WO1997017780A2

WO1997017780A2 - Optical frequency multiplexing system

Info

Publication number: WO1997017780A2
Application number: PCT/DE1996/002168
Authority: WO
Inventors: Thomas Hermes; Jürgen SANITER
Original assignee: HEINRICH-HERTZ-INSTITUT FüR NACHRICHTENTECHNIK BERLIN GMBH
Priority date: 1995-11-06
Filing date: 1996-11-06
Publication date: 1997-05-15
Also published as: DE19543044A1; WO1997017780A3

Abstract

Optical frequency multiplexing systems with heterodyne receivers have channel occupation patterns in which the positions of adjacent channels are predetermined. When the frequency spacing between adjacent channels is constant (d = IF + fb, cf. figure 5), the limited available bandwidth is not optimally used. In order to reduce channel spacing, and so increase the number of channels up to their double, alternatively different channel spacings d1 and d2 are defined between every two adjacent channels (cf. figures 1 to 4). The marginal conditions that need to be met are in any case met to such an extent that no unacceptable degradation of the transmission quality is caused.

Description

description

Optical frequency division multiplex system

description

The invention relates to an optical frequency division multiplex system in which each receiving station has a heterodyne receiver equipped with a local oscillator, for the tuning of which between two adjacent channels of a given channel assignment scheme, two unequal frequency intervals, which are constant for all receiving stations, apply, namely the distance of the intermediate frequency IF compared to the respective channel to be received and, on the other hand, the minimum frequency spacing f _b compared to the neighboring channel not to be received, which is greater than the intermediate frequency IF.

Distribution and communication systems that use optical wavelength or frequency division multiplexing techniques do have a high transmission capacity, but they are e.g. is limited by the bandwidth of optical amplifiers. In conventional optical systems with OFDM (optical frequency division multiplexing), channel assignment schemes apply with a constant distance between adjacent channels.

The prior art from which the invention is based is the article "High-Performance Dens FDM Coherent Optical Network" by B. Glance et al. (IEEE Journal on selected areas in Communications, Vol. 8, No. 6, Aug. 1990, pp 1043-1047) The channel spacing of adjacent channels is subject to the following boundary condition: To receive a channel, the local laser of the heterodyne receiver is tuned at the spacing of the intermediate frequency (IF intermediate frequency) from the channel to be received The next possible channel to be received with a higher nominal frequency is then at a minimum frequency distance f _b from the frequency of the local laser, so that reception is not disturbed by the adjacent channel technology as the narrowest possible channel spacing, the sum of the intermediate frequency IF and the minimum frequency spacing f _b .

REPLACEMENT BLAπ (RULE 26) For a bidirectional point-to-point connection, the article "Coherent 565 Mbit / s DPSK Bidirectional Transmission Experiment with Local Transceiver Lasers" by G. Heydt et al. (Conference report ThA21 -7 of the "15th European Confernce on Optical Communication" (ECOC '89) 10th to 14th September 1989, Gothenburg SE, pages 417-420) an experimental setup is known in which the local laser also supplies the frequency to be modulated on the transmitter side. In this case, both channels, the channel to be transmitted by a subscriber station and the channel to be received by it, are provided at a mutual spacing of the intermediate frequency IF. Such a system cannot readily be used for OFDM systems with a high number of channels because of the aforementioned boundary conditions.

The technical problem on which the invention is based is to increase the number of channels in OFDM systems without violating the boundary conditions to be met in a manner which could lead to an unacceptable deterioration in the transmission quality.

For an optical frequency division multiplex system of the type mentioned in the introduction, the solution according to the invention provides that alternative channel assignment schemes to be used to tune the local oscillator alternately have different, constant channel spacings d ₁ and d ₂ between two adjacent channels, the channel spacing d _{1 being} smaller than the channel spacing d ₂ is and also applies:

Alternative 1: the channel spacing d- is in the range of the difference between the minimum frequency spacing f _b and the intermediate frequency IF and the

Minimum frequency spacing f _b , the channel spacing d ₂ corresponds to the sum of the intermediate frequency IF and the minimum frequency spacing f _b , or alternative 2: the channel spacing d is equal to the intermediate frequency IF; the channel spacing d ₂ is equal to the minimum frequency spacing f _b .

Such channel assignment schemes achieve a reduction in the channel spacing at least between two adjacent channels and thus allow an increase in the number of channels within the available transmission bandwidth up to a doubling. This can be illustrated using the following equations:

REPLACEMENT BUTT (RULE 26) For the prior art, channel spacing d = IF + L applies

Invention, alternative 1 Kanalabstaπd (dOm _« = f _b - IF

Channel spacing d ₂ = IF + f _h

It generally applies to the mean channel spacing d _m = (d, + d ₂ ) / 2.

This results in an average maximum channel distance (d _m ) _max = f _b + IF / 2 an average minimum channel distance (d _m ) _min = f _b .

Invention, alternative 2: channel spacing d, = IF

Channel spacing d ₂ = f _b

This results in an average channel spacing d _m = (IF + f _b ) / 2

The number of channels within the available bandwidth is inversely proportional to the average channel spacing.

With both alternatives, of course, any desired channel can be received by tuning the local oscillator. Any superposition terms that may occur are orders of magnitude smaller than the useful signal. In the old case, if a large number of adjacent channels are occupied, it is necessary to use a receiver type that works in push-pull mode and can thus suppress interfering signals.

In principle, the channel assignment schemes according to the invention can be used for unidirectional as well as for bidirectional operation. In bidirectional operation according to alternative 1, each subscriber station requires separate tunable lasers for the transmit mode and the receive mode.

In a particularly preferred embodiment of the invention according to alternative 2, each receiving station is designed for duplex traffic and with one

Arrangement of a common laser for local oscillator and transmitter laser. In this respect, this training corresponds to that already mentioned above State of the art, however, also improves this, since now several point-to-point connections can also be implemented within an OFDM system.

In unidirectional operation according to alternative 2, another undesired signal can arise from the superimposition of the signal in a received channel with the signal from the local laser, since both signals nominally have the same nominal frequency. In this case, care must be taken when designing the bandpass of the microwave circuit that this signal does not interfere. Because of the aforementioned direct terms as a disturbing factor, such a microwave circuit must be provided in any case and optimized if necessary.

In contrast, in duplex traffic, a fixed assignment of transmit and receive channel pairs at a mutual spacing of the intermediate frequency IF can prevent the undesired signals mentioned at all from becoming effective at all. Circulators or isolators at the coupling and decoupling points of the signals are suitable for this, for example.

The invention and its preferred embodiments are explained in more detail below with reference to the figures. The following show in detail:

Figures 1 - 5 sections of channel assignment schemes for OFDM

Systems, Figure 6 is a block diagram of a transmitter-receiver unit of an OFDM system with multiple channels and

FIG. 7 shows a block diagram of an OFDM-LAN with a star coupler.

For a better understanding, the channel assignment scheme according to FIG. 5, as is known from the prior art, is first explained. The locations at which the optical frequency multiplex signals can be located are indicated at the same distance by needle tips above the frequency. The distances d between adjacent sewer sites are constant. To receive a channel, the local oscillator LO is tuned to a frequency that is at a distance from the intermediate frequency IF from the channel to be received. At this position of the frequency of the local oscillator LO, on the opposite side, at a minimum frequency distance f _{b, is} the closest adjacent channel that is not from the local laser LO

REPLACEMENT BLAπ (RULE 26) is detected. The minimum distance between two adjacent channels in the channel assignment scheme from the prior art thus corresponds to the sum of the intermediate frequency IF and the minimum frequency distance f _b .

Figures 1 to 4 show channel assignment schemes as provided by the invention. According to FIG. 1, adjacent channels have alternately different channel spacings. The channel spacing d ₂ corresponds to the channel spacing d according to FIG. 5, but the channel spacing (d -,) - ,,, is smaller than d ₂ by the intermediate frequency IF and corresponds to the minimum frequency spacing f _b in the example shown. Thus, in this channel assignment scheme, every second channel spacing is reduced by the value of the intermediate frequency IF, ie, compared to the prior art according to FIG. 5, the average channel spacing (d _m ) _max is reduced by IF / 2 in the available bandwidth.

According to FIG. 2, the channel spacing d _{2 is} the same as in FIG. 1. The channel spacing d, however, has assumed its minimum value, which corresponds to the difference between the minimum frequency spacing f _b and the intermediate frequency IF. At this channel spacing (d ^) _mln there is just no interference with the reception by a nearby, not to be received channel. The average channel spacing (d _m ) _mJn here corresponds minimally to the minimum _{frequency spacing} f _bl, ie that, compared to the prior art according to FIG. 5, the mean channel spacing (d _m ) _{mjf is reduced} in the available bandwidth by the intermediate frequency IF.

According to FIGS. 3 and 4, the channel spacing d _{2 is} reduced in comparison to that according to FIGS. 1 and 2 by the value of the intermediate frequency IF and only corresponds to the minimum frequency spacing f _b . Furthermore, the channel spacing d ₁ corresponds to the intermediate frequency IF.

This channel assignment scheme has the essential feature that a channel to be transmitted can be nominally positioned on the same frequency as a channel to be received if it is ensured that two such channels do not interfere with one another. This is possible in any case with bidirectional operation, which prescribes these two channels for the outgoing and return channels between two subscriber stations of a point-to-point connection.

REPLACEMENT BLAπ (RULE 26) The average channel spacing d _m now corresponds to half the sum of the intermediate frequency IF and the minimum frequency spacing f _b and thus, compared to the prior art according to FIG. 5, only half of the channel spacing d required there.

The block diagram of a transceiver shown in FIG. 6 shows a special form which is constructed according to the principle of "laser sharing" known per se. The local oscillator LO delivers both the local oscillator frequency required for a heterodyne receiver and the frequency, via a 3dB coupler The heterodyne receiver is labeled DEMOD in FIG. 6, the modulator of the transmitter part is labeled MOD.

In the case of a point-to-point connection of an OFDM system with several channels or channel pairs, the transceiver of the partner receiving station differs only in that its local oscillator LO delivers a different frequency. In connection with FIGS. 3 and 4, however, this means that the frequencies of the local oscillators LO of the two partner stations differ by the intermediate frequency IF. If a partner station sends (compare FIG. 3) with the frequency of its local oscillator LO in the position shown by an upright arrow in FIG. 3, it receives the channel which is higher by the intermediate frequency IF than the frequency of the local oscillator LO. Accordingly, the other partner station transmits at the frequency that the position indicated by an upright arrow in FIG. 4 and receives the channel that is below the frequency of the local oscillator LO by the intermediate frequency IF. This mode of operation represents duplex traffic.

Contrary to the illustration in FIG. 6 with two separate light guides for the signals to be received and transmitted, a common light guide with optical coupling devices can also be provided, which couple out signals to be received and couple signals to be transmitted into them.

In local optical networks LAN (local area network), the connected transceivers can be freely connected to one another via a star coupler. FIG. 7 shows an example of transceivers 1 n according to FIG. 6 with a

ERSATZBUπ (RULE 26) "Nxn" star couplers (STAR COUPLER). Such embodiments are known (compare: "Transparent, All-Optical Interconnection of Two Independent Experimental OFDM-LAN's via One Central Node" by F.-J. Westphal et al (IEEE Photonics Letters, Vol. 7, No. 1, Jan. 1995, pages 95 to 97) and "Transmissive Star Coupler based Experimental Dense WDM-LAN's interconnected via a Central Node" by F.-J. Westphal et al (1995 Digest of the LEOS Summer Topical Meetings, Aug. 7-11, 1995, Keystone, Colorado, US; MD1, pages 20 and 21).

With regard to the modulability of the OFDM channels, the frequency positions of the local laser LO for reception are indicated in FIGS. 1 and 2 by vertical arrows shown in solid lines or dashed lines. In these positions, it is irrelevant for amplitude modulation (ASK: amplitude shift keying) whether the frequency difference of the local laser LO to the signal frequency is positive or negative, as long as the amount is equal to the intermediate frequency IF. When using frequency-modulated signals (FSK: frequency shift keying), it must in principle be ensured either on the transmitting side or on the receiving side that non-negated signals are received in each case. In the case of FSK signals, the sign of the intermediate frequency IF determines whether a received pulse defines a zero or a one in the case of digital signals. If the channel assignment schemes are used in pure signal distribution systems, the transmission side is expediently the place which carries out the necessary negation of the signals. In a communication system with point-to-point connections, that transmitter laser can modulate that channel to which a position of the local laser with a nominal nominal value below that channel is assigned in the receive mode.

ERSATZBUπ (RULE 26)

Claims

claims

1.Optical frequency division multiplex system in which each receiving station has a heterodyne receiver equipped with a local oscillator, for the tuning of which between two adjacent channels of a predetermined channel assignment scheme, two unequal but constant frequency spacings apply to all receiving stations, namely on the one hand the distance between the intermediate frequency IF and the respective one receiving channel and, on the other hand, the minimum frequency spacing f _b compared to the non-receiving adjacent channel, which is greater than the intermediate frequency IF, characterized in that alternative channel assignment schemes to be used to tune the local oscillator (LO) alternately different, constant channel spacings d- between two adjacent channels, and d ₂ , the channel spacing d being smaller than the channel spacing d ₂ and the following also applies: alternative 1: the channel spacing d lies in the range of the difference between

Minimum frequency spacing f _b and intermediate frequency IF and the

Minimum frequency spacing f _b , the channel spacing d ₂ corresponds to the sum of the

Intermediate frequency IF and the minimum frequency spacing f _bl or

Alternative 2: the channel spacing d is equal to the intermediate frequency IF; the channel spacing d ₂ is equal to the minimum frequency spacing f _b .

REPLACEMENT BUTΪ (REÜLL 'S)

2. Optical frequency division multiplex system according to claim 1, alternative 2, characterized in that each receiving station is designed for duplex traffic and is equipped with an arrangement of a common laser for local oscillator (LO) and transmission laser.