Shengjie Zhao

BER analysis of TDD downlink multiuser MIMO systems with imperfect channel state information

In downlink multiuser multiple-input multiple-output (MU-MIMO) systems, the zero-forcing (ZF) transmission is a simple and effective technique for separating users and data streams of each user at the transmitter side, but its performance depends greatly on the accuracy of the available channel state information (CSI) at the transmitter side. In time division duplex (TDD) systems, the base station estimates CSI based on uplink pilots and then uses it through channel reciprocity to generate the precoding matrix in the downlink transmission. Because of the constraints of the TDD frame structure and the uplink pilot overhead, there inevitably exists CSI delay and channel estimation error between CSI estimation and downlink transmission channel, which degrades system performance significantly. In this article, by characterizing CSI inaccuracies caused by CSI delay and channel estimation error, we develop a novel bit error rate (BER) expression for M-QAM signal in TDD downlink MU-MIMO systems. We find that channel estimation error causes array gain loss while CSI delay causes diversity gain loss. Moreover, CSI delay causes more performance degradation than channel estimation error at high signal-to-noise ratio for time varying channel. Our research is especially valuable for the design of the adaptive modulation and coding scheme as well as the optimization of MU-MIMO systems. Numerical simulations show accurate agreement with the proposed analytical expressions.

Impact of imperfect channel state information on TDD downlink Multiuser MIMO system

In downlink Multiuser Multiple-Input Multiple-Output (MU-MIMO) systems, the zero-forcing (ZF) transmission is a simple and effective technique for separating users and data streams of each user at the transmitter side, but its performance depends greatly on the accuracy of the available channel state information (CSI) at the transmitter. Due to the existence of the CSI delay and channel estimation error in practical systems, CSI is always imperfect, which degrades system performance significantly. In this paper, by characterizing CSI inaccuracies due to channel delay and channel estimation error, we develop a novel bit error rate (BER) expression for M-QAM (quadrature amplitude modulation) signal in time division duplex (TDD) downlink MU-MIMO system where each user is equipped with one antenna. By simulation, we find that channel estimation error causes array gain loss while CSI delay causes diversity gain loss. Moreover, CSI delay causes more performance degradation than channel estimation error at high signal to noise ratio (SNR) for time varying channel. Especially our research is valuable for adaptive modulation and coding scheme and the optimization of MU-MIMO system. Numerical simulations show accurate agreement with the analytical expressions.

Sounding reference signal design for TDD LTE-Advanced system

In the 3rd generation partnership project (3GPP) long term evolution (LTE) time division duplex (TDD) systems, base stations use uplink sounding reference signals (SRSs) to estimate downlink channel state information (CSI) for downlink beamforming transmissions. Because SRSs sent by up to 8 users are multiplexed on the same time-frequency resources, there exist mutual interferences among these SRSs over fading channels. Thus the current scheme inevitably impacts the estimation accuracy of CSI and thereby degrades the beamforming performance. Based on the characteristics of TDD systems, we propose a novel sounding scheme to improve the quality of CSI, in which according to the CQIs (channel quality indications) periodically reported by each user and the bandwidth requirement of each user, BS dynamically schedules each user to send SRSs only on the requisite bandwidth with best CQIs instead of on full bandwidth or specified sub-bandwidth as in the current LTE TDD systems. Simulations show that the proposed scheme outperforms the current scheme in estimation accuracy for various frequency-selective and/or time-selective fading channels, and is especially robust to frequency selective channels. Therefore the proposed scheme is applicable to the uplink SRS design in LTE-Advanced and has been adopted as a candidate scheme for 3GPP LTE-Advanced.

On BER of TDD Multiuser MIMO System with Channel Estimation Error and Delay

In Multiuser Multiple-Input Multiple-Output (MU-MIMO) downlink systems, the zero-forcing algorithm is a simple and effective technique for separating users and data streams of each user at the transmitter side, but its performance depends greatly on the accuracy of the available channel state information (CSI) at the transmitter. Due to the existence of the CSI delay and channel estimation error in practical systems, CSI is always imperfect, which degrades system performance significantly. In this paper, by using the correlation between the actual channel and the estimated one as well as the channels time-correlation, we develop a novel bit error rate (BER) expression for M-QAM (quadrature amplitude modulation) signal in time division duplex (TDD) downlink MU-MIMO system with channel estimation error and CSI delay. We find that channel estimation error causes array gain loss while CSI delay causes diversity gain loss. Moreover, CSI delay causes more performance degradation than channel estimation error at high signal to noise ratio (SNR) for time varying channel. Especially our research is valuable for adaptive modulation and coding scheme and the optimization of MU-MIMO system. Numerical simulations show accurate agreement with the analytical expressions.

An Optimal Scheme for TDD Beamforming Systems with Imperfect Channel State Information

An optimal scheme on uplink pilot time interval (UPTI) to maximize average post-processing SNR (signal to noise ratio) is proposed in order to overcome the impact of channel estimation error and delay on a time division duplex (TDD) multiple input single output (MISO) beamforming system. In TDD system, the base station estimates the channel state information (CSI) at transmitter based on uplink pilots and then uses it to generate the beamforming vector in the downlink transmission. Because of the constraints of the TDD frame structure and the uplink pilot overhead, there inevitably exists delay and channel estimation error between CSI estimation and its use. In this paper, we first derive average post-processing SNR for TDD MISO beamforming system with channel estimation error and delay. We then obtain the optimal UPTI, which maximizes average post-processing SNR, given the normalized pilot overhead (the number of pilot symbols per data symbol). The simulation results validate that the optimal UPTI not only maximizes the average post-processing SNR but also minimizes the BER. Especially our research is valuable for the uplink sounding reference signal design in LTE- Advanced system.

A Frequency-Domain Sounding Scheme for LTE TDD Beamforming Systems

In the 3rd generation partnership project (3GPP) long term evolution (LTE) time division duplex (TDD) systems, base stations use uplink sounding reference signals (SRSs) to estimate downlink channel state information (CSI) for downlink beamforming transmissions. Because SRSs sent by up to 8 users are multiplexed on the same time-frequency resources, there exist mutual interferences among these SRSs over fading channels. Thus the current scheme inevitably impacts the estimation accuracy of CSI and thereby degrades the beamforming performance. Based on the characteristics of TDD systems, we propose a novel sounding scheme to improve the quality of CSI, in which according to the CQIs (channel quality indications) periodically reported by each user and the bandwidth requirement of each user, BS schedules each user to send SRSs only on the requisite bandwidth with best CQIs instead of on full bandwidth or specified sub-bandwidth as in the current LTE TDD systems. Simulations show that the proposed scheme outperforms the current scheme in estimation accuracy for various frequency-selective and/or time-selective fading channels, and is especially robust to frequency selective channels. The proposed scheme has been adopted as a candidate scheme for 3GPP LTE-Advanced.

A novel CQI-assisted sounding design for TDD LTE-Advanced system

In the long term evolution (LTE) time division duplex (TDD) systems, base stations use uplink sounding reference signals (SRSs) to estimate downlink channel state information (CSI) for downlink beamforming transmissions. Because SRSs sent by up to 8 users are multiplexed on the same time-frequency resources, there exist mutual interferences among these SRSs over fading channels. Thus this scheme inevitably impacts the estimation accuracy of CSI and thereby degrades the beamforming performance. In this paper, we propose a novel sounding scheme to improve the quality of CSI, in which according to the CQIs (channel quality indications) periodically reported by each user and the bandwidth requirement of each user, BS dynamically schedules each user to send SRSs only on the requisite bandwidth with best CQIs instead of on full bandwidth or specified sub-bandwidth as in the current LTE TDD systems. Simulations show that the proposed scheme obtains much better estimation accuracy than the current scheme for various frequency-selective and/or time-selective fading channels, and is robust to frequency selective channels. Therefore the proposed scheme is applicable to the uplink SRS design in LTE-Advanced and other MIMO-OFDM TDD systems.

Sounding reference signal pattern design for time division duplex multiple-input multiple-output and orthogonal frequency division multiplexing systems

In the long-term evolution (LTE) time division duplex (TDD) systems, base stations (BSs) use uplink sounding reference signals (SRSs) to estimate downlink channel state information (CSI) for downlink beamforming transmissions. In the current SRS pattern scheme, because SRSs of up to eight users are multiplexed on the same time-frequency resources, there exist mutual interferences among these SRSs over fading channels. Thus, the current scheme impacts the estimation accuracy of CSI and degrades the beamforming performance. Based on the characteristics of TDD systems, the authors propose a SRS pattern scheme, in which each user sends SRSs only on the allocated bandwidth instead of on full bandwidth as in the current LTE TDD systems, to eliminate the interferences among SRSs. Simulations show that the proposed scheme outperforms the current scheme in the estimation accuracy for various frequency-selective and time-selective fading channels. In addition, compared with the current SRS pattern scheme, the proposed SRS pattern scheme is robust to frequency-selective channels, and does not be impacted by the timing offset, and provides the flexibility for the choice of channel estimator at the BS side.

Practical robust uplink pilot time interval optimisation scheme for time-division duplex multiple-input-single-output beamforming system

Focusing on a time-division duplex (TDD) multiple-input-single-output (MISO) beamforming system, this study investigates the robust uplink pilot time interval (UPTI) design to overcome the impact of delay and channel estimation error. In TDD beamforming systems, the base station estimates the downlink (DL) channel state information (CSI) by exploiting the received uplink (UL) pilots and the channel reciprocity. Then, utilising the estimated DL CSI, the beamforming vector in the DL transmission is generated. Owing to the constraints of the TDD frame structure and the UL pilot overhead, there inevitably exist delay and estimation error between the estimated and the actual DLCSI, which would degrade system performance. In this study, the upper bound is first derived on ergodic rate for TDD MISO beamforming systems with channel estimation error and delay. Then, with any one given setting of the normalised UL pilot overhead, the optimal robust UPTI is designed, which maximises the upper bound on ergodic rate in the worst case of the CSI delay (i.e. in the case of maximum delay). Simulation results validate that the designed optimal robust UPTI can not on I y maximise the upper bound on ergodic rate but also be adaptive to the variation of channel conditions very well.

Capacity of TDD MISO Beamforming Systems with Channel Estimation Error and Delay

This paper examines the capacity of a time division duplex (TDD) multiple input single output (MISO) beamforming system with channel estimation error and delay over the time varying fading channel. In TDD system, the base station estimates the channel state information (CSI) at transmitter based on uplink pilots and then uses it to generate the beamforming vector in the downlink transmission. Because of the constraints of the TDD frame structure and the uplink pilot overhead, there inevitably exists delay and channel estimation error between CSI estimation and its use. In this paper, we first derive the capacity upper bound for TDD MISO beamforming system with channel estimation error and delay. We then obtain the optimal uplink pilot time interval (UPTI), which maximizes the capacity upper bound, given the normalized pilot overhead (the number of pilot symbols per data symbol). The simulation results validate that the optimal UPTI not only maximizes the capacity upper bound but also maximizes the system ergodic capacity. Especially our research is valuable for the uplink sounding reference signal design in LTE- Advanced system.