Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-04-06
2002-10-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S238000, C359S199200, C359S199200
Reexamination Certificate
active
06459521
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical communication systems and, in particular, to a transmitter for digitally modulated RF sub-carriers.
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. Optical fiber extending closer to users—Fiber to the curb (FTTC) and hybrid fiber coaxial (HFC), or fiber all the way to the user—Fiber to the Home (FTTH)—are the technologies currently being deployed to meet present and future needs. Both the existing operators and overbuilders are taking fiber as deep into their networks and closer to the customers as their costs allow.
Two different optical fiber communication systems have evolved for carrying information to homes and businesses. Each system has its own specialized equipment, its own physical plant and its own standards. One system delivers information by a digitally modulated series of light pulses. These are referred to as baseband signals.
FIG. 1
illustrates a simplified baseband modulation scheme. Typically a digital
1
is represented by a light pulse in the series and 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
.
A second system uses a plurality of radio frequency separated carriers. Each carrier is modulated to transmit a higher order digital signal. These are passband signals.
FIG. 2
a
schematically illustrates a passband lightwave transmission 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. 2
b
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. 2
c
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. It's 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 using 8×8=64 states is 64-QAM. In an optical communication system, optical transmission of sub-carrier multiplexed (SCM) multi-channel M-ary quadrature amplitude modulation (QAM) signals has many advantages over the analog amplitude modulated vestigial side band (AM-VSB) signals. Some of the advantages include: requirements of lower carrier signal-to-noise ratio (CNR); less sensitivity to nonlinear distortion; high spectral efficiency; high system transmission capacity; and, the ability to transmit all multimedia services (telephony, digital video and data). Cable companies are upgrading their Hybrid Fiber Coax (HFC) networks to create a fully interactive two-way network to carry high bandwidth multimedia services into and out of homes. Because of these advantages coupled with higher revenue generating opportunities for service providers, arrival of High Definition Television (HDTV), availability of digital televisions and set top conversion boxes, transmission of video signals in near future is likely to be all-digital.
While data can advantageously be transmitted using a baseband technique, for video transmission a passband technique is preferred by both telecom and cable TV industry as well as by overbuilders because of technical, economic and management reasons. For these reasons the full service access network (FSAN) group of global telecommunication operators has recently drafted new standards, called G983.wdm, for ITU-T for adding digital video in passband to ATMPON baseband services utilizing frequency division multiplexing (FDM) and wavelength division multiplexing (WDM) techniques which is shown in FIG.
3
. Referring to
FIG. 3
there is shown the architecture of an exemplary WDM network to deliver digital video in passband over a separate wavelength along with baseband data using wavelength division multiplexing (WDM). Digital video can be delivered by utilizing QPSK in 950-2050 MHz band or 64-QAM in MMDS 216-422 MHz band or CATV 550-800 MHz band. Located at the central office
302
is a data optical line terminator
304
, a video optical line terminator
306
and a wavelength division multiplexer
308
. The data optical line terminator
304
and the video optical line terminator
306
are coupled to the wavelength division multiplexer
308
. The wavelength division multiplexer
308
is coupled to an optical fiber transmission link
310
. The optical fiber transmission link
310
is coupled to a 1:n splitter
312
, which splits the optical signal for delivery to a home
314
. Located at the home
314
is a wavelength division demultiplexer
316
, a data optical network terminator
318
, and a video optical network terminator
320
. The data optical network terminator
318
and the video optical network terminator
320
are coupled to the wavelength division demultiplexer
316
. The wavelength division demultiplexer
316
is coupled to the 1:n splitter
312
.
One of the more expensive network elements in the video lightwave transmission system is the video laser transmitter. For a low cost per user, a video transmitter needs to be shared by as many users as possible. If by suitably designing a laser transmitter, the optical/electrical (O/E) receiver sensitivity can be increased by ~4 dBo, two and half times more users can share the same transmitter. Note that dBo represents optical dB, dBe represents electrical dB, and dBm represents power with reference to 1.0 mW. Thus dBmo and dBme will represent optical and electrical dB with reference to 1.0 mW optical and electrical power, respectively.
Presently CATV and MMDS utilize a 64-QAM format for downstream services. QPSK (4QAM) or 16 QAM formats are used for upstream services typically utilizing a 1.3 &mgr;m Febry-Perot laser. When the value of M in M-ary increases by one, for a given bit error rate the required carrier signal to noise ratio (CNR) for a sub-carrier at the receiver increases by 3-dBe which is equivalent to 1.5 dBo decrease in receiver sensitivity. A 4 dBo increase in receiver sensitivity will allow 16-QAM and 64-QAM
Bakker Laurens
Chand Naresh
van Veen Doutje Tjitske
Agere Systems Guardian Corp.
Epps Georgia
Lowenstein & Sandler PC
Thompson Timothy
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