Hybrid Analog-Digital Architectures
Large antenna systems will be employed in mmWave systems to overcome the severe path loss in this frequency band. The high cost and power consumption of mixed signal components, however, prevent dedicating a separate radio frequency (RF) chain for each antenna and using traditional MIMO baseband precoding schemes. One way to overcome these limitations is to divide the needed precoding processing between analog and digital domains by designing hybrid analog-digital precoding algorithms. In our group, we investigated the design of low-complexity hybrid analog-digital precoding and channel estimation algorithms for single-user and multi-user mmWave systems. We also studied the extension of these hybrid transmission techniques to low-frequency massive MIMO systems.
R. W. Heath Jr, N. G. Prelcic, S. Rangan, W. Roh, and A. Sayeed, “An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems,” to appear inIEEE Journal of Selected Topics in Signal Processing, April 2016.
The applications of mmWave are immense: wireless local and personal area networks in the unlicensed band, 5G cellular systems, not to mention vehicular area networks, ad hoc networks, and wearables. Signal processing is critical for enabling the next generation of mmWave communication. Due to the use of large antenna arrays at the transmitter and receiver, combined with radio frequency and mixed signal power constraints, new multiple-input multiple-output (MIMO) communication signal processing techniques are needed. Because of the wide bandwidths, low complexity transceiver algorithms become important. There are opportunities to exploit techniques like compressed sensing for channel estimation and beamforming. This article provides an overview of signal processing challenges in mmWave wireless systems, with an emphasis on those faced by using MIMO communication at higher carrier frequencies.
A. Alkhateeb, J. Mo, N. G. Prelcic, and R. W. Heath Jr, “MIMO Precoding and Combining Solutions for Millimeter-Wave Systems,” IEEE Communications Magazine, vol.52, no.12, pp.122-131, December 2014.
This article explains how beamforming and precoding are different in MIMO mmWave systems than in their lower-frequency counterparts, due to different hardware constraints and channel characteristics. Two potential architectures are reviewed: hybrid analog/digital precoding/combining and combining with low-resolution analog-to-digital converters. The potential gains and design challenges for these strategies are discussed, and future research directions are highlighted.
A. Alkhateeb and R. W. Heath Jr, “Frequency Selective Hybrid Precoding for Limited Feedback Millimeter Wave Systems,” submitted to IEEE Transactions on Communications (Invited Paper), October, 2015.
This paper considers wideband mmWave systems with a limited feedback channel between the transmitter and receiver. First, the optimal hybrid precoding design for a given RF codebook is derived. This provides a benchmark for any other heuristic algorithm and gives useful insights into codebook designs. Second, efficient hybrid analog/digital codebooks are developed for spatial multiplexing in wideband mmWave systems. Finally, a low-complexity yet near-optimal greedy frequency selective hybrid precoding algorithm is proposed based on Gram-Schmidt orthogonalization. Simulation results show that the developed hybrid codebooks and precoder designs achieve very good performance compared with the unconstrained solutions while requiring much less complexity.
R. Mendez-Rial, C. Rusu, A. Alkhateeb, N. Gozalez-Prelcic, and R. W. Heath Jr, “Hybrid MIMO Architectures for Millimeter Wave Communications: Phase Shifters or Switches?,” to appear in IEEE Access Dec. 2015.
In this paper, we propose hybrid architectures based on switching networks to reduce the complexity and the power consumption of the structures based on phase shifters. We define a power consumption model and use it to evaluate the energy efficiency of both structures. To estimate the complete MIMO channel, we propose an open loop compressive channel estimation technique which is independent of the hardware used in the analog processing stage. We analyze the performance of the new estimation algorithm for hybrid architectures based on phase shifters and switches. Using the estimated, we develop two algorithms for the design of the hybrid combiner based on switches and analyze the achieved spectral efficiency. Finally, we study the trade-offs between power consumption, hardware complexity, and spectral efficiency for hybrid architectures based on phase shifting networks and switching networks.
In this paper, a novel architecture for massive MIMO receivers, consisting of arrays of switches and constant (non-tunable) phase shifters, is proposed. This architecture applies a quasi-coherent combining in the RF domain to reduce the number of required RF chains. An algorithm that designs the RF combining for this architecture is developed and analyzed. Results show that the proposed massive MIMO combining model can achieve a comparable performance to the fully-digital receiver architecture in single-user and multi-user massive MIMO setups.
A. Alkhateeb, G. Leus, R. W. Heath Jr, “Limited Feedback Hybrid Precoding for Multi-User Millimeter Wave Systems,” IEEE Transactions on Wireless Communications, vol.14, no.11, pp.6481-6494, Nov. 2015
This paper develops low complexity two-stage hybrid analog/digital precoding for downlink multiuser mmWave systems. The proposed algorithm configures hybrid precoders at the transmitter and analog combiners at multiple receivers with a small training and feedback overhead. The performance of the proposed algorithm is analyzed in the large dimensional regime and in single path channels. When the analog and digital precoding vectors are selected from quantized codebooks, the rate loss due to the joint quantization is characterized and insights are given into the performance of hybrid beamforming compared with analog-only beamforming solutions. Analytical and simulation results show that the proposed techniques offer higher sum rates compared with analog-only beamforming solutions, and approach the performance of the unconstrained digital beamforming with relatively small codebooks.
A. Alkhateeb, O. El Ayach, G. Leus, R. W. Heath Jr, “Channel Estimation and Hybrid Precoding for Millimeter Wave Cellular Systems,” IEEE Journal of Selected Topics in Signal Processing, vol.8, no.5, pp.831-846, Oct. 2014 October, 2014
The code that implements the algorithms in the paper is available here.
This paper develops an adaptive algorithm to estimate the mmWave channel parameters that exploits the poor scattering nature of the channel. To enable the efficient operation of this algorithm, a novel hierarchical multi-resolution codebook is designed to construct training beamforming vectors with different beamwidths. For single-path channels, an upper bound on the estimation error probability using the proposed algorithm is derived, and some insights into the efficient allocation of the training power among the adaptive stages of the algorithm are obtained. The adaptive channel estimation algorithm is then extended to the multi-path case relying on the sparse nature of the channel. Simulation results show that the proposed low-complexity channel estimation algorithm achieves comparable precoding gains compared to exhaustive channel training algorithms. The results illustrate that the proposed channel estimation and precoding algorithms can approach the coverage probability achieved by perfect channel knowledge even in the presence of interference.
O. E. Ayach, S. Rajagopal, S. Abu-Surra, Z. Pi, R. W. Heath Jr, “Spatially sparse precoding in millimeter wave MIMO systems,” IEEE Transactions on Wireless Communications, vol. 99, pp. 1-15, Jan. 2014
In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.
This work is supported in part by the National Science Foundation under Grant No. 1218338 and 1319556, and by gifts from Huawei Technologies, Inc and Nokia.