GB2242105A

GB2242105A - Multi-level linecoding

Info

Publication number: GB2242105A
Application number: GB9006014A
Authority: GB
Inventors: Timothy Andrew Large
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1990-03-16
Filing date: 1990-03-16
Publication date: 1991-09-18
Anticipated expiration: 2010-03-16
Also published as: GB2242105B; GB9006014D0

Abstract

A method of linecoding binary digital non-return-to-zero (NRZ) encoded data on a multi-level code including the steps of d.c. biassing the NRZ data to change it into bipolar data and multiplying the bipolar data with a synchronous bipolar clock. This produces a 4-level code in which binary digits of one significance are encoded with biphase pulses of a first amplitude and binary digits of the other significance are encoded with biphase pulses of a second different amplitude. The resulting bit stream has no d.c. content. When applied to a conventional FSK heterodyne receiver the data will be correctly demodulated. Application is to FSK optical transmission systems using distributed feedback lasers. <IMAGE>

Description

Multi-level linecoding This invention relates to multi-level linecoding of data for FSK heterodyne optical transmission systems.

A problem arises in FSK optical transmission systems using distributed feedback (DFB) lasers in that, due to the non-uniform frequency modulation characteristics of DFB lasers, data encoded in a non-return-to-zero (NRZ) format may contain substantial dc content causing distortion when a DFB laser is modulated by NRZ data.

Various arrangements have been proposed to overcome this problem, including biphase linecoding, alternate mark inversion (AMI), and Manchester coding.

Reference is made to "Biphase linecoding in optical FSK heterodyne transmission experiment, without sensitivity degradation compared to NRZ". P.W. Hooymans et al, Electronics Letters, 2nd March 1989, Vol. 25, No. 5, pp 326-7.

According to the present invention there is provided an arrangement for multilevel linecoding of digital data stream encoded in a binary non-return-to-zero (NRZ) code format including means for applying a d.c. bias to the NRZ data stream to change it into a bipolar data stream, means for generating a clock pulse stream at the bit rate of the data stream and synchronous therewith, and means for multiplying the bipolar data stream with the clock pulse stream.

Such an arrangement produces a 4-level code in which binary digits of one significance are encoded with biphase pulsesof a first amplitude and binary digits of the other significance are encoded with biphase pulses of a second different amplitude. The resulting bit stream will always have no d.c. content.

Nevertheless, when the 4-level code is applied to a conventional FSK heterodyne receiver the data will be correctly demodulated without any modification being required in the receiver.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 illustrates the frequency of modulation response of a DFB laser, Fig. 2 illustrates a coding arrangement for a 4-level code, Fig. 3 illustrates examples of a 4-level code and a corresponding NRZ and their respective frequency spectra, Fig. t illustrates a typical optical FSK receiver, and Fig. 5 illustrates a normal binary FSK signal spectrum.

As shown in Fig. 1, the frequency modulation response of a typical 1.53 um DFB laser varies with frequency. In the modulation frequency region 10 MHz 2GHz both the phase and the magnitude characteristics are relatively fat. However, if the modulation frequency falls below 10 MHz, as is the case where a NRZ code sequence containing an unbroken succession of 'l's or 't's occurs, then the response characteristics vary unpredictably. Typically NRZ data will have substantial d.c. content when such unbroken successions occur, so significant distortion can occur in the output when a DFB laser is modulated by NRZ data.

In the coding arrangement of Fig. 2(a) binary data in a NRZ format is applied together with a d.c.

bias to a multiplier 11. The other input to the multiplier is a bit synchronous bipolar clock. The multiplier output is a 4-level bipolar code. Fig. 2(b) illustrates conventional NRZ coded binary data. When d.c. biassed the NRZ code is converted into a bipolar binary code as shown in Fig. 2(c). Fig. 2(d) shows a conventional bipolar clock which is bit synchronous with the binary data. Fig. 2(e) shows the 4-level code resulting from the multiplication of the bipolar data with the clock.

Fig. 3(a) shows a typical 4-level code trace on an oscilloscope. Fig. 3(b) shows the frequency spectrum of such 4-level code. Figs. 3(c) and 3(d) show the corresponding NRZ code trace and frequency spectrum, illustrating that the D.C. frequency content of NTZ is removed by 4-level coding.

The 4-level code is applied to the transmitter laser (not shown) as a current (or voltage) modulation which changes the laser frequency. Thus frequencies of f+l, f 1' f+0 and are transmitted. These in effect are small frequency shifts about the mean Wnmodulated laser frequency. The mean 14 laser frequency may typically be 2 x 10 Hz, with modulation of approximately 1 x 109 Hz. The receiver Fig. 4, mixes the incoming signal in 3dB coupler 41 with the output of laser 43 which acts as a local oscillator at the same mean frequency. The resulting beat frequency is detected and applied to the FSK demodulator 45. The demodulator has two parallel branches with different respective bandpass filters BPF1 and BPF2.

Since the receiver cannot detect phase, but only amplitude, it cannot distinguish between the positive and the negative encoded signals of the same frequency variation, i.e. it cannot distinguish between f f+l and -l 1 which both appear to be the same IF frequency, likewise f+0 and f0 appear to be the same IF frequency as each other but different from the IF of f+l and f1. 1 In effect the receiver has separate frequency bands for binary 'l's and 'O's respectively, as shown in Fig. 5. Therefore the two bandpass filters BPF1 and BPF2 are arranged to detect the two different intermediate frequencies, which do not coincide in time but are sequential with respect to each other. The 4-level code is therefore effectively 'folded' about the zero level and appears as a bipolar code at the output.

The two filter outputs are applied to respective envelope detectors 47, 49 followed by respective post detection filters and the two envelope waveforms are combined to give the demodulated bipolar code output.

Claims

1. An arrangement for multilevel linecoding of digital data stream encoded in a binary non-return-to-zero (NRZ) code format including means for applying a d.c. bias to the NRZ data stream to change it into a bipolar data stream, means for generating a clock pulse stream at the bit rate of the data stream and synchronous therewith, and means for multiplying the bipolar data stream with the clock pulse stream.

2. An arrangement for multi-level linecoding of digital data substantially as described with reference to Fig. 2 of the accompanying drawings.

3. A method of linecoding binary digital non-return-to-zero (NRZ) encoded data on a multi-level code including the steps of d.c. biassing the NRZ data to change it into bipolar data and multiplying the bipolar data with a synchronous bipolar clock.