Afshin Haghighat

On the Number of RF Chains and Phase Shifters, and Scheduling Design With Hybrid Analog–Digital Beamforming

This paper considers hybrid beamforming (HB) for downlink multiuser massive multiple-input multiple-output (MIMO) systems with frequency selective channels. The proposed HB design employs sets of digitally controlled phase (fixed phase) paired phase shifters (PSs) and switches. For this system, first we determine the required number of radio frequency (RF) chains and PSs such that the proposed HB achieves the same performance as that of the digital beamforming (DB) which utilizes N (number of transmitter antennas) RF chains. We show that the performance of the DB can be achieved with our HB just by utilizing rt RF chains and 2rt(N-rt + 1) PSs, where rt ≤ N is the rank of the combined digital precoder matrices of all subcarriers. Second, we provide a simple and novel approach to reduce the number of PSs with only a negligible performance degradation. Numerical results reveal that only 20-40 PSs per RF chain are sufficient for practically relevant parameter settings. Finally, for the scenario where the deployed number of RF chains (Na) is less than rt, we propose a simple user scheduling algorithm to select the best set of users in each subcarrier. Simulation results validate theoretical expressions, and demonstrate the superiority of the proposed HB design over the existing HB designs in both flat fading and frequency selective channels.This paper considers hybrid beamforming (HB) for downlink multiuser massive MIMO systems with frequency selective channels. For this system, first we quantify the required number of radio frequency (RF) chains and phase shifters (PSs) such that the proposed HB achieves the same performance as that of the digital beamforming (DB) which utilizes N (number of transmitter antennas) RF chains. We show that the performance of the DB can be achieved with our HB just by utilizing rt RF chains and 2rt(N − rt + 1) PSs, where rt ≤ N is the rank of the combined digital precoder matrices of all sub-carriers. Second, we provide a simple and novel approach to reduce the number of PSs with only a negligible performance degradation. Numerical results reveal that only 20− 40 PSs per RF chain are sufficient for practically relevant parameter settings. Finally, for the scenario where the deployed number of RF chains (Na) is less than rt, we propose a simple user scheduling algorithm to select the best set of users in each sub-carrier. Simulation results validate theoretical expressions, and demonstrate the superiority of the proposed HB design over the existing HB designs in both flat fading and frequency selective channels.

Haar Compression for Efficient CQI Feedback Signaling in 3GPP LTE Systems

Frequency selective scheduling is an attractive feature in the 3GPP LTE system that allows optimum usage of the allocated spectrum. In order to support frequency selective scheduling in the downlink, the mobile user needs to feedback channel quality indication (CQI) of the downlink channel to the base station. Several CQI feedback scheme have been proposed for 3GPP LTE systems. We propose to apply Haar compression to distributed subband groups to reduce the CQI feedback overhead. The simulation results indicate that the distributed- Haar scheme achieves the best trade-off between the throughput performance and overhead reduction compared with other CQI feedback schemes. We also observe that the sensitivity in sector throughput performance to user mobility is approximately the same for all feedback methods considered in the paper.

User scheduling for massive MIMO OFDMA systems with hybrid analog-digital beamforming

This paper proposes a new user scheduling and sub-carrier allocation algorithm for multiuser downlink massive multiple input multiple output (MIMO) orthogonal frequency division multiple access (OFDMA) systems with hybrid analogdigital beamforming (HB). We assume that the transmitter having N antennas is serving Ki decentralized single antenna receivers by sub-carrier i. The scheduling algorithm leverages the solutions of the digital beamforming (DB) result and is designed to maximize the total sum rate of all sub-carriers under per carrier power constraint. For this system and problem setup, the proposed algorithm is explained as follows: First, we express the HB matrix of sub-carrier i as a product of ABi, where the high dimensional matrix A ∈ CN×Na is common to all subcarriers whereas, Bi ∈ CNa×Ki is a low dimensional matrix which is designed for sub-carrier i, and Na is the number of RF chains satisfying N ≥ Na ≥ Ki. Second, we compute A as the first Na eigenvectors of the left singular value decomposition of the combined DB precoder matrices of sub-carriers having the highest sum rate. Finally, for fixed A, we compute Bi and its corresponding users such that the total sum rate of all sub-carriers is maximized. The performance of the proposed scheduling is studied analytically. Furthermore, the superiority of the proposed algorithm over that of the existing one is quantified analytically and demonstrated by computer simulations.

Full-Band CQI Feedback by Haar Compression in OFDMA Systems

Implementation of an efficient CQI feedback mechanism is the focus of the study presented in this paper. OFDMA-based systems such as 3GPP Long Term Evolution (LTE) downlink and WiMax require an accurate CQI feedback to allow effective operation of the adaptive modulation and coding. Besides accuracy, such mechanism should impose only a low overhead when using uplink resources for conveying the CQI information. Unlike the Best-M CQI feedback algorithms where only the M highest CQIs and their locations are reported, a full-band feedback method compresses the whole CQI vector and transmits the compressed vector in the uplink channel. In this paper, application of the Haar transform for a full-band CQI report is investigated and its performance is compared against Discrete Cosine Transform (DCT)-based compression schemes. Full-band Haar CQI feedback offers a flexible process for CQI feedback that can be easily adapted to different operating scenarios. Key features of Haar compression are incremental update and very low complexity for compression and decompression. Simulation results indicate that the full-band Haar scheme achieves significantly higher throughput than the DCT-based schemes at low speeds and about the same performance at higher speeds.

ICA-Based Signal Equalization for Digital Receivers

A technique using independent component analysis (ICA) for blind receiver signal processing is investigated. The problem of the receiver signal processing is viewed as of signal equalization and implementation imperfections compensation. Based on this, a model similar to a general ICA problem is developed for the received signal. Then, the use of ICA technique for blind signal equalization in the time domain is presented. The equalization is regarded as a signal separation problem, since the desired signal is separated from interference terms. A common problem using ICA-based techniques is to find a satisfactory number of the mixing signals to populate the mixing matrix. This problem is addressed in the paper by over-sampling of the received signal. By using ICA for equalization, besides channel equalization, other transmission imperfections such as DC bias offset, carrier phase and IQ imbalance will also be corrected. Simulation results for a system using 16-QAM modulation are presented to show the performance of the proposed scheme.

UE Calibration in MIMO Systems

The paper describes an antenna path calibration method for (Multiple Input Multiple Output) MIMO systems. In a MIMO system, the RF components used at each transmit/receive antenna path, i.e., uplink versus downlink, do not exhibit similar phase and amplitude responses. Hence, if uplink measurements are used for downlink operations such as beamforming, the incurred distortion results in performance degradation. The proposed scheme relies only on the existing uplink measurements performed by the base-station, and then it provides a feedback to the UE for calibration. It is shown that the overall scheme can be employed for both TDD and FDD systems.

Space Time Coding with Unequal Modulations

Use of unequal modulation schemes in a system utilizing space time block coding is investigated. Employing mixed modulations in a space-time system offers finer granularity in the system performance without the receiver complexity increase associated with a larger set of modulation formats. It is shown that for the optimum performance of the system, the power per stream should be adjusted accordingly. Another consideration is that the stream using the lower modulation order has a higher robustness, so it is possible to reduce the complexity of detection process by using maximum-likelihood detection only on the stream with the higher modulation order.

High Reliability Downlink Transmission with Superposition Modulated Side Information

A highly reliable transmission scheme based on a combined application of network coding and superposition modulation is introduced for a non- relay wireless communication system. It is shown that by employing superposition modulation, the side information needed for error recovery can be transmitted over the same set of resources as the original packets. A sub-constellation alignment mechanism for constructive combining of the least significant bits of the received symbols is proposed for efficient side information dissemination. The outage analysis and numerical results show that the proposed system can offer significant gains in link reliability.

Realizing Fully Digital Precoders in Hybrid A/D Architecture With Minimum Number of RF Chains

In this letter, the minimum number of required radio frequency (RF) chains for realizing any MIMO fully digital precoder (FDP) with the hybrid analog/digital precoder (HADP) architecture is investigated from a new perspective. First, in order to take full advantage of the digital domain, an extended system formulation of the hybrid structure is presented in which the digital precoder is taken from an abstract transformation space. Then, a constrained optimization problem is formulated, where the aim is to minimize the number of RF chains subject to an equality constraint on the multi-stream FDP and HADP outputs. We show that non-unique optimum solutions exist for only two RF chains. Based on these results, we further develop a single RF chain design. The implementation of the latter does not require extra hardware or computational complexity. We compare the proposed HADP design with recent works that require larger number of RF chains and demonstrate that significant improvement in performance can be achieved over direct HADP design.

On Subspace Noise Estimation for OFDM

The performance of the subspace-based noise power estimation algorithm for an OFDM receiver is investigated. It is shown that due to the correlation of the channel in the frequency domain, the dimension of the autocorrelation matrix does not need to be larger than the number of signal paths. Therefore, due to the reduction of the dimension of the signal space, the required complexity and computation for subspace decomposition is reduced. It is shown that the algorithm performs well in various speed and channel conditions.