isl3874 Intersil Corporation, isl3874 Datasheet - Page 31

• Manufacturer Part Number: isl3874
• Description: Wireless Lan Integrated Medium Access Controller With Baseband Processor With Mini-pci
• Manufacturer: Intersil Corporation

performance of the CCK modes. The losses in both figures
include RF and IF radio losses; they do not reflect the
ISL3874 losses alone. The ISL3874 baseband processing
losses from theoretical are, by themselves, a small
percentage of the overall loss.
The PRISM demodulator performs with an implementation
loss of less than 4dB from theoretical in a AWGN
environment with low phase noise local oscillators. For the
1 and 2Mbps modes, the observed errors occurred in
groups of 4 and 6 errors. This is because of the error
extension properties of differential decoding and
descrambling. For the 5.5Mbps and 11Mbps modes, the
errors occur in symbols of 4 or 8 bits each and are further
extended by the descrambling. Therefore the error patterns
are less well defined.
Clock Offset Tracking Performance
The PRISM baseband processor is designed to accept data
clock offsets of up to 25ppm for each end of the link (TX
and RX). This effects both the acquisition and the tracking
performance of the demodulator. The budget for clock offset
error is 0.75dB at 50ppm. No appreciable degradation was
seen for operation in AWGN at 50ppm. Symbol tracking is
accomplished by one of two methods. If both ends of the link
employ locked oscillators for their bit timing and carrier
frequency generation, symbol tracking is done by dividing
down the carrier frequency offset. If either one of the ends of
the link do not have locked oscillators, then symbol tracking
is done by a conventional early-late chip tracking method.
Carrier Offset Frequency Performance
The correlators used for acquisition for all modes and for
demodulation in the 1 and 2Mbps modes are time invariant
matched filter correlators otherwise known as parallel
correlators. They use two samples per chip and are tapped at
every other shift register stage. Their performance with carrier
frequency offsets is determined by the phase roll rate due to the
offset. For an offset of +50ppm (combined for both TX and RX)
will cause the carrier to phase roll 22.5 degrees over the length
of the correlator. This causes a loss of 0.22dB in correlation
magnitude which translates directly to Eb/N0 performance loss.
In the PRISM chip design, the carrier phase locked loop is
inactive during acquisition. During tracking, the carrier tracking
loop corrects for offset, so that no degradation is noted. In the
presence of high multipath and high SNR, however, some
degradation is expected.
RSSI Performance
The RSSI value is reported on CR62 in hex and is linear with
signal level in dB. Figure 21 shows the RSSI curve
measured on a whole evaluation radio. This takes into
account the full gain adjust range of all radio parts. To get
signal level in dBm on a radio, simply subtract 100 from the
RSSI value in decimal.
31
ISL3874
FIGURE 19. BER vs Eb/N0 PERFORMANCE FOR PSK MODES
FIGURE 20. BER vs Eb/N0 PERFORMANCE FOR CCK MODES
120
100
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09
80
60
40
20
7
-100
0
THY 1, 2
5
THY 11
FIGURE 21. RSSI vs SIGNAL LEVEL
8
THY 5.5
-80
6
9
7
SIGNAL LEVEL IN dBm
Eb/N0
-60
8
BER 2.0
10
9
BER 11
Eb/N0
10
-40
11
11
BER 1.0
12
12
-20
BER 5.5
RSSI
13
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
14
0