Jianhua Zhang

Throughput Analysis for a Multi-User, Multi-Channel ALOHA Cognitive Radio System

In this paper, we investigate a novel slotted ALOHA-based distributed access cognitive network in which a secondary user (SU) selects a random subset of channels for sensing, detects an idle (unused by licensed users) subset therein, and transmits in any one of those detected idle channels. First, we derive a range for the number of channels to be sensed per SU access. Then, the analytical average system throughput is attained for cases where the number of idle channels is a random variable. Based on that, a relationship between the average system throughput and the number of sensing channels is attained. Subsequently, a joint optimization problem is formulated in order to maximize average system throughput. The analytical results are validated by substantial simulations.

Small Scale Fading Characteristics of Wideband Radio Channel in the U-Shape Cutting of High-Speed Railway

High-speed railway (HSR), as an important deployment scenario for both the present and the future mobile wideband radio communication systems, has attracted more and more attention all over the world with the rapid increasing demand of the high data rate communication service on traveling. For the purpose of capturing the wideband channel characteristics of HSR, a channel measurement campaign was conducted at the center frequency of 2.35 GHz with 50 MHz bandwith in the U-shape cutting scenario of Zhengzhou--Xian HSR line in China. Based on the field measured data, we analyze the small scale characteristics in detail, which maily include path number, root mean square delay spread (rms DS), and doppler shift. It is found that the distribution of the path number is well fitted by a Gamma distribution. The statistics of rms DS in the U- shape cutting scenario are larger than the results in other scenario of HSR. In addition, an increasing tendency of rms DS against the transmitter-to- receiver distance is observed and can be modeled by a linear function. Finally, the doppler frequency shift is verified and meets the theoretical value.

Timing and Frequency Synchronization for Cooperative Relay Networks

This paper deals with timing and frequency synchronization in multi-relay cooperative networks operating with both large and small carrier frequency offset (CFO) over frequency-selective channels. A novel preamble based on constant amplitude zero auto-correlation (CAZAC) sequence is proposed, and a corresponding practical multistage scheme is presented. Joint timing and integral frequency synchronization is involved to resist multi-relay interference (MRI). Then, fractional frequency estimation is carried out, and fine timing estimation completes the synchronization scheme. The performance is evaluated in terms of the mean square error (MSE). Simulation results show that the method is robust under both flat-fading and multipath fading channels, and provides accurate estimation results in the presence of both large and small multiple CFO values.

Elevation Angle Characteristics of Urban Wireless Propagation Environment at 3.5 GHz

It is known that channel model of propagation characteristics are crucial in the research and evaluation of three-dimensional multiple-input multiple-output (3D-MIMO) technique, especially the elevation angle (EA) model. In this paper, the results of 3D-MIMO channel measurement in the urban macro-cell (UMA) scenario at 3.5 GHz are presented. Based on the measurement data, it is find that the mean value of elevation angle has a offset angle to line-of-sight (LOS) angle especially in not-line-of- sight (NLOS) situation. And the offset angle is depend on the azimuth distance between BS and MS which can be modeled as a function that use distance as a parameter. A novel way to model mean angle of elevation angle is proposed at the end of this paper. The relation between Circular angular spread (CAS) and distance is also studied, the measurement result shows that the effects caused by distance can be neglected.

Advanced International Communications

This article addresses the emerging challenges for the extensive application of IMT-Advanced MIMO channel model to the upcoming technologies, e.g., relay and CoMP. To verify the observation and analysis, some field measurements for MIMO channel have been carried out in down town Beijing for the UMi scenario. From the measured data, PL, delay, and AS model for relay are compared with those in ITU-R M.2135 and IEEE Standard 802.16j. The RS-UE PL of the proposed model is larger than those of ITU-R M.2135 and IEEE Standard 802.16j because RS is deployed differ ently from eNB. The RS-UE statistical parameters of time and angular domains also present different characteristics obtained from the measured data. Thus, the IMT-Advanced channel model in the UMi scenario cannot be directly applied to the links of RS-UE and eNB-RS. Moreover, GAS is defined to evaluate the distribution of the channel gain in the angular domain.

Multipair Two-Way Relay Networks with Very Large Antenna Arrays

We consider a multipair two-way relay network where multiple communication pairs simultaneously exchange information with the help of a single relay. Each terminal has only a single antenna, while the relay is equipped with a very large antenna array. We further assume that channel state information is available at the relay node. We investigate the power efficiency of this network when very simple signal processing, i.e., maximum ratio combining (MRC) or zero-forcing (ZF), is used at the relay. When the number of relay antennas grows infinite, the transmit power of each terminal or relay (or both) can be made inversely proportional to the number of relay antennas while maintaining a given quality-of-service. We show that with very large antenna arrays, the two-way relaying scheme outperforms both the orthogonal scheme and the one-way relaying scheme.

Energy-Efficient Resource Optimization for Relay-Aided Uplink OFDMA Systems

In this paper, the energy-efficient resource allocation in relay-aided uplink multiuser system is studied. We adopt the orthogonal frequency division multiplexing (OFDM) as the physical layer modulation technique. Assuming that the BS has all the channel state information (CSI), an energy-efficiency optimization problem through joint subcarrier assignment, bit and power allocation is formulated. Due to the computational complexity restriction, a suboptimal solution based on decomposition is presented to solve the problem with low complexity. Considering the fairness constraint, a joint resource allocation algorithm is further proposed, which is proved to achieve a considerable improvement in terms of energy-saving and simultaneously decreases rate outage probability by simulation results.

Cluster Characteristics of Wideband 3D MIMO Channels in Outdoor-to-Indoor Scenario at 3.5 GHz

In this paper, we present a cluster parameter based analysis of an outdoor-to-indoor (O2I) Multiple-Input-Multiple-Output (MIMO) measurement campaign carried out at 3.5 GHz in an office building. The measurement was performed using a wideband multi-antenna radio channel sounder. Besides azimuth angle of arrival (AoA), azimuth angle of departure (AoD) and delay domains included in traditional 2 dimensional (2D) MIMO channels, 3 dimensional (3D) MIMO channels also consider elevation angle of arrival (EoA), elevation angle of departure (EoD) when describing clusters. By introducing the elevation domain, we observe that multipath components (MPCs) within one cluster in 2D MIMO channels can be divided into several clusters in 3D MIMO channels and each cluster has MPCs with smaller power weighed multipath component distance (MCD). We also analyze the effect of height difference between transmitting and receiving ends on the clustering performance. There is a tendency that clusters show smaller parameter values when the height difference is smaller. After that, we describe all the cluster parameters by a set of probability density functions (pdfs) and give typical values in this scenario.

Pilot Aided 3D Channel Estimation for MIMO-OFDM Systems

Considering three dimensional (3D) channel with elevation effect, this paper addresses pilot spacing design in frequency, time and spatial domains for orthogonal frequency division multiplexing (OFDM) systems with multiple antennas. Employing the channel spatial correlation between transmit antennas, pilot overhead can be reduced substantially without estimation accuracy loss. On the other hand, the accuracy of pilot aided channel estimation (PACE) can be improved. This paper also verifies the performance of cascaded lower-dimensional Wiener filters in mean square error (MSE) sense. Numerical results show that the channel estimation in frequency-space domain is undecomposable.

Downlink Performance of Indoor Distributed Antenna Systems Based on Wideband MIMO Measurement at 5.25 GHz

Distributed antenna system (DAS) is a very promising technology to enhance the capacity of indoor communication system due to the inherent macro and micro diversity. With its natural characteristics of reducing the access distance and increasing the probability of line-of-sight (LoS) coverage, DAS can greatly enhance the power efficiency, which is in accordance with the goal of green communication. In this paper, the downlink performance of an indoor DAS is investigated based on a realistic wideband multiple input multiple output (MIMO) channel model constructed with measured data at 5.25 GHz. Different transmission schemes and antenna configurations are compared from the capacity point of view. It is demonstrated that with the same total transmit power DAS with selection transmission (ST) can increase the capacity significantly compared with centralized antenna system (CAS) and DAS with equal-power transmission. The results also show that MIMO technique can improve the system performance significantly even under LoS propagation conditions for indoor scenarios, which is counterintuitive to the common sense that MIMO will suffer from great performance degradation under LoS conditions.