Kushal Anand

Beamformer Design for the MIMO Interference Channels under Limited Channel Feedback

In this paper, beamforming design is considered for a K-user multiple-input-multiple-output interference channel (MIMO-IC) under limited channel feedback budget at the users. Using a geometrical framework, an analytical expression for the upper bound on the total rate loss due to limited channel feedback is derived. The rate loss is a function of the total feedback budget available to the receiver, the strength of the desired and the interfering links at the users and the network parameters. This rate loss upper bound metric is then minimized at each receiver locally by allocating the feedback bits optimally among the links observed by it. We obtain an interesting result that under limited channel feedback budget at the users, interference alignment (IA) may not be the appropriate beamforming strategy. Based on the feedback bit allocation policy to minimize the rate loss upper bound at the receiver and the corresponding channel quantization error statistics at the transmitter, an improved beamformer design is proposed. The proposed beamformer shows a higher throughput gain compared to all other existing schemes.

Sub-stream fairness and numerical correctness in MIMO interference channels

Signal-to-interference plus noise ratio (SINR) and rate fairness in a system are substantial quality-of-service (QoS) metrics. The acclaimed SINR maximization (max-SINR) algorithm does not achieve fairness between users streams, i.e., sub-stream fairness is not achieved. To this end, we propose a distributed power control algorithm to render sub-stream fairness in the system. Sub-stream fairness is a less restrictive design metric than stream fairness (i.e., fairness between all streams) thus sum-rate degradation is milder. Algorithmic parameters can significantly differentiate the results of numerical algorithms. A complete picture for comparison of algorithms can only be depicted by varying these parameters. For example, a predetermined iteration number or a negligible increment in the sum-rate can be the stopping criterion of an algorithm. While the distributed interference alignment (DIA) can reasonably achieve sub-stream fairness for the later, the imbalance between sub-streams increases as the preset iteration number decreases. Thus comparison of max-SINR and DIA with a low preset iteration number can only depict a part of the picture. We analyze such important parameters and their effects on SINR and rate metrics to exhibit numerical correctness in executing the benchmarks. Finally, we propose group filtering schemes that jointly design the streams of a user in contrast to max-SINR scheme that designs each stream of a user separately.

Precoder Designs for the Relay-Aided X Channel Without Source CSI

In this paper, precoder designs are proposed for the wireless X networks aided by a relay and with no or very limited channel state information at the transmitters (CSIT). First, we derive the general feasibility conditions for interference alignment (IA) for a relay-aided symmetric multi-input-multi-output X channel (also called the X relay channel or XRC) without any CSI at the source transmitters. Next, we propose an IA-based joint beamforming approach with very limited feedback to the transmitters (referred to as “IA-LFB” in this paper), which can attain the maximum degrees of freedom (DoF) of the network under arbitrarily low relay power. Finally, we propose an iterative minimum-sum mean-square-error (MMSE) precoder design for the single-input-single-output XRC for throughput improvement in the finite signal-to-noise ratio (SNR) region. The proposed MMSE precoder also shows better bit error rate results than the IA precoder over a wide SNR range. Since the MMSE precoder can achieve only locally optimal solutions, we propose to initialize the iterative MMSE algorithm with the IA-LFB precoder which, unlike random initializations, not only improves the throughput in the finite SNR regions but also preserves the network DoF even under limited relay power.

Balancing Weighted Substreams in MIMO Interference Channels

Substreams refer to the streams of each user in a system. Substream weighting, where the weights determine the prioritization order, can be important in multiple-input multiple-output interference channels. In this letter, a distributed algorithm is proposed for the problem of power minimization subject to weighted SINR constraint. The algorithm is based on two basic features, the well-known distributed power control algorithm by Yates in 1995 and a simple linear search to find feasible SINR targets. The power control law used in the proposed algorithm is proven to linearly converge to a unique fixed point.

Degrees of Freedom of Three-User SISO-X Networks With Hybrid CSI at the Transmitters

In this letter, we consider the degrees of freedom (DoF) of the three-user memoryless X networks with independent and identically distributed (i.i.d.) fading across antennas and channel uses and under hybrid channel state information (CSI) at the transmitters (CSIT). Hybrid CSI implies that some part of the CSI available to the transmitters may be delayed while some part may be perfect. We show that with the help of some local instantaneous CSIT, in addition to the delayed CSIT, we can obtain a DoF greater than that achievable using existing schemes based on completely delayed CSIT. In our first result, we show that with local instantaneous CSIT available to every transmitter through its own receiver only, we can achieve a DoF of 4/3. In the second result, we show that with the help of local CSIT available to the transmitter only through its local links to all the receivers, a higher DoF of 3/2 can be obtained at the expense of some more feedback compared to the first result.

Finite-SNR Precoder Designs for the Interference Relay Channel Without CSI at the transmitters

Recently, a transmission strategy was designed for the K-user interference channel (IC) without CSI at the transmitters (CSIT) but aided by a relay which knows the global CSI, where it was shown that K - 1 relay antennas are required to attain the maximum network DoF. In this paper, we derive general IA feasibility conditions for a single relay-aided IC with CSI at both the relay and the transmitters. Our result shows that except for the special case of K = 3 users, providing CSI to the transmitters still requires K - 1 relay antennas to achieve the maximum network DoF. Next, we design the minimum-sum-mean-squareerror (MSMSE) and weighted-sum-rate (WSR) maximizing precoders for the relay-aided IC network without CSIT which show higher sum-rate in the finite-SNR region compared to the IA precoding. Further, a so-called scaled-IA initialization is proposed for the MSMSE and the WSR algorithms which not only preserves the network DoF but also saves the power at the relay considerably compared to the MSMSE and the WSR schemes with random initializations. Extensive simulation results in both symmetric and asymmetric networks show that the MSMSE precoder shows better BER performance than both the IA and the WSR algorithms for low-to-moderate data rates.

Precoder design for the interference relay channel with blind transmitters

In this paper, for the K-user interference relay channel (IRC) with transmitters and relays having no and global channel state information (CSI), respectively, a minimum sum mean squared error (MSMSE) based relay precoder is proposed. It is shown that MSMSE outperforms interference alignment (IA) relay precoder in sum rate at low to mid signal-to-noise ratio (SNR) regions as well as in bit error rate (BER) at all SNR regions for small and moderate sizes of Gray encoded constellations.

Lens MIMO based millimeter wave broadcast channel

We consider the beamforming design for the millimeter wave (mmWave) broadcast channel using lens array antenna based multiple-input-multiple-output (MIMO) communication system (also referred to as “lens MIMO” in this work). Recently, lens MIMO based communication was proposed as a promising scheme for the single-user mmWave network to greatly reduce the computational and signal processing complexity of the system. In this paper, we propose a simple beamforming scheme for lens MIMO in broadcast channels which performs as good as the recently proposed hybrid beamforming (HBF), but with much reduced hardware and power consumption cost, thanks to the energy focusing property of lens array.

Semiblind Interference Alignment: A New Framework

In this letter, we provide a new framework for semiblind interference alignment (SBIA) in multi-input-single-output broadcast channels (MISO-BC) where the transmitter receives channel feedback from only one receiver (referred to as supernode). The supernode is called so because it has reconfigurable antennas as well as a feedback mechanism to the transmitters. All other receivers neither have reconfigurable antennas nor provide any kind of feedback to the transmitter. We provide a linear precoding scheme using interference alignment (IA) for this new framework for some classes of MISO-BC, and show that the degrees of freedom (DoF) gain is higher than the traditional orthogonal transmission schemes such as time-division-multiple-access (TDMA). Our proposed SBIA can also be applied to the two-user X-channel for which it is DoF-optimal.

A blind and partial interference alignment scheme based on pulse shaping for 3-user SISO interference channel

In this paper, we propose a transmit signal pulse shaping approach to improve the data rates for the 3-user single-input-single-output interference channel (SISO-IC). The proposed scheme makes use of partial interference alignment based on suitable pulse shape design and no channel state information (CSI) at the transmitters, i.e., the transmitters are blind. While a degrees of freedom (DoF) improvement over the conventional time-division-multiple-access (TDMA) scheme cannot be claimed, the proposed scheme still provides a 25% data rate improvement with quite reasonable BER. In this paper, we show the usefulness of the proposed scheme in practical situations where strict zero error rates are not required.