• ETRI Journal
• /
• v.41 no.4
• /
• pp.426-436
• /
• 2019

In time division duplex (TDD)-based multiuser massive multiple input multiple output (MIMO) systems, the uplink channel is estimated and the results are used in downlink for signal detection. Owing to noisy uplink channel estimation, the downlink channel should also be estimated for accurate signal detection. Therefore, recently, a blind method was developed, which assumes the use of a linear high-power amplifier (HPA) in the base station (BS). In this study, we extend this method to a scenario with a nonlinear HPA in the BS, where the Bussgang decomposition is used for HPA modeling. In the proposed method, the average power of the received signal for each user is a function of channel gain, large-scale fading, and nonlinear distortion variance. Therefore, the channel gain is estimated, which is required for signal detection. The performance of the proposed method is analyzed theoretically. The simulation results show superior performance of the proposed method compared to that of the other methods in the literature.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.32 no.11C
• /
• pp.1079-1086
• /
• 2007

In this paper, we propose a CCP (Companding Combined with Predistorter) scheme that maintains BER (Bit Error Rate) performance of systems, while effectively reducing PAPR (Peak-to-Average Power Ratio) of OFDM (Orthogonal Frequency Division Multiplexing) signals, independent of nonlinear property of HPAs (High Power Amplifiers). The proposed CCP scheme combines the conventional companding scheme proposed by the authors with the predistorter, in order to prevent degradation of BER performance caused by signal distortion from nonlinear property of the HPA and the compression property of the companding scheme. Hence, the proposed scheme effectively reduces the PAPR and simultaneously maintains the BER performance, independent of nonlinear property of the HPA. Simulation results showed that the proposed CCP scheme provides the lowest total degradation for all the modulations, when comparing with the companding scheme proposed by Wang and the clipping scheme.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
• /
• 2009.10a
• /
• pp.160-163
• /
• 2009

HPA is one of the most essential device in wireless communication systems. However, because of nonlinear characteristic of HPA transmit signal is distorted with both amplitude and phase, this distortion leads to deepening adjacent channel interference. So a technique to change the nonlinear characteristic with linear characteristic is needed. In this paper, Among all techniques, we adopts a polynomial pre-distortion technique. Pre-distorted signal by pre-distorter has opposite characteristic with HPA. In result, the signal passed through pre-distorter and HPA has linear characteristic. But the accuracy of opposite characteristic of HPA is decreased at near portion of saturation point. So we improve the accuracy of opposite characteristic of HPA by using PAPR reduction method. In this paper, an adaptive polynomial pre-distortion technique is introduced to counterbalance the nonlinear characteristic of the transmit power amplifier, and a PAPR reduction method is introduced to increase efficiency of polynomial pre-distorter.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.23 no.4
• /
• pp.976-987
• /
• 1998

In this paper, the effect of nonllinear distortion, caused by a high-power amplifier(HPA) in an orthogonal frequency division multiplexing (OFDM) system, on the receiver part is analyzed. Since the HPA, which can be modeled by a memeoryless Volterra system, distorts OFDM signals in a nonlinear fashion, the received signal at each subchannel includes the multiplicative distortion of 1-st order as well as additive nonlinear distortion of high-order. the nonlinear distortion can be viewed as a nonlinear interchannel interference (NICI) since it consists of harmonic distortions and intermodulation distortions, produced by oother subchannels affecting the subchannel of interest. In this paper, we analytically derive the variance of NICI in terms of average input power using the volterra model for HPA, and then calculate the bit-effor rate(BER) performance of an OFDM system. Also, we propose a simple method to compensate for the phase distortion in OFDM system amplified by HPA, OFDM system employing 16-QAM constellation input.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.29 no.2A
• /
• pp.166-173
• /
• 2004

Satellite communication system uses a high non-linear HPA(high power amplifiers) in the earth station and satellite transponder. Therefore, it is important to consider the nonlinear effect of HPA on the communication system. In this paper, we find the variation of power spectrum density by nonlinearity HPA and the change of harmonic component according to IBO (input back-off). When the BPSK is used for satellite communication system, we analyze BER performance including the external co-channel interference (CCI) and the adjacent channel interference (ACI) resulting from the HPA nonlinearity. BER degrades as ACI magnitude grows up when the uplink SNR, uplink SIR (signal to co-channel interference power ratio) and downlink SIR are constant at some level. In case there is only non-linear HPA in the satellite, it is shown that BER considerably depends on the ACI magnitude ACI. When there are two non-linear HPAs in the both earth station and satellite, much BER degradation results from the CCI and ACI.

• Journal of the Korean Institute of Telematics and Electronics B
• /
• v.31B no.7
• /
• pp.16-25
• /
• 1994

The major reason that reduces the performance of satellite communication channels is known as the nonlinearities of high power amplifiers(HPA's) which are usually modeled by memoryless nonlinear systems, in the channels. This paper proposes a nonlinear adaptive predistorter,which predistorts the input symbols such that the symbol passing through the nonlinear HPA preserves the desired constellation, and derives the region of convergence for the predistorter. The predistorter has capability of coping adaptively with any time-varying environment such as changes of operating points if HPA since the coefficients of the predistorter which has a form of truncated Tayler series are obtained by recursive optimization procedure utilizing QAM data sequence directely. Simulation results show that the proposed one with 16 QAM input has better performance than the conventional fixed predistorter with the same input.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.15 no.7
• /
• pp.685-691
• /
• 2004

In this paper, we propose the hybrid methods of SLM(selected mapping) and predistortion, and PTS(partial transmit sequence) and predistortion to reduce PAPR(peak to average power ratio) and to decrease the nonlinear distortion of the nonlinear HPA(high power amplifier) in the multi-code CDMA(code division multiple access) system. The phase rotation factors are transmitted as side information in PTS and SLM methods play an important role in the BER performance. So, we present the theoretical BER equation when the errors of side information are considered in the multi-code CDMA communication system. Simulation results show that PAPR is reduced and nonlinear distortion is compensated by hybrid methods. Therefore BER performance is enhanced.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.29 no.4
• /
• pp.299-307
• /
• 2018

Considering a nonlinear high power amplifier(HPA) and a predistorter, we have designed a four-dimensional 8-ary phase shift keying trellis-coded modulation(4D-8PSK-TCM) system, which is recommended for X-band satellite communications. Subsequently, we have evaluated and analyzed the spectrum, constellation characteristics, and BER performance of the system. In satellite communications, owing to the limited power, nonlinear characteristics that determine the operating point of the HPA must be analyzed because the HPA consumes high power. We herein report the design of the 4D-8PSK-TCM system, with efficiencies of 2 and 2.25 bits/channel-symbol. The simulation results confirmed that a 0.35 roll-off value is effective, considering the low peak-to-average power ratio(PAPR) characteristic and the narrow occupation bandwidth of the spectrum. It also confirmed that approximately 15~20 dB of output backoff(OBO) value is required at the HPA when the predistorter is not used, and approximately 1 dB of the OBO value is required when the predistorter is used.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.41 no.4
• /
• pp.415-423
• /
• 2016

High PAPR (Peak-to-Average Power Ratio) characteristic causes some problems like system performance degradation and OOB (Out-of-Band) power increasement under the HPA (High Power Amplifier) nonlinearity condition. UFMC (Universal Filtered Multi-Carrier) and FBMC (Filter Bank Multi-Carrier) are regarded as 5G(Generation) candidate waveforms. In this paper, we evaluate and analyze performance of these systems with DFT-s (Discrete Fourier Transform Spreading) technique under the nonlinear HPA environment. In this paper, we describe OFDM (Orthogonal Frequency Division Multiplexing), UFMC, FBMC, DFT-s-OFDM, DFT-s-UFMC, and DFT-s-FBMC system, and evaluate BER (Bit Error Rate) performance of these systems. As simulation results, BER performance degradation by HPA nonlinearity of DFT-s-OFDM and DFT-s-UFMC is greatly overcome by DFT spreading technique. However, BER performance degradation by HPA nonlinearity of DFT-s-FBMC system is little overcome.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.27 no.8
• /
• pp.725-734
• /
• 2016

Recently, novel candidate waveform techniques for spectral efficiency improvement was proposed in order to satisfy key performance indicators(KPIs) of 5th generation(5G) mobile communication. Multi-carrier based universal filtered multi-carrier(UFMC) and filter bank multi-carrier(FBMC) are very famous as 5G candidate waveform techniques. Also, weighted orthogonal frequency division multiplexing (W-OFDM) that has low-complexity is receiving the spotlight slowly. In this paper, firstly, we describe a basic OFDM system. And then, we also describe UFMC, FBMC, and W-OFDM system. Next, we evaluate and analyze spectrum and BER performance of these systems under the nonlinear high power amplifier(HPA) environment. As simulation results, spectrum characteristic and BER performance of UFMC, FBMC, and W-OFDM are similar to each other. Therefore, under the nonlinear HPA environment, W-OFDM system is more advantageous because W-OFDM system uses a simple time-domain windowing technique and has similar characteristics to the others.