LDPC coding in a communication system
In a method for generating a physical layer (PHY) data unit for transmission via a communication channel, a plurality of information bits to be included in the PHY data unit are received, and a number of orthogonal frequency division multiplexing (OFDM) symbols that are needed to include the plurality of information bits after encoding with a low density parity check (LDPC) encoder is determined. One or more of a, b, c, and d are performed according to a set of rules so that a receiving device can determine a number of information bits in the PHY data unit based on i) an indication of the number of OFDM symbols, and ii) the set rules: a) adding padding bits and/or shortening bits prior to encoding with the LDPC encoder, b) removing shortening bits after encoding with the LDPC encoder, c) removing parity bits after encoding with the LDPC encoder, d) repeating information and/or parity bits after encoding with the LDPC encoder. The plurality of information bits are encoded using the LDPC encoder. The PHY data unit is generated i) to include the LDPC encoded information bits in OFDM symbols and ii) to include an indication of the number of OFDM symbols in the PHY data unit.
1. A method for generating a physical layer (PHY) data unit for transmission via a communication channel, the method comprising:
receiving a plurality of information bits to be included in the PHY data unit;
determining a number of orthogonal frequency division multiplexing (OFDM) symbols that are needed to transmit the plurality of information bits after encoding with a low density parity check (LDPC) encoder;
performing one or more of a, b, c, d, e, and f according to a set of rules so that a receiving device can determine a number of information bits in the PHY data unit based on i) an indication of the number of OFDM symbols, and ii) the set rules:
a) adding padding bits prior to encoding with the LDPC encoder,
b) adding shortening bits prior to encoding with the LDPC encoder,
c) removing the shortening bits after encoding with the LDPC encoder,
d) performing puncturing,
e) repeating information bits after encoding with the LDPC encoder, and
f) repeating parity bits after encoding with the LDPC encoder;
encoding the plurality of information bits using the LDPC encoder; and
generating the PHY data unit i) to include the LDPC encoded information bits in OFDM symbols and ii) to include an indication of the number of OFDM symbols in the PHY data unit,
wherein performing one or more of a, b, c, d, e, and f according to the set of rules comprises:
determining a number of extra encoded bits needed to reach a last OFDM symbol boundary according to:
y=N SYM N CBPS −N CW min L CW −( x+L parity );
determining a number of additional codewords according to
N
CW
extra
=
⌊
x
+
L
parity
+
y
L
CW
⌋
;
determining a number of padding bits to be added before encoding according to:
p 1 =N CW extra L CW R−x ; and
adding p 1 padding bits to the plurality of information before encoding.
2. A method according to claim 1 , wherein determining the number of OFDM symbols comprises:
calculating a minimum number of LDPC codewords according to
N
CW
min
=
⌊
(
8
L
+
N
service
)
L
CW
R
⌋
;
calculating a number of remaining information bits according to
x =(8 L+N service )− N CW min L CW R;
and calculating the number of OFDM symbols according to:
N
SYM
=
{
⌈
N
CW
min
L
CW
+
(
x
+
L
parity
)
N
CBPS
⌉
x
>
0
⌈
N
CW
min
L
CW
N
CBPS
⌉
x
=
0
wherein
N CW min is the minimum number of LDPC codewords,
L is a number of octets in the plurality of information bits,
R is a code rate of the LDPC encoder,
N service is a number of service bits,
L CW is a length of an LDPC codeword in bits,
x is the number of remaining information bits after encoding information bits in the minimum number of LDPC codewords, N CW min ,
N CBPS is a number of encoded bits per OFDM symbol
L parity is a number of parity bits in each LDPC codeword in bits, and
N SYM is the number of OFDM symbols.
3. A method according to claim 1 , wherein performing one or more of a, b, c, d, e, and f according to the set of rules comprises:
determining a number z corresponding to a number of padding bits to add after encoding according to:
z =( x+L parity )+ y−N CW extra ·L CW =N SYM N CBPS −( N CW min +N CW extra ) L CW ;
when z<=L parity and z>=(N CW min +N CW extra ), repeating information and/or parity bits after encoding with the LDPC encoder in each codeword so that encoded and padded information bits fit in N SYM OFDM symbols;
when z<=L parity and z<(N CW min +N CW extra ), repeating one information or parity bit after encoding in each of a first z codewords;
when z>L parity ,
calculate a number of shortening bits according to N shrt =(L CW −z),
distributing the shortening bits among (N CW min +N CW extra +1) codewords before encoding, and
removing the shortening bits after encoding.
4. A method according to claim 1 , wherein performing one or more of a, b, c, d, e, and f according to the set of rules comprises:
determining a number z corresponding to a number of padding bits to add after encoding;
when z<=L parity performing one or both of (i) repeating information and (ii) repeating parity bits after encoding with the LDPC encoder in only a last codeword so that encoded and padded information bits fit in N SYM OFDM symbols;
when z>L parity ,
forming a codeword with z−L parity padding bits and N shrt =(L CW −z) shortening bits, and
removing the shortening bits after encoding.
5. A communication device having a network interface, wherein the network interface is configured to:
receive a plurality of information bits to be included in a physical layer (PHY) data unit for transmission via a wireless communication channel,
determine a number of orthogonal frequency division multiplexing (OFDM) symbols that are needed to transmit the plurality of information bits after encoding with a low density parity check (LDPC) encoder,
perform one or more of a, b, c, d, e, and f according to a set of rules so that a receiving device can determine a number of information bits in the PHY data unit based on i) an indication of the number of OFDM symbols, and ii) the set rules:
a) add padding bits prior to encoding with the LDPC encoder,
b) add shortening bits prior to encoding with the LDPC encoder,
c) remove the shortening bits after encoding with the LDPC encoder,
d) perform puncturing,
e) repeat information bits after encoding with the LDPC encoder, and
f) repeat parity bits after encoding with the LDPC encoder;
wherein the network interface is further configured to:
encode the plurality of information bits using the LDPC encoder,
generate the PHY data unit i) to include the LDPC encoded information bits in OFDM symbols and ii) to include an indication of the number of OFDM symbols in the PHY data unit,
determine a number of extra encoded bits needed to reach a last OFDM symbol boundary according to:
y=N SYM N CBPS −N CW min L CW −( x+L parity ),
determine a number of additional codewords according to:
N
CW
extra
=
⌊
x
+
L
parity
+
y
L
CW
⌋
,
determine a number of padding bits to be added before encoding according to:
p 1 =N CW extra ·L CW ·R−x , and
add p 1 padding bits to the plurality of information before encoding.
6. A communication device according to claim 5 , wherein the network interface is configured to:
calculate a minimum number of LDPC codewords according to
N
CW
min
=
⌊
(
8
L
+
N
service
)
L
CW
R
⌋
,
calculate a number of remaining information bits according to
x =(8 L+N service )− N CW min L CW R,
and calculate the number of OFDM symbols according to:
N
SYM
=
{
⌈
N
CW
min
L
CW
+
(
x
+
L
parity
)
N
CBPS
⌉
x
>
0
⌈
N
CW
min
L
CW
N
CBPS
⌉
x
=
0
wherein
N CW min is the minimum number of LDPC codewords,
L is a number of octets in the plurality of information bits,
R is a code rate of the LDPC encoder,
N service is a number of service bits,
L CW is a length of an LDPC codeword in bits,
x is the number of remaining information bits after encoding information bits in the minimum number of LDPC codewords, N CW min ,
N CBPS is a number of encoded bits per OFDM symbol
L parity is a number of parity bits in each LDPC codeword in bits, and
N SYM is the number of OFDM symbols.
7. A communication device according to claim 5 , wherein the network interface is configured to:
determine a number z corresponding to a number of padding bits to add after encoding according to:
z =( x+L parity )+ y−N CW extra ·L CW =N SYM N CBPS −( N CW min +N CW extra ) L CW ,
when z L parity and z>=(N CW min +N CW extra ), repeat information and/or parity bits after encoding with the LDPC encoder in each codeword so that encoded and padded information bits fit in N SYM OFDM symbols,
when z L parity and z<(N CW min +N CW extra ), repeat one information or parity bit after encoding in each of a first z codewords,
when z>L parity ,
calculate a number of shortening bits according to N shrt =(L CW −z),
distribute the shortening bits among (N CW min +N CW extra 1) codewords before encoding, and
remove the shortening bits after encoding.
8. A communication device according to claim 5 , wherein the network interface is configured to:
determine a number z corresponding to a number of padding bits to add after encoding;
when z<=L parity , perform one or both of (i) repeat information (ii) repeat parity bits after encoding with the LDPC encoder in only a last codeword so that encoded and padded information bits fit in N SYM OFDM symbols,
when z>L parity ,
form a codeword with z−L parity padding bits and N shrt =(L CW −z) shortening bits, and
remove the shortening bits after encoding.