Low-Complexity Coding Techniques for Cloud Radio Access Networks

TL;DR

This paper tackles the challenge of coding for uplink and downlink C-RANs under finite fronthaul capacities by showing that the full rate-fronthaul region achievable with joint coding can be matched by low-complexity schemes built from a small set of point-to-point codes designed for symmetric channels.The core ideas are an equivalence between joint and successive decoding (uplink) or encoding (downlink), and rate-splitting combined with quantization-splitting that reduces the problem to a higher-dimensional corner point; these are then realized with Lego-brick codes.A key quantitative result is that at most $4(K+L)-2$ point-to-point codes suffice to achieve any point on the dominant face of the joint rate region, eliminating the need for time sharing across fronthaul configurations.The findings provide practically implementable coding schemes for C-RANs that leverage existing point-to-point codes, with potential extensions to binary modulations and broader hardware-friendly deployments.

Abstract

The problem of coding for the uplink and downlink of cloud radio access networks (C-RAN's) with $K$ users and $L$ relays is considered. It is shown that low-complexity coding schemes that achieve any point in the rate-fronthaul region of joint coding and compression can be constructed starting from at most $4(K+L)-2$ point-to-point codes designed for symmetric channels. This reduces the seemingly hard task of constructing good codes for C-RAN's to the much better understood task of finding good codes for single-user channels. To show this result, an equivalence between the achievable rate-fronthaul regions of joint coding and successive coding is established. Then, rate-splitting and quantization-splitting techniques are used to show that the task of achieving a rate-fronthaul point in the joint coding region can be simplified to that of achieving a corner point in a higher-dimensional C-RAN problem. As a by-product, some interesting properties of the rate-fronthaul region of joint decoding for uplink C-RAN's are also derived.

Figures (6)

• Figure 1: (a) Uplink and (b) Downlink of a two-user, two-relay C-RAN.
• Figure 2: Rate splitting for the $K$-user multiple access channel.
• Figure 3: Quantization splitting for $L$-source distributed lossy compression.
• Figure 4: Splitting for the $K$-user $L$-relay uplink C-RAN problem.
• Figure 5: Generalized order for $K=2$ and $L=2$. Bold entries correspond to the active elements when $j_2=1$, $j_3 =1$ and $j_4 = 6$.

Theorems & Definitions (17)

• Theorem 1
• Lemma 2
• Lemma 3
• Theorem 4
• Definition 5
• Definition 6: Dominant Face
• Lemma 7
• Lemma 8
• Lemma 9
• Lemma 10