| Popis: |
For modern wireless communication systems bandwidth efficiency and energy efficiency are the vital requirements for reasonable performance. Bandwidth determined by the rate at which information can be sent via the channel and the maximum rate at which error free information can be communicated is introduced as channel capacity by Shannon. Bandwidth efficiency or spectral efficiency is often satisfied by exploiting appropriate modulation scheme. Energy efficiency or power efficiency depends on the amount of power that a system uses which is a critical issue for wireless and cellular communications. It also shows the system tolerance against the environment noise. Energy efficiency can be improved by applying error correcting codes that produces lower error probability at the receiver for a fixed signal to noise ratio. In order to achieve both requirements, the idea of coded modulation technique which combines QPSK and OFDM modulation with a low rate, short block systematic turbo code, is proposed. We aim to achieve exceptional energy efficiency in extremely noisy environments, where moderate data rates and short messages are required. For this proposed model, the performance improvement is produced by a unique mapping of trellis structure error correcting code and spectral efficiency is achieved by exploiting OFDM modulation. The designed OFDM distributes systematic and parity symbols along all sub-channels symmetrically and adjusts their power distinctively to achieve superior bit error performance. Further, we utilize four-dimensional M-ary Quadrature Amplitude Modulation (4D-MQAM) for parity symbols which effectively increases the overall rate of the system while maintaining the same level of energy efficiency. Moreover, we apply puncturing for parity symbols to increase overall rate of the system and improve bandwidth efficiency. Also, we applied non-systematic coding structure to increase coding rate with less puncturing rate while maintaining the error rate as low as possible. The resulting performance of our designed system compared with the theoretical sphere packing lower bound indicates a very small gap (less than 0.5 dB) which means a substantially close approach to the Shannon limit. |
