BURST MODE PREAMBLE

Abstract: In a burst mode communications system (figs. 1 and 2), bursts of information with a known preamble (402), fig. 4 are transmitted from a burst mode transmitter and are received by a burst mode receiver which uses the preamble to determine the presence of a burst and to determine the correct carrier frequency, carrier phase, symbol clock frequency and symbol clock phase in order to correctly recover the symbols transmitted, and thus the information contained within the burst. The present invention involves a method of burst transmission using a preamble (402) which contains an initial pulse (420) and a Barker sequence (440) following the initial pulse separated by a dead time (430) period equal to the transmission time of at least one symbol. The preamble can be transmitted on orthogonal carriers (fig. 5) and the order to the Barker sequence (440) can be reversed and multiplied by minus one on one of the carrier to reduce the probability that noise events on both carriers make the Barker sequence less detectable on each channel.

Description: Title of the invention Burst Mode Preamble Inventor Lawrence Ebπnger Cross-References This application claims the benefit of U.S Provisional Application No. 60/005,444 filed on October 12, 1995, entitled "Preamble for Burst Mode Reception. " of which Lawrence Ebπnger is the inventor, with attorney docket number NP2008. Background Burst mode transmitters generally need to transmit a training sequence or preamble pπor to the transmission of data so that a burst mode receiv er can obtain the correct clock sample phase and earner phase to recover the data There is a need for a shoπ preamble which contains the maximum amount of information regarding the burst and which can effectively be used b\ a number of different types of burst mode receivers to detect the presence of the burst and to recover clock sample phase and carrier phase Summary of the Invention In a burst mode communications system hav ing a burst mode transmitter using Quadrature Phase Shift Keying modulation, bursts containing a preamble and data are transmitted by first transmitting a pulse of one to three symbols in length, waiting during a dead time of one to three symbols in length during which time no symbols are transmitted, transmitting a Barker sequence of at least seven bits in length in both the 1 and Q channels, and transmitting the data. To reduce the effect of the noise on the Barker sequence, the sequence can be reversed and inverted in the I and Q channels such that the I channel contains a Barker sequence and the Q channel contains the same Barker sequence in reverse order and multiplied by minus 1. An advantage of the present invention is that many different types of burst mode receivers can be used to detect the burst and recover the transmitted data based on a short preamble Brief Description of the Drawings Fig 1 represents a shared media with multiple burst mode transmitters and a single burst mode receiver Fig 2 illustrates a Fiber-to-the-Curb (FTTC) network with a coaxial drop cables to the residence which provides connectivity to a number of devices in the residence and over the shared media formed by the coaxial drop cable, splitter, and in-home coaxial wiring Fig 3 illustrates a spectrum of forward and retum signals on the coaxial drop in a FTTC network Fig 4 illustrates the format of a burst transmission Fig 5 illustrates a table representing the symbol values in the preamble Description of Preferred Embodiments I. Introduction The general form of a burst mode communications system is illustrated in Fig 1 , and is formed when multiple burst mode transmitters I 10 transmit signals over a shared media 100 which are received a burst mode receiv er 120 The shared media can be of a multitude of forms including free space, twisted pair copper cable in a bus configuration, a passive optical network, or a shared coaxial cable network comprised of coaxial cable and splitters Examples of applications using shared media include local area networks cellular phone systems, and bi-directional cable TV systems Fig. 2 illustrates a Fiber to the Curb (FTTC ) network with a subscriber coaxial cable network which forms a shared media In the FTTC network illustrated, a Host Digital Terminal (HDT) 230 is connected to the Public Switched Telecommunications Network (PSTN) 210 as well as an Asynchronous Transfer Mode (ATM) network 220, and to one or more Optical Network Units (ONUs) 240 via optical fiber 260 A Broadband Interface Unit (BIU) 250 ONU 240 contains transmit and receive circuits for sendine signals to devices in a residence 290 The devices in the residence 290 are connected to BIU 250 via a subscriber coaxial cable network, which when used herein, is defined as a network comprised of a coaxial drop cable 270, splitter 280. and m-home coaxial wiring 272 which connects and individual residence 290 to the BIU 250 Use of the term subscriber indicates that the customer in the residence is a subscriber or potential subscriber to the services provided over the FTTC telecommunications network The subscriber coaxial cable network forms a shared media since it allows all devices connected to it to access the BIU 250 with no active switching As shown in Fig 3. examples of the types of devices which can be present in the residence 290 mclude a telephone 294 connected to a Premise Interface Device (PID) 296, a computer contammg a Network Interface Card 292 and a television 299 ith a settop 298 The computer containing a NIC card 292. PID 296 and settop 298 can. via the m-home coaxial wiring 272. splitter 280. and coaxial drop cable 270. all transmit signals to the BIU 250 II. Burst mode communications in a FTTC s stem ln the FTTC svstem digital signals are used to carry voice video, and data signals to the devices Asynchronous Transfer Mode (ATM) protocols and formats can be used to carrv the signals The system requires bi-directional communications between the devices in the residence 290 and the ONU 240 and information is transmitted in the retum direction, for example, from the devices to the ONU 240 for changing channels on the telev ision 299 carrying out voice conversation on the telephone 294 or using network services on the computer w ith NIC card 292 The system provides for the transpoπ of ATM cells in the return direction as w ell as the forward (ONU to devices) direction The shared media formed by the subscriber coaxial network suggests that the devices use a multiple access protocol to transmit to the BIU 250 in the ONU 240 A number of multiple access protocols can be used including Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) When a TDMA protocol is used the devices will be assigned opportunities for the transmissions to the BIU 250. and, when ATM protocols are used, w ill transmit one or more cells m the opportunities Although signals can be transmitted on the subscriber coaxial cable network in baseband form when the splitter 280 has the appropriate low frequency characteristics, it is generally more appropriate to transmit signals in both the forward and retum direction on passband signals which are centered about some non-zero frequency Fig. 3 illustrates a possible spectrum of forward and retum signals on a subscriber coaxial network, with a FTTC forward carrier 310, and a FTTC retum carrier 320 In one embodiment of an FTTC system, the forward transmission takes place on a FTTC forward carrier 310 at a frequency of 19 44 MHz, with a data rate of 51 84 Mb/s using 16 Quadrature Amplitude Modulation ( 16QAM) Transmissions in the retum direction can take place on a FTTC retum carrier 320 at a frequency of 38 88 MHz. with a data rate of 19 44 Mb/s using Quadrature Phase Shift Keying (QPSK) An advantage of this embodiment is that the spectrum of TV signals above 50 MHz is not disturbed by these transmissions, if such an analog signal is present on the subscriber coaxial network However. this embodiment is given as an example only , and various frequencies data rates and modulation formats could be used to practice the invention It should also be noted that techniques which are commonly referred to as Carπerless Amplitude Phase (CAP) modulation are subsequent to generation ofthe signal, equivalent to QPSK and QAM modulation formats III. Preamble for burst mode communications Fig 4 illustrates the structure of a burst for burst mode transmission It can be received using a number of methods, and consists of a preamble 402 followed bv data 404 The preamble has a pulse 420. a dead time 430. and a Barker sequence 440 The Barker sequence is used in a preterred embodiment ol the invention because it has low crosscorrelation and high autocorrelation properties In the preferred embodiment, the transmitter utilizes Quadrature Phase Shift Keying (QPSK) modulation in which binary symbols are modulated on to orthogonal carriers, referred to as the I and Q channels The carriers can be thought of as having different frequencies in the sense that the complex representation of the orthogonal carriers are different even when the real value of the frequency is the same The Barker sequence, consisting of 1 - 13 bits, can be transmitted in the preamble on both carriers Because noise on the orthogonal carriers can be correlated, there can be a correspondence between errors in the detected signal at the burst mode receiver To reduce the probability that noise events on both carriers make the Barker sequence less detectable on each channel, the order of the Barker sequence can be reversed in one channel with respect to the other channel To further reduce the effect of correlated noise events, the Barker sequences in the I and Q channels can be inverted with respect to each other as well as transmitted in different transmission sequences in the I and Q channels As an example, the Barker sequence can be transmitted from beginning to end in the I channel, and in inverted form from end to beginning in the Q channel Fig 5 illustrates the preamble sequence of an embodiment which is comprised of a binary ' 1 ' in both the I and Q channels followed by a dead time of two symbols duπng which time no signal is transmitted, followed by a Barker sequence of 13 bits in the I channel, and the same Barker sequence reversed and inverted in the Q channel One skilled in the an will recognize that a Barker sequence may be modified without substantially altering its cross-correlation or autocorrelation propeπies ll is also recognized that a Barker sequence may comprise more than one Barker codeword, and in one embodiment a 13 bit Barker codeword is repeated twice to form the Barker sequence In a preferred embodiment ofthe preamble, the initial pulse and dead time are followed by a 13 bit Barker codeword which is subsequently followed bv a binarv sequences which are different for the 1 channel and the Q channel In a preferred embodiment the preamble is 1= 1 0 0 1 1 1 1 1 -1 -1 1 1 -1 1 -1 1 1 1 1 1 1 1 1 1 Q=l 0 0-1 I -1 1 -I -1 1 I -1 -1 -I -I -I 1 1 I -l I -l 1 -1 The number ot ones in the pulse 420 can varv with a single occurrence ot a one being the obvious minimum for being able to detect the presence of a pulse in a receiver More than a single ' 1 can be transmitted but mav cause unnecessary lengthening ofthe preamble By transmitting a one in both the I and Q channels it is possible to use a power measurement at the receiver to detect the presence ofthe pulse Since carrier recovery has not typically occurred when the pulse arrives, only a power measurement is possible, and the use of a one in both the I and Q channel allows a noncoherent power measurement to be used to detect the pulse The dead time 430 is provided to prevent precursors which may be formed in the filtering of the Barker sequence 440 in a burst mode receiver 120 from interfering with the reception and detection of the pulse 420, and visa versa Data is subsequently transmitted in the form of binary symbols modulated onto the orthogonal carriers using QPSK modulation. In another aspect, the invention compnses a burst mode transmitter for use in a burst mode communications system. The burst mode transmitter is capable of generating bursts for transmission to a burst mode receiver. The burst mode transmitter circuitry for producing burst signals in general are known in the art. T e bursts of the present invention comprise a preamble and data which are coded into binary symbols and modulated onto a first carrier frequency and a second carrier frequency wherein said second carrier frequency is orthogonal to said first carrier frequency as described above. The preamble consists of a sequence of a data pulse followed by a Barker sequence. The data pulse and Barker sequence are separated by a transmission time period equivalent to the transmission period of at least one, preferably two. binary symbols. The burst mode signal is transmitted over a shared media, and received by a burst mode receiver which uses the preamble to determine the presence of a burst and to determine the correct carrier frequency, carrier phase, symbol clock frequency and symbol clock phase in order to correctly recover the symbols transmitted, and thus the information contained within the burst. As an example of a method of recovering a burst mode signal is the use of a cross-coupled fractionally spaced equalizer structure which is preceded by an initiator circuit which detects the presence of pulse 420 and uses the Barker sequence 440 for adjustment of the taps of the equalizer for clock and earner phase recovery Reception of the burst mode signal can be accomplished bv convening a received analog signal to a digital signal and downconveπing the digital signal to a baseband signal having an I channel and a Q channel with at least two samples per symbol The presence of a burst is determined by examining the squared values of at least three samples and determining, through the use of at least one threshold, if a burst is present, and if so. which of the samples is nearest to the center of a predetermined symbol in the preamble. The signals in the I and Q channels are passed through digital filters which have taps which can be adjusted to mmimize an error signal which is produced by a subsequent decision circuit which performs thresholding of the filtered samples and makes determinations of the symbol values and correlates the recovered symbols with a stored version of the predetermined preamble durmg training mode, or with the samples prior to thresholding during normal operation mode The correlation produces an error signal which duπng the training mode represents the initial error while the filters are bemg adjusted to obtain the proper symbol sampling phase and minimize the carrier phase offset, and which durmg normal mode of operation represents the drift in the symbol sampling and carrier phases Minimizing this error signal during the training phase results in symbol sampling phase recovery and carrier phase acquisition, and during the normal mode of operation resulting in a tracking of the symbol and carrier phases The preceding descπption of a method and receiver architecture for burst mode reception is given as an example only, and alternative architectures for burst mode receivers are possible Although the present invention has been described in considerable detail with reference to ceπain preferred versions thereof, other versions are possible The goal of the invention as a method and apparatus for the transmission of burst mode data remains the same however An application of the invention is the transmission of data transmitted from a burst mode device located in a residence 290, an example of which is a settop 298 connected to a television 299 to a BIU 250 in an ONU 240 of a FTTC svstem in this application a subscriber mav perform a channel change operation or other video related function through the remote control of a settop This information is transmitted in the form of a burst from the settop 298 to the BIU 250 to the ON 240 and to the HDT 230 which mav effect the change or request another network element of the ATM network 220 to effect the change The ability to transmit bursts from the settop 298 and be correctlv received bv a burst mode receiv er is essential to correctlv inteφret the subscriber commands Therefore the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein

Claims: What is claimed is 1. In a burst mode communications system having bursts containing a predetermined preamble and data which are coded into binary symbols and modulated onto a first earner frequency and a second carrier frequency wherein said second carrier frequency is orthogonal to said first carrier frequency, a method for transmitting said burst comprising a) transmitting at least one symbol indicating a binary one on said first carrier frequency and at least one symbol indicating a binan one on said second carrier frequency, b) subsequently waiting a period of at least one symbol during which time no symbols are transmitted; and c) subsequently transmitting a binan Barker sequence ith a length of at least seven bits and having a beginning and an end on said first carrier frequency and simultaneously transmitting said binan. Barker sequence ith a length of at least seven bits and having said beginning and said end on said second carrier frequency, d) subsequently transmitting said data in the form of binarv sv mbols modulated onto said first earner and said second carrier 2 The method of claim 1 wherein said Barker sequence has a length of at least thirteen bits 3 The method of claim 1 , wherein said Barker sequence is transmitted from said beginning to said end on said first carrier frequency and is simultaneously transmitted from said end to said beginning on said second carrier frequency 4 The method of claim 2, wherein said Barker sequence is transmitted from said beginning to said end on said first carrier frequency and is simultaneously transmitted from said end to said beginning on said second carrier frequency 5. The method of claim 1 , wherein said Barker sequence is transmitted from said beginning to said end on said first carrier frequency and is simultaneously transmitted from said end to said beginning and inverted in sign on said second carrier frequency. 6. The method of claim 2. wherein said Barker sequence is transmitted from said beginning to said end on said first carrier frequency and is simultaneously transmitted from said end to said beginning and inverted in sign on said second carrier frequency 7. A burst mode communications system comprising a burst mode transmitter, a burst mode receiver, and a transmission medium between said burst mode transmitter and said burst mode receiver, wherein bursts are transmitted from said burst mode transmitter to said burst mode receiver and said bursts comprise a preamble and data which are coded mto binary symbols and modulated onto a first carrier frequency and a second carrier frequency wherein said second carrier frequency is orthogonal to said first carrier frequency, and wherein said preamble comprises a sequence of a data pulse followed by a Barker sequence, said data pulse and Barker sequence being separated by a transmission time period equivalent to the transmission period of at least one binarv symbol 8 The burst mode communications system of ciaim 7, wherein the Barker sequence has a length of at least thiπeen bits 9 The burst mode communications system of claim 8, wherein the Barker sequence comprises a thirteen bit codeword. 10. A burst mode preamble for use in a burst mode communications system having bursts containing data which are coded into binary symbols and modulated onto a first carrier frequency and a second carrier frequency wherein said second carrier frequency is orthogonal to said first carrier frequency, said burst mode preamble comprising a sequence of a data pulse followed by a Barker sequence, said data pulse and Barker sequence being separated by a transmission time period equivalent to the transmission period of at least one binary symbol. 1 1. The burst mode preamble of claim 10, wherein the Barker sequence has a length of at least thirteen bits. 12. The burst mode preamble of claim 1 1. herein the Barker sequence comprises a thirteen bit codeword. 13. A burst mode transmitter for use in a burst mode communications system, said burst mode transmitter comprising a means for generating bursts for transmission to a burst mode receiver, said bursts comprising a preamble and data which are coded into binary symbols and modulated onto a first carrier frequency and a second carrier frequency wherein said second carrier frequency is orthogonal to said first carrier frequency, and wherein said preamble comprises a sequence of a data pulse followed by a Barker sequence, said data pulse and Barker sequence being separated by a transmission time period equivalent to the transmission period of at least one binary symbol 14. A burst mode transmitter of claim 13. w herein the Barker sequence has a length of at least thirteen bits. 15. A burst mode transmitter of claim 14. herein the Barker sequence comprises a thirteen bit codeword.

Inventors: EBRINGER LAWRENCE, EBRINGER, LAWRENCE
Applicants: NEXT LEVEL COMM, NEXT LEVEL COMMUNICATIONS
Application year: 1996
Main classification: H04L27
IPC codes: H04J3, H04J13, H04L7, H04L27