A Novel Quadrature-Tracking Demodulator for LTE-A Applications

Hindawi Wireless Communications and Mobile Computing Volume 2018, Article ID 8712414, 8 pages https://doi.org/10.1155/2018/8712414 Research Article A Novel Quadrature-Tracking Demodulator for LTE-A Applications Kang-Chun Peng and Chan-Hung Lee Department of Computer and Communication Engineering, National Kaohsiung First University of Science and Technology, 2 Jhuoyue Rd., Nanzih, Kaohsiung City 811, Taiwan Correspondence should be addressed to Kang-Chun Peng; Received 27 July 2017; Accepted 2 December 2017; Published 2 January 2018 Academic Editor: Chaojiang Li Copyright © 2018 Kang-Chun Peng and Chan-Hung Lee. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This work develops an advanced quadrature-tracking demodulation technique for coherently demodulating the orthogonal frequency-division multiplexing (OFDM) signal of LTE-A systems. To overcome the fact that traditional coherent demodulators are extremely sensitive to the quadrature imbalance of a system, especially an OFDM system, the proposed architecture uses a novel quadrature phase-locked loop (QPLL) to track simultaneously the in phase (I-phase) and the quadrature phase (Q-phase) of the received signal. This advanced quadrature-tracking demodulator is realized using TSMC 0.18 𝜇m CMOS technology and hybrid circuits. Experimental results indicate that the developed quadrature-tracking demodulator, which operates at 2.1∼2.5 GHz, can effectively demodulate an 18 Mbps LTE-A signal, even with a 15 degree quadrature imbalance. 1. Introduction Most wireless communication systems use coherent demodulation, mainly because the quality of coherent demodulation is much better than that of non-coherent demodulation [1]. Traditional coherent demodulators are based on an RF quadrature demodulator. But the RF coherent circuits are usually complex and power-hungry [2]. To simplify the receiver’s circuitry, various phase-locked loop (PLL)-based coherent demodulators are adopted in wireless communication systems. The most well-known PLL-based coherent demodulator has the Costas architecture [1, 3–5]. As depicted in Figure 1, this architecture uses a single PLL with two feedback loops. These two feedback loops demodulate the in-phase (I-phase) signal and quadrature-phase (Q-phase) signals, respectively. The demodulated signals are combined, and then tune the voltage-controlled oscillator (VCO) to track the frequency of the carrier signal. However, both the traditional quadrature demodulator and the Costas-coherent demodulator face the problem of quadrature imbalance of RF signal. Quadrature imbalance of RF signal arises from both the quadrature transmitter and the quadrature receiver. Previous investigations have showed that a slight 2.5 degree quadrature imbalance significantly degrades the demodulation quality of an OFDM signal, which is extensively used in LTE-A systems [6, 7]. Although the conventional Costas-coherent demodulator has two feedback loops for demodulation, the single-VCO design prevents tracking of more than one phase of a received signal. We [8] previously presented an alternative coherent polar demodulator without the quadrature imbalance problem of receiver. As presented in Figure 2, the received signal is divided into two paths. One of these paths uses injectionlocked oscillators (ILO) to extract the phase-modulated carrier signal and the phase information. The extracted phase-modulated carrier is then mixed with the received signal along another path. The mixing cancels out the phase information of these two input signals, and then the envelope information of the received signal is exported. The baseband processor then recovers the baseband signal from both the demodulated phase and the envelope information. However, the quadrature imbalance that is caused by the RF transmitter remains in the received signal. To overcome the quadrature imbalance problem, some works directly trimming or adjusting their RF circuits [9]. 2 Wireless Communications and Mobile Computing Demodulated I-data +1 LPF −1 BPSK QPSK/ BPSK VCO + + LPF QPSK − 90∘ +1 LPF Demodulated Q-data −1 Figure 1: Traditional Costas demodulator. Mixer Digital signal processing LPF A/D 2nd ILO BPF 1st ILO 90∘ A/D demodulation, the two tuning ports of the QVCO are shorted to make the QPLL act as a single PLL to lock the carrier frequency of the received signal. Since the effective detection range of a mixer-based phase detector is limited by ±90 degree [13], as depicted in Figure 4, an additional channelpreset frequency synthesizer is required. The additional frequency synthesizer uses an all-digital phase-frequency detector (PFD) to detect a large phase variance of up to ±360 degree. Therefore, the QPLL can track both the frequency and the phase of the received signal. After the frequency of the received signal has been locked, the channel-preset frequency synthesizer is turned off to save power and the two VCO tuning ports are disconnected, as presented in Figure 5. The QPLL can then track in real time and demodulate both the I-phase and the Q-phase of the received signal. According to PLL theory, a PLL-based demodulator attenuates the demodulated signal within the loop bandwidth of the PLL [13]. Therefore, the proposed architecture is especially suited to OFDM systems because the DC-subcarrier of the OFDM signal, as depicted in Figure 6, is not used in the LTE-A system, to mitigate the DC-offset problem [14]. Therefore, the proposed advanced QPLL-based demodulation technique can coherently demodulate the OFDM signal without attenuation if the loop bandwidth of the PLL is designed to be less than the sub-carrier space. Mixer LPF Figure 2: Polar demodulator. However, these are impractical. Another solution is basedon digital-signal process (DSP) technique. [10] and [11] respectively uses the pilot signal and a special tone to train the DSP in receiver to find out and then correct the quadrature imbalance. [9, 12] utilize adaptive algorithms to estimate the quadrature error and then compensate demodulated signal. Although these adaptive algorithms theoretically can reduce the quadrature imbalance to less than 1 degree, they take a very long computation time with about 105 iterations. To speed up the tracking process, this work proposes a novel quadrature-tracking demodulator which can real-time track the quadrature error. 3. System Analysis To analyze the proposed quadrature-tracking demodulator in the time domain, the received signal is assumed to be 𝑟 (𝑡) = 𝐼 (𝑡) cos [(𝜔0 + Δ𝜔) 𝑡 + 𝜃𝐼 ] + 𝑄 (𝑡) sin [(𝜔0 + Δ𝜔) 𝑡 + 𝜃𝑄] , where 𝐼(𝑡) and 𝑄(𝑡) denote baseband signals, and Δ𝜔 is the frequency error; 𝜃𝐼 and 𝜃𝑄 are the phase errors of the Iphase signal and the Q-phase signal, respectively. After downmixing, the signal at nodes A and B in the circuit that is dis (...truncated)