Boon Loong Ng

Distributed Downlink Beamforming With Cooperative Base Stations

In this paper, we consider multicell processing on the downlink of a cellular network to accomplish ldquomacrodiversityrdquo transmit beamforming. The particular downlink beamformer structure we consider allows a recasting of the downlink beamforming problem as a virtual linear mean square error (LMMSE) estimation problem. We exploit the structure of the channel and develop distributed beamforming algorithms using local message passing between neighboring base stations. For 1-D networks, we use the Kalman smoothing framework to obtain a forward-backward beamforming algorithm. We also propose a limited extent version of this algorithm that shows that the delay need not grow with the size of the network in practice. For 2-D cellular networks, we remodel the network as a factor graph and present a distributed beamforming algorithm based on the sum-product algorithm. Despite the presence of loops in the factor graph, the algorithm produces optimal results if convergence occurs.

Full dimension mimo (FD-MIMO): the next evolution of MIMO in LTE systems

Full dimension MIMO, or FD-MIMO, has attracted significant attention in the wireless industry and academia in the past few years as a candidate technology for the next generation evolution toward beyond fourth generation and 5G cellular systems. FD-MIMO utilizes a large number of antennas placed in a 2D antenna array panel for realizing spatially separated transmission links to a large number of mobile stations. The arrangement of these antennas on a 2D panel allows the extension of spatial separation to the elevation domain as well as the traditional azimuth domain. This article discusses features and performance benefits of FD-MIMO along with the ongoing standardization efforts in 3GPP to incorporate FD-MIMO features into the next evolution of LTE. Furthermore, a design of a 2D antenna array, which plays a key role in the implementation of FD-MIMO, is also discussed. Finally, in order to demonstrate performance benefits of FD-MIMO, system-level evaluation results are provided.

Transmit beamforming with cooperative base stations

We consider a cellular network where base stations can cooperate to determine the signals to be transmitted on the downlink. In such a scenario, it would be possible to use macroscopic transmit beamforming to improve system performance. The downlink beamformer of interest is generalised from some transmit beamformers that have been shown to meet various optimality criteria in the literature. The particular downlink beamformer structure enables us to recast our downlink beamforming problem as a virtual LMMSE estimation problem. Based on this virtual set up, we exploit the structure of the channel and develop distributed beamforming algorithms using local message passing between neighbouring base stations. Two algorithms are outlined, both of which are based on the Kalman smoothing framework. The first algorithm is a forward-backward algorithm that produces optimal performance, but it has the disadvantage of a delay that grows linearly with array size. The second algorithm, which is a limited extent algorithm, solves the delay problem by using only local information

Distributed linear multiuser detection in cellular networks based on Kalman smoothing

We consider the problem of multiuser detection in cellular networks. In particular, we present a distributed forward-backward algorithm with local message passing for efficient implementation of the linear minimum mean square error (LMMSE) receiver, for a simple model of a 1D cellular system. The distributed algorithm is based on the well-known interpretation of Kalman smoothing as a linear combination of the forward and backward filtered estimates. We also show that near-optimal performance can be achieved by only relying on information from a local linear segment of the entire array. This results in a limited extent distributed algorithm that greatly reduces processing delay, especially for large networks, yet with little loss in performance.

Distributed Downlink Beamforming in Cellular Networks

We consider a cellular network where base stations can cooperate to determine the signals to be transmitted on the downlink. Using a particular downlink beamformer structure, we recast our downlink beamforming problem as a virtual linear minimum mean square error (LMMSE) estimation problem. Based on this virtual set up, we remodel the network as a factor graph with loops and present a simple distributed cooperative scheme for base stations based on the sum-product algorithm. We study the condition for convergence for the distributed algorithm and demonstrate its performance via simulations.

Information capacity of wyner's cellular network with LMMSE receivers

We derive the Linear Minimum Mean Square Error (LMMSE) receiver for Wyners model of a linear cellular array. By exploiting the special structure of the channel model, we obtain explicit Mean Square Error (MSE) expressions and information capacity formulae for both finite-sized as well as infinite-sized linear cellular arrays. We show that the performance of an individual user becomes insensitive to the array size for large array sizes and that the proportion of users achieving almost the same performance converges to one, as the array size tends to infinity. The effects of intercell interference and noise on the performance of each user are also investigated.

On the Capacity of Cellular Networks with Global LMMSE Receiver

Frequency planning is a common intercell interference (ICI) management strategy in narrowband cellular networks. In this paper, we consider an alternative approach that allows full frequency reuse in every cell and deploys a network-wide linear minimum mean square error (LMMSE) receiver as the front-end processor to suppress ICI. Assuming equal transmit power for all users, we compare the achievable rate of the LMMSE receiver in the information-theoretic sense against the rates achieved by two different frequency reuse schemes, namely the conventional reuse scheme with single-cell processing and a reuse scheme that allows interference-free processing of signals from adjacent cells. We first compare the performance under a fixed path-gain model and then extend to a random fading model.

A Vector Quantization Based Compression Algorithm for CPRI Link

The future wireless networks, such as Centralized Radio Access Network (C-RAN), will need to deliver data rate about 100 times to 1000 times the current 4G technology. For C-RAN based network architecture, there is a pressing need for tremendous enhancement of the effective data rate of the Common Public Radio Interface (CPRI). Compression of CPRI data is one of the potential enhancements. We introduce a vector quantization based compression algorithm for CPRI links, utilizing Lloyd algorithm. Methods to vectorize the I/Q samples and enhanced initialization of Lloyd algorithm for codebook training are investigated for improved performance. Multi-stage vector quantization is considered to reduce codebook search complexity. Simulation results show that our solution can achieve compression of 4 times for uplink and 4.5 times for downlink, within 2% Error Vector Magnitude (EVM) distortion. Remarkably, vector quantization codebook proves to be quite robust against data modulation mismatch, fading, signal-to-noise (SNR) and Doppler spread.

Unified access in licensed and unlicensed bands in LTE-A Pro and 5G

Spectrum scarcity has driven enhancements of Long-Term Evolution (LTE) in utilizing unlicensed bands in conjunction with licensed bands for delivering mobile data, resulting in the introduction of LTE unlicensed technologies such as Rel-13 LTE–Licensed-Assisted Access (LAA), Rel-14 LTE–Enhanced Licensed-Assisted Access (eLAA), and LTE-Unlicensed (LTE-U). The next-generation radio access technology, 5G New Radio(NR), faces greater technical challenge due to the need to support frequency bands covering various spectrum licensing regimes and a wide range of frequencies (up to 100xa0GHz) with very different signal propagation characteristics. This paper presents an overview of LAA and eLAA technical features and 5G NR design considerations to achieve a unified access in licensed and unlicensed bands.