User's Manual

PMP 400/430 and PTP 200 Series Canopy User Guide
Supplement
Issue 4 0 – DRAFT 6 November 2009 Page 45 of 64
4.8 CYCLIC PREFIX (CONFIGURABLE ONLY ON PTP 200 BH & PMP 430
AP/SM)
OFDM technology uses a cyclic prefix, where a portion of the end of a symbol (slot) is repeated at
the beginning of the symbol (slot) to allow multi-pathing to settle before receiving the desired
data. A 1/4 cyclic prefix means that for every 4 bits of throughput data transmitted, an additional
bit is used, A 1/8 cyclic prefix means that for every 8 bit of throughput data transmitted, an
additional bit is used.
PMP 400 Series networks use a cyclic prefix of 1/4 that is not configurable by the operator.
PMP 430 Series networks use a default cyclic prefix of 1/4 that is configurable by the operator to
1/8. The Cyclic Prefix is set on the Configuration => Radio page on the AP. Changing the default
from 1/4 to 1/8 can increase throughput by ~2 Mbps (assuming 75% duty cycle) in installations
with low multipath conditions. It is recommended to test 1/8 cyclic prefix to determine actual
performance based on RF conditions.
PTP 200 Series modules (OFDM BHs) are settable for either 1/8 or 1/4 cyclic prefix. The use of
1/8 cyclic prefix provides about 11% higher maximum throughput, and is recommended for
backhaul operations in most cases.
• The Cyclic Prefix is set on the Configuration => Radio page of the BHM.
• The default on a new unit or when the unit is reset to factory defaults is 1/4 Cyclic
Prefix.
• In most deployments, 1/8 Cyclic Prefix will provide a high quality, higher throughput
link. In cases with severe multi-pathing or obstructions, 1/4 Cyclic Prefix may give
better overall results.
Procedure for setting the Cyclic Prefix
3. Set the Cyclic Prefix on the Configuration => Radio page of both the BHM and the
BHS to 1/8 before deployment.
IMPORTANT!
The Cyclic Prefix must be set the same on both the BHM and the BHS.
If they don’t match, the BHS will not register to the BHM.
4. During installation use Link Tests to confirm link quality per standard installation and
alignment procedures.
5. If a Link Test shows low throughput or efficiency, consider changing the Cyclic Prefix
to 1/4 on both the BHM and the BHS along with other standard installation
troubleshooting procedures such as re-aiming, off-axis aiming, changing location,
raising or lowering the height of the radio, adjusting Transmission Power up or
down, or identifying and mitigating sources of interference.