Khawla A. Alnajjar

Low complexity V-BLAST for massive MIMO

In an uplink massive multiple-input-multiple-output deployment with distributed single-antenna users and a large base-station array, we consider several combinations of receivers using linear combiners (maximum ratio combining (MRC) and zero forcing (ZF)) in conjunction with Vertical Bell Laboratories Layered Space Time (V-BLAST). We show that the performance loss of MRC relative to ZF can be removed in certain situations through the use of V-BLAST. Furthermore, we develop a low complexity ordering scheme which results in a V-BLAST scheme with MRC which has much less complexity than a single ZF linear combiner. An analysis of the signal-to-interference-and-noise-ratio at each stage of the V-BLAST approach is also given to support the findings of the proposed technique.

Low complexity V-BLAST for massive MIMO with adaptive modulation and power control

We consider the performance of a low complexity Vertical Bell Laboratories Layered Space Time system with a maximum ratio combining receiver (LC-VBLAST) in an uplink massive multiple-input-multiple-output (MIMO) deployment with single antenna users. This receiver is known to give a similar error rate performance to zero forcing (ZF) for simple systems while reducing complexity. In this paper, we show that LC-VBLAST remains similar to ZF for more complex adaptive modulation systems and in the presence of channel estimation error, LC-VBLAST can be superior. These results are analytically justified and we derive an exhaustive search algorithm for power control (PC) to bound the potential gains of PC. Using this bound, we demonstrate that LC-VBLAST performs well without the need for additional PC.

Design and analysis of a reduced complexity MRC V-BLAST receiver for massive MIMO

In this paper, we take a proposed low-complexity Vertical Bell Laboratories Layered Space Time receiver based on maximal ratio combining and further simplify the algorithm by replacing the channel norm ordering by a power based ordering. The receiver operates in an uplink massive multiple-input-multiple-output deployment with distributed single-antenna users and a large base station (BS) array. The novel receiver is compared with other more complex detection schemes such as linear zero forcing. Moreover, an explicit closed form analysis for error probability for both co-located and distributed BSs is provided. It is shown that the error performance of the distributed scenario is well approximated by a modified version of a co-located scenario. The simulation study demonstrates the performance of the proposed scheme and confirms the accuracy of analytical results.

Joint C-V-BLAST and DS-NOMA for Massive MIMO

We investigate the performance of modified non orthogonal multiple access (NOMA) which uses the modified low complexity Vertical Bell Laboratories Layered Space Time (C-V-BLAST) in an uplink massive multiple-input-multiple-output (MIMO) deployment with distributed single-antenna users and a large base-station array. Unlike previous work which assumes no spreading, we focus on the scenario where signal spreading is included by using the Gold Code family. It is shown that the proposed scheme provides a significant performance improvement over the conventional V-BLAST system for a large MIMO deployment when the number of transmit and receive antennas are comparable by exploiting the extra dimension added by the spreading to mitigate the interference. However, for a massive MIMO system, both schemes provide similar performance. We also show that the proposed scheme has a much better performance when the average received power for the users is the same, a scenario that the C-V-BLAST scheme struggles with due to its dependence on the ordering of users according to their power levels.

Performance of low complexity receivers for massive MIMO with channel estimation and correlation

In an uplink massive multiple-input-multiple-output (MIMO) deployment with distributed single-antenna users and a large base-station array, we investigate the performance of low complexity Vertical Bell Laboratories Layered Space Time (C-V-BLAST) and zero forcing (ZF) receivers. Unlike previous work which assumes that the channel is perfectly known, we focus on the realistic scenario where the MIMO channel is actually estimated. We consider two channel estimation strategies, pilot-based maximum likelihood (ML) which has a closed-form and semi-blind (SB) estimation which is performed iteratively using steepest-descent. To the best of our knowledge, the performance gains of SB estimation over ML estimation, and the resulting complexity tradeoffs have not been investigated in previous works in the context of massive MIMO with low complexity receivers. The motivation of our study is to fill this gap. Our work shows that the SB estimation strategy yields a significant improvement in symbol-error-rate (SER) performance, with ZF outperforming the C-V-BLAST. We also investigate the impact of channel correlation on system performance.

Inverse approximation of linear receivers for massive MIMO

To overcome computation complexity due to inverse calculation that occurs when using linear receivers such as zero forcing (ZF) in massive multiple-input-multiple-output (MIMO), we propose a simple approximation method which depends on the maximum element of the diagonal matrix multiplied by a scaling factor that controls its tuning. This method solves other inverse method problems when the system load increases in the presence of the channel correlation.

Size and Array Shape for Massive MIMO

With massive multi-input multi-output, it may be the case that large numbers of antennas are closely packed to fit in some available space. Here, channel correlations become important and it is of interest to investigate the space requirements of different array shapes. We focus on uniform square and linear arrays and consider a range of correlation models. We show that the benefits of two-dimensional arrays are dependent on the type of correlation. When the correlation decays slowly over small antenna separations then square arrays can be far more compact than linear arrays or they can offer substantial sum rate enhancements. When the correlation decays more quickly, then the main benefit is compactness.

Performance of massive MIMO V-BLAST with channel correlation and imperfect CSI

In an uplink massive multiple-input-multiple-output (MIMO) deployment with distributed single-antenna users and a large base-station array, we consider the performance of Vertical Bell Laboratories Layered Space Time (V-BLAST) with maximum ratio combining (MRC) and V-BLAST with zero forcing (ZF). In the performance evaluation, we include the effects of imperfect channel state information (CSI) and channel correlation since these system imperfections are of particular importance in massive MIMO. The main contribution is that the performance of MRC V-BLAST is shown to approach that of ZF V-BLAST under a range of imperfect CSI levels, different channel powers and different types of array, as long as the channel correlations are not too high.

Comments on How a New Engineering Field Develops: A Case Study from Iterative Learning and Repetitive Control

Iterative learning control and repetitive control try to converge to zero tracking error in the real world while performing a repetitive task. They use the error from the previous run or period to adjust the command in the current run or period. Because they want zero error, they push to the limit all of the analysis tools of control theory. The development of the field has required an intricate interplay between theoretical developments, computer simulations, and experiments. Reviewing this interplay, it is clear that each of these approaches in isolation repeatedly come up with wrong conclusions, or wrong directions for research. Advancement of the field required an interplay between each of the approaches. The paper describes this interplay, and suggests that this is an important case study of what is generally required for new fields of engineering to develop to have a practical impact.

Aligned precoder in Long Term Evolution using SISO and MIMO

In order to manage interference in Long Term Evolution (LTE) using single-input single-output (SISO) or multiuser multi-input multi-output systems (MIMO), we investigate interference alignment mitigation techniques for LTE systems and provide an analytic extension and new method. Although precoding generally is applied for each subcarrier, we observe that using a precoder in the output of OFDM signal will give no significant performance degradation. We validate the results by developing a MATLAB simulation model in order to evaluate the link-level performance.