Communication terminal apparatus, communication apparatus, and signal receiving method

A communication terminal apparatus receives a management signal at a bit rate A and a data signal at a bit rate B (B=A×M) through the same line. The communication terminal apparatus includes a signal regenerating unit, a management signal converting unit, a timing control unit, and a data signal obtaining unit. The signal regenerating unit regenerates a signal transmitted through the line as a signal of a bit rate C (C=A×N). The management signal converting unit converts N bits of the regenerated signal into the management signal of one bit. The timing control unit controls timing for obtaining a data signal based on the management signal. The data signal obtaining unit obtains the data signal according to timing control of the timing control unit.

FIELD

The embodiment(s) discussed herein is (are) directed to a communication terminal apparatus connected to a network that contains a plurality of bit rates, a communication apparatus and a signal receiving method thereof.

BACKGROUND

To respond to an increase in data traffic typically found in the Internet, construction of high-speed and large-capacity optical access networks rapidly progresses. Recently, a gigabit-passive optical network (G-PON) in which a plurality of subscribers share an optical fiber cable while high-speed upstream transmission of 1.25 Gbps and downstream transmission of 2.4 Gbps at the maximum is available is widely used as a high-speed optical access system for constructing an optical access network.

To achieve still higher speed transmission in the future, development of a bit-rate mixed PON system in which an optical access network that has already been constructed is utilized and at the same time high-speed transmission (for example, 10 Gbps) is available only to the subscribers who require high-speed transmission is expected from the economical point of view. For example, Japanese Laid-open Patent Publication No. 08-008954 discloses a technology that implements a bit-rate mixed PON system. According to the technology, a multi-rate burst circuit is provided to an optical network unit (ONU) for each subscriber. Thus, a plurality of bit rates can be mixed in the PON system.

If an existing optical access network is utilized so that the optical access network contains a plurality of bit rates, a subscriber that uses a high-speed bit rate encounters receiver sensitivity degradation. The faster a network is, i.e., the broader a band is, the lower a signal/noise (S/N) ratio becomes. Therefore, by speeding up part of an existing optical access network that has been designed such that the network has a relatively low receiver sensitivity without expecting higher speed data transmission, the S/N ratio drops due to the widening of the band, which degrades the receiver sensitivity.

For example, if part of an optical access network that has been designed to run at 2.4 Gbps is speeded up to 10 Gbps, receiver sensitivity of a communication terminal apparatus for a subscriber using the speed-up transmission drops, for example, by about 4 dB. As a result, expected high transmission speed may not be achieved.

Similarly, in a network formed from an existing optical access network so that higher speed communication is available for some of the subscribers of the network, receiver sensitivity degradation occurs also in an optical line terminal (OLT) that functions as a line concentrator located on the station side. In other words, if the OLT is speeded up (broadbanded) to receive an upstream signal from a subscriber that uses higher speed communication, the S/N ratio deteriorates as the band is broadened. As a result, reception of an upstream signal from an existing ONU designed in relatively low receiver sensitivity becomes more difficult.

SUMMARY

According to an aspect of the invention, a communication terminal apparatus that receives a management signal transmitted at a bit rate A and a data signal transmitted at a bit rate B, where B=A×M, through a common line. The communication terminal apparatus includes: a signal regenerating unit that regenerates a signal transmitted through the line as a signal of a bit rate C, where C=A×N; a management signal converting unit that analyzes the signal regenerated by the signal regenerating unit and converts N bits of the signal into the management signal of one bit; a timing control unit that controls timing for obtaining a data signal destined for the communication terminal apparatus based on the management signal obtained by the management signal converting unit; and a data signal obtaining unit that obtains the data signal from the signal regenerated by the signal regenerating unit according to timing control of the timing control unit.

DESCRIPTION OF EMBODIMENT(S)

Preferred embodiments of the present invention will be explained with reference to accompanying drawings. Although a communication apparatus including a communication terminal apparatus is described below as an optical network unit (ONU) and an optical line terminal (OLT) in an optical access network, this is by way of example only. The following embodiments are also applicable to various other communication terminal apparatuses.

An optical access network that contains a plurality of bit rates is described in detail below.FIG. 1is a diagram of an example of an optical access network which operates at a plurality of bit rates. The optical access network illustrated inFIG. 1is constructed in a PON system. In the optical access network, an optical fiber cable20connected to an optical line terminal (OLT)100provided in a station is split by a power splitter10, and is shared by ONUs2001to200Xand300on the side of subscribers.

The optical access network of the example contains the ONUs2001to200Xthat perform communication at a bit rate A and the ONU300that performs communication at a bit rate B, which is M times higher than the bit rate A. At first, the optical access network runs at the bit rate A, and later, an ONU for subscriber #z is replaced by the ONU300and an OLT at the station is replaced by the OLT100. Thus, the optical access network can contain the both bit rates.

In the network, information exchange between the OLT100, the ONUs2001to200Xand300is controlled with a time division system. For example, a downstream communication from the station to the subscribers is controlled with a time division multiplexing (TDM) system, and the OLT100transmits data while switching a destination ONU every time slot.

FIG. 2is a diagram of an example of a downstream signal. As illustrated inFIG. 2, the downstream signal is time division multiplexed, and transmitted so that data destined for a plurality of subscribers is mixed therein. The OLT100periodically generates frame synchronization/management information so that the ONUs2001to200Xand300perform frame synchronization to acquire timing to obtain data destined for each ONU, and the frame synchronization/management information is also multiplexed as is the data and is contained in the downstream signal.

In downstream communication, the same signal is transmitted to the ONUs2001to200Xand300. Each of the ONUs2001to200Xand300discards data destined for ONUs other than itself and processes only the frame synchronization/management information and the data destined thereto. Thus, a one-to-one connection is virtually established between the OLT100and the ONUs2001to200Xand300.

Focusing attention on bit rates, all the ONUs are required to read the frame synchronization/management information. Thus, the frame synchronization/management information is transmitted at the bit rate A. Time divided data is transmitted with a header, attached thereto, containing information of specifying a destination ONU. All the ONUs are required to read the header. Thus, the header is also transmitted at the bit rate A. Data itself is transmitted at a bit rate corresponding to the destination ONU.

Thus, the ONUs2001to200Xreceive all the signals at the bit rate A. On the other hand, the ONU300receives a management signal that transmits a header and frame synchronization/management information at the bit rate A and a data signal that transmits data itself at the bit rate B.

In an environment in which different bit rates are mixed, the data signal is transmitted at a bit rate corresponding to a destination ONU, and the management signal is always transmitted at the lower bit rate A. As described above, an ONU designed to communicate at a higher bit rate is affected by noise in a network designed to run at a lower bit rate, which degrades the receiver sensitivity. Especially, the degradation of receiver sensitivity can cause a problem at the time of receiving a management signal.

Only a destination ONU needs to read the data signal. Therefore, it is not necessary to consider effects to other existing ONUs. For example, by employing error correcting techniques such as forward error correction (FEC), degradation of receiver sensitivity can be compensated. On the other hand, all the ONUs are required to read a management signal. Therefore, a compensating approach that may affect the existing ONUs cannot be adapted.

To solve the receiver sensitivity degradation while a management signal is received in a network in which signals of a plurality of bit rates are transmitted without affecting the existing ONUs, according to the first embodiment, the ONU300employs a majority decision approach.

The configuration of the ONU300is described in detail below.FIG. 3is a block diagram of the ONU300. For simplicity,FIG. 3illustrates only the configuration related to this embodiment. For example, the configuration related to signal transmission is omitted.

As illustrated inFIG. 3, the ONU300includes the optical receiver310, a signal regenerating unit (hereinafter, “clock data recovery (CDR)”)320, a signal converting unit330, a management signal processing unit340, a timing controlling unit350, an error correcting unit360, and a data signal obtaining unit370.

The optical receiver310converts an optical signal received, for example, by a photodiode into an electrical signal. The CDR320generates, according to an electrical signal converted by the optical receiver310, a clock signal according to which the ONU300operates, and regenerates a digital signal. The ONU300is a communication terminal apparatus that receives data at the bit rate B. Therefore, the CDR320regenerates a digital signal of the bit rate B.

The signal converting unit330and the error correcting unit360receive a digital signal regenerated by the CDR320. The signal converting unit330is a processing unit that converts a digital signal of the bit rate B regenerated by the CDR320into a digital signal of the bit rate A so that a management signal can be read out. The signal converting unit330includes a majority deciding unit331and an identification phase synchronizing unit332.

The majority deciding unit331converts an M-bit signal into one-bit management signal while performing error correction by using majority decision logic.FIG. 4illustrates an example of the operation of the majority deciding unit331. In the example, the bit rate B is a quadruple of the bit rate A, and a four-bit management signal is converted into a one-bit management signal.

A digital signal regenerated by the CDR320is supposed to be represented by 4 bits having the same value, like “0000” or “1111”, and be able to be simply converted to “0” or “1”. However, an error may occur in some bits, like “1011” or “0001”, due to influence from noise caused by speeding up.

The majority deciding unit331converts “1011” into “1” and “0001” into “0” by using majority decision logic. Thus, by using majority decision logic, an error rate of a management signal can be significantly reduced, comparing with a case in which a value of a management signal is determined according only to a bit in a specific location.

For example, if majority decision is performed on four values as in the example ofFIG. 4, one bit of error correction is possible. In a received signal having an error rate of E (for example, 1×10−6), a probability that error occurs repeatedly is approximately E2(for example, 1×10−12). Therefore, receiver sensitivity characteristics can be improved, for example, by four to five dB.

If simple majority decision logic is employed, majority decision becomes logically indefinite when there are two identical values in a signal (for example, two bits and two bits in four bits). Even in such a case, majority decision can be performed by assigning a weight to a bit with less intersymbol interference caused by the bits therebefore and thereafter, that is, a bit with less probability of errors (for example, the third bit in the four bits). Then, the correct decision result can be obtained at a high probability.

FIG. 5illustrates an example of the operation of the majority deciding unit331when a weight is assigned to a certain bit. If majority decision is performed on four values as in the example ofFIG. 5, an error is likely to occur at the first or the fourth bit due to intersymbol interference. Thus, by assigning a weight to a bit near the center of a signal (in the example ofFIG. 5, the third bit in the four bits), precision of error correction based on majority decision can be improved and an event where majority decision becomes logically indefinite because the numbers of “0” and “1” are the same can be avoided.

In the example ofFIG. 5, a weight is assigned to the third bit. Therefore, majority decision result of “0” can be obtained even if an error occurs at the second bit and the numbers of 0 and 1 become the same in number as in “0101”.

Referring back toFIG. 3, the identification phase synchronizing unit332first identifies a phase (bit) that is a beginning point of a management signal for the majority deciding unit331to start converting every M bits of signal regenerated by the CDR320into one bit of management signal and then notifies the majority deciding unit331about the identification result.

The management signal processing unit340recognizes, by reading a management signal of a bit rate A converted by the signal converting unit330, frame synchronization/management signal and the header thereof and performs various control processes accordingly. For example, the management signal processing unit340performs frame synchronization and obtains timing for altering time slots by recognizing frame synchronization signal consisting of known signal patterns.

The management signal processing unit340determines at which time slot data is transmitted to the particular destination ONU in accordance with information contained in the frame synchronization/management information and the header, and notifies the timing controlling unit350of the result of the determination.

The timing control unit350instructs the data signal obtaining unit370to start and terminate obtaining data destined for the particular ONU according to the information notified by the management signal processing unit340.

The error correcting unit360corrects errors in a data signal transmitted to the particular ONU, thereby enhancing receiver sensitivity of a data portion. Error correction of the data signal can be performed, for example, by employing FEC. In error correction performed by error correcting techniques such as FEC, data is made redundant at a transmission source. Thus, a bit rate of the data signal after the error correction by the error correcting unit360slightly drops by the amount of data made redundant.

The error correcting unit360corrects signals that constitute management signals and data destined for other ONUs. However, these signals are discarded by the data signal obtaining unit370, and cause no problem.

The data signal obtaining unit370abstracts data destined for the particular ONU by obtaining a data signal corrected by the error correcting unit360with timing specified by the timing controlling unit350. The data signal obtaining unit370provides the obtained data to an appropriate processing unit (not illustrated) and the processing unit performs required data processing.

As described above, according to the first embodiment, the ONU300improves, by applying an error correction code such as forward error correction (FEC), receiver sensitivity of a data signal that can be modified, and improves, by applying majority decision, receiver sensitivity of a management signal that cannot be modified.

FIG. 6illustrates a specific example of a signal converting unit380. The signal converting unit380illustrated inFIG. 6implements frame synchronization which is part of the functions of the management signal processing unit340inFIG. 3in addition to the functions of the signal converting unit330inFIG. 3. The signal converting unit380includes majority deciding units3811to381m, a known code detecting unit382, and a selector383.

The majority deciding units3811to381mare circuits disposed M pieces in number, each of which performs majority decision on every M bits of digital signal regenerated by the CDR320and then converts the M bits into one bit of management signal. To start majority decision, the majority deciding units3811to381mare controlled so that a bit regarded as a beginning point of a management signal is shifted by one bit each. For example, if the majority deciding unit3811starts majority decision by regarding the first bit as a beginning point of a management signal, the majority deciding unit3812starts majority decision by regarding the second bit as a beginning point of the management signal, and the majority deciding unit381mstarts majority decision by regarding the Mth bit as a beginning point of the management signal.

The known code detecting unit382stores therein each of a predetermined bit length of a sequence of a management signal converted by each of the majority deciding units3811to381mand compares each with a known signal pattern that represents a frame synchronization signal. The known code detecting unit382notifies the selector383about the number of a majority deciding unit whose converted sequence of the management signal matches the known signal pattern at the smallest error rate (or at the highest majority decision matching).

The selector383sends out to the management signal processing unit340, which performs a succeeding processing after the selector383, an output from the majority deciding unit corresponding to the number notified by the known code detecting unit382, as a management signal. Thus, the signal converting unit380is configured to perform identification phase synchronization and frame synchronization simultaneously. Therefore, time required for the ONU to start receiving data after the power is turned on can be reduced.

The operation of the ONU300illustrated inFIG. 3is described in detail below.FIG. 7is a flowchart of the operation of the ONU300. As illustrated inFIG. 7, after the ONU is turned on, the identification phase synchronizing unit332performs identification phase synchronization for a signal regenerated by the CDR320so that the majority deciding unit331can normally convert a management signal (Step S101).

Then, the management signal processing unit340performs frame synchronization, and frame synchronization/management information and a header can be recognized normally (Step S102). Thus, the initial process is completed, and the management signal processing unit340performs the following process every time a signal corresponding to one time slot is received.

The management signal processing unit340detects a header of the signal corresponding to one time slot thus obtained (Step S103). If the signal corresponding to one time slot thus obtained is determined to be the frame synchronization/management information (Yes at Step S104), the management signal processing unit340performs a control process according to the content of the frame synchronization/management information (Step S105).

If the data corresponding to one time slot thus obtained is not the frame synchronization/management information and is determined to be data destined for the ONU500(No at Step S104and Yes at Step S106), the management signal processing unit340notifies the timing controlling unit350of information for obtaining the data and the data signal obtaining unit370obtains the data (Step S107).

If the obtained signal corresponding to one time slot is determined to be neither frame synchronization/management signal nor data destined for a particular ONU (NO at Step S104and NO at Step S106), the management signal processing unit340does not notify the timing control unit350about information for obtaining the data but discards the information (S108).

As described above, according to the first embodiment, a management signal is first regenerated at the same bit rate as a data signal, and then converted into the original management signal while performing error correction by using majority decision. Therefore, an error rate of a management signal can be reduced and receiver sensitivity thereof can be improved.

In the first embodiment, an example is described in which a signal is first regenerated by a CDR at the same bit rate as a data signal and then majority decision is performed, thereby improving receiver sensitivity of a management signal. In a second embodiment, an example is described in detail in which a signal is first regenerated by a CDR at a bit rate higher than a bit rate of a data signal and then majority decision is performed, thereby improving receiver sensitivity.

FIG. 8is a block diagram of an ONU400according to the second embodiment. Similar to the ONU300, the ONU400communicates at the bit rate B in a network in which signals of the bit rate A and the bit rate B (where the bit rate B is M times faster than the bit rate A). The ONU400includes the optical receiver310, a CDR420, a signal converting unit430, the management signal processing unit340, the timing control unit350, a majority deciding unit460, and the data signal obtaining unit370.

The optical receiver310, the management signal processing unit340, the timing control unit350, and the data signal obtaining unit370are similar to those of inFIG. 3. Therefore, their description is not repeated below. The CDR420is a processing unit that generates, according to an electrical signal converted by the optical receiver310, a clock signal to operate the ONU400and regenerates a digital signal. The CDR420regenerates a signal into a digital signal of a bit rate C that is N times faster than the bit rate A (where N is an integer multiple of M).

The signal converting unit430and the majority deciding unit460receive a digital signal regenerated by the CDR420. The signal converting unit430converts a digital signal of the bit rate C that is regenerated by the CDR420into a digital signal of the bit rate A so that a management signal can be read out. The signal converting unit430includes a majority deciding unit431and the identification phase synchronizing unit332. The identification phase synchronizing unit332is similar to that ofFIG. 3. Therefore the descriptions thereof are omitted here.

The majority deciding unit431converts N bits of signal into one bit of management signal while performing error correction by using majority decision logic. In the majority decision, as in the first embodiment, simple majority decision may be used, or majority decision assigning a weight to a particular bit may be used.

The majority deciding unit460first converts a digital signal of the bit rate C regenerated by the CDR420into a digital signal of the bit rate B so that a data signal can be read out and then sends out the conversion result to the data signal obtaining unit370. More specifically, the majority deciding unit460converts N/M bits of signal into one bit of data signal while performing error correction by using majority decision logic.

In majority decision, simple majority decision may be used or majority decision assigning a weight to a particular bit may be used. In error correction by majority decision, data is not made redundant as in error correction by FEC and the like. Therefore, the data signal obtaining unit370can receive a signal at the bit rate B without any change in the speed.

As described above, according to the second embodiment, the CDR420regenerates a signal at a bit rate higher than a bit rate of a data signal. Therefore, an error rate of not only a management signal but also a data signal can be reduced because of majority decision, and receiver sensitivity of not only a management signal but also a data signal can be improved.

The configuration of the ONU300inFIG. 1can be regarded as a modification of the ONU400configured so that N is equal to M and the error correcting unit360performs error correction of a data signal in place of the majority deciding unit460.

In a third embodiment, an example is described in detail in which an error rate of an ONU that communicates at a higher bit rate is further reduced by removing influence of intersymbol interference in an optical access network in which signals of a plurality of bit rates are transmitted.

FIG. 9is a block diagram of an ONU500according the third embodiment. Similar to the ONU300, the ONU500communicates at the bit rate B in an optical access network in which signals of the bit rates A and B are transmitted (where the bit rate B is M times faster than the bit rate A). The ONU500includes the optical receiver310, an intersymbol interference removing unit590, the CDR320, the signal converting unit330, the management signal processing unit340, the timing control unit350, the error correcting unit360, and the data signal obtaining unit370.

Comparing the ONU500with the ONU300inFIG. 3, the ONU500is configured similarly to the ONU300except that the ONU500has the intersymbol interference removing unit590provided between the optical receiver310and the CDR320.FIG. 10is a diagram for illustrating the principle of removing intersymbol interference performed by the intersymbol interference removing unit590. As illustrated inFIG. 10, after a signal corresponding to “1” is received, a decision level of 0/1 is increased, and after a signal corresponding to “0” is received, the decision level of 0/1 decreases, thereby eliminating occurrence of an error due to intersymbol interference.

The function performed by the intersymbol interference removing unit590can be implemented by, for example, using a decision feedback equalizer590aillustrated inFIG. 11. A delay circuit5911is a circuit that stores therein the previous decision result from an identifier594that performs decision of 0/1. An integrator5921is a circuit that multiplies a value stored by the delay circuit5911by a predetermined coefficient. A subtractor593is a circuit that shifts a signal to the direction of 0 by the calculation result from the delay circuit5911, thereby causing substantially the same effect as if a decision level of 0/1 is increased.

The configuration illustrated inFIG. 11is an example that intersymbol interference is removed based on one bit of signal just received. However, intersymbol interference may be removed based on a plurality of bits of signal just received.FIG. 12is a block diagram of a decision feedback equalizer590bthat removes intersymbol interference based on four bits of signal just received. As illustrated inFIG. 12, the decision feedback equalizer590bincludes delay circuits5911to5914each of which stores therein the last four bits of the decision result from the identifier594, integrators5921to5924that multiply each of the values stored by each of the delay circuits by a predetermined coefficient, and an accumulator595that first adds all the calculation results from the integrators5921to5924and then outputs the result to the subtractor593.

As described above, according to the third embodiment, intersymbol interference is removed from a signal preceding regeneration by the CDR320. Therefore, error rates of a management signal and a data signal can be reduced and the receiver sensitivity can be improved.

In the third embodiment, an example is described in which an error rate is reduced in an ONU that communicates at a high bit rate in an optical access network in which signals of a plurality of bit rates are transmitted. In the fourth embodiment, an error rate in an ONU that communicates at a lower bit rate can also be reduced.

FIG. 13is a block diagram of an ONU600according to a fourth embodiment. The ONU600is an ONU that communicates at the bit rate A in an optical access network in which signals of the bit rates A and B are transmitted (where the bit rate B is M times faster than the bit rate A). The ONU600includes the optical receiver310, the CDR320, the signal converting unit330, the management signal processing unit340, the timing control unit350, and a data signal obtaining unit670.

Comparing the ONU600with the ONU300inFIG. 3, the ONU600has the data signal obtaining unit670that receives a data signal at the bit rate A provided in place of the data signal obtaining unit370that receives a data signal at the bit rate B (−α). The majority deciding unit331performs error correction of not only a management signal but also a data signal. Therefore, the ONU600is configured so that the error correcting unit360is not provided and the data signal obtaining unit670receives a data signal from the majority deciding unit331.

As described above, according to the fourth embodiment, a management signal and a data signal are regenerated at bit rate higher than the original bit rate, and converted into the original bit rate while error correction is performed by majority decision. Therefore, an error rate can be reduced also in an ONU that communicates at a lower bit rate.

In all the above embodiments, it is assumed that the speeded up bit rate of B is an integer multiple of the bit rate A. In a fifth embodiment, an approach is described in detail that can be applied to a case in which the bit rate B is not an integer multiple of the bit rate A as in the example ofFIG. 14. In the example ofFIG. 14, the bit rate B is not an integer multiple of the bit rate A. Therefore, by performing majority decision on M bits of signal of a bit rate b, a management signal of the bit rate A can be obtained. By performing majority decision on M bits of signal of the bit rate B, however, a management signal of the bit rate A cannot be obtained.

FIG. 15is a block diagram of an ONU301according to the fifth embodiment. The ONU301is a modification of the ONU300in the first embodiment so that the ONU301can be used when the bit rate B is not an integer multiple of the bit rate A, and further includes an CDR321. The CDR321generates, according to an electrical signal converted by the optical receiver310, a clock signal to operate the ONU301and regenerates a digital signal.

The CDR320first regenerates a signal in synchronization with the bit rate B according to a signal output by the optical receiver310and then outputs the signal to the error correcting unit360. On the other hand, the CDR321regenerates a signal in synchronization with the bit rate b according to a signal output by the optical receiver310and then outputs the signal to the majority deciding unit331. Therefore, the majority deciding unit331can perform majority decision on M bits of input signal and generate a management signal of the bit rate A. By using a CDR capable of synchronizing with both a signal of the bit rate B and a signal of the bit rate b, the CDR320and the CDR321can be alternated.

As described above, according to the fifth embodiment, the CDR321that generates a signal of the bit rate b is provided in addition to the CDR320that generates a data signal of the bit rate B (where b is M multiple of A). Therefore, a management signal having a lower error rate can be obtained by using majority decision even if the bit rate B is not an integer multiple of the bit rate A.

The fifth embodiment describes a modification example of the ONU300of the first embodiment. This modification can also be applied to the ONU in the other embodiments when the bit rate B is not an integer multiple of the bit rate A.

In a sixth embodiment, configuration and the like are described in detail of an OLT in an optical access network in which signals of a plurality of bit rates are transmitted. As described above, in an optical access network in which signals of a plurality of bit rates are transmitted, S/N ratio deteriorates due to broadband not only in the ONUs provided on the subscriber side but also in the OLT provided on the station side. Therefore, the normal reception of an upstream signal from the existing ONUs becomes more difficult.

An upstream signal received by the OLT is described in detail below.FIG. 16is a diagram of an example of an upstream signal. As illustrated inFIG. 16, an upstream signal is time-division multiplexed and transmitted with data from a plurality of subscribers mixed therein. For example, upstream communication from a subscriber to the station side is controlled in time division multiple access (TDMA) system, and an OLT receives data while switching ONUs from which the data is transmitted for each time slot. In such a signal, due to the fluctuation of transmission loss for each subscriber, packets having different optical intensities are transmitted in burst. In an upstream signal, a management signal corresponding to frame synchronization/management information in a downstream signal is not transmitted and a data signal alone is transmitted.

An OLT700according to the sixth embodiment is described in detail below.FIG. 17is a block diagram of the OLT700according to the sixth embodiment. For simplicity,FIG. 17illustrates only constituent elements related to this embodiment. For example, the components related to signal transmission are omitted.

The OLT700is connected to an optical access network in which signals of the bit rates A and B are transmitted (where the bit rate B is M times faster than the bit rate A). The OLT700includes an optical receiver710, a CDR720, a signal converting unit730, a data processing unit740, an error correcting unit750, a data processing unit760, and a timing control unit770.

The optical receiver710converts an optical signal received by a photodiode and the like into an electrical signal. The CDR720generates, according to the electrical signal converted by the optical receiver710, a clock signal to operate the OLT700and regenerates a digital signal. The OLT700is a communication apparatus that supports data reception at the bit rate B. The CDR720regenerates a digital signal of the bit rate B.

The signal converting unit730and the error correcting unit750receive a digital signal regenerated by the CDR720. The signal converting unit730converts a digital signal of the bit rate B regenerated by the CDR720into a digital signal of the bit rate A to obtain a data signal from an ONU that transmitted the signal at the bit rate A. The signal converting unit730includes a majority deciding unit731and an identification phase synchronizing unit732.

The majority deciding unit731converts M bits of signal into one bit of signal while performing error correction by using majority decision logic. Thus, by converting a signal of the bit rate B into a signal of the bit rate A while performing error correction by using majority decision logic, deterioration of receiver sensitivity characteristics due to broadband can be improved.

Majority decision by the majority deciding unit731may also depend on simple comparison of the numbers of 0 and 1 or may also be performed by assigning a weight on a bit having less intersymbol interference (for example, the third bit in four bits).

When the majority deciding unit731converts every M bits of signal regenerated by the CDR720into one bit of signal, the identification phase synchronizing unit732identifies a bit (phase) at the beginning point of the conversion by the majority deciding unit731, and then notifies the majority deciding unit731about the identification result.

The data processing unit740first obtains, based on timing specified by the timing control unit770, a data signal whose bit rate is converted into the bit rate A by the signal converting unit730and then performs a predetermined data processing. The data processing unit740transfers the data signal thus processed to another device if it is required.

The error correcting unit750corrects an error that occurs in a data signal of the bit rate B, thereby improving receiver sensitivity of the data signal of the bit rate B. Error correction of a data signal can be performed by, for example, using FEC. In error correction by FEC and the like, data is made redundant by the transmission source of the data. Therefore, a bit rate of the data signal after error correction performed by the error correcting unit750is slightly lowered according to the amount of the data made redundant.

The data processing unit760first obtains, based on timing specified by the timing control unit770, a data signal of the bit rate B whose error is corrected by the error correcting unit750and then performs a predetermined data processing. The data processing unit760transfers the data thus processed to another device if it is required.

The timing control unit770specifies, according to the known timing to receive a data signal from each ONU, a timing for obtaining a data signal corresponding to each ONU to the data processing unit740and the data processing unit760.

As described above, according to the sixth embodiment, error correction is performed by majority decision for a data signal of a low bit-rate, and by using an error correction code for a data signal of a high bit-rate. Therefore, receiver sensitivity of an OLT in an optical network in which signals of a plurality of bit rates are transmitted can be improved. In the above embodiments, an optical access network is described in detail in which signals of two different bit rates are transmitted. Needless to say, however, the embodiments are similarly applicable also to an optical network in which signals of three or more different bit rates are transmitted such as an optical access network in which, for example, a signal of a bit rate A′ is contained in addition (where A′ is equal to B divided by M′).

In the sixth embodiment, an example is described in detail in which a signal is regenerated by a CDR at the same bit rate as a data signal of a higher bit rate and majority decision is performed on the regenerated signal, thereby improving receiver sensitivity of the signal of the lower bit rate. In a seventh embodiment, an example is described in detail in which a signal is regenerated by a CDR at a higher bit rate than a data signal of a higher bit rate and majority decision is performed on the regenerated signal, thereby improving receiver sensitivity of the data signal of the higher bit rate also.

FIG. 18is a block diagram of an OLT800according to the seventh embodiment. Similar to the OLT700, the OLT800is connected to an optical access network in which signals of the bit rates A and B are transmitted (where the bit rate B is M times faster than the bit rate A). The OLT800includes the optical receiver710, a CDR820, a signal converting unit830, the data processing unit740, a signal converting unit850, the data processing unit760, and the timing control unit770.

The optical receiver710, the data processing unit740, the data processing unit760, and the timing control unit770are similar to those ofFIG. 17. Therefore, their description is not repeated below. The CDR820generates, according to an electrical signal converted by the optical receiver710, a clock signal to operate the OLT800and regenerates a digital signal. More specifically, the CDR820regenerates a digital signal of a bit rate C that is N times faster than the bit rate A (where N is an integer multiple of M).

The signal converting unit830and the signal converting unit850receives a digital signal regenerated by the CDR820. The signal converting unit830converts a digital signal of the bit rate C regenerated by the CDR820into a digital signal of the bit rate A to obtain a data signal from an ONU that transmits a data at the bit rate A. The signal converting unit830includes a majority deciding unit831and an identification phase synchronizing unit732.

The majority deciding unit831converts N bits of signal into one bit of signal while performing error correction by using majority decision logic. Thus, by converting a signal of the bit rate C into a signal of the bit rate A while performing error correction by majority decision, deterioration of receiver sensitivity characteristics due to broadband can be improved. The identification phase synchronizing unit732is similar to that ofFIG. 17, the same description is not repeated below.

The signal converting unit850converts a digital signal of the bit rate C regenerated by the CDR820into a digital signal of the bit rate B to obtain a data signal from an ONU that performs data transmission at the bit rate B. The signal converting unit850includes a majority deciding unit851and an identification phase synchronizing unit852.

The majority deciding unit851converts N/M bits of signal into one bit of signal while performing error correction by using majority decision logic. Thus, by converting a signal of the bit rate C into a signal of the bit rate B while performing error correction by using majority decision logic, deterioration of receiver sensitivity characteristics due to broadband can be improved. The identification phase synchronizing unit852is similar to the identification phase synchronizing unit732ofFIG. 17.

In majority decision, simple majority decision may be used or majority decision assigning a weight to a particular bit may be used. In error correction by majority decision, data is not made redundant as in error correction by FEC and the like. Therefore, the data signal obtaining unit370can receive a signal at the bit rate B without any change in the speed.

As described above, according to the seventh embodiment, the CDR820regenerates a signal at a bit rate higher than a bit rate of a data signal. Therefore, by applying majority decision, an error rate of a data signal of not only a lower bit rate but also a higher bit rate can be reduced and receiver sensitivity thereof can be improved.

In the above embodiments, examples are described in detail in which an error rate of an OLT that receives a signal containing different bit rates is reduced. In an eighth embodiment, an error rate of an OLT that receives a signal only from an ONU that transmits a signal at lower bit rate is also reduced.

FIG. 19is a block diagram of an OLT900according to the eighth embodiment. The OLT900communicates at the bit rate A, and includes the optical receiver710, the CDR720, the signal converting unit730, the data processing unit740, and a timing control unit970.

Comparing the OLT900with the OLT700, the OLT900similarly includes the components that improve receiver sensitivity of a data signal of the bit rate A by majority decision. On the other hand, the OLT900does not include the error correcting unit750and the data processing unit760because the OLT900is not required to process a data signal of the bit rate B. The timing control unit770that specifies a data obtaining timing only to the data processing unit740is provided in place of the timing control unit770.

As described above, according to the eighth embodiment, a data signal is first regenerated at a bit rate higher than the original bit rate thereof and then converted into the original bit rate while performing error correction by majority decision. Therefore, an error rate of an OLT connected to an optical access network composed of only ONUs that support a lower bit rate can also be reduced.

In a ninth embodiment, an approach is described in detail applicable to a case in which the bit rate B is not an integer multiple of the bit rate A as in the example ofFIG. 20. In the example ofFIG. 20, the bit rate B is not an integer multiple of the bit rate A. Therefore, by performing majority decision on each M bits of signal of a bit rate b, a signal of the bit rate A can be obtained. On the other hand, by performing majority decision on each M bits of signal of the bit rate B, a signal of the bit rate A cannot be obtained.

FIG. 21is a block diagram of an OLT701according to the ninth embodiment. The OLT701is a modification of the OLT700in the sixth embodiment. The OLT701is configured to be applicable to the case where the bit rate B is not an integer multiple of the bit rate A, and further includes a CDR721. The CDR721generates, according to an electrical signal converted by the optical receiver710, a clock signal to operate the OLT701and regenerates a digital signal.

The CDR720regenerates, according to a signal output by the optical receiver710, a signal in synchronization with the bit rate B and outputs the signal thus regenerated thereby to the error correcting unit750. On the other hand, the CDR721regenerates, according to a signal output by the optical receiver710, a signal in synchronization with the bit rate b and outputs the signal thus regenerated thereby to the majority deciding unit731. Therefore, the majority deciding unit731can perform majority decision on every M bits of input signal and generate a signal of the bit rate A. By using a CDR that can synchronize with both a signal of the bit rate B and a signal of the bit rate b, the CDR720and the CDR721can be alternated.

As described above, according to the ninth embodiment, the CDR721that regenerates a signal of the bit rate b (where the bit rate b is M times faster than the bit rate A) is provided in addition to the CDR720that regenerates a signal of the bit rate B. Therefore, an error rate of a signal of a lower bit rate can be reduced by majority decision even if the bit rate B is not an integer multiple of the bit rate A.

The ninth embodiment describes in detail a modification of the OLT700in the sixth embodiment. The ONU in the other embodiments is also applicable to the case where the bit rate B is not an integer multiple of the bit rate A. For example, if the OLT800is further provided with a CDR821as in the OLT801illustrated inFIG. 22, the OLT800is applicable also to the case where the bit rate B is not an integer multiple of the bit rate A.

The specific mode of separation and integration of the above constituent elements is not limited to the ones illustrated in the embodiments, and they can be arbitrarily separated or integrated either functionally or physically. For example, the intersymbol interference removing unit in the third embodiment can be provided with the ONUs and the OLTs in the other embodiments.

As set forth hereinabove, according to one aspect, a transmitted signal is regenerated at a bit rate higher than the original bit rate thereof, and a plurality of bits is combined and analyzed to convert the bits to a low-speed signal of one bit. Therefore, an error rate of the low-speed signal can be reduced and receiver sensitivity of the signal can be improved.

In another aspect, a received signal is regenerated at the same bit rate as the high-speed signal. Therefore, the bit rate of the high-speed signal does not need to be converted, and thus, the structure of the apparatus can be simplified.

In still another aspect, a value of a signal is determined by majority decision when a plurality of bits are combined into a signal of one bit. Therefore, an error rate of the signal can be reduced and receiver sensitivity of the signal can be improved.

In still another aspect, when a majority decision is performed, a weight can be assigned to a bit in which an error is less likely to occur. Therefore, an error rate of a signal can be further reduced and receiver sensitivity thereof can be further improved.

In still another aspect, the verification of synchronization timing is carried out to all the candidates thereof concurrently in parallel. Therefore, the correct synchronization timing can be quickly detected.

In still another aspect, intersymbol interference is removed from a signal prior to regeneration thereof. Therefore, an error rate of a signal can be reduced and receiver sensitivity thereof can be improved.

In still another aspect, error correction of a high-speed signal is performed by using an error correction code. Therefore, an error rate of a high-speed signal can be reduced and receiver sensitivity thereof can be improved.

In still another aspect, a means is provided for regenerating a transmitted signal at the bit rate of the high-speed signal or at a bit rate that is an integer multiple of the bit rate of the high-speed signal. Therefore, both the high-speed signal and the low-speed signal can be correctly regenerated even if the bit rate of the higher speed signal is not an integer multiple of the bit rate of the lower speed signal.

In still another aspect, timing for obtaining data signal is not obtained from a regenerated signal. Timing for obtaining a data signal is decided according to a control signal different from a regenerated signal. Therefore, a data signal can be obtained at the correct timing even if a regenerated signal does not contain information that specifies timing for obtaining a data signal.

In still another aspect, the bit rate of the high-speed signal is an integer multiple of that of the low-speed signal. Thus, after regeneration of a transmitted signal at the same bit rate as the high-speed signal by using a single regeneration signal regenerating unit, the low-speed signal can be easily generated from the regenerated signal. This eliminates the need for a plurality of regeneration signal regenerating units, and the structure of the communication terminal apparatus can be simplified.