Millimeter Wave Cellular Systems – the Future of 5G
MmWave is a promising technology for future cellular systems. Since limited spectrum is available for commercial cellular systems, most research has focused on increasing spectral efficiency by using OFDM, MIMO, efficient channel coding, and interference coordination. Network densification has also been studied to increase area spectral efficiency, including the use of heterogeneous infrastructure (maco-, pico-, femtocells, relays, distributed antennas) but increased spectral efficiency is not enough to guarantee high per-user data rates. The alternative is more spectrum. Millimeter wave (mmWave) cellular systems, operating in the 10-300GHz band, appear to be a promising candidate for next-generation cellular systems by which multiple gigabit-per-second data rates can be supported.
Enabling mmWave cellular systems in practice, however, requires properly dealing with the channel impairments and propagation characteristics of the high frequency bands. The main propagation-related obstacles in realizing mmWave cellular are that free-space path loss is much larger in mmWave due to the higher carrier frequency, scattering is less signi?cant which reduce the available diversity, and non-line-of-sight paths are weaker making blockage and coverage holes more pronounced. Further, the noise power is larger due to the use of larger bandwidth channels. To achieve a high signal-to-noise-ratio (SNR) uniformly throughout a cell, mmWave networks must leverage high-gain electronically steerable directional antennas, i.e., they must beamform or precode data on large antenna arrays. Thanks to the small wavelength, however, arrays of practical dimensions can in fact house orders of magnitude more elements than current cellular arrays and can provide enough array gain to overcome path loss and ensure high SNR at the receiver. To overcome blockages, and in addition to using adaptive steerable arrays, mmWave networks can also be made very dense and can bene?t from the current trend pushing cellular system to a much more heterogeneous infrastructure that includes small cells and relays.
In our group, we have been studying several aspects of mmWave cellular systems. Our areas of interest include evaluating the expected coverage and capacity of mmWave systems, designing hybrid analog/digital beamforming and precoding schemes for mmWave cellular systems taking the different practical constraints into consideration, investigating and developing adaptive algorithms for beam training in mmWave cellular systems, modeling blockages in mmWave systems, and quantifying their effects on key network performance metrics.
Presentation: Millimeter Wave MIMO Communication
T. Bai, A. Alkhateeb, and R. W. Heath Jr, “Coverage and Capacity of Millimeter-Wave Cellular Networks,” IEEE Communications Magazine, vol.52, no.9, pp.70-77, September 2014
In this article, we show that dense mmWave networks can achieve comparable coverage and significantly higher data rates compared with conventional microwave networks. Sum rate gains can be achieved using more advanced beam- forming techniques that allow multiuser transmission. The insights are derived using a new theoretical network model that incorporates key characteristics of mmWave networks.
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.
In this paper, limited feedback wideband mmWave MIMO systems was considered. 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. The results show that the developed hybrid codebooks and precoder designs achieve very good performance compared with the unconstrained solutions while requiring much less complexity.
In this paper, we analyze the coverage probability in the presence of noise and both line-of-sight and non-line-of-sight interference. Performance of mmWave is then analyzed in terms of area spectral efficiency and rate coverage. The results show that mmWave networks support larger densities, higher area spectral efficiencies, and better rate coverage compared to traditional, lower-frequency ad hoc networks.
With bandwidths on the order of a gigahertz in emerging wireless systems, high-resolution analog-to-digital convertors (ADCs) become a power consumption bottleneck. One solution is to employ low resolution one-bit ADCs. In this paper, we analyze the flat fading MIMO channel with one-bit ADCs. Channel state information is assumed to be known at both the transmitter and receiver. For the MISO channel, we derive the exact channel capacity. For the SIMO and MIMO channel, we derive bounds on the high SNR capacity. Two efficient methods are proposed to design the input symbols to approach the capacity achieving solution.
A. Alkhateeb, G. Leus, and R. W. Heath Jr, “Limited Feedback Hybrid Precoding for Multi-User Millimeter Wave Systems,” to appear in the IEEE Transactions on Wireless Communications, September, 2014.
This paper develops low complexity 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 precoding compared with analog-only beamforming solutions.
T. Bai and R. W. Heath, Jr., “Analysis of Coverage and Rate in Millimeter Wave Cellular Networks”, To appear in IEEE Trans. on Wireless Commun., 2014.
In this paper, we propose a general network model to analyze SINR coverage and achievable rate in millimeter wave (mmWave) cellular networks. Leveraging concepts from stochastic geometry and random shape theory, the proposed model incorporates key features of mmWave networks, including blockage effects and directional beamforming. Our analysis shows that the SINR and rate performance in mmWave systems is much sensitive to the base station density. Numerical results indicate that due to the presence of building blockages, mmWave cellular requires dense base station deployment to achieve comparable SINR coverage to the conventional cellular systems.
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
In this paper, we develop an adaptive algorithm to estimate mmWave channels exploiting the poor scattering nature of the channel. To enable the efficient operation of this algorithm, a novel hierarchical multi-resolution hybrid precoding based 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.
S. Akoum, O. El Ayach and R. W. Heath, Jr., “Coverage and Capacity in mmWave Cellular Systems“, Proc. of the IEEE Asilomar Conference on Signals, Systems and Computers (Asilomar) , pp. 688-692, Pacific Grove, CA, November 4-7, 2012.
In this paper, we study the coverage and capacity of mmWave cellular systems with a special focus on their key differentiating factors such as the limited scattering nature of mmWave channels, and the use of RF beamforming strategies such as beam steering to provide highly directional transmission with limited hardware complexity. We show that, in general, coverage in mmWave systems can rival or even exceed coverage in microwave systems assuming that the link budgets promised by existing mmWave system designs are achieved.
O. El Ayach, R. W. Heath, Jr., S. Abu-Surra, S. Rajagopal, Z. Pi, “Low Complexity Precoding for Large Millimeter Wave MIMO Systems,” in Proc. of IEEE International Conference on Communications (ICC), pp. 3724-3729, Ottawa, Canada, June 2012.
This paper considers single user beamforming and precoding in mmWave systems with large arrays. We exploit the structure of mmWave channels to formulate the precoder design problem as a sparsity constrained least squares problem. Using the principle of basis pursuit, we develop a precoding algorithm that approximates the optimal unconstrained precoder using a low dimensional basis representation that can be ef?ciently implemented in RF hardware.
A. Alkhateeb, O. El Ayach, G. Leus, and R. W. Heath, Jr. “Hybrid Precoding for Millimeter Wave Cellular Systems with Partial Channel Knowledge,” (invited) Proc. of the Information Theory and Applications (ITA), San Diego, CA, February 10-15, 2013.
In this paper, we develop an iterative hybrid beamforming algorithm for the single user mmWave channel. The proposed algorithm accounts for the limitations of analog beamforming circuitry and assumes only partial channel knowledge at both the base and mobile stations. The precoding strategy exploits the sparse nature of the mmWave channel and uses a variant of matching pursuit to provide simple solutions to the hybrid beamforming problem.
K. Venugopal, M. C. Valenti, and R. W. Heath, Jr., “Interference in finite-sized highly dense millimeter wave networks,” in Proc. of Information Theory and Applications (ITA), San Diego, CA, Feb. 2015.
In this paper, we study the performance of mmWave wearable communication networks. The focus is on mmWave-based networks that are confined to a limited region and contain a finite number of interferers at fixed locations. We develop an approach for determining coverage and ergodic spectral efficiency in such a network modeling human bodies as the main source of blockage. As mmWave systems are likely to use compact antenna arrays, we capture the impact of key antenna parameters such as beamwidth and array gain in the expressions for coverage and rate of the system.
O. El Ayach, S. Abu-Surra, S. Rajagopal, Z. Pi, and R. W. Heath, Jr., “Spatially Sparse Precoding in Millimeter Wave MIMO Systems,” submitted to IEEE Trans. on Wireless, May 2013.
In this paper, we develop a general framework for beamforming in mmWave systems. 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.
N. Valliappan, A. Lozano, and R. W. Heath, Jr.“Antenna Subset Modulation for Secure Millimeter-Wave Wireless Communication,” to appear in IEEE Trans. on Communications.
This paper exploits the potential of large antenna arrays to develop a low-complexity directional modulation technique, Antenna Subset Modulation (ASM), for point-to-point secure wireless communication. The main idea in ASM is to modulate the radiation pattern at the symbol rate by driving only a subset of antennas in the array. Effectively, randomization occurs over multiple similar beams so that the receiver in the desired direction receives a coherent signal while intercepting receivers see a jumble of signals, thus improving the secrecy capacity of the system.