Optical communication system optically combining both...

Details Optical communication system optically combining both... Optical communication system optically combining both...

1999-11-29

2004-07-13

Pascal, Leslie (Department: 2633)

Optical communications

Multiplex

Subcarrier multiplexing

C398S091000, C398S183000, C375S261000

Reexamination Certificate

active

06763193

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical communication systems and, in particular, to an optical communication system which optically combines baseband signals and passband signals and transmits the combined signals over a common optical fiber.
BACKGROUND OF THE INVENTION
An ever increasing communication need of today is to deliver multimedia services such as voice, data, high speed internet access, video conferencing, video on demand, and broadcast television video to small businesses and residences. Cost is the prominent issue for the deployment of such networks. Among various technologies that are currently available and being deployed, optical fiber extending to users—Fiber to the Home (FTTH)—is the preferred technology to meet present and future needs. Service providers are taking fiber as deep into their networks as their costs allow.
Two different optical fiber communication systems have evolved for carrying information in digital formats to homes and businesses. One system delivers information by a digitally modulated series of light pulses. These are referred to as baseband signals. A second system uses a plurality of frequency separated carriers. Each carrier is modulated to transmit a digital signal. These are passband signals. Each system has its own specialized equipment, its own physical plant and its own standards.
FIG. 1A
schematically illustrates a baseband system
10
comprising a central office
11
providing optical fiber connections to a plurality of homes
12
and businesses
13
. High power optical signals at single or multiple wavelengths are transmitted over a plurality of access fibers
15
A,
15
B,
15
C to respective optical power splitters and/or wavelength demultiplexers
16
A,
16
B,
16
C, and at each power splitter or demultiplexer, e.g.,
16
B, the high power signal is divided into a plurality of lower power or separate wavelength signals and transmitted over a respective plurality of end user fibers
17
A and
17
B. These signals are called downstream signals. The downstream signals are typically a digitally modulated baseband series of light pulses centered in the 1.3-1.6 &mgr;m wavelength band. Signals from the end users to the central office, called upstream signals, are typically digitally modulated baseband pulses in the same 1.3-1.6 wavelength band but at different wavelength from the downstream wavelength. They are transmitted in the reverse direction over the same fibers. The upstream signals can be buffered and time division multiplexed for burst transmission at the power splitters, e.g.,
16
B. Since this system does not employ any active electronic or photonic component between the central office and the users, it is called a Passive Optical Network (PON).
FIG. 1B
illustrates a simplified baseband modulation scheme. Typically, a digital
1
is represented by a light pulse in the series. A digital
0
, by the absence of a pulse in a pulse position. Alternatively, the signal can be inverted with a pulse representing digital
0
and its absence representing 1.
FIG. 2A
schematically illustrates a passband system
20
comprising a hub
21
, and a plurality of fibers
22
A,
22
B,
22
C connecting the hub to a respective plurality of fiber nodes
23
A,
23
B and
23
C. Each node is connected, as by a plurality of fibers or coaxial cables
24
A and
24
B to a plurality of homes
12
and businesses
13
.
FIG. 2B
illustrates the radio frequency spectrum of a typical digitally modulated passband signal. The signal comprises a plurality of different radio frequency (RF) carriers spaced apart in frequency (e.g. 6 MHz spacing in the NTSC system). Each of the carriers is modulated among a plurality of states to carry a higher order digital signal to encode plural bits for each modulation state. The modulation can be amplitude modulation, frequency modulation, phase modulation or a combination of them.
Digital passband signals are conventionally transmitted using two RF carriers that are frequency locked but 90 degrees out of phase. The two carriers are said to be in quadrature. The two carriers are separately amplitude modulated (AM), and the modulated carriers are combined to form a single RF output having both amplitude information corresponding to their vector sum and phase information corresponding to their vector angle. The technique is known as quadrature amplitude modulation or QAM.
FIG. 2C
illustrates the simplest case of QAM which occurs when each of the carriers has only two states (e.g. +V and −V). One carrier, is considered the reference carrier and is called the in-phase channel. Its amplitude is represented along the horizontal axis of FIG.
2
(C). The other carrier, 90° out of phase, is called the quadrature channel. Its amplitude is represented along the vertical axis. As can be seen from the diagram, if each carrier has two states (+V, −V), then there are four possible combined outputs, each of which can represent two bits of information: (0,0), (0,1), (1,0), (1,1). This simple modulation scheme is known as quadrature phase shift keying (QPSK).
Similar modulation schemes can be based on amplitude modulation of the carriers among a larger number of states. For example if both carriers can be modulated among four amplitudes, the combined output can represent 4×4=16 states, and the modulation is called 16 QAM modulation. Modulation using 8×8=64 states is 64 QAM. With an increasing number of modulation states, the required signal-to-noise ratio also increases.
In the past few years there has been an international effort from service providers and system manufacturers to define common specifications aimed at the extension of fiber all the way to homes and businesses to deliver existing and future services. These specifications are now part of International Telecommunication Union (ITU) standard G.983.1
According to G.983.1, all services are transported in baseband format in both the upstream and downstream directions on a power splitter-based system. In one variant of the network, a shared 155-Mbps baseband signal is transported downstream in the 1.5-&mgr;m band and the same bit rate is sent upstream in the 1.3-&mgr;m band on a single fiber. For low cost, a single transmitter in the central office and a single fiber can serve up to 32 users if the fiber is all the way to the user's premises. The number of users can even be greater if the receiver is at the curb and electrical signals are distributed to multiple dwellings. The G.983.1 specification calls for a minimum logical reach of at least 20 km and an optical power budget consistent with that reach. The specified downstream receiver sensitivity at a bit error ratio of <10
10
is −30 dBm for Class B operation and −33 dBm for Class C.
A downstream capacity of 155 Mbps shared among 32 end users is more than adequate for interactive services such as voice, data, or interactive video, but can be quickly exhausted by multichannel broadcast video, especially if high definition TV (HDTV) is to be delivered. One approach to dealing with broadcast video delivery in G983.1 is to increase the downstream bandwidth from 155 to 622 Mbps. This approach is very expensive and complicates video channel switching. Alternatively, video signals can be delivered on a separate fiber using a separate transmitter and a separate receiver. This approach is even more expensive. Accordingly, there is a need for a new approach which improves the performance and lowers the cost.
An optical communication system for gracefully combining both baseband and passband signals on a common fiber is described in applicant's U.S. patent application Ser. No. 09/432,936 filed Nov. 3, 1999 and entitled “Optical Communication System Combining Both Baseband and Passband Signals”, which is incorporated herein by reference. In this system, the baseband and passband signals are electrically combined, and the combined signal modulates an optical output signal at the Central office. The optical signal can be sent over an optical fiber to

Affiliated with

Also associated with

Rating

Optical communication system optically combining both... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical communication system optically combining both..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical communication system optically combining both... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3243592