LTE - D2D

The data stream is first scrambled by a random sequence. The scrambling sequence shall be initialized at the beginning of every PSSCH/PSCCH/PSDCH/PSBCH subframe. The corresponding random seeds are

PSSCH

\(c_\text{init} = n_\text{ID}^\text{SA} \cdot 2^{14} + n_\text{ssf}^\text{PSSCH} \cdot 2^9 + 510\), where \(n_\text{ID}^\text{SA}\) is destination ID which can be obtained from PSDCH.

PSCCH/PSDCH

\(c_\text{init} = 510\).

PSBCH

\(c_\text{init} = N_\text{ID}^\text{SL}\).

The scrambled sequence is then modulated as constellation symbols. The modulation schemes for the physical channels are

There is only one layer for each physical channel above. Namely, for each physical channel, the single codeword is mapped to a single layer.

Similar to uplink transmission, SC-FDMA is adopted for sidelinks. For the 4 physical channels above, every \(M_\text{sc}^\text{PSSCH}\) / \(M_\text{sc}^\text{PSCCH}\) / \(M_\text{sc}^\text{PSDCH}\) / \(M_\text{sc}^\text{PSBCH}\) complex-valued symbols are FFT-processed.

As mentioned above, there is only one layer for each physical channel over a sidelink. Therefore, the single layer can be directly mapped to the corresponding antenna port, i.e. R1000 for PSSCH, and R1010 for PSCCH/PSDCH/PSBCH.

The complex symbols are first scaled by \(\beta_\text{PSSCH}\) / \(\beta_\text{PSCCH}\) / \(\beta_\text{PSDCH}\) / \(\beta_\text{PSBCH}\) before mapping to the physical resource blocks (PRB's). The mapping shall be in increasing order of first the index k, and then the index l, starting with the first slot in the subframe. Moreover, the PRB's used for reference signal (RS's) will be avoided in the mapping. Resource elements in the last SC-FDMA symbol within a subframe shall be counted in the mapping process but not transmitted.

Particularly for PSSCH, frequency hopping is supported. The details can be referred to the specification.

For PSBCH, the available PRB's are in the central 6 PRB's, i.e. 72 subcarriers, as synchronization signals.