Circuit) 601, such as a CMOS ASIC available from Motorola, Inc. and microprocessor 603, such as a 68HC11 microprocessor also available from Motorola, Inc., combine to generate the necessary communication protocol for operating in a cellular system. The microprocessor 603 uses RAM 605, EEPROM 607, and ROM 609, consolidated in one package 611 in the preferred embodiment, to execute the steps necessary to generate the protocol and to perform other functions for the communication unit, such as writing to a display 613, accepting information from a keypad 615, and controlling a frequency synthesizer 625. The ASIC 601 processes audio transformed by the audio circuitry 619 from a microphone 617 and to a speaker 621. Transmitter 623 transmits through an antenna 629 using carrier frequencies produced by the frequency synthesizer 625. Information received by the communication unit's antenna 629 enters the receiver 627 which demodulates the symbols comprising the message frame using the carrier frequencies from the frequency synthesizer 625. The wireless communication device may optionally include a message receiver and storage device including digital signal processing means. The message receiver and storage device could be, for example, a digital answering machine or a paging receiver. While the circuitry of FIG. 6 shows an exemplary wireless communication device, other circuitry could be employed within the scope of the present invention. Turning now to FIG. 7, a block diagram shows transmitter portion 700 of control circuit 603 according to the present invention. Transmitter control circuit 700 includes a checksum generator 702 which receives data on an application source data channel 704. The checksum generator also outputs the data blocks to be transmitted on a transmittable data channel 706. The checksum generator generates a checksum signal on checksum channel 708 which is input to a checksum sender 710. Checksum sender 710 also receives an end of data signal on end of data channel 712 and a NACK signal on a NACK channel 714. The checksum sender generates a checksum signal on a transmittable checksum channel 716 when the NACK is received. The checksum sender generates a checksum signal on a transmittable checksum channel 716 when the end of data signal is received. Turning now to FIG. 8, the receiver portion 800 of control circuit 603 is shown according to the present invention. Receiver portion 800 includes an error detector 802 which receives data blocks and checksum blocks on the received information channel 804. The data checksum detector 808 is coupled to error detector 802, last checksum detector 809 and data recovery manager 824. The data checksum detector instructs the data recovery manager if a good checksum, a bad checksum, a good last checksum or a bad last checksum is detected. A checksum detector 808, a last checksum detector 809, and a received data buffer 810 also receive the data blocks and checksum blocks on information channel 804. Error detector 802 is coupled to an incrementor 812 and bad data checksum detector 808. In particular, error detector 802 generates an error signal on bad data indication channel 813 when an error in a data block is detected. Bad data indication channel 813 is coupled to incrementor 812, which increments a counter 822 when an error is detected. A first pointer 816 is established to designate the location of the first data block having an error in received data buffer 810. Similarly, a second pointer 818 is established to designate the location of the second data block having an error. Counter 814 is coupled to a NACK generator 820 for generating a NACK signal on NACK output channel 822 after the counter indicates that two block errors have been recorded. NACK generator 820 is also coupled to a data recovery manager 824. NACK generator 820 also generates a NACK signal when instructed to do so by the data recovery manager. Data recovery manager 824 receives a signal on checksum indication channel 826 indicating a good or bad checksum or a good or bad last checksum. If a bad last checksum was received or if a bad checksum was received when two block errors had been received, the data recovery manager instructs the NACK generator to generate a NACK. If the data recovery manager is able to recover the data in an erroneous block, a decrementor 828 decrements counter 814, the value in pointer 2 is moved into pointer 1 and pointer 2 becomes empty. The present invention is at its most efficient where there are no buffer limitations in the mobile terminal, which is normally the case. However, the protocol does not prevent "Ready to Receive" type protocols from operating in addition to the present invention. "Ready to Receive" protocols are well known to those skilled in the art. Although multiple receiving terminals are shown, the present invention can be incorporated in a system having a single transmitting terminal and a single receiving terminal. Also, the present invention can be applied to any communication network, whether wireless or landline, for transmitting data of any kind. For example, the method and apparatus could be employed in cellular communication systems, wireless messaging systems, paging systems, or other communication systems. In summary, the present invention provides an efficient means for delivering data to multiple wireless destinations by the issuing of a negative acknowledge message from a receiving terminal upon detecting a second outstanding block error, followed by the sending by the transmission source of an XOR block for all the blocks up to but not including the block with the second detected error. The present invention finds particular application in point-to-multipoint transfers. While the specific implementations of the present invention are described above, variations and modifications could be made by one skilled in the art within the spirit and scope of the present invention. The present invention should be limited only by the appended claims.

Claims: CLAIMS 1. A method for transmitting data from a transmitting terminal to at least one receiving terminal in a communication system, said method comprising the steps of: sequentially transmitting blocks of data from said transmitting terminal; receiving a negative acknowledge message from a receiving terminal, said negative acknowledge message indicating that a first error in a block of data and a second error in a subsequent block of data have been detected; and transmitting an error correction block to allow the correction the first error in a block of data. 2. A method for transmitting data according to claim 1, wherein said step of transmitting an error correction block comprises transmitting a block having an XOR of all blocks before the second error. 3. A method for transmitting data according to claim 1, wherein said step of sequentially transmitting blocks of data comprises transmitting blocks of data to a plurality of receiving terminals. 4. A method for transmitting data according to claim 3, further including a step of receiving said error correction block at each said receiving terminal. 5. A method for transmitting data according to claim 4, further including a step of decoding, at each said receiving terminal, said XOR block to recover the data in a first error block detected by that terminal. 6. A method for receiving data at a receiving terminal of a communication network, said method comprising the steps of: sequentially receiving blocks of data at the terminal; detecting a first error in a block of transmitted data; detecting a second error in a subsequent block of transmitted data; and sending a negative acknowledge message upon detecting the second error. 7. A method for receiving data according to claim 6, further including a step of determining whether an error is detected in a data block or an error correction block. 8. A method for receiving data according to claim 7, further including a step of incrementing an error counter when an error is detected. 9. A method for transmitting data according to claim 6, further including a step of setting a first error pointer at a receiving terminal designating the block having the first error. 10. A method for transmitting data according to claim 9, further including a step of setting a second error pointer at a receiving terminal designating the block having the second error. 11. A method for receiving data according to claim 8, wherein said step of sending a negative acknowledge comprises sending a negative acknowledge upon receiving a bad data block prior to the error counter being incremented if said error counter is greater than zero. 12. A method for receiving data according to claim 8, wherein said step of sending a negative acknowledge comprises sending a negative acknowledge upon receiving a bad error correction block if said error counter is greater than one. 13. A method for receiving data according to claim 8, further comprising a step of withholding a negative acknowledge upon receiving a bad error correction block if said error counter is less than or equal to one. 14. A method for receiving data according to claim 6, further including a step of receiving an error correction block having data allowing the receiving terminal to calculate the correct data for the block having the first error. 15. A method for receiving data according to claim 13, wherein said step of receiving an error correction block comprises receiving a transmission of a block having an XOR of all blocks before the second error. 16. A method for transmitting data according to claim 15, further including a step of decoding said XOR block to recover the block having the first error. 17. A method for receiving data according to claim 6, further including a step of transmitting is a special error correction block which covers the complete data set 18. The method for receiving data according to claim 17, further including a step of each said mobile terminals which have an outstanding block error re-calculating the correct value of the first bad block received by that terminal. 19. A method for receiving data according to claim 18, further including a step of transmitting a negative acknowledge if said special error correction block is incorrectly received by a terminal which has a bad block to recover. 20. A method for transmitting data from a transmitting terminal to a plurality of receiving terminals in a point-to-multipoint communication system, said method comprising the steps of: sequentially transmitting blocks of data from said transmitting terminal; sequentially receiving said blocks of data at said plurality of receiving terminals; detecting a first error in a block and a second error in a subsequent block of transmitted data at one of said receiving terminals; sending a negative acknowledge message from said one of said receiving terminals upon detecting said second error; receiving a negative acknowledge message at said transmitting terminal; and transmitting an error correction block for all blocks transmitted before the block having the second error. 21. A method of dynamically defining a frame in a communication network comprising the steps of: sequentially receiving blocks of data at a terminal; detecting an error in a block of transmitted data; detecting a second error in a subsequent block of transmitted data; and defining the frame to include all blocks prior to the block having the second error. 22. A method of dynamically defining a frame in a communication network of claim 21, wherein said step of sequentially receiving blocks of data comprises receiving blocks of data at at least one terminal of a plurality of terminals in a point-to-multipoint data network. 23. An apparatus for transmitting data to at least one receiving terminal in a communication system, said apparatus comprising: a receiver for receiving a negative acknowledge message from a receiving terminal; and a transmitter for sequentially transmitting blocks of data, said transmitter having a checksum generator for generating a checksum message in response to said negative acknowledge message. 24. An apparatus for transmitting data according to claim 23, wherein said negative acknowledge message represents a message indicating that a first error in a block of data and a second error in a subsequent block of data have been detected at said receiving terminal. 25. An apparatus for transmitting data according to claim 24, further including checksum sender for sending said checksum message. 26. An apparatus for receiving data transmitted in a communication network, said apparatus comprising: a receiver for sequentially receiving blocks of data; a detector for detecting a first error in a block of transmitted data and a second error in a subsequent block of transmitted data; and a transmitter for sending a negative acknowledge message upon detecting the second error. 27. An apparatus for receiving data according to claim 26, further comprising a checksum detector. 28. An apparatus for receiving data according to claim 27, wherein said checksum detector detects error correction blocks. 29. An apparatus for receiving data according to claim 28, further comprising a last checksum detector. 30. An apparatus for receiving data according to claim 26, further including a received data buffer which receives and stores blocks of data and error correction blocks. 31. An apparatus for receiving data according to claim 26, further including an incrementor coupled to said detector to increment a counter when an error is detected. 32. An apparatus for receiving data according to claim 31, further including an decrementor coupled to said detector to increment a counter when an error is detected. 33. An apparatus for receiving data according to claim 32, further including a counter for recording errors in said block of data received by said receiver. 34. An apparatus for receiving data according to claim 33, further including a NACK generator which generates a NACK signal when a last checksum is detected by said last checksum detector or when an error in the checksum is detected by the data checksum detector after two error blocks have been received. 35. An apparatus for receiving data according to claim 26, further including a first pointer which designates the location of a first data block having an error in the received data buffer. 36. An apparatus for receiving data according to claim 26, further including a second pointer which designates the location of a second data block having an error in the received data buffer. 37. An apparatus for receiving data according to claim 26, further including a data recovery manager which is able to recover the data in an erroneous data block, and cause said decrementor to decrement said counter. 38. The apparatus for receiving data according to claim 37, wherein said data recovery manager further initiates the sending of a NACK signal by said NACK generator when said data recovery manager receives a signal indicating that an error in a last checksum block was detected, or an error in the checksum was detected when the receiver had detected two data block errors. 39. A point-to-multipoint communication system for transmitting blocks of data, said system comprising: a transmitting terminal for transmitting said blocks of data; and a plurality of receiving terminals sequentially receiving said blocks of data, each said receiver comprising a detector for detecting a first error in a block and a second error in a subsequent block of transmitted data and a transmitter for sending a negative acknowledge message from said one of said receiving terminals upon detecting said second error.

Inventors: GIBBS JONATHAN ALISTAIR, ROBINSON WILLIAM NEIL, WHINNETT NICHOLAS WILLIAM, GIBBS, JONATHAN, ALISTAIR, ROBINSON, WILLIAM, NEIL, WHINNETT, NICHOLAS, WILLIAM
Applicants: GIBBS JONATHAN ALISTAIR, MOTOROLA LTD, ROBINSON WILLIAM NEIL, WHINNETT NICHOLAS WILLIAM, GIBBS, JONATHAN, ALISTAIR, MOTOROLA LIMITED, ROBINSON, WILLIAM, NEIL, WHINNETT, NICHOLAS, WILLIAM
Application year: 1997
Main classification: H04L1
IPC codes: H04L1