Note: Descriptions are shown in the official language in which they were submitted.
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A Time Diversity Method and Apparatus for Improving Transmission Bit Rate in a
Multicarrier System
Related Annlication
This application claims the benefit of the filing date of co-pending U.S.
Provisional
Application, Serial No. 60/164,543, filed November 10, 1999, entitled "Time
Diversity Method to
Improve Data Rate in Multicarrier Systems," the entirety of which provisional
application is
incorporated by reference herein.
Field of the Invention
This invention relates to communication systems using multicarrier modulation.
More
particularly, the invention relates to a method and apparatus for improving
the transmission bit rate
in a multicarrier modulation system.
Background of the Invention
I o In a conventional multicarrier communications system, transceivers
communicate over a
communication channel using multicarrier modulation, such as discrete
multitone modulation
(DMT). A DMT transmitter, such as a DMT modem, receives an input bit stream
comprising
information bits and modulates the information bits onto carrier signals
(carriers) or sub-channels
spaced within a usable frequency band of the communication channel. The
modulation occurs at a
15 symbol transmission rate of the system. The DMT transmitter typically
modulates the phase
characteristic (or phase) and amplitude of the carrier signals using an
Inverse Fast Fourier Transform
(IFFT) to generate a time domain signal (or transmission signal) that is a
linear combination of the
carrier signals. The DMT transmitter transmits the transmission signal to a
DMT receiver over the
communication channel. The receiver demodulates the received carrier signals
using a Fast Fourier
2o Transform.
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The number of information bits that each carrier signal carries during a
single DMT symbol
depends on the signal-to-noise ratio (SNR) of that carrier signal and the
associated bit-error rate
(BER) requirement of the communication channel. Typically, DSL communication
systems operate
with a BER of 1 x 10'' (i.e., one bit in ten million is received in error on
average). Different carriers
can have different SNR and therefore may carry a different number of bits at
the same BER. For
conventional multicarner communications systems, however, if a carrier signal
has a SNR for a
given BER that is less than a minimum SNR needed to modulate a complete
information bit, the
DMT transmitter does not use that particular carrier signal. For example, to
carry at least one
complete information bit at a BER of 1 x 10'', a carrier signals needs a
minimum uncoded SNR of
11.34 dB. Any carrier signal with an SNR less than 11.34 dB is not used and
consequently, in noisy
multicarrier communication systems, many earner signals can remain unused.
Thus, there remains a need for a system and method that can use previously
unused earner
signals that have a SNR that is less than the minimum SNR needed to transmit
one information bit at
a specified BER.
Summary of the Invention
One objective of the invention is to increase the transmission bit rate of
multicarner
modulation transceivers by using carrier signals with an signal-to-noise ratio
(SNR) that precludes
transmitting one information bit at a specified BER during a single DMT symbol
period. In one
aspect, the invention features a method in a multicarner modulation system
including two
transceivers in communication with each other using a transmission signal
having a plurality of
earner signals for modulating an input bit stream. One of the carrier signals
has a signal-to-noise
ratio (SNR) that precludes transmitting at a bit-error rate a complete
information bit of the input bit
stream on that one earner signal during a single symbol period. The same
information bit of the
input bit stream is modulated during multiple symbol periods on this earner
signal, to increase the
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effective SNR of that carrier signal and achieve the specified bit-error rate.
The multiple symbol
periods can be successive in order. At least one bit is allocated to the
carrier signal having the SNR
that precludes transmitting a complete information bit at the bit-error rate.
In one embodiment, a
transmission bit rate is determined, and the modulating of the same
information bit occurs when the
transmission bit rate is less than a minimum transmission bit rate. In another
embodiment, the
modulating of the same information bit occurs if the modulating will improve
the transmission bit
rate by at least a predefined threshold percentage.
In another aspect, the invention features a method for communicating over a
communication
channel. During a symbol period, one or more information bits of an input bit
stream are transmitted
to on one or more carrier signals at a first bit-error rate. During successive
symbol periods, a same
information bit of the input bit stream is transmitted on one of the carrier
signals at a second bit-error
rate. In one embodiment, the second bit-error rate is higher than the first
bit-error rate. The
successive symbol periods can include the symbol period during which one or
more information bits
of the input bit stream are transmitted on one or more carriers at the first
bit-error rate. The number
of bits transmitted per symbol period and the transmission bit rate at the
first bit-error rate are
thereby increased.
In another aspect, the invention features a method wherein a carrier signal
having an SNR
that precludes transmitting a complete information bit at a bit-error rate
during a single symbol
period is demodulated. The Garner is demodulated for successive symbol periods
to obtain partial
2o information regarding an information bit during each of the successive
symbol periods. The partial
information is combined to produce a complete information bit.
In another aspect, the invention features a method wherein a same information
bit is received
during successive symbol periods on a previously unusable carrier signal at a
second bit-error rate.
The previously unusable carrier signal was unusable because the carrier signal
was unable to carry
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bits at a first bit-error rate. The second bit-error rate is higher than the
first bit-error rate, thereby
increasing the transmission bit rate at the first bit-error rate.
Description of the Drawings
The invention is pointed out with particularity in the appended claims. The
advantages of the
invention described above, as well as further advantages of the invention, may
be better understood
by reference to the following description taken in conjunction with the
accompanying drawings, in
which:
Fig. 1 is a block diagram of an embodiment of a digital subscriber line (DSL)
communications system including a discrete multitone modulation (DMT)
transceiver in
1 o communication with a remote transceiver; and
Fig. 2 is a flow diagram of an embodiment of a process for increasing the
transmission bit
rate of the communication system.
Detailed Description
Fig. 1 shows a digital subscriber line (DSL) communication system 2 including
a discrete
15 multitone modulation (DMT) transceiver 10 in communication with a remote
transceiver 14 over a
communication channel 18 using a transmission signal 38 having a plurality of
carrier signals. The
DMT transceiver 10 includes a DMT transmitter 22 and a DMT receiver 26 and the
remote
transceiver 14 includes a transmitter 30 and a receiver 34. Although described
with respect to
discrete multitone modulation, the principles of the invention apply also to
other types of
2o multicarrier modulation, such as, but not limited to, orthogonally
multiplexed quadrature amplitude
modulation (OQAM), discrete wavelet multitone (DWMT) modulation, and
orthogonal frequency
division multiplexing (OFDM).
The communication channel 18 provides a downstream transmission path from the
DMT
transmitter 22 to the receiver 34, and an upstream transmission path from the
transmitter 30 to the
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DMT receiver 26. In one embodiment, the communication channel 18 is a pair of
twisted wires of a
telephone subscriber line. In other embodiments, the communication channel 18
is a fiber optic
wire, a quad cable, consisting of two pairs of twisted wires, or a quad cable
that is one of a star quad
cable, a Dieselhorst-Martin quad cable, and the like. In a wireless
communication system wherein
the transceivers 10, 14 are wireless modems, the communication channel 18 is
the air through which
the transmission signal 38 travels between the transceivers 10, 14.
As shown in Fig. 1, the DMT transmitter 22 includes a QAM encoder 42, a bit
allocation
table (BAT) 44, and a modulator 46. The transmitter 30 of the remote
transceiver 14 comprises
equivalent components as the DMT transmitter 22. Although this embodiment
specifies a detailed
l0 description of the DMT transmitter 22, the inventive concepts apply also to
the receivers 34, 36,
which have similar components to that of the DMT transmitter 22, but perform
inverse functions in a
reverse order.
The QAM encoder 42 maps an input serial data bit stream 54, which consists of
information
bits, into N parallel QAM symbols 58, where N represents the number of carrier
signals generated
by the modulator 46. The modulator 46 uses an inverse fast Fourier transform
(IFFT) to change the
QAM symbols 58 into a transmission signal 38 comprised of a sequence of DMT
symbols 70. Each
Garner signal of the transmission signal 38 is modulated with a different QAM
symbol 58. In one
embodiment, a pilot tone is included in the transmission signal 38 to provide
a reference signal for
coherent demodulation of the carrier signals at the receiver 34 during
reception of the transmission
signa138.
In general, the modulator 46 modulates information bits on carrier signals on
a DMT symbol
by DMT symbol basis. Each DMT symbol period is approximately 250 ms, which
corresponds to a
4 kHz DMT symbol rate. The number of bits modulated on a particular Garner
signal during a DMT
symbol period depends on the signal-to-noise ratio (SNR) of that carrier
signal at a specified BER.
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The modulator 46 can modulate several information bits on each carrier signal
when the
communication channel 18 has low noise (i.e., a high SNR on each carrier
signal) and thus achieves
a high system transmission bit rate. If the conditions of the communication
channel 18 are poor (i.e.,
noisy), the SNR can be low and the number of information bits modulated on
each carrier signal
few, resulting in a low system transmission bit rate. For example, Table 1
below shows the required
uncoded SNR at a 1 x 10'~ BER for modulating one through eight information
bits on a carrier signal
during one DMT symbol period using QAM.
TABLE 1: Required Uncoded SNR on a Carrier During One Symbol Period for BER =
1 x 10''
Bits er carrier Re uired SNR for BER
= 1 x 10''
1 11.34 dB
2 ~ 14.32 dB
3 19.11 dB
4 21.31 dB
24:46 dB
6 27.54 dB
7 30.59 dB
8 33.61 dB
Typically, in a DMT system with many carrier signals, the SNR of one or more
carrier
to signals is too low to carry a full information bit at the specified bit-
error rate. For example, when the
SNR of a carrier signal is less than 11.34 dB, such a SNR precludes the
transmission of a complete
bit of information on that carrier signal at a BER of 1 x 10''. In accordance
with the principles of the
invention, the transceivers 10, 14 make use of these carrier signals that
might otherwise remain
unused for conveying information over the communication channel 18. Hereafter,
such carrier
signals are referred to as recovered carrier signals. As described further
below, the transmitter 22
transmits one or more information bits on the recovered carrier signal for
successive DMT symbol
periods at a higher BER than the specified BER. The effect of transmitting an
information bit on a
recovered carrier signal for successive DMT symbol periods at the higher BER,
although that Garner
signal cannot transmit a complete information bit at the specified BER, is to
increase the effective
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SNR of the recovered carrier signal and achieve the specified BER.
For example, if the SNR of a recovered carrier signal is substantially
equivalent to 8.34 dB
and the SNR required to modulate one information bit during one DMT symbol
period is
approximately 11.34 dB to achieve a 1 x 10-7 BER, the modulator 46 modulates
one information bit
over two successive DMT symbols 70. Table 2 below illustrates an embodiment of
the required
SNR to modulate an information bit in a DMT communication system 2 with a BER
of 1 x 10-x. As
shown in column 2, another way of expressing the transmission of one bit over
successive DMT
symbol periods is as a fraction of a bit being transmitted on a carrier in a
single DMT symbol period.
TABLE 2: Required SNR on a carrier during one symbol period for BER = 1 x 10-~
Number of DMT SymbolsBits per Garner signalRequired SNR for
for 1 per one
bit transmission DMT s bol eriod BER = 1 x 10~~
2 '~Z 8.34 dB
4 '~4 5.34 dB
8 1~8 2.34 dB
In other embodiments, the transmitter 22 transmits one or more information
bits on the
recovered carrier signal during non-successive DMT symbol periods at a higher
BER than the
specified BER. For example, the transmitter 22 can transmit the same
information bit on a
recovered carrier signal during every other DMT symbol period (e.g., during
the first and third DMT
t 5 symbol periods). Other examples include transmitting the same information
bit over every third,
fourth, fifth DMT symbol period, and so on. Thus, the transmitter 22 can
transmit the complete
information bit on a recovered carrier signal using non-successive DMT symbol
periods as long as
the receiver 34 knows which DMT symbol periods the transmitter 22 is using to
transmit that
information bit.
2o In yet other embodiments, the transmitter 22 can transmit more than one bit
on a recovered
carrier signal during a single DMT symbol period. For such embodiments, the
BER at which the
multiple bits are transmitted is higher than if only one information bit was
transmitted on that
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recovered carrier signal. Further, the transmitter 22 transmits the same
information bits on the
recovered carrier signal for a greater number of successive DMT symbols than
needed for
transmitting one information bit. For example, if the SNR is 8.34 dB, the
recovered Garner signal
can carry two bits over four successive DMT symbols 70 to achieve a BER of 1 x
10-x.
The BAT 44 is in communication with the modulator 46 to specify the number of
bits carried
by each carrier signal. For recovered Garner signals, the BAT 44 in one
embodiment allocates one
bit and specifies additional information that indicates the number of DMT
symbol periods needed to
convey the complete information bit. Thus, the BATs 44, 44' identify which
carrier signals are being
used to convey information bits over more than one DMT symbol period and the
number of DMT
1 o symbol periods required to transmit information bits on that carrier
signal. In another embodiment,
the BAT specifies fractions of bits to indicate the number of DMT symbol
periods needed to convey
the complete information bit.
Fig. 2 shows embodiments of a process used by the DMT transceiver 10 and the
remote
transceiver 14 for communicating over the communication channel 18. During
initialization of the
15 transceivers 10, 14, the remote receiver 34 determines (step 204) the
number of bits to be carried by
each carrier signal. For each carrier signal, the remote receiver 34 measures
the SNR for a specified
bit-error rate. The measured SNR limits the number of bits that the carrier
signal can carry and
achieve the specified bit-error rate. For example, Table 1 described above
shows the SNR required
for a carrier signal to convey one through eight bits at a bit-error rate of 1
x 10-x.
20 The receiver 34 then determines (step 206) from the measured SNR of the
recovered carrier
signal how many DMT symbol periods are needed to convey the information bit
completely at the
specified BER. For example, when the measured SNR is 8.34 dB, the receiver 34
determines that
two DMT symbol periods are needed to carry the complete information bit at the
specified bit-error
rate of 1 x 10-x. Approximately one-half of the bit information can be
conveyed during each of the
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two symbol periods; in effect, during each symbol period the carrier signal
conveys partial
information or a fraction of the information bit. If, for example, the
measured SNR is instead 5.34
dB, then the receiver determines that four DMT symbol periods are needed to
convey the complete
information bit.
The remote receiver 34 then communicates (step 208) the number of bits
allocated to each
carrier signal to the transmitter 22. For carrier signals with an SNR that
precludes conveying a
complete bit during a single symbol period, the remote receiver 34 can specify
the number of DMT
symbol periods needed to convey the complete information bit or the fraction
of the information bit
conveyed during each DMT symbol period.
l0 The transmitter 22 and remote receiver 34 each create (step 212) its copy
of the bit allocation
table 44, 44', which specify the number of bits allocated to each carrier
signal, in accordance with
the information determined by the receiver 34 and communicated to the
transmitter 22. For
recovered carrier signals, the BAT 44 in one embodiment allocates one bit and
specifies additional
information that indicates the number of DMT symbol periods needed to convey
the complete
15 information bit. In another embodiment, the BAT specifies the fraction of
the information bit
conveyed during each DMT symbol period for recovered carrier signals.
In one embodiment, shown in phantom, the transmitter 22 determines (step 216)
whether to
use recovered carrier signals to carry information bits based on one or a
combination of the
following factors: (1) the transmission bit rate of the communication system;
or (2) the amount of
2o improvement in the transmission bit rate gained by using recovered carrier
signals for carrying
information bits. With respect to the first factor, the transceiver 10
compares the transmission bit
rate of the DSL communication system 2 to a minimum transmission bit rate
(e.g., 256 kilobits per
second (kb/s)). If the transmission bit rate of the DSL communication system 2
is greater than or
equal to the minimum transmission bit rate, the transmitter 22 does not use
(step 218) recovered
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carrier signals to carry information bits. If, instead, the transmission bit
rate is less than the
minimum bit rate, the transmitter 22 can modulate information bits on
recovered carrier signals.
With respect to the second factor, the transmitter 22 can modulate information
bits on recovered
carrier signals if using of such recovered carrier signals will increase the
transmission bit rate of the
DSL communication system 2 by a predetermined amount. For example, if the
increase in the
transmission bit rate is equal to or exceeds a predefined threshold percentage
(e.g., 10%), the
transmitter 22 then uses recovered carrier signals for transmitting
information bits.
When using recovered carrier signals to convey information, the transmitter 22
modulates
(step 226) the same information bits) on the recovered carrier signals for two
or more successive
to DMT symbols. The number of successive DMT symbols over which the same
information bit is
transmitted is based on the SNR of the recovered earner signal and the BER at
which the modulated
bits are transmitted. Thus, some carrier signals carry information bits that
are transmitted
completely in a single DMT symbol period at a first BER, and one or more
recovered carrier signals
carry information bits over two or more successive DMT symbols 70 at a second
BER (per single
DMT period) that is greater than the first BER. By combining within a single
DMT symbol period
information bits on recovered carrier signals with information bits on the
other carrier signals, the
transmitter 22 achieves an increase in the transmission bit rate at the first
BER.
For example, one embodiment of the BAT 44 can allocate one bit to carrier
signal #l, one bit
to carrier signals #2 and #3, and three bits to carrier signals #4 and #5.
Further, the BAT 44 can
2o identify earner signal #1 as a recovered earner signal that carries the
same information bit for two
successive DMT symbols. Thus, carrier signals #2, #3, #4, and #5 carry new
information bits during
each of the two successive DMT symbols 70 at the first BER, while the
recovered carrier signal #1
carries the same information bit over both of the two successive DMT symbols
70 at the second
BER.
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In another embodiment, the second of the two successive DMT symbol periods can
be used
to carry only the information bits on the recovered carrier signals (e.g.,
carrier signals #2, #3, #4, and
#5 are unused). In still other embodiments, the transmitter 22 transmits the
information bits at the
first BER during one DMT symbol 70, and subsequently transmits the information
bits on the
recovered Garner signals at the second BER during the two successive DMT
symbol periods.
Although each of such embodiments may provide little or no transmission bit
rate improvement,
such embodiments may simplify the design of the transmitter 22 or receiver 34.
The DMT transmitter 22 then transmits (step 228) the transmission signal 38 to
the receiver
34. The receiver 34 demodulates (step 230) the transmission signal 38 for
successive DMT symbol
1o periods to obtain partial information about the information bit during each
of the successive DMT
symbol periods. The receiver 34 then linearly combines (step 232) the partial
information of the
information bit to generate the complete information bit.
In one embodiment, the receiver 34 uses the first DMT symbol period in the
series of
successive DMT symbol periods to determine whether the same information bit is
modulated over
the successive DMT symbol periods. For example, if the receiver 34 determines
that the phase of
the carrier signal is approximately 90°, the receiver 34 anticipates
that the information bit has a value
that is equal to one. The receiver 34 therefore does not need to wait for the
following DMT
symbols) to bring additional partial bit information so that the receiver 34
can generate the complete
information bit. Thus, in this embodiment receiving an information bit over
multiple DMT symbols
does not increase the delay of demodulating that information bit. If, however,
the phase of the
carrier signal is less than 90°, such as 45°, the receiver 34
may need additional information provided
by subsequent DMT symbols) to determine the value of the information bit. For
instance, the 45°
phase might correspond to a bit value of one or might be the result of noise.
In such cases, the
receiver 34 can still decide from this phase that the information bit has a
value of 1 and then use
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error checking to determine later if the decision was erroneous.
During the operation of the communications system 2, the conditions of the
communication
change may change to affect the SNRs of one or more carrier signals. In one
embodiment, the
receiver 34 and the transmitter 22 dynamically exchange bit allocation
information corresponding to
the new communication channel 18 conditions. For Garner signals with SNRs that
have fallen below
the minimum SNR for the specified BER, and for recovered carrier signals with
SNRs that change
but remain below the minimum SNR, such exchanged information may include the
number of DMT
symbol periods needed to transmit a complete information bit, as described
above. The exchange of
the information can occur at the boundary of complete information bit (e.g.,
after the second DMT
l0 symbol 70 conveying an information bit that is completely transmitted in
two DMT symbols).
While the invention has been shown and described with reference to specific
preferred
embodiments, it should be understood by those skilled in the art that various
changes in form and
detail may be made therein without departing from the spirit and scope of the
invention as defined
by the following claims. For example, although the specification uses DSL to
describe the
invention, it is to be understood that various other forms of DSL can be used,
i.e., ADSL, VDSL,
SDSL, HDSL, HDSL2, or SHDSL. It is also to be understood that the principles
of the invention
apply to various types of applications transported over DSL systems (e.g.,
telecommuting, video
conferencing, high speed Internet access, video-on demand).
What is claimed is:
