A novel frequency domain training sequence and the corresponding carrier frequency offset (CFO) estimator are proposed for orthogonal frequency division multiplexing (OFDM) systems over frequency-selective fading channels. The proposed frequency domain training sequence comprises two types of pilot tones, namely distinctively spaced pilot tones with high energies and uniformly spaced ones with low energies. Based on the distinctively spaced pilot tones, integer CFO estimation is accomplished. After the subcarriers occupied by the distinctively spaced pilot tones and their adjacent subcarriers are nulled for the sake of interference cancellation, fractional CFO estimation is executed according to the uniformly spaced pilot tones. By exploiting a predefined lookup table making the best of the structure of the distinctively spaced pilot tones, computational complexity of the proposed CFO estimator can be decreased considerably. With the aid of the uniformly spaced pilot tones generated from Chu sequence with cyclically orthogonal property, the ability of the proposed estimator to combat multipath effect is enhanced to a great extent. Simulation results illustrate the good performance of the proposed CFO estimator.

In this paper, a distributed cooperative multicell beamforming algorithm is proposed, and a detail analysis and solving method for instantaneous and statistical channel state information (CSI) are presented. Firstly, an improved distributed iterative beamforming algorithm is proposed for the multiple-input single-output interference channel (MISO IC) scenario which chooses virtual signal-to-interference-and-noise (SINR) as decision criterion to initialize and then iteratively solves the constrained optimization problem of maximizing the virtual SINR for a given level of generated interference to other users. Then, the algorithm is generalized to the multicell date sharing scenario with a heuristics power allocation scheme based on a viewpoint of the layered channel. Finally, the performance is illustrated through numerical simulations.

In this letter, an improved channel estimator for MIMO-SCBT systems is proposed. Pilot blocks are constructed using quadriphase complementary sequences (QCSs) which enable both one-sided (OSD) and two-sided (TSD) channel estimation (CE). And OSD-CE and TSD-CE are combined to provide improved performance in frequency-selective fast and slow fading channels and to maintain low-complexity implementations. Simulation results demonstrate the performance merits of the proposed scheme.

In this paper, we study the spectral efficiency of the uplink multi-user large-scale distributed antenna systems (DAS) with imperfect channel state information. We propose the system model of multi-user DAS and illustrate the necessity of pilot reuse. Then, we derive the sum-rate of the system under pilot contamination. Furthermore, we investigate the asymptotical performance when the number of antennas goes to infinity. To reduce the pilot contamination, we present two novel pilot assignment algorithms to improve the spectral efficiency. Finally, we evaluate our proposed strategies through extensive simulations which show that compared with random pilot reuse, the min-max algorithm shows impressive performance with low complexity.

A polynomial expansion linear detector (PELD) based turbo receiver is proposed for single-carrier zero-padding block transmission (ZP-SCBT) systems over the single-input single-output (SISO) and multiple-input multiple-output (MIMO) frequency selective channels. The performance is compared with the minimum mean square error (MMSE) and match filter (MF) based turbo receivers. It is shown that the PELD-based turbo receiver provides a good trade-off between performance and complexity compared with the other two alternatives.

Both multiplexing and multi-user diversity are exploited based on Round Robin (RR) scheduling to achieve tradeoffs between average throughput and fairness in distributed antenna systems (DAS). Firstly, a parallel Round Robin (PRR) scheduling scheme is presented based on the multi-user multiplexing in spatial domain to enhance the throughput, which inherits the excellent fairness performance of RR. Then a parallel grouping Round Robin (PGRR) is proposed to exploit multi-user diversity based on PRR. Due to the integration of multi-user diversity and multi-user multiplexing, a great improvement of throughput is achieved in PGRR. However, the expense of the improvement is at the degradation of fairness since the "best channel criteria" is used in PGRR. Simulations verify analysis conclusions and show that tradeoffs between throughput and fairness can be achieved in PGRR.

In this paper we investigate a low complexity channel estimation and data transmission scheme for bi-directional relaying networks. We also propose a semi-orthogonal pilot structure for channel estimation to increase the efficiency of data transmission between the Base Station (BS) and Mobile Station (MS) via a fixed Relay Node (RN).

We analyze a power adaptation method to maximize the achievable rate under the finite block length regime, for MIMO block fading channel with channel state information available at both the transmitter and receiver side. We find a convex approximation to the lower bound of the achievable rate, and it leads to a simple power and rate adaptation method. We show that the method achieves near optimal channel rate under the finite block length regime. Compared to the classical waterfilling method, the proposed method can further improve achievable rate especially for short block lengths.

In this letter, we first investigate the bias of Doppler shift estimator based on autocorrelation function (ACF). Then we derive a signal-to-noise ratio (SNR) independent condition for Doppler shift estimation and achieve this condition by a adaptive process. Moreover, we present theoretical analysis about the convergency of our adaptive Doppler shift estimator, and derive a close-form expression for its mean square error (MSE). We verify the proposed estimator by computer simulation, the results of which are in agreement with the analysis, i.e., the proposed method achieves a good SNR-independent performance in a wide range of velocities and SNRs.

In this letter, signal to interference plus noise ratio (SINR) performance is analyzed for orthogonal frequency division multiplexing (OFDM) based amplify-and-forward (AF) relay systems in the presence of carrier frequency offset (CFO) for fading channels. The SINR expression is derived under the one-relay-node scenario, and is further extended to the multiple-relay-node scenario. Analytical results show that the SINR is quite sensitive to CFO and the sensitivity of the SINR to CFO is mainly determined by the gain factor and the different power of the direct link channel and relay link channel.

In this letter, Doppler spread estimation in different Doppler spectra is investigated and some efficient methods are proposed to calculate the maximum Doppler frequency from autocorrelation function easily.

In this letter, we propose a low-complexity sparse channel estimation method for orthogonal frequency division multiplexing (OFDM) systems. The proposed method uses a discrete Fourier transform (DFT)-based technique for channel estimation and a novel sorted noise space discrimination technique to estimate the channel length and tap positions. Simulation results demonstrate that the reduction in signal space improves the channel estimation performance.

The information of channel impulse response (CIR) length and noise variance play an important role in blind identification and equalization of wireless multipath channels. In orthogonal frequency division multiplexing (OFDM) systems, multipath fading channels introduce interference between adjacent symbols which can be prevented by inserting a cyclic prefix (CP) before each symbol. In this letter, we find that the interference power in the cyclic prefix (CP) interval and its variation can be used to estimate the CIR length and noise variance jointly and blindly.

We consider a time-division multiple access (TDMA) transmission scheme in MIMO broadcast channels. To cope with the fairness issue in heterogeneous networks with slow fading, an opportunistic scheduling algorithm based on the channel eigen-direction is investigated. In the system with sparse users, the mismatch between a random beamforming vector and the principle eigenvector of the channel incurs a throughput penalty. To reduce such a throughput loss, a multiple random beams selection (MRBS) scheme exploiting chordal distances is proposed. Two feedback schemes (unquantized or quantized chordal distances) are considered. The closed-form throughput expressions of the proposed schemes are derived.

The problem of joint orthogonal precoding and user scheduling in a multi-user multi-input multi-output (MU-MIMO) downlink system is considered. Based on the theoretics of subspace and vector projection, a novel orthogonal precoding matrix is designed to achieve high sum-rate capacity with low to moderate number of active users and in low SNR regions. With respect to sum-rate capacity, numerical simulations show that the proposed algorithm outperforms the zero-forcing beam-forming (ZFBF) and linear orthogonal beam-forming (OLBF).

In this letter, we study the impact of feedback error on transmit beamforming systems with finite rate feedback. The feedback channel is modeled as a uniform symmetric channel. The outage probability, bit error rate (BER), diversity gain, and array gain formulas are given. Both analytical and simulation results show that feedback error with small probability will make the system behave badly at high signal-to-noise ratios (SNR).

The application of millimeter wave (mmWave) directional transmission technology in high-speed railway (HSR) scenarios helps to achieve the goal of multiple gigabit data rates with low latency. However, due to the high mobility of trains, the traditional initial access (IA) scheme with high time consumption is difficult to guarantee the effectiveness of the beam alignment. In addition, the high path loss at the coverage edge of the millimeter wave remote radio unit (mmW-RRU) will also bring great challenges to the stability of IA performance. Fortunately, the train trajectory in HSR scenarios is periodic and regular. Moreover, the cell-free network helps to improve the system coverage performance. Based on these observations, this paper proposes an efficient IA scheme based on location and history information in cell-free networks, where the train can flexibly select a set of mmW-RRUs according to the received signal quality. We specifically analyze the collaborative IA process based on the exhaustive search and based on location and history information, derive expressions for IA success probability and delay, and perform the numerical analysis. The results show that the proposed scheme can significantly reduce the IA delay and effectively improve the stability of IA success probability.

In this letter, a joint estimation algorithm of Doppler spread and frequency offset for OFDM systems in Rayleigh fading channels is proposed based on the autocorrelation function between the last part of the received OFDM signal and its copy in guard interval. It is shown by computer simulations that the proposed algorithm performs well for different Doppler spread values and carrier frequency offsets.

Reducing the energy consumption of wireless communication systems with new technologies and solutions continues to be an important concern in developing future standards. In this paper, we study the routing strategies in multi-hop relaying networks. For a 2-way assignment routing method, an efficient feedback scheme is presented to minimize the power consumption over the whole system. Compared with the full channel information in traditional feedback scheme, only the backward accumulated feedback metrics are required. If the proposed routing calculation is used, there is no performance loss. When the number of the hops and the relays is large, the new scheme achieves a significant feedback overhead reduction. Moreover, we show a proof for the optimality of the presented routing strategy based on mathematical induction.

In this letter, we propose an FFT-based SNR estimation method for wireless OFDM systems, and analyze the impact of the proposed SNR estimation method on adaptive OFDM performance in slow Rayleigh fading channels. Numerical and simulation results show that the proposed method is effective and feasible for adaptive modulation in slow Rayleigh fading channels.