ISL3874 Datasheet, PDF (23/33 Pages) Intersil Corporation – Wireless LAN Integrated Medium Access Controller with Baseband Processor with Mini-PCI

ISL3874
Scrambling is done by division with a prescribed polynomial
as shown in Figure 14. A shift register holds the last quotient
and the output is the exclusive or of the data and the sum of
taps in the shift register. The transmit scrambler seed for the
long preamble or for the short preamble can be set with
CR48 or CR49.
SERIAL DATA
IN
XOR
Z^-1 Z^-2 Z^-3 Z^-4
SERIAL
DATA OUT
Z^-5 Z^-6 Z^-7
XOR
FIGURE 14. SCRAMBLING PROCESS
For the 1Mbps DBPSK data rates and for the header in all
rates using the long preamble, the data coder implements
the desired DBPSK coding by differential encoding the serial
data from the scrambler and driving both the I and Q output
channels together. For the 2Mbps DQPSK data rate and for
the header in the short preamble mode, the data coder
implements the desired coding as shown in the DQPSK
Data Encoder table. This coding scheme results from
differential coding of dibits (2 bits). Vector rotation is
counterclockwise although bits 6 and 7 of configuration
register CR1 can be used to reverse the rotation sense of
the TX or RX signal if desired.
TABLE 16. DQPSK DATA ENCODER
PHASE SHIFT
DIBIT PATTERN (d0, d1)
d0 IS FIRST IN TIME
0
00
+90
01
+180
11
-90
10
Spread Spectrum Modulator Description
The modulator is designed to generate DBPSK, DQPSK, and
CCK spread spectrum signals. The modulator is capable of
automatically switching its rate where the preamble is
DBPSK modulated, and the data and/or header are
modulated differently. The modulator can support date rates
of 1, 2, 5.5 and 11Mbps. The programming details to set up
the modulator are given at the introductory paragraph of this
section. The ISL3874 utilizes Quadraphase (I/Q) modulation
at baseband for all modulation modes.
In the 1Mbps DBPSK mode, the I and Q Channels are
connected together and driven with the output of the
scrambler and differential encoder. The I and Q Channels
are then both multiplied with the 11-bit Barker word at the
spread rate. The I and Q signals go to the Quadrature
upconverter (HFA3724) to be modulated onto a carrier.
Thus, the spreading and data modulation are BPSK
modulated onto the carrier.
For the 2Mbps DQPSK mode, the serial data is formed into
dibits or bit pairs in the differential encoder as detailed
above. One of the bits from the differential encoder goes to
the I Channel and the other to the Q Channel. The I and Q
Channels are then both multiplied with the 11-bit Barker
word at the spread rate. This forms QPSK modulation at the
symbol rate with BPSK modulation at the spread rate.
Transmit Filter Description
To minimize the requirements on the analog transmit
filtering, the transmit section shown in Figure 16 has an
output digital filter. This filter is a Finite Impulse Response
(FIR) style filter whose passband shape is set by tap
coefficients. This filter shapes the spectrum to meet the
radio spectral mask requirements while minimizing the peak
to average amplitude on the output. To meet the particular
spread spectrum processing gain regulatory requirements in
Japan on channel 14, an extra FIR filter shape has been
included that has a wider main lobe. This increases the 90%
power bandwidth from about 11MHz to 14MHz. It has the
unavoidable side effect of increasing the amplitude
modulation, so the available transmit power is compromised
by 2dB when using this filter (CR11, bit 5).
CCK Modulation
For the CCK modes, the spreading code length is 8 complex
chips and based on complementary codes. The chipping
rate is 11Mchip/s. The following formula is used to derive the
CCK code words that are used for spreading both 5.5 and
11Mbps:
c
=


e^j(ϕ1
+
ϕ₂
+
ϕ₃
+
ϕ₄ )_,
j
e
(ϕ₁
+
ϕ₃
+
ϕ₄)_,
e^j(ϕ1
+
ϕ₂
+
ϕ₄
)
,

j
–e
(ϕ₁
+
ϕ₄)_,
j
e
(ϕ₁
+
ϕ₂
+
ϕ₃)_,
e^j(ϕ1
+
ϕ ₃ )_,
j(
–e
ϕ₁
+
ϕ₂)_,
e^jϕ1



(LSB to MSB), where c is the code word.
The terms: ϕ1, ϕ2, ϕ3, and ϕ4 are defined below for
5.5Mbps and 11Mbps.
This formula creates 8 complex chips (LSB to MSB) that are
transmitted LSB first. The coding is a form of the generalized
Hadamard transform encoding where the phase ϕ1 is added
to all code chips, ϕ2 is added to all odd code chips, ϕ3 is
added to all odd pairs of code chips and ϕ4 is added to all
odd quads of code chips.
The phase ϕ1 modifies the phase of all code chips of the
sequence and is DQPSK encoded for 5.5 and 11Mbps. This
will take the form of rotating the whole symbol by the
appropriate amount relative to the phase of the preceding
symbol. Note that the last chip of the symbol defined above
is the chip that indicates the symbol’s reference phase.
For the 5.5Mbps CCK mode, the output of the scrambler is
partitioned into nibbles. The first two bits are encoded as
differential symbol phase modulation in accordance with
Table 17. All odd numbered symbols of the MPDU are given
an extra 180 degree (π) rotation in addition to the standard
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