Not Applicable.
The present invention relates generally to optical communication systems, and in particular to coherent optical hubbing in an optical communication network.
Referring to
As is known in the art, the optical channel signal can be demultiplexed and routed through the optical communications network using filter based DeMUX devices or Wavelength Selective Switches (WSSs) known in the art.
It is known to have network topologies beyond simple point to point connections. Well known examples include Optical drop and continue, broadcast, rings, mesh, etc. In each of these topologies, a channel signal transmitted from a single modem (or, equivalently, electro-optical interface) is received by two or more remote modems at respective different sites. In many instances, each site is interested in only a portion of the content modulated on the channel signal. Typically, this requirement is addressed by means of a multiple access technology, in which a portion of the optical channel's information rate is allocated to each site.
Various multiple access techniques are known. For example, Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA) are techniques that are commonly used in wireless communications to enable multiple remote terminals (in this case, wireless handsets) to transmit and receive signals that utilize an assigned portion of the bandwidth of a given communications channel. At least some of these techniques have been proposed for use in optical communications.
However, all of the techniques suffer a disadvantage in that the remote modem must be capable of transmitting and receiving the entire bandwidth of the communications channel. For example, in TDMA, during the modem's assigned timeslot(s), the modems must send and receive data at the full symbol rate of the communications channel. Similarly, CDMA requires the modem to transmit and receive a spread spectrum signal spanning the entire width of the communications channel, while using a code to identify the portion of the spectrum assigned to the remote modem. In order to maintain orthogonality, a remote receiver must sample its assigned OFDM signal at a sample rate sufficient to receive the entire channel signal. Similarly, a remote transmitter must generate its OFDM signal that is both coherent to and sampled at the same rate as the entire channel signal.
In all of these cases, the transmitters and receivers must be substantially symmetrical, in that (referring back to
In many applications the full channel bandwidth represents more capacity than is needed. For example, an optical channel signal with a baud rate of 20 GHz can achieve a data rate of 100 Giga-bits/second (Gb/s). However, a central office serving a given town or neighbourhood may need only 40 Gb/s or less.
Techniques which overcome limitations of the prior art remain highly desirable.
An optical communications system includes a hub modem and a set of two or more remote modems. Each remote modem includes a transmitter stage for transmitting a respective uplink data stream within a selected one of a set of two or more sub-channels. The hub modem optically communicates with the set of remote modems. The hub modem includes a receiver stage having an optical front-end for receiving an uplink optical channel signal within a spectral range that encompasses the set of two or more spectral sub-bands; a photodetector for detecting modulation components of the received uplink optical channel signal and for generating a corresponding high bandwidth analog signal; and a digital signal processor for processing the high bandwidth analog signal to recover the respective uplink data stream transmitted by each remote modem.
Representative embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:
a and 1b are block diagrams schematically illustrating elements of an optical communications system known in the art.
a-4e are spectral diagrams illustrating operation of the signal generator of
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
In very general terms, the present invention provides a technique in which a single optical channel signal can be sub-divided into two or more sub-channels, each of which may be terminated at a respective independent remote modem. This enables the implementation of a hub-and spoke topology within a single optical channel of the network, while enabling the use of low cost components in each remote modem.
Applicant's copending U.S. patent application Ser. No. 12/692,065, filed Jan. 22, 2010, and entitled High Speed Signal Generator, teaches techniques for generating a high-bandwidth optical signal using multiple parallel lower speed Digital to Analog converters. The entire content of U.S. patent application Ser. No. 12/692,065 is incorporated herein by reference.
As described in U.S. patent application Ser. No. 12/692,065 the signal generator 4 of a transmitter 2 includes a DSP 14 that operates to process the input digital signal X(n) to generate a corresponding digital drive signal X′(m) in the form of a set of N parallel sub-band signals νx[m], which are subsequently processed to yield the desired high-speed analog signal S(t).
As may be seen in
During each clock cycle, a set of M/2 successive symbols output from the encoder block 34 are deserialized (at 36) to generate a parallel input vector {rNEW}. This input vector is combined with the input vector of the previous cycle {rOLD} 38, and the resulting M-valued input array supplied to an FFT block 40, which computes an array {R} representing the spectrum of the M-valued input array. The FFT output array {R} is then supplied to a frequency-domain processor (FDP) 42, which implements a periodic convolution algorithm to generate corresponding sub-band arrays {A} and {B} containing the respective complex amplitudes of the spectral components for each digital sub-band signal. Each of the sub-band arrays {A} and {B} is processed using a respective IFFT block 44A, 44B to generate corresponding M-valued output vectors {vA} and {vB} 46A, 46B. The low-band output vector {vA} can be divided into a pair of M/2-valued low sub-band vectors {vAOLD} and {vANEW} respectively representing the sub-band signal νA[m] for the current and previous clock cycles. Similarly, the high-band output vector {vB} can be divided into a pair of M/2-valued high sub-band vectors {vBOLD} and {vBNEW} respectively representing the sub-band signal νB[m] for the current and previous clock cycles. Accordingly, the respective sub-band signals νA[m] and νB[m] for the current clock cycle can be obtained by serializing the respective sub-band vectors {vANEW} and {vBNEW}, and discarding the vectors {vAOLD} and {vBOLD} for the previous clock cycle.
If desired the resulting sub-band signals νA[m] and νB[m] can be retimed, for example by using a decimation function (not shown), to match the DAC symbol rate.
In accordance with the present invention, the flexibility of this signal generator is exploited to implement a hub modem designed to generate a optical channel signal which is subdivided into N≧2 sub-channel signals, each of which occupies a respective portion of the spectral range of the optical channel signal.
As maybe seen in
As noted above, within the signal generator 48, each sub-channel data signal x[n] is processed by a respective encoder 34, which may implement any of a variety of algorithms including, but not limited to: encoding the subscriber data signal x[n] using a desired encoding scheme such as M-ary PSK or QAM; applying Forward Error Correction (FEC); and pre-distortion to compensate link impairments such as dispersion. In some embodiments, each encoder may implement the same algorithms, so that, for example, all of the sub-channel signals will be encoded with the same encoding scheme. However, this is not essential. In some embodiments, respective different encoding schemes may be used for different sub-channels. Furthermore, each of the sub-channels may have the same or different bandwidths. Similarly, some sub-channels may be compensated for more dispersion than others, and in fact some sub-channels may not be dispersion compensated at all. Thus, the specific encoding scheme and dispersion compensation implemented for each sub-channel may be selected based on the capabilities of each remote modem, and the respective distance between the hub modem and each remote modem.
As may be appreciated, the receiver stage of the hub modem can be constructed to effectively mirror that of the transmitter stage.
As noted above, in the embodiment of
The hub modem described above is capable of transmitting and receiving an optical channel signal that contains two or more sub-channels, which are independently encoded and occupy a respective sub-range of the full spectrum of the optical channel signal. Since the sub-channels all lie within the spectral range of the optical channel signal, the optical communications network will route all of the sub-channels together through the network. per-sub-channel routing, by definition, is not possible. By using known wavelength switching, drop and continue, and power splitting techniques, the network can operate to route the optical channel signal to each one of a set of remote modems, each of which is designed to terminate a respective one of the sub-channels. Thus,
Preferably, each remote modem 70 is configured to send and receive data signal traffic within a respective one of the sub-channels of the optical channel signal. Thus, in an embodiment in which the optical channel signal (or, equivalently, the high speed analog signal S(t)) has a total capacity of 100 Gb/sec, and comprises five sub-channels of 20 Gb/sec, a total of five remote modems 70 may provided, each of which is configured to transmit and receive optical signals within at a line speed 20 Gb/sec.
In some embodiments, each remote modem 70 may use known coherent receiver techniques to detect and receive the desired sub-channel, while having sufficient Common Mode Rejection Ratio (CMRR) to avoid interference from the adjacent sub-channels. For example,
As may be seen in
As may be appreciated, each remote modem that communicates with a hub modem using via a given optical channel signal, will transmit a respective optical sub-channel signal that occupies a limited (and substantially non-overlapping) portion of the entire spectrum of the optical channel signal. As the multiple optical sub-channel signals propagate towards the hub modem, the network routing equipment will inherently combine the sub-channel signals together, so that the hub modem receives the entire optical channel signal.
In the embodiment of
The use of a hub modem with multiple remote modems achieves economy of scale in the hub modem to get lowest cost per bit at the hub, and lowest cost per site at the remote sites by minimizing the bandwidth of the remote modems.
In some embodiments, each remote modem may utilize a directly modulated laser, or an integrated laser-modulator to transmit an optical signal within the modem's designated sub-band. In such cases, the receiver of the hub modem is preferably configured as a coherent receiver capable of compensating at least dispersion of the received sub-band. As may be appreciated, one consequence of terminating each sub-band at a respective different remote modem is that, at the hub modem, each sub-band of the incoming wavelength channel may be subject to a respective different amount of dispersion. This can be overcome by configuring the hub modem to apply a respective different amount of dispersion compensation on each sub-band. Such a receiver may also be configured to compensate impairments due to low-cost optical elements of the remote modem. An advantage of this arrangement is that it enables the transmitter stage of the remote modem to be constructed using low-cost and low power consumption components.
Alternatively, the remote mode could use the same silicon as the hub modem but low bandwidth coherent electro-optics. In this arrangement, the DSP of the remote modem has the same dispersion compensation capabilities as the hub modem, and so is capable of both compensating dispersion in a received signal, as well as pre-compensating uplink signals prior to transmission to the hub modem. This arrangement achieves economies of scale in terms of utilizing the same electronic components in both the hub and remote modems. Additional cost savings are obtained in the remote modems by way of the use of lower-cost optical components, which can be used in the remote modem in view of its lower bandwidth requirements.
In the embodiments described above fractional access is provided using a Frequency Division Multiple Access scheme, in which each remote modem is tuned to a respective sub-band. Alternative methods of fractional access such as time division access, or code division access, can be used where the bandwidths required in the remote modem can be achieved cost effectively.
Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.
This application is based on, and claims benefit of, U.S. Provisional patent Application No. 61/313,330, filed Mar. 12, 2010, the entire contents of which are hereby incorporated herein by reference.
Number | Date | Country | |
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61313330 | Mar 2010 | US |