Shihai Shao

Optimal antenna location for STBC-OFDM downlink with distributed transmit antennas in linear cells

This letter focuses on where to place transmit antennas so as to obtain the best performance for a linear cell downlink distributed antenna system with space-time block coded orthogonal frequency division multiplexing. Aiming to solve this problem, the area averaged bit error probability (AABEP) is derived, considering the effects of path loss, correlated shadowing, multipath fading, background noise and mobile station location. Our analysis shows that the antennas should be located symmetrically about the cell center in order to achieve the minimum AABEP

Analysis of Carrier Utilization in Full-Duplex Cellular Networks by Dividing the Co-Channel Interference Region

We focus on the intra-cell co-channel interference between users in a full-duplex (FD) cellular network where FD enabled base stations are deployed with traditional half-duplex (HD) mobile stations. In order to suppress the co-channel interference, a cell partitioning method is proposed to divide the whole interference region into several partitions and allocate the frequency resources to them properly. The carrier utilization of such orthogonal frequency division multiple access (OFDMA)-based cellular networks is also analyzed theoretically to verify the proposed partitioning scheme. Our results show that the achievable gain in spectral efficiency over traditional HD systems increases with the number of users and decreases with the number of the partitions.

A General Digital Predistortion Architecture Using Constrained Feedback Bandwidth for Wideband Power Amplifiers

Digital predistortion (DPD) is one of the most effective techniques to mitigate the distortions caused by power amplifier (PA) nonlinearity and memory effects. As the input signal bandwidth increases, the required bandwidth on the DPD feedback channel becomes even larger, i.e., normally five times the signal bandwidth. However, the DPD feedback bandwidth is often restricted by the nonideal electronic components, e.g., the anti-aliasing filter and associated circuits, which therefore introduce bandwidth mismatch between the PA model basis functions and the feedback signal, and thus degrade the linearization performances of the DPD. This paper presents a general DPD architecture for wideband PA systems with constrained feedback bandwidth. By using linear operations to cancel the bandwidth mismatch between the proposed model and the PA feedback signal, the full-band PA model parameters can be estimated with bandwidth-limited observations. This estimated PA model is subsequently used with the PA input signal to extract the DPD function by applying the direct learning algorithms. The proposed DPD architecture reduces the feedback bandwidth to less than two times that of the input signal, while it maintains its linearization performance, as in the full-band case. Experiments are performed on the 20- and 100-MHz long-term evolution advanced signals to demonstrate the effectiveness of the proposed PA behavior modeling and DPD linearization performances with limited feedback bandwidth.

A New Digital Predistortion for Wideband Power Amplifiers With Constrained Feedback Bandwidth

This letter presents a new digital predistortion (DPD) solution for wideband power amplifiers (PAs) with restricted feedback bandwidth and relatively low sampling rate. By integrating a microwave cavity filter into the feedback path, the feedback bandwidth is reduced efficiently. A PA parameter extraction method is then proposed to identify the PA model, using the bandwidth-constrained signals. By applying the extracted parameters to the direct learning architecture, the DPD function can be obtained, which can linearize nonlinear distortion over the sampling bandwidth. Experiments demonstrate that a 22 dB adjacent channel leakage ratio improvement is acquired for a 100 MHz Long Term Evolution-advanced signal, even when the feedback bandwidth is restricted from 500 MHz to 100 MHz, which remarkably reduces the ADC sampling rate from 1105.92 Msps to 368.64 Msps.

Performance Analysis of RF Self-Interference Cancellation in Full-Duplex Wireless Communications

An expression of radio frequency (RF) interference cancellation ratio (ICR) is derived for full duplex wireless communications, considering the amplitude and phase errors in Rician fading channel. The results show that the amplitude and phase estimation errors of line-of-sight (LOS) components are the significant factors that limited the performance when Rician factor K > 1. ICR 55 dB can be achieved when the standard deviation of LOS component relative amplitude error is less than 0.1% and the standard deviation of LOS component phase error is less than 0.1°.

A Wideband Doherty Power Amplifier With 100 MHz Instantaneous Bandwidth for LTE-Advanced Applications

In this letter, a 698-862 MHz wideband Doherty power amplifier (DPA) with 100 MHz instantaneous bandwidth is presented. The electrical memory effect of the wideband DPA is reduced by the LC resonant circuits employed in the drain bias networks. The influence of the integrated Doherty combiner is compensated by the broadband output matching networks, which were designed by applying the simplified real frequency technique. Driven with 100 MHz LTE-advanced signals, the adjacent channel leakage ratio (ACLR) and ACLR asymmetry of the proposed DPA at 20 MHz offset are improved by about 4 dB and 3 dB respectively, compared to the wideband DPA with conventional bias networks. After digital predistortion (DPD) linearization, the proposed DPA shows an ACLR of lower than -48 dBc at an average output power of 42.5 dBm, and high drain efficiency of over 42% across the operating frequency range.

Correction of Carrier Frequency Offsets in OFDM-Based Spatial Multiplexing MIMO With Distributed Transmit Antennas

This correspondence focuses on the correction of carrier frequency offsets (CFOs) in orthogonal frequency-division multiplexing (OFDM)-based spatial multiplexing multi-input-multi-output (MIMO) with distributed transmit antennas, where the CFOs between the receiver and each transmit antenna are possibly different. Considering macroscopic fading and multipath Rayleigh fading, the average power of the intercarrier interference (ICI) caused by the CFOs is derived, and the optimal CFO correction value is then obtained by maximizing the average signal-to-interference-and-noise ratio (SINR). In particular, when the difference between the minimum and maximum values of the CFOs is no more than half of the adjacent subcarrier spacing, an upper bound of the average ICI power can be obtained, and a closed-form solution of the optimal CFO correction value is derived.

Performance Analysis of TDD Reciprocity Calibration for Massive MU-MIMO Systems With ZF Beamforming

This study analyzes a two-way signaling method for time division duplex reciprocity calibration (RC) in multiuser beamforming systems. The system model of RC is established, and the estimation error of the RC coefficient is derived. The effect of this error is examined in terms of the user signal-to-interference-plus-noise ratio (SINR) and capacity. Results reveal the relationship between the user SINR and RC errors as well as the number of antennas and users. Moreover, the findings show that the colocated antenna array is less sensitive to calibration errors than the case of distributed antennas even if the calibration signal-to-noise ratio (SNR) is identical. The interference resulting from imperfect calibration becomes the determinant that constrains the performance, particularly when a high user SNR is required.

Self-mixed self-interference analog cancellation in full-duplex communications

Rather than using existing self-interference cancellation methods, which essentially consist of reconstruction and subtraction, this paper proposes a novel approach, based on multiplication, to cancel selfinterference in the analog domain in full-duplex communications. This approach is called self-mixed selfinterference analog cancellation (SM-SIAC). Moreover, rather than using an individual analog cancellation circuit in existing self-interference cancellation methods, SM-SIAC can merge the analog cancellation circuit and the receiver. SM-SIAC is configured with three auto-tuning loops, consisting of one delay loop and two gain loops. SM-SIAC is further simplified with the Gaussian minimum shift keying (GMSK) self-interference signal. When these loops converge, the paper analyzes the cancellation capacity and derives a closed-form expression for the quadrature amplitude modulation self-interference signal and the GMSK self-interference signal. Simulation results illustrate the convergence of the gain loops and the cancellation capacity in the presence of engineering errors.

Optimization of Power Allocation and Relay Placement for Decode-and-Forward ARQ Relaying

In this letter, we investigate the power allocation and relay placement for decode-and-forward (DF) automatic repeat request (ARQ) relaying over fast Rayleigh fading channels, where the channel gains are constant during each ARQ transmission round and change independently from round to round. Specifically, when the relay decodes correctly, the source and the relay send the Alamouti space-time block coded (STBC) signals to the destination. The maximum number of ARQ rounds is represented by L, and an outage event occurs when the destination decodes the packet unsuccessfully after L ARQ rounds. Based on these assumptions, we derive the asymptotically tight approximation of outage probability in the high signal-to-noise ratio (SNR) regime. Furthermore, three optimization problems are formulated to minimize the obtained asymptotic outage probability, namely, optimization of the power allocation at the source and the relay with fixed relay placement, optimization of the relay placement with fixed power allocation, and joint optimization of the power allocation and relay placement. It is shown that joint optimization obtains the best outage performance.