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 à Large path loss à 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 à higher frequency, shorter coherence time
⎻ Connectivity will be highly intermittent and
communication will need to be rapidly adaptable
⎻ Channel estimation should be performed frequently à 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
à Extremely narrow beams
mmWave radios use phased antenna arrays to focus the power along one direction
𝑁 : number of possible directions
Client AP
𝑁 directions 𝑁
Beam Searching
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
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
