Patent Application
20130272721
This application is directed, in general, to optical network devices and, more specifically, to the use of duobinary modulation.
Optical fiber is the backbone of many modern access networks. Optical fiber has several advantages over copper media, namely, lower attenuation and interference. Those advantages combined with falling costs and a broadening user base, have created a demand for more reliable and increasingly higher-bandwidth fiber optic networks.
Optical access networks, or simply optical networks, assume a variety of architectures, including passive optical networks (PONS) and active optical networks. Both PONS and active optical networks assume a star topology, where an optical line termination (OLT) at a service provider, or “upstream,” provides network access to multiple “downstream” end users via optical network units (ONUs). The distinction between PONS and active optical networks are where the optical lines from the service provider split into individual lines to reach end users. An active optical network uses electrically powered, or “active,” devices, or “nodes,” in the field to split and relay optical signals among the service provider and the end users; a PON uses unpowered, or “passive,” splitter devices. The trade-off is often perceived as reliability versus range. Generally speaking, a PON lacks the range capability of an active optical network, but is less vulnerable to failures because it employs more robust parts. In short, powered devices boost range, but are more susceptible to failure.
Cost is a significant factor in optical access networks. Architecting an optical access network may hinge on this alone. As the demand for bandwidth rises, so does cost. High-bandwidth components are expensive, and, when on the leading edge of high-bandwidth optical systems, scarce. Passive components can be less expensive than powered alternatives, but the nature and inherent obstacles of high-bandwidth optical access networks tends to dominate the cost structure.
One aspect provides an optical network receiver module, including: (1) an optical input configured to receive an optical non-return-to-zero (NRZ) signal having a data rate, and (2) an encoder having a maximum bandwidth, coupled to the optical input and configured to receive and encode the optical NRZ signal into an electrical three-level duobinary signal, the maximum bandwidth being related to, but substantially less than, the data rate.
Another aspect provides a method of generating a three-level duobinary signal having a data rate, including: (1) modulating a data signal onto an optical carrier signal, (2) employing a bandwidth limited device to at least partially generate the three-level duobinary signal, the bandwidth limited device having a maximum bandwidth related to but substantially less than the data rate, and (3) launching the three-level duobinary signal into an optical medium from which a bandwidth limited receiver receives and completes generation of the three-level duobinary signal, the bandwidth limited receiver having a maximum bandwidth related to but substantially less than the data rate.
Yet another aspect provides an optical network unit (ONU), including: (1) an optical input configured to receive, from an optical medium, an optical non-return-to-zero (NRZ) signal having a data rate, and (2) an avalanche photo-detector (APD) having a maximum bandwidth, coupled to the optical input and configured to receive and encode the optical NRZ signal into an electrical three-level duobinary signal, the maximum bandwidth being substantially equal to one-fourth of the data rate.
Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
A conventional optical network includes an optical line terminal (OLT) on the service provider's end (upstream) and multiple optical network units (ONUs) near the end users (downstream). Bandwidth demands in most optical access networks are asymmetric, meaning there is a greater demand for bandwidth in one direction than in the opposite. Bandwidth demand for data flowing downstream, toward the ONUs, tends to be greater than the bandwidth demand for upstream data flow, toward the OLT. There are several avenues for increasing the downstream data rates of optical networks, including higher performance components upstream and down, increasing the order of the modulation format or using multiple wavelengths.
When optical networks step up bandwidth and data rate capacity, they potentially expose themselves to additional noise and chromatic dispersion, which lead to bit errors. These issues are often overcome by utilizing higher order modulation schemes and data encoding, and likely require higher performance, higher quality components. Simply put, high bandwidth, high data rate optical networks can be expensive. Some approaches to increasing data rates include wavelength division multiplexing (WDM) and orthogonal frequency division multiplexing (OFDM) to mitigate obstacles like chromatic dispersion (CD) and low signal-to-noise ratio. These approaches generally require more expensive components than time division multiplexing (TDM). Increasing bit-rates downstream faces several obstacles: chromatic dispersion, sufficient optical power and cost. Non return to zero (NRZ) is a conventional modulation scheme in TDM optical networks. Increasing bit-rates of an NRZ signal requires increases in kind of downstream bandwidth. For instance, a rate, R, of 25.8 Gigabits per second (Gbps) requires ONUs containing transmitter and receiver components with a bandwidth of at least three-quarters of rate R, or about 20 GHz. Such high bandwidth components are prohibitively expensive for PON systems, if available at all.
Duobinary is an alternate modulation scheme to traditional NRZ modulation. Duobinary modulation allows for transmitting R bits per second (bps) using a modulation bandwidth of about 0.5 R, as opposed to the 0.75 R required for NRZ. Nyquist's theory implies duobinary signals will exhibit intersymbol interference (ISI). In fact, duobinary modulation does introduce ISI, but in a controlled fashion such that all data is recoverable. By allowing some ISI, the duobinary signal becomes smoother in the time domain, but narrower in the frequency domain, which also lends itself to improved dispersion performance (see, e.g., Shankar, “Duobinary Modulation for Optical Systems,” White Paper, Inphi Corporation, Pages: 1-10, December 2002).
An electrical duobinary modulated signal can be generated by passing an NRZ signal through an electrical delay and add filter, yielding a three-level signal existing in {1,0,−1}. The delay and add filter can be approximated by a well-chosen low pass filter having a bandwidth of about one-quarter the bit rate, R. It is realized herein three-level duobinary modulation, as an alternate to NRZ modulation, can yield high bit-rates by employing limited bandwidth components.
Optical networks may employ three-level duobinary signals by converting upstream and transmitting a three-level duobinary optical signal, or, it is realized herein, by transmitting a traditional NRZ modulated optical signal and converting to a three-level duobinary electrical signal downstream at the receiver. It is fundamentally realized herein that a transmitted three-level duobinary optical signal having a bit rate, R, requires receiver components with roughly half the bandwidth (R/2) of two-level duobinary or conventional NRZ signals at equivalent bit-rates. It is further realized herein that receiving an optical NRZ signal having a bit rate, R, with a bandwidth-limited receiving component, such that the signal is translated into a three-level duobinary electrical signal, requires that receiving component have a bandwidth equivalent to roughly one-quarter the bit rate. It is realized herein the bandwidth limited components in either embodiment are less susceptible to chromatic dispersion and are readily available.
It is realized herein that conversion at the transmitter yields slightly better chromatic dispersion performance while conversion at the receiver yields better optical receiver sensitivity due to lower noise equivalent bandwidth. It is also realized herein that chromatic dispersion introduced by increased bit-rates may be alleviated by transmitting in the O-frequency band (wavelengths ranging from 1260 nanometers to 1380 nanometers).
It is fundamentally realized herein the ONUs may employ bandwidth limited avalanche photo-detector (APD) based receivers as NRZ-to-duobinary converters. It is further realized herein that receiver performance is finely tunable by selecting appropriately bandwidth limited components.
It is realized herein that in duobinary systems, precoding data at the transmitter is necessary to reduce error propagation and to simplify the receiver circuitry. It is also realized herein that employing wavelength stacking in combination with the three-level duobinary modulation allows bit-rates in excess of 100 Gbps.
Before describing various embodiments of the optical network devices or method introduced herein, an optical network within which the optical network devices and method introduced herein may be embodied or carried out will be generally described.
Having generally described a PON within which the optical network devices or method introduced herein may be embodied or carried out, various embodiments of the optical network devices and method will be described.
On the transmit side, laser 202 is a continuous wave light source onto which NRZ data signal 206 is modulated by modulator 204. In certain embodiments, laser 202 is an O-band distributed feedback (DFB) laser. Other embodiments may employ direct modulation of the DFB laser current, thus obviating modulator 204. Various alternate embodiments may employ a variety of other types of lasers operable in various other bands in the light spectrum, including: E-band, S-band, C-band, L-band and U-band.
Optical networks are often employed over long distances, which makes it necessary to amplify the modulated signal. SOA 208 serves that purpose here. SOA 208 amplifies the modulated optical signal from modulator 204 and injects it into fiber line 210. SOA 208, in other embodiments, may be substituted for any optical amplifier appropriately rated for the length of transmission and wavelength. In alternate embodiments that experience low attenuation, for example, over short distances, SOA 208 may be omitted altogether.
Receiver 218 receives the transmitted optical signal via APD/TIA 212. APD/TIA 212 is a bandwidth limited device, having a maximum bandwidth of roughly one-quarter the data rate: R/4 Hz. As the attenuated optical signal passes through APD/TIA 212, it is effectively low pass filtered, yielding a three-level duobinary electrical signal. In alternate embodiments, the bandwidth may be larger than R/4 Hz, but maintains low pass filter characteristics. Other embodiments may employ different optical receivers, such as a positive intrinsic negative (PIN) diode or other photo-diode.
In the embodiment of
Continuing the embodiment of
At step 340 the NRZ or partially converted three-level duobinary signal is launched into an optical medium towards a bandwidth limited optical receiver. Similar to the bandwidth limited device mentioned above, upon receipt of the partially converted three-level duobinary optical signal, the bandwidth limited receiver operates as a filter, completing the duobinary conversion. The bandwidth limited receiver has a maximum bandwidth that is related to but substantially less than the data rate of the data signal. The filtering effects of the bandwidth limited device and the bandwidth limited receiver combine to approximate the low-pass filter necessary for duobinary conversion. The method ends at step 450. In certain embodiments the optical medium is a standard single-mode fiber (SSMF), however other media are possible.
Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.
This application claims the benefit of U.S. Provisional Application Ser. No. 61/623,942, filed by van Veen, et al., on Apr. 13, 2012, entitled “25.8 G-PON Transmission over 40 km using Duobinary Detection with a Low Cost 7.5 GHz APD-Based Receiver,” commonly assigned with this application and incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61623942 | Apr 2012 | US |
