{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,14]],"date-time":"2026-03-14T18:06:31Z","timestamp":1773511591684,"version":"3.50.1"},"reference-count":37,"publisher":"MDPI AG","issue":"11","license":[{"start":{"date-parts":[[2023,5,23]],"date-time":"2023-05-23T00:00:00Z","timestamp":1684800000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Chip and Module Project of 5G Reduced Capability (RedCap) 2022","award":["TC220H07A"],"award-info":[{"award-number":["TC220H07A"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Pilot contamination due to pilot reuse seriously mitigates the performance of the cell-free massive multiple-input multiple-output (MIMO) systems. In this paper, we propose a joint pilot assignment scheme that employs user clustering and graph coloring (UC-GC) to reduce pilot contamination. The proposed method consists of two steps: firstly, we utilize AP selection to classify all users; secondly, we assign pilots to users with more severe pilot contamination using the graph coloring algorithm and then assign pilots to the remaining users. The numerical simulation results show that the proposed scheme outperforms existing pilot assignment schemes and significantly improves throughout with low complexity.<\/jats:p>","DOI":"10.3390\/s23115014","type":"journal-article","created":{"date-parts":[[2023,5,24]],"date-time":"2023-05-24T01:29:26Z","timestamp":1684891766000},"page":"5014","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Joint User Clustering and Graph Coloring Based Pilot Assignment for Cell-Free Massive MIMO Systems"],"prefix":"10.3390","volume":"23","author":[{"given":"Xinyu","family":"Huang","sequence":"first","affiliation":[{"name":"School of Microelectronics and Communication Engineering, Chongqing University, Chongqing 400044, China"}]},{"given":"Yubo","family":"Wang","sequence":"additional","affiliation":[{"name":"Beijing Smart-Chip Microelectronics Technology Co., Ltd., Beijing 100192, China"}]},{"given":"Shiyong","family":"Chen","sequence":"additional","affiliation":[{"name":"School of Microelectronics and Communication Engineering, Chongqing University, Chongqing 400044, China"}]},{"given":"Yan","family":"Li","sequence":"additional","affiliation":[{"name":"School of Microelectronics and Communication Engineering, Chongqing University, Chongqing 400044, China"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1116-7706","authenticated-orcid":false,"given":"Yucheng","family":"Wu","sequence":"additional","affiliation":[{"name":"School of Microelectronics and Communication Engineering, Chongqing University, Chongqing 400044, China"}]}],"member":"1968","published-online":{"date-parts":[[2023,5,23]]},"reference":[{"key":"ref_1","first-page":"77","article-title":"Making cell-free massive MIMO competitive with MMSE processing and centralized implementation","volume":"19","author":"Sanguinetti","year":"2019","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"6141","DOI":"10.1109\/TWC.2022.3146624","article-title":"Topological pilot assignment in large-scale distributed MIMO networks","volume":"21","author":"Yu","year":"2022","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"634","DOI":"10.1109\/LWC.2020.3044301","article-title":"Grant-free massive connectivity in massive MIMO systems: Collocated versus cell-free","volume":"10","author":"Wang","year":"2020","journal-title":"IEEE Wirel. Commun. Lett."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"611","DOI":"10.1109\/COMST.2021.3135119","article-title":"User-centric cell-free massive MIMO networks: A survey of opportunities, challenges and solutions","volume":"24","author":"Ammar","year":"2021","journal-title":"IEEE Commun. Surv. Tutor."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"1482","DOI":"10.1109\/TWC.2021.3104456","article-title":"Downlink resource allocation in multiuser cell-free MIMO networks with user-centric clustering","volume":"21","author":"Ammar","year":"2021","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"3523","DOI":"10.1109\/TVT.2020.2970018","article-title":"Performance analysis of cell-free massive MIMO systems: A stochastic geometry approach","volume":"69","author":"Papazafeiropoulos","year":"2020","journal-title":"IEEE Trans. Veh. Technol."},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Zhou, M., Zhang, Y., Cao, H., Qiao, X., and Yang, L. (2019, January 9\u201313). Enhanced power allocation algorithms for uplink mixed ADCs massive MIMO systems. Proceedings of the 2019 IEEE Globecom Workshops (GC Wkshps), Waikoloa, HI, USA.","DOI":"10.1109\/GCWkshps45667.2019.9024464"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"43","DOI":"10.1109\/LCOMM.2019.2947462","article-title":"On the performance of cell\u00adfree massive MIMO with mixed\u00adADC under Rician fading channels","volume":"24","author":"Zhang","year":"2019","journal-title":"IEEE Commun. Lett."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"162","DOI":"10.1561\/2000000109","article-title":"Foundations of user-centric cell-free massive MIMO","volume":"14","author":"Demir","year":"2021","journal-title":"Found. Trends Signal Process."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"695","DOI":"10.1016\/j.dcan.2021.12.005","article-title":"A survey on user-centric cell-free massive MIMO systems","volume":"8","author":"Chen","year":"2022","journal-title":"Digit. Commun. Netw."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"1834","DOI":"10.1109\/TWC.2017.2655515","article-title":"Cell-free massive MIMO versus small cells","volume":"16","author":"Ngo","year":"2017","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"4445","DOI":"10.1109\/TWC.2017.2698449","article-title":"Precoding and power optimization in cell-free massive MIMO systems","volume":"16","author":"Nayebi","year":"2017","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"1250","DOI":"10.1109\/TWC.2019.2952117","article-title":"User-centric 5G cellular networks: Resource allocation and comparison with the cell-free massive MIMO approach","volume":"19","author":"Buzzi","year":"2019","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"706","DOI":"10.1109\/LWC.2017.2734893","article-title":"Cell-free massive MIMO: User-centric approach","volume":"6","author":"Buzzi","year":"2017","journal-title":"IEEE Wirel. Commun. Lett."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"25","DOI":"10.1109\/TGCN.2017.2770215","article-title":"On the total energy efficiency of cell-free massive MIMO","volume":"2","author":"Ngo","year":"2017","journal-title":"IEEE Trans. Green Commun. Netw."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"40860","DOI":"10.1109\/ACCESS.2021.3056051","article-title":"Performance of cell-free massive MIMO with joint user clustering and access point selection","volume":"9","author":"Wang","year":"2021","journal-title":"IEEE Access"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"2225","DOI":"10.1109\/LCOMM.2022.3186350","article-title":"User-centric access point selection in cell-free massive MIMO systems: A game-theoretic approach","volume":"26","author":"Wei","year":"2022","journal-title":"IEEE Commun. Lett."},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Biswas, S., and Vijayakumar, P. (2021, January 25\u201327). AP selection in Cell-Free Massive MIMO system using Machine Learning Algorithm. Proceedings of the 2021 Sixth International Conference on Wireless Communications, Signal Processing and Networking (WiSPNET), Chennai, India.","DOI":"10.1109\/WiSPNET51692.2021.9419450"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"108127","DOI":"10.1109\/ACCESS.2020.3001270","article-title":"Effective channel gain-based access point selection in Cell-Free massive MIMO systems","volume":"8","author":"Dao","year":"2020","journal-title":"IEEE Access"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Zhang, Y., Cao, H., Zhong, P., Qi, C., and Yang, L. (2018, January 7\u201310). Location-based greedy pilot assignment for cell-free massive MIMO systems. Proceedings of the 2018 IEEE 4th International Conference on Computer and Communications (ICCC), Chengdu, China.","DOI":"10.1109\/CompComm.2018.8780756"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Dang, X.T., Lai-Thuc, T., Nguyen, A.T., Vu-Huy, T., Tran, N.H.A., and Han, H.D. (2021, January 13\u201315). A Genetic Algorithm based Pilot Assignment strategy for Cell-Free massive MIMO system. Proceedings of the 2020 IEEE Eighth International Conference on Communications and Electronics (ICCE), Phu Quoc Island, Vietnam.","DOI":"10.1109\/ICCE48956.2021.9352116"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"2286","DOI":"10.1109\/TVT.2019.2956217","article-title":"Tabu-search-based pilot assignment for cell-free massive MIMO systems","volume":"69","author":"Liu","year":"2019","journal-title":"IEEE Trans. Veh. Technol."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Ding, J., Kong, D., and Qu, D. (July, January 28). Improved Tabu-search preamble assignment in cell-free massive MIMO systems. Proceedings of the 2021 International Wireless Communications and Mobile Computing (IWCMC), Harbin City, China.","DOI":"10.1109\/IWCMC51323.2021.9498617"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"5153","DOI":"10.1109\/TWC.2019.2933831","article-title":"Downlink training in cell-free massive MIMO: A blessing in disguise","volume":"18","author":"Interdonato","year":"2019","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1698","DOI":"10.1109\/JSAC.2020.3000810","article-title":"On the spectral and energy efficiencies of full-duplex cell-free massive MIMO","volume":"38","author":"Nguyen","year":"2020","journal-title":"IEEE J. Sel. Areas Commun."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"34","DOI":"10.1109\/LWC.2020.3020003","article-title":"Pilot assignment in cell-free massive MIMO based on the Hungarian algorithm","volume":"10","author":"Buzzi","year":"2020","journal-title":"IEEE Wirel. Commun. Lett."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"16893","DOI":"10.1038\/s41598-022-21241-0","article-title":"Mitigation pilot contamination based on matching technique for uplink cell-free massive MIMO systems","volume":"12","author":"Sambo","year":"2022","journal-title":"Sci. Rep."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"9180","DOI":"10.1109\/TVT.2020.3000496","article-title":"Graph coloring based pilot assignment for cell-free massive MIMO systems","volume":"69","author":"Liu","year":"2020","journal-title":"IEEE Trans. Veh. Technol."},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Hmida, W.H., Meghdadi, V., Bouallegue, A., and Cances, J.P. (2020, January 7\u201311). Graph coloring based pilot reuse among interfering users in cell-free massive MIMO. Proceedings of the 2020 IEEE International Conference on Communications Workshops (ICC Workshops), Dublin, Ireland.","DOI":"10.1109\/ICCWorkshops49005.2020.9145111"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"999","DOI":"10.1109\/TCOMM.2021.3129928","article-title":"Coexistence of D2D communications and cell-free massive MIMO systems with low resolution ADC for improved throughput in beyond-5G networks","volume":"70","author":"Masoumi","year":"2021","journal-title":"IEEE Trans. Commun."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Lin, X., Xu, F., Fu, J., and Wang, Y. (2022). Resource Allocation for TDD Cell-Free Massive MIMO Systems. Electronics, 11.","DOI":"10.3390\/electronics11121914"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"537","DOI":"10.1109\/TWC.2015.2475746","article-title":"The application of MIMO to non-orthogonal multiple access","volume":"15","author":"Ding","year":"2015","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"950","DOI":"10.1109\/LWC.2018.2841375","article-title":"NOMA-aided cell-free massive MIMO systems","volume":"7","author":"Li","year":"2018","journal-title":"IEEE Wirel. Commun. Lett."},{"key":"ref_34","doi-asserted-by":"crossref","unstructured":"Hao, Y., Xin, J., Tao, W., Tao, S., Yu-Xiang, L., and Hao, W. (2020, January 11\u201314). Pilot Allocation Algorithm Based on K-means Clustering in Cell-Free Massive MIMO Systems. Proceedings of the 2020 IEEE 6th International Conference on Computer and Communications (ICCC), Chengdu, China.","DOI":"10.1109\/ICCC51575.2020.9345177"},{"key":"ref_35","doi-asserted-by":"crossref","unstructured":"Marzetta, T.L., and Yang, H. (2016). Fundamentals of Massive MIMO, Cambridge University Press.","DOI":"10.1017\/CBO9781316799895"},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Cramer, H. (1970). Random Variables and Probability Distributions, Cambridge University Press.","DOI":"10.1017\/CBO9780511470936"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"333","DOI":"10.1109\/VETECS.2001.944859","article-title":"Mobile propagation loss with a low base station antenna for NLOS street microcells in urban area","volume":"Volume 1","author":"Tang","year":"2001","journal-title":"Proceedings of the IEEE VTS 53rd Vehicular Technology Conference, Spring 2001, Proceedings (Cat. No. 01CH37202)"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/23\/11\/5014\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T19:40:36Z","timestamp":1760125236000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/23\/11\/5014"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2023,5,23]]},"references-count":37,"journal-issue":{"issue":"11","published-online":{"date-parts":[[2023,6]]}},"alternative-id":["s23115014"],"URL":"https:\/\/doi.org\/10.3390\/s23115014","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2023,5,23]]}}}