Product Description 13
Time-division duplexing
TDD cycle
ePMP links operate using Time Division Duplexing (TDD). The links employ a TDD cycle in which the AP
determines the Subscriber Modules to transmit and the time based on the configured downlink/uplink
ratio (duty cycle). Three fixed Downlink/Uplink frame ratios are available – 75/25, 50/50, and 30/70.
Adaptive modulation
Force 4600C can transmit a data over the wireless link using several different modulation modes ranging
from 4096-QAM to QPSK. For a given channel bandwidth and TDD frame structure, each modulation
mode transmits data at a fixed rate. Also, the receiver requires a given signal-to-noise ratio to
successfully demodulate a given modulation mode. Although the more complex modulations such as
4096-QAM transmits data at a more higher rate than the less complex modulation modes, the receiver
requires a much higher signal-to-noise ratio.
Force 4600C provides an adaptive modulation scheme where the receiver constantly monitors the
quality of the received signal and notifies the far end of the link of the optimum modulation mode with
which to transmit. In this way, optimum capacity is achieved at all times.
Radar avoidance
In regions where the protection of radars is a part of the local regulations, ePMP must detect interference
from radar-like systems and avoid co-channel operation with these systems.
To meet this requirement, ePMP implements the following features:
l
The equipment can only transmits on available channels, of which there are none at initial power-
up. The radar detection algorithm always scan a usable channel for 60 seconds for radar
interference before making the channel an available channel.
l
This compulsory channel scan means that there is at least 60 seconds service outage every time
radar is detected, and that the installation time is extended by at least 60 seconds even if there is
no radar on the channel.
There is a secondary requirement for bands requiring radar avoidance. Regulators have mandated that
products provide a uniform loading of the spectrum across all devices. In general, this prevents operation
with fixed frequency allocations as follows:
l
ETSI regulations allow frequency planning of networks (as that has the same effect of spreading
the load across the spectrum).
l
The FCC allows the channels to be avoided if there is actual interference on them.
Note
When operating in a region that requires DFS, ensure that the AP is configured
with alternate frequencies and the SM is configured to scan for these frequencies
to avoid long outages.
Encryption
ePMP supports optional encryption for data transmitted over the wireless link. The encryption algorithm
used is the Advanced Encryption Standard (AES) with a 128-bit key size. AES is a symmetric encryption
algorithm approved by U.S. Government organizations (and others) to protect sensitive information.
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