About the Radio | 31
Aprisa SRi User Manual 1.1.0
Interference Avoidance / Immunity
Aprisa SRi is designed to avoid interference, mainly from other unlicensed radios and deploys a few
mechanisms for better interference immunity, to increase performance and maintain robust
communication.
Using frequency hopping with 8 zones and a narrower radio channel results in better power density and
reduces the chance the channel will be hit with interference.
The noise floor and statistics of each zone and hop set channel is being logged and can be used to find
frequencies that have constant interference mainly from other DTS radios. Using this information, the user
can navigate in SuperVisor to Radio > Channels and deactivate the noisy zone / channels.
If a beacon isn’t received due to frequency inte
rference or being occupied, remotes will automatically move
to the next frequency hop before sending an access request (AR). This prevents occupied channels being
used with no major impact on throughput.
For reliable link in a noisy environment, remotes will buffer transmitted packets, and perform retries using
ARQ mechanism. ARQ (Automatic Repeat reQuest) is a well-known data integrity mechanism used in the
Aprisa SRi as it adds a layer of interference recovery on top of the powerful Aprisa FEC (Forward Error
Correction). Some frequencies may be subject to more interference than others so if packet retries are
enabled in the Aprisa SRi and interference on a specific frequency overwhelms the FEC, then any missing
packets are automatically retransmitted on another frequency. Packet retries for uplink and downlink
direction will work as follows;
In uplink direction:
Packet retries will continue
until the packets TTL time expires, or packet has been re sent per ‘Remote to
Base Packet’ parameter settings retries times (if 0 no retries will be made). Remote radio
s will check the
next downlink ack flag in the beacon or data packet to determine if retransmission is required (if the remote
‘unicast packet’
is set to auto, the retries parameter is received by the remote from base during
registration).
In downlink direction:
Downlink packet retries are used for unicast packets. Retries will continue until the packets TTL time
expires, or packet has been re sent ‘Unicast Packet’ parameter settings retries times (if 0 no retries will be
made). The base radio will check the next uplink ack flag from the remote radio to determine if
retransmission is required.
To ensure the integrity of some broadcast packets (e.g. OTA firmware upgrade), they would automatically
be sent per
‘
Broadcast Packet
’ parameter settings
times.
