Enabling Grant-Free URLLC: An Overview of Principle and Enhancements by Massive MIMO
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Citations
Toward immersive communications in 6G
Predictive Precoder Design for OTFS-Enabled URLLC: A Deep Learning Approach
Next-Generation URLLC With Massive Devices: A Unified Semi-Blind Detection Framework for Sourced and Unsourced Random Access
Next-Generation URLLC with Massive Devices: A Unified Semi-Blind Detection Framework for Sourced and Unsourced Random Access
Improved Grant-Free Access for URLLC via Multi-Tier-Driven Computing: Network-Load Learning, Prediction, and Resource Allocation
References
Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas
Massive MIMO for next generation wireless systems
LTE - The UMTS Long Term Evolution: From Theory to Practice
Channel Coding Rate in the Finite Blocklength Regime
Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems
Related Papers (5)
Frequently Asked Questions (17)
Q2. What is the key to achieving the maximum mMIMO performance?
It is also important to remark that accurate acquisition of instantaneous CSI is a key to fully exploiting large spatial9 diversity and multiplexing gains of mMIMO.
Q3. How many retransmissions can be achieved in GF URLLC?
Suppose that a URLLC application aims to achieve an error probability of 10−5 and its required latency only allows a maximum K = 4 retransmissions in GF URLLC.
Q4. What are some of the applications that are emerging in the field of URLLC?
Emerging URLLC applications include virtual/augmented reality, public safety, factory automation, and autonomous vehicles, among others [7].
Q5. How can the BS estimate the CSI of preambles?
In order to further improve the spectral efficiency or shorten the latency of coded random access, a superposition of multiple orthogonal preambles [103] can be considered, which can effectively increase the number of active devices whose CSI can be estimated by the BS using SIC.
Q6. What is the main reason why retransmissions are required in random access?
In particular, in random access, retransmissions are required as packet collisions are inevitable due to the nature of uncoordinated transmissions.
Q7. What is the effect of a noisy superposition of multiple channel vectors?
Since the estimated CSI is a noisy superposition of multiple channel vectors of the collided devices, it brings two effects in mMIMO, leading to unsuccessful decoding [89], [94]–[96]: 1) it reduces the coherent array gain of the desired received signal, and 2) it introduces a coherent interference that gets stronger as M grows.
Q8. How many URLLC devices can be served simultaneously?
with target reliability of = 10−5, it is evident that only 1 URLLC device can be served in MIMO with 8 antennas, while the number of URLLC devices that can be served simultaneously increases with M and can reach 55 when M = 128.
Q9. What are the performance limitations for HARQ retransmission schemes in GF random access?
It is important to remark that the performance limitations for the HARQ retransmission schemes in GF random access are fundamentally originated from the multi-user interference as well as preamble collision when multiple devices compete for the same channel resource for uplink transmissions.
Q10. What is the main reason why multiple competing devices can transmit data over the same channel?
In GF URLLC, since multiple competing devices can transmit data over the same channel, it results in potential transmission collision, which is detrimental to the reliability.
Q11. What are the disadvantages of GF URLLC?
it incurs additional latency and undesirable signalling overheads, which hinder achieving the required level of latency constraints for URLLC.
Q12. What is the reason why LTE is not applicable to the uplink URLLC?
Earlier studies on the long-term evolution (LTE) systems to support URLLC [9] reported that LTE is not applicable to meet low-latency requirements.
Q13. What is the main reason for the lack of GF URLLC?
with limited latency budget and wireless resources, their resulting reliability levels and spectral efficiency still needs to be enhanced to meet what is required for emerging URLLC services, particularly when the URLLC access load is relatively high [6].
Q14. How can a fixed N be used to reduce the need for retransmissions?
As observed, with a fixed N , the decoding error probability can be significantly reduced as M grows, which reveals that the need for retransmissions can be dramatically reduced in GF URLLC.
Q15. How long does a TTI take to deliver a data packet?
In Fig. 3, a mini-slot based TTI of 21User-plane latency is the time it takes to successfully deliver a data packet at the radio protocol layer from the transmitter to the receiver.
Q16. What is the difference between GF and the 4-step random access procedure?
Compared to the 4-step random access, GF random access can be more efficient thanks to low signaling overhead when devices have short packets to transmit.
Q17. What is the decoding error probability of receiving q bits of data within a channel?
Based on [14], [91], [92], the decoding error probability of receiving q bits of data within d channel uses can be well approximated by(γ) ≈ E{γ}[ Q ( d log2(1 + γ)− q√V (γ)d)] , (1)where E{x}[·] is the expectation operation over variable x, V (γ) is the channel dispersion that is given by V (γ) =( 1− 1(1+γ)2 ) log22(e).