LTE Emulator

LTE Emulator version 1.0 – technical report
The length of an LDPC codeword can be modified by shortening the code, i.e. the removal
of a number of information bits, which leads to a decrease of the code’s coding rate. The codes
employed are shortened, so that the lengths of the codewords would match the lengths of the
transport frames, Lframe. The coding rate of the shortened codes, Rc, is expressed by:
R
c
L frame − J ⋅ P
= (3)
L
frame
Fig. 4.3. Setting the parameters of the adaptive modulations, of the error-correcting codes and of the
data structure in the simulator interface.
Five codes are defined by default for each length of the transport frame, but more codes can
be defined in simulator.
The codes employed for various lengths of the transport frame are selected so that they
should ensure about the same coding rate.
The parameters „Initial_rate_index” and „No_retransm” see figure 4.3., are related to the
H-ARQ procedure, [Fau06] [See06] [Val06].
The first parameter indicates the code that is employed in the first transmission of a transport
frame, while the second shows the maximum number of retransmission attempts of a transport
frame, if bit-errors occur. For each retransmission the FEC code is changed, by using the next
code on the list, code that has smaller coding rate. The maximum number of retransmission
should be smaller with 1 than the number of codes defined for each transport frame length. The
number of transport frame lengths (”No_transp_frames”), the number of FEC codes employed
for each length of a transport frame (“No_codes/frame”), and the parameters of the FEC
employed can be set in the simulator interface, see fig. 4.3., by filling in the appropriate boxes.
Several default groups of codes, defined for several transport frame lengths are also presented in
fig. 4.3.
Note: the selection of the set of LDPC codes employed for each Lframe (and for each
modulation) requires a careful evaluation of the throughputs provided by the
resulting configurations (code +modulation).
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