A Review on Cell-Free Massive MIMO Systems
Abstract
:1. Introduction
1.1. Bandwidth Expansion
1.2. Network Densification
1.3. Cell-Free Massive MIMO Systems
1.4. Paper Organization
2. Scalable Cell-Free mMIMO Systems
2.1. Limited Fronthaul Capacity
2.2. AP–UE Association Techniques
3. Beamforming for Cell-Free mMIMO Systems
3.1. Sub-6 GHz
3.1.1. Conventional Cell-Free
3.1.2. User-Centric Cell-Free
3.2. Millimeter Wave
3.2.1. Wideband Channel Model
3.2.2. Narrowband Channel Model
- Conventional Cell-Free
- User-Centric Cell-Free
4. Cell-Free Promising Technologies
4.1. Reconfigurable Intelligent Surface (RIS)
- Since RIS acts as antenna arrays, it can boost network capacity and data rates in inexpensive and energy-efficient way due to the tuning of various passive reflecting elements.
- As RIS is passive, it is not possible to use the traditional channel estimation techniques to estimate the reflected channels of RIS-aided wireless communication systems.
4.2. Radio Stripe (RS)
- Flexible deployment and cable routing in real-world applications to achieve more practical and scalable systems.
- The network can be fault-tolerant by using routing mechanisms that might reduce the impact of node failures.
- RSs improve robustness and resilience, and thus decrease maintenance expenses.
- Low heat dissipation simplifies and reduces the cost of cooling systems.
- The crucial requirement of CF network with RSs is the specific and accurate synchronization and coordination between antennas and antenna processing units.
4.3. Large Intelligent Surfaces (LIS)
- LIS provides maximum data rates with a traditional large antenna array for a certain surface area.
- LIS has high performance when the number of terminals increases and can mitigate interference. Therefore, it is a promising candidate for data transmission in wireless networks that go beyond mMIMO technology.
4.4. Unmanned Aerial Vehicle (UAV)
- Having solid connections between APs and UAVs, to ensure particular QoS criteria in terms of data rates, latency, and reliability, can be considered a prerequisite to fulfill the anticipated benefits of UAVs.
- It is necessary to build new communication protocols, taking into consideration the risk of sparse and intermittent network connectivity.
- The issues resulted from the high signal processing complexity and the high costs associated with power consumption make it highly expensive to use multiple antennas in UAVs due to the size, weight, and power constraints.
4.5. Artificial Intelligence (AI)
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Abbreviations
ADC | analog-to-digital converter |
AI | artificial intelligence |
AoA | angle of arrival |
AoD | angle of departure |
AP | access point |
BCD | block coordinate descent |
BER | bit error rate |
BS | base station |
CF | mMIMO cell-free massive MIMO |
CoMP | coordinated multi-point |
CPU | central processing unit |
CSI | channel state information |
DAC | digital-to-analog converter |
DAS | distributed antenna syste |
EE | energy efficiency |
ICI | inter-cell interference |
LIS | large intelligent surface |
LSF | large-scale fading |
MIMO | multiple-input multiple-output |
mmWave | millimeter Wave |
MR | maximum ratio |
MSE | mean square error |
OFDM | orthogonal frequency division multiplexing |
PL | path-loss |
QoS | quality of service |
RF | radio frequency |
RIS | reconfigurable intelligent surface |
RS | radio stripe |
SC | small cell |
SE | spectral efficiency |
SINR | signal to interference and noise ratio |
SNR | signal-to-noise ratio |
UAV | unmanned aerial vehicle |
UC | user-centric |
UDN | ultra-dense network |
UE | user equipment |
ZF | zero-forcing |
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Ref. | Year | Transmission | Channel Model | Coordinated Beamforming | Performance |
---|---|---|---|---|---|
[25] | 2018 | Downlink | Block fading channel | Centralized | Improve power weight allocation |
[26] | 2019 | Uplink | Block fading channel | Distributed | Improve SE and EE |
[27] | 2019 | Uplink / Downlink | mmWave | Semi-centralized | Improve average max–min user rate |
[28] | 2020 | Uplink / Downlink | Block fading channel | Distributed | Improve average max–min user rate |
[29] | 2021 | Uplink | Rayleigh fading channel | Distributed | Improve BER |
[30] | 2021 | Uplink | Flat fading channel | Distributed | Improve overall system SINR |
[31] | 2021 | Uplink | Rician fading channel | Distributed | Improve average max–min user rate |
[32] | 2022 | Downlink | Rayleigh fading channel | Distributed | Improve average max–min user rate |
[33] | 2022 | Uplink | mmWave | Centralized | Improve SE and EE |
[34] | 2022 | Uplink | mmWave | Distributed | Improve average max–min user rate |
[35] | 2022 | Downlink | Block fading channel | Distributed | Improve coverage user rate and EE |
Literature Review | Conventional CF mMIMO | User-Centric CF mMIMO | Both | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
[50] 2017 | [51] 2019 | [52] 2019 | [53] 2022 | [54] 2021 | [55] 2022 | [36] 2018 | [37] 2020 | [38] 2020 | [39] 2022 | [40] 2022 | [41] 2022 | [42] 2021 | ||
Data Transmission | Uplink | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |||||||
Downlink | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |||||
Channel Model | Rayleigh fading | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||
Flat fading | ✓ | ✓ | ✓ | ✓ | ||||||||||
Digital Beamforming | at UEs | ✓ | ✓ | |||||||||||
at APs/CPU | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |
Coordinated Beamforming | Distributed | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||
Semi-centralized | ✓ | ✓ | ||||||||||||
Centralized | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
Literature Review | Conventional CF mMIMO | Both Conv. & UC CF mMIMO | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
[27] 2019 | [58] 2020 | [59] 2021 | [60] 2022 | [61] 2022 | [62] 2022 | [63] 2022 | [43] 2017 | [44] 2018 | [45] 2019 | [46] 2021 | [47] 2022 | ||
Data Transmission | Uplink | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |||||
Downlink | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |||
Channel Model | Narrow-band | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||
Wide-band | ✓ | ✓ | |||||||||||
Hybrid Beamforming | at UEs | ✓ | ✓ | ✓ | ✓ | ||||||||
at APs/CPU | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |
Coordinated Beamforming | Distributed | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||
Semi-centralized | ✓ | ✓ | ✓ | ||||||||||
Centralized | ✓ | ✓ |
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Kassam, J.; Castanheira, D.; Silva, A.; Dinis, R.; Gameiro, A. A Review on Cell-Free Massive MIMO Systems. Electronics 2023, 12, 1001. https://doi.org/10.3390/electronics12041001
Kassam J, Castanheira D, Silva A, Dinis R, Gameiro A. A Review on Cell-Free Massive MIMO Systems. Electronics. 2023; 12(4):1001. https://doi.org/10.3390/electronics12041001
Chicago/Turabian StyleKassam, Joumana, Daniel Castanheira, Adão Silva, Rui Dinis, and Atílio Gameiro. 2023. "A Review on Cell-Free Massive MIMO Systems" Electronics 12, no. 4: 1001. https://doi.org/10.3390/electronics12041001