Wireless Communication Systems
@CS.NCTU
Lecture 15: mmWave
Lecturer: Kate Ching-Ju Lin ( 林靖茹 )
Millimeter Wave Bands
• Huge amount of available bandwidth (λ=C/f)
mmWave Wireless Applications
5G Cellular Networks Wireless Data Centers Wireless LANs 802.11ad
Wireless Virtual/ Augmented Reality
Connected Vehicles
• Between 30GHz and 300GHz
• Offers much greater bandwidths combined with further gains via beamforming and spatial
multiplexing
• Antenna arrays: Enable large numbers (32
elements) of miniaturized antennas placed in small dimensions
• Increasing omnidirectional path loss due to the higher frequencies of mmWave transmissions
⎻Compensated through suitable beamforming and directional transmissions
⎻Severely vulnerable to shadowing (blockage)
Challenges
• Directional communications
• Shadowing
• Channel fluctuation
• Multiuser coordination
• Power consumption
Directional Transmissions
• Path loss grows with the square of the frequency
• Small wavelength Large path loss Short transmission range
• Leverage antenna array and beamforming to steer directional beam with a stronger
power
• Deafness occurs when the main lobes at both Tx and Rx do not point to each other
Shadowing
• mmWave signals are extremely susceptible to shadowing
⎻High penetration loss due to obstacles
⎻Brick can attenuate signals by as much as 40–80 dB
Channel Fluctuation
• For a given mobile velocity, channel coherence time is linear in the carrier frequency higher frequency, shorter coherence time
⎻Connectivity will be highly intermittent and
communication will need to be rapidly adaptable
⎻Channel estimation should be performed frequently large overhead
Multiuser Coordination
• Directional transmissions imply more spatial reuse opportunities
• Challenges
⎻How to locate users?
⎻How to quickly switch the beam directions and widths?
Power Consumption
• Power consumption generally scales
⎻linearly in the sampling rate
⎻exponentially in the number of bits per samples
• Hard to achieve high-resolution quantization at wide bandwidths and large numbers of
antennas
• Efficient RF power amplification and
combining will be needed for phased array antennas
Phase Array
Small Wavelength enables thousands of antennas to be packed into small space
Extremely narrow beams
mmWave radios use phased antenna arrays to focus the power along one direction
: number of possible directions
Clien AP t
�directions
�
Beam Searching
Naïve solution: Exhaustive search
802.11ad: Multi-Stage Scan
• Stage 1: Client uses omni-directional; AP scans directions
AP
Client
802.11ad: Multi-Stage Scan
• Stage 2: AP uses omni directional; client scans directions
AP
Client
O ) Beacon Packets (�
Still Too Slow [MOBICOM’14, SIGMETRICS’15,
Hybrid Precoding
• Iteratively reduce the size of lobes as scanning
AP
Client
Hybrid Precoding
• Iteratively reduce the size of lobes as scanning
AP
Client
Hybrid Precoding
• Iteratively reduce the size of lobes as scanning
• Until the narrowest beam pointing to each other
AP
Client
