CA2084884A1 - Method for error correction of a transmitted data word - Google Patents
Method for error correction of a transmitted data wordInfo
- Publication number
- CA2084884A1 CA2084884A1 CA002084884A CA2084884A CA2084884A1 CA 2084884 A1 CA2084884 A1 CA 2084884A1 CA 002084884 A CA002084884 A CA 002084884A CA 2084884 A CA2084884 A CA 2084884A CA 2084884 A1 CA2084884 A1 CA 2084884A1
- Authority
- CA
- Canada
- Prior art keywords
- message
- dsat
- received
- data
- word
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/08—Arrangements for detecting or preventing errors in the information received by repeating transmission, e.g. Verdan system
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F11/00—Error detection; Error correction; Monitoring
- G06F11/07—Responding to the occurrence of a fault, e.g. fault tolerance
- G06F11/08—Error detection or correction by redundancy in data representation, e.g. by using checking codes
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/37—Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35
- H03M13/43—Majority logic or threshold decoding
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
- H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
- H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
- H04L1/1812—Hybrid protocols; Hybrid automatic repeat request [HARQ]
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F11/00—Error detection; Error correction; Monitoring
- G06F11/07—Responding to the occurrence of a fault, e.g. fault tolerance
- G06F11/14—Error detection or correction of the data by redundancy in operation
- G06F11/1402—Saving, restoring, recovering or retrying
- G06F11/1415—Saving, restoring, recovering or retrying at system level
Abstract
The process of the present invention error corrects subaudible data words that have been transmitted over a voice channel. A data word may be transmitted up to three times (515), if an acknowledge is not received by the transmitter. If any of the three possible repeats of the received data word are decodable an acknowledgment (525) is transmitted. If all three repeats of the data word contain uncorrectable errors a bitwise majority vote (519) is performed on the three data words. If the result of majority vote is decodable, an acknowledgment (525) is transmitted to the sender.
Description
- 1 - 2 ~ 8 ~
METHOD FOR ERROR CORRECTION OF A TRANSMlITED
DATA WORD
Fielsl of the In~ention s The present invention relates generally to the field of communications and par~icularly to error correction of sub-audible data words tran~imitted over 8 radio frequency cban-nel.
~und of the Invention W~ile operating in a cellular radiotelephone system, data messages are continuously transmittect between a ra-15 diotelephone and a base station. These me3sages includeorders requesting the radiotelephone transceiver to change transmit power level, to change channet assignment, to re-lease the call, or other similar requests. Some of these data messages are sent on the forward and re~rerse voice channels.
:2 0 In systems using subaudible data, the me~sages are inter-leaved in a continuous stream of data A system as described in U.S. Patent No. 4,984,290 (Le~ine et at.), assigned to the as-signee of t~e pre~ent invention, is an esample of Auch a sys-tem. Narrowb~nd Advanced Mobite Service (NAMPS) is an-2 5 other e~ample of such a system and i8 described in more ds-tail in Motorola NA~S Air Interface Specification Revision D, available from the assignee of the present invention. F IG. 1 and sections 2.7.2.2 and 3.7.2.2 of the above spe~ification give a more detuted description of the forward and reverse voice 3 0 channels. The format of the messa~es transmitted on both the forward and reverse channels is itentical, the difference b~
tween the two channels is the data word fo~nat.
The data word format uset OIl the narrow forward voice channel (base to radiotelephone) consists of 28 bits of content 3 5 encodet into a (40,28) e~or correct~g Bose-Chauthun-Hocquenghem (BCH) coded Manche~ter modulated data wort . , .- .
- . . . ~. . ~ ~ -.. .. . .
-: . .-.. i : ~ , 2 ~ 8 !~ 8 ~
with a distance of five sent at 100 bits/second. The BCH code allows for the correction of one bit error and is a shortened version of the standard (63,51) BCH code. The forward voice channel mes~age is transmittet after a single transmission of S a 30 bit wort synchroDization pattern sent at 200 bits/second using non-return to zero (NRZ) modulation. Four errors are allowed in the synchronization pattern. Bit synchronization is accomplishet by receiving one of seven 24-bit continuously transmittet 200 bits/second Digital Supervisory Audio Tone 10 (DSAT) sequences and maintaining 3ynchronization to it. A
(48,36) BCH coded Manchester motulated data word is used on the narrow reverse voice channel (radiotelephone to base).
The reverse voice channel message i~ similarly transInittet - -af~er a single transmission of a 30 bit word synchronization 15 pattern. Bit syncbronization on the reverse voice channel i~
accomplished with a DS~T sequencs similar to that of the forward voice channel.
Instead of multiple repeats~ as used in some typical cel-lular systems, an automatic repeat request procedure (ARQ) 2 0 is implemented for fading protection. NAMPS messages are transmitted only once with ARQ due to the low baud rate of the subaudible data. When a subaudible data word is received, an acknowledgement is sent by the receiving unit to the transmit-ting unit as is illustrated in FIG. 1. The message may be 2 5 transmitted a total of three times if an acknowledgement is not received by the sending unit. This process is illustrated in FIG. 2.
If the ratiotelephone is in a low signal level, multipath, or Rayleigh fading environment, the data word may contain 3 0 uncorrectaUe errors when it is received. As shown in FIG. 2, if the message i8 received three time~ with uncorrectable er-ror6, the message is ignored and no acknowledgement i~ sent.
There is a resulting need to correct these uncorrectable er-rors, thereby reducing the message error rate for subaudible 3 5 data communications.
.. , . ~:
.. . .
; ,.
.. : : .. ,...... .; : , .-- 3 - 2 ~
The proce~s of the preslent invention error corrects sub-audible data words that have been transmitted over a voice S channel. If any of the the three receivet possible transmis-~ion~ of the data word is decodable, an aclmowledgement is transmitted to the sender. If the data words were received three times with uncorrectable errors, a bitwise majority vote is performed on the three words. If the result from thi~ opera-tion can be decoded, an acknowledgement is transmitted to the sender. An algorithm i~ used to prevent inappropriate utiliza-tion of the majority vote process.
~IG. 1 3hows the format of the narrow analog fonvard and re~erse voice channel subaudible data streams.
PIG. 2 shows a flowc~art of t~e prior art process of re-ceiving and ac~no~vledging subaudible data word~ on a nar-2 0 row voice channel.
PlGs. 3A and B show a flowchart of the process of the present invention.
~lG. 4 shows a Uocl~ d;agram of a typical radiotele-phone in accordanoe with the present in~ention.
2 5 ~IG. 5 ~hows an e~ample of the operation of the bitwise myority vote uset in the process of the present inYention.
~lG. 6 shows esamples of the operation of the me3sage repeat counter and window timer used in the present inven-tion.
3 0 EIG. 7 shows message error probability plots for the prior art and present invention.
netailed De~cnotion of the Preferred Embodiment 3 5 The process of the present invention reduces the mes-sage error rate for subaudible data words tran~mitted on a . ~
., , ~. . ~
208~8~
voice channel of a cellular communication system. This is accomplished by performing a bitwise majority vote on three received data worts that contain more than one tmcorrectable error. To prevent inappropriate use of the majority vote S scheme, the algorithm incorporstes two t mers and a mes-sage repeat counter.
The message repeat counter is uaed to count the re-ceived trsnsmission of a data message. This counter i8 in-cremented if a synchronization pattern i3 decoded but the data 1 0 word has uncorrectable errors. When the counter reaches a value of three within the e~pected time for three transmis-sions, the ma~orit~ vote operation ia performed. The counter - - is reset after arn~ing on a voice channel, sending an AC~, when the hole or window timers espire or when the majority 1 5 vote faila.
DSAT may be 108~ due to a fade or a transition to a syn-chronization pattern. A hole timer measures the time elapsed aflcer DSAT is lost. When DSAT i~ reaquired, the timer i~
e~aluated. If the elapsed time is greater than the time it takes 2 0 to r~ce*e a combined synchronization pattern and data word (message) but less than the DSAT 1088 time, it is assumed that a mes~age may have been sent and the message repeat counter is re~et. If DSAT is lost for fi~e or more seconds, the cellular telephone call is terminated.
2 5 A window timer runs for the time it takes to receive two messages. The window timer is started when the first trans-mission of a message is detected. This timer is then active for the appro~imate length of time it ta~es to receive two addi-tional messages. If the message repeat counter has not 3 0 reached t~ree prior to the windov~ timer espinng, the mes-sage repeat counter and window timer are reset.
A flowchart of the process of the present invention is il-lustrated in FIGs. 3A and B. Referring to FIG. 3A, the pro-cess begins when the radiotelephone arrives on a nalTow voice 3 5 channel (401) and resets the message repeat counter and vrin-dow timer (403). The radiotelephone then monitors DSAT
:'~ ~' ,: ' . , . :
" . ,.:' .~' '' ' . :
2~38~9~
(405). If DSAT is not lost (409), the window timer is checked to determine if the window time is esceeded (407). If the window time is esceeded, the message repeat counter and window timer are re~et (403). If the ~indow time is not esceeded, DSAT i~ monitored (405). If DSAT is lost (409), an attempt is made to decode a synchro~ization pattern (411). If a synchro-nization pattern is decodet (413), the process receiYes a data wort (501). If no synchronization pattern is decoded (413), the DSAT hole timer is started (415). The process monitors the in-1 0 coming data stream for the DSAT pattern to return (417). If DSAT is not detected (419), the DSAT hole timer i8 checked to determine if the DSAT 1088 time has been esceeded (421). If - the DSAT 1088 time is esceeded, the call is terminated (425). If the DSAT 1088 time i3 not esceeded, DSAT is monitored (417).
1 5 If DSAT is detectsd (419), the hole timer is checlced (423) to de-termine if ths hole time i~ e~ceeded. If the hole time i~ es-ceeded, the message repeat counter and window tuner are re-set (403) and the proce~ returns to monitor DSAT (405). If the hole time i~ not e~ceeded, the process return~ to monitor 2 0 DSAT (405).
The process of the present invention continues in FIG.
3B when the ratiotelephone receives the first subautible data wort preceded by a synchronization pattern (501) and per-forms BCH error correction (503). If no uncorrectable errors 2 5 are found (505), the radiotelephone tran~ib an acknowl-edgement (ACR) to the base (525). The ACK is desc2ibed in the Motorola NAMPS Air Interface Specification Re~ision D in sections 2.4.5.1, 2.4.5.2, 2.4.5.8 and 2.7.2.2. The radiotelephone will then act on the message received and reset the repeat 3 0 counter and window timer (403). If the word contains uncor-rectable errors, t~e ra~ word is stored (507) and no ACK i8 sent to the base station. The data word messsge repeat counter is then incremented by one (509). This counter is checl~ed to determine if this i the Srst transmission of the 3 5 current data word (511). Since this is the first transmission, .;
....
: . . . .
~; ;
2~ 8~.~
ths window timer i8 startet (513) ant the proces~ returns to monitor DSAT (405).
Af'cer an appropriate time intenral, the base ~tation re-peat~ the messsge. The radiotelephone then receives the re-5 peat of the data word t501) and performs BCH error correction(503). If no uncorrectable errors are found (505), the radiotele-phone transmits an ACR to the base and acts on the message received (525). If the word contains uncorrectable errors, it is stored (507) and no ACR i~ sent. The message repeat colmter 10 is incremented agam (509). The counter is chec~ed to deter-mine if this is the first transmission of thi3 message (511) and to determine if three data words have been stored (515). If three data words with uncomctable error~ have not yet been stored, the process again returns to moDitor DSAT (405).
1 5 The radiotelephone recei~es the last repeat of the data word (501) and performs BCH error correction (503). If no un-correctable errors are fount (505), the radiotelephone trans-mits an AC~ to t~e ba~e and ac~ on the message mei~ed (525). If this wort contains uncorrectable errors, it is storet 2 0 (507), the mesulge repeat counter i8 incremented (509), and checked to determine if this is the f~rst transmission of this me~sage (511) and to detennine if three words have been stored (515). Now that the three word~ have been stored, the window timer i~ checl~ed to determine if the window time has 2 5 been esceeded (517). If the window time has been esceeded, the message repeat counter and window timer are reset (403) and the proces~ returns to monitor DSAT (405). If the window time has not been esceeded, a two-out-of-three majority vote is perfonnet on the three stored words (519). T'ne boolean for-3 0 mula for the majority ~rote operation is as follows:
error corrected result = (word1 ~ word2) + (wordl ~ word3) +
(word2 ~ word3).
3 5 An esample of the majority vote process is illustrated in FIG. 5. The first line ~hows the error free transmitted data ... . . , ~. ..-.. . - ~ .,.
.. . . . ..
7 2 ~ o ~ L9 word sent by the base station. The ne~t three lines ~how the data words as received by the radiotelephone. It can be seen that these words contain uncorrectable errors, each error des-ignated by an "~c", in various locations. Starting at bit position 0 (701) of the data words, the m~ority vote of the process of the present invention logically ANDs the logical 1 of wordl with the error (logical 0) of word2. This results in a logic~l value of 0. Nest, the logical 1 of wordl is logically ANDed with the log-ical 1 of word3 resulting in a logical 1. Last, the error of word2 1 0 i~ logically ANDed with the logical 1 of word3 result~g in a logical 0. The logical results of the ANDing operations sre then logically ORed to produce an error free logical 1 for bit position 0. These bitwise logical operations, using the above boolean formula, are represented as:
` 15 error corrected bit result = (1~ 0) + (1- 1) + (0 1) =0+1+0 =1.
2 0 This operation i8 repeated for each bit position of the three data worts. If an error is contained in the same bit position on two or three of the repeab, the error ~ill be contsined in the resulting voted word. If only one e~or i~ contained in the ~oted result, it will be corrected by the BCH decoder.
2 5 Tho result of the majority vote operation then goes through BCH error correction (521) which can correct one bit error. If the process of the present invention produces a result that can be decoded by the radiotelephone (523), an ACK is tran~mitted to the sender and the mesBage i8 acted upon (525).
3 0 If the result of the proce~s does not produce a decodable result, the message repeat counter and window timer are reset (403) and the data word is ignored. The process then returns to monitor DSAT (405).
FIG. 6A shows what would happen if a window timer ~ r 3 5 were not used. Although three transmis~ions of me~sage 1 were attempted, the second transmission of message 1 was , , , . , . i . -;
: . . : . , , - 8 - 2 ~ 8 i~
108t, thus cau~g the hole (601). The me~sage repeat counter would not be reset and the two received transmissions of mes-sage 1(602 and 603) would be rnajority voted with the first re-ceived transmission of message 2 (604). If this occurs, mes-sage 1 will be missed and there will be no possibility for mes-~age 2 to be majority voted.
An esample of the operation of the window timer and message repeat counter is shown in FIG. 6B. To prevent im-proper utilization of the majority vote operation, the majority vob i~ only u8ed when the message repeat counter is equal to three and the window timer i8 active and not timed out.
FIG. 6C illustrates a case where one of the three ~- - transmissions of a message is missed (610). The mes~age re-peat counter does not reach a value of three before the window timer times out and no vote occurs. Although mes~age 1 i8 missed, message 2 may ~till be received.
The followi~g calculation deter~ines the NAMPS
Me~sage Error Rate ta}ing into accoullt the transmission and detection of both the synchronization word and the (40,28) 2 0 modified BCII encoded data word.
S~chronization in the NAMPS system is accomplished by the detection of a 30-bit sync~ronization word ba~ing t~e fol-lo~nng sequence: "195A99A6~. T'ni~ syn~hronization word is sent at 200 bpc, is NRZ coded, and its decoding allows for up to 2 5 four bit err~ra Upon successful s~nchronization to the syn-~hroDization word, the t40,28) BCH Manchester coded data word, w~ich is sent at 100 bp~, is detected. The Manchester coded data i8 u~ed for the BCH coded words and the NRZ coded data is u~ed for the synchroniza~on word.
3 0 The a~erage static bit error rate can be shown to be a function of the recei~ed RF ~ignal le~rel. As the RF signal level is increased, the average static bit error rate decreases. For a gnen average static bit error rate for the 200 bps NRZ coded synchronization word, the probability of a successfill 3ynchro-3 5 nization detect, allo~nng for four or fewer errors, i~ Psd.
.
.. . . . . . . .
. ~, ,, . . .
,.
2 ~ 8 f' P~d = (1-P2b)30 + 30 P2b(1-P2b)2~ + 435 P2b2(1-P2b)28 + 4060 P2b3(1-P2b)2~ + 27405 P2b4(1-P2bY~
S After succ~ssful synchronization to the synchronization word, the data wort i8 read in. The probability of a word error is Pword.
Pword = 1- (1-Pb)~ - 40 Pb (1-Pb)3 The probability of a me~sage error without the ARQ is Pw~. It is a function of both synchronization and BCH word detection. Pws i9 also the probability of a message error oc-curring during ~e first ARQ attempt.
Pws s 1 (P~dX1- Pword) The ARQ is talcen into account b~r as~um~g that the three message attempts are independent of each other.
2 0 Pmer2nt i~ the probability of a message error for both the com-bined first and second ARQ attempt.
Pmer2nd = (p~8)2 2 5 Pmer il~ the probability of mes~age error with all tllree message attempt~ taken into account.
Pmer = (PWB)9 3 0 The probability of a message error du~ing the first at-tempt, the combined first and second ARQ attempt, and the r - combined Srst, second and third ARQ attempt in the NAMPS
system, with respect to meived RF signal level, ~lave been plotted in FIG. 7.
3 5 The following calculation determines th~ NAMP~3 Message Error Rate filrther taking into account the redun-.. , -10- 2t!~4?~' dancy scheme of the present illvention which makes use of two~out-of-three majority-vote poet processing on each bit.
Also taken into account i9 the correlation between ARQ de-coder failure and the majority-~rote process.
The stalldard static NAMPS ARQ message error proba-bility remains as stated abo~e. Should the three (ARQ) mes-sage attempta fail to detect a ~ralid data word, then a final at-tempt can be made to recover the message by doing a bit-by-bit two-out-of-three majority-vote on the three received data I O words. For the mobile to realize t~at it failed to detect any of the three ARQ attempt~ implies a special condition in which the bit error rate was higb enough to produce at least two er-rors in each of the individual three word attempts but not high enough to prohibit synchronization to the ~ynchronization 1 5 word. With at least two bits in error for each forty bit data word9 t}lis result~ in a per bit error probability (Pbw) of at least 0.05 even when the channel bit error rate is les~ than 0.05.
T'nerefore, for the calculation the ~alue of Pbw is set to 0.05 for a~ Pb ~ 0.06.
2 0 Applying two-out-of-three voting on each of the indi~id-ual bih in the three 40 bit word~ results in the improved bit er-ror rate Pb2ors~
Pb2 a~-3 = 3 Pbw2 (1 - Pbw) + Pbw9 where: Pbw = P~ for Pb ~ 0.05 and Pbw = 0.05 for Pb ~= O.OS
The above calculation can not use the ~ame bit error 3 0 probability Pb from the earlier calculations that were u~ed for the standard ARQ calculations. This i8 becau~e the bit error probability Pbw i8 now related to the failure of t~e standard ARQ algorithm which forces the post proce~g to be con-ducted. ~ other words, the bit error probability Pbw may be 3 5 statistically muc~ worse than the channel bit error probability .,. . .-: ,. : - .
. . ; . ., .: ~ . : . --~: . -~84884 Pb, because the failure of the error control coding on these words which allows the majolity vote to proceed bia3es Pbw.
With the calculation of Pbw in the majority-voted word, the probability of a word error allowing for one bit of error cor-5 rection i8 P'word.
P'word = 1 - (l-pb2 of 3)4 - 40 Pb2 of 3 (l-Pb2 of 3)39 The combined me3sage error probability using standard 1 0 A~Q ant majority voting is P'mer i~ then calculated wing:
P'mer z (Pws~3 (P'word) The probability of a message error includ;ng the above l 5 improvements (P'mer) for the NA~ system has been plot-ted in Fig 7 for compan~on to the message error rates of the standard NAMP8 syotem. A~ illustrated in FIG. 7, the NA~ message error rate employiDg the two-out-of-three majority-voting scheme of the present inve~tion prondes im-2 0 proved message error rates o~er the original NAMPS scheme.
The preferred embodiment of the process of the presentinvention i8 perfo~ed by the microprocessor (401) of a ra-diotelephone or other communication dence, howe~er, the process could be performed by hardware. A simple bloclc dia-2 5 gram of a typical radiotelephone is illustrated in ErIG. 4 anddescribed in Motorola Micro T A-C manual #68P81150E49 a~ailable from the assignee of the present invention.
In 3um~ar~, the process of the present in~ention can den~e a decodable data word from three received data words 3 0 hanng uncorrectable errors. A~ seen by the above calcula-tions and graph, this improves the message elmr rate on nar-row analog voice channels.
., -- .
.. . . . . - . . . . ..
,:
, : :
METHOD FOR ERROR CORRECTION OF A TRANSMlITED
DATA WORD
Fielsl of the In~ention s The present invention relates generally to the field of communications and par~icularly to error correction of sub-audible data words tran~imitted over 8 radio frequency cban-nel.
~und of the Invention W~ile operating in a cellular radiotelephone system, data messages are continuously transmittect between a ra-15 diotelephone and a base station. These me3sages includeorders requesting the radiotelephone transceiver to change transmit power level, to change channet assignment, to re-lease the call, or other similar requests. Some of these data messages are sent on the forward and re~rerse voice channels.
:2 0 In systems using subaudible data, the me~sages are inter-leaved in a continuous stream of data A system as described in U.S. Patent No. 4,984,290 (Le~ine et at.), assigned to the as-signee of t~e pre~ent invention, is an esample of Auch a sys-tem. Narrowb~nd Advanced Mobite Service (NAMPS) is an-2 5 other e~ample of such a system and i8 described in more ds-tail in Motorola NA~S Air Interface Specification Revision D, available from the assignee of the present invention. F IG. 1 and sections 2.7.2.2 and 3.7.2.2 of the above spe~ification give a more detuted description of the forward and reverse voice 3 0 channels. The format of the messa~es transmitted on both the forward and reverse channels is itentical, the difference b~
tween the two channels is the data word fo~nat.
The data word format uset OIl the narrow forward voice channel (base to radiotelephone) consists of 28 bits of content 3 5 encodet into a (40,28) e~or correct~g Bose-Chauthun-Hocquenghem (BCH) coded Manche~ter modulated data wort . , .- .
- . . . ~. . ~ ~ -.. .. . .
-: . .-.. i : ~ , 2 ~ 8 !~ 8 ~
with a distance of five sent at 100 bits/second. The BCH code allows for the correction of one bit error and is a shortened version of the standard (63,51) BCH code. The forward voice channel mes~age is transmittet after a single transmission of S a 30 bit wort synchroDization pattern sent at 200 bits/second using non-return to zero (NRZ) modulation. Four errors are allowed in the synchronization pattern. Bit synchronization is accomplishet by receiving one of seven 24-bit continuously transmittet 200 bits/second Digital Supervisory Audio Tone 10 (DSAT) sequences and maintaining 3ynchronization to it. A
(48,36) BCH coded Manchester motulated data word is used on the narrow reverse voice channel (radiotelephone to base).
The reverse voice channel message i~ similarly transInittet - -af~er a single transmission of a 30 bit word synchronization 15 pattern. Bit syncbronization on the reverse voice channel i~
accomplished with a DS~T sequencs similar to that of the forward voice channel.
Instead of multiple repeats~ as used in some typical cel-lular systems, an automatic repeat request procedure (ARQ) 2 0 is implemented for fading protection. NAMPS messages are transmitted only once with ARQ due to the low baud rate of the subaudible data. When a subaudible data word is received, an acknowledgement is sent by the receiving unit to the transmit-ting unit as is illustrated in FIG. 1. The message may be 2 5 transmitted a total of three times if an acknowledgement is not received by the sending unit. This process is illustrated in FIG. 2.
If the ratiotelephone is in a low signal level, multipath, or Rayleigh fading environment, the data word may contain 3 0 uncorrectaUe errors when it is received. As shown in FIG. 2, if the message i8 received three time~ with uncorrectable er-ror6, the message is ignored and no acknowledgement i~ sent.
There is a resulting need to correct these uncorrectable er-rors, thereby reducing the message error rate for subaudible 3 5 data communications.
.. , . ~:
.. . .
; ,.
.. : : .. ,...... .; : , .-- 3 - 2 ~
The proce~s of the preslent invention error corrects sub-audible data words that have been transmitted over a voice S channel. If any of the the three receivet possible transmis-~ion~ of the data word is decodable, an aclmowledgement is transmitted to the sender. If the data words were received three times with uncorrectable errors, a bitwise majority vote is performed on the three words. If the result from thi~ opera-tion can be decoded, an acknowledgement is transmitted to the sender. An algorithm i~ used to prevent inappropriate utiliza-tion of the majority vote process.
~IG. 1 3hows the format of the narrow analog fonvard and re~erse voice channel subaudible data streams.
PIG. 2 shows a flowc~art of t~e prior art process of re-ceiving and ac~no~vledging subaudible data word~ on a nar-2 0 row voice channel.
PlGs. 3A and B show a flowchart of the process of the present invention.
~lG. 4 shows a Uocl~ d;agram of a typical radiotele-phone in accordanoe with the present in~ention.
2 5 ~IG. 5 ~hows an e~ample of the operation of the bitwise myority vote uset in the process of the present inYention.
~lG. 6 shows esamples of the operation of the me3sage repeat counter and window timer used in the present inven-tion.
3 0 EIG. 7 shows message error probability plots for the prior art and present invention.
netailed De~cnotion of the Preferred Embodiment 3 5 The process of the present invention reduces the mes-sage error rate for subaudible data words tran~mitted on a . ~
., , ~. . ~
208~8~
voice channel of a cellular communication system. This is accomplished by performing a bitwise majority vote on three received data worts that contain more than one tmcorrectable error. To prevent inappropriate use of the majority vote S scheme, the algorithm incorporstes two t mers and a mes-sage repeat counter.
The message repeat counter is uaed to count the re-ceived trsnsmission of a data message. This counter i8 in-cremented if a synchronization pattern i3 decoded but the data 1 0 word has uncorrectable errors. When the counter reaches a value of three within the e~pected time for three transmis-sions, the ma~orit~ vote operation ia performed. The counter - - is reset after arn~ing on a voice channel, sending an AC~, when the hole or window timers espire or when the majority 1 5 vote faila.
DSAT may be 108~ due to a fade or a transition to a syn-chronization pattern. A hole timer measures the time elapsed aflcer DSAT is lost. When DSAT i~ reaquired, the timer i~
e~aluated. If the elapsed time is greater than the time it takes 2 0 to r~ce*e a combined synchronization pattern and data word (message) but less than the DSAT 1088 time, it is assumed that a mes~age may have been sent and the message repeat counter is re~et. If DSAT is lost for fi~e or more seconds, the cellular telephone call is terminated.
2 5 A window timer runs for the time it takes to receive two messages. The window timer is started when the first trans-mission of a message is detected. This timer is then active for the appro~imate length of time it ta~es to receive two addi-tional messages. If the message repeat counter has not 3 0 reached t~ree prior to the windov~ timer espinng, the mes-sage repeat counter and window timer are reset.
A flowchart of the process of the present invention is il-lustrated in FIGs. 3A and B. Referring to FIG. 3A, the pro-cess begins when the radiotelephone arrives on a nalTow voice 3 5 channel (401) and resets the message repeat counter and vrin-dow timer (403). The radiotelephone then monitors DSAT
:'~ ~' ,: ' . , . :
" . ,.:' .~' '' ' . :
2~38~9~
(405). If DSAT is not lost (409), the window timer is checked to determine if the window time is esceeded (407). If the window time is esceeded, the message repeat counter and window timer are re~et (403). If the ~indow time is not esceeded, DSAT i~ monitored (405). If DSAT is lost (409), an attempt is made to decode a synchro~ization pattern (411). If a synchro-nization pattern is decodet (413), the process receiYes a data wort (501). If no synchronization pattern is decoded (413), the DSAT hole timer is started (415). The process monitors the in-1 0 coming data stream for the DSAT pattern to return (417). If DSAT is not detected (419), the DSAT hole timer i8 checked to determine if the DSAT 1088 time has been esceeded (421). If - the DSAT 1088 time is esceeded, the call is terminated (425). If the DSAT 1088 time i3 not esceeded, DSAT is monitored (417).
1 5 If DSAT is detectsd (419), the hole timer is checlced (423) to de-termine if ths hole time i~ e~ceeded. If the hole time i~ es-ceeded, the message repeat counter and window tuner are re-set (403) and the proce~ returns to monitor DSAT (405). If the hole time i~ not e~ceeded, the process return~ to monitor 2 0 DSAT (405).
The process of the present invention continues in FIG.
3B when the ratiotelephone receives the first subautible data wort preceded by a synchronization pattern (501) and per-forms BCH error correction (503). If no uncorrectable errors 2 5 are found (505), the radiotelephone tran~ib an acknowl-edgement (ACR) to the base (525). The ACK is desc2ibed in the Motorola NAMPS Air Interface Specification Re~ision D in sections 2.4.5.1, 2.4.5.2, 2.4.5.8 and 2.7.2.2. The radiotelephone will then act on the message received and reset the repeat 3 0 counter and window timer (403). If the word contains uncor-rectable errors, t~e ra~ word is stored (507) and no ACK i8 sent to the base station. The data word messsge repeat counter is then incremented by one (509). This counter is checl~ed to determine if this i the Srst transmission of the 3 5 current data word (511). Since this is the first transmission, .;
....
: . . . .
~; ;
2~ 8~.~
ths window timer i8 startet (513) ant the proces~ returns to monitor DSAT (405).
Af'cer an appropriate time intenral, the base ~tation re-peat~ the messsge. The radiotelephone then receives the re-5 peat of the data word t501) and performs BCH error correction(503). If no uncorrectable errors are found (505), the radiotele-phone transmits an ACR to the base and acts on the message received (525). If the word contains uncorrectable errors, it is stored (507) and no ACR i~ sent. The message repeat colmter 10 is incremented agam (509). The counter is chec~ed to deter-mine if this is the first transmission of thi3 message (511) and to determine if three data words have been stored (515). If three data words with uncomctable error~ have not yet been stored, the process again returns to moDitor DSAT (405).
1 5 The radiotelephone recei~es the last repeat of the data word (501) and performs BCH error correction (503). If no un-correctable errors are fount (505), the radiotelephone trans-mits an AC~ to t~e ba~e and ac~ on the message mei~ed (525). If this wort contains uncorrectable errors, it is storet 2 0 (507), the mesulge repeat counter i8 incremented (509), and checked to determine if this is the f~rst transmission of this me~sage (511) and to detennine if three words have been stored (515). Now that the three word~ have been stored, the window timer i~ checl~ed to determine if the window time has 2 5 been esceeded (517). If the window time has been esceeded, the message repeat counter and window timer are reset (403) and the proces~ returns to monitor DSAT (405). If the window time has not been esceeded, a two-out-of-three majority vote is perfonnet on the three stored words (519). T'ne boolean for-3 0 mula for the majority ~rote operation is as follows:
error corrected result = (word1 ~ word2) + (wordl ~ word3) +
(word2 ~ word3).
3 5 An esample of the majority vote process is illustrated in FIG. 5. The first line ~hows the error free transmitted data ... . . , ~. ..-.. . - ~ .,.
.. . . . ..
7 2 ~ o ~ L9 word sent by the base station. The ne~t three lines ~how the data words as received by the radiotelephone. It can be seen that these words contain uncorrectable errors, each error des-ignated by an "~c", in various locations. Starting at bit position 0 (701) of the data words, the m~ority vote of the process of the present invention logically ANDs the logical 1 of wordl with the error (logical 0) of word2. This results in a logic~l value of 0. Nest, the logical 1 of wordl is logically ANDed with the log-ical 1 of word3 resulting in a logical 1. Last, the error of word2 1 0 i~ logically ANDed with the logical 1 of word3 result~g in a logical 0. The logical results of the ANDing operations sre then logically ORed to produce an error free logical 1 for bit position 0. These bitwise logical operations, using the above boolean formula, are represented as:
` 15 error corrected bit result = (1~ 0) + (1- 1) + (0 1) =0+1+0 =1.
2 0 This operation i8 repeated for each bit position of the three data worts. If an error is contained in the same bit position on two or three of the repeab, the error ~ill be contsined in the resulting voted word. If only one e~or i~ contained in the ~oted result, it will be corrected by the BCH decoder.
2 5 Tho result of the majority vote operation then goes through BCH error correction (521) which can correct one bit error. If the process of the present invention produces a result that can be decoded by the radiotelephone (523), an ACK is tran~mitted to the sender and the mesBage i8 acted upon (525).
3 0 If the result of the proce~s does not produce a decodable result, the message repeat counter and window timer are reset (403) and the data word is ignored. The process then returns to monitor DSAT (405).
FIG. 6A shows what would happen if a window timer ~ r 3 5 were not used. Although three transmis~ions of me~sage 1 were attempted, the second transmission of message 1 was , , , . , . i . -;
: . . : . , , - 8 - 2 ~ 8 i~
108t, thus cau~g the hole (601). The me~sage repeat counter would not be reset and the two received transmissions of mes-sage 1(602 and 603) would be rnajority voted with the first re-ceived transmission of message 2 (604). If this occurs, mes-sage 1 will be missed and there will be no possibility for mes-~age 2 to be majority voted.
An esample of the operation of the window timer and message repeat counter is shown in FIG. 6B. To prevent im-proper utilization of the majority vote operation, the majority vob i~ only u8ed when the message repeat counter is equal to three and the window timer i8 active and not timed out.
FIG. 6C illustrates a case where one of the three ~- - transmissions of a message is missed (610). The mes~age re-peat counter does not reach a value of three before the window timer times out and no vote occurs. Although mes~age 1 i8 missed, message 2 may ~till be received.
The followi~g calculation deter~ines the NAMPS
Me~sage Error Rate ta}ing into accoullt the transmission and detection of both the synchronization word and the (40,28) 2 0 modified BCII encoded data word.
S~chronization in the NAMPS system is accomplished by the detection of a 30-bit sync~ronization word ba~ing t~e fol-lo~nng sequence: "195A99A6~. T'ni~ syn~hronization word is sent at 200 bpc, is NRZ coded, and its decoding allows for up to 2 5 four bit err~ra Upon successful s~nchronization to the syn-~hroDization word, the t40,28) BCH Manchester coded data word, w~ich is sent at 100 bp~, is detected. The Manchester coded data i8 u~ed for the BCH coded words and the NRZ coded data is u~ed for the synchroniza~on word.
3 0 The a~erage static bit error rate can be shown to be a function of the recei~ed RF ~ignal le~rel. As the RF signal level is increased, the average static bit error rate decreases. For a gnen average static bit error rate for the 200 bps NRZ coded synchronization word, the probability of a successfill 3ynchro-3 5 nization detect, allo~nng for four or fewer errors, i~ Psd.
.
.. . . . . . . .
. ~, ,, . . .
,.
2 ~ 8 f' P~d = (1-P2b)30 + 30 P2b(1-P2b)2~ + 435 P2b2(1-P2b)28 + 4060 P2b3(1-P2b)2~ + 27405 P2b4(1-P2bY~
S After succ~ssful synchronization to the synchronization word, the data wort i8 read in. The probability of a word error is Pword.
Pword = 1- (1-Pb)~ - 40 Pb (1-Pb)3 The probability of a me~sage error without the ARQ is Pw~. It is a function of both synchronization and BCH word detection. Pws i9 also the probability of a message error oc-curring during ~e first ARQ attempt.
Pws s 1 (P~dX1- Pword) The ARQ is talcen into account b~r as~um~g that the three message attempts are independent of each other.
2 0 Pmer2nt i~ the probability of a message error for both the com-bined first and second ARQ attempt.
Pmer2nd = (p~8)2 2 5 Pmer il~ the probability of mes~age error with all tllree message attempt~ taken into account.
Pmer = (PWB)9 3 0 The probability of a message error du~ing the first at-tempt, the combined first and second ARQ attempt, and the r - combined Srst, second and third ARQ attempt in the NAMPS
system, with respect to meived RF signal level, ~lave been plotted in FIG. 7.
3 5 The following calculation determines th~ NAMP~3 Message Error Rate filrther taking into account the redun-.. , -10- 2t!~4?~' dancy scheme of the present illvention which makes use of two~out-of-three majority-vote poet processing on each bit.
Also taken into account i9 the correlation between ARQ de-coder failure and the majority-~rote process.
The stalldard static NAMPS ARQ message error proba-bility remains as stated abo~e. Should the three (ARQ) mes-sage attempta fail to detect a ~ralid data word, then a final at-tempt can be made to recover the message by doing a bit-by-bit two-out-of-three majority-vote on the three received data I O words. For the mobile to realize t~at it failed to detect any of the three ARQ attempt~ implies a special condition in which the bit error rate was higb enough to produce at least two er-rors in each of the individual three word attempts but not high enough to prohibit synchronization to the ~ynchronization 1 5 word. With at least two bits in error for each forty bit data word9 t}lis result~ in a per bit error probability (Pbw) of at least 0.05 even when the channel bit error rate is les~ than 0.05.
T'nerefore, for the calculation the ~alue of Pbw is set to 0.05 for a~ Pb ~ 0.06.
2 0 Applying two-out-of-three voting on each of the indi~id-ual bih in the three 40 bit word~ results in the improved bit er-ror rate Pb2ors~
Pb2 a~-3 = 3 Pbw2 (1 - Pbw) + Pbw9 where: Pbw = P~ for Pb ~ 0.05 and Pbw = 0.05 for Pb ~= O.OS
The above calculation can not use the ~ame bit error 3 0 probability Pb from the earlier calculations that were u~ed for the standard ARQ calculations. This i8 becau~e the bit error probability Pbw i8 now related to the failure of t~e standard ARQ algorithm which forces the post proce~g to be con-ducted. ~ other words, the bit error probability Pbw may be 3 5 statistically muc~ worse than the channel bit error probability .,. . .-: ,. : - .
. . ; . ., .: ~ . : . --~: . -~84884 Pb, because the failure of the error control coding on these words which allows the majolity vote to proceed bia3es Pbw.
With the calculation of Pbw in the majority-voted word, the probability of a word error allowing for one bit of error cor-5 rection i8 P'word.
P'word = 1 - (l-pb2 of 3)4 - 40 Pb2 of 3 (l-Pb2 of 3)39 The combined me3sage error probability using standard 1 0 A~Q ant majority voting is P'mer i~ then calculated wing:
P'mer z (Pws~3 (P'word) The probability of a message error includ;ng the above l 5 improvements (P'mer) for the NA~ system has been plot-ted in Fig 7 for compan~on to the message error rates of the standard NAMP8 syotem. A~ illustrated in FIG. 7, the NA~ message error rate employiDg the two-out-of-three majority-voting scheme of the present inve~tion prondes im-2 0 proved message error rates o~er the original NAMPS scheme.
The preferred embodiment of the process of the presentinvention i8 perfo~ed by the microprocessor (401) of a ra-diotelephone or other communication dence, howe~er, the process could be performed by hardware. A simple bloclc dia-2 5 gram of a typical radiotelephone is illustrated in ErIG. 4 anddescribed in Motorola Micro T A-C manual #68P81150E49 a~ailable from the assignee of the present invention.
In 3um~ar~, the process of the present in~ention can den~e a decodable data word from three received data words 3 0 hanng uncorrectable errors. A~ seen by the above calcula-tions and graph, this improves the message elmr rate on nar-row analog voice channels.
., -- .
.. . . . . - . . . . ..
,:
, : :
Claims (8)
1. A method for error correction of a plurality of data words transmitted over a communication channel having a digital supervisory audio tone (DSAT) and a synchronization pattern, each data word comprising a plurality of data bits, the method comprising the steps of:
a) monitoring DSAT;
b) if DSAT is no longer received, decoding the synchro-nization pattern;
c) if the synchronization pattern is decodable, receiving a data word;
d) if the data word is received substantially error free, transmitting an acknowledge word;
e) if the data word is not received substantially error free, storing the data word;
f) if the data word is not received substantially error free, repeating from step a until the plurality of data words have been received;
g) if the plurality of data words are not received substan-tially error free, performing a bitwise majority vote on the plu-rality of data words, thereby producing a corrected data word;
and h) if the corrected data word is substantially error free, transmitting the acknowledge word.
a) monitoring DSAT;
b) if DSAT is no longer received, decoding the synchro-nization pattern;
c) if the synchronization pattern is decodable, receiving a data word;
d) if the data word is received substantially error free, transmitting an acknowledge word;
e) if the data word is not received substantially error free, storing the data word;
f) if the data word is not received substantially error free, repeating from step a until the plurality of data words have been received;
g) if the plurality of data words are not received substan-tially error free, performing a bitwise majority vote on the plu-rality of data words, thereby producing a corrected data word;
and h) if the corrected data word is substantially error free, transmitting the acknowledge word.
2. The method of claim 1 and further including the step of if DSAT is no longer received, determining a DSAT loss time.
3. The method of claim 2 and further including the step of if the DSAT loss the is greater than a predetermined time, terminating communication.
4. The method of claim 1 wherein the plurality of data words is equal to three data words.
5. A method for detecting a missed message of a plurality of messages in a radio frequency communication system having a synchronization pattern, each message and synchronization pattern being transmitted to a communication device for a predetermined time, the communication device having a message repeat counter, the method comprising the steps of:
monitoring a digital supervisory audio tone (DSAT);
if DSAT is no longer received, decoding the synchro-nization pattern; and if the synchronization pattern is not decodable and if DSAT is no longer received for the predetermined time, reset-ting the message repeat counter.
monitoring a digital supervisory audio tone (DSAT);
if DSAT is no longer received, decoding the synchro-nization pattern; and if the synchronization pattern is not decodable and if DSAT is no longer received for the predetermined time, reset-ting the message repeat counter.
6. The method of claim 5 wherein the predetermined time is equal to a time required to received a combined message and synchronization pattern.
7. A method for prevention of inappropriate use of a majority vote scheme in a radio frequency (RF) communication system comprising a plurality of communication devices, a plurality of messages being transmitted within the RF communication system, at least one communication device having a message repeat counter and a message window timer for determining a window time, the method comprised of the steps of:
a) starting the window timer upon receiving a first message; and b) if the plurality of messages are not received during the window time, resetting the message repeat counter and window timer.
a) starting the window timer upon receiving a first message; and b) if the plurality of messages are not received during the window time, resetting the message repeat counter and window timer.
8. The method of claim 7 wherein the window time is a time required for the communication device to receive two mes-sages.
Applications Claiming Priority (2)
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|---|---|---|---|
| US07/704,508 US5241548A (en) | 1991-05-23 | 1991-05-23 | Method for error correction of a transmitted data word |
| US704,508 | 1991-05-23 |
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| CA2084884A1 true CA2084884A1 (en) | 1992-11-24 |
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| CA002084884A Abandoned CA2084884A1 (en) | 1991-05-23 | 1992-04-17 | Method for error correction of a transmitted data word |
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| JP (1) | JP2661368B2 (en) |
| KR (1) | KR950015190B1 (en) |
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| GB (1) | GB2261802B (en) |
| MX (1) | MX9202376A (en) |
| WO (1) | WO1992021085A1 (en) |
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| JP3229070B2 (en) * | 1993-06-01 | 2001-11-12 | 三菱電機株式会社 | Majority circuit and control unit and majority integrated semiconductor circuit |
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| US5603013A (en) * | 1993-06-03 | 1997-02-11 | Texas Instruments Incorporated | Method and apparatus for noise reduction of cyclic signal by selecting majority logic state of corresponding portions of plural cycles |
| US5426653A (en) * | 1993-07-28 | 1995-06-20 | Motorola, Inc. | Method and apparatus for performing error correction on a signal received by a radio communication device |
| US5995559A (en) * | 1995-08-31 | 1999-11-30 | Telefonaktiebolaget Lm Ericsson | Methods for improved communication using repeated words |
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| SE511310C2 (en) * | 1997-05-20 | 1999-09-06 | Ericsson Telefon Ab L M | Method for bit detection in a radio communication system |
| US6044486A (en) * | 1997-09-11 | 2000-03-28 | Uniden America Corporation | Method and device for majority vote optimization over wireless communication channels |
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| US6523148B1 (en) * | 1999-08-19 | 2003-02-18 | Koninlijke Philips Electronics N.,V. | Majority decision method for improved frame error rate in the digital enhanced cordless telecommunications system |
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| WO2007019533A2 (en) * | 2005-08-04 | 2007-02-15 | R2Di, Llc | System and methods for aligning capture and playback clocks in a wireless digital audio distribution system |
| US8392810B2 (en) * | 2010-01-25 | 2013-03-05 | Qualcomm Incorporated | Majority vote error correction |
| KR102050896B1 (en) * | 2013-02-19 | 2019-12-02 | 삼성전자주식회사 | Memory controller and operating method of the same |
| FR3024932B1 (en) * | 2014-08-14 | 2017-12-01 | Sagem Defense Securite | METHOD FOR TRANSMITTING DATA WITH IMPROVED ROBUSTNESS AND SET OF DEVICES FOR IMPLEMENTING IT |
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-
1991
- 1991-05-23 US US07/704,508 patent/US5241548A/en not_active Expired - Lifetime
-
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- 1992-04-17 BR BR9205293A patent/BR9205293A/en not_active Application Discontinuation
- 1992-04-17 JP JP5500030A patent/JP2661368B2/en not_active Expired - Lifetime
- 1992-04-17 CA CA002084884A patent/CA2084884A1/en not_active Abandoned
- 1992-04-17 WO PCT/US1992/003153 patent/WO1992021085A1/en active Application Filing
- 1992-04-17 GB GB9301030A patent/GB2261802B/en not_active Expired - Fee Related
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1993
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1994
- 1994-04-17 KR KR1019930700063A patent/KR950015190B1/en not_active IP Right Cessation
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| GB9301030D0 (en) | 1993-03-17 |
| MX9202376A (en) | 1992-11-01 |
| KR930701790A (en) | 1993-06-12 |
| KR950015190B1 (en) | 1995-12-23 |
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| WO1992021085A1 (en) | 1992-11-26 |
| GB2261802A (en) | 1993-05-26 |
| JPH05508508A (en) | 1993-11-25 |
| JP2661368B2 (en) | 1997-10-08 |
| US5241548A (en) | 1993-08-31 |
| BR9205293A (en) | 1994-06-28 |
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