Information Technology Reference
In-Depth Information
Shortening bits
Shortened
BCH Encoder
(40,28)
PHR
Frame
Head Check
Sequence
shift
registers
Additive
Scrambler
BCH
Encoder
MPDU
FIFO
Bit
Interleaver
Shortening bits
Pad
Bits
487 KHz
CLK
generator
Data from
MAC layer
Fig. 2.
Block diagram of UWB baseband transmitter
After scrambling, the data goes to the channel coding block. The channel cod-
ing employed in the baseband processor is BCH(n=63, k=51, t=2), a systematic
cyclic code which has very simple encoding and decoding structures. The gen-
erator polynomial employed for BCH(63,51) is given by equation 3. The parity
bits are determined by computing the remainder polynomial
r
(
x
)asshownin
50
m
i
x
i
.
equation 4, where
m
(
x
) is the message polynomial
m
(
x
)=
i
=0
g
(
x
)=1+
x
3
+
x
4
+
x
5
+
x
8
+
x
10
+
x
12
(3)
11
r
i
x
i
=
x
12
m
(
x
)
modg
(
x
)
r
(
x
)=
(4)
i
=0
The total number of bits in the MPDU is shown in the equation 5 where
N
MACheader
is the number of octets in the MAC header,
N
MACframebody
is the
number of octets in the MAC frame body, and
N
FCS
is the number of octets
of the FCS. The number of codewords (
N
CW
) in a PSDU frame is given by the
equation 6. Bit stung is performed such that modulo ((MPDU+ stung), 51)
is zero. This is done so that the BCH encoder always has full blocks to process.
Mathematically, if the rem(
N
MPDU
,k)
=0, the last codeword requires
N
bs
bits
stung which is shown in equation 7. The total number of bits before encoding
is given by equation 8. After sending 51 information bits to BCH encoder, it will
add 12 parity bits to the output stream. So, the pipeline should be stalled for
12 clock cycles.
N
MPDU
=8(
N
MACheader
+
N
MACframebody
+
N
FCS
)
(5)
N
CW
=
N
MPDU
k
(6)
