ETSI
ETSI EN 301 790 V1.5.1 (2009
Spreading shall be applied on a PLFRAME basis, following the conventional DVB-S2 physical layer scrambling
process. Each symbol in a PLFRAME, including the PLHEADER and pilot symbols if used, shall be spread by a
repetition of a real-valued spreading code C(i). The output of the spreading for each symbol on the I and Q branches
shall thus be a sequence of SF chips corresponding to the spreading code chip sequence, multiplied by the
corresponding, real-valued symbol component value. The spreading code sequence shall be time-aligned time with the
symbol boundary.
If {d[k]}, k = 0, 1,…, N
PLFRAME
-1, represents the (I+jQ) symbols of the PLFRAME, where N
PLFRAME
is the number of
symbols in one PLFRAME, then the spreading operation yields the spread sequence s(i):
⎦
()( )
),mod(/)( SFiCSFidis = for i = 0, 1,..., (N
PLFRAME
× SF)-1
Spreading codes C(i) are defined for spreading factors of 1, 2, 3 and 4 and are signalled in the Satellite Forward Link
Descriptor in clause 8.5.5.10.11. In terms of the reference modulator signal flow defined in DVB-S2 [16], the spreading
shall be performed immediately prior to the physical layer scrambling.
The second operation, scrambling, shall be achieved through the use of the same method as that defined for physical
layer scrambling in clause 5.5.4 of [16], except that:
• the length of the scrambling sequence is here equal to N
PLFRAME
× SF, rather than N
PLFRAME
; and
• the scrambling sequence is applied to the entire spread PLFRAME, including the PLHEADER and pilots if
used.
The scrambling sequence shall be aligned with the PLFRAME epoch, and it shall be re-initialized at the beginning of
each PLFRAME.
The sequence of complex valued chips shall be scrambled (complex chip-wise multiplication) by the complex-valued
scrambling code, w(i), defined in [16], clause 5.5.4, when SF is greater than 1. The Spread PLFRAME duration depends
on the selected modulation and the adopted spreading factor. The scrambled symbols, z(i), shall be obtained by directly
multiplying the spread symbols, s(i), by the scrambling sequence, w(i), as follows:
z(i) = s(i) × w(i modulo 66420), i=0,1,2,…,(N
PLFRAME
× SF)-1
After scrambling, the signal {z(i)} shall be square root raised cosine filtered as described in [16], clause 5.6.
It is mandatory to define an explicit scrambling sequence in the corresponding satellite forward link descriptor
(clause 8.5.5.10.11) when SF is greater than 1.
6 Return link base-band physical layer specification
and multiple access definition
Specifications for the base-band physical layer are given in this clause. Figure 6 represents the generic digital signal
processing to be performed at the RCST transmitter side, from the burst formatting of the serial information bit-stream,
to the modulation representing the digital to analogue conversion. The signal processing to be performed by each subset
is described in the following clauses.
Energy
Dispersal
Information data
MAC data
Channel
Coding
Burst
formatting
I/Q burst
modulation
Modulated signal
Synchronization
Figure 6: Block diagram of the RCST return link baseband signal processing