PRODUCTS & SERVICES
AEROSPACETESTINGINTERNATIONAL.COM // MARCH 103
CODING FOR
ERROR CORRECTION
In aeronautical telemetry, if it
looks too good to be true, it
probably is – unless it’s LDPC
Now that everyone has a radio in their
pocket or purse, we all know that radio
connections are prone to errors. On a cell
phone, these errors manifest as garbled voice
or slow downloads. When these same
dropouts occur on a telemetry link between a
test article and the ground station, the
telemetry data is simply lost, with no
opportunity of recovery. The best strategy
available is to make the link more reliable –
that is, to reduce the number of errors.
This is exactly what forward error
correcting codes do. By introducing carefully
constructed “redundant” bits into the
transmitted data stream, it is possible to
detect and correct received errors using a
suitably equipped receiver.
There are many different coding schemes
employed in communications systems. In the
aeronautical mobile telemetry (AMT) market,
low-density parity-check (LDPC) codes have
emerged as a clearly superior choice. As a
result, LDPC codes were adopted as the IRIG
standard for the AMT community in 2015.
LDPC codes are linear block codes, first
developed by Robert G. Gallager at MIT in 1960
and published as a monograph in 1963. But
their high computational complexity made
implementations
impossible at that time.
LDPC codes were
re-discovered by David J.C. MacKay in 1996.
With the use of large field-programmable gate
arrays (FPGAs), compact implementations
became possible. For a given bandwidth
expansion, LDPC codes provide more coding
gain than previous alternatives. For this
reason, LDPC codes began displacing turbo
codes in the late 1990s and are now widely
deployed in applications demanding high
performance, such as the DVB-S2 standard for
satellite digital TV and Wi-Fi 802.11 High
Throughput PHY.
The primary figure of merit for a coding
scheme is called coding gain. This is the
difference between the signal-to-noise ratio
that is required with and without coding to
achieve a particular target bit error rate.
depends on
circumstances such as
available bandwidth,
all are vastly superior
to no coding. The
typical gain with LDPC
coding is around 9dB, which has
extraordinary real-world implications. It is
like amping up a 20-watt transmitter to 160
watts, expanding a 6ft receiving antenna to
17ft, or extending reception range from 50 to
over 140 miles.
Alternately, LDPC coding is a huge win for
links like compressed video that suffer
degradation when even a few bits are in error.
The result of LDPC coding is such a steep bit
error rate curve that the channel has nearly
binary performance: perfection or highly
errored, with perfection being achieved deep
into the area where bit errors normally occur.
Since the LDPC encoder and decoder can
be implemented entirely as computations in a
suitable FPGA, there is no size or weight
impact and insignificant power increase. For
Quasonix’s first LDPC units
were delivered to Patuxent
River Naval Air station in 2011,
and LDPC coding was
introduced to the telemetry
community in 2013 with
LDPC capability.
Errors in a telemetry link can be costly,
requiring more test time to reproduce the lost
data, if replicating the test is even possible.
LDPC coding reduces the number of errors in
a telemetry link, and it does so more cost
effectively than any other approach. \\
1 // Low-density parity-
check (LDPC) coding is
optionally available on all
Quasonix transmitters
2 // Quasonix RDMS
receivers offer many
industry firsts, including
LDPC decoding
3 // LDPC coding gain
dramatically increases
link margin
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There are six IRIG standard LDPC variants.
While the optimal selection
the rare cases where LDPC decoding is not
available on the ground, the LDPC encoder
can be bypassed in the transmitter.
publications at the ETTC and ITC
conferences. Quasonix has now shipped over
800 transmitters and 1,000 receivers with
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