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KTH ROYAL INSTITUTEOF TECHNOLOGY
Beam-searching and Transmission Scheduling
in Millimeter Wave Communications
Hossein Shokri-Ghadikolaei, Lazaros Gkatzikis, and Carlo Fischione
Automatic Control Department and ACCESS KTH Royal Institute of Technology,
Stockholm, Sweden
Email: [email protected], [email protected], and [email protected]
The Sixth Nordic Workshop on System and Network Optimization for Wireless (SNOW),
Norway, 2015
Why millimeter wave frequencies?
• Ever growing demand for multi-Gbps data rates
• scarce spectrum resources in microwave bands
However, as spectral efficiency is approaching its fundamental limits, we need to
add more spectrum and increase deployment density without increasing the
interference footprint.
Enhancing spectral efficiency
• massive multiple input multiple output (MIMO)
• cognitive networks
• interference cancelation
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explore centimeter/millimeter Wave (cmWave/mmWave)
Outline
• Introduction
• Problem formulation
• Numerical results
• Concluding remarks and future works
Characteristics of mmWave frequencies
• mmWave frequencies: 30–300 GHz
6–30 GHz are also often referred to as mmWave
• large bandwidth
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• high interaction with atmospheric constituents such as oxygen
• high path-loss (distance-dependent component of attenuation)
typical coverage10 to 20 m in 60 GHz
• high attenuation through obstacles
line of sight (LoS) communications
• short wavelength
large number of antenna elements in the current size of radio chips
high directivity gain T. Baykas, C. Sum, Z. Lan, J. Wang, M. Azizur Rahman, H. Harada, and S. Kato, "IEEE 802.15. 3c: the first IEEE wireless standard for data rates over 1 Gb/s," IEEE Commun. Mag., v. 49, no. 7, pp. 114-121, 2011.
Standardization activities
• WirelessHD consortium, wireless gigabit alliance (WiGig), ECMA 387
• IEEE 802.15.3c (WPAN), IEEE 802.11ad (WLAN)
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Beacon CSMA/CA TDMA
• A coordinator schedules transmissions in a centralized manner
• channel access requests are registered with CSMA/CA protocol, and served with TDMA scheme
Each time slot is assigned to a single transmitter-receiver pair inefficient in mmWave networks with pencil-beam operation
Beamforming (1/2)
• Deafness: the main beams of a transmitter and the intended receiver are not
aligned
• massive number of antennas (32/64 elements in existing WPAN products), so
no digital beamforming!
• Analog beamforming
only phase shifters and one RF chain, so only directivity gain
finite size codebooks each covering a certain direction
an exhaustive search over all possible directions
select the combination of vectors that maximizes the signal-to-noise ratio
no need for instantaneous CSI
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Beamforming (2/2)
• Three-step alignment of the current standards a) a quasi-omnidirectional level search
b) a coarse grained sector-level sweep
c) a beam-level alignment phase
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• Alignment overhead: the time required to find the best beams
• It depends on the number of directions that have to be searched,
which in turn depends on the selected transmission and reception
beamwidths.
J. Wang, et al. "Beam codebook based beamforming protocol for multi-Gbps millimeter-wave WPAN systems," IEEE J-SAC, v. 27, no. 8, pp. 1390-1399, 2011.
Contributions
• Identifying alignment-throughput tradeoff in mmWave communications
• enabling concurrent transmissions in mmWave networks
• translating the proposed framework into protocols
• evaluating the performance gains arising from the proposed protocols
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1. using extremely narrow beams (or excessively increasing the beamforming
codebook size) is not beneficial due to the increased alignment overhead
2. very simple contention-based resource allocation may substantially outperform
complicated contention-free resource allocation
• Introduction
• Problem formulation
• Numerical results
• Concluding remarks and future works
Alignment-throughput tradeoff
• Narrower beamwidths
significant alignment overhead
higher transmission rate due to higher directivity gains
• Larger beamwidths
less alignment overhead
reduced transmission rate
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Alignment Data transmission
iTime slot duration T
Problem formulation (1/3)
• A mmWave network with one coordinator and N links
• a path after the alignment procedure
• established sector-level alignment prior to the alignment phase
• per slot beam-level alignment
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Alignment time:
sector-level beamwidths of the transmitter and receiver of link i
beam-level beamwidths of the transmitter and receiver of link i
Pilot transmission time
Problem formulation (2/3)
• Discard the non-continuous ceiling function
• the alignment time cannot exceed total time slot duration T
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• ideal sectored antenna model:
Problem formulation (3/3)
• SINR at the receiver i
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• Sum-rate maximization Alignment Data transmission
Antenna beamwidths affect both τi
and SINRi
transmission powers only affect
SINRi
the optimization problem is non-
convex
SINRi depends on the network
topology
Single link scenario
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• The derivative has up to one root, so the throughout has up to one extremum.
• This extremum is a maximum (a simple gradient descent algorithm)
• This maximum is inside the feasible set, and not on the boundaries
Adopting an extremely narrow or wide beam is not throughput optimal.
Multiple links scenario (1/4)
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• Adding spatial gain to the current standards by allowing concurrent
transmissions
• proposing two topology-agnostic approaches to solve (6)
• decomposing a multiple links scenario into multiple single link
scenarios
substantial reduction of computational complexity
a performance loss compared to (6)
no power allocation, so a scheduling problem
1. Overestimation of interference conservative interference avoidance
2. Underestimation of interference ignore multiuser interference (noise-limited regime)
Multiple links scenario (2/4)
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using OFDM transceivers (standard compliant)estimation of interference at sector-level, higher than actual interference
Multiple links scenario (3/4)
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using OFDM transceivers (standard compliant)ignore multiuser interference (noise-limited network)
Multiple links scenario (4/4)
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Interference over-estimator Interference under-estimator
Finding all independent setsNP-hard problem
-
Light computational complexity given independent sets
*multiple executions of gradient descent algorithm
Light computational complexity
*multiple executions of gradient descent algorithm
Underutilization of spatial resources
*over-estimation of interference
Some level of multiuser interference
*mmWave systems are not necessarily noise-limited.
Higher signaling overhead Lower signaling overhead
• Introduction
• Problem formulation
• Numerical results
• Concluding remarks and future works
• A WPAN scenario with several mmWave devices on 60 GHz
• random location in an area of 10x10
• maximum transmission power: 2.5 mW
• pilot transmission time : 20 μs
• time slot duration T: as high as 65,535 μs
• sector-level beams: 90 degrees
• Monte Carlo simulations over 100 random topologies due to very high computational complexity for the optimal solution
Simulation setup
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• Contours of the throughput of a single link against transmission and reception beamwidths. Optimal hyperbola:
Single link scenario (1/2)
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• Existence of the alignment-throughput tradeoff
• High beam-searching overhead with narrow beams (do not use pencil-beams!)
• Low directivity gain with wide beams
• Performance improvement with reduced pilot transmission overhead 22/27
Single link scenario (2/2)
• Allocating only one channel per time slot (existing standards) is significantly inefficient.
• Inefficiency increases with the number of links.
• With 10 links, 525%, 401%, and 177% performance enhancement can be achieved by the Oracle, interference under-estimator, and over-estimator, respectively.
• MmWave WPANs are not noise-limited (gap between under-estimator and Oracle) 23/27
Single Link Activation: only the link of the highest SNR is activated Oracle: the solution of optimization problem (6)
Multiple links scenario
• Introduction
• Problem formulation
• Numerical results
• Concluding remarks and future works
• Existing standards do not leverage full potential of mmWave communications
• We need to optimize the alignment-throughput
• Joint beamwidth selection and resource allocation: very high computational complexity exact network topology as an input
• Interference over-estimator: very high computational complexity substantially suboptimal
• Interference under-estimation:
some level of interference increased optimality gap with the number of links
Concluding remarks
25/27
• Overhead of connection management (establishment, maintenance,
recovery) with mobile users
Frequent execution of connection recovery with pencil-beam operation
• Validity of noise-limited regime
activating all links at the same time and frequency! Why do we need
complicated contention-free resource allocations in mmWave networks?
mmWave networks show a transitional behavior from a noise- to an
interference-limited regime
Future works
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[1] H. Shokri-Ghadikolaei, L. Gkatzikis, and C. Fischione, “Beam-searching and transmission
scheduling in millimeter wave communications,” Proc. IEEE ICC'15, 2015.
[2] H. Shokri-Ghadikolaei, C. Fischione, G. Fodor, P. Popovski, and M. Zorzi, “Millimeter
wave cellular networks: A MAC layer perspective”, submitted to IEEE Trans. Commun.
(invited paper), Feb. 2015.
[3] H. Shokri-Ghadikolaei, C. Fischione, P. Popovski, and M. Zorzi, “Design aspects of short
range millimeter wave networks: A MAC layer perspective”, KTH, Tech. Rep., 2015.
[4] H. Shokri-Ghadikolaei and C. Fischione, “Transitional behavior of millimeter wave
networks”, KTH, Tech. Rep., 2015.
[5] T. Baykas, C. Sum, Z. Lan, J. Wang, M. Azizur Rahman, H. Harada, and S. Kato, "IEEE
802.15. 3c: the first IEEE wireless standard for data rates over 1 Gb/s," IEEE Commun.
Mag., v. 49, no. 7, pp. 114-121, 2011.
[6] T. BaykasJ. Qiao, X. Shen, J. Mark, Q. Shen, Y. He, and L. Lei, “Enabling device to
device communications in millimeter-wave (5G) cellular networks,” IEEE Commun.
Mag., vol. 53, no. 1, pp. 209–215, Jan. 2015.
References
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KTH ROYAL INSTITUTEOF TECHNOLOGY
Beam-searching and Transmission Scheduling
in Millimeter Wave Communications
Hossein Shokri-Ghadikolaei, Lazaros Gkatzikis, and Carlo Fischione
Automatic Control Department and ACCESS KTH Royal Institute of Technology,
Stockholm, Sweden
Email: [email protected], [email protected], and [email protected]
The Sixth Nordic Workshop on System and Network Optimization for Wireless (SNOW),
Norway, 2015
