Multi-antenna techniques capable of exploiting the elevation dimension are anticipated to be an important air-interface enhancement targeted to handle the expected growth in mobile traffic. In order to enable the development and evaluation of such multi-antenna techniques, the 3rd Generation Partnership Project (3GPP) has recently developed a three-dimensional (3D) channel model. The existing two-dimensional (2D) channel models do not capture the elevation channel characteristics, making them insufficient for such studies. This article describes the main components of the newly developed 3D channel model and the motivations behind introducing them. One key factor is the ability to model channels for users located on different floors of a building (at different heights). This is achieved by capturing a user height dependency in modelling some channel characteristics including pathloss, lineof- sight (LOS) probability, etc. In general, this 3D channel model follows the framework of WINNERII/WINNER+ while also extending the applicability and the accuracy of the model by introducing some height dependent and distance dependent elevation related parameters.

This paper reports the methodology and results from our latest field measurement campaign for characterizing the 3-dimensional (3D) MIMO channel, particularly in the elevation domains, in a typical urban macro (UMa) environments in Beijing, China. Stochastic channel model parameters are obtained based on the high-resolution multi-path parameter estimates. In addition, a distance dependent elevation angular spread model is proposed based on field observations. These works enables the realistic evaluation of 3D MIMO system performance and extends the applicability of the ITU SCM models in 3D MIMO system study.

This paper demonstrates a simulation methodology based on a reasonable 3D channel model. System level simulations on various 3D antenna port configurations are performed, and results are compared with those of 2D MIMO system. Several interesting observations are made from the evaluation results: (1) the vertically distributed active antenna elements provide less MIMO gain than the horizontally distributed active antenna elements; (2) with the same number of antenna elements, the 3D active antenna configuration can provide better performance than the 2D one, and the performance improvement is much larger for MU-MIMO than SU-MIMO; (3) even when the 2D antenna configuration has more antenna elements (providing more antenna array gain) than the 3D active antenna configuration, the 3D configuration can still outperform the 2D one if there are enough transmitter ports.

virtual basestation (vBS) live migration is an enabling technology for reducing the power consumption of the baseband unit (BBU) pool of the cloud radio access network (Cloud RAN). Our previous work shows that using the existing virtual machine live migration solutions for vBS migration will introduce a service interruption time of several seconds, which is unacceptable for real-time services carried by basestations. In this paper we propose and develop a seamless and lossless mechanism for vBS live migration. The experiment results show that the proposed solution for vBS live migration introduces a service interruption time of tens of milliseconds and ensures no loss of user data during vBS migration, which significantly outperforms the commercial virtual machine live migration solutions.

A new time-domain transmit beamforming algorithm is proposed for cancelling inter-channel-interference (ICI) due to Doppler frequency shift under high speed train communication scenario. Simulation results show that by employing the algorithm a high speed train communication system is capable of providing continuous 100Mbps data rate for passengers at a speed of 450km/h. This would guarantee continuous data-intensive services for today's high speed train passengers.

We propose a novel high-speed train communication using baseband cloud (C-HSTC) system framework for providing continuous broadband services to highly mobile users. This framework is featured with a new virtualized single cell design which mitigates the impact of conventional handover failures and guarantees continuous communication services. Through exploiting the baseband units (BBU) cloud and the ful frequent frequency reuse in the virtualized single cell, we also proposed a highly efficient joint transmit beamforming algorithm targeting at compensating the inter-carrier interference (ICI) caused by severe Doppler frequency shift due to mobility. Numerical analysis shows that the new architecture and corresponding algorithms are suitable for high-speed train communication and can provide a continuous data rate of more than 100 megabits per second (Mbps) for passengers at a speed of 450 kilometers per hour (kmph). This would help to achieve satisfactory mobile broadband services for high speed train passengers.

Channel reciprocity is regarded as one of the most important advantages of TDD systems. However, the RF front-ends contribute to the channel experienced, and their TX/RX mismatches might cause reciprocity error, and thus degrade system throughput. Therefore, channel reciprocity calibration is a crucial issue. Among the existing calibration methods, Over-the-air (OTA) calibration is a promising approach because no extra hardware cost is needed. Since the OTA calibration precision highly depends on the link quality between Base Station (BS) and the calibration User Equipment (UE), how to select the most proper UE for calibration is an important issue. In this paper, efficient calibration UE selection schemes are proposed to enhance OTA calibration for both single-cell and Coordinated Multi-Point Transmission (CoMP) scenarios. System level simulation results demonstrate that the proposed scheme can well improve the system throughput. For single-cell scenario, with the proposed scheme, the system average throughput almost achieves the ideal reciprocal case. For CoMP scenario, although system throughput is more sensitive to reciprocity error, the proposed scheme can still improve the average throughput around 13%.

Channel reciprocity is regarded as one of the most important advantages of TDD systems. However, the uplink (UL) and downlink (DL) channels are not actually reciprocal because of different characteristics of transmitters (TXs) and receivers (RXs). When the antenna elements are placed with large distance, the Radio Frequency (RF) gain mismatch of TXs and RXs is the dominant un-reciprocal factor, but if the antenna elements are placed close, such as compact antenna array, mutual coupling (MC) should be considered. In general, the MC effects for a transmitting and receiving array are different, even if the physical geometry of the array remains unchanged. Conventional reciprocity calibration methods mainly target at compensating RF gain mismatch and regard reciprocity error as diagonal matrix. However, the reciprocity error is no longer diagonal for compact antenna array due to the un-reciprocal MC of TXs and RXs, and thus the conventional calibration methods do not work in this scenario. In this paper, a comprehensive reciprocity error model is proposed firstly, and a combined MMSE and dot-division calibration scheme is then presented for calibrating MC and RF mismatch at the same time. Link level simulation results demonstrate the efficiency of the proposed scheme. The calibration method can well calibrate the UL and DL channel reciprocity, and the performance can be boosted up to around 1dB~2dB less than the ideal reciprocal case.

Retransmission (ReTX) mechanisms, i.e. Automatic Retransmission reQuest (ARQ) protocols are widely used for error control in mobile communications. Using ARQ, corrupted data frames are retransmitted in several attempts for eventual error recovery at the receiver. Error correction coding and soft combining have also been used in Hybrid ARQ (HARQ) protocols to increase efficiency. However, the evolution of ARQ protocols to date has been limited to the paradigm of ONE ReTX attempt for (the recovery of) ONE data frame, which often leads to excessive redundancies. In this paper we propose a new N-in-1 ReTX scheme. Using Random Network Coding, the scheme further improves the efficiency of ReTX by transforming a data frame into a series of equally-useful blocks for error recovery and sharing every ReTX attempt among multiple frames. Results from extensive simulations show that the proposed scheme achieves a significant throughput gain of up to 106% against the conventional HARQ-CC protocol.

Cross-entropy (CE) in information theory is introduced as a method to analyse iterative decoding. The maximum a posteriori probability decoding algorithm is shown to minimise the CE between the a priori and the extrinsic information under given code constraints. The error-correcting ability of the constituent decoders is evaluated in terms of CE. Analysis on turbo decoding is carried out based on theoretical findings from several aspects, including analysis of convergence rate, derivation of the E-b/N-0 threshold for convergence, evaluation of error performance in the 'error-floor' region and a design example of asymmetric turbo codes. Compared with conventional methods, the new technique provides stricter prediction on the E-b/N-0 threshold for convergence and quicker error performance evaluation. An asymmetric turbo code designed with the guidance of our new method exhibits more than 0.1 dB of gain over that guided by classical bounding techniques in both high and low bit error rate regions. Particularly, since no information on the source is required and the density of the a priori/extrinsic information can be arbitrary, the new technique is valuable for both offline design and online evaluation in practical systems.

The use of cross-entropy to analyse the convergence behaviour of a turbo decoder is proposed. Based on the new method, E-b/N-0 thresholds are predicted and compared with those predicted by existing techniques. Simulation results show that the new technique is effective and advantageous in practical applications.

We propose several bandwidth efficient HARQ schemes based on multilevel coded modulation in this paper. These schemes combine the bandwidth efficiency of coded modulation with power efficiency of forward error control to provide a high efficient transmission. Simulations are carried out over the wireless channels. Both throughput and reliability are found to be improved by employing the HARQ schemes based on the multilevel coded modulation instead of the conventional trellis coded modulation. Simulation and performance analysis on code combining in multilevel HARQ is also presented. Code combining in the different multilevel HARQ schemes is found to influence the multistage decoding differently. Suggestion on scheme choosing for various requirements is presented at the end.