Fronthaul is defined as the link between BBUs and RRHs.
[1] “Fronthaul-Constrained Cloud Radio Access Networks: Insights and Challenges”
The heterogeneous cloud radio access network (H-CRAN) as a 5G paradigm toward green and soft themes is briefly presented in [3] to enhance C-RAN.
To alleviate the capacity constraint on the fronthaul links, a multi-service small-cell wireless access architecture based on combining radio-over-fiber with optical wavelength division multiplexing (WDM) techniques is proposed in [4].
- C-RAN System Architectures:
A. C-RAN components: RRH, BBU pool, Fronthaul.
Non-ideal fronthaul: bandwidth, time latency and jitter constraints.
B. C-RAN System Structures: Full centralization, Partial centralization, Hybrid centralization.
- Signal Compression and Quantization
An interesting result shows that by simply setting the quantization noise power proportional to the background noise level at each RRH, the quantize-and-forward scheme can achieve a capacity within a constant gap to a throughput performance upper bound. [6]
A. Compression and Quantization in the Uplink
Distributed Wyner-Ziv lossy compression; independent compression.
B. Compression and Quantization in the Downlink
Hybrid compression and message-sharing strategy for DL transmission is presented in [10].
- Coordinated Signal Processing and Clustering
A. Precoding Techniques
Two types of IQ-data transfer methods: after-precoding & before-precoding.
Sparsity: individual sparsity, group sparsity.
B. Clustering Techniques ？
Two types of RRH clustering schemes: disjoint clustering and user-centric clustering.
An explicit expression for the successful access probability (SAP) for clustered RRHs is derived by applying stochastic geometry in [12].
- Radio Resource Allocation and Optimization
There are mainly three approaches to deal with the delay-aware RRAO problem: equivalent rate constraint, Lyapunov optimization, and Markov decision processes (MDPs).
In [13], a hybrid coordinated multi-point transmission (H-CoMP) scheme is presented for downlink transmission in frontal constrained C-RANs, which fulfills the flexible tradeoff between large-scale cooperation processing gain and frontal consumption.
- Challenging Work and Open Issues
A. C-RANs with SDN
B. C-RANs with NFV
C. C-RANs with Inter-Connected RRHs
[2] “Joint Power Control and Fronthaul Rate Allocation for Throughput Maximization in OFDMA-based Cloud Radio Access Network”
[3] “Joint Precoding and Multivariate Backhaul Compression for the Downlink of Cloud Radio Access Networks”
[4] “Robust and Efficient Distributed Compression for Cloud Radio Access Networks”
[5] “Joint Decompression and Decoding for Cloud Radio Access Networks”
[6] “Performance Evaluation of Multiterminal Backhaul Compression for Cloud Radio”
[7] “Inter-Cluster Design of Precoding and Fronthaul Compression for Cloud Radio Access Networks”
Compared inter-cluster with intra-cluster.
[8] “Hybrid Compression and Message-Sharing Strategy for the Downlink Cloud Radio-Access Network”
