Meilong Jiang

The case for re-configurable backhaul in cloud-RAN based small cell networks

Small cells have become an integral component in meeting the increased demand for cellular network capacity. Cloud radio access networks (C-RAN) have been proposed as an effective means to harness the capacity benefits of small cells at reduced capital and operational expenses. With the baseband units (BBUs) separated from the radio access units (RAUs) and moved to the cloud for centralized processing, the backhaul between BBUs and RAUs forms a key component of any C-RAN. In this work, we argue that a one-one mapping of BBUs to RAUs is highly sub-optimal, thereby calling for a functional decoupling of the BBU pool from the RAUs. Further, the backhaul architecture must be made re-configurable to allow the mapping between BBUs and RAUs to be flexible and changed dynamically so as to not just optimize RAN performance but also energy consumption in the BBU pool. Towards this end, we design and implement the first OFDMA-based C-RAN test-bed with a reconfigurable backhaul that allows 4 BBUs to connect flexibly with 4 RAUs using radio-over-fiber technology. We demonstrate the feasibility of our system over a 10 km separation between the BBU pool and RAUs. Further, real world experiments with commercial off-the-shelf WiMAX clients reveal the performance benefits of our reconfigurable backhaul in catering effectively to heterogeneous user (static and mobile clients) and traffic profiles, while also delivering energy benefits in the BBU pool.

MIMO Transmission with Rank Adaptation for Multi-Gigabit 60GHz Wireless

In this paper, we propose a practical and systematic approach to implement the MIMO transmission with rank adaptation for 60 GHz systems. In the 60 GHz system with multiple antennas, the transmit and receive (Tx-Rx) antenna arrays are grouped into a number of subarrays with a predetermined subarray separation based on the derived geometrical criteria of creating high rank MIMO in LoS environments. We first apply an enhanced blind beamforming technique based on a stochastic gradient algorithm (SGA) for the inner-subarray antennas, which does not require channel state information (CSI) at either the transmitter or the receiver. Secondly, the composite MIMO channel, as a joint effect of Tx-Rx beamforming and the channel impulse response, can be estimated with much reduced complexity. Finally, the MIMO transmission with rank adaptation is performed by adaptively selecting the better scheme out of the high-rank spatial multiplexing and the rank-1 beamforming whichever gives higher system throughput. Simulation results show that high-rank spatial multiplexing and rank-1 beamforming outperform each other at different geometrical placements and transmit power settings. The proposed MIMO transmission with rank adaptation offers significant performance gain especially at high signal-to-noise ratio (SNR) regions.

Design of high performance MIMO receivers for LTE/LTE-A uplink

In this paper we design high performance multiple-input-multiple-output (MIMO) receivers for the DFT-Spread-OFDM based long term evolution (LTE) cellular uplink. In the LTE uplink multiple single-antenna users can be scheduled on the same time-frequency resource block via space division multiple access. The designed receivers are also applicable to the LTE-Advanced cellular uplink wherein simultaneous transmission of multiple streams by a single user will be possible. Two types of advanced non-linear receivers are considered and optimized, namely, a receiver based on a two-symbol max-log soft-output demodulator (two-symbol MLD) and a turbo minimum mean squared error successive interference cancelation (turbo MMSE-SIC) receiver. Based on extensive simulations, it is shown that both the two-symbol MLD and the turbo MMSE-SIC receivers exhibit superior performance compared to the conventional linear MMSE (LMMSE) receiver. In general, the turbo MMSE-SIC receiver is robust to timing offsets and offers the best performance but also introduces larger latency and higher computational complexity. Upon employing a proposed new pairing method, the two-symbol MLD based receiver is also found to yield a good performance that is robust to timing offsets and which entails a moderate complexity and latency.

Enhanced DFT-Based Channel Estimation for LTE Uplink

Discrete Fourier transform (DFT) based channel estimation (CE) has been widely studied as a practical CE scheme over the OFDM based wireless systems. The conventional DFT-based channel estimation utilizes a transform domain cut-off filter to suppress the noise in the time domain. However, this method can suffer significant performance loss due to the channel impulse response (CIR) energy leakage, especially when the available pilot sub-carriers are confined to a small portion of the system bandwidth. In this paper, we propose an enhanced DFT-based channel estimation technique for the long term evolution (LTE) based cellular uplink. A sinc-null based noise power estimation method in conjunction with a dynamic noise removal technique is proposed to suppress the noise in the time domain and achieve better performance while keeping the complexity in check. Simulation results show that the proposed scheme not only achieves better mean square error (MSE) and block error rate (BLER) performance but also exhibits robustness to timing offsets compared to existing DFT based CE schemes.

Resource Allocation in 4G MIMO Cellular Uplink

In this paper, we consider resource allocation in the the fourth generation multi antenna (4G MIMO) cellular uplink. In particular, we consider the two 4G standards, IEEE 802.16m and 3GPP LTE-A, that have recently been approved. We show that the uplink resource allocation problems in both 802.16m and LTE-A networks are NP-hard. We then propose constant-factor polynomial-time approximation algorithms for both these problems. We also provide linear programming (LP) based upper bounds to benchmark the performance of our proposed approximation algorithms. Simulations reveal that the proposed algorithms have excellent performance, much superior to their worst-case guarantees.

A novel wireless over fiber access architecture employing moving chain cells and RoF technique for broadband wireless applications on the train environment

We have demonstrated a hybrid optical/wireless system based on moving chain cells and radio-over-fiber techniques. This scheme can improve power efficiency, cell-edge coverage, system throughput and flexibility for Internet users on the trains.

Efficient Link Adaptation for Precoded Multi-Rank Transmission and Turbo SIC Receivers

In this paper, an efficient closed-loop link adaption scheme consisting of adaptive modulation and coding as well as adaptive precoding is proposed for the 3GPP LTE-A uplink. The uplink envisaged in the LTE-A cellular network will support precoded multi-rank transmission from the users and have base stations with advanced non-linear receivers. Our proposed scheme considers one such advanced receiver, namely, the turbo successive interference cancelation (Turbo-SIC) receiver. The main difficulty in designing link adaptation schemes for these advanced non-linear receivers is that the soft-outputs of such receivers cannot be simply modeled using a scalar Gaussian channel characterized by a signal-to-interference-plus-noise ratio (SINR). To circumvent this difficulty, we employ SINRs corresponding to an ordered hard-decision SIC receiver, that can be explicitly expressed in closed form. The proposed scheme accurately predicts the turbo SIC performance and results in a perfect match between the predicted and actually simulated performance in terms of block error rate (BLER) and the spectrum efficiency. Realistic and extensive simulations reveal that it provides the highest actual spectrum efficiency compared to other competing schemes.

Improving downlink multiuser MIMO throughput in LTE-advanced cellular systems

In this paper, we consider a downlink (DL) multiuser (MU) multi-input-multi-output (MIMO) channel with linear precoding where the base station simultaneously schedules several user terminals on the same frequency sub-band. We assume imperfect (or quantized) per-user channel state information at the base station and present two types of channel state information (CSI) reports from user terminals, namely, the CSI report that assumes the single-user (SU) MIMO transmissions and the enhanced CSI feedback report that assumes the MU-MIMO transmissions, and in the latter case we consider both uniform and nonuniform power allocations. To improve the MU-MIMO system performance, we propose signal-to-interference-plus-noise ratio (SINR) approximation techniques that utilize the quantized CSI available at the base station and improve the rate matching. We also introduce user pooling techniques which enable a reduction in feedback signaling overhead via per-user feedback mode selection. The proposed techniques also allow unconstrained user pairing at the base station scheduler and hence enable dynamic switching between SU and MU MIMO transmissions. The simulation results demonstrate the efficiency of the proposed MU-MIMO enhancement techniques.

Enhancing Multiuser MIMO in Practical Cellular Systems

We consider a downlink multi-user multi-input-multi-output (MU-MIMO) fading channel wherein the base station can schedule several user terminals on the same time-frequency resource. A severe practical problem in MU-MIMO is that when computing its feedback report, a user does not have an accurate estimate of the interference it might see (if scheduled) from the signals intended for the other co-scheduled users. This results in a mismatch between the user reported signal-to-interference-plus-noise-ratio (SINR) and the one it actually observes in the aftermath of scheduling. To alleviate this problem we propose to inform each user (in a slow or semi-static manner) about the rank of the precoding matrix that it should report, along with an estimate of the total number of streams that the base station expects to co-schedule on a time-frequency resource. The suggested rank and the expected total number of streams can be user-specific and together convey the expected total number of co-scheduled interfering streams to the intended user. Each user then computes one or more SINRs for all the precoding matrices having the suggested rank and reports its preferred precoding matrix along with the corresponding SINRs. The SINRs are computed after assuming that the co-scheduled interfering streams will be transmitted along vectors isotropically distributed in the orthogonal complement of the range of the precoding matrix being examined. Alternatively, the SINRs can be computed after assuming that the co-scheduled interfering streams will be transmitted along the worst-case choice of mutually orthogonal vectors that lie in the orthogonal complement. We show that the proposed solutions, while requiring negligible additional signalling overhead, mitigate the mismatch problem to a large extent and result in significant improvements in system throughput.

Link adaptation in LTE-A uplink with Turbo SIC receivers and imperfect channel estimation

In this paper, we propose an efficient link adaptation scheme for the 3GPP Long Term Evolution Advanced (LTEA) uplink with turbo successive interference cancelation (Turbo-SIC) receiver and imperfect channel estimation. Link adaptation for non-linear MIMO receivers such as Turbo SIC is not straightforward because the soft-outputs of such receiver cannot be simply modeled using a scalar Gaussian channel and the post-processing signal-to-interference-plus-noise ratio (SINR) does not have an explicit closed-form expression. In addition, the lack of perfect channel state information due to estimation errors imposes further uncertainty and challenges in the link adaptation design. To circumvent these difficulties, we propose a prediction method that can effectively characterize the Turbo SIC performance and derive an approximate post-detection SINR for each layer by considering the channel estimation error statistic. We particularly consider the link adaptation for precoded multi-rank MIMO transmission with both inter-codeword(CW) and intra-codeword(CW) interference cancelation. Realistic and extensive simulations reveal that the proposed link adaptation scheme accurately predicts the turbo SIC performance and results in highest spectrum efficiency and robustness compared to other competing schemes.