U.S. patent application number 10/305594 was filed with the patent office on 2003-06-19 for system and method for performing combined multi-rate convolutional coding.
This patent application is currently assigned to HUGHES ELECTRONICS CORPORATION. Invention is credited to Antia, Yezdi, Hammons, A. Roger JR., Ritterbush, Olga, Shi, Zhen-Liang.
Application Number | 20030112879 10/305594 |
Document ID | / |
Family ID | 23523966 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030112879 |
Kind Code |
A1 |
Antia, Yezdi ; et
al. |
June 19, 2003 |
System and method for performing combined multi-rate convolutional
coding
Abstract
A system and method for establishing an integrated forward error
correction (FEC) scheme to perform multi-rate encoding on different
priority data bits of a channel access message transmitted on a
random access channel between devices of a communications network,
such as between an access terminal and a base station of a
satellite-based communications network. The channel access message
includes a first data group representing first information and a
second data group representing second information, which is
transmitted between an access terminal and a base station in a
satellite-based communications network. The system and method
encodes the second data group at an encoding rate to provide a
second encoded data group, and encodes the first data group at the
same encoding rate to provide a first encoded data group. The
encoding of the first and second data groups is performed by a
single encoder, such as a rate 1/4 convolutional encoder. The
second encoded data group is transmitted from the access terminal
to the base over a random access channel. The second encoded data
group further can be punctured during transmission to in effect
decrease its coding rate, for example, to rate 1/2 coding. The
first encoded data group is transmitted from the access terminal to
the base station, and is then retransmitted from the access
terminal to the base station to in effect increase the rate of
coding of the first encoded data group to, for example, 1/8 coding.
At the base station, a combiner/demodulator combines the
transmitted and retransmitted first encoded data group, and the
combined first encoded data groups and the second encoded data
group are then decoded by a decoder.
Inventors: |
Antia, Yezdi; (Gaithersburg,
MD) ; Shi, Zhen-Liang; (Germantown, MD) ;
Hammons, A. Roger JR.; (North Potomac, MD) ;
Ritterbush, Olga; (Germantown, MD) |
Correspondence
Address: |
Hughes Electronics Corporation
Patent Docket Administration
Mail Stop A109, Bldg. 1
P.O. Box 956
El Segundo
CA
90245-0956
US
|
Assignee: |
HUGHES ELECTRONICS
CORPORATION
|
Family ID: |
23523966 |
Appl. No.: |
10/305594 |
Filed: |
November 26, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10305594 |
Nov 26, 2002 |
|
|
|
09386053 |
Aug 30, 1999 |
|
|
|
6487251 |
|
|
|
|
Current U.S.
Class: |
375/259 |
Current CPC
Class: |
H04L 1/007 20130101;
H04L 1/0041 20130101; H04L 1/0068 20130101; H04L 1/08 20130101 |
Class at
Publication: |
375/259 |
International
Class: |
H04L 027/00 |
Claims
What is claimed is:
1. A method for encoding data transmitted between a transmitter and
a receiver in a communications network, said data including a first
data group representing first information and a second data group
representing second information, the method comprising the steps
of: encoding said second data group at an encoding rate to provide
a second encoded data group; encoding said first data group at said
encoding rate to provide a first encoded data group; transmitting
said second encoded data group between said transmitter and said
receiver; transmitting said first encoded data group between said
transmitter and said receiver; and retransmitting said first
encoded data group between said transmitter and said receiver to in
effect increase a rate of coding of said first encoded data
group.
2. A method as claimed in claim 1, wherein: said retransmitting
step in effect doubles said rate of coding of said first encoded
data group at which said first encoded data group was encoded by
said first data group encoding step.
3. A method as claimed in claim 1, further comprising the step of:
puncturing said second encoded data group to in effect decrease a
rate of coding of said second encoded data group.
4. A method as claimed in claim 3, wherein: said puncturing step in
effect halves said rate of coding of said second encoded data group
at which said second encoded data group was encoded by said second
data group encoding step.
5. A method as claimed in claim 3, wherein: said puncturing step is
performed prior to or while performing said second encoded data
group transmitting step.
6. A method as claimed in claim 3, wherein: after performing said
transmitting steps, said retransmitting step and said puncturing
step, said rate of coding of said second encoded data group is in
effect 1/4 of said rate of coding of said first encoded data
group.
7. A method as claimed in claim 1, wherein: said transmitter is at
an access terminal of a satellite-based communications network and
said receiver is at a base station of said satellite-based
communications network; and said step of transmitting said second
encoded data group and said steps of transmitting and
retransmitting said first encoded data group transmit their
respective said first and second encoded data groups between said
access terminal and said base station.
8. A method as claimed in claim 7, wherein: said step of
transmitting said second encoded data group and said steps of
transmitting and retransmitting said first encoded data group
transmit their respective said first and second encoded data groups
from said access terminal to said base station.
9. A method as claimed in claim 1, wherein: said second data group
encoding step is performed prior to said first data group encoding
step.
10. A method as claimed in claim 1, wherein said data includes a
third data group, and the method further comprises the steps of:
encoding said third data group at said encoding rate to provide a
third encoded data group; transmitting said third encoded data
group between said transmitter and said receiver; and
retransmitting said third encoded data group between said
transmitter and said receiver to in effect increase a rate of
coding of said third encoded data group.
11. A method as claimed in claim 10, wherein: said second data
group encoding step is performed before said first data group
encoding step, and said first data group encoding step is performed
before said third data group encoding step.
12. A method as claimed in claim 1, further comprising the step of:
receiving at said receiver said first encoded data groups
transmitted and retransmitted by said first encoded data group
transmitting and retransmitting step; and combining said received
first encoded data groups.
13. A system for encoding data transmitted between a first and
second devices in a communications network, said data including a
first data group representing first information and a second data
group representing second information, the system comprising: an
encoder, adapted to encode said second data group at an encoding
rate to provide a second encoded data group, and to encode said
first data group at said encoding rate to provide a first encoded
data group; and a transmitter, adapted to transmit said second
encoded data group between said first and second devices, to
transmit said first encoded data group between said first and
second devices, and to retransmit said first encoded data group
between said first and second devices to in effect increase a rate
of coding of said first encoded data group.
14. A system as claimed in claim 13, wherein: said transmitter in
effect doubles said rate of coding of said first encoded data group
at which said first encoded data group was encoded by said encoder
by retransmitting said first encoded data group.
15. A system as claimed in claim 13, wherein: said transmitter is
adapted to apply a puncturing mask to said second encoded data
group to in effect decrease a rate of coding of said second encoded
data group.
16. A system as claimed in claim 15, wherein: said puncturing mask
applied by said transmitter in effect halves said rate of coding of
said second encoded data group at which said second encoded data
group was encoded by said encoder.
17. A system as claimed in claim 15, wherein: after said
transmitter has transmitted and retransmitted said first encoded
data, and has punctured and transmitted said second encoded data,
said rate of coding of said second encoded data group is in effect
1/4 of said rate of coding of said first encoded data group.
18. A system as claimed in claim 13, wherein: said first and second
device include an access terminal and a base station, respectively,
of a satellite-based communications network.
19. A system as claimed in claim 13, wherein: said transmitter is
at an access terminal of a satellite-based communications network
to transmit said second encoded data group and transmit and
retransmit said first encoded data group from said access terminal
to a base station of said satellite-based communications
network.
20. A system as claimed in claim 13, wherein: said encoder is
adapted to encode said second data group prior to encoding said
first data group.
21. A system as claimed in claim 13, wherein: said data includes a
third data group; said encoder is adapted to encode said third data
group at said encoding rate to provide a third encoded data group;
and said transmitter is adapted to transmit said third encoded data
group between said first and second devices, and retransmit said
third encoded data group between said first and second devices, to
in effect increase a rate of coding of said third encoded data
group.
22. A system as claimed in claim 21, wherein: said encoder is
adapted to encode said second data group before encoding said first
data group, and is adapted to encode said first data group before
encoding said third data group.
23. A system as claimed in claim 13, further comprising: a
combiner, adapted to receive said first encoded data groups
transmitted and retransmitted by said transmitter, and to combine
said received first encoded data groups.
24. A system as claimed in claim 23, wherein: said combiner is at
said second device, which includes a base station of a
satellite-based communications network.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a system and method, for
use with a network, such as a satellite-based communications
network, which establishes an integrated forward error correction
(FEC) scheme to perform multi-rate encoding/decoding on different
priority data bits transmitted, for example, on a random access
channel. More particularly, the present invention relates to a
system and method for using a single encoder to perform multi-rate
encoding/decoding on different priority data bits of a channel
request message transmitted on a random access channel from an
access terminal to a base station of a satellite-based
communications network.
[0003] 2. Description of the Related Art
[0004] A satellite-based communications network, such as a
geosynchronous earth orbit mobile (GEM) satellite communications
network, comprises at least one geosynchronous earth orbit
satellite, a ground-based advanced operations center (AOC) and
spacecraft operations center (SOC) associated with the satellite,
at least one ground-based gateway station (GS), and at least one
access terminal (AT), which is typically a hand-held or vehicle
mounted mobile telephone. The satellite enables the access terminal
to communicate with other access terminals, or with other
telephones in the terrestrial public switched telephone network
(PSTN), via the gateway stations under the control of the gateway
stations. The AOC provides system-wide resource management and
control functions for its respective satellite, and the SOC
controls on-orbit satellite operations for its respective
satellite.
[0005] When an access terminal is operated to establish a radio
resource connection, it generates and transmits a channel request
message to the network on a random access channel (RACH) at a
frequency assigned by the gateway station to a spot beam covering
an area in which the access terminal is located. A channel request
message includes data used to represent the mobile access terminal,
as well as contention resolution and timing synchronization
information.
[0006] Typically, the data bits of a channel request message are
classified into two classes, namely, Class I bits and Class II
bits. The Class I bits generally consists of a short block of data
bits, and represent high priority information, such as data for
contention resolution and timing synchronization between the access
terminal and the base station, which is essential in enabling the
access terminal to gain access to the satellite communications
network. Therefore, the Class I data bits must be received with
high probability by the base station the first time they are
transmitted.
[0007] On the other hand, the Class II bits typically consist of a
longer block of data bits, and are not as crucial as the Class I
bits for call set up. Class II bits thus have a lower priority than
the Class I bits. Class II bits can include, for example, data for
accelerating call set up time, which can include information such
as the called party number, location of the access terminal placing
the call, identification of the service provider for the access
terminal, and so on.
[0008] During transmission of a channel request message, an access
terminal will encode the Class I and Class II bits to increase the
probability that they will be received intact by the base station.
Class II bits can be encoded using, for example, a GEM baseline
rate 1/2 convolutional code. However, because the Class I bits are
necessary for call set up and have a higher priority than the Class
II bits, the access terminal will encode the Class I bits at a
higher rate convolutional code.
[0009] Because the block of Class I bits generally has a short
length, it is difficult to convolutionally encode the block of
Class I bits by itself. Therefore, to encode the Class I bits, an
access terminal can include a block code encoder in addition to the
convolutional encoder used to encode the Class II bits. In this
event, the base station receiving the encoded channel request
message would require a block code decoder to decode the encoded
Class I bits in addition to a convolutional decoder for decoding
the Class II bits. These additional block code encoders and
decoders in the access terminal and base station increase the
overall complexity of the system, which can result in additional
cost of the system as well.
[0010] Accordingly, a need exist for a satellite-based
communications network capable of encoding and decoding data bits
of different priorities in a channel request message at different
coding rates without using additional encoders and decoders.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a system
and method for establishing an integrated forward error correction
(FEC) scheme to perform multi-rate encoding/decoding on different
priority data bits of a channel request message transmitted over a
random access channel in a satellite-based communications network,
without using multiple encoders and multiple decoders.
[0012] Another object of the invention is to provide a system and
method which uses an encoder having a single coding rate to encode
different priority bits of a channel request message transmitted
over a random access channel in a mobile satellite communications
network at different coding rates which are dependent on the
respective priorities of the data bits.
[0013] A further object of the invention is to provide a system and
method which uses a single encoder in an access terminal of a
satellite-based communications network to encode different priority
data bits of a channel request message at different respective
encoding rates, and which uses a single decoder in a base station
of the communications network to decode the differently encoded
bits of the channel request message received from the access
terminal over a random access channel.
[0014] These and other objects of the invention are substantially
achieved by providing a system and method for encoding data,
including a first data group representing first information and a
second data group representing second information, which is
transmitted between an access terminal and a base station in a
satellite-based communications network. The system and method
encodes the second data group at an encoding rate to provide a
second encoded data group, and encodes the first data group at the
same encoding rate to provide a first encoded data group. The
encoding of the first and second data groups is performed by a
single encoder, such as a rate 1/4 convolutional encoder. The
second encoded data group is transmitted between the access
terminal and the base station or, in particular, from the access
terminal to the base station over a random access channel, for
example. The second encoded data group further can be punctured
during transmission to in effect decrease its coding rate, for
example, to rate 1/2 coding. The first encoded data group is
transmitted from the access terminal to the base station, and is
then retransmitted from the access terminal to the base station to
in effect increase the rate of coding of the first encoded data
group to, for example, 1/8 coding. As can be appreciated by one
skilled in the art, the transmission of the second encoded data
group, and the transmission and retransmission of the first encoded
data group, occurs within a single burst. At the base station, a
combiner/demodulator combines the transmitted and retransmitted
first encoded data group, and the combined first encoded data
groups and the second encoded data group are then decoded by a
decoder at the base station.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] These and other objects, advantages and novel features of
the invention will be more readily appreciated from the following
detailed description when read in conjunction with the accompanying
drawings, in which:
[0016] FIG. 1 is a block diagram of a satellite communications
network employing a system and method according to an embodiment of
the present invention;
[0017] FIG. 2 is a detailed block diagram illustrating an example
of an access terminal shown in FIG. 1 employing an encoder and
transmitter arrangement according to an embodiment of the present
invention;
[0018] FIG. 3 is an exemplary block diagram illustrating Class I,
Class II and tail bits of a channel request message to be encoded
by the encoder of the access terminal shown in FIG. 2; and
[0019] FIG. 4 is a detailed block diagram illustrating an example
of a base station shown in FIG. 1 employing a combiner/demodulator
and decoder arrangement according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] FIG. 1 is a block diagram illustrating an example of a
satellite-based communications network 100 employing a system and
method according to an embodiment of the present invention. The
satellite-based communications network 100 includes at least one
satellite 102 which is, for example, a geosynchronous earth orbit
satellite, at least one base station 104, and at least one access
terminal 106.
[0021] As discussed above, during call set up, the access terminal
106 transmits a channel request message to the base station 104
over a random access channel 108. A channel request message
includes Class I data bits, Class II data bits and Tail bits. The
Class I data bits can be a short block of 23 bits representing
information necessary for call set up which, as discussed above,
can include data for contention resolution and timing
synchronization between the access terminal 106 and the base
station 104. The Class II data bits can be a longer block of, for
example, 135 bits, representing information less crucial for call
set up, such as information for accelerating call set up time.
[0022] As shown in FIG. 2, an access terminal 106 includes, among
other things, an encoder 110, a transmitter 112, and a controller
114 which controls the encoder 110 and transmitter 112 to encode
and transmit the Class I, Class II and tail bits of the channel
request message. In this example, the encoder can be a rate 1/4
convolutional encoder as known in the art. In this example, the
rate 1/4 convolutional code encoder 110 is set to be
rate-compatible with a GEM baseline rate 1/2 code of constraint
length 5, and is defined the following generator polynomials:
g.sub.0(D)=1+D.sup.3+D.sup.4
g.sub.1(D)=1+D+D.sup.2+D.sup.4
g.sub.2(D)=1+D.sup.2+D.sup.4
g.sub.3(D)=1+D+D.sup.2+D.sup.3+D.sup.4
[0023] where generator polynomials g.sub.0 and g.sub.1 define the
baseline rate 1/2 convolutional code.
[0024] The encoding of the Class I, Class II and Tail bits are
performed according to an integrated forward error correction (FEC)
scheme to achieve multi-rate convolutional encoding for the
different classes of bits. Specifically, the controller 114
includes or controls, for example, a channel request message
generator which generates the Class I, Class II and Tail bits of
the channel request message, and inputs those bits into the rate
1/4 convolutional encoder 110 in blocks. As shown in FIG. 3, the
Class II bits (C II) are input first into the encoder 110, followed
by the Class I bits (C I) and the Tail bits (T). The encoding is
performed in a block mode using zero-value tail bits to flush the
encoder 110. The Class I, Class II and Tail bits are each encoded
at rate 1/4 by the convolutional encoder 110, and are output by the
encoder 110 to the transmitter 112 as a block of rate 1/4 coded
bits.
[0025] To achieve the desired coding rates for the Class I, Class
II and Tail bits, the controller 114 controls the transmitter 112
as follows. When the transmitter 112 transmits the Class II data
bits, only the Class II bits corresponding to the two arms of the
baseline rate 1/2 convolutional code are transmitted, while the
bits produced by the other two arms are punctured by applying a
puncturing mask [1100].sup.T to the encoded Class II data bits. The
controller 114 then controls the transmitter 112 to transmit the
encoded Class I data bits twice, and afterward, controls the
transmitter to transmit the encoded tail bits. The puncturing of
the Class II data bits effectively lowers the coding rate of the
Class II data bits to rate 1/2 convolutional coding, while the
retransmission of the Class I data bits effectively increases the
coding rate to rate 1/8 convolutional coding for the Class I data
bits.
[0026] As discussed above, the encoded Class I, Class II and Tail
bits are transmitted in a channel request message by the access
terminal 106 to the base station 104 over a random access channel
108. As shown in FIG. 4, the base station 104 includes, among other
things, a combiner/demodulator 116, a decoder 118, and a controller
120. The decoder 118 can be, for example, a rate 1/4 Viterbi
decoder as known in the art.
[0027] When the base station 104 receives the encoded channel
request message, the controller 120 controls the
combiner/demodulator 116 to combine the two received groups of
encoded Class I bits (the encoded Class I bits were transmitted
twice by the transmitter 112 of the access terminal 106), and to
output the combined Class I data bits to the decoder 118. The
received encoded Class II bits and received encoded Tail bits are
also input to the decoder 118. Accordingly, the Class I bits are in
effect received and decoded at 1/8 rate coding, the Class II bits
are in effect received and decoded at 1/2 rate coding, and the Tail
bits are in effect received and decoded at rate 1/4 coding.
[0028] As can be appreciated by one skilled in the art, the order
in which data blocks are encoded at different coding rates is very
crucial. Technically, convolutional codes are not equal error
protecting codes for all bits, because the initial bits being input
into the encoder 110 and the last bits being input into encoder 110
will have a lower bit error rate (BER) than the middle bits
entering the encoder. Because the Class I bits require high
priority encoding, it is conceivable that instead of inputting the
Class I, Class II and Tail bits to the encoder 110 in the order
shown in FIG. 3, the Class I bits can be divided into two data
blocks and input to the encoder 110 in a different order.
[0029] For example, the first half of the Class I bits could be
input to the encoder 110 and encoded first, followed by the Class
II bits, the second half of the Class I bits, and the Tail bits, in
that order. This order of encoding would be effective if the Class
I and Class II bits were to ultimately be encoded by the same code
rates. However, because a combiner in the combiner/demodulator 116
at the base station 104 performs a maximum ratio combining on the
repeated Class I coded sequence, the demodulated Class II bit error
rate is 3 dB poorer than the demodulated Class I bits error rate.
Thus, at low signal to noise ratios (SNR), high BERs in the Class
II demodulated bits cause the trellis traceback at the decoder 118
to be incorrect, which adversely affects decoding of the first half
of Class I bits and worsens the bit error rate of the decoded Class
I data bits.
[0030] In addition, because the Class II data bits have been
encoded with the weaker code of rate 1/2, neutral data bits are
inserted into the encoded Class II to compute the branch metric for
the rate 1/4 decoder 118. The insertion of neutral bits affects the
transition boundaries between the Class I and Class II data bits,
and therefore, would further worsen the bit error rate of the
decoded Class I data bits. However, by encoding the Class I, Class
II and Tail data bits in the order shown in FIG. 3, the traceback
and boundary problems are eliminated.
[0031] Working Example
[0032] The continuous transmission of RACH burst was simulated for
AWGN and two Rician channels (a slow fading channel with a k factor
of 9 and a fading bandwidth (BW) of 10 Hz, and a fast fading
channel with a k factor of 12 and a fading BW of 200). The error
rates obtained for Class I and Class II bits, encoded and decoded,
together using an integrated FEC scheme described above, were
compared to those of conventional rate 1/2 and rate 1/4
decoders.
[0033] The results have shown that the integrated FEC scheme did
not degrade the error rate performance of either convolutional
code, and the desired protection for both Class I and Class II bits
was realized.
[0034] Accordingly, the integrated FEC scheme as described above
for coding of the RACH information bits achieves the desired rate
1/2 protection for Class II bits, and extra, effective rate 1/8,
protection for Class I bits. The use of the same encoder and
decoder to encode and decode the Class I and Class II bits
eliminates the need to complicate the system by using an additional
encoder and decoder. Eliminating the need for an additional encoder
and decoder also eliminates any problems which could arise when
switching between the different encoders and decoders. Furthermore,
the computational overhead of using a rate 1/4 decoder employed by
the integrated scheme is minimal over a standard rate 1/2 decoder
used in GEM. In addition, by grouping the Class I and Class II bits
together during encoding enables the system to use a rate 1/4
convolutional code for a block of Class I bits, that could not, by
itself, be protected with convolutional code because of its short
length.
[0035] Although only a few exemplary embodiments of this invention
have been described in detail above, those skilled in the art will
readily appreciate that many modifications are possible in the
exemplary embodiments without materially departing from the novel
teachings and advantages of this invention. Accordingly, all such
modifications are intended to be included within the scope of this
invention as defined in the following claims.
* * * * *