Red
MAX
™
Base Station
user manual
Doc. #70-00058-01-01-DRAFT
Proprietary Redline Communications © 2006
November
29,
2006
Page 82 of 106
1. Equalizing the received signal based on an estimate of the channel fading conditions
(restoring the faded sub-channels).
2. Interleaving the data before modulation onto the individual sub-channels (so that
adjacent data symbols do not necessarily end up on adjacent sub-channels, and thus
deep fades do not impair large chunks of the signal).
3. Convolutional encoding of the input data introduces correlations between signal
samples that are echoed on sub-channels occurring in separate parts of the spectrum.
This allows the receiver to infer the values of the signal that were transmitted on
fatally faded sub-carriers. The key advantage of multiple carriers is that the signal is
more robust to multipath disturbance, as described below.
OFDM equalization and coding techniques cannot overcome all forms of sharp,
multipath-related fading. For additional robustness, the OFDM implementation uses a
guard interval between each of the sub-channels. This guard interval is filled with
redundant data taken from the end of the same channel symbol and repeated at the
beginning of each channel symbol. Multipath echoes from each individual sub-channel
overlap the guard interval, rather than overlapping data in other symbols. Used in
conjunction with adaptive equalization, coding, and modulation, use of the guard interval
provides reliable operation in a wide class of non line-of-sight environments.
6.2.2 Calculating Receive Sensitivity (WiMAX Testing)
Overview
The WiMAX test for receive sensitivity modifies the methodology from 802.16 to allow
results to be obtained easier and faster. The standard Bit Error Rate (BER) of 1x10-6
remains the benchmark for these calculations. The BER is obtained by measuring Packet
Error Rate (PER), but PER cannot truly be measured (it is a limit) and requires that a test
be performed to estimate the PER value. Larger sample sizes will produce more accurate
values for PER. There is a practical limit to the size of the test sample, and WiMAX has
chosen the number of packets in the sample to enable result that will approximate a BER
lower than 1x10-6.
Sample Test for Subscriber Receive Sensitivity
1. Connect packet generator to the BS<->SS link and then configure packet generator
to send 50,000 Ethernet packets with payload of 288 Bytes (i.e., total Ethernet packet
size is 288 + 18 = 306 Bytes). Payload should not be chosen by the packet generator,
but must be filled with the pattern defined in the WiMAX standard. The packets
should be sent at a rate to fill at least 80% of the downlink (DL) frame.
2. Count the number of packets received. If less than 97 packets are lost, it can be
stated with 95% confidence that the BER is 1x10-6 or lower.
3. Repeat the test by sending 20,000 Ethernet packets with payload of 864 Bytes. If less
than 118 packets are lost, it can be stated with 95% confidence that the BER is 1x10-
6 or lower.
4. You repeat the test by sending 10,000 Ethernet packets with payload of 1488 Bytes.
If less than 100 packets are lost, it can be stated with 95% confidence that the BER is
1x10-6 or lower.