Error Correction for
Digital Terrestrial Television Broadcasting
Contributed by Paul G.M. de Bot
and Flavio Daffara
For Digital Terrestrial Television Broadcasting (DTTB),
error correcting coding is used to guarantee at the input of the demultiplexer
virtual error-free performances, i.e., a bit error rate (BER) less than 10^-10. To
achieve this, the same concatenated error correcting code as in the
DVB digital satellite television standard is used. This concatenated code consists of
-
a nu = 6, 64-state (punctured) convolutional inner code with code rates 1/2,
2/3, 3/4, 5/6 and 7/8,
- a Reed-Solomon code with length 204 and dimension 188
over GF(2^8), and appropriate interleaving.
Convolutional Inner Code
The inner convolutional code has the advantage of being decodable with the
Viterbi algorithm, which is an implementable Maximum Likelihood (ML) decoding
algorithm. This makes the code a powerful tool to reduce a BER from 10^-1
to 10^-2 at the output of the channel to 10^-3 to 10^-4. Since
the Viterbi decoder is only able to correct random bit errors, and has no burst
error correction capabilities, inner bit interleaving in the frequency domain
is applied, to scatter the frequency selective
behavior of the channel and
provide random bit errors at the input of the Viterbi decoder. When the
Viterbi decoder fails to correct certain errors, it typically produces burst
errors of 5 to 15 times nu (equivalent to 30 to 90) bits, depending on the
puncture rate of the code and of the channel.
Reed-Solomon Outer Code
Since the outer RS-decoder is able to
correct random byte errors (but not bursts of byte errors), the bits at the
Viterbi decoder output are organized in bytes, on which outer byte interleaving
is applied using a Forney interleaver of a depth of 12 bytes.
Thanks to its large Hamming distance of 17, which makes correction possible
of up to 8 random byte errors per codeword, the Reed-Solomon decoder is able
to reduce the bit error rate further e.g. from 10^-3 to 10^-4 down to
10^-10 to 10^-11.
Soft Decision Decoding
The Viterbi algorithm can be implemented for either hard-decision or
soft-decision decoding. Soft-decision decoding, relative to hard-decision
decoding,
of a nu = 6, rate = 1/2
convolutional code with BPSK modulation can yield
a gain of up to 4 dB in the region of BER around 10^-4 on a Rayleigh fading
channel. A soft-decision Viterbi decoder uses
the channel state information.
Explicit knowledge of
Channel State Information (CSI), can yield some additional gain. In case of
frequency-selectivity,
the optimal method for combining
this CSI with the received signal, is by
applying maximal ratio combining. Using this method, strong
signals are made stronger, while weak signals are made even weaker. The use of
CSI in this way, can yield an additional gain of 2 dB in the region of
BER = 10^-4 on a Rayleigh fading channel.
Further, it is
interesting to see that if one uses hard-decision Viterbi decoding with 1-bit CSI
(comparable with an erasure flag), we closely approach the performances of
soft-decision Viterbi decoding without CSI.
Interference
If the channel is primarily subject to interference, rather than to multipath,
it is essential that this interference be estimated in order to apply
soft-decision decoding.
