Senior Research Engineer Huawei Technologies Canada
October 10, 2022
ECE5884 Wireless Communications @ Monash Uni.
October 10, 2022
Copyright By PowCoder代写 加微信 powcoder
ECE5884 Wireless Communications
Week 11: Multi-antenna systems: From theory to standardization in 5G-NR
Multi-antenna systems From theory to standardization in 5G-NR
● Motivation examples
● Analog & digital multi-antenna processing
● Multi-antenna precoding (Downlink and uplink)
● Beam management
Motivation example 1: Interpretation of BER curves
Scenario 1: 1T2R
Scenario 2: 1T1R
• Q1: Two curves for T1R1 and T1R2 system. Label them?
• Q2: Which curve is better, Red or blue? Channel estimation overhead, cost/complexity
• Q3: What is the significance of value BER1, SNR1, SNR2?
• Q4: What is the significance of value BER2, SNR3, SNR4?
Motivation example 2: Closed loop transmission
𝑠𝑠 x+ĥ𝑦𝑦=h𝑠𝑠+𝑛𝑛 h𝑛𝑛�
● Set up: Improve the BER/Capacity of the fading channel by knowing h at the transmitter where h� is an estimate of h
● Q1: Knowing an estimate of h at the transmitter better or not?
○ The performance of the closed loop transmission is better than
open loop transmission?
○ How to account for feedback overhead?
○ What is the impact in MIMO?
Multi-Antenna Processing Diversity gain:
Multiple antennas at the transmitter and/or receiver side can provide diversity against fading Channels experienced by different antennas may be at least partly uncorrelated
Sufficient inter-antenna distance
Different polarization between the antennas
Spatial multiplexing:
• Multiple antennas at both the transmitter and the receiver sides can be used to enable transmission of
multiple “layers/streams” in parallel using the same time/frequency resources Transmit beamforming:
Adjust the phase and/or the amplitude of each antenna element, provide directivity to signals Directivity: Focus the overall transmitted power in a certain direction, i.e, beam forming
Increase the achievable data rates and range due to higher power reaching the target receiver Directivity can also reduce the interference to other links (improves the overall spectrum efficiency)
Receive beamforming:
• Multiple receive antennas to provide receiver-side directivity (focusing the reception in the direction of
a target signal while suppressing interference arriving from other directions)
Higher the operating frequency, what happens to the propagation loss?
In high frequency, what is more important, diversity, spatial multiplexing or beam-forming?
Background
Higher frequencies are associated with higher propagation loss
• Correspondinglyreducedcommunicationrange
Receive antennas
• Thedimensionsofthereceiverantennascalewiththewavelength(theinverseofthecarrierfrequency) • Tentimesincreaseinthecarrierfrequency,correspondingtoa?reductioninthewavelength
• Assumetentimesreductioninthephysicaldimensionsofthereceiverantennaorafactorof100
reduction in the physical antenna area
• Thiscorrespondstoa20dBreductionintheenergycapturedbytheantenna
Receive antenna size vs directivity
• If the receiver antenna size would instead be kept unchanged as the carrier frequency increases, the
reduction in captured energy could be avoided
• Imply that the antenna size would increase relative to the wave length
• Directivity of an antenna is proportional to the physical antenna area normalized with the square of the
wave length
• The gain with the larger antenna size can thus be realized if the receive antenna is well directed
towards the target signal
• In practice increasing the transmit-antenna directivity, the link budget at higher frequencies can be
improved Transmit antennas
• In practice increasing the transmit-antenna directivity, the link budget at higher frequencies can be
Multi-antenna processing
Any linear multi-antenna transmission scheme can be modeled according • 𝑁𝑁 layers (vector 𝒙𝒙)
• Mapped to 𝑁𝑁𝑇𝑇 transmit antennas (vector 𝒚𝒚)
• Multiplicationwithamatrix𝑾𝑾ofsize𝑁𝑁𝑇𝑇×𝑁𝑁𝐿𝐿
Analog and Digital Multi-Antenna Processing
• The multi-antenna processing is applied within the analog part of the transmitter chain (after digital-to-analog conversion (left))
• The multi-antenna processing is applied within the digital part of the transmitter chain (before digital-to-analog conversion (right))
Analog and Digital Multi-Antenna Processing
• Implementation complexity DAC per antenna element is the main drawback of digital processing
• At higher frequencies with large number of closely spaced antenna elements, analog multi-
antenna processing is the most common case
• The multi-antenna transmission will typically be limited to per-antenna phase shifts providing beam forming
• High frequencies: Not a severe limitation as operation at higher frequencies is typically more power-limited than bandwidth-limited, making beam forming more important than, high-order spatial multiplexing
• Lower frequencies: The spectrum is a more sparse resource with less possibility for wide transmission bandwidths
Downlink Multi-Antenna Precoding
• In the case of digital processing (control both amplitude and phase), the transmission matrix 𝑾𝑾 is referred as a precoder matrix (multi-antenna processing is referred as multi-antenna precoding)
• Coherent demodulation is achieved by demodulation reference signal (DMRS)
• Precoder-related measurements and reporting are part of the more general CSI reporting framework:
1. Rank Indicator (RI): What the device believes is a suitable transmission rank (number of layers 𝑁𝑁𝐿𝐿) 2. Precoder-Matrix Indicator (PMI): What the device believes is a suitable precoder matrix for given RI 3. Channel-Quality Indicator (CQI): What the device believes is a suitable channel-coding rate and
modulation scheme, given the selected precoder matrix.
• MU-MIMO: Simultaneously transmit to different devices taking the PMI reports into account
• Suppress the interference to other devices
Higher the number of antennas, higher the reference signal overhead?
What is the channel state information known at the transmitter means?
Uplink Multi-Antenna Precoding
• Support precoding in uplink direction
• Coherent demodulation is achieved by demodulation reference signal (DMRS)
• Codebook-based precoding: The scheduling includes information about a precoder and the device is
assumed to use the precoder provided by the network
• Device configured to transmit a reference signal (i.e., SRS) and based on the measurements from
transmitting SRS, precoder is chosen by the network
• Non-codebook-based precoding: Based on device measurements on downlink reference signal (CSI-
RS) and precoder indications to the network
Precoding based transmission only has overhead of DMRS (demodulation reference signal) for coherent detection?
Beam Management
Transmit and receiver beam for uplink and downlink transmission
Beam management consists mainly three parts:
• Initial beam establishment
• Beam adjustment: Primarily to compensate for movements and rotations of the
mobile device, but also for gradual changes in the environment
• Beam recovery: To handle the situation when rapid changes in the environment
disrupt the current beam pair
Initial beam establishment
• MultipleSSblocksbeingtransmittedinsequencewithindifferentdownlinkbeams
• Subsequentuplinkrandom-accesstransmissioncanbeusedbythenetworktoidentifythedownlink
beam acquired by the device and establishes the initial beam pair • CorrespondenceofSSblocktothebeam
Beam adjustment
Why beam adjustment: Once an initial beam pair has been established, there is a need to regularly reevaluate the selection of transmitter-side and receiver-side beam directions
• Movements and rotations of the mobile device block or unblock different beam pairs
• Even for stationary devices, movements of other objects in the environment may
block or unblock different beam pairs
Beam adjustment may also include refining the beam shape
• Example: Making the beam more narrow compared to a relatively wider beam used
for initial beam establishment
SSB/CSI-RS
Beam recovery
Why beam recovery:
• In some cases, movements in the environment or other events, may lead to a
currently established beam pair being rapidly blocked without sufficient time for the
regular beam adjustment to adapt
• The NR specification includes specific procedures to handle such beam-failure
events/beam (failure) recovery
Beam failure/recovery steps
• Beam-failure detection: The device detecting that a beam failure has occurred
• Candidate-beam identification: The device trying to identify a new beam (i.e., a new
beam pair by means of which connectivity may be restored)
• Recovery-request transmission: The device transmitting a beam recovery request to
the network
• Network response to the beam-recovery request
• Beam failure can happen because of the user mobility?
• Beam management has some overheads in uplink and downlink directions?
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