Modulation and Coding Schemes for Variable-Rate Parallel Sequence Spread Spectrum

This paper investigates coding and modulation schemes for wireless communication based on variable-rate parallel sequence spread spectrum (PSSS). PSSS can adapt the spreading gain according to channel conditions. When this modulation technique is combined with external forward error correction (FEC), a challenge arises in adjusting the gains from FEC and PSSS to achieve the highest spectral and energy efficiency. We profile the bit error rate (BER) and energy efficiency as a function of the signal-to-noise ratio (SNR) for PSSS combined with low-density parity-check (LDPC) codes implemented in 28 nm CMOS technology. This analysis reveals that the gain provided by PSSS should be used only in exceptional cases, whereas LDPC codes provide the gain at a lower spectral- penalty. Moreover, we simplify our system by reducing the number of LDPC code rates. As compared to IEEE 802.11n, we exclude the 2/3 and 3/4 FEC code rates from the hardware and compensate these modes by using the variable-rate PSSS.