Compensation of Phase Noise in Uplink Massive MIMO OFDM Systems

Compensation of Phase Noise in Uplink Massive MIMO OFDM Systems

Abstract: Due to the imperfection of practical oscillators, phase noise (PHN) inevitably occurs in the phase of desired signals, and may incur significant degradation in the performance of wireless communications. This paper addresses the PHN compensation in uplink massive multiple-input multiple output orthogonal frequency division multiplexing systems, where multiple single-antenna users simultaneously communicate with a base station equipped with a large antenna array. The proposed compensation scheme considers a two-stage transmission protocol. In the first stage, training symbols are transmitted, and the channels of multiple users are estimated along with the PHN sequences involved. In the second stage, information-conveying data symbols are transmitted, and the channel estimates previously obtained are used to estimate the data symbols in the presence of unknown PHN. For both stages, the estimation methods are elaborately derived based on the variational expectation maximization methodology. Moreover, some simplifications are conducted to significantly reduce the computational complexity of the proposed scheme. The effectiveness of the proposed PHN compensation scheme is corroborated through extensive simulations.

Existing system:

The scheme proposed herein provides a systematic solution for the PHN compensation in multi-user massive MIMO OFDM systems. Specifically, it consists of a channel estimation stage and a data transmission stage. In the channel estimation stage, the training symbols are transmitted, and the multipath channels of users are estimated, along with the transmitter and receiver PHN sequences.

When conducting the joint estimation of channels and PHN sequences, we exploit the structural sparsity inherent in MIMO channels and the statistical property of PHN. In the data transmission stage, information-conveying data symbols are transmitted, and the channel estimates already obtained are used to estimate the data symbols in the presence of PHN. Similar to the channel estimation stage, we jointly estimate the data symbols and the PHN sequences involved.

Proposed system:

That there is no PHN compensation scheme provided. To alleviate the adverse effect of PHN and enhance the performance of massive MIMO, two PHN compensation schemes are developed infor uplink and downlink SC systems, respectively. However, the two schemes are based on the flat-fading channel assumption. That is, there is only one path in the channel between each user and each BS antenna. This greatly limits the application scope of the two schemes in practice. In this paper, we propose a novel PHN compensation scheme for uplink massive MIMO OFDM systems. Multiple single-antenna users simultaneously communicate with the BS over frequency-selective channels, and the PHN effects at both the users and the BS are considered. Our previous work in investigated the case of one user communicating with the BS. Advantages: Despite the aforementioned advantages, massive MIMO is still faced with some practical challenges, one of which is the phase noise (PHN) problem caused by the imperfection of oscillators.

The PHN manifests itself as a random and time varying phase rotation of the information-conveying signal. If not properly compensated, the PHN can cause

significant performance degradation for wireless communications, especially for orthogonal frequency division multiplexing (OFDM) systems.

Disadvantages: Despite the aforementioned advantages, massive MIMO is still faced with some practical challenges, one of which is the phase noise (PHN) problem caused by the imperfection of oscillators. The PHN manifests itself as a random and time varying phase rotation of the information-conveying signal.

If not properly compensated, the PHN can cause significant performance degradation for wireless communications, especially for orthogonal frequency division multiplexing (OFDM) systems.

Modules: Channel state information: RECENTLY, massive multiple-input multiple-output (MIMO) has been widely recognized as an essential technology for next-generation wireless communications, due to various advantages. By deploying a large number of base station (BS) antennas, massive MIMO allows the BS to simultaneously communicate with multiple users in the same time-frequency slot, which can achieve significantly high spectrum efficiency. Denote the number of BS antennas as M. With massive MIMO, the transmit power per user can be scaled down at the ratio of 1/M when the perfect channel state information (CSI) is available, or at the ratio M when the CSI is estimated from the uplink training symbols. This means that massive MIMO also enjoys high energy efficiency. Besides, when M increases without bound, the channel vectors associated with different users become asymptotically orthogonal. As a result, low-complexity linear processing techniques, such as zero-forcing (ZF) combining/precoding, maximum-ratio combining/transmission (MRC/MRT) and minimum mean square error (MMSE) schemes, can be used to approach the uplink/downlink capacity upper-bound. In contrast, for small-scale MIMO, the realization of uplink/downlink capacity requires formidably high-complexity nonlinear technique, i.e., successive interference cancelation (SIC)/dirty-paper precoding (DPC).

Minimum mean square error:

This means that massive MIMO also enjoys high energy efficiency. Besides, when M increases without bound, the channel vectors associated with different users become asymptotically orthogonal. As a result, low-complexity linear processing techniques, such as zero-forcing (ZF) combining/precoding, maximum-ratio combining/transmission (MRC/MRT) and minimum mean square error (MMSE) schemes can be used to approach the uplink/downlink capacity upper-bound. In contrast, for small-scale MIMO, the realization of uplink/downlink capacity requires formidably high-complexity nonlinear technique, i.e., successive interference cancelation (SIC)/dirty-paper precoding (DPC). Despite the aforementioned advantages, massive MIMO is still faced with some practical challenges, one of which is the phase noise (PHN) problem caused by the imperfection of oscillators.

Orthogonal frequency division multiplexing: The PHN manifests itself as a random and time varying phase rotation of the information-conveying signal. If not properly compensated, the PHN can cause significant performance degradation for wireless communications, especially for orthogonal frequency division multiplexing (OFDM) systems. Although the effect and compensation of PHN have been extensively studied for conventional single input single-output (SISO) systems and small-scale MIMO systems, the investigation for PHN-impaired massive MIMO is far from mature and only a few references are available up till now. In, the derived hardware scaling laws show that massive MIMO is much more sensitive to PHN than to other impairments, such as the additive distortion noise caused by non-linear circuitry, the quantization error and the signal leakage. In , a lower bound on the sum capacity is derived for uplink massive MIMO single carrier (SC) systems subject to PHN impairment, where a time-reversal MRC reception strategy is considered.

Signal to – interference – plus – noise ratio: In, closed-form achievable rate expressions are rigorously derived for uplink massive MIMO OFDM systems in the presence of PHN. The presented expressions correspond to two different operations, namely, the case of the BS array sharing a common oscillator, and the case of each BS antenna having an independent oscillator. For downlink massive MIMO SC systems, the analysis in

reveals that the impact of PHN on the signal to- interference-plus-noise ratio (SINR) can be quantified as an effective reduction in the CSI quality, compared with the ideal system without PHN. For downlink massive MIMO OFDM systems, the degradation in terms of SINR and the achievable rate are analytically obtained.