LINEARIZER DEVELOPMENT FOR WIDEBAND SIGNAL TRANSMISSION USING SOFTWARE DEFINED RADIOS

Abstract

The imperative demand of hungry data consumer is motivating research activities around the world to develop wideband systems. In response to meet this demand, the wireless communication industry steadily marches towards the next generation of network technology. The upcoming fifth generation (5G) promises data rates that are one hundred times faster than the existing networks, vastly improved connection and signal quality. However, the path towards 5G is not an easy one; as there are many significant challenges facing the design of radio frequency (RF) system and its real-world implementation; such as the use of Ultra-Wideband (UWB) signaling and inter-/multi-band transmission. These challenges led to the introduction of spectral efficient complex modulation schemes. These modern wireless waveforms result in the high peak to average power ratio of the signal that pushes the power amplifier (PA) into the compression region. In the compression region, it yields maximum efficiency, however it also causes signal distortion due to the non-linearity of PA. The non-linearity of the PA causes unwanted signal spilling into the adjacent channels as well as deterioration of the in-band performance. In a wireless communication system, the spill-over effect is particularly important, and adjacent channel leakage ratio- or ACLR as it is termed-is controlled and tightly specified. The requirement of highly linear transmitter front end along with the high efficiency mandates the deployment of linearization technique to neutralize the trade-off between linearity and efficiency of the nonlinear transmitter. This work focus on an efficient analog and hybrid circuit implementation, which are specifically tailored for PA linearization. Being analog in nature, the predistortion linearization models do not require access to the baseband information as in Digital Predistortion (DPD). In addition, the proposed predistorters also eliminate the constraint on the system bandwidth of the conventional DPD. The novelty and focus of this work lies in the system inversion of PA nonlinearity with a custom RF components as well as circuit design and hardware implementation of analog circuits. The analog circuits are chosen to alleviate the power dependency and complexity of the digital circuits. This work investigates three analog models which are RF-in RF-out Analog Predistorter (APD), Ultra-Broadband RF- Predistorter (UBB RF-PD) and Ultra-Broadband Multipath RF- Predistorter (UBB MRF-PD) that aim to provide linearization to UWB signals using low cost and energy ii efficient passive RF components. These models alleviate the need of Field programmable Gate Array (FPGA), Analog-to-Digital converter (ADC), Digital -to- Analog converter (DAC), Mixers, wideband transmitter and receiver chains etc. and still provide commendable linearization to UWB signals. The performance of the proposed RF-in RF-out APD and UBB RF-PD models are demonstrated using 8 component carrier (CC) 160 MHz Long-Term Evolution (LTE) signal through experimental measurements. It is also reported with measurement results that the proposed UBB MRF-PD model works efficiently for inter-/multi-band transmission. However, the performance of the proposed analog models can also be affected by the analog imperfections in the transmitter, which are introduced by the analog components; such as analog filters and Intermodulation generators. To circumvent the limitations, we further proposed a combinational Hybrid RF- Digital Predistortion (HRF-DPD) linearization method to take the best of both the analog and digital predistortion techniques, which in turn improve the overall predistortion performance. The linear operations are controlled digitally using FPGA, which provides flexibility in terms of digital compensation of delay, gain and phase control of the signal. Moreover, the proposed resulting combinational HRF-DPD considerably reduces the hardware complexity of digital system and provides a compromise between linearity and efficiency at higher power levels. The HRF-DPD performance is evaluated and compared with the proposed RF-in RF-out APD using contiguous and non-contiguous 8CC 160 MHz LTE signal along with several other state-ofthe- art signals such as two-tone, 10 MHz, 20 MHz LTE signals as well as carrier aggregated LTE signals. Experimental results validate the dexterity of the proposed HRF-DPD linearization. The linearizability of the proposed HRF-DPD is reported to be better than the proposed RF-in RF-out APD due to digitally compensation of gain and phase of the signal.