Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
2000-07-07
2003-09-02
Liu, Shuwang (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
C375S340000, C375S341000, C714S786000, C714S790000
Reexamination Certificate
active
06614850
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to a method and apparatus for puncturing code symbols to provide improved performance in a communications system.
II. Description of the Related Art
In a typical digital communications system, data is processed, modulated, and conditioned at a transmitter unit to generate a modulated signal that is then transmitted to one or more receiver units. The data processing may include, for example, formatting the data into a particular frame format, encoding the formatted data with a particular coding scheme to provide error detection and/or correction at the receiver unit, puncturing (i.e., deleting) some of the code symbols to fit within a particular frame size, channelizing (i.e., covering) the encoded data, and spreading the channelized data over the system bandwidth. The data processing is typically defined by the system or standard being implemented.
At the receiver unit, the transmitted signal is received, conditioned, demodulated, and digitally processed to recover the transmitted data. The processing at the receiver unit is complementary to that performed at the transmitter unit and may include, for example, despreading the received samples, decovering the despread samples, inserting “erasures” in place of punctured symbols, and decoding the symbols to recover the transmitted data.
A digital communications system typically employs a convolutional code or a Turbo code to provide error correction capability at the receiver unit. The ability to correct transmission errors enhances the reliability of a data transmission. Conventionally, convolutional and Turbo coding is performed using a particular polynomial generator matrix that generates a particular number of code symbols (e.g., 2, 3, or more code symbols) for each input data bit. For example, a rate ½ encoder generates two code symbols for each data bit.
A multiple access communications system typically transmits data in frames or packets of predetermined sizes to allow for efficient sharing of system resources among active users. For example, some communications systems support frame sizes that are multiple times a basic frame size (e.g., 768·K bits, where K=1, 2, . . . ). For efficiency, some communications systems also support multiple data rates. Depending on a number of factors, a variable number of data bits (i.e., X) may be provided to the encoder, which then generates a corresponding number of code symbols (e.g., 2X).
In certain instances, the number of code symbols generated is not exactly equal to the capacity of the frame Symbol repetition and puncturing are then used to fit the generated code symbols into a frame of a particular size. For example, if the number of code symbols is less than the frame capacity, some or all of the code symbols may be repeated (i.e., duplicated) a particular number of times. Conversely or additionally after the symbol repetition, if the number of code symbols is greater than the frame capacity, some of the code symbols may be deleted (i.e., punctured).
One conventional method for puncturing code symbols is to systematically puncture one symbol out of every D
th
symbols until the required number of symbol punctures is achieved. The remaining symbols are then sent unmodified. In certain situations, this method can puncture symbols unevenly throughout an entire frame, which results in more symbols being punctured in one portion of the frame and less or no symbols being punctured in some other portion of the frame. When symbols are unevenly punctured, performance may be compromised.
As can be seen, techniques that can be used to puncture symbols in a manner to provide improved performance are highly desirable.
SUMMARY OF THE INVENTION
The present invention provides various techniques for puncturing symbols to achieve a more even distribution of symbol punctures throughout an entire frame, which can result in improved system performance. Generally, a number of puncture distances are computed, and the required symbol punctures are performed using the computed distances. A puncture distance can be defined as the periodicity of the symbol punctures. By properly selecting the puncture distances, and using the selected distances at the appropriate time, the desired puncture results can be achieved.
An embodiment of the invention provides a method for puncturing symbols in a communications system (e.g., a system that conforms to CDMA-2000, W-CDMA, or 1XTREME standard, which are identified below). In accordance with the method, S symbols are received for a frame having a capacity of N symbols, with S being greater than N. P symbols need to be punctured from the S received symbols such that the remaining unpunctured symbols fit into the frame. A number of puncture distances, D
1
through DN, are then computed based on the S received symbols and the P symbol punctures. Next, a particular number of symbol punctures is determined for each computed puncture distance. P
1
through PN symbol punctures are then performed at the puncture distances of D
1
through DN, respectively. For a more even distribution of the symbol punctures, each of the distances D
1
through DN can be selected to be greater than or equal to a minimum puncture distance Dmin defined as:
D
min
=
⌊
S
P
⌋
,
where └ ┘ denotes a floor operator.
In a simple implementation, two puncture distances, D
1
and D
2
, can be computed based on S and P as follows:
D1
=
⌊
S
P
⌋
,
and
D2
=
{
D1
if
D1
*
P
=
S
D1
+
1
otherwise
.
P
1
and P
2
can then be computed as:
P
2
=
S−P*D
1
, and
P
1
=
P−P
2
.
The symbol puncturing can be achieved by (1) selecting either the puncture distance of D
1
or D
2
to be used to determine which symbol should be punctured next, (2) puncturing the next symbol based on the selected puncture distance, and (3) decrementing P
1
or P
2
based on the selected puncture distance. Steps (1) through (3) can be repeated until all P
1
and P
2
symbol punctures are achieved. The puncture distance can be selected such that the P
1
symbol punctures at the distance of D
1
are distributed among the P
2
symbol punctures at the distance of D
2
. For example, if the ratio of P
1
to P
2
is equal to R, then the puncture distance can be selected such that, on an average, R symbol punctures are performed at the distance of D
1
for each symbol puncture at the distance of D
2
. Alternatively, P
1
symbol punctures at the distance of D
1
can be performed, followed by P
2
symbol punctures at the distance of D
2
. The method can thus be used to provide a rich set of patterns of puncture distances D
1
and D
2
that can provide improved performance.
The above concepts for two puncture distances can be applied to the general case in which N puncture distances are computed and used. The symbol punctures at each computed distance can be performed together or distributed with symbol punctures at other distances.
Prior to the symbol puncturing, the code symbols may have been repeated to generate the S received symbols. For example, in the CDMA-2000 system, each code symbol may be repeated M times, with M being an integer greater than or equal to one and selected such that S is greater than or equal to N. Also, the code symbols are typically generated by coding a number of data bits with a particular coding scheme (e.g., a convolutional or Turbo code).
Another embodiment of the invention provides a method for decoding symbols in a communications system. In accordance with the method, N symbols are initially received. It is then determined that P symbol punctures had been performed on S symbols to generate the N received symbols. A number of puncture distances, D
1
through DN, is then computed based on S and P, and P
1
through PN symbol punctures at the distances of D
1
through DN, respectively, are also determined. A puncturing patt
Ling Fuyun
Razoumov Leonid
Baker Kent D.
Harnois Albert J.
Liu Shuwang
Qualcomm Incorporated
Wadsworth Philip R.
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