Hybrid Pulse Shaping with Fractional Oversampling for Ultra High Data Rate Wireless Communication
Recent advances in the area of ultra-fast wireless transmission systems demand new techniques for precise control and manipulation of the shape of pulses to avoid Inter-Symbol-Interference (ISI). Considering the limited power budget for battery operated portable consumer electronics, the conventional pulse shaping techniques can not meet data rate and power consumption specifications for multi-gigabit wireless applications. A system concept that was developed in the context of EASY-A project introduces a Single-Carrier based architecture with directional antenna for a symbol detection without an equalizer and enables low-cost and low-power receivers for ultra high data rates. The key prerequisite for that is a suitable pulse shaping method in the transmitter.
For an optimal receiver performance, transmitted pulses must satisfy the first and second Nyquist criterion. By using a hybrid, digital-analog pulse shaping filter we gain a full control over the form of generated pulses. However, additional design issue is introduced by imperfections of reconstruction filter and requires oversampling to avoid ISI. Commonly utilized, integer-valued oversampling methods lead in multi-gigabit systems to excessive system clock frequencies. Thus, we propose an architecture for pulse shaping with fractional oversampling technique. Our highly optimized and flexible concept allows to optimally adopt to the system constraints and decrease power wastage caused by high resource usage and toggling rates in conventional algorithms. In addition, the signal characteristic is appropriately adjusted to fulfil the requirements of power spectral density mask.
As a proof-of-concept we developed a Stratix III FPGA-based demonstrator board. The system architecture exploits an 8-fold parallel structure with fractional oversampling factor of 4/3. The FPGA system clock of 288 MHz provides sampling frequency of 2.304 GHz. The shape of the transmitted pulses is optimally adjusted to the transfer function of the analog reconstruction filter and digital-to-analog-converter. Thus, the eye-diagram shows a minimal ISI influence in the received symbols. The symbol rate of 1.728 GSpS in case of QAM-16 modulation allows data transfer rates with 6.912 Gbps.