Comtech EF Data MD2401 - Manual (Page 30)

MD2401 L-Band Multi Demod Installation and Operation Manual Theory of Operation
MN-MD2401 3–6
Revision 7
3.3.2 EXT REF: External Reference, J2
This is not actually a clock, but does have some clocking implications. When the external
reference is used, the master oscillator within the modem is locked to the external reference, and
the internal accuracy and stability of the unit assumes that of the External Reference. Therefore,
not only are the transmit frequencies locked to the external reference, but the modem’s internal
SCT Oscillator is locked to the external reference as well.
External reference port input is specified at 0 to +6 dBm.
3.4 MD2401 Reed-Solomon
3.4.1 Reed-Solomon Codec
Utilizing a Reed-Solomon (RS) Outer Codec concatenated with a convolutional inner codec is an
effective way to produce very low error rates even for poor signal-to-noise ratios while requiring
only a small increase in transmission bandwidth. Typically, concatenating an RS Codec requires
an increase in transmission bandwidth of only nine to twelve percent while producing a greater
than 2 dB improvement in Eb/No. RS is a block codec where K data bytes are fed into the
encoder which adds 2t = (N - K) check bytes to produce an N byte RS Block. The RS Decoder
can then correct up to “t” erred bytes in the block (refer to Figure 3-4 and Table 3-1).
3.4.2 Reed-Solomon Operation in the MD2401 Demodulator
When the Reed-Solomon Decoder is enabled, the signal is received and demodulated by the
receiving unit, fed to a Viterbi Decoder for the first layer of error correction. After error correction
by the Viterbi Decoder, the unique words are located and the data is deinterleaved and reformed
into blocks. The R-S Decoder then corrects the leftover errors in each block. The data is then
descrambled and output from the R-S Section.
3.4.3 Reed-Solomon Code Rate
The R-S Code Rate is defined by (N, K) where N is the total R-S block size in bytes (data + check
bytes) and K is the number of data bytes input into the R-S Encoder. The transmission rate
expansion required by the R-S Codec is then defined by N/K. The MD2401 allows any N or K
setting up to N = 255, and K = 235 to allow tailoring of the code rate to meet system
requirements, with the following restrictions:
1. N/K must be less than 1.25 (25% maximum overhead).
2. N-K must be between 2 and 20, and must be even.
3. Maximum N = 255, Minimum N = 25.
4. Maximum K = 253, Minimum K = 23.