U.S. patent number 5,243,627 [Application Number 07/748,594] was granted by the patent office on 1993-09-07 for signal point interleaving technique.
This patent grant is currently assigned to AT&T Bell Laboratories. Invention is credited to William L. Betts, Edward S. Zuranski.
United States Patent |
5,243,627 |
Betts , et al. |
September 7, 1993 |
Signal point interleaving technique
Abstract
Viterbi decoder performance in a data communication system using
2N-dimensional channel symbols N>1 can be further enhanced by an
interleaving technique which uses a distributed trellis encoder in
combination with a signal point interleaver.
Inventors: |
Betts; William L. (St.
Petersburg, FL), Zuranski; Edward S. (Largo, FL) |
Assignee: |
AT&T Bell Laboratories
(Murray Hill, NJ)
|
Family
ID: |
25010113 |
Appl.
No.: |
07/748,594 |
Filed: |
August 22, 1991 |
Current U.S.
Class: |
375/222; 375/262;
375/265; 375/298; 375/341; 714/795 |
Current CPC
Class: |
H03M
13/25 (20130101); H04L 27/3416 (20130101); H03M
13/27 (20130101) |
Current International
Class: |
H03M
13/25 (20060101); H04L 27/34 (20060101); H03M
13/00 (20060101); H03M 13/27 (20060101); H04L
005/12 () |
Field of
Search: |
;375/39,58,60,99
;371/43,37.5,2.1,45 ;341/81 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kuntz; Curtis
Assistant Examiner: Bocure; Tesfaldet
Attorney, Agent or Firm: Slusky; Ronald D. deBlasi; Gerard
A.
Claims
We claim:
1. Apparatus for forming a stream of trellis encoded signal points
in response to input information, said apparatus comprising
means for generating a plurality of streams of trellis encoded
channel symbols in response to respective portions of said input
information, each of said channel symbols being comprised of a
plurality of signal points, and
means for interleaving the signal points of said generated channel
symbols to form said stream of trellis encoded signal points, said
interleaving being carried out in such a way that the signal points
of each channel symbol are non-adjacent in said stream of trellis
encoded signal points and such that the signal points of adjacent
symbols in any one of said channel symbol streams are non-adjacent
in said stream of trellis encoded signal points.
2. The apparatus of claim 1 wherein said means for generating
generates three of said streams of trellis encoded channel symbols,
and wherein said means for interleaving causes there to be
interleaved between each of the signal points of each channel
symbol at least two signal points from other channel symbols of
said streams of trellis encoded channel symbols.
3. The apparatus of claim 1 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said means for
interleaving causes every N.sup.th signal point in said interleaved
signal point stream to be the N.sup.th signal point of a respective
one of said channel symbols.
4. The apparatus of claim 2 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said means for
interleaving causes every N.sup.th signal point in said interleaved
signal point stream to be the N.sup.th signal point of a respective
one of said channel symbols.
5. A modem comprising
means for receiving a stream of input bits,
means for dividing said stream of input bits into a stream of
uncoded bits and a plurality of streams of trellis bits,
means for independently trellis encoding each of said plurality of
streams of trellis bits to generate respective streams of data
words each identifying one of a plurality of predetermined subsets
of the channel symbols of a predetermined 2N-dimensional
constellation, N being an integer greater than unity, each of said
channel symbols being comprised of a plurality of signal
points,
means for selecting an individual channel symbol from each
identified subset in response to said stream of uncoded bits to
form a stream of channel symbols, and
means for generating a stream of output signal points, said signal
point stream being comprised of the signal points of the selected
channel symbols, the signal points of said signal point stream
being sequenced in such a way that signal points that are either a)
part of the same channel symbol, or b) part of channel symbols that
are adjacent to one another in said channel symbol stream, are
separated in said output stream by at least one other signal
point.
6. The apparatus of claim 5 wherein said trellis encoding means
includes a plurality of trellis encoder stage means for trellis
encoding respective ones of said streams of trellis bits.
7. The apparatus of claim 5 wherein said means for selecting
includes means for modulus converting said stream of uncoded
bits.
8. The apparatus of claim 5 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said means for
generating causes every N.sup.th signal point in said stream of
output signal points to be the N.sup.th signal point of a
respective one of said channel symbols.
9. Receiver apparatus for recovering information from a received
stream of trellis encoded signal points, said signal points having
been transmitted to said receiver apparatus by transmitter
apparatus which generates said signal points by generating a
plurality of streams of trellis encoded channel symbols in response
to respective portions of said information, each of said channel
symbols being comprised of a plurality of signal points, and by
interleaving the signal points of said generated channel symbols to
form said stream of trellis encoded signal points, said
interleaving being carried out in such a way that the signal points
of each channel symbol are non-adjacent in said stream of trellis
encoded signal points and such that the signal points of adjacent
symbols in any one of said channel symbol streams are non-adjacent
in said stream of trellis encoded signal points,
said receiver apparatus comprising
means for deinterleaving the interleaved signal points to recover
said plurality of streams of trellis encoded channel symbols,
and
a distributed Viterbi decoder for recovering said information from
the deinterleaved signal points.
10. The apparatus of claim 9 further comprising
a phase tracking loop, and
means for adapting the operation of said phase tracking loop in
response to minimum accumulated path metrics in said distributed
Viterbi decoder.
11. A method for forming a stream of trellis encoded signal points
in response to input information, said method comprising the steps
of
generating a plurality of streams of trellis encoded channel
symbols in response to respective portions of said input
information, each of said channel symbols being comprised of a
plurality of signal points, and
interleaving the signal points of said generated channel symbols to
form said stream of trellis encoded signal points, said
interleaving being carried out in such a way that the signal points
of each channel symbol are non-adjacent in said stream of trellis
encoded signal points and such that the signal points of adjacent
symbols in any one of said channel symbol streams are non-adjacent
in said stream of trellis encoded signal points.
12. The method of claim 11 wherein said generating step generates
three of said streams of trellis encoded channel symbols, and
wherein said interleaving step causes there to be interleaved
between each of the signal points of each channel symbol at least
two signal points from other channel symbols of said streams of
trellis encoded channel symbols.
13. The method of claim 11 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said
interleaving step causes every N.sup.th signal point in said
interleaved signal point stream to be the N.sup.th signal point of
a respective one of said channel symbols.
14. The method of claim 12 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said
interleaving step causes every N.sup.th signal point in said
interleaved signal point stream to be the N.sup.th signal point of
a respective one of said channel symbols.
15. A method for use in a modem, said method comprising the steps
of
receiving a stream of input bits,
dividing said stream of input bits into a stream of uncoded bits
and a plurality of streams of trellis bits,
independently trellis encoding each of said plurality of streams of
trellis bits to generate respective streams of data words each
identifying one of a plurality of predetermined subsets of the
channel symbols of a predetermined 2N-dimensional constellation, N
being an integer greater than unity, each of said channel symbols
being comprised of a plurality of signal points,
selecting an individual channel symbol from each identified subset
in response to said stream of uncoded bits to form a stream of
channel symbols, and
generating a stream of output signal points, said signal point
stream being comprised of the signal points of the selected channel
symbols, the signal points of said signal point stream being
sequenced in such a way that signal points that are either a) part
of the same channel symbol, or b) part of channel symbols that are
adjacent to one another in said channel symbol stream, are
separated in said output stream by at least one other signal
point.
16. The method of claim 15 wherein in said trellis encoding step a
plurality of trellis encoder stages trellis encode respective ones
of said streams of trellis bits.
17. The method of claim 15 wherein said selecting step includes the
step of modulus converting said stream of uncoded bits.
18. The method of claim 15 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said generating
step causes every N.sup.th signal point in said stream of output
signal points to be the N.sup.th signal point of a respective one
of said channel symbols.
19. A method for use in a receiver to recover information from a
received stream of trellis encoded signal points, said signal
points having been transmitted to said receiver apparatus by a
method which includes the steps of
generating a plurality of streams of trellis encoded channel
symbols in response to respective portions of said information,
each of said channel symbols being comprised of a plurality of
signal points, and
interleaving the signal points of said generated channel symbols to
form said stream of trellis encoded signal points, said
interleaving being carried out in such a way that the signal points
of each channel symbol are non-adjacent in said stream of trellis
encoded signal points and such that the signal points of adjacent
symbols in any one of said channel symbol streams are non-adjacent
in said stream of trellis encoded signal points,
said method comprising the steps of
deinterleaving the interleaved signal points to recover said
plurality of streams of trellis encoded channel symbols, and
using a distributed Viterbi decoder to recover said information
from the deinterleaved signal points.
20. The method of claim 19 wherein said receiver includes a phase
tracking loop and wherein said method comprises the further step of
adapting the operation of said phase tracking loop in response to
minimum accumulated path metrics in said distributed Viterbi
decoder.
21. Data communication apparatus comprising
means for receiving input information,
means for generating a plurality of streams of trellis encoded
channel symbols in response to respective portions of said input
information, each of said channel symbols being comprised of a
plurality of signal points,
means for interleaving the signal points of said generated channel
symbols to form a stream of trellis encoded signal points, said
interleaving being carried out in such a way that the signal points
of each channel symbol are non-adjacent in said stream of trellis
encoded signal points and such that the signal points of adjacent
symbols in any one of said channel symbol streams are non-adjacent
in said stream of trellis encoded signal points,
means for applying the stream of trellis encoded signal points to a
transmission channel,
means for receiving the stream of trellis encoded signal points
from the channel,
means for deinterleaving the interleaved signal points to recover
said plurality of streams of trellis encoded channel symbols,
and
a distributed Viterbi decoder for recovering said information from
the deinterleaved signal points.
22. The apparatus of claim 21 wherein said means for generating
generates three of said streams of trellis encoded channel symbols,
and wherein said means for interleaving causes there to be
interleaved between each of the signal points of each channel
symbol at least two signal points from other channel symbols of
said streams of trellis encoded channel symbols.
23. The apparatus of claim 21 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said means for
interleaving causes every N.sup.th signal point in said interleaved
signal point stream to be the N.sup.th signal point of a respective
one of said channel symbols.
24. The apparatus of claim 22 wherein said channel symbols are
2N-dimensional channel symbols, N>1, and wherein said means for
interleaving causes every N.sup.th signal point in said interleaved
signal point stream to be the N.sup.th signal point of a respective
one of said channel symbols.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the transmission of digital data
over band-limited channels.
Over the years, the requirements of modern-day digital data
transmission over band-limited channels--such as voiceband
telephone channels--have resulted in a push for higher and higher
bit rates. This push has led to the development and introduction of
such innovations as adaptive equalization, multi-dimensional signal
constellations, echo cancellation (for two-wire applications), and
trellis coding. Today, the data rates achieved using these and
other techniques are beginning to approach the theoretical limits
of the channel.
It has been found that various channel impairments, whose effects
on the achievable bit rate were relatively minor compared to, say,
additive white Gaussian noise and linear distortion, have now
become of greater concern. These include such impairments as
nonlinear distortion and residual (i.e., uncompensated-for) phase
jitter. Such impairments are particularly irksome in systems which
use trellis coding. Indeed, it has been found that the theoretical
improvement in Gaussian noise immunity promised by at least some
trellis codes is not realized in real-world applications where
these impairments are manifest. The principal reason this is so
appears to be that the noise components introduced into the
received signal samples are such as to worsen the effectiveness of
the Viterbi decoder used in the receiver to recover the transmitted
data.
U.S. Pat. No. 4,677,625, issued Jun. 30, 1987 to Betts et al,
teaches a method and arrangement in which, through the use of a
distributed trellis encoder/Viterbi decoder, the effects of many of
these impairments can be reduced. The invention in the Betts et al
patent recognizes that a part of the reason that the performance of
the Viterbi decoder is degraded by these impairments is the fact
that the noise components of channel symbols which closely follow
one another in the transmission channel are highly correlated for
many types of impairments. And it is that correlation which worsens
the effect that these impairments have on the Viterbi decoder.
Among the impairments whose noise is correlated in this way are
impulse noise, phase "hits" and gain "hits." All of these typically
extend over a number of adjacent channel symbols in the channel,
and thus all result in channel symbol noise components which are
highly correlated. The well-known noise enhancement characteristics
of linear equalizers also induce correlated noise in adjacent
channel symbols, as does uncompensated-for phase jitter. Also, the
occurrence of one of the relatively high power points of the signal
constellation can, in pulse code modulation (PCM) systems, for
example, give rise to noise on adjacent channel symbols which,
again, is correlated.
The Betts et al patent addresses this issue by distributing the
outgoing data to a plurality of trellis encoders in round-robin
fashion and interleaving the trellis encoder outputs on the
transmission channel. In the receiver, the stream of received
interleaved channel symbols is correspondingly distributed to a
plurality of trellis decoders. Since the successive pairs of
channel symbols applied to a particular trellis decoder are
separated from one another as they traverse the channel, the
correlation of the noise components of these channel symbol pairs
is reduced from what it would have otherwise been.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been realized that
the Viterbi decoder performance in a data communication system
using 2N-dimensional channel symbols can be further enhanced by an
interleaving technique which uses, in combination, a) the
aforementioned distributed trellis encoder/Viterbi decoder
technique and b) a signal point interleaving technique which causes
the constituent signal points of the channel symbols to be
non-adjacent as they traverse the channel.
In preferred embodiments of the invention, the interleaving is
carried out in such a way that every N.sup.th signal point in the
signal point stream traversing the channel is the N.sup.th signal
point of a respective one of the channel symbols. This criterion
enhances the accuracy with which the phase tracking loop in the
receiver performs its function.
Also in preferred embodiments, we have found that the use of three
parallel trellis encoders in conjunction with a signal point
interleaving regime in which the signal points of each channel
symbol are separated from one another by three signaling intervals
(bauds) provides an optimum or near-optimum tradeoff between signal
point/channel symbol separation and the decoding delay that is
caused by the interleaving.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing,
FIG. 1 is a block diagram of the transmitter section of a prior art
modem;
FIG. 2 is shows a signal constellation used by the transmitter of
FIG. 1;
FIG. 3 is a block diagram of the transmitter section of a modem
employing four-dimensional channel symbols and embodying the
principles of the invention;
FIG. 4 is a block diagram of the receiver section of a modem
embodying the principles of the invention which processes the
received four-dimensional channel symbols generated by the
transmitter of FIG. 3;
FIG. 5 is a signal point timing/sequencing chart helpful in
explaining the principles of the present invention;
FIG. 6 is a signal point interleaver which can be used in the
transmitter of FIG. 3 to interleave the signal points of
eight-dimensional channel symbols; and
FIG. 7 is a signal point deinterleaver which can be used in the
receiver of FIG. 4 to deinterleave the signal points of
eight-dimensional channel symbols.
DETAILED DESCRIPTION
FIG. 1 depicts the transmitter section of a prior art modem
employing a 2N-dimensional signaling scheme, N.gtoreq.1. The modem
receives input information in the form of a serial bit stream from
data terminal equipment (DTE) 111--illustratively a host computer.
That bit stream is then scrambled, or randomized, by randomizer 113
whose output bits are provided in serial form to serial-to-parallel
(S/P) converter 115.
Serial-to-parallel converter 115, in turn, provides, during each of
a succession of symbol intervals (comprised of N baud intervals),
some predetermined number of parallel bits on lead 109 and some
number of parallel bits on lead 108. (It will be appreciated that
whenever bits are provided in parallel in the modem, separate leads
are required to carry each of the bits.) The bits on lead 109 are
applied to trellis encoder 119.alpha., and are referred to as the
"trellis bits." The bits on lead 108 are applied to modulus
converter 116, and are referred to as the "uncoded bits."
To better understand how trellis encoder 119.alpha. and modulus
converter 116 work, reference is made to FIG. 2, which shows the
two-dimensional signal constellation that forms the basis of the
2N-dimensional signaling scheme illustratively used by the modem.
This constellation is comprised of 32 signal points, which are
divided into four subsets, A through D, each comprised of eight
signal points. The eight points of subset A are explicitly labeled
as A.sub.0 through A.sub.7. It may be noted that subsets C, B and D
can be arrived at by clockwise rotation of subset A by 90, 180 and
270 degrees, respectively. (Conventional differential encoding
circuitry within trellis encoder 119.alpha. exploits this
symmetry.) For reference, a single signal point of each of those
subsets is also shown on FIG. 2.
Consider, first, the case of N=1, i.e., a two-dimensional signaling
scheme. In this case, one trellis bit on lead 109 would be expanded
to two bits by trellis encoder 119.alpha. on lead 121. The four
possible values of those three bits 00, 01, 10, and 11 identify
subsets A, B, C and D, respectively. The successive 2-bit words on
lead 121 are represented as .alpha..sub.n, n=0,1,2. . . , where n
is an index that advances at the baud rate. At the same time, three
parallel bits would be provided on lead 108. These are converted by
modulus converter 116 into an index having a value within the range
(decimal) 0 to 7. The index value, represented in binary form on
lead 117, selects a particular signal point from the subset
identified on lead 121. Thus if lead 121 carries the two bits 00
while lead 117 carries the three bits 001, then signal point
A.sub.1 of the FIG. 2 constellation has been selected. The words on
leads 117 and 121 are applied to QAM encoder 124 which generates,
on lead 125, values representing the I (in-phase) and Q
(quadrature-phase) components of signal point A.sub.1. The signal
point generated on lead 125 in the n.sup.th baud interval is
denoted X.sub.n.sup..alpha., which is passed on to modulator 128 to
generate a passband line signal which is applied to the
communication channel. The superscript, .alpha., indicates that the
trellis encoder that was used to identify the subset for any
particular signal point was trellis encoder 119.alpha.. That is, of
course, a trivial notation as far as FIG. 1 goes inasmuch as
trellis encoder 119.alpha. is the only trellis encoder in the
modem. However, it is useful to introduce this notation because
more than one trellis encoder stage is used in preferred
embodiments of modems incorporating the principles of the present
invention as shown in later FIGS.
In the case of N>1, the operation is similar. Now, however, the
words on lead 109 are used by trellis encoder 119.alpha. to
sequentially identify on lead 121N subsets, while the words on lead
108 are used to generate N corresponding index values on lead 117.
The N signal points identified in this way are the component signal
points of a 2N-dimensional channel symbol, the first such symbol
being comprised of the signal points X.sub.0.sup..alpha., . . .
X.sub.(N-1).sup..alpha.. For example, a modem in which the
transmitter of FIG. 1 could be used may be a 14,400 bit per second
modem using four-dimensional coding (i.e., N=2) and a baud rate of
3200. In this case, nine bits from S/P converter 115 are used for
each four-dimensional symbol. Specifically, three parallel bits on
lead 109 are expanded into four bits on lead 121 to identify a pair
of subsets while six bits on lead 108 are used to select particular
signal points from those two subsets. Those two signal points are
thereupon communicated over the channel by QAM encoder 124 and
modulator 128 as described above.
Note that, implementationally, the 2N-dimensional channel symbol is
generated by having the trellis encoder identify, interdependently,
N subsets of the two-dimensional constellation of FIG. 2, then
select a two-dimensional signal point from each of the subsets thus
identified. The concatenation of the N two-dimensional signal
points thus selected is the desired 2N-dimensional channel symbol.
This process, however, can be understood as involving the direct
selection of a 2N-dimensional channel symbol. Viewed in this
context, the set of all possible combinations of N of the
two-dimensional subsets identified by N successive trellis encoder
outputs can be understood to be a set of 2N-dimensional subsets of
a 2N-dimensional constellation, the latter being comprised of all
possible combinations of N of the signal points of the
two-dimensional constellation. A succession of N outputs from the
trellis encoder identifies a particular one of the 2N-dimensional
subsets and a succession of N outputs from the modulus converter
selects a particular 2N-dimensional signal point from the
identified 2N-dimensional subset.
Modulus converter 116 is illustratively of the type disclosed in
co-pending, commonly-assigned U.S. patent application Ser. No.
588,658 filed Sep. 26, 1990 and allowed on May 21, 1991, hereby
incorporated by reference. Modulus converter 116 provides the modem
with the ability to support data transmission at various different
bit rates. Assume, for example, that the rate at which bits are
provided by DTE 111 decreases. The serial-to-parallel converter
will continue to provide its outputs on leads 108 and 109 at the
same baud rate as before. However, the upper limit of the range of
index values that are provided by modulus converter 116 on lead 117
will be reduced, so that, effectively, each of the four subsets A
through D, instead of having eight signal points, will have some
smaller number. Conversely if the rate at which bits are provided
by DTE 111 should increase over that originally assumed, the upper
limit of the range of index values, and thus the number of parallel
bits, that appear on lead 117 will be increased beyond eight and
the constellation itself will be expanded to accommodate the larger
number of signal points thus being selected. As an alternative to
using a modulus converter, fractional bit rates can be supported
using, for example, the technique disclosed in L. Wei,
"Trellis-Coded Modulation with Multidimensional Constellations,"
IEEE Trans. on Communication Theory, Vol. IT-33, No. 4, July 1987,
pp. 483-501.
Turning now to FIG. 3, the transmitter portion of a modem embodying
the principles of the invention is shown. This embodiment
illustratively uses the aforementioned four-dimensional, i.e., N=2,
signaling scheme. Many of the components are similar to those shown
in FIG. 1. Thus, in particular, the transmitter of FIG. 3--which
receives its input information in the form of a stream of input
bits from DTE 311--includes randomizer 313, which supplies its
output, on lead 314, to S/P converter 315. The latter outputs
uncoded bits to modulus converter 316. The transmitter further
includes four-dimensional QAM encoder 324 and modulator 328. The
trellis bits, on lead 309, are provided not to a standard single
trellis encoder, but to a distributed trellis encoder comprised of
three trellis encoder stages: trellis encoder stage 319.alpha.,
trellis encoder stage 319.beta., and trellis encoder stage
319.gamma..
Such a distributed trellis encoder, which is described in the
aforementioned Betts et al patent, generates a plurality of streams
of trellis encoded channel symbols in response to respective
portions of the input information. Specifically, a three-bit word
on lead 309 is supplied to trellis encoder stage 319.alpha.. The
next three-bit word on lead 309 is supplied to trellis encoder
stage 319.beta.. The next three-bit word is supplied to trellis
encoder stage 319.gamma., and then back to trellis encoder stage
319.alpha.. This distribution of the trellis bits to the various
trellis encoder stages is performed by switching circuit 331
operating under the control of symbol clock 325. The initial data
word outputs of the trellis encoders are subset identifiers
.alpha..sub.0 and .alpha..sub.1 for encoder stage 319.alpha.,
.beta..sub.2 and .beta..sub.3 for encoder stage 319.beta., and
.gamma..sub.4 and .gamma..sub.5 for encoder stage 319.gamma.,
followed by .alpha..sub.6 and .alpha..sub.7 for encoder stage
319.alpha., and so forth. These are supplied to four-dimensional
QAM encoder 324 by switching circuit 337--also operating under the
control of symbol clock 325--on lead 338 through a one-symbol delay
364 and lead 363, in order to compensate for a one-symbol delay
caused by modulus converter 316. Thus, the stream of subset
identifiers on lead 338 is .alpha..sub.0, .alpha..sub.1,
.beta..sub.2, .beta..sub.3, .gamma..sub.4, .gamma..sub.5,
.alpha..sub.6 . . . . Using the notation introduced above, then,
the output of encoder 324 on lead 325 is the stream of signal
points X.sub.0.sup..alpha., X.sub.1.sup..alpha.,
X.sub.2.sup..beta., X.sub.3.sup..beta., X.sub.4.sup..gamma.,
X.sub.5.sup..gamma., X.sub.6.sup..alpha. . . . , which is comprised
of three interleaved streams of trellis encoded channel symbols,
these streams being X.sub.0.sup..alpha., X.sub.1.sup..alpha.,
X.sub.6.sup..alpha., X.sub.7.sup..alpha., X.sub.12.sup..alpha. . .
. ; X.sub.2.sup..beta., X.sub.3.sup..beta., X.sub.8.sup..beta.,
X.sub.9.sup..beta., X.sub.14.sup..beta. . . . ; and
X.sub.4.sup..gamma., X.sub.5.sup..gamma., X.sub.10.sup..gamma.,
X.sub.11.sup..gamma., X.sub.16.sup..gamma. . . . . These, in turn,
are supplied, in accordance with the invention, to signal point
interleaver 341 which applies alternate ones of the signal points
applied thereto to lead 3412--which signal points appear
immediately at the interleaver output on lead 342--and to
one-symbol (Z.sup.-1) delay element 3411, which appear on lead 342
after being delayed therein by one symbol interval. The resulting
interleaved stream of trellis encoded signal points is
X.sub.0.sup..alpha., X.sub.-1.sup..gamma., X.sub.2.sup..beta.,
X.sub.1.sup..alpha., X.sub.4.sup..gamma., X.sub.3.sup..beta.,
X.sub.6.sup..alpha., X.sub.5.sup..gamma., X.sub.8.sup..beta.,
X.sub.7.sup..alpha., X.sub.10.sup..gamma., X.sub.9.sup..beta. . . .
(the signal point X.sub.-1.sup..gamma. being, of course, the signal
point applied to interleaver 341 just ahead of signal point
X.sub.0.sup..alpha.).
A discussion and explanation of how the interleaving just described
is advantageous is set forth hereinbelow. In order to fully set the
stage for that explanation, however, it will be first useful to
consider the receiver section of a modem which receives the
interleaved signal point stream.
Thus referring to FIG. 4, the line signal transmitted by the
transmitter of FIG. 3 is received from the channel and applied to
demodulator/equalizer 455 which, in conventional fashion--including
an input from phase tracking loop 457--generates a stream of
outputs on lead 456 representing the demodulator/equalizer's best
approximation of the values of the I and Q components of the signal
points of the transmitted interleaved signal point stream. These
outputs are referred to herein as the "received signal points."
(Due to distortion and other channel impairments that the
demodulator/equalizer is not able to compensate for, the I and Q
components of the received signal points, instead of having exact
integer values, can have any value. Thus a transmitted signal point
having coordinates (3, -5) may be output by the
demodulator/equalizer as the received signal point (2.945,
-5.001).) The stream of received signal points on lead 456 is
denoted X.sub.0.sup..alpha., X.sub.-1.sup..gamma.,
X.sub.2.sup..beta., X.sub.1.sup..alpha., X.sub.4.sup..gamma.,
X.sub.3.sup..beta., X.sub.6.sup..alpha., X.sub.5.sup..gamma.,
X.sub.8.sup..beta., X.sub.7.sup..alpha., X.sub.10.sup..gamma.,
X.sub.9.sup..beta. . . . .
The successive received signal points are deinterleaved in signal
point deinterleaver 441, which provides the opposite function to
interleaver 341 in the transmitter. The output of deinterleaver 441
on lead 442 is thus X.sub.0.sup..alpha., X.sub.1.sup..alpha.,
X.sub.2.sup..beta., X.sub.3.sup..beta., X.sub.4.sup..gamma.,
X.sub.5.sup..gamma., X.sub.6.sup..alpha., . . . , etc. (Although
not explicitly shown in the drawing, the same well-known techniques
used in modems of this general kind to identify within the stream
of received signal points the boundaries between successive symbols
is used to synchronize the operation of signal point deinterleaver
441 to ensure that received signal points X.sub.0.sup..alpha.,
X.sub.2.sup..beta., X.sub.4.sup..gamma. . . . are applied to delay
element 4411 while received signal points X.sub.1.sup..alpha.,
X.sub.3.sup..beta., X.sub.5.sup..gamma. . . . are applied to lead
4412.)
The received signal points on lead 442 are then distributed by
switching circuit 431 under the control of symbol clock 425 to a
distributed Viterbi decoder comprised of 4D Viterbi decoder stages
419.alpha., 419.beta. and 419.gamma.. Specifically, received signal
points X.sub.0.sup..alpha. and X.sub.1.sup..alpha. are applied to
decoder stage 419.alpha.; received signal points X.sub.2.sup..beta.
and X.sub.3.sup..beta. are applied to decoder stage 419.beta.; and
received signal points X.sub.4.sup..gamma. and X.sub.5.sup..gamma.
are applied to decoder stage 419.gamma.. The outputs of the three
decoder stages are then combined into a serial stream on lead 438
by switching circuit 437, also operating under the control of
symbol clock 425. Those outputs, representing decisions as to the
values of the transmitted signal points, are denoted X.sub.0,
X.sub.1, X.sub.2, X.sub.3, X.sub.4, X.sub.5, X.sub.6, . . . , the
.alpha., .beta. and .gamma. superscripts no longer being
needed.
In conventional fashion, the bits that represent each of the
decisions on lead 438 can be divided into bits that represent a)
the trellis bits that appeared on transmitter lead 309 and b) the
index values that appeared on transmitter lead 317. Those two
groups of bits are provided in the receiver on leads 461 and 462,
respectively. The latter group of bits are deconverted by modulus
deconverter 416 (also disclosed in the aforementioned '658 patent
application) back to uncoded bit values on lead 414. The operation
of the modulus deconverter imparts a one-symbol delay to the bits
on lead 414. Accordingly, the bits on lead 461 are caused to be
delayed by one symbol by delay element 464. The resulting combined
bits on lead 415 thus represent the stream of bits that appeared at
the output of randomizer 313 in the transmitter. These are
derandomized in the receiver by derandomizer 413 and the resulting
derandomized bit stream is applied to DTE 411 which may be, for
example, a computer terminal.
Referring to FIG. 5, one can see the improvement that is achieved
by the present invention.
Line I shows the stream of output signal points generated and
launched into the channel using one stage of trellis encoding and
no signal point interleaving. This is, of course, the prior art
arrangement shown in FIG. 1. Line II shows the effect of providing
a three-stage distributed trellis encoder but still no signal point
interleaving. This is the arrangement shown in the aforementioned
Betts et al patent. Note that the signal points of each channel
symbol operated on by a particular trellis encoder stage are
adjacent in the output signal point stream. For example, the second
signal point of the symbol X.sub.0.sup..alpha. X.sub.1.sup..alpha.
--namely signal point X.sub.1.sup..alpha. --is separated by five
baud intervals from the first (closer) signal point of the symbol
X.sub.6.sup..alpha. X.sub.7.sup..alpha. --namely signal point
X.sub.6.sup..alpha.. As noted earlier, such separation is
advantageous because the channel symbols which are processed one
after the other in a particular Viterbi decoder stage have noise
components which are not highly correlated.
Note, however, that the individual signal points of each channel
symbol, e.g., X.sub.0.sup..alpha. and X.sub.1.sup..alpha., are
adjacent to one another as they pass through the channel; and since
all the signal points of a channel symbol must be processed
serially in the same Viterbi decoder stage, this means that the
Viterbi decoder must process adjacent signal points that have
highly correlated noise components.
It is to this end that signal point interleaver 341 is included
within the transmitter in accordance with the invention. Firstly,
it may be noted from Line III that using the signal point
interleaver without the distributed trellis encoder--an arrangement
not depicted in the drawing--will, advantageously, cause the signal
points from the same channel symbol to be non-adjacent. Moreover,
there is further advantage in that a pair of channel symbols
processed serially by Viterbi decoder stage 419.alpha. traverses
the channel separated by five baud intervals rather than three,
thereby providing greater decorrelation of the noise components
thereof. Compare, for example, the span of baud intervals occupied
by signal points X.sub.0.sup..alpha. and X.sub.1.sup..alpha.,
X.sub.2.sup..alpha. and X.sub.3.sup..alpha. in Line I and the span
of baud intervals occupied by the same signal points in Line III.
Disadvantageously, however, the use of a single trellis encoding
stage brings back the problem that the distributed trellis encoder
solves, as described above. Thus, for example, although signal
points X.sub.0.sup..alpha. and X.sub.1.sup..alpha., which are from
the same channel symbol, are separated from one another when
traversing the channel, we find that, disadvantageously, signal
points X.sub.2.sup..alpha. and X.sub.1.sup..alpha., which are
signal points from two different channel symbols which will be
processed serially by the Viterbi decoder, traverse the channel
adjacent to one another.
Line IV shows that using the signal point interleaver with a
two-stage trellis encoder--also an arrangement not depicted in the
drawing--provides some improvement. Firstly, it may be noted that,
as in Line III, signal points from the same channel symbol remain
separated by three baud intervals. Additionally, pairs of channel
symbols processed sequentially by a given Viterbi decoder
stage--such as the channel symbols comprised of signal points
X.sub.0.sup..alpha. and X.sub.1.sup..alpha., X.sub.4.sup..alpha.
and X.sub.5.sup..alpha. --are still non-adjacent and, indeed, are
now separated by seven baud intervals, which is even greater than
the separation of five baud intervals provided in Line III.
Moreover, certain signal points that traverse the channel adjacent
to one another and which are from channel symbols which would have
been decoded sequentially in the one-trellis-encoding-stage case
are, in the two-trellis-encoding-stage case of Line IV, processed
by different Viterbi decoding stages. Signal points
X.sub.2.sup..beta. and X.sub.1.sup..alpha. are such a pair of
signal points. Note, however, that, disadvantageously, signal
points X.sub.1.sup..alpha. and X.sub.4.sup..alpha. traverse the
channel serially, and are from channel symbols which are serially
processed by the ".alpha." Viterbi decoder stage.
Referring, however, to Line V, which depicts the stream of signal
points output by the transmitter of FIG. 3, it will be seen that,
in accordance with the invention, there is still a
non-adjacency--indeed, a separation of at least three baud
intervals--between a) the signal points which belong to any
particular channel symbol (and which, therefore, are processed
serially by a particular Viterbi decoder stage) and b) the signal
points which belong to channel symbols which are processed serially
by a Viterbi decoder stage. Thus, for example, signal points
X.sub.1.sup..alpha. and X.sub.4.sup..gamma. are now processed by
different Viterbi decoder stages. Moreover, pairs of channel
symbols processed sequentially by a given Viterbi decoder
stage--such as the channel symbols comprised of signal points
X.sub.0.sup..alpha. and X.sub.1.sup..alpha., X.sub.6.sup..alpha.
and X.sub.7.sup..alpha. --are now separated by none baud
intervals.
Using more than three trellis encoder stages in the distributed
trellis encoder and/or a signal point interleaver that separates
signal points from the same channel symbol by more than three baud
intervals would provide even greater separation and could,
therefore, potentially provide even greater improvement in Viterbi
decoding. However, such improvement comes at a price--that price
being increased decoding delay--particularly as the number of
trellis encoders is increased beyond three. An engineering
trade-off can be made, as suits any particular application.
Moreover, it is desirable for the signal point interleaver to
provide a sequence in which every N.sup.th signal point in the
interleaved signal point stream is the N.sup.th signal point of a
channel symbol. (The reason this is desirable is described in
detail hereinbelow.) In the case of an N=2, four-dimensional
signaling scheme, this means that every second, that is "every
other," signal point in the interleaved stream is the second signal
point of the channel symbol from which it comes. In the case of an
N=4, eight-dimensional signaling scheme, this means that every
fourth signal point in the interleaved stream is the fourth signal
point of the channel symbol from which it comes. Indeed, this
criterion is in fact satisfied in the embodiment of FIG. 3. Note
that each one of signal points X.sub.0.sup..alpha.,
X.sub.2.sup..beta., X.sub.4.sup..gamma., X.sub.6.sup..alpha., . . .
, which appear as every other signal point in the interleaved
stream, is the second signal point of one of the four-dimensional
channel symbols. Note that not all rearrangements of the signal
points will, in fact, satisfy this criterion, such as, if the two
signal points of a channel symbol are separated by two, rather than
three, baud intervals.
Satisfying the above criterion is advantageous because it enhances
the accuracy with which phase tracking loop 457 performs its
function. This is so because the arrival of an N.sup.th signal
point of a given symbol means that all the signal points comprising
that channel symbol have arrived. This, in turn, makes it possible
to form a decision as to the identity of that channel symbol by
using the minimum accumulated path metric in the Viterbi decoder
stages. (Those decisions are fed back to the tracking loop by
decoder stages 419.alpha., 419.beta. 419.gamma. on leads 494, 495
and 496, respectively, via switching circuit 456.) Without having
received all of the signal points of a channel symbol, one cannot
take advantage of the accumulated path metric information but,
rather, must rely on the so-called raw sliced values, which is less
accurate. By having every N.sup.th signal point in the interleaved
stream be the N.sup.th signal point of a channel symbol, we are
guaranteed that the time between adjacent such path metric
"decisions" supplied to the phase tracking loop is, advantageously,
never more than N baud intervals.
The foregoing merely illustrates the principles of the invention.
Thus although the illustrative embodiment utilizes a
four-dimensional signaling scheme, the invention can be used with
signaling schemes of any dimensionality. In the general,
2N-dimensional, case each stage of the distributed trellis encoder
would provide N two-dimensional subset identifiers to switching
circuit 337 before the latter moves on to the next stage. And, of
course, each stage of the distributed Viterbi decoder would receive
N successive received signal points. The distributed trellis
encoder and distributed Viterbi decoder can, however, continue to
include three trellis encoders and still maintain, independent of
the value of N, a separation of three baud intervals in the channel
between signal points that are from channel symbols that are
adjacent in the trellis encoder. If a greater separation of such
signal points is desired, more stages can be added to the
distributed trellis encoder/Viterbi decoder, just as was noted
above for the four-dimensional case. However, when dealing with
2N-dimensional signaling where N>2, it is necessary to add
additional delay elements to the signal point
interleaver/deinterleaver in order to maintain a
three-baud-interval separation among the signal points from any
given channel symbol.
Consider, for example, the case of N=4, i.e., an eight-dimensional
case. Looking again at FIG. 3, the three (8D) stages of the
distributed trellis encoder would generate the three streams of
subset identifiers .alpha..sub.0 .alpha..sub.1 .alpha..sub.2
.alpha..sub.3 .alpha..sub.12 . . . , .beta..sub.4 .beta..sub.5
.beta..sub.6 .beta..sub.7 .beta..sub.16 . . . , and .gamma..sub.8
.gamma..sub.9 .gamma..sub.10 .gamma..sub.11 .gamma..sub.20 . . . ,
respectively. This would lead to the following stream of signal
points of eight-dimensional trellis encoded channel symbols at the
output of the QAM encoder on lead 325: X.sub.0.sup..alpha.
X.sub.1.sup..alpha. X.sub.2.sup..alpha. X.sub.3.sup..alpha.
X.sub.4.sup..beta. X.sub.5.sup..beta. X.sub.6.sup..beta.
X.sub.7.sup..beta. X.sub.8.sup..gamma. X.sub.9.sup..gamma.
X.sub.10.sup..gamma. X.sub.11.sup..gamma. X.sub.12.sup. .alpha. . .
. Signal point interleaving could be carried out by substituting
signal point interleaver 641 of FIG. 6 for interleaver 341.
Interleaver 641, in addition to direct connection 6414, includes
one-, two-, and three-symbol delay elements 6413, 6412 and 6411,
respectively.
The signal points on lead 325, after passing through interleaver
641, would appear on lead 342 in the following order:
X.sub.0.sup..alpha. X.sub.-3.sup..gamma. X.sub.-6.sup..beta.
X.sub.-9.sup..alpha. X.sub.4.sup..beta. X.sub.1.sup..alpha.
X.sub.-2.sup..gamma. X.sub.-5.sup..beta. X.sub.8.sup..gamma.
X.sub.5.sup..beta. X.sub.2.sup..alpha. X.sub.-1.sup..gamma.
X.sub.12.sup..alpha. X.sub.9.sup..gamma. X.sub.6.sup..beta.
X.sub.3.sup..alpha. X.sub.16.sup..beta. X.sub.13.sup..alpha.
X.sub.10.sup..gamma. X.sub.7.sup..beta. . . . where signal points
with negative subscripts are, of course, signal points that arrived
before signal point X.sub.0.sup..alpha. and were already stored in
the delay elements 6411, 6412 and 6413. Examination of this signal
point stream will reveal that there is either a three- or five-baud
separation between signal points of channel symbols that are
processed sequentially by the same trellis encoder stage, e.g.,
X.sub.3.sup..alpha. and X.sub.12.sup..alpha. ; that adjacent signal
points of any one channel symbol, e.g., X.sub.0.sup..alpha. and
X.sub.1.sup..alpha., are separated by five baud intervals; and that
the four signal points comprising any particular one channel symbol
are separated by fifteen baud intervals.
FIG. 7 shows the structure of a deinterleaver 741 that could be
used in the receiver of FIG. 4 in place of deinterleaver 441 in
order to restore the signal points of the eight-dimensional channel
symbols to their original order. This structure, which is the
inverse of interleaver 641, includes delay stages 7411, 7412 and
7413, as well as direct connection 7414.
It will be appreciated that, although various components of the
modem transmitter and receiver are disclosed herein for pedagogic
clarity as discrete functional elements and indeed--in the case of
the various switching circuits--as mechanical elements, those
skilled in the art will recognize that the function of any one or
more of those elements could be implemented with any appropriate
available technology, including one or more appropriately
programmed processors, digital signal processing (DSP) chips, etc.
For example, multiple trellis encoders and decoders can be realized
using a single program routine which, through the mechanism of
indirect addressing of multiple arrays within memory, serves to
provide the function of each of the multiple devices.
It will thus be appreciated that those skilled in the art will be
able to devise numerous arrangements which, although not explicitly
shown or described herein, embody the principles of the invention
and are within its spirit and scope.
* * * * *