GB2376158A

GB2376158A - In a receiver for signals with ISI the trial sequence of symbols which minimises an error function is considered to represent the original sequence

Info

Publication number: GB2376158A
Application number: GB0113088A
Authority: GB
Inventors: Salah A Al-Chalabi
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-05-30
Filing date: 2001-05-30
Publication date: 2002-12-04
Anticipated expiration: 2021-05-30
Also published as: GB0113088D0; GB2376158B

Abstract

The present application is concerned with a receiver for receiving signals from a transmission channel which introduces inter-symbol interference (ISI). The receiver tries a number of trial binary sequences X<SB>n</SB> and determines which of these sequences most accurately represents an originally transmitted sequence. This involves generating a value Y<SB>n</SB>, by passing the received signal through an integrator with integration time equal to the maximum time period over which one symbol can interfere with another, using a channel estimator to produce a sequence a<SB>n</SB> representing the channel response, multiplying each element of the response sequence, a<SB>n</SB> , with an element of a trial sequence, X<SB>n</SB> , and subtracting Y<SB>n</SB> from the result to produce an error function E<SB>n</SB>. The trial sequence which minimises E<SB>n</SB> is considered to provide the most accurate representation of the transmitted sequence. Block decoding and sequential decoding embodiments are disclosed. The invention is preferably used with optically transmitted signals suffering from chromatic and/or polarisation dispersion.

Description

Title : Opto-electronic Receiver Based on Block and Sequential Decoding to Compensate for Dispersion (Chromatic or Polarisation Mode Dispersion) in Digital Optical Communication Systems Description This invention relates to opto-electronic decoding techniques to recover the information carried by an optical signal distorted by the dispersion (chromatic or/and polarisation mode dispersion) caused by the optical transmission media linking the transmitter to the receiver.

It is well known that chromatic dispersion and polarisation mode dispersion cause a broadening of an optical pulse as it travels through an optical communication channel; such as an optical fibre. The dispersion causes several consecutive pulses to overlap, making it difficult for conventional optical receivers in an optical communication system to decide whether the transmitted pulse was I or 0. Overcoming the effect of dispersion (chromatic or polarisation mode dispersion) becomes particularly important in high speed transmission systems as the overlap of the consecutive pulses becomes a more significant part of the duration of the transmitted pulse.

The present invention is an opto-electronic receiver which consists of : An optical receiver which converts the optical signal at the end of the communication channel to an electrical signal using a photodetector.

An estimator which is an opto-electronic system whose output signal is the response to an input sequence representing a possible transmitted message. The exact response of the estimator is determined in co-operation with the transmitter before the actual data is transmitted over the optical communication channel. The optimum estimator should have a response which matches the response of the optical communications channel. In the case of an optical communication channel which changes its characteristics slowly with time, the estimation of the optical communication channel is performed adaptively over periods of time agreed between the transmitter and the receiver.

A comparator which compares the output of the estimator and the actual output of the photodetector to evaluate the error in the estimation.

A signal processor which processes the signal in an a specific manner to minimise the error in estimation.

A decision making system to decode the message and detect and correct errors.

Data storage devices to store the results of decoding the message and the information about the errors encountered in the decoding processing.

The embodiment of the invention will first be described in terms of the receiver system diagram with reference to the accompanying drawings in which: Figure 1 shows the general system diagram of an opto-electronic receiver for compensating for dispersion (chromatic and/or polarisation mode dispersion) The specific embodiments of the invention are then described with reference to the accompanying drawing in which: Figure 2 shows a digital receiver for decoding an optical signal with consecutive overlapping pulses due to dispersion (chromatic and/or polarisation mode dispersion) using the Block Decoding scheme.

Figure 3 shows a digital receiver for decoding an optical signal with consecutive overlapping pulses due to dispersion (chromatic and/or polarisation mode dispersion) using the Sequential Decoding scheme.

Referring to the drawing of Fig. I, the receiver consists of a photodetector (2) which converts the optical signal carrying the information from the output of the optical fibre (1) to an electrical signal. The electrical signal is then compared by a comparetor (3) to the outputs of several estimators and the input to each estimator is one of the message possibly sent by the system's transmitter (not shown in the diagram). Each estimator has an impulse response which matches the impulse response of the optical fibre, The output of each comparator is then passed through the signal processor (4) whose output is used by the Decoder (5) to select the sequence of the opto-electronic system (6) which resulted in minimum error at the output of the signal processor. For estimating the nth bit and the overlapping pulses, the signal processor (4) consists of an integrator which starts integrating the output of each comparator at the beginning of the nth pulse and stops after a period equals to the total duration of the nth pulse plus the broadening in pulse duration due to dispersion caused by the optical communications channel. This increase in integration time from pulse width without dispersion to pulse width after dispersion is essential to ensure that the estimation of the nth pulse is based on the total energy of the nth pulse. The output of the integrators are compared and the input sequence to the estimator resulting in minimum error at the output of the integrator is stored in the output storage system (7) as the decoded message. The decision system also compares the output of the integrator to threshold levels to determine the confidence in the estimation and the magnitude of the integrated difference between the electrical signal and the output of the estimator with least error. The error function is stored in storage device (7) and then used to indicate the location of possible errors in the decoded sequence. Two types of error functions are proposed in this invention, the first is the absolute value error function and the second is the square value error function. An accumulative error function is also generated by the decoder where the errors of several consecutive pulses are added to indicate the occurrence of errors and their location in the estimator sequence. These error functions are also used for correcting any errors that occurred in the initial decoding process.

The receiver essentially decodes the message by performing a division of two sequences; namely dividing the sequence {Yk} by the sequence {an} to produce the estimate of the

transmitted sequence {Xm}, where {Yk} is the output sequence of the integrator of the electrical signal representing the optical signal, {Xm} is the estimator sequence or the possible transmitted sequence, and {an} is the response of the estimator which should match as closely as possible the response of the optical communication channel. This division operation is given below for the case where each pulse overlaps with the pulse before it and the pulse after it (i. e. the sequence {an} has only three elements)

The sequence {Xm} consists of Is and Os only. The error function at the output of the integrator is given by the equation

Where

Is is the electrical signal representing the optical signal, 1m is the output of the mth estimator and is given by the following equation

In the absence of noise, the correct estimator would have an output error equals to zero.

However, noise will always be present in a communication system, and the best receiver is the one which chooses the estimating sequence that minimises the error function.

The response of the estimator {an} is estimated by co-operation between the transmitter and the receiver.

The specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which:

Figure 2 shows a digital receiver for decoding an optical signal with consecutive overlapping pulses due to dispersion (chromatic and/or polarisation mode dispersion) using the Block Decoding scheme.

The first embodiment is the Block Decoding scheme and the second embodiment is the Sequential Decoding. l. The Block Decoding embodiment scheme The first embodiment of this invention is the Block Decoding scheme shown in Fig 2 where the decoder (12) generates all possible messages that could represent the nth pulse and the pulses that overlap the nth pulse and each message is stored in a register (11).

Each sequence is weighted by the same estimator sequence {a, ao al}, The difference (3) between the electrical signal representing the optical signal and the output of each estimator is integrated by the integrator (9) over the period Tc. The resulting error functions can then be represented as:

Where: Ekn is the error at the output of the kth integrator resulting from using the Xk Xk Xkn+ sequence as an input to the estimator for estimating the nth pulse together with the overlapping pulses. This scheme is a block decoder because it decodes the overlapping pulses in a block. The values ofXn and Xn+l can be checked with the result obtained from a similar calculation to estimate the nth and (n+l) th pulses. This double checking effectively detects and corrects error. The estimate of the message is the combination of {Xn} consisting of I s and Os which results in minimum value of error. The results are stored in register (13).

The confidence in the estimation is determined by the value of the error. If this value is greater than a certain threshold, then the confidence in the decoded sequence is low.

The error function is stored in register (14) and then used to indicate the location of possible errors in the decoded sequence. Two types of error functions are proposed in this invention, the first is the absolute value error function and the second is the square value error function. An accumulative error function is also generated by the decoder where the errors of several consecutive pulses are added to indicate the occurrence of errors and their location in the estimator sequence.

2. The Sequential Decoding embodiment scheme: The second embodiment of this invention is the Sequential Decoding scheme shown in Fig. 3 where the sequential decoder performs the division operation using the following equations:

where the sequence Xn-2, Xn-l consists of Is and Os only and are obtained from the estimates of the (n-2) and (n-1) pulses which are stored in the first two cells of the shift register (15). The error in estimation is given by

The error function is stored in register (14) and then used to indicate the location of possible errors in the decoded sequence. The value of the error function is compared to a threshold level (or levels) to indicate the degree of confidence in its output and the value of the error function. If the value of error is too high for one error to occur, this will be an indication that an error has occurred in the estimation of the previous pulses which can then be corrected to minimise the error function. Two types of error functions are proposed in this invention, the first is the absolute value error function and the second is the square value error function. An accumulative error function is also generated by the decoder where the errors of several consecutive pulses are added to indicate the occurrence of errors and their location in the estimator sequence.

Claims

Claims 1. An opto-electronic receiver which uses a block decoder or a sequential decoder to recover the information on an optical signal which suffered dispersion (chromatic or polarisation mode dispersion) after passing through an optical communication channel. The receiver uses an opto-electronic system with an estimator that matches the response of the optical communication channel and compares the optical signal and the output of the estimator and the difference after passing through an integrator (or an equivalent low pass filter) with an integration time equals the total of the transmitted pulse plus the spread in time due to dispersion. The receiver also uses an error function to estimate the error associated with the decoded sequence as well as the accumulative error over several decoded pulses. These error functions are used to detect and correct errors in the decoding and indicate the degree of confidence in the estimate of the transmitted message.
2. The Block Decoder uses error functions to decode a block of pulses that overlap due to dispersion by selecting the input sequence to the estimator that yields the minimum error at the output of the integrator. The error unction is used to detect and correct errors and indicate the degree of confidence in the decoded message.
3. The Sequential Decoder uses error functions to decode one pulse at a time by choosing the input to the estimator which gives minimum error as the decoded message. The sequential decoder also uses the error functions to detect and correct errors and indicate the degree of confidence in decoding the transmitted message.
4. The receiver, in corporation with the transmitter, estimates a model of the optical communication channel. This optical channel estimation process is performed before transmitting the message and conducted adaptively if the characteristics of the optical communication channel varies with time.